The effect of growth rate,size, and season on oocyte development and maturity of Atlantic cod (Gadus morhua L.)

The effect of growth rate, body weight, age, and season on ovarian development and maturation was investigated for Atlantic cod (Gadus morhua L.) reared in the laboratory over 10 months, for each of two consecutive years, 1978–1980. Cod were also collected from the Gulf of St. Lawrence, the Scotian Shelf, Georges Bank, the Flemish Cap, and the N.E. Newfoundland Shelf.The state of maturity was recognized by oocyte size. Stage I oocytes did not vary in size with growth rate, season, age, or maturity, while the size of stage II oocytes was positively correlated with maturity and negatively correlated with maximum stage of development achieved. Ovarian wall thickness was positively correlated with age and maturity.The frequency distribution of stage I and II oocytes distinguished the state of maturation, with cod that would mature by the next spawning season having a minimum of 20% ($\text{x}\text{?}\text{= 42\%}$) of their oocytes at stage II or greater level of development.Maturing 3-yr-old cod had greater life specific growth rates than immature 3-yr-olds, but growth rates during the third year itself were not significantly different. A hypothesis of a three-part density-dependent mechanism controlling fecundity is postulated. Future reductions in partial recruitment and total fecundity are predicted for the Gulf of St. Lawrence cod stock based on calculated growth rates for Gulf cod in 1979.     

Use of parasite and genetic markers in delineating populations of winter flounder from the central and south‐west Scotian Shelf and north‐east Gulf of Maine

Discriminant function analyses of infection parameters of parasitic helminths revealed that abundances of seven helminth species contributed significantly to the delineation of four host populations of winter flounder Pleuronectes americanus from the central and south‐west Scotian Shelf and the north‐east Gulf of Maine (NAFO subdivision 4WX‐5Z). These were adult digeneans, Derogenes varicus , Genolinea laticauda , Steganoderma formosum and Steringophorus furciger , metacercariae of the digenean, Stephanostomum baccatum , and larval nematodes, Anisakis simplex and Hysterothylacium aduncum . The correct classification rate was 84% overall, with Georges Bank (5Z) and Sable Island Bank (4W) winter flounder being the most accurately classified samples at 98 and 88%, respectively. Winter flounder from south‐west Nova Scotia (4X), an inshore sample from St Marys Bay and offshore fish from Browns Bank, had the lowest rates of correct classification (76 and 71%, respectively) due, primarily, to cross‐misclassification between the two samples. Winter pairwise comparisons of four microsatellite markers identified significant genetic differences between all populations sampled with the Georges Bank population being the most genetically distinct overall, and St Marys Bay and Browns Bank fish being the least dissimilar.     

Bay-scale population structure in coastal Atlantic cod in Labrador and Newfoundland, Canada

Polymorphisms at five microsatellite DNA loci provide evidence that Atlantic cod Gadus morhua inhabiting Gilbert Bay, Labrador are genetically distinguishable from offshore cod on the north-east Newfoundland shelf and from inshore cod in Trinity Bay, Newfoundland. Antifreeze activity in the blood suggests that Gilbert Bay cod overwinter within the Bay. Gilbert Bay cod are also smaller (weight and length) for their age and consequently less fecund for their age, than cod elsewhere within the northern cod complex. The productivity and recruitment potential of coastal cod off Labrador may thus be much lower than that of offshore northern cod or of inshore cod farther south, implying that a more conservative management strategy may be required for cod from coastal Labrador than traditionally practised for northern cod inhabiting less harsh environments. Relatively high F _ST and R _ST measures of population structure suggest that important barriers to gene flow exist among five components that include two inshore (Gilbert and Trinity Bay) and three offshore cod aggregations on the north-east Newfoundland Shelf and the Grand Bank. D _A and D _SW estimates of genetic distance that involve Gilbert Bay cod are approximately three- and 10–fold larger, respectively, than estimates not involving Gilbert Bay cod. The differences between inshore cod from Gilbert Bay and Trinity Bay raise the possibility that other genetically distinguishable coastal populations may exist, or may have existed prior to the northern cod fishery collapse. Harvesting strategies for northern cod should recognize the existence of genetic diversity between inshore and offshore components as well as among coastal components.     

