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Food habits of Steller sea lions (Eumetopias iubatus) off Oregon and northern California, 1986-2007.

Abstract--We described the diet of the eastern stock of Steller sea lions (Eumetopias jubatus) from 1416 scat samples collected from five sites in Oregon and northern California from 1986 through 2007. A total of 47 prey types from 30 families were identified. The most common prey was Pacific hake (Merluccius productus), followed by salmonids (Oncorhynchus spp.), skates (Rajidae), Pacific lamprey (Lampetra tridentata), herrings (Clupeidae), rockfish (Sebastes spp.), and northern anchovy (Engraulis mordax). Steller sea lion diet composition varied seasonally, annually, and spatially. Hake and salmonids were the most commonly identified prey in scats collected during the summer (breeding season), whereas hake and skate were most common in the non-breeding season. Continued research on Steller sea lion diet and foraging behavior in the southern extent of their range is necessary to address issues such as climate change, interaction with competing California sea lions, and predation impacts on valuable or sensitive fish stocks.

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Knowledge of an animal's diet is important for understanding its foraging behavior, habitat use, and population dynamics, and this knowledge is of particular importance when considering threatened and endangered species. Steller sea lions (Eumetopias jubatus) are a case in point. Ranging throughout the North Pacific Rim from California to Japan (Loughlin et al., 1984; Pitcher et al., 2007), Steller sea lion populations in western Alaska underwent dramatic declines from the late 1970s to early 1990s (Braham et al., 1980; Merrick et al., 1987; Loughlin et al., 1992; Trites and Larkin, 1996). This population was listed as "threatened" under the Endangered Species Act (ESA) in 1990, and later the western stock was listed as "endangered" (Loughlin, 1997; NMFS, 2008). The primary hypothesis for the decline has been chronic nutritional stress related to changes in diet (Springer, 1992; Merrick and Loughlin, 1997; Trites and Donnelly, 2003; NMFS, 2008).

With the nutritional stress hypothesis (Springer, 1992; Merrick et al., 1997; Trites and Donnelly, 2003), and its successor, the ocean climate hypothesis (Trites et al., 2007a), declines in the Steller sea lion western distinct population segment (WDPS) were proposed to be the result of changes in the quantity, quality, and availability of prey, brought about by an ocean climate regime shift in 1976-77 (but see Fritz and Hinckley, [2005]). This shift is hypothesized to have forced Steller sea lions to change their diet and to have resulted in chronic nutritional stress manifested by reductions in body size, productivity, and juvenile and pup survival (York, 1994; Trites and Donnelly, 2003). Other explanations for the decline of the WDPS that were considered but rejected included population redistribution, commercial and subsistence harvest, predation, pollution, and entanglement in marine debris (Merrick et al., 1987).

Although the Steller sea lion WDPS experienced annual declines in abundance ranging between 1.6% and 5.2% (Merrick et al., 1987), the abundance in the eastern distinct population segment (EDPS) increased at 3.1% per year from 1977 through 2002 (Pitcher et al., 2007; NMFS, 2008). The hypothesized role of poor diet in the decline of the WDPS, contrasted with the increasing EDPS, begs the question as to what type of prey the Steller sea lion EDPS consumes and how does it compare with that of the WDPS. The majority of information on Steller sea lion diet, however, has come from Alaska (e.g., Pitcher, 1981; Merrick et al., 1997; Sinclair and Zeppelin, 2002; Womble and Sigler, 2006; Trites et al., 2007b; McKenzie and Wynne, 2008). In this study, we provide data on Steller sea lion diet from the southern extent of the EDPS range based on 1416 scat (fecal samples) collected from five sites in Oregon and northern California from 1986 through 2007. We tested for seasonal, annual, and spatial differences in diet composition and discuss our results in relation to findings from Alaska.

Materials and methods

Field and laboratory

We collected scat from four locations off Oregon and one location off northern California from 1986 through 2007 (Fig. 1; Table 1). Three of the five locations were occupied seasonally as rookeries (Orford Reef, Rogue Reef, St. George Reef), whereas the other two were strictly nonbreeding haul-outs (Columbia River South Jetty, Cascade Head). Scats collected from May through August were classified samples from the "breeding season" and scats from the remainder of the year, as samples from the "nonbreeding season." Scats were collected opportunistically as part of other research activities or during dedicated food habit collection trips.

Scat samples were collected and processed according to the method described in Lance et al. (1) Collections made after 2003 were processed with a standard washing machine according to collection and processing procedures described in Orr et al. (2003). Recovered hard parts were examined with a dissecting microscope and identified to the lowest possible taxonomic level by comparing all identifiable prey remains (e.g., bones, otoliths, cartilaginous parts, lenses, teeth, and cephalopod beaks) with a comparative reference collection of fish from the northeastern Pacific Ocean and Oregon estuaries. Individual samples that contained both identified prey and remains too eroded to be identified (unidentified fish) were included in this analysis, whereas samples with only unidentified remains (n=11) or no remains (n=22) were not.

Data analysis

We summarized the relative importance of prey in sea lion diet by calculating the frequency of occurrence (FO) of each prey type. Frequency of occurrence was defined as the number of scat containing a given prey type divided by the number of scat with identifiable prey. Although other summary statistics are possible, FO is a simple calculation, widely used, and probably least affected by differences in prey recovery (Tollit et al., 2010). We calculated exact 95% confidence intervals for FO by assuming that the number of scat in a collection containing a given prey was binomially distributed.

[FIGURE 1 OMITTED]

In addition to univariate summaries, we were also interested in testing whether multivariate diet composition differed between collections. Wright (2010) and Lemons et al. (2010) noted that the common practice of using chi-square tests to compare diets violates the assumption of independence for that test by ignoring the nesting of multiple prey items within a scat. Violation of the assumption of independence results in psuedoreplication and biased chi-square statistics. More appropriate alternatives for comparing multivariate diet composition between groups include distance-based permutation methods (e.g., Luo and Fox, 1996; Anderson, 2001; Berry and Mielke, 2003); multiple-response categorical variable methods (e.g., Agresti and Liu, 1999; Bilder and Loughlin, 2009; Nandram et al., 2009); and mark-recapture methods (Lemons et al., 2010). We chose the distance-based Mantel test (Mantel, 1967; Luo and Fox, 1996) because it could be formulated to address our questions of interest, was easy to implement in existing software, and has been used by other researchers studying animal diets (e.g., Hudon and Lamarche, 1989; Green and Burton, 1993; Jones and Barmuta, 1998).

