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Effect of demography on spatial distribution: movement patterns of the Albemarle Sound--Roanoke River stock of Striped Bass (Morone saxatilis) in relation to their recovery.

Abstract--We analyzed tag returns from a long-term tagging program to evaluate the movement patterns of the Albemarle Sound-Roanoke River (AR) stock of Striped Bass (Morone saxatilis) during a period of stock recovery in 1991-2008. The AR stock was found to increase its movement outside the Albemarle Sound estuary (from <4% to 15-31%) as it recovered from 1991 to 2008. Analysis with multinomial logistic regression where recapture area was modeled as a function of fish size and stock abundance indicated that Striped Bass from the AR stock exhibit a strong size-dependent emigration pattern. Larger (older) adults >600 mm in total length (TL) were much more likely to emigrate to ocean habitats (after spawning) than were smaller adults (350-600 mm TL), which mostly remained in inshore estuarine habitats. Smaller adults showed evidence of density-dependent movement and were recaptured only in adjacent estuarine systems, the Pamlico Sound and lower Chesapeake Bay, during periods of increased stock abundance. Assessment and management strategies for the AR stock of Striped Bass could be improved by accounting for movement (and hence harvest) outside the currently assumed stock boundary. More broadly, this study illustrates that changes in the demographics, such as size structure and total abundance, within fish populations can result in major shifts in their distribution and that long-term tagging data are useful in detection of such population-level changes in movement patterns.


The demographics of fish populations can be important in shaping their movement patterns. Numerous species have been shown to increase their distributional range or movement distances as population abundance increases (Swain and Wade, 1993; Brodie et al., 1998; Overholtz, 2002; Abesamis and Russ, 2005; Dunning et al., 2006), a response presumably due to density-dependent mechanisms (e.g., intraspecific competition for food or the saturation of optimal habitats) (MacCall, 1990). In addition, changes in movement patterns with ontogenetic changes in fish are common because habitat requirements change as species age (Werner and Gilliam, 1984; Dahlgren and Eggleston, 2000).

The demographics of fish populations are continually shifting for reasons that include changes in fishing pressure and the natural environment (e.g., recruitment variation) that can alter age structure and abundance (Longhurst, 2002; Berkeley et al., 2004; Hutchings and Baum, 2005) and in turn cause population-level changes in movement patterns. Understanding if and how population-level movements (and distribution) change over time is of particular importance for exploited fishery species because such changes can pose challenges for assessment and management techniques, for which stock boundaries are often assumed to be static and not dynamic (Winters and Wheeler, 1985; Hammer and Zimmermann, 2005; Link et al., 2011).

Striped Bass (Morone saxatilis) occur throughout the East Coast of the United States and have supported important fisheries there for centuries (Merriman, 1941). Tagging studies clearly have shown that spawning populations (or stocks) of Striped Bass in the mid-Atlantic region, which includes the Hudson River, Delaware River, and Chesapeake Bay, generally exhibit an anadromous life-history strategy and undergo extensive seasonal migrations. After spawning in the freshwater portion of their respective estuaries, many adults emigrate to Atlantic Ocean waters from New Jersey to Maine in early summer, move south in the fall to overwintering habitats in coastal waters from New Jersey to Cape Lookout in North Carolina, then return to their natal estuary in subsequent springs to spawn (Boreman and Lewis, 1987; Waldman et al., 1990; Dorazio et al., 1994; Welsh et al., 2007). In contrast, the Albemarle Sound-Roanoke River (AR) stock of Striped Bass, hereafter referred to as the "AR stock," has historically been viewed as a nonmigratory stock, and most fish are believed to remain in their natal estuarine system, the Albemarle Sound estuary, throughout their lives (Merriman, 1941; Hassler et al. (1)). Indeed, in the most extensive tagging study to date on the AR stock by Hassler et al. (1), virtually all (99%) of the 2428 returns of the 9220 adults tagged in the Roanoke River during the springs of 1959-77, occurred within the Albemarle Sound estuary. The few returns that occurred outside Albemarle Sound (<1% of the total) were from an adjacent estuary (Pamlico Sound); remarkably, no returns were from ocean waters (Hassler et al. (1)).