Microsatellite variation in cod Gadus morhua throughout its geographic range

Previous genetic studies using neutral markers such as allozymes, mtDNA and minisatellite loci have demonstrated varying amounts of population structure in cod Gadus morhua throughout the Atlantic. Microsatellite loci, which are potentially the most informative of presently available neutral genetic markers, have been applied extensively within western and eastern Atlantic areas but not on a range-wide basis. In the present study, six microsatellite DNA loci were used to screen cod samples from nine locations throughout the geographic range from the Scotian Shelf in the West Atlantic to the Barents and Baltic Seas in the east. Overall F _ST value was 0·03 ( P = < 0·001) across all samples. Statistically significant population differences over all loci combined were evident between more geographically distant samples, using either heterogeneity tests or F _ST analysis, with at least one locus showing significant differences between all samples (prior to Bonferroni correction). A significant correlation was observed between genetic and geographical distance, suggesting a higher level of historical and contemporary gene flow between adjacent populations than more distant populations. Samples from either end of the geographic range (Scotian Shelf and Baltic Sea) were particularly distinct when analysed using the STRUCTURE programme and also showed a high level of self-assignment when individuals of either the Scotian Shelf or Baltic Sea were tested against the entire data set. The present microsatellite study demonstrates a high level of geographic population structure between the western Atlantic, middle and eastern Atlantic and Baltic Sea, and thus, the findings should be useful in devising overall management and conservation strategies for the species.     

Microsatellite analysis indicates an absence of population structure among Hippoglossus hippoglossus in the north‐west Atlantic

To explore the population structure of Atlantic halibut Hippoglossus hippoglossus , 160 fish from four locations in the north‐west Atlantic (Bay of Fundy, Scotian Shelf, Gulf of St Lawrence and Iceland) were examined for evidence of population structure using 18 microsatellite markers. Pair‐wise F _ST and a model‐based cluster analysis revealed no significant differentiation between samples, although uncertainties surrounding Atlantic halibut reproductive behaviour made it difficult to ascertain that only a single breeding population had been sampled at each location     

Mitochondrial DNA sequence variation and genetic stock structure of Atlantic cod (Gadus morhua) from bay and offshore locations on the Newfoundland continental shelf

Bay cod, Atlantic cod (Gadus morhua) that over-winter in the deep-water bays of northeastern Newfoundland, have historically been regarded as distinct in migration and spawning behaviour from offshore (Grand Bank) cod stocks. To investigate their genetic relationships, we determined the DNA sequence of a 307-base-pair portion of the mitochondrial cytochrome b gene for 236 adult cod taken from the waters off northeastern Newfoundland, including fish found over-wintering and spawning in Trinity Bay. Although 17 genotypes were found, a single common genotype occurs at a frequency of greater than 80% in all samples, and no alternative genotype occurs at a frequency of greater than 3%. Genotype proportions did not differ significantly among samples. Measures of genetic subdivision among sampling locations are nil. Cod over-wintering in Trinity Bay are not genetically distinct from offshore cod. In combination with tagging and physiological studies, these data suggest that there is sufficient movement of cod between bay and offshore locations to prevent the development or maintenance of independent inshore stocks. Adult cod that over-winter in Trinity Bay appear to represent an assemblage of temporarily nonmigratory fish that have become physiologically acclimated to cold-water inshore environments. The pattern of genetic variation in northern cod suggests a recent population structure characterized by extensive movement of contemporary individuals superimposed on an older structure characterized by a bottleneck in the population size of cod in the north-western Atlantic.     

