Haddock Larvae Research Articles

Evaluation of microdiets versus live feeds on growth, survival and fatty acid composition of larval haddock ( Melanogrammus aeglefinus)

Two newly developed microdiets for the culture of marine fish larvae were compared to the control ICES Standard Weaning diet and the traditional live foods, rotifers ( Brachionus plicatilis) and Artemia nauplii, in a feeding study with larval haddock. Haddock larvae were reared from hatch to 20 days post hatch (dph) in a 5000-l tank and fed algae-enriched rotifers. Between 21 and 24 dph, 1000 larvae were transferred to each of the 20 tanks. From 25 until 45 dph, the larvae were fed five different diets: (1) B. plicatilis enriched with mixed algae species; (2) Artemia nauplii enriched with Algamac 2000 (Aquafauna-BioMarine, California, USA); (3) ICES Standard Weaning Diet; (4) IFREMER-INRA microdiet; and (5) microdiet produced by Louis D'Abramo, Mississippi State University, MS, USA. Survival was significantly higher ( P<0.001) in the rotifer and Artemia nauplii treatments (mean±S.E.; 24.9±4.2% and 21.3±6.3%, respectively) than in the microdiet treatments (ICES, 2.2±1.1%; IFREMER-INRA, 4.3±1.8%; and D'Abramo, 4.0±1.2%). Survival was not significantly different ( P<0.05) among the microdiet treatments. Mean larval weights were significantly different ( P<0.001) between rotifer (1.61±0.12 mg) and Artemia nauplii (2.14±0.14 mg) treatments. The weights of larvae fed microdiets (ICES, 0.75±0.05 mg; IFREMER-INRA, 0.85±0.04 mg; and D'Abramo, 0.86±0.05 mg) were not significantly different from each other; however, all microdiet treatments were significantly smaller than the live feed treatments ( P<0.01). There were also significant differences in the fatty acid composition of the larvae fed the different diets although no correlation was found between dietary fatty acid composition and growth or survival of the larvae. The highly unsaturated fatty acid (HUFA) 22:6n-3 showed a dramatic proportional increase in larval tissue relative to its proportional composition in both the live and formulated diets. Dietary proportions of the HUFA 20:4n-6 were similar among the Artemia, ICES and D'Abramo diets (average 1.2–1.3%); however, the proportions of 20:4n-6 in the larvae were significantly higher in larvae fed Artemia (mean±S.E.; 5.4±0.11%) compared to larvae fed either ICES or D'Abramo diets (mean±S.E.; 4.0±0.04% and 4.4±0.08%, respectively). Proportional increases of other HUFA, specifically 20:5n-3 and 22:5n-6 in larvae relative to dietary HUFA, suggest important physiological roles for these fatty acids. It appears that under the current rearing conditions, none of the microdiets examined is a suitable replacement for live feeds in the culture of haddock larvae from 25 to 45 dph and factors other than fatty acid composition must be considered.

The effect of different HUFA enrichment emulsions on the nutritional value of rotifers ( Brachionus plicatilis ) fed to larval haddock ( Melanogrammus aeglefinus ).

A feeding study was conducted in the winter 2001 to determine the effects of feeding rotifers (Brachionus plicatilis) enriched with various levels of essential fatty acids on the growth and survival of haddock larvae (Melanogrammus aeglefinus). Rotifer enrichment treatments were: 1) mixed algae, 2) high DHA (docosahexaenoic acid, 22:6n-3), 3) high DHA and EPA (eicosapentaenoic acid, 20:5n-3), and 4) DHA, EPA, and AA (arachidonic acid, 20:4n-6). Larvae were fed rotifers enriched with the different treatments from days 1 to 16 post-hatch. From day 17 until 25 all treatment groups were fed rotifers reared on mixed algae and then weaned onto the International Council for Exploration of the SEA (ICES) Standard Reference Weaning diet (http://allserv.rug.ac.t/aquaculture/rend/rend.htm) over a five day period. The experiment was terminated on day 41 post-hatch. The enrichment treatments affected the fatty acid composition of the rotifers and correlated with the accumulation of these fatty acids in the haddock larvae. However, no significant differences in larval growth or survival to 40 days post hatch were detected, suggesting that all treatments provided the minimal essential fatty acid requirements for haddock.

