Protracted Spawning Season Research Articles

Reproduction of Merluccius merluccius (Actinopterygii: Merlucciidae) from the northern Atlantic coasts of Morocco based on histological analysis of gonads

The hake (Merluccius merluccius) fishery occupies an important place in the Moroccan trawl fishery. Despite the ecological and commercial value of this species, the reproductive biology of European hake populations in Moroccan coastal waters has been little studied. Here, we describe the seasonal variations of gonad histology of hake collected from August 2017 to August 2018 along the northern Atlantic coast of Morocco. The histological changes were compared with macroscopical changes, and the body length at first maturity (L50) was calculated. The results revealed differences between macroscopic and histological maturity staging of the gonads. The L50 was estimated to be 34.7 cm for females and 28.6 cm for males (histology-based) and 30.6 cm for females and 26.8 cm for males (macroscopy-based). In addition, the observations of ovarian maturity indicated the presence of a protracted spawning season throughout the year with a peak around January and in midsummer.

Diet composition and reproductive biology of spineless cuttlefish Sepiella inermis (Orbigny, 1848) from Ratnagiri (Arabian Sea, North-west coast of India)

In the present study, we investigated some biological aspects of the spineless cuttlefish,Sepiella inermis (Orbigny, 1848) along the Ratnagiri coast of Maharashtra, India. A study ondiet composition and reproductive biology of the species was carried out. Dorsal mantlelength (DML) ranged from 35.9 to 93.3 mm for males and 29.3 to 98 mm for females.Feeding intensity was noted to be low during most of the study period. S. inermis was foundto have a protracted spawning season. The overall male: female ratio was 1:1.02 during thestudy period. The average fecundity was found to be 358 eggs. Length at sexual maturitywas estimated to be 42 mm and 53 mm for males and females respectively. The informationobtained from the study will help the fishery agencies to formulate management measuresin sustaining the landings of S. inermis in the region. Keywords:Biological studies, Cephalopods, Maharashtra,Ratnagiri coast, Sepiella inermis, Spinelesscuttlefish

Biology of the cleftbelly trevally Atropus atropos (Bloch and Schneider, 1801) from Ratnagiri coast of India

The cleftbelly trevally Atropus atropos (Bloch and Schneider, 1801) is one of the important carangid species landed mostly as trawl bycatch along Ratnagiri coast. Investigations were carried out on A. atropos with respect to length-weight relationship, condition factor, morphometrics, feeding and reproductive biology. The length-weight relationship indicated negative allometric growth in A. atropos. Morphometric measurements showed significant correlation with total length. The gut content analysis revealed A. atropos to be a carnivore fish feeding mainly on Acetes spp., small teleostean fishes, cephalopods, and squilla. Sex ratio showed dominance of males. Gonadosomatic index and maturity studies indicated that A. atropos has a protracted spawning season extending from May to December. Length at sexual maturity for females was estimated to be 171 mm. Fecundity ranged from 16458 to 138688 eggs with an average of 70097 eggs. Ova diameter studies showed that A. atropos spawns twice in a protracted spawning season. Keywords: By-catch, Carangidae, Carnivore, Reproductive biology, Spawning season

Variation in Prairie Chub hatch relationships across wet and dry years in the upper Red River basin

AbstractObjectiveThe Prairie Chub Macrhybopsis australis is a poorly studied minnow species endemic to the upper Red River basin and is of both state and federal conservation interest due to uncertainty about its life history and potential listing status. The upper Red River basin of Oklahoma and Texas is a harsh environment where drought and extreme flow events are exacerbated by human alterations. As an assumed pelagic‐broadcast‐spawning minnow, the Prairie Chub is capable of a protracted spawning season and larval fish survival is assumed to be linked to discharge and streamflow variability.MethodsWe systematically collected 2,017 age‐0 Prairie Chub from seven sites (North Fork Red River, Salt Fork Red River, Pease River, Red River, Prairie Dog Town Fork Red River, North Wichita River, and South Wichita River) with variable flow patterns during April–September 2019 and May–August 2020. We used otolith age estimates and back calculations to determine successful spawning dates. We used a hurdle model framework to examine relationships between hatch probability and hatch frequency.ResultHatch probability had a negative relationship with calendar day and declined as calendar day increased. Hatch counts peaked in late June and early July and declined thereafter in 2019 but showed no discernible peak during the spawning season in 2020. Hatch probability during the spawning season increased with relative flow and air temperature. Increased hatch counts were also positively related to discharge variability (CV) for the 10 d prior to hatch dates.ConclusionOur findings indicated that successful hatches had considerable spatial and temporal variability, with some sites contributing minimally to the population during some years. Spatial and temporal variability of hatch probability and hatch frequencies pose a variety of considerations for future conservation and management efforts, particularly given the pending federal listing status of the species.