We implemented Mantel tests, using package "vegan" (Oksanen et al., 2009) in R (R Development Core Team, 2009). We tested whether diet composition differed by month (after controlling for year and site), year (after controlling for month and site), or site (after controlling for month and year). Distances among scat samples were computed using the Jaccard coefficient which is an asymmetrical binary coefficient commonly used to compare sampling units using species presence-absence data (Legendre and Legendre, 1998). A Jaccard distance of zero indicates that two scat shared all of the same prey items, whereas a distance of one indicates that they had no prey items in common. We paired the Jaccard distance matrix with a design matrix consisting of zeros for between-population distances and 1/ ([n.sub.i]-1) for within-population distances (where indicates population membership; see Manly, 1997). When used with a design matrix the Mantel test is equivalent to a nonparametric multivariate analysis of variance (Sokal and Rohlf, 1995).

We compared diets based on prey identified to the lowest possible taxon in order to limit the potential for spurious differences arising from an arbitrary categorization of prey types, although this procedure resulted in some comparisons where data were not at an equivalent taxonomic level. Analysis based on additional categorization of prey--such as size, ecology, or abundance--although potentially useful, was beyond the limits of what the data could support. We restricted statistical comparisons to selected unpooled collections with at least 30 samples. For each test, 9999 randomizations were used to obtain the distribution for the Mantel test statistic ([r.sub.M]) and to calculate probability (P) values. A significance level of [alpha]=0.002 was used based on a Bonferroni adjustment of [alpha]=0.05 for 26 multiple comparisons.

Results

We collected 1416 Steller sea lion scat samples during 42 collection trips from 1986 through 2007. The number of scat collected per trip ranged from 2 to 78 (mean of 34) (Table 1). Of the 1416 scat, 22 were discarded from analysis because they had no prey and 11 were discarded because they contained only unidentified prey, resulting in a working data set of 1383 scat. The majority of samples came from Rogue Reef during the breeding (n=526) and nonbreeding (n=290) seasons, followed by collections at the Columbia River South Jetty (n=219) and St. George Reef (n=165) during the breeding season. Only minor collections were made at Orford Reef during the breeding season (n=56), and Cascade Head (n=24) and St. George Reef (n=7) during the nonbreeding season (Table 1).

A total of 47 Steller sea lion prey taxa from 30 families were identified (33 to species) (Table 2). Overall percent frequency of occurrence for the most common (FO>10%) prey in decreasing order were Pacific hake (Merluccius productus, FO = 78.6%), salmonids (Oncorhynchus spp.; FO = 28.6%), skates (Rajidae; FO = 23.4%), Pacific lamprey (Lampetra tridentata; FO = 20.8%), clupeids (Clupeidae; FO = 18.7%), rockfish (Sebastes spp.; FO = 17.4%), northern anchovy (Engraulis mordax; FO = 13.2%), and unidentified teleost fishes (FO = 10.8%) (Table 2, Fig. 2). Scat during the breeding season were dominated by hake (87.1%), followed by salmonids (27.1%) and Pacific lamprey at 20.1%. Hake, with an FO of 59%, was also a primary prey in samples collected during the nonbreeding season and skate species increased in frequency to 40.3%, followed by salmonids (32.1%), and rockfish (29.7%) (Table 2). Prey diversity within scat samples ranged from one to 25 types, although 64% of all samples had <3 prey types. Of the 222 scat collected during the breeding season that contained a single prey item, 85.1% contained Pacific hake and 4.1% contained rockfish. Scat collected during the nonbreeding season that contained a single prey species (n = 63), 49.2% contained hake and 22.2% rockfish.

By site and season (Fig. 2), Pacific hake occurred in more scats than any other prey taxa among all sites and seasons except at Cascade Head and St. George Reef during the nonbreeding season. For example, Pacific hake was the dominant prey in scats collected at Rogue Reef, the largest rookery in the study area, both during breeding (87.3%) and nonbreeding (62.1%) seasons. Although salmonids occurred with high frequency at all sites and seasons, except at Cascade Head, the highest frequency was found at Rogue Reef during the nonbreeding season. Skates, although consumed at all sites and seasons, occurred most frequently in scats collected during the nonbreeding season. For example, skate FO increased at Rogue Reef from 16.2% to 45.5% during the breeding and nonbreeding seasons, respectively. Pacific staghorn sculpin (Leptocottus armatus) was common only at the northern sites (i.e., Columbia River and Cascade Head), whereas rockfish were common only at the southern sites (particularly Rogue Reef).

In general, diet composition varied seasonally, annually, and spatially. After controlling for site and year (10 of 11 comparisons; Table 3), we found that diet differed by month; after controlling for site and month, we found that diet differed by year (10 of 12 comparisons; Table 4); and after controlling for year and month (2 of 3 comparisons; Table 5), we found that diet differed by site. Average Jaccard distance within collections ranged from 0.206 to 0.807 (median of 0.724), whereas average Jaccard distance between collections ranged from 0.425 to 0.911 (median of 0.771).

Discussion

Like other researchers (e.g., Pitcher, 1981; Merrick et al., 1997; Sinclair and Zeppelin, 2002; Womble and Sigler, 2006; Trites et al., 2007b; McKenzie and Wynne, 2008), we found that Steller sea lion diet was diverse yet dominated by only one or two species (Fig. 3). In Oregon and northern California the diet was dominated by Pacific hake, whereas in Alaska diet was dominated by walleye pollock (Theragra chalcogramma) in the Bering Sea and Gulf of Alaska, and Atka mackerel (Pleurogrammus monopterygius) in the Aleutians Islands. Prey types shared between Alaskan and Oregon-northern California collections included salmonids, clupeids (e.g., Pacific herring [Clupea pallasii]), rockfish, and skate.