The relationship between plankton blooms, the hatching of fish larvae, and recruitment

Models of fish recruitment must include a realistic description of the underlying prey population dynamics. In the example presented here a model for the growth of haddock larvae ( Melanogrammus aeglefinus) and recruitment to the population of juvenile fish is coupled to an excitable medium representation of the planktonic ecosystem. The results elucidate and quantify non-trivial interactions between the larval and planktonic populations. Fish spawning can act to influence the date of onset of the spring phytoplankton bloom, and to increase bloom duration. Simulations show the effects of different reproductive strategies in a stochastic and evolving environment.

The effect of light intensity and spectrum on the incidence of first feeding by larval haddock

Under full–spectrum white light, feeding success of haddock Melanogrammus aeglefinus first feeding larvae, as measured both by proportion of larvae feeding and mean prey consumed, peaked at 1·7‐18 μmol s‐1 m‐2. Feeding was significantly reduced at lower and higher intensities. A similar result was observed for larvae feeding under blue (470 nm) light, with significantly greater feeding success at intermediate light intensity (1·8 μmol s‐1 m‐2). When different light qualities were compared, larvae had significantly greater feeding success when exposed to blue (470 nm) light than either full‐spectrum white or green (530 nm) light. Haddock larvae were capable of prey capture under all light treatments tested, indicating a necessary degree of adaptive flexibility in feeding response. The results are consistent with predisposition of haddock larvae to optimal feeding in a visual environment comparable with open ocean nursery grounds. Information on the impact of light on haddock first feeding can be incorporated into models of larval growth, survival, year‐class strength and recruitment, and assist in developing husbandry protocols to maximize larval survival in aquaculture.

Profiling of Bacterial Species Associated with Haddock Larviculture by PCR Amplification of 16S rDNA and Denaturing Gradient Gel Electrophoresis

Abstract A major limitation to marine finfish culture is the sudden and substantial increase in mortality frequently referred to as a “larval crash.” Contamination of the production system with opportunistic pathogenic bacteria has frequently been implicated as a cause of this mortality. In studies designed to test this hypothesis, 15 larval haddock from 16 tanks were sampled weekly between hatching and weaning in spring 2000 for their bacterial 16S rDNA profiles by polymerase chain reaction amplification combined with denaturing gradient gel electrophoresis (PCR−DGGE). The profiles obtained were generally dominated by fewer than five bands. A well-defined trend in bacterial succession was observed during the growth of the haddock larvae, the diversity of bacterial species being greater at the onset but invariably ending with a singular predominance by a species with 16S rDNA homology to various α-Proteobacteria such as Roseobacter. Sequence analysis of a larger fragment of 16S rDNA (1,437 nucleotides) of...

Modelling the retention and survival of Browns Bank haddock larvae using an early life stage model

Browns Bank, the principal spawning ground for haddock on the south‐west Nova Scotian shelf, is composed of two distinct production zones: the inshore Bay of Fundy (BoF) region and the offshore south‐west Nova (SWN) bank region. Fish growing in the BoF are larger at age than those in the SWN region. Analysis of research vessel (RV) data shows that the majority of age‐2 haddock have the size of south‐west Nova Scotia fish, suggesting that the majority of surviving fish were retained in SWN. We used an early life stage (ELS) model to address two questions. First, we asked whether the length‐at‐age difference between Bof and SWN is evident at the larval stage. Using a temperature‐based growth model, we found that predicted size differences for late larvae would be less than 0.5 mm. From consideration of the average growth curves to age‐2 for the two regions, we showed that this difference was not the seed for the size difference in later life stage. The second question we addressed was how well the ELS model predicted partitioning of late larvae between SWN and the BoF corresponding to the ratio of SWN/BoF age‐2 juveniles found in the RV data. We ran the model on a representative number of years between 1973 and 1992, and found a significant correlation between the model simulation and the data (cor=0.71, P=0.047). This indicates that the age‐2 size distribution is a reflection of retention and survival occurring during the first two months of life. Model/data discrepancies are explained in terms of differential mortality between the two regions.