These differences in migration patterns may have been due to differences in life-history strategies (non-anadromous vs. anadromous) between the AR stock and more northerly stocks, or it could have been a result of a historic lack of larger, older fish (>600 mm in total length [TL]) in the AR stock because of high harvest levels. Differences in life-history strategy would be perplexing given that these stocks occur in the same zoogeographic province (mid-Atlantic coast of the United States) and given that some of them are in close latitudinal proximity (e.g., the AR and Chesapeake Bay stocks). In 1988, the North Carolina Division of Marine Fisheries (NCDMF) began a cooperative tagging program with the North Carolina Wildlife Resources Commission (NCWRC) to address this question and to further investigate the migration dynamics of the AR stock of Striped Bass.

Much of the past work of tagging individuals from the AR stock was done when Striped Bass were at low levels of abundance and overfished (NCDMF and NCWRC (2)). In more recent years (1991-2008), the AR stock, as well as the Chesapeake Bay stock (Richards and Rago, 1999), made a dramatic recovery from their depleted state in the late 1970s and 1980s. The estimated total abundance of the AR stock nearly doubled during the 1990s, increasing from 1.0 to 1.9 million fish, and remained at high levels (>1.8 million fish) throughout the 2000s (NCDMF and NCWRC (2)). In addition, the age and size structure of the stock expanded as larger (>600 mm TL) and older (age 9+) fish became more prevalent as the stock recovered (NCDMF and NCWRC2). The recovery of the AR stock was a result of a combination of factors, namely more stringent fishing regulations that increased development to older age classes and improvements in environmental conditions that enhanced spawning habitat and recruitment of young Striped Bass (e.g., regulated river flows that were more conducive for the transport and survival of eggs and larvae) (Rulifson and Manooch, 1990; NCDMF and NCWRC (2)).

For this study, we first addressed the following question: Have Striped Bass of the AR stock increased their movement outside of the Albemarle Sound estuary since population rebuilding in the 1990s? After showing that the movement of the AR stock out of the estuary has indeed increased, we then related recapture locations of tagged individuals to both fish size and total annual stock abundance (density) in an effort to explain this increase in emigration over the past 2 decades (1991-2008). Lastly, we discuss the management implications of this increased movement given the stock is currently considered to be resident.

Materials and methods

Fish tagging

During the springs of 1991-2008, 42,534 adult Striped Bass from the AR stock (mostly >350 mm TL; Fig. 1) were tagged and released on their well-described spawning grounds (Hassler et al. (1)) ~200 km upstream of the mouth of the Roanoke River in North Carolina (Fig. 2A). During weekly sampling events throughout April and May, Striped Bass were collected with an electrofishing boat and transported to a tagging vessel, where they were held in a "live well" until processing. Fish in good condition were measured (TL to the nearest millimeter), weighed (to the nearest gram), and sex was determined by expression of gonadal products. The fish were then tagged just above the posterior tip of the pelvic fin with a Floy (model FM-84 (3)) internal anchor tag (Floy Tag, Inc., Seattle, WA). Fish were immediately released after tagging. The streamer of the tags indicated a "reward" (US $5 or a baseball cap) would be offered for reporting information on recaptured Striped Bass (e.g., recovery date and location, and tag number) to the NCDMF, whose contact information was printed on the tag.

Data analysis

We used multinomial logistic regression to evaluate the effects of fish size and stock abundance on the recapture location (i.e., to evaluate the movements) of Striped Bass of the AR stock. For this analysis, recapture locations of tagged fish were assigned to 1 of 4 broad geographic areas: 1) the Albemarle Sound estuary, 2) the Pamlico Sound estuary, 3) ocean waters of North Carolina, or 4) northern coastal waters from Virginia to Massachusetts (Fig. 2B). Therefore, recapture area constituted a multicategory response variable. Explanatory variables were fish size (TL at tagging) and total annual abundance of the AR stock (1991-2008). Annual abundance estimates (of age 1+ fish) were obtained from a statistical catch-at-age model from the most recent AR stock assessment (NCDMF and NCWRC2) and served as a proxy for the annual densities of conspecifics (AR stock only) with which tagged Striped Bass were expected to interact each year. Sex was not included as an explanatory variable because it was confounded with fish size beyond 800 mm TL because all but 4 tag returns from this size range were from females. However, across smaller sizes (400-800 mm TL), over which sexes were more equally represented, similar-size males and females were generally recaptured in the same areas, indicating that movements differed little between sexes.

