An ecological analysis of rockfish (Sebastes spp.) assemblages in the North Pacific Ocean along broad-scale environmental gradients.

Abstract--Environmental variability affects the distributions of most marine fish species. In this analysis, assemblages of rockfish (Sebastes spp.) species were defined on the basis of similarities in their distributions along environmental gradients. Data from 14 bottom trawl surveys of the Gulf of Alaska and Aleutian Islands (n=6767) were used. Five distinct assemblages of rockfish were defined by geographical position, depth, and temperature. The 180-m and 275-m depth contours were major divisions between assemblages inhabiting the shelf, shelf break, and lower continental slope. Another noticeable division was between species centered in southeastern Alaska and those found in the northern Gulf of Alaska and Aleutian Islands. The use of environmental variables to define the species composition of assemblages is different from the use of traditional methods based on clustering and nonparametric statistics and as such, environmentally based analyses should result in predictable assemblages of species that are useful for ecosystem-based management.

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Ecosystem-based management of marine fish species requires all ecosystem components to be accounted for in the management framework (Livingston et al., 2005). This necessitates knowing the major environmental gradients along which important species are organized within marine systems. Rockfish (Sebastes spp.) comprise an important component of marine ecosystems; they are abundant, diverse, and a widely dispersed group found across a wide range of habitats on the Pacific coast of North America. Rockfish management is problematic because of the different habitat requirements of these species at different life history stages (Love et al., 1991; Rooper et al., 2007), their episodic recruitment (Ralston and Howard, 1995; Field and Ralston, 2005), and their susceptibility to overfishing (Parker et al., 2000). Knowledge of the environmental gradients upon which rockfish organize themselves should be useful in predicting where different species and age groups co-occur and, thus, the best strategies for managing rockfish as an important component of the marine ecosystem.

Analyses used to define fish assemblages typically are based on methods that cluster characteristics of the catch, such as the Bray-Curtis dissimilarity index (Weinberg, 1994; Williams and Ralston, 2002) and other measures of co-occurrence (Weinberg, 1994; Mueter and Norcross, 2002), or that classify stations into categories of similar catch (Methratta and Link, 2006; Zimmermann, 2006). These analyses do not explicitly take into account the distribution of species across environmental gradients, even though fish species are known to respond to characteristics such as water depth, temperature, salinity, and sediment type (Friedlander and Parrish, 1998; Rooper et al., 2005). According to ecological theory, species will inhabit a preferred niche of environmental conditions (Hutchinson, 1957). Application of this principle can be useful in predicting the biological basis for co-occurrence of rockfish species (Murawski and Finn, 1988), as well as for defining groups of species with similar habitat requirements. Additionally, the species comprising assemblages defined by ecological relationships may be expected to respond to environmental changes in a predictable fashion.

The objective of this study was to analyze the distribution of rockfish species across two large ecosystems: the Gulf of Alaska and the Aleutian Islands. Relationships based on niche theory were developed for rockfish life history stages, sexes, and species subgroups with depth, temperature, and geographical position in order to test for important overlaps and to infer common distributions among species. The co-occurrence of species in bottom trawl catches was compared to their environmental overlap to determine whether species with similar distributions were likely to be captured together. Finally, the major gradients along which rockfish species organize themselves were examined in relation to the varying life history stages of the species. These methods should allow for a more robust analysis of the assemblage of similar species, as well as documentation of the important differences among life-stage and species subgroups along environmental gradients.

