More rapid shift to a benthic niche in larger Gadus morhua juveniles

Trophic use by Atlantic cod Gadus morhua juveniles was examined early and late in the shift from pelagic to benthic habitats. Changes in the proportion of pelagic copepods, estimates of benthic prey indicated by isotope mixing models and stable‐isotope values between sample periods suggested a gradual shift towards a benthic niche. Values of the trophic proxies, however, changed most markedly in the largest juvenile group, suggesting a more rapid trophic niche shift, and in turn competitive advantage, of larger juveniles.     

Cross-shoal variability in the feeding habits of migrating Atlantic cod (Gadus morhua)

Prey intake and selection were related to within-shoal position for Atlantic cod (Gadus morhua) engaged in annual migration across the Newfoundland shelf in the northwest Atlantic. Comparisons made among fish occupying five regions from the front to rear of a large (>10 km across) migrating shoal indicated that leading fish, or scouts, were larger, ate more food by weight, and had a more varied diet than did fish at other positions. Also, scouts consumed more preferred prey types (fish and pelagic invertebrates) than did fish at other positions. In contrast, trailing fish consumed few fish prey but a larger proportion of benthic invertebrates. Our results are the first to document systematic heterogeneous feeding success among members of a free-ranging and migrating fish shoal in the open ocean.     

Examination of fine‐scale spatial‐temporal overlap and segregation between two closely related congeners Gadus morhua and Gadus ogac in coastal Newfoundland

The spatial and temporal movement patterns of sympatric juvenile Atlantic cod Gadus morhua and Greenland cod Gadus ogac were studied using high‐resolution radio‐acoustic positioning in a coastal area of Newfoundland during the summers of 2009 and 2010. A total of 20 fish (10 G. ogac and 10 G. morhua) were equipped with acoustic transmitters and monitored for periods up to 23 days. Most fishes showed high site fidelity with mean residence times of 12·4 (G. morhua) and 14·4 days (G. ogac). A few individuals showed a transient use of the study area, ranging distances up to c. 4 km. Mean daily home ranges [95% kernel utilization distributions (KUDs)] and core activity areas were significantly larger for G. morhua (3·8 and 0·5 ha) than for G. ogac (2·7 and 0·3 ha). Home ranges were not related to total length (L_T) for G. morhua but showed a weak positive relationship for G. ogac. Gadus morhua occupied larger areas during the day while G. ogac occupied slightly larger areas at night. Mean rates of movement for G. ogac and G. morhua ranged from 0·83 to 1·24 and 0·76 to 1·76 L_T s^?1, respectively, and were highest during crepuscular periods. Overall, G. morhua were wider ranging, moved at faster rates and were active throughout the diel cycle compared to G. ogac of the same size. It is suggested that differential use of space and activity periods plays an important role in the successful coexistence of these two species.     

Escape performance in three teleosts from West Greenland

Escape performance was examined in three similarly sized teleosts caught in Arctic waters: the Atlantic cod, Gadus morhua, the Greenland cod, Gadus ogac, and the short-horned sculpin, Myoxocephalus scorpius. Escape responses were elicited by a mechanical stimulus and followed by recordings of velocity and acceleration over the first 320 ms. A significant difference in escape performance was confirmed between the three species. G. morhua always exhibited the fastest and M. scorpius the slowest responses. Maximal measures of acceleration and velocity revealed no difference between G. morhua (12.6 m s^–2 and 1.27 m s^–1) and G. ogac (11.9 m s^–2 and 1.16 m s^–1) but an overall reduced performance in M. scorpius (6.5 m s^–2 and 0.71 m s^–1). The escape performance of the gadid and cottid species generally correlate well with their behavioural lifestyle, but it is surprisingly similar to Antarctic notothenioid fishes at lower temperatures.     