Predation risk on larval gadids by pelagic fish in the Georges Bank ecosystem. I. Spatial overlap associated with hydrographic features

We investigated the relationship between hydrographic variables on Georges Bank and the spatial distribution of larval Atlantic cod (Gadus morhua), larval haddock (Melanogrammus aeglefinus), and two planktivorous fish, Atlantic herring (Clupea harengus) and Atlantic mackerel (Scomber scombrus). We conducted cruises during April-May of 1990, 1994, and 1995 to document spatial distributions and employed logistic regression and a spatial-overlap index to assess relationships between hydrographic variables, spatial distributions and spatial overlap between gadid larvae and planktivorous fish. Cod larvae were more abundant in shallow well-mixed areas, while both haddock larvae and herring were more abundant in stratified areas. Atlantic mackerel occurred in deeper water and had little spatial overlap with the other species. In 1995, the presence of Scotian Shelf water and an intrusion of continental slope water altered temperature and salinity distributions and increased spatial overlap between cod larvae and herring. The stratified areas offer a higher concentration of prey for larval haddock, however there is increased exposure to planktivorous fish predators, creating a potential trade-off between predation and starvation. Due to the high abundance of planktivorous fish and high spatial and temporal overlap, predation is likely an important factor influencing survival of gadid larvae on Georges Bank.

The drift-retention dichotomy on Browns Bank: a model study of interannual variability

Interannual variability in drift and retention in the Browns Bank region in spring was investigated by tracking particles in numerical-model estimates of the seasonal mean circulation and monthly mean flow anomalies computed using wind and sea-level data. The circulation changes associated with the variable forcing tend to reinforce either drift to the Bay of Fundy or retention in the vicinity of Browns Bank. Regular spring spawning can lead to widely different distributions of 1-month-old larvae, supporting the concept of a drift-retention dichotomy for Atlantic cod (Gadus morhua) and haddock (Melanogrammus aeglefinus) larvae in the region. Analysis of interannual variability indicates that retention has bounded properties that must be accounted for when relating retention variability to forcing indices. As well, the nonlinearities associated with calculating particle trajectories from velocity fields contribute about 10% of the variance in retention indices. Comparison with biologically based indices of haddock survival indicates that circulation variability alone is not the dominant influence on survival to ages 1 and 2 years. However, it can be expected to have a significant influence in conjunction with biological factors such as the location and timing of peak spawning and the variable vertical position of eggs and larvae.

Reply to Buckley et al.’s "Comment: Larval Atlantic cod and haddock growth models, metabolism, ingestion, and temperature effects"

In reexamining recent modeling work on Atlantic cod (Gadus morhua) and haddock (Melanogrammus aeglefinus) larvae, Buckley et al.1 conclude that the procedure used in Laurence (1985) to calculate daily metabolic rates may have been incorrect. Buckley et al. suggest that the actual daily metabolic rates should be nearly three times higher for both Atlantic cod and haddock. At the same time, they also point out that total assimilation may be much higher (70% rather than <50%) based on more recent data, including laboratory experiments conducted on larval fish of other species (Kiorboe et al. 1987; Wieser and Medgyesy 1990; Letcher et al. 1996). Besides these points, Buckley et al. suggest that the value of Qa, a temperature-dependent “overall” assimilation term used by Leising and Franks (1999), was unrealistically high (we used a value of 1.6). Instead, Buckley et al. suggest that this value should be closer to 1.07–1.14 based on their analysis of data from several experiments on other species of fish. At the time of creation of our model (Leising and Franks 1999), we used our best estimates of these parameters. However, in light of Buckley et al.’s arguments, we agree that their estimates for these parameters may be more appropriate for Atlantic cod and haddock larvae. We therefore reran our model for Atlantic cod only using their suggested parameters to determine the impact on our model results, with specific attention to the effects of temperature versus food availability.