[FIGURE 1 OMITTED]

Materials and methods

Study area

The data used for these analyses were collected during bottom trawl surveys of the Gulf of Alaska and Aleutian Islands ecosystems (Britt and Martin, 2001; Zenger, 2004). The Aleutian Islands are a chain of volcanic islands stretching from the Alaska Peninsula in southwest Alaska across the North Pacific Ocean and dividing the western Gulf of Alaska from the Bering Sea. The upper continental slope in the Aleutian Islands is narrow and steep. In the Gulf of Alaska, the continental shelf ranges in width from 20 km to greater than 200 km and the continental slope is steep and features periodic gullies and submarine canyons extending into the continental shelf. These two ecosystems encompass a large area of the Alaska continental shelf, from Dixon Entrance (133[degrees]W) in the southeastern Gulf of Alaska to Stalemate Bank (170[degrees]E) at the far western end of the Aleutian Islands (Fig. 1). The Gulf of Alaska bottom trawl survey is conducted from the Islands of Four Mountains (170[degrees]W) to Dixon Entrance. The Aleutian Islands bottom trawl survey is conducted along the island chain from 165[degrees]W to Stalemate Bank on the Bering Sea side and from the Islands of Four Mountains (170[degrees]W) to Stalemate Bank on the Gulf of Alaska side.

The Aleutian Islands and Gulf of Alaska ecosystems are connected by oceanic currents over the shelf. The Alaska Coastal Stream and Alaska Coastal Current flow westward (counter-clockwise) around the Gulf of Alaska from Dixon Entrance to the end of the Aleutian Island chain, whereas on the Bering Sea side of the Aleutian Islands the current flows eastward and provides extensive transport through passes in the chain from the Gulf of Alaska to the Bering Sea (Stabeno et al., 1999, 2002). The Islands of Four Mountains area is thought to be an area of change in both oceanographic properties (a higher influence of marine waters to the west) and biological properties (Ladd et al., 2005; Logerwell et al., 2005).

Trawl survey data

The National Marine Fisheries Service (NMFS) Alaska Fisheries Science Center (AFSC) has conducted standard bottom trawl surveys in the Gulf of Alaska and the Aleutian Islands region since 1980 (Britt and Martin, 2001; Zenger, 2004). Surveys were conducted triennially between 1990 and 2000 and biennially thereafter (Table 1). For this analysis, the AFSC bottom trawl data from 1990 through 2005 were combined across years and survey areas in order to maximize the number of useful data points for each species. A Poly Nor'Eastern high-opening bottom trawl (with 24.2-m roller gear equipped with 36-cm rubber bobbins that are separated by 10-cm rubber disks) was used in these AFSC bottom trawl surveys. Trawl hauls were conducted at a speed of 5.6 km/h (3 knots) for 15 or 30 minutes. Bottom contact and net dimensions were recorded throughout each trawl haul by using net mensuration equipment. In a few cases, net width was not recorded (n=35); therefore the overall average net width (15.84 m, standard error [SE]=0.01) was used. For these analyses, records were used only if trawl performance was satisfactory and if the distance fished, geographical position, average depth, and bottom water temperature were recorded (n=6767). Trawl hauls were deemed satisfactory if the net opening was within a predetermined normal range, the roller gear maintained contact with the seafloor, and the net suffered little or no damage during the tow (Zenger, 2004).

All fish captured during a survey tow were sorted by species, counted, measured for total or fork length, and the total weight of each species in the catch was determined. For large catches, the total catch was weighed and subsampled for length data. Catch per unit of effort (CPUE, no of fish/ha) for each rockfish species was calculated by using the area swept computed from the net width for each tow multiplied by the distance towed recorded with geographical positioning systems. The rockfish catch data were transformed by using natural log (CPUE+1) before analyses, hereafter shortened to CPUE.

