Gordon H. Kruse

Spatial and Temporal Variability in Size at Maturity of Walleye Pollock in the Eastern Bering Sea

Abstract Size at maturity is incorporated into stock assessments of walleye pollock Theragra chalcogramma. For eastern Bering Sea walleye pollock, however, this important biological variable has not been examined since 1976 and possible interannual and geographic variations have never been evaluated. Maturity condition and fish length data were collected from 10,197 walleye pollock aboard factory trawlers during winter 2002 and 2003. We also analyzed a smaller, previously unanalyzed data set on maturity collected annually by National Marine Fisheries Service scientists during eight echo integration-trawl surveys over 1989-2002. Length at 50% maturity (L 50) was estimated by year and area by logistic regression using maximum likelihood methods. Spatial (within subareas) and temporal (interannual) variability were found. For instance, fish matured at the smallest lengths north of the Pribilof Islands and in the years 1989, 1991, and 1995 and at the largest lengths in the southeastern Bering Sea and in the y...

Demographic and risk analyses of spiny dogfish (Squalus suckleyi) in the Gulf of Alaska using age- and stage-based population models

Demographic models are useful tools for assessing data-limited species and may be an appropriate alternative to cohort analyses for sharks due to their long-lived, slow-growing nature. In this study, age- and stage-based demographic analyses were conducted to examine the intrinsic rebound potential (r) and potential risk of fishing for spiny dogfish (Squalus suckleyi) in the Gulf of Alaska. Monte Carlo simulations were conducted to incorporate input parameter uncertainty. For an unfished population, r was estimated to be 0.02–0.03 year–1. Fishing mortalities (F) of F = 0.04 and 0.03 (age- and stage-based models respectively), resulted in r = 0, indicating that populations fished at higher F are not sustainable. Harvest strategies targeting juveniles (age-based model) and subadults (stage-based model) caused the highest risk of the population falling below defined thresholds (BMSY, B40% and B50%) after 20 years. The age- and stage-based models provided similar estimates of r and sustainable fishing mortality, suggesting that the stage-based model is an appropriate substitute for the age-based model in this case. S. suckleyi and the closely related S. acanthias are often harvested around the world and this modelling approach could be useful to the management of these species and other sharks where data is limited.

Red King Crab, Paralithodes camtschaticus, Size-Fecundity Relationship, and Interannual and Seasonal Variability in Fecundity

ABSTRACT Stock assessment of Alaska red king crab, Paralithodes camtschaticus, can be improved by incorporating embryo production, which requires an understanding of the size-fecundity relationship and an understanding of interannual and seasonal variability in fecundity. We collected red king crab egg clutches from Bristol Bay, AK, during summer 2007 to 2010 and autumn 2007 to 2009 and estimated individual fecundity using dry weight methods. Fecundity increased with female size up to 138 mm carapace length (CL), at which point the slope decreased by 40%, suggesting senescence. Fecundity varied significantly but slightly (maximum, 5%) among years. Fecundity was consistently lower in autumn than spring, suggesting brood loss, with a 6% decrease between seasons in females smaller than 138 mm CL and a 10% decrease in larger females. When incorporating embryo production in stock assessments, differences in the size-fecundity relationship should be accounted for and rates of brood loss can be used to predict larval output more effectively if fecundity cannot be measured shortly before hatching. To include potential environmental effects on the relationship, time series of fecundity and female size should be extended over a wider range of future temperature conditions.

Analysis of a Stock-Recruit Relationship for Red King Crab off Kodiak Island, Alaska