Life-history traits facilitate the population success of the sergeant major Abudefduf saxatilis (Linnaeus, 1758, Perciformes: Pomacentridae) in the Caribbean Sea

ABSTRACT Trade-offs among life-history traits (e.g. growth, reproduction, and survival) are fundamental to the population success of living beings in the wild. This study describes the life-history traits of the sergeant major, Abudefduf saxatilis, one of those fishes most abundant in the coral reefs of the Caribbean Sea. The reproductive biology, age, growth, and abundance of a particular population of A. saxatilis from Mexico was explored using 12 monthly samplings to collect 414 specimens, and abundance monitoring through visual censuses for age-based demographic analysis. Our results indicated that A. saxatilis would have an opportunistic strategy with a type II survivorship curve since the studied population was characterized by fast growth, maturation at intermediate sizes (≈11 cm L T or 2.5 yrs. old), with batch spawning and indeterminate fecundity, protracted spawning season (January to September), high abundance of recruits, low instantaneous natural mortality (M = 0.114 year −1), high life expectancy at birth (e 0 ≈ 4.7 yrs.) and maturity (e α ≈ 2.8 yrs.), and high annual survival of adults and high survival to maturity (). All these life-history traits facilitate the population success of A. saxatilis, showing a high reproductive effort and a rapid population turnover, making this species an efficient colonizer.

Reproductive biology and feeding dynamics of the shrimp scad Alepes djedaba (Forsskal, 1775) from Ratnagiri, central west coast of India

The shrimp scad Alepes djedaba (Forsskal, 1775) belonging to family Carangidae is one of the important components of trawl bycatch landed at Ratnagiri, Maharashtra. Length-weight relationship, condition factor, morphometrics, feeding and reproductive biology of the species was investigated in the present study. Length-weight relationship indicated isometric growth in males, females and pooled individuals. All morphometric characters showed a moderate to high degree of correlation with total length. The qualitative and quantitative analysis of food revealed that A. djedaba is a carnivorous fish feeding mainly on Acetes spp., small fishes, copepods and shrimp larvae. The male:female sex ratio was found to be 1:0.85. Higher gonadosomatic index (GSI) values for females were observed during post-monsoon season. The ova diameter study indicated that A. djedaba follows a protracted spawning season with the individual spawning twice in a season. The absolute fecundity ranged from 15004 to 640800 eggs with an average of 309916 eggs. Length at sexual maturity for female fish was estimated to be 149 mm.Keywords: Arabian Sea, Feeding biology, Pelagic fish, Spawning season, Tropical fish

Life History of the Gray Snapper at the Warm Edge of Its Distribution Range in the Caribbean

AbstractKnowledge of the life history of populations at the warm edge of their distributional range can provide a better understanding of how they will adapt to climate warming, including potential poleward redistribution. The range of Gray Snapper Lutjanus griseus has the potential to expand along its northern temperate fringe, but little is known about this species in the warmest portion of its range. We studied the age, growth, reproduction, and mortality of commercially caught Gray Snapper in the Guatemalan Caribbean, where sea surface temperature consistently exceeds 26°C. Longevity was estimated as 10 years, and von Bertalanffy growth parameters that were consolidated through Bayesian estimation incorporating earlier estimates from the Caribbean region were as follows: asymptotic length (L∞) was 35 cm, the growth coefficient (K) was 0.56 year−1, and the theoretical age at zero length (t0) was −0.7 year. Gray Snapper grew slowest in April, prior to the rainy season, and at the onset of the reproductive season, which lasted to September. Fifty percent of the Gray Snapper matured at 31 cm and at 3.5 years of age. Gray Snapper had a lower maximum size, longevity, and peak reproductive investment, a protracted spawning season and reproductive life span, and elevated natural mortality at the warm edge of their distribution relative to temperate climates. Despite the plasticity in life history of Gray Snapper observed in this study, their potential to further adapt to warming remains unknown.