The dominance of Pacific hake in Steller sea lion diets in Oregon and northern California is probably related to the widespread abundance of this species in the California current (as is the case with the widespread distribution of walleye pollock in Alaskan waters). Dorn et al. (2) reported that Pacific hake, ranging from southern California to the Queen Charlotte Sound, British Columbia, was the most abundant groundfish in the California Current system. During summer months adult Pacific hake move north along the Oregon coast while juveniles stay further south off central California (Bailey et al., 1982). From 1966 to 2007 the Pacific Coast (U.S, and Canadian waters) Pacific hake fishery landings averaged 219,000 metric tons (t), with a low of 90,000 t in 1980 and a peak harvest of 364,000 t in 2006 (Helser et al., 2008). Pacific hake are similar in caloric density to cod and pollock, which are prominent in the diet of Steller sea lions in the WDPS. This gadid diet has been hypothesized to result in chronic nutritional stress and ultimately population declines (Trites and Donnelly, 2003, Trites et al., 2007a). However, despite the dominance of Pacific hake in the diet from Oregon and northern California, Steller sea lions in the EDPS have been increasing at approximately 3% per year since the 1970s (Pitcher et al., 2007). This fact was cited by Fritz and Hinckley (2005) as evidence that was inconsistent with the nutritional stress hypothesis.

[FIGURE 2 OMITTED]

How important a specific type of prey is to the survival of an opportunistic marine pinniped predator is unknown. Abundance of prey may have more impact on survival when a predator feeds on schooling fish rather than on more solitary types of prey, such as flatfish and sculpin. Furthermore, a diversity of prey types may be important in sustaining populations and help buffer the effects of ocean climate changes. For example, analyses by Merrick et al. (1997) and Trites et al. (2007b) showed a strong positive correlation between diet diversity and rate of population change. Our data are consistent with this finding; we identified 17 primary ([greater than or equal to] 5%) prey types (Fig. 3) and the population has been growing at approximately 3% per year (Pitcher et al., 2007).

Perhaps as a reflection of their diverse diet, we found a surprisingly high number of statistical differences in diet composition between months (Table 3), years (Table 4), and sites (Table 5). Although simulation exercises (not presented) indicated that the Mantel test was not overly sensitive (e.g., it did not reject a null hypothesis simply due to a large difference in a single prey type), the procedure is only a hypothesis test and does not lend itself to estimation of effect sizes or biologically interpretable parameters. Nonetheless, it does indicate that researchers should be cautious about pooling samples across space and time before investigating whether those samples differ.

Athough analysis of pinniped fecal matter is a standard technique for studying diet (e.g., Pitcher, 1980; Beach et al. (3); Olesiuk et al., 1990; Orr et al., 2004), there are some limitations. For example, the use of otoliths to identify prey can lead to biased diet composition estimates (Jobling and Breiby, 1986). We minimized this problem by including all bony skeletal structures (vertebrae, gillrakers, etc.) to identify prey. Another potential bias can occur when drawing inference to a particular population from opportunistically collected samples. For example, scat that we collected on rookeries (i.e., Rogue Reef, Orford Reef, St. George Reef) during the summer breeding season primarily reflect adult female diet because males often fast during the breeding season and juveniles are not generally present at rookery sites.

[FIGURE 3 OMITTED]

The Steller sea lion recovery plan (NMFS, 2008) notes that although several factors affecting the endangered WDPS also affect the threatened EDPS, those threats do not appear to be affecting the sustained growth or recovery of the EDPS. It is noted in the plan, however, that concerns regarding climate change, particularly on the southern part of the species range, warranted continued research and monitoring. Population growth in California sea lions (Carretta et al., 2010) may also be a concern for the EDPS because these sympatric ottariids potentially compete for prey resources and habitat. Steller sea lions in the Channel Island rookeries in California experienced a similar situation in the late 1950s as California sea lion populations increased and potentially out-competed Steller sea lions for food and habitat (Bartholomew and Boolootian, 1960). Additionally, as the Steller sea lion EDPS increases, its real and perceived impacts on sport and commercial fish harvests; as well as threatened and endangered fish populations, will likely increase. For example, Steller sea lion abundance at Bonneville Dam on the Columbia River (Fig. 1), 235 km from the ocean, increased from zero individuals in 2002 to at least 53 in 2010 and these sea lions have consumed hundreds of threatened and endangered salmonids and thousands of white sturgeon (Acipenser transmontanus) (Stansell et al. (4)). Ongoing uncertainties over the role of diet in the decline of the WDPS, impacts of climate change on the EDPS, and emerging management concerns all argue for continued and refined research on Steller sea lion diet and foraging behavior in the southern extent of their range.

Conclusions

Identification of prey from 1383 Steller sea lions scats collected in Oregon and northern California during 1986-2007 resulted in a list of 47 prey taxa consumed. Primary prey items included Pacific hake, Pacific salmon, skate, Pacific lamprey, rockfish and clupeid species. Prey identified from scat during the breeding and nonbreeding seasons were fairly similar but rockfish and skate species had a higher frequency of occurrence during the nonbreeding season. Data analysis showed that, in general, diet composition varied seasonally, annually, and spatially. When compared to previous diet studies for Steller sea lions in Alaska, this population was shown to depend on hake as the primary prey rather than on the gadid and hexagrammid species identified in the northern populations studied. Salmonids were important prey in all the studies compared. Continued study of Steller sea lion food habits is necessary to evaluate their interactions with important fish populations (such as salmonids and rockfish), to assess the increasing pressure from migrating California sea lions for limited prey resources, and to begin to address the effects of climate change on population abundance.

Acknowledgments

This article summarizes over 25 years of research on Steller sea lions in Oregon that would not have been accomplished without the help of many individuals and agencies. We would like to thank the following for support, guidance, and assistance: Oregon Department of Fish and Wildlife, National Marine Fisheries Service (J. Scordino, P. Gearin, R. DeLong), U.S. Fish and Wildlife Service Oregon Coast National Wildlife Refuge (P. Sekora, R. Lowe, D. Pitkin), M. Tennis, J. Jenniges, J. Scordino, M. Dhruv, A. Ougzin, S. Jeffries, J. Harvey, J. Stein, and many others who assisted in the field with collection of data and samples over the years. R. Emmett, P. Gearin, D. Fox, and R. Stauff and three anonymous reviewers provided beneficial comments on the manuscript. This research was conducted under Marine Mammal Protection Act scientific research permit numbers 499, 835, 854, 782-1446, 434-1669, and 434-1892.

