Homing, population structure and management of Atlantic cod (Gadus morhua), with emphasis on spawning at Bar Haven in Placentia Bay, Newfoundland

Robichaud, Dave (2001) Homing, population structure and management of Atlantic cod (Gadus morhua), with emphasis on spawning at Bar Haven in Placentia Bay, Newfoundland. Doctoral (PhD) thesis, Memorial University of Newfoundland.

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Abstract

I have assessed the potential for small-scale population structure within a cod stock on the south cost of Newfoundland, since decreases in productivity can occur if managers fail to match the scale of the management unit to that of the population. A group of cod (Gadus morhua) that spawns at the Bar Haven ground (Placentia Bay. Newfoundland) were studied in detail. I examined the homing of adults, and the retention of spawning products. Over three consecutive spawning seasons, all relocations of spawning cod, tagged acoustically at Bar Haven, were within 10 km of the tagging site, the majority within a few hundred meters. No tagged fish were relocated at other known spawning grounds or elsewhere in the bay. Navigation while homing was most likely towards an omnidirectional attractor at the spawning ground that dissipates with distance, such as a characteristic sound or geophysical signature. Movements during spawning seasons were sex-specific, and suggested that females move in and out of male-dominated spawning aggregations. Local retention of eggs and larvae was observed, but was greater in warmer water, in which eggs and larvae develop faster, thus settling before drifting with currents out of the bay. Given exacting homing, and local retention, there is a strong possibility that population sub-structure exists within Placentia Bay. However, a review of the literature shows that cod migratory behaviour ranges from sedentary to highly migratory, and no behaviour is limited to inshore or offshore environments, or to any part of the North Atlantic range. -- Although management of cod at Bar Haven may benefit from recruitment predictions resulting from simple age 0 cod surveys, predictions more quantitative than a ranking of year-class strengths were complicated by density-dependent site-use. However, important, temporally stable nursery grounds were recognisable within the bay. Acoustic assessment of Bar Haven spawners was complicated by high rates of turnover of individuals within a spawning season. Thus, acoustically determined abundance estimates from serial surveys must be adjusted to account for the proportion of individuals present during more than one survey. -- Overall, I review the diversity of cod migratory behaviours, and illustrate the potential for small-scale population structure, specifically where cod perform precise homing migrations and eggs are retained near spawning areas. I discuss how managers can use information about population structure to hinder local depletions and to help avoid overall reductions in productivity.

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