For the purpose of our analyses, we included only tag returns that occurred after the first 2 weeks but within the first calendar year at liberty. By restricting returns to those returns that occurred within the first calendar year at liberty (on or before 31 December), movement between tagging and recapture locations could be known to occur during a given year. This restriction allowed movements (recapture area) to be directly related to stock abundance, which was estimated on an annual basis (i.e., each calendar year) from 1991 to 2008, the terminal year in the assessment. In addition, restriction of returns to a relatively short time period at liberty (<9 months) minimized the opportunity for growth between tagging and recapture, thereby ensuring that fish lengths at tagging (the size variable used in our analyses) were representative of the size of fish when movement occurred.

To reach another recapture area (outside the Albemarle Sound estuary), tagged fish would have had to travel a considerable distance (>300 km) from their release site in the upper Roanoke River. Therefore, to reduce the likelihood of underestimation of fish movement, we excluded tag returns from the first 14 days at liberty, affording tagged fish a more realistic period of time to complete movement or migration to another system. Indeed, the earliest tag return from outside the Albemarle Sound estuary (in North Carolina ocean waters) occurred at 16 days after tagging, providing justification for our 14-day exclusion window. Finally, data from 1994 were excluded from analyses because of reduced tagging efforts in that year (only 9 fish were tagged and 1 returned).

To determine which explanatory variables affected movements of Striped Bass and to assess their relative importance, we used an information-theoretic approach. A multinomial logistic regression model was run for each of the 5 possible combinations of explanatory variables: 1) length, abundance, lengthxabundance (interaction model), 2) length and abundance, 3) length only, 4) abundance only, and 5) intercept only (no effects model). Akaike's information criterion (AIC) values were obtained for each candidate model. We considered the model with the lowest AIC value as the most parsimonious or "best," but we also computed additional diagnostics, Akaike differences ([DELTA].sub.i]) and Akaike weights ([w.sub.i] ), to assess how other models performed in comparison to this single best model (Burnham and Anderson, 2002). The first of these other diagnostics was calculated as

[[DELTA].sub.i] = [AIC.sub.i] - [AIC.sub.min], where [AIC.sub.i] = the AIC value of a given model (i); and

[AIC.sub.min] = the AIC value of the best model (minimum AIC).

As a general guideline, models with [[DELTA].sub.i] close to zero have considerable empirical support, models with [[DELTA].sub.i]; of 4-7 have much less support, and models with [[DELTA].sub.i]; of 9-14 have little support (Anderson, 2008). The following equation was used to calculate [w.sub.i] values:

[w.sub.i] = exp(-1/2[[DELTA].sub.i]/[summation].sup.R.sub.r=1]exp(-1/2[[DELTA].sub.r]

Note that R refers to the set of models being evaluated. Values of [w.sub.i] can be interpreted as the probability that a particular model (i) is the best model for the data set given that one of the models must be selected as the best (Anderson, 2008).

We based our inferences on parameter estimates from the model (i.e., on the combination of explanatory variables) deemed most parsimonious from AIC diagnostics. For example, if the third model (length effect only) was determined to be the best model, values of the parameters (i.e., regression coefficients) that represented the effect of fish length were used to calculate the predicted relative probability of Striped Bass being recovered in each recapture area as a function of their size at tagging. We assessed the fit of the best model through the use of both Pearson and deviance goodness-of-fit tests. Because explanatory variables were continuous, it was necessary to group data for these tests (Agresti, 1996). For this purpose, we used 100-mm bins and abundance bins of 0.1 million fish. All statistical analyses were performed in SAS, vers. 9.1.3 (SAS Institute, Inc., Cary, NC) with [alpha] = 0.05.


Tag return summary

From 1991 to 2008, 1197 tagged Striped Bass were reported as having been recaptured within their first 9 months at liberty (late April-December); analyses conducted for this study were based on data from these individuals. Hook-and-line (recreational) anglers accounted for a majority (84%) of tag returns. Although most returns (80%) were from fish 400-600 mm TL (at tagging), fish lengths ranged from 287 to 1105 mm TL. Moreover, nearly all tag returns (154 of 156) of larger Striped Bass (>600 mm TL) were from years in which stock abundance exceeded 1.5 million fish (Table 1).

