Arrowtooth Flounder Atheresthes Stomias Research Articles

Spatially varying coefficients can improve parsimony and descriptive power for species distribution models

Species distribution models (SDMs) are widely used to relate species occurrence and density to local environmental conditions, and often include a spatially correlated variable to account for spatial patterns in residuals. Ecologists have extended SDMs to include spatially varying coefficients (SVCs), where the response to a given covariate varies smoothly over space and time. However, SVCs see relatively little use perhaps because they remain less known relative to other SDM techniques. We therefore review ecological contexts where SVCs can improve the interpretability and descriptive power from SDMs, including local responses to regional indices that represent ecological teleconnections; density‐dependent habitat selection; spatially varying detectability; and context‐dependent covariate responses that represent interactions with unmeasured covariates. We then illustrate three additional examples in detail using the vector autoregressive spatio‐temporal (VAST) model. First, a spatially varying decadal trends model identifies decadal trends for arrowtooth flounder Atheresthes stomias density in the Bering Sea from 1982 to 2019. Second, a trait‐based joint SDM highlights the role of body size and temperature in spatial community assembly in the Gulf of Alaska. Third, an age‐structured SDM for walleye pollock Gadus chalcogrammus in the Bering Sea contrasts cohorts with broad spatial distributions (1996 and 2009) and those that are more spatially constrained (2002 and 2015). We conclude that SVCs extend SDMs to address a wide variety of ecological contexts and can be used to better understand a range of ecological processes, e.g. density dependence, community assembly and population dynamics.

Climate‐informed models benefit hindcasting but present challenges when forecasting species–habitat associations

Although species distribution models (SDMs) are commonly used to hindcast fine‐scale population metrics, there remains a paucity of information about how well these models predict future responses to climate. Many conventional SDMs rely on spatially‐explicit but time‐invariant conditions to quantify species distributions and densities. We compared these status quo ‘static' models with more climate‐informed 'dynamic' SDMs to assess whether the addition of time‐varying processes would improve hindcast performance and/or forecast skill. Here, we present two groundfish case studies from the Bering Sea – a high latitude system that has recently undergone considerable warming. We relied on conventional statistics (R2, % deviance explained, UBRE or GCV) to evaluate hindcast performance for presence–absence, numerical abundance and biomass of arrowtooth flounder Atheresthes stomias and walleye pollock Gadus chalcogrammus. We then used retrospective skill testing to evaluate near‐term forecast skill. Retrospective skill testing enables direct comparisons between forecasts and observations through a process of fitting and forecasting nested submodels within a given time series. We found that the inclusion of time‐varying covariates improved hindcasts. However, dynamic models either did not improve or decreased forecast skill relative to static SDMs. This is likely a result of rapidly changing temperatures within the ecosystem, which required models to predict species responses to environmental conditions that were outside the range of observed values. Until additional model development allows for fully dynamic predictions, static model forecasts (or persistence forecasts from dynamic models) may serve as reliable placeholders, especially when anomalous conditions are anticipated. Nonetheless, our findings demonstrate support for the use of retrospective skill testing rather than selecting forecast models a priori based on their ability to quantify species–habitat associations in the past.

Shifting fish distributions impact predation intensity in a sub‐Arctic ecosystem

An abundance of studies in marine systems have documented species range shifts in response to climate change, and many more have used species distribution models to project species ranges under future conditions. However, there is increasing interest in moving beyond a single‐species focus to understand how species redistribution alters ecosystem dynamics via changes in trophic interactions. We employed spatiotemporal models to characterize decadal‐scale changes in spatial overlap between the distribution of juvenile walleye pollock Gadus chalcogrammus and the distributions of four of its groundfish predators: arrowtooth flounder Atheresthes stomias, Pacific cod Gadus macrocephalus, Pacific halibut Hippoglossus stenolepis and adult walleye pollock. These fishes represent ecologically and commercially important species in a rapidly changing sub‐Arctic ecosystem, the eastern Bering Sea, Alaska, USA. We then examined whether changes in spatial overlap corresponded to changes in predation, using spatiotemporal models of predator stomach contents. We found marked shifts in spatial overlap between juvenile pollock and two predators (arrowtooth flounder and Pacific halibut) over 34 years, with changes in overlap corresponding to increases in population‐scale predation pressure. By contrast, we did not find clear relationships between spatial overlap and predation for Pacific cod and adult pollock, the two predators for which juvenile pollock constitute a much smaller diet proportion. Our findings highlight the complexity of predicting predation dynamics for generalist marine species and suggest a need for better process‐based methods for understanding the potential future ecological impacts of coupled species range shifts. However, simple metrics of spatial overlap between relatively specialized predators and their prey offer promise as a means to integrate predictions from species distribution models into ecosystem‐based fisheries management.

