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The condition of Atlantic cod Gadus morhua L. larvae reared in densities of 0, 2, 5 and 10 zooplankton ml-l was examined after the transition to exogenous feeding. Fulton's K (weight/length3) was not correlated with prey density in which larvae were reared, indicating that this index of condition is inappropriate for larvae immediately after yolk sac absorption. Fulton's K was also not positively correlated with the ability to avoid a simulated predator. However, predation avoidance was positively correlated with higher prey density. A second measure of condition, body height standardized for length, appeared to be a more sensitive indicator of condition but also did not correlate with the ability of larvae to avoid predation. The results of a buoyancy experiment indicated that poorly fed larvae may occur higher in the water column, and thus may be more vulnerable to predation or cause a sampling bias in ichthyoplankton studies.

The role of the ectoparasite Caligus sp. (Copepoda: Caligidae) in the northeast Georges Bank cod-haddock ecosystem was examined. Vertical distribution of free-living Caligus elongatus adults and host-parasite relationships of juvenile CaLigus sp. and juvenile gadids are described at 2 locations with contrasting oceanographc properties, one thermally stratified and the other well-mixed. Cod Gadus morhua had both greater prevalence and number of Caligus sp. ectoparasites than did haddock Melanogran~mus aeylefinus at both locations. Preferred sites of attachment on the host also differed. While no direct evidence of reduced fish condition as a function of parasite infestation was found, circumstantial evidence is offered in support of the hypothesis that Caligus sp. ectoparasitism is a source of mortality for young haddock. Free-living C. elongatus were demonstrated to b e a significant component of fishes' diet, particularly for cod at the stratified site where zooplankton were less abundant.

No abstract

Freeze resistance of eggs and larvae of Atlantic cod (Gadus morhua) from the northern cod stock was investigated to determine whether ice contact could affect survival during the spring spawning season off Newfoundland. Egg and larval homogenates did not appear to contain antifreeze proteins (mean freezing points −0.78 and −0.88 °C, respectively). However, cod eggs did not freeze at −1.8 °C in icy aerated seawater, could be undercooled to −4.0 °C in ice, and froze at temperatures between −4.1 and −1 7 °C; freeze resistance depended on the integrity of the chorion. Larvae withstood undercooling to −1.8 °C, provided they were not brought into direct contact with ice crystals, if directly touched with ice, larvae froze at −1.36 °C (feeding stage) or −1.34 °C (yolk-sac), approximately 0.5 °C lower than would be expected from the freezing temperatures of their body fluids. The nature of their external epithelium and delayed development of sensitive gill structures below 0 °C may contribute to larval freeze resistance. Cod eggs and larvae are found in spring off Newfoundland and Labrador, when sea temperatures can be as low as −1.8 °C and ice cover extensive. While cod eggs are remarkably freeze resistant, such environmental conditions may cause freezing mortalities in larval cod.