Population genetics of Atlantic cod using amplified single locus minisatellite VNTR analysis

Atlantic cod, Gadus morhua , have been examined extensively over the last two decades using allozyme electrophoresis. More recently, several populations have been studied using mitochondrial DNA (RFLP and sequence) analysis, together with multilocus minisatellite DNA and also microsatellite DNA analyses. The declining status of cod populations in many areas highlights the need for powerful genetic markers capable of discriminating between cod populations, in order to facilitate the design of effective management strategies. Single locus minisatellite DNA analysis offers a potentially powerful alternative to the already mentioned techniques, by combining the power of detection of hypervariable DNA, with non-radioactive techniques in a cost-effective way. As a preliminary investigation into the feasibility of using this approach, four Atlantic cod samples (North Norway, Irish Sea, Scotian Shelf and Northern Cod) were screened at a single polymerase chain reaction (PCR) amplified minisatellite locus ( Mmer -AMP2), using primers designed for the flanking regions of a whiting Merlangius merlangus L., minisatellite DNA locus. PCR products separated by agarose electrophoresis and viewed by ethidium bromide fluorescence under UV illumination, consisted of one or two bands per individual (corresponding to homozygotes and hétérozygotes, respectively). Twenty-two alleles were resolved in 119 cod, and sample heterozygosity ranged from 0.76 to 0.90. Samples from opposite sides of the Atlantic showed highly significant differences in allelic composition. The results suggest that single locus minisatellite DNA analysis may be of substantial benefit to furthering our knowledge of the population genetics of Atlantic cod.     

A nuclear DNA basis for shelf- and bank-scale population structure in northwest Atlantic cod (Gadus morhua): Labrador to Georges Bank

Variation at five microsatellite DNA loci scored in ≈ 1300 individuals provided evidence of genetic structure among 14 cod ( Gadus morhua ) populations spanning the range of the species in the northwest (NW) Atlantic. Using D _A and D _SW measures of genetic distance, as well as F _ST and R _ST measures of population structure, differences are revealed among populations at continental shelf scales (NE Newfoundland Shelf, Grand Banks, Flemish Cap, Scotian Shelf, Georges Bank) where regions are separated by submarine saddles, channels and trenches. However, we also provide evidence of genetic structure at spawning-bank scales consistent with variation in oceanographic features and in the spatiotemporal distribution of spawning, each of which may represent barriers to gene flow among geographically contiguous populations inhabiting a highly advective environment. The differences described are consistent with postdispersal spawning fidelity to natal areas, a behaviour that may be facilitated by topographically induced gyre-like circulations that can act as retention mechanisms. Significant degrees of substructure among neighbouring and contiguous cod populations may be most easily explained by the associated oceanographic features and processes that conceivably form the template for the evolution of the structure. We suggest that bathymetric and hydrodynamic structure represents a rational starting point for developing hypotheses to examine the processes that lead to the genetic structuring of marine fish species.     

Feeding ecology of silver hake larvae on the Western Bank, Scotian Shelf, and comparison with Atlantic Cod

The feeding ecology of the larvae of silver hake Merluccius bilinearis was examined during two time periods (October 1998 and December 1992) on the Western Bank, Scotian Shelf, north‐west Atlantic, and compared with the feeding ecology of Atlantic cod Gadus morhua larvae collected in the same samples in December 1992. During both time periods silver hake exhibited strong selection for late stage copepodids and adult copepods at a small size (>3·5 mm total length, L _T). The niche width measured as the diet breadth index ( I _DB) of silver hake declined rapidly as they increased in size and remained relatively constant from 3 to 11 mm L _T, during each time period. Atlantic cod larvae exhibited a broader niche width that was curvilinear over the same L _T. Atlantic cod were also less selective than silver hake, incorporating both naupliar and early stage copepodids in their diets throughout the length classes examined. Simple isometric relationships did not explain the differences in diet, as Atlantic cod larvae continued to feed on early stages of copepods at large size, while silver hake larvae quickly switched to large prey items. The strong selection and narrow I _DB observed for silver hake probably reflects adaptation to spawning during the periods between major secondary production peaks in temperate waters.     