Temporal recapture trends

The AR stock of Striped Bass increased their movement outside of the Albemarle Sound estuary as the population rebuilt over the past 2 decades (1991-2008). In the early 1990s, few tag returns occurred outside the Albemarle Sound estuary: <4% annually across the years of 1991-96, with the exception of 1995 (Fig. 3). However, as the stock increased in abundance and its age structure expanded, returns from regions outside the Albemarle Sound estuary increased considerably and ranged from 15% to 31% annually during the years of 1997-2008 (Fig. 3).

Effects of fish size and stock abundance on recapture area

Fish size and stock abundance affected recapture area. The best multinomial logistic regression model included the main effects of both fish length and stock abundance but not their interaction (Table 2). Goodness-of-fit tests indicated this model fitted the sample data well (Pearson goodness of fit, [chi square]=117, degrees of freedom=126, P=0.70; deviance goodness of fit, [chi square]=104, degrees of freedom=126, P=0.93). Although the best model showed that recapture area depended on both fish size and stock abundance, AIC diagnostics across the suite of models indicated that fish length exerted a much stronger effect than abundance. Specifically, the model that included only fish length had moderate empirical support ([[DELTA].sub.i]=3.8, [w.sub.i]=0.12), but the model that included stock abundance alone had very little support ([[DELTA].sub.i]=462.2, [w.sub.i]=0) (Table 2).

Striped Bass of the AR stock exhibit a strong size-dependent migration pattern, whereby both the incidence of emigration and the distance emigrants move increase with fish size. The best model predicted that the probability of emigration from (i.e., recapture outside) the Albemarle Sound estuary increased dramatically with fish size. Specifically, the probability of recapture within Albemarle Sound declined sharply (from values >90%) beyond 600 mm TL, the size at which recapture probabilities began to increase in other areas, such as Pamlico Sound and ocean waters (Fig. 4). The model predicted that Striped Bass 700-800 mm TL in length were most likely to be recaptured in ocean waters of North Carolina (Fig. 4C) and that the largest fish (>850 mm TL) were most likely to be recaptured in the northern coastal region (Fig. 4D).

Empirical tag return data supported the movement pattern indicated by the best model. Nearly all (92%) of the tag returns of smaller fish (<600 mm TL; n=1040) occurred within the Albemarle Sound estuary (Fig. 5A). Yet, only 47% of returns of fish 600-799 mm TL (n=102) and 2% of returns of fish >800 mm TL (n=55) occurred in Albemarle Sound; most tag returns of these larger fish occurred in ocean waters (Fig. 5, B and C). Interestingly, the majority (78%) of tag returns of the largest fish in this study (800-1105 mm TL) occurred in distant coastal waters from New Jersey to Cape Cod, 780 to 1250 km from the release site (Fig. 5C).

Stock abundance also affected the areas in which Striped Bass were recaptured. The best model predicted a slight increase (~5%) in recapture of small Striped Bass (<600 mm TL) in the Pamlico Sound region as stock abundance increased from 1 to 2 million fish (Fig. 4B). This trend also was evident in empirical tag return data. Returns from the Pamlico Sound estuary, -6% of all returns, occurred only during years in which stock abundance exceeded 1.4 million fish. There were no returns from the Pamlico Sound estuary during years of lower abundance (1.0-1.1 million fish) (Fig. 6)


Continuous tagging over a 20-year period, a length of effort that is rare in most fisheries, allowed us to determine the strong effect of fish size and relatively smaller effect of stock abundance on a fish stock's spatial distribution. Multiple stocks of Striped Bass co-occur along the East Coast of the United States during nonspawning periods. Therefore, by tagging fish on their natal spawning grounds (when stocks are separated), we were able to investigate stock-specific movements and spatial distribution--information that could otherwise not have been resolved with approaches such as fisheries-independent surveys (e.g., trawl surveys). In this section, we provide further details on the effects of fish size and stock abundance on the spatial distribution of the AR stock of Striped Bass and on the implications for management of Striped Bass.

