Feeding habits the Pacific cod <Emphasis Type="Italic">Gadus macrocephalus</Emphasis> in oceanic waters of the Northern Kuril Islands and Southeast Kamchatka

This study examines the feeding habits of the Pacific cod Gadus macrocephalus in waters off the eastern coast of the northern Kuril Islands and southern Kamchatka. In November–December 1996, the cod primarily consumed fish, which made up 47.6% of the total food mass. The proportion of cephalopods, fishery offal discarded from fishing vessels, and decapods did not exceed 18.5, 17.4, and 12.2%, respectively. Among fishes, the main prey item of the cod was atka mackerel (15.4%); among cephalopods, octopus (16.8%); among fishery offal, heads of atka mackerel (14.2%); and among decapods, majid crabs (6.4%). The rather low percentage of walleye pollock (7.3%) in the cod diet was due to the decline of the east-Kamchatka walleye pollock stock.     

Quantifying food web interactions in the North Pacific – a data-based approach

As part of its annual bottom-trawl survey program, the Alaska Fisheries Science Center (AFSC) has been collecting and analyzing the stomach contents of groundfish predators since 1981. Between 1981 and 2011, a total of 233,451 fish stomachs were collected and analyzed from the eastern Bering Sea, the Gulf of Alaska, and the Aleutian Islands large marine ecosystems; these data are now available online as AFSC’s Groundfish Trophic Interactions Database. Here, we discuss features of the survey and data to aid in the interpretation and use of this extensive dataset for the Alaska region. The primary fish sampled include walleye pollock (Gadus chalcogrammus), Pacific cod (Gadus macrocephalus), Pacific halibut (Hippoglossus stenolepis), and arrowtooth flounder (Atheresthes stomias), although 159 predator species have been included in the stomach content analysis. Prey length measurements are included for important commercial prey and can identify age or size classes of prey prior to their recruitment into fisheries and most other surveys. With these data, one can track time trends in growth, mortality, and prey composition as ecosystem indicators, and include food web interactions in fish stock assessments for ecosystem-based fisheries management.     

Variation in the diet of Arctic Cod (<Emphasis Type="Italic">Boreogadus saida</Emphasis>) in the Pacific Arctic and Bering Sea

Arctic cod, Boreogadus saida (Lepechin, 1774), is a nodal species in Arctic marine foodwebs as an important prey of many birds, marine mammals, and other fishes, as well as an abundant predator of zooplankton and epibenthic fauna. We examined the summer diet of Arctic cod across a latitudinal gradient extending from the southern limit of their distribution in the eastern Bering Sea to the northern margins of the eastern Chukchi Sea (ECS) continental shelf. Specimens were collected from demersal and pelagic trawls conducted between 1999 and 2012, and across a range of predator sizes (3–26 cm). Arctic cod diets vary with body size and between regions within the study area, and appear to vary between years in the eastern Bering Sea, indicating opportunistic feeding habits. Constrained Analysis of Principal Coordinates was conducted on ECS demersal samples and revealed consumption of fish and decapod crustacea were positively correlated with Arctic cod length while consumption of euphausiids and copepods had the opposite relationship. The demersal Arctic cod diet in the northern latitudes of the ECS was dominated by copepod consumption (47% by weight, %W), but copepods were less important (12–26%W) in the central and southern latitudes of the ECS and in the northern and eastern Bering Sea—areas where diets were more varied in their composition. High levels of variation in the diet of Arctic cod highlights the need to monitor Arctic cod diets to identify consistently dominant prey types and potential future changes to trophic relationships related to climate change or increasing anthropogenic activity.     

Assessment of Competition between Fisheries and Steller Sea Lions in Alaska Based on Estimated Prey Biomass,Fisheries Removals and Predator Foraging Behaviour

A leading hypothesis to explain the dramatic decline of Steller sea lions (Eumetopias jubatus) in western Alaska during the latter part of the 20th century is a change in prey availability due to commercial fisheries. We tested this hypothesis by exploring the relationships between sea lion population trends, fishery catches, and the prey biomass accessible to sea lions around 33 rookeries between 2000 and 2008. We focused on three commercially important species that have dominated the sea lion diet during the population decline: walleye pollock, Pacific cod and Atka mackerel. We estimated available prey biomass by removing fishery catches from predicted prey biomass distributions in the Aleutian Islands, Bering Sea and Gulf of Alaska; and modelled the likelihood of sea lions foraging at different distances from rookeries (accessibility) using satellite telemetry locations of tracked animals. We combined this accessibility model with the prey distributions to estimate the prey biomass accessible to sea lions by rookery. For each rookery, we compared sea lion population change to accessible prey biomass. Of 304 comparisons, we found 3 statistically significant relationships, all suggesting that sea lion populations increased with increasing prey accessibility. Given that the majority of comparisons showed no significant effect, it seems unlikely that the availability of pollock, cod or Atka mackerel was limiting sea lion populations in the 2000s.     