An ensemble approach to understand predation mortality for groundfish in the Gulf of Alaska

There is increasing consensus of the need for ecosystem-based fisheries management (EBFM), which accounts for trophic interactions and environmental conditions when managing exploited marine resources. Continued development and testing of analytical tools that are expected to address EBFM needs are essential for guiding the management of fisheries resources in achieving and balancing multiple social, economic, and conservation objectives. To address these needs, we present and compare alternative climate-informed multi-species statistical catch-at-age models to account for spatio-temporal differences in stock distributions, with application to four groundfish species (walleye pollock Gadus chalcogrammus, Pacific cod Gadus macrocephalus, arrowtooth flounder Atheresthes stomias, and Pacific halibut Hippoglossus stenolepis) in the Gulf of Alaska, USA. We integrate across multiple forms of uncertainty regarding the data and distribution of Pacific halibut using an ensemble modelling approach. Models developed here can be used to supplement current tactical fisheries management and inform on the trade-offs between harvesting across groundfish in the Gulf of Alaska. This approach may be applicable for other situations where spatial and temporal overlap is extensive among closely coupled species.

The Role of Size in Trophic Niche Separation between Two Groundfish Predators in Alaskan Waters

AbstractTo minimize negative effects of competition, predators must divide resources along one or more niche dimensions. Trophic niche separation is a principal method of resource partitioning in marine environments and is often assessed by quantifying diets of fishes with similar lengths. Taxonomic differences in allometric growth, however, suggest that the degree of gape limitation at a given body size varies among species. We studied resource overlap between two potential competitors in Southeast Alaska: Pacific Halibut Hippoglossus stenolepis and Arrowtooth Flounder Atheresthes stomias. We quantified length–gape relationships and assessed effects of fork length (FL; cm), gape height (GH; mm), and gape width (GW; mm) on interpretations of trophic niche separation. Gape sizes for Arrowtooth Flounder were nearly twice those of Pacific Halibut at similar FLs. Predators with smaller gapes exhibited moderate to high dietary overlap ( = 0.554; = 0.731), whereas those with larger gapes had lower dietary overlap and distinct diet compositions ( = 0.421; = 0.265). These findings suggest that the greatest potential for food competition is among small‐bodied Arrowtooth Flounder and relatively large‐bodied Pacific Halibut, with trophic niche separation increasing as these predators progress throughout their ontogeny. When comparing resource use among predators with similar FLs, we found high dietary overlap but significantly different diet compositions ( = 0.657). Thus, we postulate that GW is more appropriate than FL when quantifying trophic niche separation among sympatric fishes, especially when disparate length–gape relationships are detected. Studies of resource partitioning would benefit from examining multiple size metrics and explicitly considering differences in allometric growth, thereby addressing potential mechanisms for resource overlap among hypothesized competitors.

Spatio-temporal patterns in juvenile habitat for 13 groundfishes in the California Current Ecosystem.