Exercise metabolism in two species of cod in arctic waters

The northern range of Atlantic cod (Gadus morhua), overlaps the southern range of the Greenland cod (Gadus ogac), in the coastal waters of Western Greenland. The availability of a temperate water species (G. morhua) in the same area and oceanographic conditions as a polar species (G. ogac) presented us with the ideal circumstances to test the hypothesis of metabolic cold adaptation (MCA) since many of the problems associated with MCA studies (adaptation of the animals beyond their normal temperature range or mathematical extrapolation of data to common temperatures) could thus be avoided. We therefore used a swim tunnel to measure oxygen consumption in fish at 4°C over a range of swimming speeds and following exhaustion, monitored the size of the oxygen debt and time of oxygen debt repayment. There were no significant differences in standard (60–72 mg O₂ kg^–1· hr^–1), routine (76 mg O₂ kg^–1·hr^–1), active (137mg O₂ kg^–1·hr^–1), or maximal (157 mg O₂ kg^–1·hr^–1) metabolic rate, metabolic scope (2.5) or critical swimming speed (2.2 BL·s^–1) between the two species. Following exhaustive swimming, however, the half-time for oxygen debt repayment in G. ogac (43 min) was almost twice that of G. morhua (25 min). Despite its circumpolar distribution, therefore, there was no evidence of MCA in G. ogac.     

THE TROPHIC ROLE OF MARINE MAMMALS IN THE NORTHERN GULF OF ST. LAWRENCE

The trophic role of apex predators was evaluated in the northern Gulf of St. Lawrence ecosystem. An Ecopath model was developed for the period 1985–1987 prior to the collapse of commercially exploited demersal fish stocks in this area. Marine mammal trophic levels were estimated by the model at 4.1 for cetaceans, 4.4 for harp seals (Pagophilus groenlandicus), 4.7 for hooded seals (Cystophora cristata), 4.5 for gray seals (Halichoerus grypus), and 4.3 for harbor seals (Phoca vitulina). Harp seals were the third most important predator on vertebrate prey following large Atlantic cod (Gadus morhua) and redfish (Sebastes spp.). Different seal species preyed on different levels of the food chain. Harp seals preyed on most trophic groups, whereas larger seals, such as gray seals and hooded seals, mainly consumed higher trophic levels. The model suggested that apex predators had a negative effect on their dominant prey, the higher trophic level fish, but an indirect positive feedback on the prey of their preferred prey, mainly American plaice (Hippoglossoides platessoides), flounders, skates, and benthic invertebrates. Our results suggest that both marine mammals and fisheries had an impact on the trophic structure.     

Distribution and diet of 0-group cod (<Emphasis Type="Italic">Gadus morhua</Emphasis>) and haddock (<Emphasis Type="Italic">Melanogrammus aeglefinus</Emphasis>) in the Barents Sea in relation to food availability and temperature

Distribution of 0-group cod (Gadus morhua) and haddock (Melanogrammus aeglefinus) in August–September 2005 and 2006 was mainly restricted to the Atlantic waters of the western and central areas of the Barents Sea. The main distribution of 0-group fish overlapped largely with areas of high biomass (>7 gm⁻² dry weight) of zooplankton. The copepod Calanus finmarchicus and krill Thysanoessa inermis, which are dominant zooplankton species in both Atlantic and boreal waters of the Barents Sea, were the main prey of 0-group cod and haddock. The main distribution, feeding areas and prey of 0-group cod and haddock overlapped, implying that competition for food may occur between the two species. However, though their diet coincided to a certain degree, haddock seems to prefer smaller and less mobile prey, such as Limacina and appendicularians. As 0-group fish increased in size, there seems to be a shift in diet, from small copepods and towards larger prey such as krill and fish. Overall, a largely pelagic feeding behaviour of 0-group cod and haddock was evident from this study.     

Is the poleward expansion by Atlantic cod and haddock threatening native polar cod, <Emphasis Type="Italic">Boreogadus saida</Emphasis>?

During a recent period of increased influx of warm Atlantic water to the western coast of Svalbard, we have observed a northward expansion of boreal Atlantic cod (Gadus morhua) and haddock (Melanogrammus aeglefinus) into areas dominated by the native polar cod (Boreogadus saida). To determine the potential impact of new ecological interactions, we studied the diet of co-occurring juvenile gadoids in fjords, open water, and sea ice around Svalbard. We also reviewed the available literature on polar cod feeding in different habitats across the Arctic to determine whether region, habitat, or fish size may influence diet. Feeding by polar cod in the pelagic zone was size dependent, with small fish primarily consuming Calanus spp. and smaller copepods, with an increasing ration of Themisto spp. at larger sizes. In benthic habitats, diets were more varied and included considerably more unidentified material and sediment. Less than 40% dietary overlap was detected among the three species when they were found together. Stable isotope analyses indicated these patterns were representative of longer-term assimilation. The low interspecific dietary overlap suggests little direct competition. Future increases in abundance and the high predation potential of the boreal taxa, however, may impact the persistence of polar cod on some Arctic shelves.     