Adaptation to hypoxic environments; bearded gobies Sufflogobius bibarbatus in the Benguela upwelling ecosystem

The oceans of the world are slowly losing oxygen, in part because of climate change and in part because of anthropogenic eutrophication. This deoxygenation affects marine organisms in species-specific ways. This paper reviews what is known on how hypoxia tolerant species respond to low dissolved oxygen, using the bearded goby Sufflogobius bibarbatus as a model system. This species is endemic to the Benguela upwelling ecosystem, where, off Namibia, 9000 km2 of the shelf is hypoxic. Here, the species is now considered central to ecosystem functioning and in recent decades it has sustained commercial fisheries. Unlike their predators, S. bibarbatus has strong anti-predator responses as they can remain alert and escape threats in anoxic and severely hypoxic waters and can cope with sulphide shocks. Populations display diel vertical migration and shuttle between suboxic seabed refugia during the day to more oxygenated waters higher up in the water column at night and whilst they may share the water column with predators they may reduce their predation risk by associating with jellyfish (Aequorea forskalea and Chrysaora fulgida). The spatial distribution of S. bibarbatus varies with changes in available oxygen and populations expand and contract with climate-induced changes in the distribution of hypoxic waters. This species has an unusual diet and plays an important role in ecosystem functioning. It is a batch-spawner, with a protracted spawning season. Remarkably, males display alternative reproductive tactics and reproduction can take place at oxygen levels <0·5ml l-1 dissolved oxygen on the shelf break. Gaps in knowledge are identified and future works are proposed.

Oceanic, Latitudinal, and Sex-Specific Variation in Demography of a Tropical Deepwater Snapper across the Indo-Pacific Region

Deepwater tropical fisheries provide an important source of income and protein to Pacific and Indian Ocean coastal communities who are highly dependent on fish for food security. The development of quantitative assessments and management strategies for these deepwater fisheries has been hindered by insufficient biological and fisheries data. We examine the age-specific demography of the pygmy ruby snapper Etelis carbunculus, an important target species in tropical deepwater fisheries, across 90° of longitude and 20° of latitude in the Pacific and Indian Oceans. Our results show that growth of E. carbunculus varies significantly between oceans and sexes and across latitudes in both oceans. Estimates of natural and fishing mortality were similar between oceans, but higher for females than males in both oceans. Evidence of greater fishing pressure on females than males is likely due to the larger size-at-age of females compared to males, assuming that selectivity of the fishing gear is related directly to fish size. Sex ratios were significantly female biased in both oceans despite this species being gonochoristic, and maturity schedules were similar between sexes in the Pacific Ocean. This species exhibits a protracted spawning season from mid-spring to autumn (i.e., October to May) in the Pacific Ocean. These results represent the first estimates of age-specific demographic parameters for E. carbunculus, and provide the foundation for the development of the first species-specific assessment models and harvest strategies for the species. Future stock assessment models for E. carbunculus should consider sex-specific demographic parameters and spatial variation in demography. Our results reveal substantial differences in biology between E. carbunculus and the giant ruby snapper E. sp., a cryptic congeneric species, and thus contribute to greater clarity in managing fisheries that are dependent on these two species. Furthermore, the improved information on the life history of E. carbunculus contributes to the broader sustainable management and improved food security for deepwater snapper fisheries in the Indo-Pacific region.

Responses of bay anchovy (Anchoa mitchilli) larvae under lethal and sublethal scenarios of crude oil exposure

Bay anchovy (Anchoa mitchilli) is an ecologically important zooplanktivorous fish inhabiting estuaries of the Gulf of Mexico and eastern North America from Maine to Florida. Because they have a protracted spawning season (spring through fall) and are abundant at all life stages in coastal estuaries, their eggs and larvae likely encountered oil that reached the coast during the Deepwater Horizon oil spill. We compared responses to oil exposure at different life stages and at lethal and sublethal conditions using acute, 24h exposures. In a series of experiments, bay anchovy larvae were exposed to high energy water accommodated fractions (HEWAF) and chemically-enhanced WAF (CEWAF) at two stages of larval development (5 and 21 days post hatch, dph). HEWAF oil exposures induced significantly greater life stage dependent sensitivity at 5 dph than at 21 dph but chemically dispersed (CEWAF) exposure mortality was more variable and LC50s were not significantly different between 5 and 21dph larvae. Acute exposure to two low-level concentrations of CEWAF did not result in significant mortality over 24h, but resulted in a 25–77% reduction in larval survival and a 12–34% reduction in weight specific growth after six days of post-exposure growth following the initial 24h exposure. These results show that younger (5 dph) bay anchovy larvae are more vulnerable to acute oil exposure than older (21 dph) larvae, and that acute responses do not accurately reflect potential population level mortality and impacts to growth and development.