Identifying juvenile habitats is critical for understanding a species’ ecology and for focusing spatial fishery management by defining references like essential fish habitat (EFH). Here, we used vector autoregressive spatio-temporal models (VAST) to delineate spatial and temporal patterns in juvenile density for 13 commercially important species of groundfishes off the US west coast. In particular, we identified hotspots with high juvenile density. Three qualitative patterns of distribution and abundance emerged. First, Dover sole Microstomus pacificus, Pacific grenadier Coryphaenoides acrolepis, shortspine thornyhead Sebastolobus alascanus, and splitnose rockfish Sebastes diploproa had distinct, spatially-limited hotspots that were spatially consistent through time. Next, Pacific hake Merluccius productus and darkblotched rockfish Sebastes crameri had distinct, spatially limited hotspots, but the location of these hotspots varied through time. Finally, arrowtooth flounder Atheresthes stomias, English sole Parophrys vetulus, sablefish Anoplopoma fimbria, Pacific grenadier Coryphaenoides acrolepis, lingcod Ophiodon elongatus, longspine thornyhead Sebastolobus altivelis, petrale sole Eopsetta jordani, and Pacific sanddab Citharichthys sordidus had large hotspots that spanned a broad latitudinal range. These habitats represent potential, if not likely, nursery areas, the location of which will inform spatial management.

Illuminating the Headrope of a Selective Flatfish Trawl: Effect on Catches of Groundfishes, Including Pacific Halibut

AbstractThis study evaluated how illuminating the headrope of a selective flatfish trawl can affect catches of groundfishes, including Pacific Halibut Hippoglossus stenolepis, in the U.S. West Coast limited‐entry (LE) groundfish bottom trawl fishery. Over the continental shelf, fishermen engaged in the LE bottom trawl fishery target a variety of flatfishes, roundfishes, and skates. Green LED fishing lights (Lindgren‐Pitman Electralume) were used to illuminate the headrope. The lights were grouped into clusters of three, with each cluster attached ~1.3 m apart along the 40.3‐m‐long headrope. Catch comparisons and ratios of mean fish length classes were compared between tows conducted with (treatment) and without (control) LEDs attached along the trawl headrope. Fewer Rex Sole Glyptocephalus zaphirus, Arrowtooth Flounder Atheresthes stomias, and Lingcod Ophiodon elongatus were caught in the treatment than in the control trawl, though not at a significant level. Pacific Halibut catches differed between the two trawls, with the treatment trawl catching an average of 57% less Pacific Halibut. However, this outcome was not significant due to a small sample size. For Dover Sole Microstomus pacificus 31–44 cm in length and Sablefish Anoplopoma fimbria 43–61 cm in length, significantly fewer fish were caught in the treatment than in the control trawl. On average, the treatment trawl caught more rockfishes Sebastes spp., English Sole Parophrys vetulus, and Petrale Sole Eopsetta jordani, but not at a significant level. These findings show that illuminating the headrope of a selective flatfish trawl can affect the catch comparisons and ratios of groundfishes, and depending on fish length and species the effect can be positive or negative.

Temporal and Ontogenetic Variability in Trophic Role of Four Groundfish Species—Walleye Pollock, Pacific Cod, Arrowtooth Flounder, and Pacific Halibut—around Kodiak Island in the Gulf of Alaska

AbstractChanges in trophic position estimates of commercial fishery catches are used as an ecosystem‐based indicator for sustainability, but often these estimates do not incorporate species‐specific seasonal feeding dynamics and ontogenetic diet changes. Using stable isotope analysis, we obtained a fine‐scale resolution of ontogenetic and temporal (interannual and seasonal) variations in the trophic roles of four commercially and ecologically important groundfish species in the Gulf of Alaska: walleye pollock Theragra chalcogramma, Pacific cod Gadus macrocephalus, arrowtooth flounder Atheresthes stomias, and Pacific halibut Hippoglossus stenolepis. For each groundfish taxon, the nitrogen stable isotope signature (δ15N) increased with total length. In contrast, the lipid‐normalized carbon stable isotope signature (δ13C′) significantly varied with size‐class only for walleye pollock. There were species‐specific differences in trophic position; adult Pacific cod fed at the highest trophic position, and walleye pollock fed at the lowest trophic position. Walleye pollock also had the lowest δ13C′, indicating a mainly pelagic diet, while Pacific halibut and Pacific cod had the highest δ13C′, indicating a mostly benthic diet. Interannual differences in trophic position were detected for each species. Pacific cod, arrowtooth flounder, and Pacific halibut fed at a significantly lower trophic position in 2003 than in 2001, 2002, or 2004. All species had a significantly lower average δ13C′ (i.e., a more benthic diet) in 2001 and a higher average δ13C′ (a more pelagic diet) in 2003. Pacific cod, Pacific halibut, and walleye pollock had a significantly more pelagic diet in the summer. Walleye pollock, arrowtooth flounder, and Pacific halibut showed a significantly more benthic diet in the fall, which probably corresponded with their seasonal migrations. Temporal variations in stable isotope signatures were observed but were relatively small (<1.25‰). Length‐based ontogenetic diet shifts, however, were much larger (up to 6‰) and must be considered when trophic position is used as an ecosystem indicator of sustainability.