The trophic ecology of Atlantic cod: insights from tri-monthly, localized scales of sampling

The meso-scale trophic dynamics of cod Gadus morhua were examined based upon tri-monthly stomach sample collections from a nearshore, localized ( c. 800 km²) region off Cape Cod, Massachusetts, U.S.A. The major objective for this work was to relate any changes in cod diet and amount of food eaten to seasonal variations in prey availability, water temperature and spawning at a spatial scale between broad-scale and laboratory studies. Results suggested that the type and amount of food eaten by cod was generally consistent throughout a year and repeatable across years. Cod feeding was marked by two periods of increased feeding, corresponding to the arrival of small pelagic fishes in the area. This pelagic migration and subsequent increased feeding by cod occurred during important periods in the life history of cod ( e.g . spawning and overwintering). Similar annual patterns in food consumption and diet composition were remarkably consistent over the 2·5 years of the project, suggesting important feeding periods for cod that correspond to environmental and biological events. The diet of cod was composed primarily of several species of forage fishes [ e.g. herrings (predominantly Atlantic herring Clupea harengus ), sand lance Ammodytes sp. and Atlantic mackerel Scomber scombrus ], ophiuroids, Cancer sp. crabs and other small crustaceans. It was inferred that cod exhibited a maintenance diet on local forage fishes and benthic macroinvertebrates, augmenting their diet by seasonally gorge feeding upon migrating pelagic species.     

Does diet in Celtic Sea fishes reflect prey availability?

Feeding preferences of Celtic Sea fishes were investigated using a database of stomach content records, collected between 1977 and 1994. The diet of cod Gadus morhua, hake Merluccius merluccius, megrim Lepidorhombus whiffiagonis, whiting Merlangius merlangus and saithe Pollachius virens changed markedly as the animals grew larger, and although large predators generally chose larger bodied prey, the variability of prey sizes consumed also increased. Large predators continued to select small, low value, benthic prey (e.g. Callionymus spp. and Trisopterus spp.) which were easier to catch, rather than larger, more energy lucrative pelagic prey (e.g. mackerel Scomber scombrus), even though these pelagic prey‐fishes were nearly always available and were often very abundant. Stock estimates of the International Council for the Exploration of the Sea and U.K. groundfish survey catches were used as indices of prey abundance. Blue‐whiting Micromesistius poutassou and other small pelagic fishes (Argentina spp. and clupeoids) were identified as being particularly important, and were consumed by some predators more often than would be expected given the abundance of these prey in the environment. There was no evidence for density‐dependent feeding by predators on mackerel and only hake exhibited density‐dependent feeding on horse‐mackerel. Hake, cod and megrim consumed more blue‐whiting when this prey was at higher abundance in the environment. In choosing what prey to consume, predators must balance costs and benefits, considering the quality of prey and the energy expended during search, capture and handling.     

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.     

Oxygen consumption in four species of teleosts from Greenland: no evidence of metabolic cold adaptation

Standard metabolic rate of Greenland cod or uvak, Gadus ogac, polar cod, Boreogadus saida, Atlantic cod, Gadus morhua, and sculpin, Myxocephalus scorpius, caught in the same geographical area on the west coast of Greenland was measured at 4.5°C, the temperature at which the fish were caught. The present data does not support the Metabolic Cold Adaptation theory in the traditional sense of the standard metabolic rate being 2–4 times higher for Arctic fishes than for temperate species. The standard metabolic rate of the two exclusively Arctic species of teleosts was only 10% and 26% higher, respectively, than the two species that occur in temperate as well as Arctic areas. The critical oxygen tension, with respect to oxygen consumption, of resting uvak was between 50 and 60 mmHg, and the lethal oxygen tension 20–25 mmHg at 4.5°C, which is considerably higher than for Atlantic cod from a temperate area measured at the same temperature.     

