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Bay cod, Atlantic cod (Gadus morhua) that over-winter in the deep-water bays of north-eastern 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 north-eastern 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.

During three consecutive years of observation 23 Atlantic cod (Gadus morhua) were tagged with ultrasonic transmitters as part of two experiments to determine the movement patterns of adult cod in the genetically distinctive population inhabiting Gilbert Bay, Labrador. Individual cod were relocated for periods up to 15 months, indicating year-round residency within the bay despite unimpeded access to the open ocean. The tracking data show that individual cod have a strong homing tendency. In experiment 1 displaced cod returned (homed) to their place of capture in a small arm separated from the rest of the bay by a shallow sill while non-displaced fish remained at their capture site. In experiment 2 some cod remained within 2 km of their capture/release site, a spawning area, while others ranged widely in Gilbert Bay before returning to this site. Whether an individual ranged widely or had more restricted movements was not related to size. Cod exhibited wider ranging movements in the spring and early summer, following spawning, than in the late summer and early autumn. The return of Gilbert Bay cod to specific locations in the inner part of the bay to overwinter and spawn is evidence of one mechanism that could have led to their genetic distinctiveness.

No abstract

Mathematical equations describing marine plankton dynamics are solved for the climatological oceanographic conditions during the month of May in the North Atlantic. Geographical distributions of phytoplankton, zooplankton and limiting nutrient (nitrate) concentration in the surface mixed layer of the ocean are predicted from historical mixed layer depth and the total nutrient made locally available to the plankton ecosystem by convective mixing. The effects of major ocean currents are parameterized in the model through the geographic distribution of nitrate and its vertical gradient. Major upwelling and downwelling circulations control the proximity of high nutrient concentrations to the surface. Model solution of the phytoplankton field with 1° longitude and latitude grid resolution is compared to a recently produced composite of Coastal Zone Color Scanner images of surface chlorophyll in the North Atlantic during May 1979 [W.E. Esaias et al., 1986]. Large‐scale chlorophyll patterns seen in the CZCS composite can be explained as transition zones in the supply of plant nutrients to the surface layer by vertical mixing or localities of light limitation of phytoplankton growth by a deep mixed layer.