GEOGRAPHICAL VARIATION IN METRIC SKULL CHARACTERS AMONG PROPOSED SUBPOPULATIONS AND STOCKS OF HARBOR PORPOISE, PHOCOENA PHOCOENA, IN THE WESTERN NORTH ATLANTIC

A total of 433 skulls of harbor porpoises, Phocoena phocoena , were examined to evaluate geographical variation within the western North Atlantic, using analysis of covariance and discriminant analysis based on 45 measurements of skull components. The relationship between condylobasal length and body length was sigmoid in both sexes. Condylobasal length is generally less in adult males than in adult females.
Despite some overlap, morphological differences between different regions and even adjacent local stocks could be recognized by ANCOVA and discriminant analysis. The percentage of correct classification by discriminant functions was 65.2% for the four regional samples: (1) North Canadian Atlantic (which includes Gulf of St. Lawrence and Newfoundland), (2) Bay of Fundy, (3) Gulf of Maine-New York, and (4) Maryland-South Carolina. It was 65.9% for the four local geographical areas (10s of km) within the Bay of Fundy; Quoddy-region porpoises revealed a closer relationship with the southwestern Nova Scotia population than with other adjacent stocks. The results at this level imply that gene flow is restricted to some degree even among closely adjacent geographical units. Management of this species should aim, therefore, for conservation of a variety of local stocks.     

Microsatellite loci reveal highly significant genetic differentiation among Atlantic salmon (Salmo salar L.) stocks from the east coast of Canada

Allele frequency data from eight microsatellite loci provide evidence of highly significant genetic differentiation among stocks of Atlantic salmon Salmo salar L. from the Bay of Fundy, eastern and north-western Nova Scotia and Newfoundland. Estimates of genetic structure ( R _ST and θ) were significant both among all samples taken from the different geographical locations and among samples from geographical regions for which more than one stock was sampled. Samples from the Bay of Fundy taken from stocks which are phenotypically and behaviourally diverse showed particularly high levels of genetic structure. Rogers', allele sharing and (δμ)² distances also revealed significant differences among stock samples and were significantly correlated [Rogers' and (δμ)²] with sea distance between rivers. Results suggest that stocks of Atlantic salmon in eastern Canada are highly diverse genetically and that this should be an important consideration in any management programme for stocks in the area.     

Evolutionary impacts differ between two exploited populations of northern bottlenose whale (Hyperoodon ampullatus)

Interpretation of conservation status should be informed by an appreciation of genetic diversity, past demography, and overall trends in population size, which contribute to a species' evolutionary potential and resilience to genetic risks. Low genetic diversity can be symptomatic of rapid demographic declines and impose genetic risks to populations, but can also be maintained by natural processes. The northern bottlenose whale Hyperoodon ampullatus has the lowest known mitochondrial diversity of any cetacean and was intensely whaled in the Northwest Atlantic over the last century, but whether exploitation imposed genetic risks that could limit recovery is unknown. We sequenced full mitogenomes and genotyped 37 novel microsatellites for 128 individuals from known areas of abundance in the Scotian Shelf, Northern and Southern Labrador, Davis Strait, and Iceland, and a newly discovered group off Newfoundland. Despite low diversity and shared haplotypes across all regions, both markers supported the Endangered Scotian Shelf population as distinct from the combined northern regions. The genetic affinity of Newfoundland was uncertain, suggesting an area of mixing with no clear population distinction for the region. Demographic reconstruction using mitogenomes suggests that the northern region underwent population expansion following the last glacial maximum, but for the peripheral Scotian Shelf population, a stable demographic trend was followed by a drastic decline over a temporal scale consistent with increasing human activity in the Northwest Atlantic. Low connectivity between the Scotian Shelf and the rest of the Atlantic likely compounded the impact of intensive whaling for this species, potentially imposing genetic risks affecting recovery of this population. We highlight how the combination of historical environmental conditions and modern exploitation of this species has had very different evolutionary impacts on structured populations of northern bottlenose whales across the western North Atlantic.     