Scales of dispersal and the biogeography of marine predator-prey interactions

Striking differences in the dispersal of coexisting species have fascinated marine ecologists for decades. Despite widespread attention to the impact of dispersal on individual species dynamics, its role in species interactions has received comparatively little attention. Here, we approach the issue by combining analyses of simple heuristic predator-prey models with different dispersal patterns and data from several predator-prey systems from the Pacific coasts of North and South America. In agreement with model predictions, differences in predator dispersal generated characteristic biogeographic patterns. Predators lacking pelagic larvae tracked geographic variation in prey recruitment but not prey abundance. Prey recruitment rate alone explained more than 80% of the biogeographic variation in predator abundance. In contrast, predators with broadcasting larvae were uncorrelated with prey recruitment or adult prey abundance. Our findings reconcile perplexing results from previous studies and suggest that simple models can capture some of the complexity of life-history diversity in marine communities.     

Potential for bowhead whale entanglement in cod and crab pot gear in the Bering Sea

Bowhead whales (Balaena mysticetus) of the western Arctic stock winter in ice‐covered continental shelf regions of the Bering Sea, where pot fisheries for crabs (Paralithodes and Chionoecetes spp.) and Pacific cod (Gadus macrocephalus) pose a risk of entanglement. In the winter of 2008–2009 and 2009–2010 the spatial distribution of 21 satellite tagged bowhead whales partially overlapped areas in which pot fisheries for cod and blue king crab (Paralithodes platypus) occurred. However, these fisheries ended before whales entered the fishing areas, thus avoiding temporal overlap. A fishery for snow crab (Chionoecetes opilio) typically runs from January to May and provides the greatest potential for bowhead whales to encounter active pot gear. Tagged whales did not enter the area of the snow crab fishery during this study and generally remained in areas with >90% sea ice concentration, which is too concentrated for crab boats to penetrate. Pack ice sometimes overruns active fishing areas, resulting in lost gear, which is the most likely source of entanglement. The western Arctic stock of bowhead whales was increasing as of 2004; as such, incidental mortality from commercial pot fisheries is probably negligible at this time. Regardless, entanglement may increase over time and should be monitored.     

Brief morphological characteristics of cod <Emphasis Type="Italic">Gadus macrocephalus</Emphasis> (Gadidae) from coastal waters of the Komandor Islands

The results of the studies on morpho-biological characteristics of Pacific cod Gadus macrocephalus of Bering Island coastal waters (Komandor Islands) are presented. As assessed by several morphometric indices, the studied cod differs from the cod of the Bering Sea.     

The influence of feeding behaviour on growth of Atlantic cod (Gadus morhua,Linnaeus, 1758) in the North Sea

The objective of this study was to resolve key mechanisms driving individual growth patterns of Atlantic cod (Gadus morhua). Growth dynamics were analysed by linking growth patterns with stomach content composition and environmental temperature. Samples were collected in August/September of the years 2009, 2010 and 2011 in the north‐eastern part of the central North Sea. Prey selection was assessed by identification of individual prey items in the stomach content to species. Ten feeding groups were identified consisting of individuals with one prey type dominating their stomach contents (≥75% by mass), of which six were used for growth analyses: “Sandeel”, “Clupeids”, “Norway pout”, “Flatfishes”, “Crustaceans” and “Brittle stars”. For each group, growth patterns were estimated based on measures of otolith growth increments. The stomach contents showed that cod as a species are opportunistic in their prey selection, but at the same time indicated that the total, broad feeding niche width of the population is dominated by individual diet specialization and that many individuals temporally show a preference for a particular prey type. The contribution of invertebrates and particularly crustaceans decreased with increasing cod size, whereas that of fish and predominantly herring increased. Prey type had a significant effect on growth, while temperature had no effect. Slowest growth was observed in the cod group preying on sandeel, while cod preying on Norway pout showed the fastest growth. No significant difference was observed between groups preying on brittle stars, crustaceans, flatfishes and herring. Growth in the year before capture did however not differ between any of these groups. Across sampling years, growth chronology patterns were similar but not significantly influenced by temperature.     