The effect of photoperiod, tank colour and light intensity on growth of larval haddock

In two separate experiments, haddock (Melanogrammus aeglefinus) larvae were raised under different photoperiods (24L : 0D or 15L : 9D), or different combinations of tank colour (black or white) and light intensity (1.1 μmol s−1 m−2 or 18 μmol s−1 m−2). Growth (0.8% day−1 in standard length; 2.9% day−1 in body area) and survival (2%) were not significantly different between photoperiod treatments after 35 days. Larval survival was greater in white versus black tanks after 41 days (2% versus l%, respectively). Growth of larvae was impaired in black tanks at low (1.1 μmol s−1 m−2) light intensity (0.8% day−1 in standard length and 2.2% day−1 in body area versus 1.1% day 21 in standard length and 3.1% day−1 in body area, for all other treatments). Transmission and reflection of light was low in black tanks at low incident light, and there was very little upwelling light. The resultant poor prey to background contrast probably resulted in larvae being unable to consume sufficient food to sustain a level of growth comparable to that in other treatments.

Estimates of vertical heat flux and stratification from Southern Georges Bank, interannual variability, 1985–1995

Interannual variability (IAV) of net vertical heat flux ( Q), potential energy anomaly or “stratification index” ( V) and date of onset for water column stratification were studied using five-day averaged model output over an 11 yr (1985–1995) period for three depth zones which span water depths from less than 60 to 100 m on southern Georges Bank. Comparison between five-day averaged model Q estimates from this study and Q measurements from US-GLOBEC mooring ST1 during winter–summer 1995 shows that nearly all of the difference between uncalibrated model estimates and calibrated ST1 measurements is attributable to the difference between the sum of the latent, sensible and long-wave components ( Q e+ Q s+ Q lw) from each data set. Furthermore, there is virtually no IAV in residual five-day averaged Q e+ Q s+ Q lw values relative to the long-term (1985–1995) mean seasonal cycle, suggesting strongly that the calibration of model Q estimates using measurements from mooring ST1 during 1995 may be applied to other years (1985–1994). Calibrated long-term (1985–1995) monthly mean Q values from this study show a similar seasonal cycle to climatological values determined from ship-of-opportunity data. However, values from this study are higher relative to the ship-of-opportunity long-term monthly mean climatology, with largest differences occurring during winter. IAV of monthly mean Q for zones 1–3 on southern Georges Bank is significant with the magnitude of residuals increasing from zones 1 to 3. Long-term (1985–1995) monthly mean model-derived V computed for zones 1–3 shows zone 1 (<60 m depth) remaining well-mixed throughout the year with no stratification development and therefore no detectable IAV for V. However, within zone 2 (60–80 m depth) and zone 3 (80–100 m depth) significant permanent stratification begins to develop during May and April, respectively, increasing almost linearly from May–July and reaching a maximum during July and August, respectively. IAV of V is large for zones 2 and 3 with typical standard deviations of 1000–5000 J m −2 occurring from late-spring-summer with the largest coefficient of variation during spring and fall. Results from linear regression for zones 2–3 show that IAV of V is controlled largely by IAV of Q, explaining up to ∼80% of the variance, with IAV of wind mixing being of secondary importance. Results from model-derived five-day averaged V show that temporary or “transient” stratification nearly always occurs in early-May and mid-April for zones 2 and 3, respectively, over the 11 yr study period and is associated with reduced tidal stirring during periods of neap tide. The mean date of first transient stratification for zone 3 coincides with the historical maximum abundance of early-stage (<6 mm) cod larvae and copepod nauplii for Georges Bank. In addition, the historical maximum abundance of haddock larvae occurs on Georges Bank at the same time that permanent or “seasonal” stratification is established in zone 3.