Abstract Waters of the northern Gulf of Alaska around Kodiak Island once supported the worlds largest fishery for the red king crab Paralithodes camtschaticus. Commercial fisheries began at low levels in the 1930s, increasing rapidly in the 1960s to a peak harvest of 42,800 metric tons in 1965. Stock abundance declined sharply in the late 1960s, moderated in the 1970s, and crashed in the early 1980s. A commercial fishery closure since 1983 has not resulted in stock recovery. To improve understanding of stock dynamics, we examined spawner–recruit (S-R) relationships for the Kodiak red king crab stock. The shape of the S-R relationship helps describe average stock productivity at different stock levels, thus relating directly to fishery management objectives. Due to limited female data, we used only male data and two currencies of male abundance as a proxy for spawners: either (1) all males greater than or equal to 125 mm carapace length (CL) or (2) legal males (≥145 mm CL). Due to age uncertainty, we considered lag times of 5 to 8 years between reproduction and recruitment. Residuals from fitting a standard Ricker model were strongly negative for brood years from the mid-1970s to the mid-1980s and positive from the early to mid-1990s. A Ricker model with autocorrelated errors resulted in better fits with corrected Akaikes information criterion (AICc) values minimized for a 5-year lag using all males. Among model configurations, estimates of peak production ranged from 2.0 to 3.4 million recruits from a range of 11.3 to 37.7 million males. A model separating the time series into three productivity periods corresponding to different ecological regimes further improved model fit. Although abundances of both spawners and recruits have been less than 1.0 million males since 1985, depensation at low stock sizes was not detected. Future analyses will examine the contribution of ecological and environmental factors to crab recruitment.

Does maternal size affect red king crab, Paralithodes camtschaticus, embryo and larval quality?

Stock assessment of Alaskan red king crab, Paralithodes camtschaticus (Tilesius, 1815), can be improved by incorporating reproductive output, which requires an understanding of maternal size effects on embryo and larval quality. In June 2009 and 2010, we collected clutches of recently extruded red king crab embryos in Bristol Bay, Alaska, to assess embryo quality based on dry weight, carbon and nitrogen content. To assess larval quality, we collected ovigerous females from Bristol Bay in 2007 and reared them in the laboratory until larval hatching in 2008. Larval quality based on dry weight, carbon and nitrogen content, and times to 50% mortality under starvation conditions were assessed. All samples were collected in years that were colder than the 15-year average in the eastern Bering Sea. Among the measures of embryo quality, only nitrogen content was significantly different, increasing with maternal size. Carbon and nitrogen content were significantly higher for embryos in 2009 than in 2010, suggesting inter-annual differences in maternal investment. No effect of maternal size with larval quality was found. Our results indicate that maternal size does not have a biologically significant effect on embryo and larval quality in colder-than-average years, and therefore maternal size effects on embryo and larval quality does not need to be explicitly incorporated into reproductive output estimates in stock assessments under these conditions. We are, however, cautious to extrapolate our results to years with different environmental conditions. Further study is needed to fully resolve the possible interaction of environment with maternal size effects on embryo and larval quality for red king crab.

Are Pacific spiny dogfish lying about their age? A comparison of ageing structures for Squalus suckleyi

Historically, Pacific spiny dogfish (Squalus suckleyi) have been aged using dorsal fin spines, a method that was validated through bomb radiocarbon analysis and oxytetracycline tagging. However, ages generated using this method generally have poor precision and require estimation of missing growth bands in eroded spines, prompting a search for improved age determination methods. In the present study, spiny dogfish were aged using the historical spine method and a new method involving stained thin sections of vertebral centra. Results of an inter-laboratory exchange demonstrated the need for readers to calibrate ageing criteria with a reference collection before reading structures, a practice that yielded significant improvements in between-reader precision of spine band pair counts. After calibration, the primary readers examined the full sample set. The two structures yielded similar age estimates for younger animals, but centrum estimates were consistently younger than spine estimates after age-10. Although further work is necessary to fully explore potential reasons for the observed bias, such as centrum size and location within the vertebral column, at the present time centra are not a suitable alternative to dorsal fin spines for age determination of Pacific spiny dogfish >10 years of age.