Variability in the diet of New Zealand sea lion (Phocarctos hookeri) at the Auckland Islands, New Zealand

We examined the stomach contents of 121 New Zealand (NZ) sea lions ( Phocarctos hookeri ) caught by the squid fishery during the summer/autumn 1997–2006 around the Auckland Islands (51°S, 166°E). Dietary variation was assessed among juveniles, lactating females, nonlactating females and males, and between areas on the Auckland Islands shelf. The digested fraction of the contents consisted mostly of opalfish ( Hemerocoetes spp.) (50.1% by number [ N ], 4.7% by mass [ M ]), rattail ( Coelorinchus spp.) (12.0% N , 2.4% M ), arrow squid ( Nototodarus sloani ) (14.1% N , 17.9% M ), octopus ( Enteroctopus zealandicus ) (2.1% N , 27.8% M ), and red cod ( Pseudophycis bachus ) (3.8% N , 4.3% M ). Opalfish was found in greater proportions in the stomachs of females (lactating: 58.1% N , nonlactating: 62.4% N ) and juveniles (56.9% N ) than males (14.5% N ). Juveniles caught smaller opalfish and rattail than adults did. Over all classes, sea lions ate larger prey in the east than in the north of the Auckland Islands shelf. The common prey—arrow squid and rattail—constitute an abundant resource at the edges of the Auckland Islands shelf, where lactating NZ sea lions forage. Although these key areas are far from the rookeries and impacted by the squid fishery, they may provide the only reliable resource able to support the cost of benthic foraging behavior in the deepest diver of all otariids.     

Ontogenetic and spatial variability in trophic biomarkers of juvenile saffron cod (<Emphasis Type="Italic">Eleginus gracilis</Emphasis>) from the Beaufort,Chukchi and Bering Seas

Climate models indicate the Arctic will undergo dramatic environmental change with forecasted increases in temperature and river runoff. Saffron cod (Eleginus gracilis) is abundant in nearshore waters and appears in the diet of many Arctic sea birds and marine mammals; however, little is known about its early ecology and consequently how they might be affected by environmental changes. We aimed to characterize the mechanisms of spatial and ontogenetic variation in trophic biomarkers (lipid classes, fatty acids and bulk C and N stable isotopes) of saffron cod from the Western Arctic, Chukchi and Bering Seas. Size-standardized analyses showed a significant difference in lipid condition metrics and trophic biomarkers as a function of survey location. Both ontogeny and sampling location played an important role in determining lipid stores with elevated levels in both small offshore juveniles (<55 mm) and larger inshore juveniles (>75 mm). Higher lipid storage in Arctic juveniles was associated with elevated levels of diatom fatty acid markers, but not with nearshore carbon input. Increased lipids were found in age-1 juveniles from Prudhoe Bay in the Western Beaufort that were feeding at a lower trophic level than similarly sized age-0 juveniles from surface trawls in the Bering Sea. The use of otolith annuli revealed two discrete patterns of growth that help explain the trade-offs between energy storage and rapid growth that diverge between the Arctic and Bering Sea. Laboratory temperature-growth experiments confirmed that saffron cod have a eurythermal growth response and are able to store excess lipids at temperatures as high as 20 °C.     

Temperature-dependent growth and behavior of juvenile Arctic cod (<Emphasis Type="Italic">Boreogadus saida</Emphasis>) and co-occurring North Pacific gadids

The thermal sensitivity of Arctic fish species is poorly understood, yet such data are a critical component of forecasting and understanding ecosystem impacts of climate change. In this study, we experimentally measured temperature-dependent growth and routine swim activity in the juvenile stage of two Arctic gadids (Arctic cod, Boreogadus saida and saffron cod, Eleginus gracilis) and two North Pacific gadids (walleye pollock, Gadus chalcogrammus and Pacific cod, Gadus macrocephalus) over a 6-week growth period across five temperatures (0, 5, 9, 16 and 20 °C). Arctic cod demonstrated a cold-water, stenothermic response in that there was relatively high growth at 0 °C (0.73 % day^?1), near-maximal growth at 5 °C (1.35 % day^?1) and negative impacts on activity, growth and survival at 16 °C. In contrast, saffron cod demonstrated a warmer-water, eurythermic response, and temperature had a positive effect on growth and condition beyond 16 °C. However, despite these distinct thermal responses, walleye pollock and Pacific cod grew 2–3 times faster than Arctic gadids across a relatively broad temperature range above 5 °C. These results, coupled with possible northward expansion by both Pacific cod and walleye pollock, suggest Arctic cod are highly vulnerable to continued climate change in the Arctic, especially in coastal areas of the Beaufort and Chukchi Seas where temperatures already exceed 14 °C in the summer growth period.     