Spawn date explains variation in growth rate among families of hatchery reared Hood River steelhead (Oncorhynchus mykiss)

Body size at release for hatchery-reared steelhead is positively correlated with probability of survival to return as an adult. Although the relationship between body size and survival is well documented, little is known about what factors influence growth in the hatchery at the family level. We test if parent length, parent type (hatchery or natural), parent run date, or date of spawning correlate with among-family variation in offspring growth in the hatchery. Using Hood River winter steelhead (Oncorhynchus mykiss), two experiments were performed over two brood years. No effect of father length, parental type, or parental run date was found on offspring length in either year. Mother length was positively correlated with offspring length during both brood years and egg size data suggest this result occurs because longer females have larger eggs (i.e., it is a maternal effect). Although run date of parents was not correlated with offspring size, the date of spawning was negatively correlated with offspring size in a year with a protracted spawning season. Families spawned later in the season were smaller than those spawned earlier even though all fish began feeding on the same date. The spawn date effect lasted surprisingly long, being correlated with offspring size one year after spawning. A possible explanation for the spawn date effect is that eggs from early-spawned families were reared in chilled water to slow development so all families would begin feeding on the same date (a common hatchery practice). Colder water could have reduced metabolic costs and provided a surplus of energy to be used for somatic growth. This study highlights a little-studied environmental effect that could have large effects on long-term growth rates for hatchery-reared O. mykiss.

Reproductive parameters in native and non-native areas of Padogobius bonelli and comparison with P. nigricans (Actynopterigii, Gobiidae)

The introduced goby Padogobius bonelli has established abundant populations in the River Tiber basin (Central Italy) and its expansion coincided with the decrease of the endemic P. nigricans. P. bonelli exhibits a more aggressive behaviour than P. nigricans, but other biological attributes may advance its colonization success. To test this hypothesis, some reproductive parameters (gonado-somatic index, age at maturity, egg size, absolute and relative fecundity) of a long-time established non-native population of P. bonelli were analysed and compared to a population from the native area as well as to those of P. nigricans inhabiting the same river. As a result, no differences between non-native and native populations of P. bonelli were observed. On the contrary, non-native P. bonelli showed higher values of gonado-somatic index for both sexes, a more protracted spawning season and produced a greater number of eggs than P. nigricans. These results indicated a greater reproductive output of P. bonelli that can enhance its invasion process and favour the expansion outside of its native range, causing further decline of P. nigricans populations.

Reproductive tactics of the freshwater fish Brycon guatemalensis (Teleostei: characidae) in the lake Nicaragua

The Machaca Brycon guatemalensis is a trop- ical freshwater fish widely distributed in Central America continental water, and recognized to have potentiality for further fisheries development and recreational fishing opportunities in the Lake Nicaragua. The species presents a protracted spawning season that last 8 months. Both small and large females start spawning activity in July, but this is females size-dependent. The female length at 50 % maturity based on histological procedures was 27.3 cm, which differs from the ones previously reported based on the gonad macroscopic observations (34.9 cm). The present study indicates that B. guatemalensis has determinate fecundity and group-synchronous type ovar- ian organization. The studies of the potential annual fecundity of the species show that this allometrically increases above three with female size, indicating that larger females produce more eggs than predicted by weight. These results can be incorporated in the current fishery management to promote sustainability.

Reproductive traits and seasonal variability ofMerluccius merlucciusfrom the Tunisian coast

Merluccius merlucciusis an economically and ecologically important demersal species inhabiting the Atlantic Ocean and the Mediterranean Sea; however, reproductive knowledge about this species, along the North African coast is still missing. Monthly samples were collected from November 2010 to October 2011 along the North Tunisian coast. Morphometric parameters were measured, and histological preparations of gonadic tissues provided detailed information on the reproductive biology. Ovary maturity data showed a protracted spawning season throughout the year, with peaks in January, April and August. In females ofM. merluccius, seasonal variability in weight and length were noticed, with higher values in the first part of the study period (November–April) than from May to October. Analysing separately these two periods, significant relationships were observed between gonadic maturation indices and the somatic and gonadic indices, suggesting a seasonal differential use of energy intake. From November to May the fish alternate the use of their energy for body growth and reserves for later uses (i.e fat, muscle and liver mass) or for gonadic growth. From June to October the energy is converted mainly into gonad weight and less to somatic growth. The maturity ogives obtained by macroscopic and histological maturity staging methods showed significant differences; however, in both cases the length at first maturity estimates were around 29 cm. Nonetheless, when selecting the main spawning period, theL50estimation was heavily reduced to 24 cm, suggesting careful attention in the selection of the samples to be used specially for fishery management purposes.