Seasonal distribution of juvenile Atlantic cod in the southern Gulf of St Lawrence

Seasonal changes in the geographic distribution of juvenile (ages 1–4) Atlantic cod Gadus morhua were determined from research surveys conducted in the southern Gulf of St Lawrence during 1986–87, 1990–91, and 1994. From late June to early October, juvenile cod were most abundant in shallow water (<50 m deep). By early November, all age groups (adults and juveniles) had migrated to deep (>100 m) offshore waters for the winter. For most juveniles, this represented a minimum migration of >225 km (each way). Age 3 and 4 cod migrated greater distances than age 1 and 2 fish. Some age 3 and 4 cod migrated as far as 650 km from their summer and early autumn areas. Although there was partial geographic segregation of juvenile and adult fish during summer and winter, all age groups were found together during the migration. No cod were present in the shallow waters (<80 m deep) of the south-western Gulf during April due, in part, to later than average ice-melt. Large numbers of all age-classes were present in most of the south-eastern Gulf by mid-May.     

Gene flow and lack of population differentiation in Atlantic cod, Gadus morhua L., from Iceland, and comparison of cod from Norway and Newfoundland

Restriction analysis of mitochondrial DNA was used to study genetic variation and geographic population structure of Atlantic cod from localities around Iceland. Gene phylogenies were constructed and geographic locations superimposed on these. The variation was not localized. Estimated gene flow was large. Thus, Atlantic cod in Iceland belong to a single genetic population. Analyses of published sequence variation of cod from Norway and Newfoundland showed that the extensive continuity of intraspecific phylogeny of cod in Iceland, which is very similar to other marine organisms with a similar life history, extends from Norway to Newfoundland and possibly to larger areas of the Atlantic.     

A hitchhiker’s guide to the Maritimes: anthropogenic transport facilitates long‐distance dispersal of an invasive marine crab to Newfoundland

Aim To determine timing, source and vector for the recent introduction of the European green crab, Carcinus maenas (Linnaeus, 1758), to Newfoundland using multiple lines of evidence. Location Founding populations in Placentia Bay, Newfoundland, Canada and potential source populations in the north‐west Atlantic (NWA) and Europe. Methods We analysed mitochondrial and microsatellite genetic data from European and NWA populations sampled during 1999–2002 to determine probable source locations and vectors for the Placentia Bay introduction discovered in 2007. We also analysed Placentia Bay demographic data and shipping records to look for congruent patterns with genetic analyses. Results Demographic data and surveys suggested that C. maenas populations are established and were in Placentia Bay for several years (c. 2002) prior to discovery. Genetic data corroboratively suggested central/western Scotian Shelf populations (e.g., Halifax) as the likely source area for the anthropogenic introduction. These Scotian Shelf populations were within an admixture zone made up of genotypes from both the earlier (early 1800s) and later (late 1900s) introductions of the crab to the NWA from Europe. Placentia Bay also exhibited this mixed ancestry. Probable introduction vectors included vessel traffic and shipping, especially vessels carrying ballast water. Main conclusions Carcinus maenas overcame considerable natural barriers (i.e., coastal and ocean currents) via anthropogenic transport to become established and abundant in Newfoundland. Our study thus demonstrates how non‐native populations can be important secondary sources of introduction especially when aided by human transport. Inference of source populations was possible owing to the existence of an admixture zone in central/western Nova Scotia made up of southern and northern genotypes corresponding with the crab’s two historical introductions. Coastal vessel traffic was found to be a likely vector for the crab’s spread to Newfoundland. Our study demonstrates that there is considerable risk for continued introduction or reintroduction of C. maenas throughout the NWA.     