Constant proportion harvest policies: dynamic implications in the Pacific halibut and Atlantic cod fisheries

Overfishing, pollution and other environmental factors have greatly reduced commercially valuable stocks of fish. In a 2006 Science article, a group of ecologists and economists warned that the world may run out of seafood from natural stocks if overfishing continues at current rates. In this paper, we explore the interaction between a constant proportion harvest policy and recruitment dynamics. We examine the discrete-time constant proportion harvest policy discussed in Ang et al. (2009) and then expand the framework to include stock-recruitment functions that are compensatory and overcompensatory, both with and without the Allee effect.We focus on constant proportion policies (CPPs). CPPs have the potential to stabilize complex overcompensatory stock dynamics, with or without the Allee effect, provided the rates of harvest stay below a threshold. If that threshold is exceeded, CPPs are known to result in the sudden collapse of a fish stock when stock recruitment exhibits the Allee effect. In case studies, we analyze CPPs as they might be applied to Gulf of Alaska Pacific halibut fishery and the Georges Bank Atlantic cod fishery based on harvest rates from 1975 to 2007. The best fit models suggest that, under high fishing mortalities, the halibut fishery is vulnerable to sudden population collapse while the cod fishery is vulnerable to steady decline to zero. The models also suggest that CPP with mean harvesting levels from the last 30 years can be effective at preventing collapse in the halibut fishery, but these same policies would lead to steady decline to zero in the Atlantic cod fishery. We observe that the likelihood of collapse in both fisheries increases with increased stochasticity (for example, weather variability) as predicted by models of global climate change.     

Development and characterization of novel di‐ and tetranucleotide microsatellite markers in Pacific cod (Gadus macrocephalus)

The Pacific cod (Gadus macrocephalus) supports large commercial fisheries in the northern Pacific Ocean and the Bering Sea. Here we characterize 10 polymorphic microsatellite loci isolated from enriched genomic libraries. Loci were screened on a sample of 96 spawning adults. The average number of alleles per locus was 25.3 (range 12–44), with expected heterozygosities (H_E) ranging from 0.54 to 0.97. No significant deviations of genotypic proportions from Hardy–Weinberg equilibrium or linkage equilibrium were detected. These markers will be used in future studies of population structure and mixed stock analysis of this important gadid species.     

Effects of acute and chronic temperature changes on the functional responses of the dogfish <Emphasis Type="Italic">Scyliorhinus canicula</Emphasis> (Linnaeus, 1758) towards amphipod prey <Emphasis Type="Italic">Echinogammarus marinus</Emphasis> (Leach, 1815)

Predation is a strong driver of population dynamics and community structure and it is essential to reliably quantify and predict predation impacts on prey populations in a changing thermal landscape. Here, we used comparative functional response analyses to assess how predator-prey interactions between dogfish and invertebrate prey change under different warming scenarios. The Functional Response Type, attack rate, handling time and maximum feeding rate estimates were calculated for Scyliorhinus canicula preying upon Echinogammarus marinus under temperatures of 11.3 °C and 16.3 °C, which represent both the potential daily variation and predicted higher summer temperatures within Strangford Lough, N. Ireland. A two x two design of “Predator Acclimated”, “Prey Acclimated”, “Both Acclimated”, and “Both Unacclimated” was implemented to test functional responses to temperature rise. Attack rate was higher at 11.3 °C than at 16.3 °C, but handling time was lower and maximum feeding rates were higher at 16.3 °C. Non-acclimated predators had similar maximum feeding rate towards non-acclimated and acclimated prey, whereas acclimated predators had significantly higher maximum feeding rates towards acclimated prey as compared to non-acclimated prey. Results suggests that the predator attack rate is decreased by increasing temperature but when both predator and prey are acclimated the shorter handling times considerably increase predator impact. The functional response of the fish changed from Type II to Type III with an increase in temperature, except when only the prey were acclimated. This change from population destabilizing Type II to more stabilizing Type III could confer protection to prey at low densities but increase the maximum feeding rate by Scyliorhinus canicula in the future. However, predator movement between different thermal regimes may maintain a Type II response, albeit with a lower maximum feeding rate. This has implications for the way the increasing population Scyliorhinus canicula in the Irish Sea may exploit valuable fisheries stocks in the future.     