Use of Fish Hydrolysates and Fish Meal Byproducts of the Alaskan Fishing Industry in Diets for Pacific White Shrimp Litopenaeus vannamei

AbstractWe determined the suitability of four fish hydrolysates and two fish meals (Pacific halibut Hippoglossus stenolepus and arrowtooth flounder Atheresthes stomias; all derived from byproducts of the Alaskan fishing industry) as replacements for menhaden fish meal in shrimp diets. A control diet (30% crude protein; 8.5% crude lipid) was produced with menhaden meal (13% of diet). Experimental diets were manufactured by using each hydrolysate or fish meal to replace 50% of the menhaden meal on an isonitrogenous basis. Each diet was fed to Pacific white shrimp Litopenaeus vannamei in an outdoor, zero‐water‐exchange system for 8 weeks. Final weight, survival, feed efficiency, and growth rate were compared among treatments by analysis of variance. The final weights and growth of Pacific white shrimp fed two of the hydrolysates (one acidified to pH 3.8 and left in a liquid state; the other acidified, then neutralized to pH 6.5 and drum‐dried) and the two fish meals were not different from those of shrimp fed the control diet. Shrimp that were given the other two hydrolysates (one acidified to pH 3.8 and then drum‐dried; the other hydrolyzed [but not acidified] and then drum‐dried) had lower final weights and growth than shrimp fed the control diet. These data indicate that the two fish meals and two of the hydrolysates in this trial can replace 50% of menhaden meal in diets for shrimp, although there is some loss of growth potential in some cases. The lack of a diet effect on feed efficiency indicates that the growth differences were mostly the result of differences in feed intake rather than nutritional quality.

Recruitment forecast models for walleye pollock Theragra chalcogramma fine-tuned from juvenile survey data, predator abundance and environmental phase shifts

MEPS Marine Ecology Progress Series Contact the journal Facebook Twitter RSS Mailing List Subscribe to our mailing list via Mailchimp HomeLatest VolumeAbout the JournalEditorsTheme Sections MEPS 417:237-248 (2010) - DOI: https://doi.org/10.3354/meps08817 Recruitment forecast models for walleye pollock Theragra chalcogramma fine-tuned from juvenile survey data, predator abundance and environmental phase shifts Tianyang Zhang1, Kevin M. Bailey2, Kung-sik Chan1,* 1Department of Statistics and Actuarial Science, University of Iowa, 263 Schaeffer Hall, Iowa City, Iowa 52242, USA 2Alaska Fisheries Science Center, National Marine Fisheries Service, NOAA, 7600 Sand Point Way NE, Seattle, Washington 98115, USA *Corresponding author. Email: kchan@stat.uiowa.edu ABSTRACT: Forecasting recruitment of marine fishes from environmental effects acting upon larvae has proven difficult due to multiple, nonlinear, interacting factors influencing larval survival. We used another approach, which circumvents the period of high egg and larval mortality, and instead improves forecasts from juvenile abundance indices. We compared several statistical recruitment forecast models and demonstrate that an increasing abundance of predators on juvenile walleye pollock Theragra chalcogramma, particularly the arrowtooth flounder Atheresthes stomias, which now dominates the groundfish biomass in the Gulf of Alaska, and autocorrelation caused by intercohort interactions strongly affect pollock recruitment during the juvenile phase. Furthermore, the weight of predictor variables changes with threshold criteria, which are linked to phase shifts in the marine environment. Our results indicate that forecasting recruitment of marine fishes can be improved by considering factors that influence survival after the juvenile period, but also needs to account for changes in community structure and phase shifts in the environment, as opposed to only environmental correlates. Inclusion of these factors is consonant with biological knowledge of the species. KEY WORDS: Population dynamics · Fisheries · Environment · Ocean research · Regime shift · Ecological community · Fish recruitment · Forecast Full text in pdf format PreviousNextCite this article as: Zhang T, Bailey KM, Chan Ks (2010) Recruitment forecast models for walleye pollock Theragra chalcogramma fine-tuned from juvenile survey data, predator abundance and environmental phase shifts. Mar Ecol Prog Ser 417:237-248. https://doi.org/10.3354/meps08817 Export citation RSS - Facebook - Tweet - linkedIn Cited by Published in MEPS Vol. 417. Online publication date: November 04, 2010 Print ISSN: 0171-8630; Online ISSN: 1616-1599 Copyright © 2010 Inter-Research.

