A drift study of larval fish survival

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Abstract
The early survival of capelin (Mallotus villosus) was studied by tracking a cohort of larvae and monitoring the decline in their abundance during the transition from yolk-sac nutrition to exogenous feeding. A free-drifting drogue equipped with temperature and salinity probes was launched in the surface layer of the lower St. Lawrence estuary [Canada]. Larval capelin abundance around the drogue was measured at 2 h intervals for 5.6 d. The T-S (temperature-salinity) diagram for the duration of the experiment revealed that the drifter crossed 7 distinct water masses. Capelin larvae transported by each of the different water masses were considered as distinct subpopulations. One water mass was monitored for a period of 46 h beginning on the second day following the final resorption of the vitelline reserves in 50% of the subpopulation. Over these 46 h, capelin abundance decreased from 34 larvae m-3 to 5 larvae m-3. Progressive mixing of the surface layer with deep water containing no larvae explained 5.6% of the reduction in larval density. The decline in capelin number was adequately described by an exponential-decay model with mortality constant over time. The estimated hourly mortality rate was 0.0339 h-1 (SD = 0.0061 h-1) corresponding to a daily survival rate of 43.7% d-1. A logistic model reflecting the sigmoid survival curve proposed by Hjort (1914, 1926), also fitted the observations, and predicted more realistic values of capelin density at emergence than the constant exponential decay model. Partial results from the other water masses confirmed that capelin mortality was high at the resportion of the yolk sac. The isolation of capelin larvae into subpopulations transported by independent ecological units (the distinct water masses) suggested that meso-scale (1 to 10 km) spatial variations in survival could be important.

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