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Manuscript submitted 27 October 2010.

Manuscript accepted 18 May 2011.

Fish. Bull. 109:369-381 (2011).

The views and opinions expressed or implied in this article are those of the author (or authors) and do not necessarily reflect the position of the National Marine Fisheries Service, NOAA.

Susan D. Riemer (1) (contact author)

Bryan E. Wright (2)

Robin F. Brown (2)

Email address for contact author: susan.d.riemer@state.or.us

(1) Oregon Department of Fish and Wildlife

Marine Mammal Research Program

149S E. Gregory Road

Central Point, Oregon 97502

(2) Oregon Department of Fish and Wildlife

Marine Mammal Research Program

7116 NE Vandenberg Avenue

Corvallis, Oregon 97330

(1) Lance, M. M., A. J. Orr, S. D. Riemer, M. J. Weise, and J. L. Laake. 2001. Pinniped food habits and prey identification techniques protocol. AFSC (Alaska Fisheries Science Center) Proc. Rep. 2001-04, 36 p. Alaska Fisheries Science Center, NMFS, NOAA, 7600 Sand Point Way NE, Seattle, WA 98115.

(2) Dorn, M. W., M. W. Saunders, C. D. Wilson, M. A. Guttormsen, K. Cooke, R. Kieser, and M. E. Wilkins. 1999. Status of the coastal Pacific hake/whiting stock in U.S. and Canada in 1998, 102 p. [Available at Pacific Fishery Management Council, 7700 NE Ambassador Place, Suite 101, Portland, OR. 97220 1384.]

(3) Beach, R. J., A.C. Greiger, S. J. Jeffries, S. D. Treacy, and B. L. Troutman. 1985. Marine mammals and their interactions with fisheries of the Columbia River and adjacent waters, 1980-1982: third annual report, March 1, 1980 to October 31, 1982. National Marine Mammal Laboratory, Northwest and Alaska Fisheries Center, NMFS, NOAA Proc. Rep. 85-03, 316 p. [Available at www.lib.noaa.gov, accessed May 2011.]

(4) Stansell, R. J, K. M. Gibbons, and W. T. Nagy. 2010. Evaluation of pinniped predation on adult salmonids and other fish in the Bonneville Dam tailrace, 2008-2010. U.S. Army Corps of Engineers, Cascade Locks, OR. [Available online at http:// www.nwd-wc.usace.army.mil/tmt/documents/fish/2010/ 2008-2010_Pinniped_Report.pdf, accessed 2 March 2011.]
Table 1
Total number of Steller sea lion (Eumetopias jubatus) scat collected
by site, month, and year from haul-outs and rookeries in northern
California and Oregon, 1986-2007.

Location              Year     Breeding season month

                             May   Jun   Jul   Aug

Columbia Rivers (1)   2004          55          38
                      2006                      48
                      2007                20    61
Cascade Head (2)      2003
Orford Reef (3)       1990                41
                      2002                15
Rogue Reef (4)        1986          18
                      1987          40
                      1988          20
                      1990                47
                      1993          36
                      1994          33
                      1995          12
                      1996    60
                      2001          70          48
                      2002                33    37
                      2003                12
                      2004                33
                      2005     2          13
                      2006                25
St. George Reef (5)   1990                 4
                      1994          37
                      2002                35
                      2003                29
                      2004                34
                      2006                33
Total                         62   321   374   232

Location              Year   Nonbreeding season month

                             Sep   Oct   Nov   Dec

Columbia Rivers (1)   2004    45
                      2006
                      2007     3
Cascade Head (2)      2003                13
Orford Reef (3)       1990
                      2002
Rogue Reef (4)        1986
                      1987
                      1988
                      1990
                      1993
                      1994
                      1995
                      1996
                      2001
                      2002
                      2003                54
                      2004
                      2005
                      2006
St. George Reef (5)   1990
                      1994
                      2002
                      2003
                      2004
                      2006
Total                         48    67     0     0

Location              Year   Nonbreeding season month

                             Jan   Feb   Mar   Apr   Total

Columbia Rivers (1)   2004                      51     189
                      2006                              48
                      2007                              84
Cascade Head (2)      2003    11                        24
Orford Reef (3)       1990                              41
                      2002                              15
Rogue Reef (4)        1986                              18
                      1987                              40
                      1988                              20
                      1990                              47
                      1993                              36
                      1994                              33
                      1995                              12
                      1996                              60
                      2001                      46     164
                      2002                42    78     190
                      2003                      57     123
                      2004                              33
                      2005                      20      35
                      2006                              25
St. George Reef (5)   1990                               4
                      1994                              37
                      2002                              35
                      2003                       7      36
                      2004                              34
                      2006                              33
Total                         11     0    42   259    1416

(1) South Jetty (46.233 sat. N, 124.070 long. W).

(2) Sea Lion Cove (45.067 sat. N, 124.013 long. W).

(3) Long Brown Rock (42.791 sat. N, 124.605 long. W).

(4) Primarily Pyramid Rock (42.444 lat. N, 124.469 long. W), but also
surrounding sites including Needle Rock (42.448 sat. N, 124.483 long.
W), Double Rock (42.449 lat. N, 124.490 long. W), and South Seal Rock
(42.436 lat. N, 124.465 long. W).

(5) South Seal Rock (41.813 sat. N, 124.351 long. W).

Table 2
Sample information and frequency of occurrence (FO) of prey identified
from Steller sea lion (Eumetopias jubatus) scat collected in Oregon
and northern California from 1986 through 2007. FO is presented by
collection site (CR = Columbia River, OR = Orford Reef, RR = Rogue
Reef, and SGR = St. George Reef; CH = Cascade Head, see Fig. 1 and
Table 1) and season (breeding = May-August, nonbreeding = September-
April). Prey are sorted by family in decreasing order of total FO.

