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Newport Laboratory: Fisheries Behavorial Ecology Program

Recruitment Signals and Post-Settlement Processes of Juvenile Gadids in Coastal Nursery Areas of Kodiak

sampling for juvenile Pacific cod
Figure 1.  Sampling for juvenile Pacific cod caught by beach seine (top) and baited camera (bottom) across multiple fixed-site locations in Kodiak.  The survey occurred over two sampling periods (mid-July and late-August) across 5 years (200610).

Age-0 fish that successfully transition through early life stages may be the earliest reliable indicators of year-class strength, yet few survey data are available to test these predictions for commercially important groundfish species in Alaska. Over the past 5 years, we examined trends in annual abundance and seasonal growth and mortality in age-0 Pacific cod (Gadus macrocephalus) across multiple fixed-sites in two nursery embayments off Kodiak Island using beach seines (n=320 hauls) and baited cameras (n=410 deployments) (Fig.1).

The beach seine targeted age-0 fish in 2-4 m depth whereas the baited camera targeted older conspecifics (age 1 2) across a broader range of depths (2-20 m) adjacent to seine site locations. We used these survey data to address the following questions: 1) does post-settlement growth and mortality vary annually, and if so, 2) what processes control such variation? and 3) can bay-resident populations of age-1 and age-2 juvenile cod be predicted from the age-0 abundance or do post-settlement processes mask initial recruitment signals?

Question 1: The interannual abundance of age-0 gadids recruiting to nurseries was highly variable, but it was interesting to note that the variability was positively correlated among co-occurring gadids, age-0 saffron cod (Eleginus gracilis) (r2 = 71%; Fig. 2a) and age-0 walleye pollock (Theragra chalcogramma)(r2 = 69%; Fig. 2b). These patterns suggest a common mechanism regulates pre-settlement survival and/or successful delivery of larvae to coastal nurseries.

Question 2: Two notable trends emerged from the size and growth data. First, size-at-settlement was positively related to spring temperatures experienced by larvae, but more importantly, these differences in size were directly related to post-settlement mortality (r2 = 47%, p = 0.047; Fig. 3); i.e., larger fish in July were more likely to survive to late August in that year. Second, although age-0 density did not appear to impact mortality (Fig. 4a) there was a negative impact on post-settlement growth (r2 = 65%; Fig.4b), suggesting nurseries may have limited resources (e.g., habitat, food) and could be a population bottleneck in years of high age-0 abundance.

Figure 2, see caption
Figure 2.  Interannual regional abundance of age-0 Pacific cod compared to a) age-0 saffron cod (left) and b) age-0 walleye pollock (right) during the same period and region.  Data are based on seine catches of age-0 fish in mid-July at multiple sites across two regions, Anton Larson Bay (ALB) and Cook Bay (CB), over a 5-year period (200610).

Figure 3, see caption
Figure 3.  Influence of settlement size on mortality in age-0 Pacific cod over a 6-week period in two Kodiak nurseries areas.  Size data (x-axis) are based on mean size of age-0 Pacific cod collected in mid-July in seines at multiple sites across two regions, Anton Larson Bay (ALB) and Cook Bay (CB), over a 5-year period (200610).  Mortality estimates are based on relative change in abundance (catch-per-haul) from mid-July to late August, respectively.

Question 3: The regression analysis of age-0 abundance on subsequent year-class strength yielded mixed results. For Pacific cod, age-0 abundance significantly predicted age-1 and age-2 abundance (p<0.05 in all instances) and there was model improvement using late estimates of age-0 abundance from August compared to July. However, all the regressions were highly leveraged by the abundant 2006 year class in ALB (hii = 0.76 0.93). The removal of this data point resulted in no significant relationship between age-0 and age-1 or age-0 and age-2 abundance (p>0.05).

A more cautious interpretation is that age-0 estimates of low abundance (using beach seines) are reliable indicators of relatively low year-class strength in subsequent years whereas abundant year classes may still be prone to high rates of mortality to be considered sound indicators of recruitment strength.

Collectively, these data suggest age-0 Pacific cod abundance is a reasonable predictor of year-class strength of resident juveniles, but post-settlement sources of mortality (e.g., overwintering, predation, etc.) may be significant enough to consider the existence of additional critical periods in juvenile gadids resident to Kodiak coastal areas.

Figure 4, see caption
Figure 4.  Influence of annual age-0 Pacific cod abundance on a) growth (left) and b) mortality (right) over a 6-week period in two Kodiak nurseries areas.  Abundance data (x-axis) are based on seine catches of age-0 Pacific cod in mid-July at multiple sites across two regions, Anton Larson Bay (ALB) and Cook Bay (CB), over a 5-year period (2006-2010).  Growth and mortality estimates are based on relative change mean length and abundance from mid-July to late August, respectively.

Further studies will undoubtedly need to determine how the population dynamics of juvenile cod in coastal nurseries integrate with the broader population dynamics of the Gulf of Alaska. With additional time-series data, an examination of parallels between coastal and offshore abundance data (e.g., age-0 seine data vs. age-3 trawl data) could be one means of examining such links. In the meantime, the supporting vital rate information (derived in coastal nurseries) could be used for provisional estimates of growth and natural mortality where other such data are absent or assumed in the Gulf of Alaska stock assessment.

By Benjamin J. Laurel

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