Effects of fish size on recapture area

The increase in tag returns of the AR stock from regions outside its natal estuary over the past 2 decades was largely due to expansion of the age and size structure of the stock as it recovered. The majority of returns (67%) that occurred outside the Albemarle Sound estuary during the stock recovery period were from ocean waters. Model results and empirical data both showed the probability of Striped Bass being recaptured in ocean waters increased dramatically with fish size beyond 600 mm TL, to the point where the largest individuals (>800 mm TL) were almost exclusively captured in ocean waters. Therefore, it is not surprising that returns from ocean waters increased over the past 2 decades as more fish from this largest size class (which was the class most likely to emigrate to ocean habitats) became available for tagging and recapture as the age and size structure of the AR stock expanded.

The strong size-dependent emigration pattern of Striped Bass revealed by this study helps explain the lack of recaptures in ocean waters by Hassler et al. (1), who also focused on the AR stock. To collect fish for tagging, Hassler et al. (1) primarily used small-mesh (<150 mm stretched) gill nets that likely selected for smaller fish. Indeed, of the 2428 returns in their study, most (86%) were from fish 400-550 mm TL at tagging, and only 2 returns (<0.1%) were from fish >800 mm TL at tagging. Moreover, the vast majority (88%) of tag returns in their study occurred within the first year at liberty. Therefore, given the small sizes of tagged fish and short-term nature of returns (i.e., small tagged fish did not have time to grow into larger size categories because of high harvest), the lack of ocean recaptures by Hassler et al. (1) is not surprising. Nearly all fish recaptured in their study (>99%) were smaller than the size at which appreciable ocean emigration occurs (>800 mm TL), as indicated in our study.

Although other factors, such as prey availability and susceptibility to predation, may be involved, water temperature appears to be a salient factor in explanation of the size-dependent migration and distribution patterns of the AR stock. A change in temperature preferences with fish size has been hypothesized to be the main driver of the size-dependent emigration pattern observed previously for other stocks of Striped Bass (Coutant, 1985), especially the Chesapeake stock (Dorazio et al., 1994; Secor and Piccoli, 2007).

Decreases in temperature optima with fish size can be explained by bioenergetic principles. Specifically, the temperature threshold beyond which the increase in total metabolic load starts to become stressful (i.e., the point at which the scope for activity and growth begins to decline) occurs at progressively lower temperatures as fish size increases because larger individuals have a greater total metabolic demand than smaller individuals on the basis of body size alone (Hartman and Brandt, 1995). Therefore, after spawning, most large Striped Bass may emigrate, as we found, to cooler northern ocean habitats, which would provide a metabolic reprieve, rather than spend their summers in warm estuarine waters.

Interestingly, Striped Bass of the AR stock in the intermediate size range of 700-850 mm TL, were mainly recaptured in ocean waters off North Carolina, from the Oregon Inlet north to the border of North Carolina and Virginia. No Striped Bass were recaptured in ocean waters south of Cape Hatteras, where summer temperatures (>26[degrees]C;, Station#41036) are similar to summer temperatures in Albemarle Sound. Therefore, nearby ocean waters may provide an adequate thermal refuge (23-26[degrees]C;, Station#44100) during summer for Striped Bass in the size range of 700-850 mm TL. One intriguing question is whether the size at which the onset of ocean emigration occurs will shift to a smaller size as inshore estuarine waters, which already approach 30[degrees]C in summer (, Gage#0208114150), are expected to continue warming under current projections for climate change (IPCC, 2007). Continuation of the long-term tagging program on the AR stock of Striped Bass could help address this question.

Previous research on northern stocks of Striped Bass has provided evidence for diverse lifetime migration patterns: some members of a given population reside in freshwater or estuarine environments throughout their life (resident contingent) and others are more exploratory and engage in large-scale coastal migrations (migratory contingent) (Clark, 1968; Secor, 1999). There is particularly strong evidence for this "contingent" behavior in Striped Bass in the Hudson River (Secor and Piccoli, 1996; Secor et al., 2001; Zlokovitz et al., 2003). Our study, however, provides little indication of this phenomenon in the AR stock of Striped Bass. If contingent behavior had been prevalent, one would have expected that some large fish would have remained and been recaptured in the Albemarle Sound after spawning. Yet, of the 50 fish exceeding 855 mm TL that were recovered in our study, none were recaptured within Albemarle Sound and, instead, all were taken in the ocean. It is possible that contingent behavior is not beneficial, and, therefore, it does not manifest in the AR stock because of high inshore water temperatures during summer that would be unsuitable for "resident" fish once they attain a large size. The possibility for latitudinal differences in the frequency of contingent behavior in Striped Bass and other fishes warrants future investigation.