                                                         Season

                                         Total   Breeding   Nonbreeding
Samples
  Total scat collected                    1416        989           427
  Scat containing [greater than or        1383        966           417
    equal to] 1 identifiable prey
  Scat containing no identifiable prey      11          6             5
  Empty scat                                22         17             5
Prey item
  Hakes: family Merlucciidae
    Pacific hake (Merluccius              78.6       87.1          59.0
      productus)
  Salmon: family Salmonidae
    Pacific salmon (Oncorhynchus spp.)    28.6       27.1          32.1
  Skate: family Rajidae
    Skate, unidentified                   23.4       16.1          40.3
  Lamprey: family Petromyzontidae
    Pacific lamprey (Lampetra             20.8       20.1          22.3
      tridentata)
  Herring, shad, sardine: family
      Clupeidae
    Unidentified clupeid                  18.7       17.7          20.9
    Pacific herring (Clupea pallasii)      9.9       12.4           4.1
    Pacific sardine (Sardinops sagax)      9.2       10.5           6.2
    American shad (Alosa sapidissima)      2.2        1.9           3.1
  Rockfish: family Sebastidae
    Rockfish (Sebastes spp.)              17.4       12.1          29.7
  Anchovies: family Engraulidae
    Northern anchovy (Engraulis           13.2       11.9          16.1
      mordax)
  Class Osteichthyes
    Teleost fishes, unidentified          10.8       10.1          12.5
  Sculpins: family Cottidae
    Pacific staghorn sculpin               9.4        7.2          14.4
      (Leptocottus armatus)
    Sculpins, unidentified                 5.5        5.8           4.8
    Irish lord (Hemilepidotus spp.)        1.6        2.0           0.7
    Buffalo sculpin (Enophrys bison)       0.2        0.1           0.5
  Sand lances: family Ammodytidae
    Pacific sand lance                     9.0        5.6          16.8
      (Ammodytes hexapterus)
  Smelts: family Osmeridae
    Smelts, unidentified                   6.8        6.3           7.9
    Eulachon (Thaleichthys pacificus)      0.1        0.0           0.2
    Surf smelt (Hypomesus pretiosus)       0.1        0.0           0.2
  Stickleback: family Gasterosteidae
    Threespine stickleback                 6.1        5.5           7.4
      (Gasterosteus aculeatus)
  Squids and octopus: class
      Cephalopoda
    Squid and octopus, unidentified        5.7        5.6           6.0
    Squids, unidentified                   4.3        3.9           5.0
    Octopus, unidentified                  2.8        2.6           3.4
    Jack mackerels: family Carangidae
    Pacific jack mackerel                  5.4        7.1           1.2
      (Trachurus synimetricus)
  Codfishes: family Gadidae
    Pacific tomcod (Microgadus             4.3        3.0           7.4
      proximus)
    Codfishes, unidentified                1.5        1.2           2.2
    Pacific cod (Gadus macrocephalus)      0.1        0.0           0.2
  Righteye flounders: family
      Pleuronectidae
    Starry flounder (Platichthys           4.3        3.3           6.7
      stellatus)
    Dover sole (Microstomus pacificus)     2.1        2.1           2.2
    Rigbteye flounder, unidentified        1.9        1.6           2.6
    Rex sole (Glyptocephalus zachirus)     1.2        1.1           1.4
    Sand sole (Psettichthys                1.2        0.8           2.2
      melanostictus)
    Slender sole (Lyopsetta exilis)        1.2        1.0           1.7
    Butter sole (Isopsetta isolepis)       1.2        0.6           2.4
    English sole (Parophrys vetulus)       0.7        0.8           0.5
    Arrowtooth flounder                    0.1        0.1           0.2
      (Atheresthes stomias)
    Rock sole (Lepidopsetta bilineata)     0.1        0.1           0.0
  Flatfishes: order Pleuronectiformes
    Flatfishes, unidentified               3.1        2.6           4.3
  Dogfish sharks: family Squalidae
    Spiny dogfish (Squalus acanthias)      2.7        2.8           2.4
  Sanddabs: family Paralichthyidae
    Sanddabs (Citharichthys spp.)          2.7        2.2           4.1
  Hagfishes: family Myxinidae
    Pacific hagfish (Eptatretus            1.6        2.1           0.5
      stoutii)
  Mackerel and tuna: family Scombridae
    Pacific chub mackerel                  1.4        2.1           0.0
      (Scomber japonicus)
  Greenlings: family Hexagrammidae
    Greenling/Lngcod, unidentified         0.7        0.8           0.2
    Lingcod (Ophiodon elongatus)           1.3        0.6           2.9
    Greenling (Hexagrammos spp.)           0.3        0.2           0.5
  Poachers: family Agonidae
    Poachers, unidentified                 1.3        1.6           0.7
    Sturgeon poacher                       0.1        0.1           0.0
      (Podothecus accipenserinus)
  Subclass Elasmobranchii
    Sharks and rays, unidentified          1.2        1.3           1.0
    Sharks, unidentified                   0.3        0.1           0.7
  Surfperch: family Embiotocidae
    Surfperch, unidentified                1.1        0.7           1.9
  Gunnel: family Pholidae
    Gunnel, unidentified                   1.0        0.9           1.2
  Wolffishes: family Anarhichadidae
    Wolf eel (Anarrhichthys ocellatus)     0.8        1.1           0.0
  Snailfish: family Liparidae
    Snailfish and lumpfish,                0.7        0.9           0.2
      unidentified
  Cusk-eels: family Ophidiidae
    Spotted cusk-eel (Chilara taylori)     0.3        0.3           0.2
  Prickleback: family Stichaeidae
    Pricklebacks, unidentified             0.3        0.4           0.0
  Class Agnatha
    Jawless fishes, unidentified           0.1        0.1           0.0
  Clingfish: family Gobiesocidae
    Clingfishes, unidentified              0.1        0.2           0.0
    Northern clingfish 0.1                 0.1        0.0           0.0
      (Gobiesox maeandricus)
  Eelpout: family Zoarcidae
    Eelpouts, unidentified                 0.1        0.1           0.2
  Pipefish: family Syngnathidae
    Bay pipefish                           0.1        0.1           0.0
    (Syngnathus leptorhynchus)
  Sandfish: family Trichodontidae
    Pacific sandfish (Trichodon            0.1        0.1           0.0
      trichodon)
    Sandfishes, unidentified               0.1        0.0           0.2