The role of flatfishes in the organization and structure of the eastern Bering Sea ecosystem

We evaluated the role of flatfishes in the organization and structure of the eastern Bering Sea ecosystem using the Ecopath/Ecosim approach. As basic input data for the Ecopath/Ecosim model, we used estimates of biomass from bottom trawl surveys and age-structured population models, production/biomass (P/B) ratio, consumption/biomass (Q/B) ratio, diet composition (DC), and fisheries harvests for each component of species or species groups. We estimated the trophic level of each component, niche overlaps among flatfishes, and the impacts of competition and predation on flatfish species in the eastern Bering Sea ecosystem. Based on those estimates, we developed the tropho-dynamic structure of the ecosystem, and the model was used to simulate ecological effects of fishery exploitation patterns. No single flatfish species appeared to have a profound and uniquely important role in the organization and structure of the ecosystem. Instead, the most important component among the guild of flatfish species appeared to be yellowfin sole Pleuronectes asper, which had greater biomass than other flatfish and a relatively diverse diet among the small flatfish species. Pacific halibut Hippoglossus stenolepis, Greenland turbot Reinhardtius hippoglossoides, and arrowtooth flounder Atheresthes stomias were important keystone predators in the eastern Bering Sea ecosystem together with some groups of marine mammals and sea birds. Intra flatfish complex cannibalism was not observed, however, substantial diet overlaps were common in the flatfish guild system.

Introduction to a Special Section: Atka Mackerel Distribution, Life History, Ecology, and Management