Environmental variables

Three environmental variables were included in the analyses: depth, temperature, and geographic position along the Alaska coastline. Temperature measurements were collected throughout each trawl haul with a calibrated Branker bathythermograph, either a SeaBird-19 or SeaBird-39 microbathythermograph (Sea-Bird Electronics, Inc., Bellevue, WA) attached to the net. Depth was also recorded during each trawl haul either off the vessel echosounder or from the microbathythermograph attached to the net. The average bottom temperature and average depth from each trawl haul were used in the analyses. The final environmental variable included in the analyses was the relative position of each trawl haul along the Alaska coastline. Because the major axes of the Alaska coastline are from south to north in the southeastern Gulf of Alaska and from east to west in the remainder of the Gulf of Alaska and Aleutian Islands (Fig. 1), a reference point was chosen to standardize the spatial patterns in trawl hauls. The reference point was chosen in the central Gulf of Alaska at the eastern side of Hinchinbrook Island, (146.08[degrees]W, 60.37[degrees]N). The distance from this point to each trawl haul provided the change in spatial distribution (both by latitude and longitude) of rockfish species and the variable is hereafter referred to as the position of each trawl. Thus, a negative position indicates a trawl haul occurring in southeastern Alaska, and a positive position indicates a trawl haul occurring west of Hinchinbrook Island. Although gcographical position is not actually an environmental variable, it was used as a proxy fbr longitudinal and latitudinal gradients that affect fish distribution and ranges.

Data analyses

In every bottom trawl haul, the CPUE of each rockfish species was divided into adult and juvenile stages by fish length (Table 2). The juvenile stage was broadly defined to include all subadult fish with lengths less than the size at 50cA maturity from literature sources (Paraketsov, 1963; McDermott, 1994; NPFMC, 1998, 2005; Love et al., 2002; Pearson and Gunderson, 2003). The adult rockfish of each species were further divided into male and female CPUE components. Trawl hauls from which no length or sex data were collected were eliminated from the analyses for that species. Species for which catches occurred in fewer than 100 trawl hauls were also excluded from the analyses. For each trawl haul, these divisions resulted in three estimates of CPUE for each species: juveniles, adult females, and adult males.

In order to determine the species composition of rockfish assemblages, the CPUE-weighted distribution of each of the species subgroups was computed for each environmental variable with the formulation given in May (1973) and Murawski and Finn (1988) (Fig. 2). A weighted mean value for each environmental variable (depth, temperature, and position) was computed as

Mean = [summation]([f.sub.i][x.sub.i])/[summation][f.sub.i]

where [f.sub.i] = the CPUE of each rockfish species group in tow i; and

[x.sub.i] = the value of the environmental variable at tow i.

The weighted standard deviation (SD) was then computed as

SD = [square root of ([summation]([f.sub.i][x.sup.2.sub.i]) - [([summation][f.sub.i])*[mean.sub.2])]/([summation][f.sub.i]) - 1.

These calculations yielded the niche dimensions for each species group defined along each of the three environmental gradients.

[FIGURE 2 OMITTED]

The overlap of each species group across each environmental gradient (A) can then be calculated as

[A.sub.ij] = [C.sub.ij] exp[-[d.sub.2]/2([w.sup.2.sub.i] + [w.sup.2.sub.j])],

where the normalization constant (C.sub.ij) is calculated by

[C.sub.ij] = [square root of 2[w.sub.i][w.sub.j]/([w.sup.2.sub.i] + [w.sup.2.sub.j])];

where d = the distance between means for a pair of species i and j;

[w.sub.i] = the standard deviation (SD) for species i; and

[w.sub.j] = the SD for species j (Murawski and Finn, 1988).

Overlap indices for each variable were calculated for each species and within each species, by the groupings of males, females, and juveniles. The overlaps between males and females were first examined and if the overlap index was greater than 0.9 across all three environmental gradients, these males and females were combined and the means and SDs for each environmental gradient were recalculated. The resulting groupings (adults and juveniles) were then compared and, again, where the indices exceeded 0.9, the catches for the entire species (all size and sex classes) were combined and the means and SDs for each environmental gradient were recalculated.