Interannual and Spatial Variability in Maturity of Walleye Pollock Gadus chalcogrammus and Implications for Spawning Stock Biomass Estimates in the Gulf of Alaska

Catch quotas for walleye pollock Gadus chalcogrammus, the dominant species in the groundfish fishery off Alaska, are set by applying harvest control rules to annual estimates of spawning stock biomass (SSB) from age-structured stock assessments. Adult walleye pollock abundance and maturity status have been monitored in early spring in Shelikof Strait in the Gulf of Alaska for almost three decades. The sampling strategy for maturity status is largely characterized as targeted, albeit opportunistic, sampling of trawl tows made during hydroacoustic surveys. Trawl sampling during pre-spawning biomass surveys, which do not adequately account for spatial patterns in the distribution of immature and mature fish, can bias estimated maturity ogives from which SSB is calculated. Utilizing these maturity data, we developed mixed-effects generalized additive models to examine spatial and temporal patterns in walleye pollock maturity and the influence of these patterns on estimates of SSB. Current stock assessment practice is to estimate SSB as the product of annual estimates of numbers at age, weight at age, and mean maturity at age for 1983-present. In practice, we found this strategy to be conservative for a time period from 2003–2013 as, on average, it underestimates SSB by a 4.7 to 11.9% difference when compared to our estimates of SSB that account for spatial structure or both temporal and spatial structure. Inclusion of spatially explicit information for walleye pollock maturity has implications for understanding stock reproductive biology and thus the setting of sustainable harvest rates used to manage this valuable fishery.

Autumn distribution of Bristol Bay red king crab using fishery logbooks

Spatial distributions of fished species must be well characterized to avoid local depletions, identify critical habitat, and predict and mitigate interactions with other fisheries. The Bristol Bay red king crab (Paralithodes camtschaticus) fishery is one of the largest crab fisheries in Alaska. Summer crab distributions have been well documented by decades of bottom trawl surveys. However, crab movement and distribution are poorly understood outside the summer survey period, which creates several management challenges. One important component of fishery management is the existence of no-trawl zones, which are intended to protect crab from bottom trawl fisheries. However, it is difficult to evaluate the placement of no-trawl zones, because most crab bycatch occurs in trawl fisheries during winter when crab distributions are unknown. Daily fishing logs, kept by skippers in the red king crab fleet since 2005, contain detailed information on the spatial distribution of catch and effort of legal sized male crab during the autumn crab fishery. However, data contained in these hand-written logbooks have not been readily accessible. We digitized daily fishing logs from 2005 to 2016 and used spatial information on catch and effort to infer geographic distributions of legal sized male king crab during the crab fishing season. Changes in distribution were tracked across this 12-yr period and comparisons were made between warm and cold temperature regimes. In warm years (2005, 2014–2016), crab aggregated in the center of Bristol Bay, Alaska, while in cold years (2007–2013) they were closer to the Alaska Peninsula. The majority of crab were caught in no-trawl areas (63.4% on average), but variations occurred among years and with temperature regime (40.0–86.8% in no-trawl zones). As temperatures continue to shift in the Bering Sea, it will be important to continue monitoring crab distributions outside the summer survey period.

Do abiotic and ontogenetic factors influence the diet of a generalist predator? Feeding ecology of the Pacific spiny dogfish (Squalus suckleyi) in the northeast Pacific Ocean

Stomach contents from 1221 Pacific spiny dogfish (Squalus suckleyi) were collected from the Gulf of Alaska over 2004–2006 and analyzed to determine predominant prey species. Pacific spiny dogfish in the Gulf of Alaska have a variable diet suggesting that they are generalist feeders. Of the 68.30% of stomachs with prey contents, the most important prey groups based on the percent prey-specific index of relative importance (%PSIRI) were shrimp (27.06%), cephalopods (17.16%), and forage fish (17.11%). Remaining components of the diet were inconsistent. Commercially valuable species, such as salmon, Oncorhynchus spp., and rockfish, Sebastes spp. constituted only 2.37% and 1.51% of the diet of Pacific spiny dogfish, respectively. Diet diversity was not significantly influenced by spatial, temporal or ontogenetic factors; however, trophic level of prey and average prey item weight varied both interannually and with ontogeny. With increasing size, Pacific spiny dogfish incorporate larger prey items into their diet, and diet composition may be driven more by seasonal availability and prey size than any other factors.