Effects of stock abundance on recapture area

Stock abundance in the year of release was included in the best model explaining where Striped Bass were recaptured. This effect was primarily a result of smaller Striped Bass being recaptured in the adjacent estuarine systems of Pamlico Sound and lower Chesapeake Bay only in the years of highest abundance (Fig. 6C). Also, evidence of recapture patterns within the Albemarle Sound estuary were indicative of a density effect. Namely, tag returns were much more common in the eastern portions of Albemarle Sound, particularly in Currituck Sound (6% vs. 1% of returns) and Croatan and Roanoke sounds (32% vs. 6%), during years in which stock abundance exceeded 1.4 million fish in contrast to years when it was below this level (Fig. 6). Therefore, although adults generally may remain inshore until they reach larger sizes (>600 mm TL), the distances they disperse within estuarine habitats, after spawning, tend to increase with the abundance of conspecifics, presumably because of density-dependent mechanisms. These movements likely are important ecologically to prey of Striped Bass because the smallest size groups (<600 mm TL) are the most numerous in this population (i.e., predation effects may change with stock abundance). Future research should investigate these possibilities and better isolate the effects of density by controlling for environmental covariates, such as the abundance of competitor species and changing habitat suitability, as suggested by Shepherd and Litvak (2004).

Management implications

Results from this study have important implications for the management of Striped Bass along the East Coast of the United States. With current assessment strategies, Striped Bass from the AR stock are assumed not to contribute to the Atlantic Ocean mixed stock fishery (ASMFC (4)). However, this study revealed that some members of the AR stock, those fish surviving to sizes >800 mm TL, are indeed migratory and, therefore, unequivocally contribute to (i.e., are harvested by) the mixed stock fishery of the Atlantic coast. Because management benchmarks for the mixed stock fishery, such as the threshold fishing mortality ([F.sub.MSY]=0-41; ASMFC (4)), currently are based on data from Chesapeake, Hudson, and Delaware stocks that are potentially more productive than the AR stock, it is possible that the mixed stock fishery could affect the AR stock disproportionately. Accordingly, future research should establish the productivity of the AR stock of Striped Bass in relation to other stocks. If the AR stock is found to be less productive, then future work also should determine the implementation costs of more stringent fishing regulations in the mixed stock fishery, namely the amount and value of harvest that would be lost from more productive stocks (Chaput, 2004; Crozier et al., 2004; Hilborn et al., 2004).

Results from this study also have implications for the assessment and management of Striped Bass within North Carolina. Currently, landings of Striped Bass outside the Albemarle Sound estuary (region 1; Fig. 2B) are not included in the AR stock assessment (NCDMF and NCWRC (2)). Stock status is based on the estimate of fishing mortality ([F.sub.threshold]=0-27) for fully recruited Striped Bass of age 4-6 and 400-600 mm TL, a size group for which fish were found in this study to increase their movement to adjacent estuarine systems outside the stock boundary as they increased in abundance. Therefore, by not including fish that move to and are harvested in adjacent systems, the AR stock assessment underestimates fishing mortality. Accordingly, future research should examine the sensitivity of fishing mortality estimates from the AR stock assessment to additional landings of age-4-6 Striped Bass of AR origin outside the Albemarle Sound estuary.


It is important to note that the analyses in this study indicate the probability of recapture location; movements are inferred from these data. Fishermen behavior (e.g., spatiotemporal differences in fishing effort or size targeting because of regulations and economic value) can affect and potentially bias tag returns and inferences about movement patterns (Hilborn, 1990; Gillanders et al., 2001). The size-dependent migration pattern that we observed could be due to differences in selectivity between ocean and estuarine fisheries; that is, small tagged fish could have migrated to the ocean but not been caught in the fishery. However, fisheries independent data indicate that it is predominantly the large Striped Bass of the AR stock that migrate to ocean waters. In a mobile telemetry study, Haeseker et al. (1996) searched the Albemarle Sound during summer (May-August) for the presence of 26 telemetered Striped Bass (all but 1 fish <600 mm TL) that participated in the April Roanoke River spawning run. They relocated 25 (96%) of these fish in the Albemarle Sound at least 1 month after spawning, providing evidence that smaller Striped Bass mostly remain in the estuary after spawning. Furthermore, in an ongoing telemetry study, 163 Striped Bass ranging in length from 445 to 1146 mm TL (mean=580 mm TL) were telemetered in the Roanoke River during spring, beginning in 2011, by Harris and Hightower. (5) Most large fish in their study (15 of the 18 individuals >900 mm TL at tagging) have been detected by coastal receiver arrays in Massachusetts, New York, New Jersey, Delaware, and Virginia, but no smaller individuals have been detected in these northern ocean waters (Harris and Hightower (5)). Hence, results from these fisheries-independent telemetry studies corroborate the strong size-dependent emigration pattern of the AR stock of Striped Bass that we inferred from tag recaptures in our study.