This suite of papers on the life history, ecology, and management of Atka mackerel was originally presented at a research symposium (29–30 April 2008) in Seattle, Washington. The studies were done as a collaborative effort between the Alaska Fisheries Science Center, the University of Alaska–Fairbanks, the Alaska Sea Life Center, and the North Pacific Fisheries Foundation. This research was funded by the National Marine Fisheries Service, the North Pacific Research Board (project 522), and the North Pacific Fisheries Foundation. Atka mackerel Pleurogrammus monopterygius are widely distributed along the continental shelf extending from the Kuril Islands in Asia through the Gulf of Alaska. They are the most abundant groundfish in the Aleutian Islands region, with an estimated exploitable biomass of 388,500 metric tons in 2010 (Lowe et al. 2009), and are a key component of the Aleutian Islands ecosystem. Within Alaska waters they are concentrated in the Aleutian Islands chain, where they form dense aggregations in island passes. Atka mackerel are obligate spawners on hard rocky bottoms and exhibit a polygamous mating system, elaborate sexual dichromatism, and social behaviors that have been documented in other hexagrammids. Nesting colonies are widespread across the continental shelf and are associated with areas of strong currents and hard substrate as deep as 144 m (Lauth et al. 2007). In June, the reproductive phase begins with adult males establishing individual territories within nesting colonies. Adult females aggregate in dense patches close to the nesting colonies during the spawning season, presumably to feed (Cooper and McDermott, unpublished). In July spawning starts, and during a 12-week period ending in mid-October females spawn four to five batches of adhesive eggs in the territories of different males (McDermott et al. 2007). After spawning, females disperse and males guard the nests until the eggs hatch following an incubation period of 39–75 d in the eastern and central Aleutian Islands, with maximum hatching occurring in late November (Lauth et al. 2007). Atka mackerel exhibit a size cline across their range, with individuals having smaller sizes at age in the western Aleutian Islands area and larger sizes at age in the eastern areas. Historical morphological and meristic studies provide some indications of separate populations within Alaska. However, a genetic study of allozyme variation showed no evidence of discrete stocks (Lowe et al. 1998), a result that is reexamined in this special section using another class of nuclear markers. Atka mackerel’s highly localized aggregations and their overall large biomass make them an important prey species in the Aleutian Islands ecosystem. Atka mackerel have been identified as the main prey item for the endangered Steller sea lion Eumetopias jubatus and fish species such as Pacific cod Gadus macrocephalus and arrowtooth flounder Atheresthes stomias (I. Ortiz and E. Logerwell, National Marine Fisheries Service, unpublished). Adult Atka mackerel consume a variety of prey but principally feed on calanoid copepods, euphausiids, and myctophid fishes (Rand 2007). Egg cannibalism during the spawning period has been observed for both sexes. Heterocannibalism by both sexes and male filial cannibalism are common phenomena in species that have evolved to provide exclusive paternal care. Atka mackerel are the target of a large offshore commercial fishery in the Aleutian Islands. The spatial patterns of the fishery generally reflect the distribution and behavior of the species; the fishery is highly localized and focuses on the same locations each year, almost exclusively at depths shallower than 200 m. The fishery is currently managed by the North Pacific Fishery Management Council under a quota system whereby the overall total allowable catch (TAC) is divided into two equal seasonal allowances: an ASubject editor: Donald Noakes, Thompson Rivers University, British Columbia, Canada

Sex‐Specific Natural Mortality of Arrowtooth Flounder in Alaska: Implications of a Skewed Sex Ratio on Exploitation and Management

AbstractConservative fisheries management in Alaskan waters typically limits harvest to maintain a target biomass of female spawners (40% of the female biomass observed in the absence of fishing). This harvest calculation does not consider the male population level for a species. If natural mortality rates (M) differ between sexes and if females outlive males, then a reduction in males relative to females could diminish the reproductive potential of the stock. This problem was examined in the present study by estimating separate sex‐specific M‐values for the arrowtooth flounder Atheresthes stomias in Alaska, where females are almost always found at higher proportions than males during trawl surveys. Age data indicated that females outlive males, and estimates of M ranged from 0.10 to 0.33 for females and from 0.16 to 0.51 for males. Based on four estimation methods, male M was consistently higher than female M. Simulated harvest scenarios in which male M and selectivity were varied indicated that increasing male M and lowering the age of males at first capture skewed the sex ratio even more toward females; in the most extreme harvest scenario, the percentage of females in the population was as high as 89% (Gulf of Alaska) and 96% (Bering Sea). Because reproductive success under a skewed sex ratio is probably linked to reproductive behavior, we developed a simple conceptual model to consider three reproductive strategies that differed in the number of females (1–3) that could be paired with a single male. Although a population's reproductive potential was much more resilient to changes in sex ratio for 1:2 or 1:3 (male : female) pairings (reduced by only 15% when the population was 70% female), the fishing mortality rate (F) that corresponded to a reduction in egg fertilization was much higher than the target F (i.e., that maintained biomass at 40% of the unfished biomass) and exceeded overfishing levels when the population was over 80% female.