Diel variability of feeding activity in haddock ( Melanogrammus aeglifinus ) larvae in the East Shetland area, North Sea

Investigations of factors affecting feeding success in fish larvae require knowledge of the scales of variability of the feeding process itself and the indices used to assess this variability. In this study, we measured short-term (diel) variability in feeding rates of wild haddock (Melanogrammus aeglifinus) larvae four times per day during a 10-d cruise in the northern North Sea. Feeding activity was evaluated using indices of gut fullness, prey digestive state and biochemical measurements (tryptic enzyme activity). The gut fullness and the enzyme activity indices indicated moderate to high rates of food consumption throughout the cruise. Time series analysis of the three indices showed significant diel variability in all indices and enabled identification of significant lags between food uptake and peak digestive enzyme activity. The typical pattern of food consumption and digestion was characterized by maximal ingestion of prey early in the evening (19:00 hrs) and peak digestive enzyme activity at 01:00 hrs. The time scale over which enzyme activities reacted to prey ingestion was ca. 6 h, and is consistent with expectations from controlled laboratory experiments with other larval fish species. Significant diel variability in tryptic enzyme activity suggests that attempts to relate this measure of feeding success to other variables (e.g. food concentrations) should take care to accommodate natural cycles in feeding activity before making statistical comparisons.

Variability in growth rates of larval haddock in the northern North Sea

The large‐scale distribution of haddock (Melanogrammus aeglefinus) larvae in the northern North Sea was mapped in a grid survey carried out in late April 1996. A drifting buoy was deployed in the centre of one of the areas of concentration of larvae located off the east coast of the Shetland Isles, where intensive sampling was carried out for ≈ 10 days. Daily larval haddock growth variability, estimated from otolith microstructure analysis, was independent of the measured variability of the physical and biological environment of the larvae. The survey coincided with the onset of the spring plankton production bloom, and a likely explanation for the absence of environmental effects on larval growth was high food availability and larval feeding rates. Nevertheless, differences in growth were observed between cohorts, with larvae hatched later in the spring displaying higher growth at age than those hatched earlier. Particle‐tracking modelling suggested that differences in temperature history between cohorts, on their own or compounded by a potential interaction between temperature and the development of plankton production, may explain the higher growth rate of the larvae hatched later in the season.

Bio‐physical modelling of the early life stages of haddock, Melanogrammus aeglefinus, in the North Sea

An individual‐based modelling approach was developed to investigate the spatial and temporal patterns in the recruitment processes of North Sea haddock, Melanogrammus aeglefinus. The approach was based on the realization that the survivors to recruitment of an annual cohort are most probably not drawn at random from the initial population of eggs, but represent the fastest‐growing individuals. Individual growth rates reflect the unique exposure of each larva to the environment along its drift trajectory. In this context, the environment refers to a wide range of factors affecting growth such as food, turbulence and temperature. A combination of a model of egg production by the adult stock, a particle‐tracking scheme, and a model of larval growth and mortality rate was used to simulate the dispersal trajectories, and the survival of haddock larvae spawned at different times and locations on the continental shelf. The particle tracking was driven by flowfields from a climatological implementation of the Hamburg Shelf–Ocean Model (HAMSOM) for the North Sea and NE Atlantic. The system was able to resolve spatial and temporal patterns in the recruitment process and indicated that the surviving population of larvae was drawn from a restricted part of the spawning distribution. The results have the potential to guide the development of future conservation measures in fisheries management.

ENVIRONMENTAL VARIABILITY EFFECTS ON MARINE FISHERIES: FOUR CASE HISTORIES

The changing nature of marine fisheries requires management approaches that recognize and include ecosystem and environmental effects. Therefore, we review some examples of exploited fishery stocks in which environmental control is a major contributor to structuring the abundance and distribution of the stock. Four examples, taken from studies of northern cod (Gadus morhua), cod and haddock (Melanogrammus aeglefinus) larvae, the eastern oyster (Crassostrea virginica), and Antarctic krill (Euphausia superba), are given that clearly illustrate environmental control on the fishery. From these examples, we argue that future management strategies for exploited fisheries must include effects of environmental variability. In particular, management strategies must be flexible enough to include delayed responses to environmental variations that result from the transfer of per- turbations from larger to smaller scales and vice versa. This capability requires an under- standing of where linkages between the physical environment and the species of interest occur. Development of this knowledge requires input from a variety of disciplines, coor- dinated research programs, and considerable cooperation at national and international levels.