An analysis of temporal variability in abundance, diversity and growth rates within the coastal ichthyoplankton assemblage of South Georgia (sub-Antarctic)

The study of spatial and temporal distribution and diversity of ichthyoplankton (fish eggs and larvae) can provide fisheries-independent information on the population dynamics and recruitment processes of marine fish species. Ichthyoplankton studies in the Southern Ocean have to date been largely constrained to the summer months. We analysed ichthyoplankton data collected from a year round, long term (2002–2008), plankton trawl sampling programme in a large fjord system (Cumberland Bay) at South Georgia, sub-Antarctic (54.25°S, 36.5°W) to assess temporal changes in larval fish diversity and abundance. Larvae of 22 species, representing nine families, were identified although three, Krefftichthys anderssoni (Myctophidae), Lepidonotothen nudifrons/Trematomus hansoni (Nototheniidae) and Champsocephalus gunnari (Channichthyidae), dominated abundance in all years. Significant seasonal and interannual differences in the larval fish assemblage were revealed by multivariate analyses. Estimates of larval growth are provided for five abundant species. Considerable inter-specific differences in relative larval growth rate were recorded but interannual variability within species was small. However, in the commercially important C. gunnari, multiple larval cohorts, representing a protracted spawning season, were observed to grow at different rates, and this may be related to temperature and/or food availability. A comparison with historical growth data from South Georgia suggests there has been little change in growth rate for the main species over the last three decades.

Atlantic cod (Gadus morhua) feed during spawning off Newfoundland and Labrador

Abstract Krumsick, K. J., and Rose, G. A. 2012. Atlantic cod (Gadus morhua) feed during spawning off Newfoundland and Labrador. – ICES Journal of Marine Science, 69: 1701–1709. We test a current assumption that Atlantic cod (Gadus morhua) off Newfoundland and Labrador, Canada, do not feed during the protracted spawning season (March–September). Stomach contents were analysed from 10 473 cod from four Northwest Atlantic Fisheries Organization regions (2J, 3K, 3L, and 3Ps) over 9 years from which gonads were also analysed to determine sex and maturity status. Adult cod in spawning condition did feed in all regions, usually at rates equivalent to or even greater than non-spawning fish and juveniles. Both sexes fed during spawning, though females consumed lesser amounts. Regional differences were evident. The total fullness index was greater in the northern (2J) than the southern (3Ps) region, with no consistent differences between spawners and non-spawners. The most southerly region (3Ps) exhibited the greatest prey diversity, the northern region (2J) the least. Shrimp was the major diet item in the northern regions. Capelin, zooplankton, crab, and other fish increased in importance to the south. Differences in prey items between non-spawning and spawning individuals of both sexes were possibly related to spawning behaviour. Models using consumption rates should not assume that cod do not feed during the protracted spawning season in these waters.

Life History Aspects of a Relict Ironcolor Shiner Notropis chalybaeus Population in a Novel Spring Environment

Ironcolor shiner Notropis chalybaeus is generally absent from groundwater-dominated systems throughout its range; however, a relict disjunct population occurs within the spring-fed upper reaches of the San Marcos River in central Texas. We conducted monthly seine collections within the restricted 2.2 km headwater range of the species to assess food habits and reproductive life history within a unique spring-run environment. Prey items were dominated by aquatic insects including Diptera (16% by weight), Ephemeroptera (13%), and Odonata (5%), as well as terrestrial insects (9%). The population consisted of four age groups with a maximum life span of 2.5 y. Reproductive ecology showed a protracted spawning season ranging Mar.–Dec. during which multiple clutches were produced. Reproductive maturity was reached at approximately 1 y (36 mm SL), mean mature oocyte diameter was ∼0.8 mm, and number of mature oocytes per clutch ranged 46–326. Comparisons between ironcolor shiner populations in the San Marcos River and thermally dynamic Marshalls Creek of Pennsylvania revealed mature female size was larger (T38 = 10.48, P < 0.01) and mature oocyte diameter smaller (T38 = 16.87, P < 0.01) in the upper San Marcos River. Literature accounts regarding ironcolor shiner reproductive ecology suggest a latitudinal trade-off between reproductive season length and oocyte size. Our findings provide further evidence for the roles of photoperiod and water temperature in structuring the reproductive seasonality of spring-dwelling fishes, specifically the lack or delay of terminating cues in stenothermal waters. In this manner, aquifer depletion and alteration of thermal regimes threaten spring-dwelling fishes by disrupting naturally occurring reproductive cues.