Finally, the overlap indices were compared to determine the amount of separation among both species and the remaining groupings by size and sex. The multinomial intersection in the overlap indices across the three environmental gradients was calculated by multiplying the individual overlap coefficients together. This was used as a measure of the relative similarity in environmental preferences computed for each pair of species subgroups and resulted in a matrix of overlap coefficients for all species-group pairs. The matrix was then clustered into assemblages with similar distributions across environmental gradients by using Primer Analysis software (PRIMER-E Ltd., Plymouth, UK). The combined overlap index was used as a measure of similarity and the average cluster linkage method was used to determine the species composition of rockfish assemblages that had similar distributions along the environmental gradients. The combined overlap indices for each species pair were also compared to the frequency of co-occurrence for the species pair in trawl hauls by using linear regression to determine if the distribution of species across environmental variables was directly linked to their co-occurrence in trawl hauls.

Results

There was little difference in the distributions of the examined rockfish species among sexes. For all species, the overlap indices exceeded 0.9 across all three environmental gradients between males and females and, thus, the CPUE data were combined across sexes. Some differences between adults and juveniles were observed in their distributions across all three environmental variables (Table 3). Juvenile Pacific ocean perch (POP) (Sebastes alutus) and silvergray rockfish (S. brevispinis) were distributed at shallower depths than were adults of the same species. There was also a distinct separation between juvenile and adult rougheye rockfish (S. aleutianus) along the temperature gradient; juveniles were found at slightly higher temperatures than were adults. Juvenile northern rockfish (S. polyspinis) were found farther west (approximately 600 km) along the Alaska Peninsula than were adults. In total, these preliminary analyses indicated that there were four species where juveniles and adults were found to be separate and nine species that did not have different distributions between either sexes or life stages for any of the three environmental gradients. As a result, 17 species subgroups were analyzed (Table 3).

The cluster analysis resulted in five assemblages of rockfish species subgroups (Fig. 3). There was a relatively shallow-water assemblage (Aleutian Islands-shelf) containing northern rockfish, juvenile POP, and dusky rockfish (S. variabilis). These species had mean weighted depths from 134 to 164 m and were widely distributed around the northern Gulf of Alaska and Aleutian Islands (mean weighted distances from 757 to 1819 km). A similar assemblage (central Gulf of Alaska-shelf) had mean weighted depths from 144 to 160 m and was distributed from 200 to -186 km. This assemblage contained three species subgroups: yelloweye rockfish (S. ruberrimus), harlequin rockfish (S. variegatus), and juvenile silvergray rockfish. The third assemblage (southeastern Alaska-break) consisted of a group of species found predominantly in southeastern Alaska (mean weighted distance -430 to -779) at depths centered around 208 m. These species included adult silvergray rockfish, redbanded rockfish (S. babcocki), rosethorn rockfish (S. helvomaculatus), sharpchin rockfish (S. zacentrus), and redstripe rockfish (S. proriger). The fourth assemblage (Aleutian Islands-break) included juvenile rougheye rockfish and adult POP. These species were distributed at the shelf break at depths of 244 and 212 m, in the north Gulf of Alaska and Aleutian Islands (mean weighted distances 1205 and 685 km). A final assemblage (Aleutian Islands-slope) was composed of adult rougheye rockfish, shortraker rockfish (S. borealis), and shortspine thornyhead (Sebastolobus alascanus). These species were generally distributed in the north Gulf of Alaska and Aleutian Islands, at depths from 318 to 355 m. Bivariate plots of the three environmental variables indicate that the assemblages had very different distributions across the depth and position variables, and less distinct separation around the temperature variable (Fig. 4). There were clear lines of division along the depth variable between shelf and shelf break assemblages at 180 m, and between shelf break and slope assemblages at 275 m. There were also divisions along the position variable between southeastern Alaska species, central Gulf of Alaska species, and the Aleutian Island species at approximately Icy Strait (-350 km) and Seward, Alaska (250 km).