Dependence of survival on growth in larval pollock Pollachius virens and haddock Melanogrammus aeglefinus:a field study based on individual hatchdates

The hatchdate frequency distributions (HFD) of pollock and haddock larvae sampled at monthly intervals west of Sable Island (Scotian Shelf, northwest Atlantic) in 1992 and 1993 were reconstructed for different age intervals (0-20, 21-40, 41-80 d) and corrected for aliasing due to sampling discontinuity and mortality-dispersion. The ratio of the HFD at a given age to the HFD at an earlier age was used as an index of the relative survival of larvae grouped into 5 d hatchdate cohorts. Pollock hatched from November to March and haddock from February to June. In pollock, seasonal variations in relative survival of the cohorts over the 21-40 and 41-80 d age intervals were correlated to Sorong vanations in growth. In haddock, growth varied little over the hatching season and there was no significant link with survival. For pollock, slow growth invariably resulted in low survival but fast growth resulted in either low or high survival, indicating that fast growth is a necessary but not sufficient condition for survival. Increased predation pressure late in the hatching season of both species Could explain the decoupling of growth and survival in cohorts hatched in spring and early summer.

Vertical distribution of cod and haddock eggs and larvae, feeding and condition in stratified and mixed waters on southern Georges Bank, May 1992

In late May 1992, cod and haddock eggs and larvae were sampled at discrete depths using MOCNESS on the southern flank of Georges Bank when seasonal stratification of the water column was developing. A deeper, stratified site (80-m bottom) was sampled over 7 days in relation to the temporal evolution of vertical structure and compared to a nearby shoal, mixed site (50 m). Sampling also was conducted to the west along two transects in the Great South Channel. Larval biochemical composition was measured and nutritional condition inferred from RNA:DNA ratios.During the period 22–24 May 1992, surface warming resulted in a temperature gradient from 6 to 10°C (0.6 δt units) in the upper 20 m at the stratified site. Gadid eggs (93% haddock) were most abundant (6–14 per 100 m3, on average) in the surface 20 m, with maximum density just above the base of the thermocline at 20-10 m. The few recently-hatched larvae (5–6 mm) caught were broadly distributed in the water column, with maximum average densities (1–3 per 100 m3) deeper at 10–40 m. The naupliar and copepodite stages of Pseudocalanus spp. were principal prey of these larvae, and also were more abundant (3–7 prey 1−1) in the surface 20 m, with a maximum density at 20-10 m. A moderate storm on 25 May mixed the upper part of the water column to 5–6°C. The day after the storm, gadid eggs and copepods had highest densities in the surface 10 m.At the shoal (mixed site) egg densities were low (1–3 per 100 m3) and distributed evenly through the water column. Cod and haddock larvae were larger (7–8 mm modal length) and more abundant than at the stratified site. At night, they were evenly distributed at 4–6 per 100 m3 on average, but by day, maximum density increased to 6–16 per 100 m3 in the deepest strata sampled (40–30 m). The larger larvae at this site preyed predominantly on copepodite stages of Pseudocalanus spp., which were uniformly distributed in the water column.Of all the larvae sampled for biochemical analysis, 10% of haddock and 1.5% of cod had ratios below 4.1. Based on laboratory studies, fish larvae with RNA:DNA ratios below 4.1 are considered to have been in poor condition. There were no larvae in poor condition at the Great South Channel Transect, where their mean size was greatest; no larvae were collected at the stratified site for comparison. At the mixed site, cod larvae sampled before and after the storm had identical ratios, while haddock larvae had significantly higher ratios after the storm. Larvae from different sampling depths had significantly different ratios, inferring that they remained together long enough to acquire a unique signature.