Eelgrass patch size and proximity to the patch edge affect predation risk of recently settled age 0 cod (Gadus)

Postsettled age 0 Atlantic cod (Gadus morhua) seek refuge from predation in eelgrass (Zostera marina) habitat within shallow, coastal nurseries. Laboratory and field experiments have demonstrated that predation risk on small fish is reduced in habitats of greater structure compared to less complex or barren environs. To determine if predation risk is linked to the areal extent of eelgrass coverage, we tested the hypothesis that predation risk of age 0 cod decreases with increasing eelgrass patch size. Predation on tethered age 0 cod (G. morhua and G. ogac) was quantified in eelgrass patches (1-80 m²) at three sites in northeastern Newfoundland, Canada, during September and October 1999 and 2000. Based on evidence of edge effects in terrestrial landscapes, we also tested the hypothesis that predation is elevated at ecotones separating eelgrass from adjacent barren mud substrate. We examined predation at the edge (i.e., 0 m) and both 5 and 10 m from this edge into and away from eelgrass cover along an 18 m long barren mud-gravel and eelgrass boundary, at two sites. Logistic regression analysis showed that the risk of predation, as measured by the odds ratio, increased with area over a small range of patch sizes (1-35 m² in 1999). When the study was extended to a wider range of patch sizes (1-80 m²) in 2000, a parabolic relationship emerged, with patches on the order of 25 m² providing the least safety and the largest patches (80 m²) providing the most safety. Predation on tethered cod was highest at the edge of eelgrass patches, compared with barren and eelgrass locations; predation generally decreased with distance from the eelgrass boundary. Our results are consistent with the hypothesis that predators are drawn to large patches of eelgrass because of increased prey numbers, that predators increase their success by searching edges, and that this results in greatest predation risk to prey in isolated patches of intermediate size.     

The forgotten feeding ground: patterns in seasonal and depth-specific food intake of adult cod Gadus morhua in the western Baltic Sea

This study presents the diet composition of western Baltic cod Gadus morhua based on 3150 stomachs sampled year-round between 2016 and 2017 using angling, gillnetting and bottom trawling, which enhanced the spatio-temporal coverage of cod habitats. Cod diet composition in shallow areas (<20 m depth) was dominated by benthic invertebrate species, mainly the common shore crab Carcinus maneas. Compared to historic diet data from the 1960s and 1980s (limited to depth >20 m), the contribution of herring Clupea harengus decreased and round goby Neogobius melanostomus occurred as a new prey species. Statistical modelling revealed significant relationships between diet composition, catch depth, fish length and season. Generalized additive modelling identified a negative relationship between catch depth and stomach content weight, suggesting reduced food intake in winter when cod use deeper areas for spawning and during peak summer when cod tend to avoid high water temperatures. The results of this study highlight the importance of shallow coastal areas as major feeding habitats of adult cod in the western Baltic Sea, which were previously unknown because samples were restricted to deeper trawlable areas. The results strongly suggest that historic stomach analyses overestimated the role of forage fish and underestimated the role of invertebrate prey. Eventually, this study shows the importance of a comprehensive habitat coverage for unbiased stomach sampling programmes to provide a more reliable estimation of top predator diet, a key information for food web analyses and multispecies models.     

The interrelated effects of body size and choroid rete development on the ocular O 2 partial pressure of Atlantic ( Gadus morhua) and Greenland cod ( Gadus ogac)

The interrelated effects of body size and choroid rete development on the ocular partial pressure of oxygen (PO₂) of the mainly temperate Atlantic cod, Gadus morhua, and the exclusively polar Greenland cod, G. ogac, were investigated using micro-optode O₂ sensors. Due to a difference in geographical distribution, it is hypothesized that G. ogac will possess features favouring visual-metabolic processes in a cold, dark environment. The relative size of the eye was identical between species and exhibited negative body-size scaling. The oxygen-secreting retia of both gadid species are extremely well developed and, although variable, recordings of ocular PO₂ were consistently and often greatly in excess of atmospheric pressures (27.7–138.3 kPa). The choroid rete was slightly more developed in G. ogac, but ocular PO₂ was not significantly different between the two species. Choroid rete development scaled isometrically with body size in both species and may explain why differences in body size did not account for any of the variation in ocular PO₂ measures. The hypothesis that polar-fish species exhibit marked visual-metabolic adaptations as a result of their cold, dark environment is not wholly supported by the current data.     