                                                 Breeding season

                                          CR      OR      RR      SGR
Samples
  Total scat collected                     222      56     539     172
  Scat containing [greater than or         219      56     526     165
    equal to] 1 identifiable prey
  Scat containing no identifiable prey       0       0       4       2
  Empty scat                                 3       0       9       5
Prey item
  Hakes: family Merlucciidae
    Pacific hake (Merluccius              80.8    98.2    87.3    90.9
      productus)
  Salmon: family Salmonidae
    Pacific salmon (Oncorhynchus spp.)    27.9    21.4    27.6    26.7
  Skate: family Rajidae
    Skate, unidentified                   21.5     8.9    16.2    11.5
  Lamprey: family Petromyzontidae
    Pacific lamprey (Lampetra             12.3     3.6    28.5     9.1
      tridentata)
  Herring, shad, sardine: family
      Clupeidae
    Unidentified clupeid                  27.4     7.1    14.8    17.6
    Pacific herring (Clupea pallasii)      6.8     5.4    18.1     4.2
    Pacific sardine (Sardinops sagax)     10.5    19.6     6.7    19.4
    American shad (Alosa sapidissima)      4.6     1.8     1.1     0.6
  Rockfish: family Sebastidae
    Rockfish (Sebastes spp.)               3.2     5.4    14.8    17.6
  Anchovies: family Engraulidae
    Northern anchovy (Engraulis           33.8     8.9     4.2     8.5
      mordax)
  Class Osteichthyes
    Teleost fishes, unidentified           5.9     5.4    11.6    12.7
  Sculpins: family Cottidae
    Pacific staghorn sculpin              26.5     1.8     1.7     1.2
      (Leptocottus armatus)
    Sculpins, unidentified                 8.7     5.4     5.7     2.4
    Irish lord (Hemilepidotus spp.)        0.0     1.8     1.1     7.3
    Buffalo sculpin (Enophrys bison)       0.0     0.0     0.0     0.6
  Sand lances: family Ammodytidae
    Pacific sand lance                     1.4    17.9     6.5     4.2
      (Ammodytes hexapterus)
  Smelts: family Osmeridae
    Smelts, unidentified                   3.7     0.0     8.2     6.1
    Eulachon (Thaleichthys pacificus)      0.0     0.0     0.0     0.0
    Surf smelt (Hypomesus pretiosus)       0.0     0.0     0.0     0.0
  Stickleback: family Gasterosteidae
    Threespine stickleback                 0.0     7.1     4.6    15.2
      (Gasterosteus aculeatus)
  Squids and octopus: class
      Cephalopoda
    Squid and octopus, unidentified        0.5     3.6     6.8     9.1
    Squids, unidentified                   0.0     5.4     5.9     2.4
    Octopus, unidentified                  0.0     0.0     4.0     2.4
    Jack mackerels: family Carangidae
    Pacific jack mackerel                  2.3    14.3     5.9    15.2
      (Trachurus synimetricus)
  Codfishes: family Gadidae
    Pacific tomcod (Microgadus             0.5     1.8     4.2     3.0
      proximus)
    Codfishes, unidentified                0.0     0.0     1.3     3.0
    Pacific cod (Gadus macrocephalus)      0.0     0.0     0.0     0.0
  Righteye flounders: family
      Pleuronectidae
    Starry flounder (Platichthys          12.8     0.0     0.6     0.6
      stellatus)
    Dover sole (Microstomus pacificus)     3.2     0.0     1.7     2.4
    Rigbteye flounder, unidentified        3.7     0.0     1.0     1.2
    Rex sole (Glyptocephalus zachirus)     0.9     0.0     1.1     1.8
    Sand sole (Psettichthys                0.5     0.0     1.0     1.2
      melanostictus)
    Slender sole (Lyopsetta exilis)        0.0     0.0     0.8     3.6
    Butter sole (Isopsetta isolepis)       0.5     0.0     0.8     0.6
    English sole (Parophrys vetulus)       0.9     0.0     1.0     0.6
    Arrowtooth flounder                    0.0     0.0     0.0     0.6
      (Atheresthes stomias)
    Rock sole (Lepidopsetta bilineata)     0.0     0.0     0.2     0.0
  Flatfishes: order Pleuronectiformes
    Flatfishes, unidentified               2.3     1.8     3.2     1.2
  Dogfish sharks: family Squalidae
    Spiny dogfish (Squalus acanthias)      5.0     0.0     2.5     1.8
  Sanddabs: family Paralichthyidae
    Sanddabs (Citharichthys spp.)          2.3     0.0     1.7     4.2
  Hagfishes: family Myxinidae
    Pacific hagfish (Eptatretus            0.0     0.0     2.9     3.0
      stoutii)
  Mackerel and tuna: family Scombridae
    Pacific chub mackerel                  0.0     0.0     3.4     1.2
      (Scomber japonicus)
  Greenlings: family Hexagrammidae
    Greenling/Lngcod, unidentified         0.0     1.8     0.6     2.4
    Lingcod (Ophiodon elongatus)           0.0     0.0     1.0     0.6
    Greenling (Hexagrammos spp.)           0.0     0.0     0.0     1.2
  Poachers: family Agonidae
    Poachers, unidentified                 3.7     0.0     0.4     3.0
    Sturgeon poacher                       0.5     0.0     0.0     0.0
      (Podothecus accipenserinus)
  Subclass Elasmobranchii
    Sharks and rays, unidentified          3.7     0.0     0.8     0.6
    Sharks, unidentified                   0.0     0.0     0.2     0.0
  Surfperch: family Embiotocidae
    Surfperch, unidentified                0.0     0.0     1.1     0.6
  Gunnel: family Pholidae
    Gunnel, unidentified                   0.5     0.0     0.8     2.4
  Wolffishes: family Anarhichadidae
    Wolf eel (Anarrhichthys ocellatus)     0.0     1.8     1.3     1.8
  Snailfish: family Liparidae
    Snailfish and lumpfish,                1.4     1.8     1.0     0.0
      unidentified
  Cusk-eels: family Ophidiidae
    Spotted cusk-eel (Chilara taylori)     0.0     0.0     0.2     1.2
  Prickleback: family Stichaeidae
    Pricklebacks, unidentified             0.0     0.0     0.8     0.0
  Class Agnatha
    Jawless fishes, unidentified           0.0     0.0     0.2     0.0
  Clingfish: family Gobiesocidae
    Clingfishes, unidentified              0.0     0.0     0.4     0.0
    Northern clingfish 0.1                 0.0     0.2     0.0     0.0
      (Gobiesox maeandricus)
  Eelpout: family Zoarcidae
    Eelpouts, unidentified                 0.0     0.0     0.0     0.6
  Pipefish: family Syngnathidae
    Bay pipefish                           0.0     0.0     0.0     0.6
    (Syngnathus leptorhynchus)
  Sandfish: family Trichodontidae
    Pacific sandfish (Trichodon            0.0     0.0     0.2     0.0
      trichodon)
    Sandfishes, unidentified               0.0     0.0     0.0     0.0