Trophodynamic and advective influences on Georges Bank larval cod and haddock

Using a model-based approach, the relative effects of advective and trophodynamic (feeding and growth) processes are considered on populations of larval cod (Gadus morhua) and haddock (Melanogrammus aeglefinus) on Georges Bank. Building on previous studies that describe the role of advection, this study incorporates trophodynamic relationships to examine starvation mortality and growth rates at the level of individual larvae on the Bank. Estimates of prey concentrations and flow fields appropriate for late winter/early spring are used. Both trophodynamic processes and advection influence larval losses from the Bank where, in the absence of starvation, advective losses are on the order of one-fifth of the eggs and larvae spawned on the Bank. Starvation is most important in the first feeding larvae and is much reduced for older larvae. The contact rates between larval fish and zooplankton prey when turbulence is included are 2–5 times greater than the contact rates with no turbulence, and allow the model cod larvae to achieve growth rates similar to those observed on the Bank, although mean rates for larval haddock are still lower than observed. Turbulence-enhanced contact rates are thus determined to be a necessary component in our description of the growth of cod and haddock larvae on Georges Bank. Model cod larvae with growth rates comparable to those observed in the field are located below the surface layer (deeper than 25 m) and inside the 60 m isobath. The region of highest retention due to circulation processes (Werner et al., 1993; Fisheries Oceanography, 2, 43–64) coincides with the region of highest growth rates and highest larval survival. Therefore, there is a complementary interaction between trophodynamic and circulation processes, with those larvae most likely to remain on the Bank also being those in the most favorable feeding regions. Haddock larvae require higher prey densities than cod larvae to survive.

Influences of mean advection and simple behavior on the distribution of cod and haddock early life stages on Georges Bank

ABSTRACTResults of a modeling study designed to explore the influences of physical advection and certain biological mechanisms on the distribution of cod (Gadus morhua) and haddock (Melarwgrammus aeglefinus) early life stages on Georges Bank are described. Using a late‐winter/early‐spring 3‐D circulation field driven by the M2 tidal current, mean wind stress and Scotian Shelf inflow, we examine the distribution of cod and haddock larvae spawned on the Northeast Peak of the Bank. The sensitivity to a March‐April baroclinic field is also explored. Results indicate that larvae remaining in the surface Ekman layer are generally advected off‐bank. However, downwelling associated with Ekman layer convergence near the shelf break provides a mechanism for larvae to exit from the off‐bank surface drift. Larvae below the surface layer are transported south‐westward along the southern flank of Georges Bank and are retained on the Bank if their position immediately upstream of the Great South Channel is shoalward of (roughly) the 70 m isobath. Within the Great South Channel region and between the 50 and 70 m isobaths, retention can depend on the phase of the tide. Spawning shoalward of the 50 m isobath on the Northeast Peak greatly increases the chances of retention. These results apply to passive larvae and to those with specified vertical distributions and migration based on observations. Directional on‐bank swimming at rates of 0.5 to 1 body length per second would substantially enhance shoalward displacement, resulting in larval distributions during the first 2 months that are consistent with field observations.

Recruitment of the 1987 Year Class of Georges Bank Haddock (Melanogrammus aeglefinus): The Influence of Unusual Larval Transport

Unusually strong along-shelf surface flow in the spring of 1987 transported haddock (Melanogrammus aeglefinus) larvae westward from spawning grounds on Georges Bank into the Middle Atlantic Bight, some as far as 400 km beyond the limits of their normal distribution. 0-Group haddock survived in the Bight in record numbers. Their center of abundance occurred along the outer half of the shelf from Delaware Bay to Martha's Vineyard where more than 97% of the 0-group fish resided in the fall of 1987. Juveniles remained more abundant in the Bight than on the bank through the spring of 1988. Collective evidence suggests that survivors returned to Georges Bank during the late spring/early summer of 1988. The estimated number of 0-group fish in the 1987 year class exceeded all previous estimates from a 27-yr time series which included the 1963 year class, the largest recruited to the fishery since record keeping began in 1931.