Spatial and temporal variation in the diet of Steller sea lions in the Kodiak Archipelago, 1999 to 2005

MEPS Marine Ecology Progress Series Contact the journal Facebook Twitter RSS Mailing List Subscribe to our mailing list via Mailchimp HomeLatest VolumeAbout the JournalEditorsTheme Sections MEPS 360:265-283 (2008) - DOI: https://doi.org/10.3354/meps07383 Spatial and temporal variation in the diet of Steller sea lions in the Kodiak Archipelago, 1999 to 2005 Jane McKenzie*, Kate M. Wynne University of Alaska Fairbanks, School of Fisheries and Ocean Science, 118 Trident Way, Kodiak, Alaska 99615, USA *Email: janemckenzie@malpage.com ABSTRACT: Spatial and temporal variation in the diet of Steller sea lions Eumetopias jubatus was investigated using scat (fecal) samples collected from 4 regions in the Kodiak Archipelago. Over 2700 scats with identifiable prey were collected from the northern, eastern, southern and western sides of Kodiak Island from 1999 to 2005. Of 76 prey types identified using hard remains, the most important species in terms of frequency of occurrence and numerical abundance were Pacific sand lance Ammodytes hexapterus, walleye pollock Theragra chalcogramma, arrowtooth flounder Atheresthes stomias, Pacific cod Gadus macrocephalus, salmon Oncorhynchus spp., and Pacific herring Clupea pallasi. Significant differences in diet composition were found among regions, seasons, and years, suggesting that the diet of sea lions is strongly influenced by local and temporal distributions and abundances of prey. Herring dominated scat collections from the west coast and accounted for most (14 to 30%) of the differences in regional diet in spring and winter. Annual variation in diet was relatively low in winter, whereas spring and fall diets varied from year to year, with regional-specific shifts in dominant prey. Results from our study generally agree with diet studies conducted in the 1990s, but differ markedly in the relative importance of the major prey species. KEY WORDS: Eumetopias jubatus · Prey · Seasonal · Diet overlap · Forage fish · Gulf of Alaska · Theragra chalcogramma · Ammodytes hexapterus Full text in pdf format Supplementary appendix PreviousNextCite this article as: McKenzie J, Wynne KM (2008) Spatial and temporal variation in the diet of Steller sea lions in the Kodiak Archipelago, 1999 to 2005. Mar Ecol Prog Ser 360:265-283. https://doi.org/10.3354/meps07383Export citation RSS - Facebook - Tweet - linkedIn Cited by Published in MEPS Vol. 360. Online publication date: May 22, 2008 Print ISSN: 0171-8630; Online ISSN: 1616-1599 Copyright © 2008 Inter-Research.

Assemblage Structure of Eastern Pacific Groundfishes on the U.S. Continental Slope in Relation to Physical and Environmental Variables

AbstractWe analyzed data from the National Marine Fisheries Service continental slope trawl surveys (1999–2002) to examine patterns in eastern Pacific groundfish assemblage structure in relation to longitude, latitude, depth, temperature, and interannual variation. The slope surveys spanned approximately 32–48°N and 200–1,200‐m depths. We focused on the 26 most abundant species, which made up more than 95% of the catch in terms of biomass. Assemblage structure was strongly correlated with depth and latitude. For the most part, changes in assemblage structure appeared to be fairly continuous, although there were more‐abrupt changes at approximately 500–600 m and in the vicinity of Point Conception and Cape Mendocino, California, and Cape Blanco, Oregon. There was also an interaction between depth and latitude; more northerly sites had deeper scores on the canonical axes than did more southerly sites of comparable depth. Shallower sites also showed more variation in assemblage structure with latitude than did deeper sites. We were able to identify five assemblages on the continental slope. Pacific grenadiers Coryphaenoides acrolepis, giant grenadiers Albatrossia pectoralis, deepsea soles Embassichthys bathybius, longspine thornyheads Sebastolobus altivelis, California slickheads Alepocephalus tenebrosus, and roughtail skate Bathyraja trachura formed a deepwater assemblage with a broad latitudinal range. Southern species that inhabited very shallow waters included splitnose rockfish Sebastes diploproa, spotted ratfish Hydrolagus colliei, English soles Parophrys vetulus, stripetail rockfish Sebastes saxicola, chilipeppers Sebastes goodei, and shortbelly rockfish Sebastes jordani. Shallow‐water, midlatitude species included spiny dogfish Squalus acanthias, longnose skate Raja rhina, Pacific hakes Merluccius productus, rex soles Glyptocephalus zachirus, bigfin eelpout Lycodes cortezianus, and sandpaper skate Bathyraja interrupta. Darkblotched rockfish Sebastes crameri, arrowtooth flounder Atheresthes stomias, and Pacific Ocean perch Sebastes alutus were identified as a shallow, northerly assemblage. The final group—sablefish Anoplopoma fimbria, brown cat sharks Apristurus brunneus, aurora rockfish Sebastes aurora, Dover soles Microstomus pacificus, and shortspine thornyheads Sebastolobus alascanus—occurred at different depths over a range of latitudes and might be best described as a shallow–middepth and midlatitude–southern grouping.