[FIGURE 3 OMITTED]

According to the trawl survey CPUE, the most abundant 10 species subgroups captured were adult POP, shortspine thornyhead, juvenile POP, juvenile northern rockfish, juvenile rougheye rockfish, adult northern rockfish, adult rougheye rockfish, shortraker rockfish, sharpchin rockfish, and dusky rockfish. For these species the co-occurrence in trawl survey catches had a strong linear relationship to the amount of overlap in their distributions across environmental variables, and over 50% of the variance in co-occurrence was explained by species group overlap (Fig. 5). When all species subgroups were considered, the relationship was significant, but the overlap index explained only 32% of the variance in co-occurrence among pairs of species subgroups. This demonstrates that the co-occurrence of rockfish in the trawl survey data is positively correlated to their overlap in environmental preferences.

[FIGURE 4 OMITTED]

Discussion

The method used in this study provided resolution of the species composition of rockfish assemblages across large-scale environmental gradients that influence fish distribution (depth and geographical position). The results of these analyses were similar to findings on fish assemblages in other areas. Rockfish (and other species) on the west coast of North America distribute themselves by depth and latitude into distinct assemblages (Weinberg, 1994; Williams and Ralston, 2002; Tolimieri and Levin, 2006) In the Gulf of Alaska, an analysis conducted on 72 groundfish species (including rockfish species) with data from five NMFS trawl surveys revealed that the major gradients for variation in species diversity were depth and alongshore distance, whereas temperature and temporal gradients had only minor effects on species composition (Mueter and Norcross, 2002). As in the current study, Mueter and Norcross (2002) and Williams and Ralston (2002) found a peak in groundfish species richness near the shelf break (200 m), in the region of overlap between shallower species and those occurring at deeper depths.

The method of defining rockfish assemblages described here is very different from more commonly used methods where trawl survey catches or stations with similar components are clustered together. More commonly used approaches define assemblages by comparing patterns of catch by species in trawl hauls (i.e., Weinberg, 1994; Williams and Ralston, 2002; Zimmermann, 2006). Although these methods are highly effective for determining patterns in catches, they typically are based on complex analyses such as nonmetric multidimensional scaling, principle components, and cluster analyses that can make interpretation difficult. Determining the scaling method to apply to catch data for abundant versus rare species, the patchy distribution of some species, and inherent differences in catchability (such as between small and large fish of the same species) are all problems that must be addressed with these methods. By first defining the relationship of a species to environmental variables, and then comparing the parameters of that relationship to parameters for other species or life history stages, this analysis method may avoid some of those potential pitfalls. For example, the absolute abundance or catchability should not matter in the identification of the correct assemblage membership for a species. If the proper distribution of a species along a depth gradient is known from the trawl data; the species will be placed within a group of species with similar depth distributions regardless of its total abundance.

[FIGURE 5 OMITTED]

The issue of whether trawl survey data can be used to determine the underlying relationship of rockfish species to large-scale environmental gradients may have some limitations. Less sampling effort was directed at shallow inshore depths (0-50 m) than at greater depths; therefore shallow-water species were likely under-represented in the catches. Although this analysis included three environmental gradients, there are obviously more variables needed to fully describe the niche dimensions for any of the rockfish species. One important feature that was omitted from this analysis was the effect of small-scale habitat features on rockfish distribution. Variability in rockfish species assemblages on a small scale is often related to the local habitat, where higher habitat heterogeneity is correlated with higher diversity and abundance (Matthews, 1990; Stein et al., 1992; Yoklavich et al., 2000). The data used in this analysis was collected only on trawlable ground, and large tracts of untrawlable area were unsampled. Species composition and abundance can be starkly different between trawlable and untrawlable locations (Matthews and Richards, 1991). Even within trawlable areas, habitat characteristics such as presence of epibenthic invertebrates can be correlated with increased catches of some life history stages of rockfish, such as juveniles that seek out complex habitat features (Rooper and Boldt, 2005; Reoper et al., 2007). Larger-scale phenomena, such as patterns in prey productivity and the effects of local currents on rockfish distribution, were also absent from this analysis and can certainly affect the distribution of those species that prey on planktonic organisms. Geographical position was used as a proxy for these large-scale phenomena and other unknown variables influencing rockfish distributions. By pooling the data, I did not take into account interannual changes in geographical position due to climate events, such as El Nine or the Pacific decadal oscillation shifts. The effects of El Nine events are typically exhibited through changes in water temperature and there was only a weak response of the species to temperature gradients, and major climate shifts were not detected in Alaska during the years examined. Knowledge of the effects of climate shifts and local habitat features would further improve future assemblage analyses.