Fishing,predation, and temperature drive herring decline in a large marine ecosystem

Since 1960, landings of Atlantic herring have been the greatest of any marine species in Canada, surpassing Atlantic cod and accounting for 24% of the total seafood harvested in Atlantic Canada. The Scotian Shelf‐Bay of Fundy herring fisheries (NAFO Division 4VWX) is among Canada''s oldest and drives this productivity, accounting for up to 75% of the total herring catch in some years. The stocks’ productivity and overall health have declined since 1965. Despite management measures to promote recovery implemented since 2003, biomass remains low and is declining. The factors that drive the productivity of 4VWX herring are primarily unresolved, likely impeding the effectiveness of management actions on this stock. We evaluated potential drivers of herring variability by analyzing 52 time‐series that describe the temporal and spatial evolution of the 4VWX herring population and the physical, ecological, and anthropogenic factors that could affect them using structural equation models. Variation in herring biomass was best accounted for by the exploitation rate''s negative effect and the geographic distribution of fishing and recruitment. Thermal phenology and temperature adversely and egg predation positively impacted the early life stage mortality rate and, ultimately, adult biomass. These findings are broadly relevant to fisheries management, but particularly for 4VWX herring, where the current management approach does not consider their early life stage dynamics or assess them within the ecosystem or climate change contexts.     

Genetic Structure of Capelin (Mallotus villosus) in the Northwest Atlantic Ocean

Capelin (Mallotus villosus) is a commercially exploited, key forage-fish species found in the boreal waters of the North Pacific and North Atlantic Oceans. We examined the population structure of capelin throughout their range in the Canadian northwest Atlantic Ocean using genetic-based methods. Capelin collected at ten beach and five demersal spawning locations over the period 2002 through 2008 (N = 3,433 fish) were genotyped using six polymorphic microsatellite loci. Temporally distinct samples were identified at three beach spawning locations: Chance Cove, Little Lawn and Straitsview, Newfoundland. Four capelin stocks are assumed for fisheries management in the northwest Atlantic Ocean based on meristics, morphometrics, tag returns, and seasonal distribution patterns. Our results suggested groupings that were somewhat different than the assumed structure, and indicate at least seven genetically defined populations arising from two ancestral populations. The spatial mosaic of capelin from each of the two basal cluster groups explains much of the observed geographic variability amongst neighbouring samples. The genetic-defined populations were resolved at Jost’s D _est ≥ 0.01 and were composed of fish collected 1) in the Gulf of St. Lawrence, 2) along the south and east coasts of Newfoundland, 3) along coastal northern Newfoundland and southern Labrador, 4) along coastal northern Labrador, 5) near the Saguenay River, and at two nearshore demersal spawning sites, 6) one at Grebes Nest off Bellevue Beach on the east coast of Newfoundland, and 7) one off the coast of Labrador at Domino Run. Moreover, the offshore demersal spawners on the Scotian Shelf and Southeast Shoal appeared to be related to the inshore demersal spawners at Grebes Nest and in Domino Run and to beach spawners from the Gulf of St. Lawrence.     

Thermal sensitivity of metabolic rates and swimming performance in two latitudinally separated populations of cod, <Emphasis Type="Italic">Gadus morhua</Emphasis> L.