Evaluation of a Selective Flatfish Trawl and Diel Variation in Rockfish Catchability as Bycatch Reduction Tools in the Deepwater Complex Fishery off the U.S. West Coast

AbstractWe tested the potential of a selective flatfish trawl to reduce bycatch of slope rockfish in the upper continental slope bottom‐trawl fishery (250–500 m) on the U.S. West Coast. The trawl we tested differed from typical slope trawls in that it was a low‐rise, two‐seam trawl with a severely cut back headrope. We used an alternate haul, randomized block design to compare catches of the experimental trawl with those of a typical four‐seam, high‐rise trawl and to examine diel changes in catch rates for both trawls. The experimental trawl produced catches similar to the control trawl for all commercially valuable flatfish, except arrowtooth flounder Atheresthes stomias, which was reduced 24%. Catches of most rockfish and roundfish were significantly reduced in the experimental trawl (50–94% depending on species). However, the catches of darkblotched rockfish Sebastes crameri and redbanded rockfish S. babcocki were not reduced significantly in the experimental trawl. Nighttime catches were reduced 30–99% for most rockfish species, the greatest reductions occurring in the experimental trawl. The large nighttime catch reduction for several rockfish species for both trawls along with no catch reduction in Dover sole Microstomus pacificus, suggests that fishing only at night may be a viable bycatch‐reduction strategy for some rockfish species. The diel and depth‐related variation in catch rates we noted for the selective flatfish trawl indicates that catches by this trawl may be especially sensitive to physical and environmental factors such as time of day, depth, or turbidity.

The effect of trawl speed on the footrope capture efficiency of a survey trawl

The effect of speed through water on the footrope capture efficiency of a survey bottom trawl was experimentally investigated by repetitively towing at three vessel speeds (2.5, 3.0 and 3.5 knots) with an auxiliary net attached underneath the trawl footrope to capture fish escaping beneath the trawl. Length-dependent capture efficiencies were then computed from the number of fish caught in both the trawl and auxiliary net. Capture efficiencies for Pacific cod Gadus macrocephalus, walleye pollock Theragra chalcogramma and Pacific halibut Hippoglossus stenolepis were not affected by fish length or trawl speed. Capture efficiency for skates Bathyraja spp. decreased with increasing trawl speed but was not affected by fish length. Capture efficiencies for arrowtooth flounder Atheresthes stomias and flathead sole Hippoglossoides elassodon increased with fish length and decreased with increasing trawl speed. Our results indicate that in areas of variable current, variation in survey catch per unit effort could be reduced for some species by standardizing towing speed to speed through water or by reducing footrope lift by the addition of sufficient weight to the trawl footrope.

Control of Muscle Softening and Protease‐Parasite Interactions in Arrowtooth Flounder Atheresthes stomias

ABSTRACTDistinct proteolytic activity was observed in the incubated muscle tissue of arrowtooth flounder. Absorbance measurements of the TCA filtrates indicated maximum proteolytic activity between 55°C and 60°C, and minimum activity below 40°C. Instron puncture tests of raw, baked, steamed, deep‐fat fried and microwaved samples of arrowtooth demonstrated that rapid inactivation of the protease(s) by microwave cooking significantly improved textural properties. Myxosporean parasites tentatively classified as Kudoa thyrsitis were identified in all but one of the arrowtooth samples. However, no clear relationship between parasite density and textural deterioration of the cooked fillets was observed.