Reproductive biology of Argyrosomus regius (Asso, 1801) inhabiting the south eastern Mediterranean Sea, Egypt

The aim of the current work is to study the reproductive biology of Argyrosomus regius (Asso, 1801) collected from the wild catch of Egyptian Mediterranean coast. This is to give a clear image about the spawning season, length at first sexual maturity, and to identify gonadal maturation using histological analysis in respect to steroid hormone concentration throughout 1 year period. The present results revealed that wild meagre has a protracted spawning season starting in April and ending in July. The length at first sexual maturity of female fishes started at larger lengths than males 47 cm for females and 45 cm for males. By comparing calculated gonadosomatic index (GSI) to histological analysis it is clear that male fishes started gonadal maturation before female fishes. Plasma steroid level showed fluctuations in its mean concentration as Testosterone showed massive increase during male gonadal maturation and ripening stages. Plasma Progesterone and Estradiol levels displayed different patterns in their mean concentration during developmental stages throughout the maturation period of ovaries and testes. They showed significant declination in mature fish plasma followed by an insignificant increase during nearly ripe and ripening periods. The current data showed that wild meagre displays a gonochoristic group-synchronous oocyte maturation pattern. More investigation on this important fish species is recommended to establish a breeding protocol for aquaculture purpose.

Reproductive aspects of the yellowtail snapper Ocyurus chrysurus from the southern Gulf of Mexico

In this study, sex ratio, spawning season, fork length (L(F)) at maturity (L(F50)), batch fecundity and spawning frequency were characterized for the continental population of Ocyurus chrysurus from the Campeche Bank, in the southern Gulf of Mexico. A total of 1657 specimens were collected from February 2008 to January 2009. The overall sex ratio (male:female) and sex ratios by size-class showed no significant differences from an expected 1:1 ratio. The Campeche Bank population did not conform to the reproductive seasonality pattern characteristic of a continental population. A protracted spawning season that extended from January to September with peaks occurring mainly between April and May and additionally in September was observed. The population conformed, however, to the sexual maturity pattern observed for populations and species associated with a continental margin. Fish of both sexes reached the onset of sexual maturity at a similar and small L(F) of c. 14 cm, and L(F50) (L(F) at which 50% of females and males become mature) was 21·3 and 19·4 cm. Asynchronous-type ovarian development was observed for this species and batch fecundity estimates ranged from 14,102 to 164,756 oocytes (mean ±S.D. = 43,852 ± 32,684 oocytes). The overall spawning frequency estimate was once every 8·3 days or 26 times during the 9 month spawning season.

Diet and feeding niches of juvenile Gadus morhua, Melanogrammus aeglefinus and Merlangius merlangus during the settlement transition in the northern North Sea

A study on the feeding ecology of juvenile cod Gadus morhua, haddock Melanogrammus aeglefinus and whiting Merlangius merlangus during the pelagic to demersal transition was carried out on fishes sampled throughout their settlement season at a local nursery ground in the north-western North Sea, off the Scottish east coast. A comprehensive quantitative taxonomic analysis of the diets, as described in the paper, showed the emergence of distinctive feeding niches, minimizing the potential for competition between species and size categories. The diet of the juveniles changed with fish size, water depth, time of year and distance offshore. Small G. morhua were present in the study area earlier in the season, settled further inshore and ate a higher proportion of pelagic prey (copepods) and as size increased they moved into deeper waters and targeted larger, more benthic prey. As M. aeglefinus grew larger and moved into deeper waters, a diet of largely copepods, amphipods, pelagic Ammodytes spp., cyprids and pelagic gastropods evolved to one dominated predominantly by fishes and benthic invertebrates. In the case of M. merlangus, widespread ages and sizes throughout the sampling season, a consequence of their more protracted spawning season, resulted in dietary changes which were more likely to be influenced by seasonal changes in the prey field, in addition to developmental (size) changes, than the diets of the other two species.