A limitation of our study was that nearly all tag returns (99%) from larger fish (>600 mm TL) occurred during years of higher stock abundance (>1.5 million fish). Therefore, it is possible that the observed ocean emigration of larger fish was due in part to the higher abundance of similar size conspecifics (i.e., density-dependent mechanisms). However, ocean emigration of the AR stock of Striped Bass appears to be a size-dependent phenomenon related to bioenergetics as described and is probably largely independent of ambient population density or abundance. Two lines of evidence support this notion. First, data on large Striped Bass (>600 mm TL) across the more restricted range of annual values of stock abundance (1.5-2.0 million fish) indicate that density had little effect (an increase <3%) on the probability of large fish being recaptured in ocean waters. Second, just as we found in our study, Dorazio et al. (1994) found a strong size-dependent emigration pattern for the Chesapeake Bay stock: most fish >800 mm TL were recovered in northern ocean waters from New Jersey to Maine. Their study occurred in 1988-91, years when the Chesapeake Bay stock was at relatively low abundance levels and still rebuilding, demonstrating that substantial ocean emigration of large fish, albeit from a different stock, still occurs at low densities.


Our study revealed major changes in the movements and associated distribution of a fish stock as it recovered from a depleted state. During the early phases of rebuilding, the stock was largely confined to its natal estuary but dramatically expanded its distribution, and degree of anadromy, as recovery continued. This major shift in distribution was due to changes in the demographics--namely size structure and total abundance--of the stock as it recovered. Size structure has received little attention in the fisheries literature in regard to its effects on stock distribution but appears to be important.

Although the recovery of Striped Bass often is regarded as one of the few success stories in fisheries management (Richards and Rago, 1999), many global fish stocks are either currently experiencing rebuilding or have recently recovered, for example, nearly one-third of the 166 stocks examined worldwide by Worm et al. (2009). It is possible that the spatial dynamics of these and other rebuilding stocks will differ from their depleted state. For instance, as stocks recover and more individuals are allowed to reach larger sizes (e.g., through a reduction in fishing mortality; Berkeley et al., 2004), the spatial distribution of stocks may shift or expand because larger, older fish often have different migratory behaviors and habitat preferences than smaller, younger individuals (Heifetz and Fujioka, 1991; Macpherson and Duarte, 1991; Shepherd et al., 2006; Gruss et al., 2011). Such changes in the movement and distribution of fish populations can have important consequences for stock assessments, as argued previously, and also affect ecosystem dynamics (e.g., as predators move into new areas, they can exert top-down changes in community structure; Casini et al., 2012). Therefore, resource managers should be aware of potential changes in the movement and distribution of recovering fish stocks and account for them accordingly if they manifest. As indicated in our study, long-term tagging and monitoring data are useful for detection of population-level changes in the movement and distribution of fishes.

Manuscript submitted 18 April 2013.

Manuscript accepted 12 February 2014.

Fish. Bull. 112:131-143 (2014).

doi:10.7755/FB. 112.2-3.3

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.


We thank the many individuals from the NCDMF and North Carolina Wildlife Resources Commission who collected and tagged Striped Bass and also compiled and processed tag return data. We also are grateful to the many fishermen who provided tag return information. Analyses for this study were completed while J. Callihan was a Marine Fisheries Management Fellow supported by NCDMF (Coastal Recreational Fisheries License fund award no. 3210) and North Carolina Sea Grant award E/GS-6. The manuscript was improved by comments from J. Harris, J. Finn, P. McGrath, and R. Laney.