Winter feeding ecology of cod (Gadus morhua) in a fjord of southern Norway

We investigated the winter feeding ecology of cod ( Gadus morhua ) in the Risørfjord and Flødevigen areas on the Skagerrak coast, southern Norway. Diets from the ice-covered Risørfjord were compared with diets from the more coastal Flødevigen area. In the Risørfjord area the diet featured numerically both decapods (56.0%) and fish (27.8%), but fish dominated by mass (75.1%). The most numerous dietary items from the Flødevigen area were decapods (48.1%) and isopods (30.6%), although fish still made up the bulk of the diet's mass (52.6%). Diets at Flødevigen shifted from winter to spring, as polychaetes became important numerically (67.8%) and also contributed substantially by weight (53.6%). Since cod feed opportunistically, differences between areas and seasons probably reflected differences in prey diversity and abundance. Prey size variability increased with increasing predator length, but maximum prey size relative to predator length was constant at about 9%. Prey fish increased in both length and numbers with the length of cod, particularly in the Risørfjord area, where fish were more important in the diet than at Flødevigen. However, the majority of fish consumed in both areas were small gobiids. Based on growth patterns observed in otoliths, with opaque zones (indicating faster growth) formed during winter, the quality and quantity of the winter diet may determine annual growth rates of cod on the Skagerrak coast. Fish and decapods constituted important, high energy food sources, and there was little indication that cod were food limited during winter in this area.     

Influence of cruising and ambush predators on 3-dimensional habitat use in age 0 juvenile Atlantic cod Gadus morhua

Predators and prey often co-exist at high densities within the same habitat, yet the behavioural and spatial dynamics underlying this co-existence are not well known. To better understand small-scale, predator-prey co-occurrence, the spatial patterns and behaviour of age 0 juvenile cod Gadus morhua 75-88 mm SL and two of their known predators, age 2+ cod and short-horn sculpin Myoxocephalus scorpinus, were examined in two habitats (i.e., sand and eelgrass) using three-dimensional video analysis. Both habitat and predator type interacted to result in unique spatial patterns of prey. Spatial overlap between predators and prey was highest in open habitat in the presence of the cruising predator but lowest in the presence of sculpin in the same habitat. In eelgrass, age 0 cod avoided predators primarily along the vertical axis (i.e., distance off bottom). Age 0 cod stayed above eelgrass in the presence of sculpin but lowered themselves into the eelgrass while in the presence of predator cod. Anti-predator behaviour (i.e., predator-prey distance, prey cohesion and freezing) was significantly reduced over eelgrass compared to sand, suggesting eelgrass has lower ‘inherent risk’ than open habitats. However, predator consumption was similar across all treatments, suggesting that, 1) complex habitat also impairs the visual cues needed for anti-predator behaviour (e.g., schooling) and assessing the location of predators, and 2) predators change their behaviour with habitat to enhance their opportunities for finding and capturing prey.     

Distribution and feeding ecology of the Greenland shark (Somniosus microcephalus) in Greenland waters

Greenland sharks are widely distributed and most likely a highly abundant predator in arctic waters. Greenland sharks have previously been considered scavengers, but recent studies suggest that Greenland sharks also predate on live prey. In this study, distribution and feeding ecology in Greenland waters were investigated. Based on data from 25 years of surveys, Greenland sharks were usually caught at 400–700 m but were found at all depths between 100 and 1,200 m. Based on examination of stomachs from 30 Greenland sharks (total length of 258–460 cm), the most important prey items were Atlantic cod (65.6 % IRI), harp seal (9.9 % IRI), skates (5.2 % IRI) and wolffish (4.4 % IRI), but large geographical variations were observed. Prey composition and qualitative observations support the hypothesis of active predation. Consistent with other studies, the results of this work support the notion that the Greenland shark is an apex predator with the potential to influence trophic dynamics in the Arctic.     