Because these analyses were based on rockfish relationships with environmental variables, they should result in predictable species assemblages useful for ecosystem-based management. For example, species that co-occur in trawl catches due to overlapping distributions with environmental variables would be likely to experience similar fishing mortalities. Additionally, based on these assemblage analyses, marine protected areas could be designed for specific depth and geographical areas that would protect portions of rockfish populations. A series of marine protected areas has been suggested for shortraker and rougheye rockfishes in the Gulf of Alaska as a first step towards spatial management (Sohet al, 2000). The spatial and depth separation of juveniles and adults in many of the species examined here could also provide information to implement spatially based management systems for some species, where bycatch of smaller, immature members of a population would be protected from harvest.

Acknowledgments

This work would not have been possible without the assistance of numerous scientists and fishermen who have collected data on the trawl surveys of the Gulf of Alaska and Aleutian Islands. J. Boldt provided assistance in the use of the PRIMER software in the data analyses. The manuscript was improved with reviews by R. Reuter, D. Somerton, M. Wilkins, M. Zimmermann, and four anonymous reviewers.

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Manuscript submitted 7 February 2007.

Manuscript accepted 1 August 2007.

Christopher N. Rooper

Email address: [e-mail]

Alaska Fisheries Science Center

National Marine Fisheries Service

7600 Sand Point Way NE

Seattle, Washington 98115

Table 1

Summary of the number of Alaska Fishery Science Center
bottom trawl survey hauls used in the analysis of rockfish
(Sebaates spp.) assemblages in the North Pacific Ocean.
Data tram trawl hauls with no temperature, depth, latitude
longitude, and inadequate gear performance were
not used in the analyses. The Gulf of Alaska and Aleutian
Islands bottom trawl surveys were the data source for
each ecosystem, although in some years (i.e., 19981 a full
survey of the ecosystem was not conducted.

                                Number of
Year         Ecosystem         trawl hauls

1990      Gulf of Alaska           286
1991      Aleutian Islands          63
1993      Gulf of Alaska           727
1994      Aleutian Islands         389
1996      Gulf of Alaska           716
1997      Aleutian Islands         399
1998      Gulf of Alaska             5
1999      Gulf of Alaska           765
2000      Aleutian Islands         415
2001      Gulf of Alaska           550
2002      Aleutian Islands         417
2003      Gulf of Alaska           795
2004      Aleutian Islands         415
2005      Gulf of Alaska           825
Total                             6767

Table 2

Species of rockfish captured in the Gulf of Alaska and Aleutian
Islands trawl surveys across all years (1990-2005). The number
captured in all years combined, the number of trawl hauls with
catch, and the length distributions (FL, in mm) of juvenile and
adult rockfish captured in the bottom trawl surveys are also
provided. Dashed lines indicate either no catch in that category
or insufficient length data were collected to distinguish juveniles
and adults.