Feeding interactions and diet of carnivorous fishes in the Shelikhov Bay of the Sea of Okhotsk

The food spectra, trophic statuses, and feeding interrelations of three most abundant benthic carnivorous fish species inhabiting the Shelikhov Bay—the Pacific cod Gadus macrocephalus, the great sculpin Myoxocephalus polyacanthocephalus, and the Okhotsk sculpin M. ochotensis—are considered based on materials collected during the complex survey of the RV Professor Kaganovsky of the TINRO-Center, in September 2004. It was found that these species were facultative predators with wide food spectra. The significance of prey objects in the diet of the Okhotsk sculpin was as follows: crustaceans, fish, and mollusks. Great sculpin and Pacific cod preyed mostly on fish, then on crustaceans, and mollusks. Pacific cod ate equal proportions of fish and decapods. All the species had age-related variability of diet. The potential competition of great sculpin and Okhotsk sculpin for food was mitigated by the difference in the depths of their ranges, as well as by morphological (body size) and behavioral peculiarities in the areas where their habitats overlapped, and in microecosystems. The most probable competition was among Pacific cod 30–60 cm long and Okhotsk sculpin 20–50 cm in length, as well as among cod and great sculpins of all sizes.     

Rapid Evolution of the Intergenic T–P Spacer in the mtDNA of Arctic Cod <Emphasis Type="Italic">Arctogadus glacialis</Emphasis>

A noncoding intergenic spacer has previously been reported in mtDNA of Gadiformes. Here we present sequence information from two other cod species and variation within three species to clarify the evolution of this region. A general feature of the T–P spacer is high variation and folding into two or three hairpins. The variation among species both in structure of the region and sequence variation reflects the phylogenetic relationship of the species. A unique pattern is found within Arctic cod, Arctogadus glacialis, in which tandem repeat motifs result in new stable secondary structures. There is large variation in size of the region both within (heteroplasmy) and among individuals. A duplicated insertion is found in Greenland cod, Gadus ogac, at the same position as a corresponding duplication in Atlantic cod, Gadus morhua.     

Personality-dependent inter- and intraspecific foraging competition in the invasive round goby,Neogobius melanostomus

This study examines the impact of boldness on foraging competition of the highly invasive round goby Neogobius melanostomus Pallas 1815. Individual risk tolerance, or boldness, was measured as the time to resume movement after a simulated predation strike. Fish that resumed movement faster were categorized as “bold,” fish that took more time to resume movement were categorized as “shy” and those that fell in between these two categories were determined to have “intermediate” boldness. Competitive impacts of boldness in N. melanostomus were determined in a laboratory foraging experiment in which interspecific (juvenile Atlantic cod Gadus morhua Linnaeus 1758) and intraspecific (intermediate N. melanostomus) individuals were exposed to either bold or shy N. melanostomus competitors. G. morhua consumed fewer prey when competing with bold N. melanostomus than when competing with shy N. melanostomus, whereas intermediately bold N. melanostomus foraging was not affected by competitor boldness. Bold and shy N. melanostomus consumed similar amounts of prey, and the number of interactions between paired fish did not vary depending on the personality of N. melanostomus individuals. Therefore, intraspecific foraging competition was not found to be personality dependent. This study provides evidence that individual differences in boldness can mediate competitive interactions in N. melanostomus; nonetheless, results also show that competition is also governed by other mechanisms that require further study.     

Biology of Greenland cod,Gadus ogac,at Saqvaqjuac,northwest coast of Hudson Bay

Synopsis The distribution and relative abundance, life history parameters, food habits, and metabolic rate were determined forGadus ogac in Sagvagjuac Inlet, northwest coast of Hudson Bay (63° N). Fish were demersal, non-schooling, and distributed evenly down to 35 m depth. Growth was slow (maximum age 12 y) and mortality relatively low (0.5 y^–1).G. ogac first spawned at 2–3 y and spawned annually thereafter, in late March – early April. They tended to remain in the inlet and were not taken on the open coast. They are top carnivores, taking primarily capelin when available, benthic crustacea (crabs, amphipods) when not. The metabolic rate ofG. ogac is intermediate between the elevated rate of Arctic cod,Boreogadus saida, and eurythermal temperate species. Available data indicate they are not important in marine mammal and bird food webs. Their biology is contrasted with that of Arctic cod, which are short-lived, cryopelagic, feed on pelagic crustacea, and are an extremely important component of Arctic marine food webs.Direct reprint requests