                                                 Nonbreeding season

                                          CR      CH      RR      SGR
Samples
  Total scat collected                      99      24     297       7
  Scat containing [greater than or          96      24     290       7
    equal to] 1 identifiable prey
  Scat containing no identifiable prey       0       0       5       0
  Empty scat                                 3       0       2       0
Prey item
  Hakes: family Merlucciidae
    Pacific hake (Merluccius              64.6     4.2    62.1    12.9
      productus)
  Salmon: family Salmonidae
    Pacific salmon (Oncorhynchus spp.)    22.9     4.2    37.9    14.3
  Skate: family Rajidae
    Skate, unidentified                   26.0    25.0    45.5    71.4
  Lamprey: family Petromyzontidae
    Pacific lamprey (Lampetra             19.8     0.0    25.2    14.3
      tridentata)
  Herring, shad, sardine: family
      Clupeidae
    Unidentified clupeid                  18.8    37.5    20.3    14.3
    Pacific herring (Clupea pallasii)      2.1     4.2     4.8     0.0
    Pacific sardine (Sardinops sagax)     16.7     8.3     2.8     0.0
    American shad (Alosa sapidissima)      5.2     8.3     2.1     0.0
  Rockfish: family Sebastidae
    Rockfish (Sebastes spp.)               6.3     0.0    40.3    14.3
  Anchovies: family Engraulidae
    Northern anchovy (Engraulis           24.0    33.3    11.4    42.9
      mordax)
  Class Osteichthyes
    Teleost fishes, unidentified          13.5     4.2    12.8    14.3
  Sculpins: family Cottidae
    Pacific staghorn sculpin              35.4    54.2     4.5     0.0
      (Leptocottus armatus)
    Sculpins, unidentified                 1.0     4.2     6.2     0.0
    Irish lord (Hemilepidotus spp.)        0.0     0.0     1.0     0.0
    Buffalo sculpin (Enophrys bison)       0.0     0.0     0.7     0.0
  Sand lances: family Ammodytidae
    Pacific sand lance                     4.2     8.3    20.7    57.1
      (Ammodytes hexapterus)
  Smelts: family Osmeridae
    Smelts, unidentified                   6.3     8.3     8.3    14.3
    Eulachon (Thaleichthys pacificus)      1.0     0.0     0.0     0.0
    Surf smelt (Hypomesus pretiosus)       0.0     0.0     0.3     0.0
  Stickleback: family Gasterosteidae
    Threespine stickleback                 0.0     0.0     9.7    42.9
      (Gasterosteus aculeatus)
  Squids and octopus: class
      Cephalopoda
    Squid and octopus, unidentified        3.1     0.0     7.6     0.0
    Squids, unidentified                   0.0     0.0     7.2     0.0
    Octopus, unidentified                  1.0     8.3     3.8     0.0
    Jack mackerels: family Carangidae
    Pacific jack mackerel                  1.0     0.0     1.4     0.0
      (Trachurus synimetricus)
  Codfishes: family Gadidae
    Pacific tomcod (Microgadus             6.3    12.5     7.6     0.0
      proximus)
    Codfishes, unidentified                1.0     0.0     2.8     0.0
    Pacific cod (Gadus macrocephalus)      0.0     0.0     0.3     0.0
  Righteye flounders: family
      Pleuronectidae
    Starry flounder (Platichthys          18.8     4.2     3.1     0.0
      stellatus)
    Dover sole (Microstomus pacificus)     0.0     0.0     3.1     0.0
    Rigbteye flounder, unidentified        1.0     8.3     2.8     0.0
    Rex sole (Glyptocephalus zachirus)     0.0     0.0     2.1     0.0
    Sand sole (Psettichthys                2.1     0.0     2.4     0.0
      melanostictus)
    Slender sole (Lyopsetta exilis)        0.0     0.0     2.4     0.0
    Butter sole (Isopsetta isolepis)       4.2     4.2     1.4    14.3
    English sole (Parophrys vetulus)       1.0     0.0     0.3     0.0
    Arrowtooth flounder                    1.0     0.0     0.0     0.0
      (Atheresthes stomias)
    Rock sole (Lepidopsetta bilineata)     0.0     0.0     0.0     0.0
  Flatfishes: order Pleuronectiformes
    Flatfishes, unidentified               5.2     4.2     3.8    14.3
  Dogfish sharks: family Squalidae
    Spiny dogfish (Squalus acanthias)      2.1     0.0     2.8     0.0
  Sanddabs: family Paralichthyidae
    Sanddabs (Citharichthys spp.)          5.2    20.8     2.1    14.3
  Hagfishes: family Myxinidae
    Pacific hagfish (Eptatretus            0.0     0.0     0.7     0.0
      stoutii)
  Mackerel and tuna: family Scombridae
    Pacific chub mackerel                  0.0     0.0     0.0     0.0
      (Scomber japonicus)
  Greenlings: family Hexagrammidae
    Greenling/Lngcod, unidentified         0.0     0.0     0.0    14.3
    Lingcod (Ophiodon elongatus)           1.0     4.2     3.4     0.0
    Greenling (Hexagrammos spp.)           1.0     0.0     0.3     0.0
  Poachers: family Agonidae
    Poachers, unidentified                 1.0     0.0     0.7     0.0
    Sturgeon poacher                       0.0     0.0     0.0     0.0
      (Podothecus accipenserinus)
  Subclass Elasmobranchii
    Sharks and rays, unidentified          1.0     0.0     1.0     0.0
    Sharks, unidentified                   0.0     0.0     1.0     0.0
  Surfperch: family Embiotocidae
    Surfperch, unidentified                3.1     0.0     1.7     0.0
  Gunnel: family Pholidae
    Gunnel, unidentified                   0.0     0.0     1.7     0.0
  Wolffishes: family Anarhichadidae
    Wolf eel (Anarrhichthys ocellatus)     0.0     0.0     0.0     0.0
  Snailfish: family Liparidae
    Snailfish and lumpfish,                0.0     0.0     0.3     0.0
      unidentified
  Cusk-eels: family Ophidiidae
    Spotted cusk-eel (Chilara taylori)     0.0     0.0     0.3     0.0
  Prickleback: family Stichaeidae
    Pricklebacks, unidentified             0.0     0.0     0.0     0.0
  Class Agnatha
    Jawless fishes, unidentified           0.0     0.0     0.0     0.0
  Clingfish: family Gobiesocidae
    Clingfishes, unidentified              0.0     0.0     0.0     0.0
    Northern clingfish 0.1                 0.0     0.0     0.0
      (Gobiesox maeandricus)
  Eelpout: family Zoarcidae
    Eelpouts, unidentified                 0.0     0.0     0.3     0.0
  Pipefish: family Syngnathidae
    Bay pipefish                           0.0     0.0     0.0     0.0
    (Syngnathus leptorhynchus)
  Sandfish: family Trichodontidae
    Pacific sandfish (Trichodon            0.0     0.0     0.0     0.0
      trichodon)
    Sandfishes, unidentified               0.0     0.0     0.3     0.0

