Density of juvenile cod in coastal Newfoundland habitats : multiscale analyses of spatial and temporal variation

Methven, David Alan (1997) Density of juvenile cod in coastal Newfoundland habitats : multiscale analyses of spatial and temporal variation. Doctoral (PhD) thesis, Memorial University of Newfoundland.

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Abstract

This thesis examines the relative importance of small-scale spatial and temporal variation in density of coastal populations of juvenile cod, Gadus morhua and how identification of this variation may contribute to a better understanding of the early life history and to the design of better sampling programs. This thesis also considers sampling variation due to fishing equipment and species identification. Populations of marine fish exhibit strong variation in yearclass strength. Hence annual variation in density was expected to exceed variation at monthly and hourly scales. Contrary to expectation, temporal variation was highest at a monthly scale for two intensively sampled sites, and was attributed to cod settling in coastal habitats in pulses during May-June, August-September, and after mid October. Settlement in May-June and after mid October was due solely to G. morhua. Settlement in August-September was by G. morhua and G. ogac, a sibling species that is presently difficult to identify at <50 mm. Larger cod G. morhua (87-135 mm) identified in this study by electrophoresis and discriminant function analyses had larger eyes and were not as deep and heavy-bodied as G. ogac. Highest spatial variation occurred at the scale of individual sites and was consistent with the expectation that cod were selecting specific habitats. Cod density, however, varied markedly at the same sites from year to year, indicating factors other than just habitat selection were determining local density. Variation in density of juvenile cod decreased with increasing size indicating older individuals were more evenly dispersed than younger cod. Small-scale spatial and temporal changes in catch were independent of fishing equipment. Density of juvenile cod was higher at night than during the day and higher at 4-7 m than at greater depths. These observations establish that the coastal zone represents the centre, and not the edge, of the distribution of age 0 cod during autumn. Nursery areas in the early 1990s, a time of low stock biomass must be identified as the coastal zone, not offshore.

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