                                                              No. of
                                                              trawl
                                                              hauls
                                                     No.       with
Species                        Common name        captured    catch

Sebastes alutus           Pacific ocean perch     1,651,753    3059
Sebastes aleutianus       Rougheye rockfish          36,250    1894
Sebastes polyspinis       Northern rockfish         459,372    1547
Sebastolobus alascanus    Shortspine thornyhead     171,405    1515
Sebastes borealis         Shortraker rockfish        14,549     676
Sebastes variabilis       Dusky rockfish             22,622     633
Sebastes babcocki         Redbanded rockfish           2985     509
Sebastes variegutus       Harlequin rockfish         28,834     462
Sebastes zacentrus        Sharpchin rockfish         63,343     442
Sebastes brevispinis      Silvergray rockfish        11,279     340
Sebastes helvomaculatus   Rosethorn rockfish           2447     158
Sebastes proriger         Redstripe rockfish         17,868     151
Sebastes ruberrimus       Yelloweye rockfish            287     125
Sebastes elongatus        Greenstriped rockfish         750      77
Sebastes ciliatus         Dark rockfish                1607      65
Sebastolobus altivelis    Longspine thornyhead         4463      46
Sehastes wilsuni          Pygmy rockfish                514      44
Sebastes crameri          Darkblotched rockfish         122      43
Sebastes melanops         Black rockfish                542      34
Sebastes maliger          Quillbaek rockfish             98      30
Sebastes reedi            Yellowmouth rockfish          690      20
Sebastes pinniger         Canary rockfish               182      20
Sebastes diploproa        Splitnose rockfish             55      20
Sebastes flavidus         Yellowtail rockfish           423      17
Sebastes entomelas        Widow rockfish                 86      14
Sebastes paucispinis      Bocaccio                        9       8
Adelosebastes latens      Aleutian scorpionfish           6       3
Sebastes emphaeus         Puget Sound rockfish           11       2
Sebastes saxicola         Stripetail rockfish             9       2
Sebastolobus macrochir    Broadfin thornyhead             4       2
Sebastes miniatus         Vermilion rockfish              2       1
Sebastes nigrocinctus     Tiger rockfish                  1       1

                              Juvenile            Adult
                               length             length
Species                     distribution       distribution

Sebastes alutus                40-250            250-620
Sebastes aleutianus            50-439            439-820
Sebastes polyspinis            50-361            361-500
Sebastolobus alascanus         40-215            215-770
Sebastes borealis              50-449            449-1030
Sebastes variabilis           130-428            428-560
Sebastes babcocki             110-420            420-630
Sebastes variegutus            60-230            230-420
Sebastes zacentrus        Females 70-265,    Females 265-660,
                            Males 70-240      Males 240-660
Sebastes brevispinis      Females 150-415,   females 415-720,
                           Males 150-395      Males 395-720
Sebastes helvomaculatus       100-215            215-360
Sebastes proriger         Females 100-290,   Females 290-480,
                           Males 100-280      Males 280-480
Sebastes ruberrimus       Females 90-450,    Females 450-740,
                            Males 90-540      Males 540-740
Sebastes elongatus        Females 100-220,   Females 220-380,
                           Males 100-240      Males 240-380
Sebastes ciliatus             210-428            428-500
Sebastolobus altivelis         60-190            190-340
Sehastes wilsuni                 --                 --
Sebastes crameri          Females 120-390,   Females 390-500,
                           Males 120-,i70     Males 370-500
Sebastes melanops             230-400            400-590
Sebastes maliger              230-290            290-480
Sebastes reedi            Females 240-380,   Females 380-550,
                           Males 240-370      Males 370-550
Sebastes pinniger         Females 390-510,   Females 510-630,
                           Males 390-430      Males 430-630
Sebastes diploproa             90-270            270-270
Sebastes flavidus         Females 330-405,   Females 405-580,
                           Males 330-380      Males 380-580
Sebastes entomelas        Females 350-365    Females 365-570,
                                              Males 345-570
Sebastes paucispinis             --              690-720
Adelosebastes latens             --                 --
Sebastes emphaeus                --              170-180
Sebastes saxicola                --              100-270
Sebastolobus macrochir           --                 --
Sebastes miniatus                --                 --
Sebastes nigrocinctus            --                 --

Table 3

The estimated mean weighted values and niche width (w) across
each of the three resource gradients (position, depth, and
temperature) for the species-subgroups examined in this analysis.
Although the species were initially split into male, Female, and
juvenile components, they were recombined if overlaps within a
species (i.e., between sexes or between juveniles and adults)
were greater than 0.9 across all three environmental variables.