Table 3
Comparison of Steller sea lion (Eumetopias jubatus) diet composition
by month, after controlling for collection site and year. Sample size
(n = number of scat analyzed) and individual number of prey types (D)
are given for each diet; pooled number of unique prey types
([D.sub.p]), correlation coefficient ([R.sub.M]), and permutation-
based P-value (based on 9999 replications) are given for each
comparison. * indicates significance at the a = 0.002 level (based on
Bonferroni adjustment of a = 0.05 for 26 multiple comparisons).

                            Diet 1                 Diet 2

Site          Year   Month     n     D    Month        n     D

Columbia R.   2004   June     53    25    August      37    13
                     June     53    25    September   43    17
                     August   37    13    September   43    17
Rogue Reef    2001   April    45    32    August      48    32
              2002   March    39    27    April       49    30
                     March    39    27    July        33    27
                     March    39    27    August      37    35
                     April    49    30    July        33    27
                     April    49    30    August      37    35
                     July     33    27    August      37    35
              2003   April    57    29    October     53    31

                                     Mantel test

Site          Year   [D.sub.p]   [R.sub.M]   P value

Columbia R.   2004      27         0.061     0.0017 *
                        28         0.145     0.0001 *
                        19         0.047     0.0105
Rogue Reef    2001      44         0.089     0.0001 *
              2002      36         0.264     0.0001 *
                        35         0.409     0.0001 *
                        38         0.251     0.0001 *
                        36         0.173     0.0001 *
                        39         0.147     0.0001 *
                        40         0.117     0.0002 *
              2003      42         0.101     0.0001 *

Table 4
Comparison of Steller sea lion (Eumetopias jubatus) diet composition
by year, after controlling for collection site and month. Sample size
(n) and individual number of prey types (D) are given for each diet;
pooled number of unique prey types ([D.sub.p]), correlation
coefficient ([R.sub.M]), and permutation-based P-value (based on 9999
replications) are given for each comparison. * indicates significance
at the [alpha] = 0.002 level (based on Bonferroni adjustment of
[alpha] = 0.05 for 26 multiple comparisons).

                                 Diet 1             Diet 2

Site              Month    Year    n     D    Year    n     D

Columbia R.       August   2004   37    13    2007   31    16
Rogue Reef        April    2001   45    32    2002   49    30
                           2001   45    32    2003   57    29
                           2002   49    30    2003   57    29
                  June     1987   34    14    1993   36    29
                  July     1990   43    16    2002   33    27
                           1990   43    16    2004   33    20
                           2002   33    27    2004   33    20
                  August   2001   48    32    2002   37    35
St. George Reef   July     2002   33    13    2004   33    21
                           2002   33    13    2006   33    21
                           2004   33    21    2006   33    21

                                           Mantel test

Site              Month    [D.sub.P]   [R.sub.M]   P value

Columbia R.       August      20         0.105     0.0004 *
Rogue Reef        April       40         0.109     0.0001 *
                              40         0.018     0.0682
                              38         0.126     0.0001 *
                  June        32         0.222     0.0001 *
                  July        29         0.199     0.0001 *
                              26         0.216     0.0001 *
                              29         0.097     0.0001 *
                  August      41         0.123     0.0001 *
St. George Reef   July        22         0.008     0.2515
                              25         0.122     0.0001 *
                              29         0.203     0.0001 *

Table 5
Comparison of Steller sea lion (Eumetopias jubatus) diet composition
by site, after controlling for collection year and month. Sample size
(n) and individual number of prey types (D) are given for each diet;
pooled number of unique prey types ([D.sub.p]), correlation
coefficient ([r.sub.M]), and permutation-based P-value (based on 9999
replications) are given for each comparison. * Indicates significance
at the [alpha] = 0.002 level (based on Bonferroni adjustment of
[alpha] = 0.05 for 26 multiple comparisons).

                         Diet 1

Year   Month      Site        n     D

1990   July    Orford Reef   41     6
2002   July    Rogue Reef    33    27
2004   July    Rogue Reef    33    20

                    Diet 2

Year        Site          n     D

1990   Rogue Reef        43    16
2002   St. George Reef   33    13
2004   St. George Reef   33    21

                        Mantel test

Year   [D.sub.p]   [r.sub.M]   P value

1990      17         0.079     0.0002 *
2002      28         0.075     0.0023
2004      25         0.096     0.0008 *
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Author:Riemer, Susan D.; Wright, Bryan E.; Brown, Robin F.
Publication:Fishery Bulletin
Article Type:Report
Geographic Code:1USA
Date:Oct 1, 2011
Words:8708
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