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Jody L. Callihan (contact author) [1]

Charlton H. Godwin [2]

Jeffrey A. Bucket [3]

Email address for contact author:

[1] Department of Applied Ecology North Carolina State University Campus Box 7617 Raleigh, North Carolina 27695

[2] Northern District Office North Carolina Division of Marine Fisheries 1367 US 17 South Elizabeth City, North Carolina 27909

[3] Center for Marine Sciences and Technology Department of Applied Ecology North Carolina State University 303 College Circle Morehead City, North Carolina 28557

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(3) Mention of trade names of commercial companies is for identification purposes only and does not imply endorsement by the National Marine Fisheries Service, NOAA.

(4) ASMFC (Atlantic States Marine Fisheries Commission). 2003. Amendment 6 to the Interstate Fishery Management Plan for Atlantic Striped Bass. Fishery Management Report No. 41 of the Atlantic States Marine Fisheries Commission, 63 p. [Available from file/sbAmendment6.pdf.]

(5) Harris, J. E., and J. E. Hightower. 2013. Unpubl. data. North Carolina Cooperative Fish and Wildlife Research Unit, U. S. Geological Survey, and Department of Applied Ecology, North Carolina State Univ., Raleigh, NC 27695.

Table 1

Number of tag returns of Striped Bass (Morone saxatilis) per
combination of total annual stock abundance (millions of fish)
and interval of total length (TL) at tagging. Annual abundance
estimates (1991-2008) of Albemarle Sound-Roanoke River Striped
Bass were obtained from a statistical catch-at-age model (NCDMF
and NCWRC2). Only those tag returns occurring after the first 2
weeks but within the first calendar year at liberty were included
in data analyses and are enumerated here. "--" = no tag returns for
that year.

                    Number of returns per size (mm TL) interval
(millions)   Year     <400      400-499   500-599   600-699

1.035        1993         --        37        15        --
1.101        1991         10        17         5        --
1.104        1992          2        48         7         1
1.388        1995         --         4        17        --
1.518        2005          1        68        36         7
1.569        1996          4         7        13         1
1.673        2004         --        17         8        --
1.752        1997          6        38        28         4
1.803        2006         --        54        80         6
1.828        2003          7        20        36         7
1.829        2001          1        35        38        12
1.836        2000          1        29        16         4
1.860        2002          2        27        39         4
1.877        1998          4        41        23         7
1.895        2008         38        42        13         3
1.907        1999          1        29        17         5
2.051        2007         --        19        40        11

                    Number of returns per size (mm TL) interval
(millions)   Year   700-799   800-899   900-999     >1000

1.035        1993       --        --        --          --
1.101        1991       --        --        --          --
1.104        1992        1        --        --          --
1.388        1995       --        --        --          --
1.518        2005        2        --         6          --
1.569        1996       --        --        --          --
1.673        2004        1         3         1          --
1.752        1997        1        --        --          --
1.803        2006        5         3         9           3
1.828        2003        2         1         3          --
1.829        2001        2         2        --          --
1.836        2000        2        --        --          --
1.860        2002        7         1        --           1
1.877        1998        3        --        --          --
1.895        2008       --         3         5           6
1.907        1999        1        --        --          --
2.051        2007        3         1         5           2

Table 2

Diagnostics with Akaike's information criterion (AIC) for
candidate multinomial logistic regression models that relate the
recapture area (Albemarle Sound estuary, Pamlico Sound estuary,
North Carolina ocean waters, or northern coastal waters from
Virginia to Massachusetts) of tagged Striped Bass (Morone
saxatilis) to fish length and total annual stock abundance for
the years 1991-2008. Each model represents a different
combination of these explanatory variables. Note that
[[DELTA].sub.i]=Akaike's differences and [w.sub.i]=Akaike's
weights, where lower values of [[DELTA].sub.i] and higher values
of [w.sub.i] indicate greater relative empirical support for a

Model                   AIC     [[DELTA].sub.i]   [w.sub.i]

Length + abundance     1003.2         0.0           0.80
Length only            1007.0         3.8           0.12
Length + abundance +   1007.8         4.6           0.08
  length x abundance
Abundance only         1465.4        462.2           0.0
Intercept only         1496.1        492.9           0.0
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Author:Callihan, Jody L.; Godwin, Charlton H.; Buckel, Jeffrey A.
Publication:Fishery Bulletin
Geographic Code:0ATLA
Date:Apr 1, 2014
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