Atlantic cod populations live in a wide thermal range and can differ genetically and physiologically. Thermal sensitivity of metabolic capacity and swimming performance may vary along a latitudinal gradient, to facilitate performance in distinct thermal environments. To evaluate this hypothesis, we compared the thermal sensitivity of performance in two cod stocks from the Northwest Atlantic that differ in their thermal experience: Gulf of St Lawrence (GSL) and Bay of Fundy (BF). We first compared the metabolic, physiological and swimming performance after short-term thermal change to that at the acclimation temperature (7°C) for one stock (GSL), before comparing the performance of the two stocks after short-term thermal change. For cod from GSL, standard metabolism (SMR) increased with temperature, while active metabolism (AMR, measured in the critical swimming tests), EMR (metabolic rate after an exhaustive chase protocol), aerobic scope (AS) and critical swimming speeds (U _crit and U _b–c) were lower at 3°C than 7 or 11°C. In contrast, anaerobic swimming (sprint and burst-coasts in U _crit test) was lower at 11 than 7 or 3°C. Factorial AS (AMR SMR⁻¹) decreased as temperature rose. Time to exhaustion (chase protocol) was not influenced by temperature. The two stocks differed little in the thermal sensitivities of metabolism or swimming. GSL cod had a higher SMR than BF cod despite similar AMR and AS. This led factorial AS to be significantly higher for the southern stock. Despite these metabolic differences, cod from the two stocks did not differ in their U _crit speeds. BF cod were better sprinters at both temperatures. Cod from GSL had a lower aerobic cost of swimming at intermediate speeds than those from BF, particularly at low temperature. Only the activity of cytochrome C oxidase (CCO) in white muscle differed between stocks. No enzymatic correlates were found for swimming capacities, but oxygen consumption was best correlated with CCO activity in the ventricle for both stocks. Overall, the stocks differed in their cost of maintenance, cost of transport and sprint capacity, while maintaining comparable thermal sensitivities.     

Stock-specific changes in growth rates, food conversion efficiencies, and energy allocation in response to temperature change in juvenile Atlantic cod

Under common environments, populations of laboratory reared (Grand Banks, GB and Gulf of Maine, GOM) and wild caught (Fortune Bay, FB and Bonavista Bay, BB; Newfoundland) juvenile cod Gadus morhua responded similarly to temperature change in specific growth rates, food conversion efficiencies, condition factors, liver water content, and muscle water content. However, GOM cod had higher condition factors, and showed differences from GB cod in phenotypic plasticity of hepatosomatic index to temperature. These differences were not present in a different population comparison between FB and BB cod. All populations had higher growth rates and food conversion efficiencies at warmer temperatures, and exhibited compensatory growth when temperature was increased. The results suggest relatively larger genetic differences between GB and GOM cod than between FB and BB cod, and indicate that the faster growth of southern populations in the wild is not due to a higher genetic capacity for growth rate.     

Genetic and environmental components of phenotypic variation in body shape among populations of Atlantic cod (Gadus morhua L.)

A common-garden experiment was conducted on larvae to test for genetic differences in body shape among populations of Atlantic cod ( Gadus morhua ). Offspring from four north-west Atlantic regions were reared from hatching to postmetamorphosis at two temperatures (7 ± 1 °C and 11 ± 1 °C) and two food levels (1500 and 4500 prey L⁻¹). Body shape differed between populations and treatments. Population differences were greatest between south-west Scotian Shelf cod and those further north; the former were characterized by a deeper body, larger head, and longer caudal peduncle than cod from the other populations. Significant differences were also observed between two putative populations on the south-west Scotian Shelf, suggesting genetic divergence between spawning aggregations at small spatial scales (< 100 km). Temperature and food supply also influenced body shape, with the effect of the former being more pronounced. Individuals reared at the higher temperature or food level had a deeper body and a larger head than those reared at the lower temperature or food supply. Phenotypic responses to changes in the rearing environment also differed among populations, indicating genetic differences in phenotypic plasticity. Differences between populations in morphology and in phenotypic plasticity suggest genetic divergence at both large (> 1000 km) and small (< 100 km) spatial scales. The genetic differences at large spatial scales counteracted the expected effects of temperature differences in the wild, suggesting countergradient variation in morphology among these populations.  © 2006 Her Majesty the Queen in Right of Canada. Journal compilation © 2006 The Linnean Society of London, Biological Journal of the Linnean Society , 2006, 88 , 351–365.