Pre-winter distribution and habitat characteristics of polar cod (Boreogadus saida) in southeastern Beaufort Sea

Polar cod was shown to form dense under-ice winter aggregations at depth in the Amundsen Gulf (southeastern Beaufort Sea). In this paper, we verify the premises of the aggregation mechanism by determining the distribution and habitat characteristics of polar cod prior to the formation of winter aggregations. Multifrequency split-beam acoustic data collected in October–November 2003 revealed that polar cod split into two distinct layers. Age-0 polar cod formed an epipelagic layer between 0 and ~60 m depth without any clear large-scale biomass trend. In contrast, adult polar cod tended to distribute into an offshore mesopelagic layer between ~200 and 400 m that shoaled into a denser (1–37 g m^?2) benthopelagic layer on sloping bottoms (between 150 and 600-m isobaths) along the Mackenzie shelf and into the Amundsen Gulf basin. Concentrations peaked in the Amundsen Gulf where estimated total biomass reached ~250 kt. Both age-0 and adult polar cod distributed in the warmer waters (>?1.4 °C). We hypothesise that polar cod concentration over slopes is governed by the combined actions of (1) local currents concentrating both depth-keeping zooplankton and polar cod at the shelf-break and basin slopes and (2) trophic association with these predictable topographically trapped aggregations of zooplankton prey. During freeze-up, these slope concentrations of polar cod are thought to constitute the main source of the observed dense under-ice winter aggregations. The hypothesis of active short-distance displacements combined with prevailing mean currents is retained as the likely aggregation mechanism.     

The Sun,Moon, Wind,and Biological Imperative–Shaping Contrasting Wintertime Migration and Foraging Strategies of Adult Male and Female Northern Fur Seals (Callorhinus ursinus)

Adult male and female northern fur seals (Callorhinus ursinus) are sexually segregated in different regions of the North Pacific Ocean and Bering Sea during their winter migration. Explanations for this involve interplay between physiology, predator-prey dynamics, and ecosystem characteristics, however possible mechanisms lack empirical support. To investigate factors influencing the winter ecology of both sexes, we deployed five satellite-linked conductivity, temperature, and depth data loggers on adult males, and six satellite-linked depth data loggers and four satellite transmitters on adult females from St. Paul Island (Bering Sea, Alaska, USA) in October 2009. Males and females migrated to different regions of the North Pacific Ocean: males wintered in the Bering Sea and northern North Pacific Ocean, while females migrated to the Gulf of Alaska and California Current. Horizontal and vertical movement behaviors of both sexes were influenced by wind speed, season, light (sun and moon), and the ecosystem they occupied, although the expression of the behaviors differed between sexes. Male dive depths were aligned with the depth of the mixed layer during daylight periods and we suspect this was the case for females upon their arrival to the California Current. We suggest that females, because of their smaller size and physiological limitations, must avoid severe winters typical of the northern North Pacific Ocean and Bering Sea and migrate long distances to areas of more benign environmental conditions and where prey is shallower and more accessible. In contrast, males can better tolerate often extreme winter ocean conditions and exploit prey at depth because of their greater size and physiological capabilities. We believe these contrasting winter behaviors 1) are a consequence of evolutionary selection for large size in males, important to the acquisition and defense of territories against rivals during the breeding season, and 2) ease environmental/physiological constraints imposed on smaller females.     

EVOLUTION OF ATLANTIC AND PACIFIC COD: LOSS OF GENETIC VARIATION AND GENE EXPRESSION IN PACIFIC COD

An allozyme investigation of 41 protein-coding loci in two morphologically similar fishes, Atlantic and Pacific cod, indicates that Pacific cod experienced a severe population bottleneck that led to the loss of gene diversity and gene expression. Pacific cod possesses a significantly lesser amount of gene diversity (H = 0.032) than Atlantic cod (H = 0.125) and lacks gene expression for Me-3. Allele-frequency distributions differ between species as predicted by neutral theory: Atlantic cod has a U-shaped distribution, which is expected for populations in drift-mutation equilibrium, whereas Pacific cod has a J-shaped distribution with an excess of low-frequency alleles. This excess may be explained by the appearance of new alleles through mutation which have not yet reached intermediate frequencies through drift. The population bottleneck in Pacific cod was most likely associated with founder populations that dispersed into the Pacific Ocean after the Bering Strait opened. Under the molecular-clock hypothesis a Nei genetic distance of 0.415 (based on 41 loci) suggests that Pacific cod dispersed into the Pacific Ocean soon after the Bering Strait opened in the mid-Pliocene, 3.0 to 3.5 million years ago.