Species                         Common name             Group

Sebastes variabilis         Dusky rockfish            All
Sebastes variegatus         Harlequin rockfish        All
Sebastes polyspinis         Northern rockfish         Adults
                                                      Juveniles
Sebastes alutus             Pacific ocean perch       Adults
                                                      Juveniles
Sebastes babcocki           Redhanded rockfish        All
Sebastes proriger           Redstriped rockfish       All
Sebastes helvomaculatus     Rosethorn rockfish        All
Sebastes aleutianus         Rougheye rockfish         Adults
                                                      Juveniles
Sebastes zacentrus          Sharpchin rockfish        All
Sebastes borealis           Shortraker rockfish       All
Sebastolubus alascanus      Shortspine thornyhead     All
Sebastes brevispinis        Silvergray rockfish       Adults
                                                      Juveniles
Sebastes ruberrimus         Yelloweye rockfish        All

                                             Position
                                               (km)

Species                       Group        mean       w

Sebastes variabilis         All             757       780
Sebastes variegatus         All             200      1124
Sebastes polyspinis         Adults         1265       818
                            Juveniles      1819      1180
Sebastes alutus             Adults         1205      1176
                            Juveniles       857      1231
Sebastes babcocki           All            -443      1532
Sebastes proriger           All            -676      1720
Sebastes helvomaculatus     All            -779      1760
Sebastes aleutianus         Adults         1112      1043
                            Juveniles       685       987
Sebastes zacentrus          All            -430      1482
Sebastes borealis           All            1325      1124
Sebastolubus alascanus      All             468      1160
Sebastes brevispinis        Adults         -713      1737
                            Juveniles      -186      1319
Sebastes ruberrimus         All              66      1101

                                             Depth (m)

Species                       Group       mean        w

Sebastes variabilis         All           141.3      57.0
Sebastes variegatus         All           159.8      50.3
Sebastes polyspinis         Adults        134.1      54.4
                            Juveniles     137.6      56.0
Sebastes alutus             Adults        211.8      75.9
                            Juveniles     164.2      47.9
Sebastes babcocki           All           232.5      78.5
Sebastes proriger           All           193.8      42.3
Sebastes helvomaculatus     All           215.1      54.7
Sebastes aleutianus         Adults        315.7     164.6
                            Juveniles     244.3     112.9
Sebastes zacentrus          All           195.9      45.9
Sebastes borealis           All           354.4     194.5
Sebastolubus alascanus      All           318.4     192.9
Sebastes brevispinis        Adults        202.4      49.1
                            Juveniles     153.0      54.2
Sebastes ruberrimus         All           143.8      45.8

                                           Temperature
                                           ([degrees]C)

Species                       Group       mean        w

Sebastes variabilis         All            5.4       0.9
Sebastes variegatus         All            5.6       0.7
Sebastes polyspinis         Adults         5.0       0.9
                            Juveniles      4.7       1.0
Sebastes alutus             Adults         4.7       0.9
                            Juveniles      5.1       0.9
Sebastes babcocki           All            5.4       0.6
Sebastes proriger           All            5.6       0.6
Sebastes helvomaculatus     All            5.6       0.6
Sebastes aleutianus         Adults         4.4       1.0
                            Juveniles      5.0       0.9
Sebastes zacentrus          All            5.6       0.6
Sebastes borealis           All            4.2       1.1
Sebastolubus alascanus      All            4.7       0.9
Sebastes brevispinis        Adults         5.7       0.7
                            Juveniles      6.0       1.0
Sebastes ruberrimus         All            5.7       0.7