When assessed with light microscopy, daily increment formation did not appear to occur in the otoliths of known-age larval herring (Clupea harengus). Increment counts underestimated age in all larvae. The age-increment discrepancy increased curvilinearly with age and appeared to stabilize after 50–60 d. Both the magnitude and rate of increase of the discrepancy were consistent with a hypothesis of resolution-limited increment visibility; models of daily otolith growth indicated that discrete daily growth increments would not be resolvable with a light microscope for the first 15–20 d after hatch. The hypothesis was also consistent with the observed effects of otolith polishing, a systematic difference in increment counts between different-sized sagittae in the same larvae, and other published reports of apparent nondaily increment formation in slow-growing pelagic larvae. Previous reports of growth rate limited increment formation appear to provide an empirical description of the same phenomenon. Otolith-based age, growth and mortality estimates can be expected to be biased if resolution effects are ignored. However, various procedures are available for the identification of potentially sensitive species and samples.

The winter/spring vertical distributions of polar cod, copepods, and ringed seal were monitored at a 230-m station in ice-covered Franklin Bay. In daytime, polar cod of all sizes (7-95 g) formed a dense aggregation in the deep inverse thermocline (160-230 m,-1.0 to 0°C). From December (polar night) to April (18-h daylight), small polar cod \25 g migrated into the isothermal cold intermediate layer (90-150 m,-1.4°C) at night to avoid visual predation by shallow-diving immature seals. By contrast, large polar cod (25-95 g), with large livers, remained below 180 m at all times, presumably to minimize predation by deep-diving mature seals. The diel vertical migration (DVM) of small polar cod was precisely synchronized with the light/dark cycle and its duration tracked the seasonal lengthening of the photoperiod. The DVM stopped in May coincident with the midnight sun and increased schooling and feeding. We propose that foraging interference and a limited prey supply in the deep aggregation drove the upward redistribution of small polar cod at night. The bioluminescent copepod Metridia longa could have provided the light needed by polar cod to feed on copepods in the deep aphotic layers.

No abstract

Abstract.We investigated the impact of neurotoxins produced by the dinoflagellate Alexandriurn excavatum on survival of Atlantic mackerel (Scomber scombrus) and American lobster (Homarus americanus) larvae, respectively reared from eggs and from female lobster, collected in 1988 from the southern Gulf of St. Lawrence, Canada. Sensitivity to the toxins was first verified by exposing larvae of both species to various concentrations of toxic A. excavatum (treatment) and non-toxic A. tamarense (control). Daily mortality rates ranged from 65 to 96% among mackerel larvae directly fed upon toxic cells and reached 36% in postlarvae exposed to toxic microzooplankton. Lobster larvae were apparently immune to the toxins, which they concentrated up to five times relative to vector toxicities. Bioassays conducted on mackerel larvae by exposure to natural plankton samples collected in situ during a bloom of toxic A. excavatum confirmed that exposure to the toxins could also have lethal effects in natural ecosystems. We conclude that the current proliferation of toxic dinoflagellates threatens early survival of finfish larvae and their recruitment to adult populations.

The relative success of the 1985 spring and fall cohorts of Atlantic herring (Clupea harengus) in the St. Lawrence estuary was studied in relation to food availability (match/mismatch hypothesis) and hydrography (member/vagrant hypothesis). The development of the spring cohort (early June) matched remarkably well the development of suitable prey and larval abundance within the estuary decreased slowly (7.8%∙d−1). The fall cohort (mid-September) hatched in a period of low food availability and dwindled rapidly (28.9%∙d−1). The initial growth of both cohorts appeared limited by food. The stratification front limiting seaward drift was strong in the spring (0–80 j∙m−3) and weak in the fall (0–20 J∙m−3), suggesting that advection out of the estuary could also have contributed to the observed differences in the fate of the two cohorts. We conclude from this case study that spawning times and locations may have evolved to favor the initial cohesion of larval fish cohorts by limiting dispersion and that both transport and energetic processes can influence the success of a population at colonizing its dispersion area. The relative contribution of initial vagrancy and subsequent trophic interactions in determining final recruitment remains to be assessed.