Seasonal biological cycles in Atlantic cod (Gadus morhua) and implications for fisheries and management: a simulation approach with application to the Placentia Bay cod fishery (NAFO subdivision 3Ps)

Atlantic cod (Gadus morhua) is a demersal fish found across the North Atlantic Ocean in a variety of habitats from the shoreline to the continental shelf slope. Throughout its range of distribution, cod experience a great variety of biotic and abiotic conditions, particularly in relation to thermal and feeding regimes. Such variations affect life history characteristics such as growth, physiological condition, distribution and migration patterns at a variety of scales including decadal, annual and seasonal. In this study I investigate how these processes vary on a seasonal scale level and affect fisheries, stock assessment and management of cod from Placentia Bay, Newfoundland (Northwest Atlantic Fisheries Organization Subdivision 3Ps). The results of this study show that cod have marked seasonal variability in traits such as weight, physiological condition, growth, distribution and aggregations patterns, in addition to the commercial yield and quality of fish products. These changes were related to changes in thermal and feeding regimes through the year, in addition to spawning, migration and intermixing of cod from different geographic regions. Cod ages 4-9 experienced a rapid increase in weight and condition during late spring and summer when capelin (Mallotus villosus) comprised an important component of the diet, despite of cold water temperatures and moderate to high spawning activity and peaked in the fall. Seasonal variations of biological cycles, distribution and mixing of different groups of cod resulted in large within-year variations in stock abundance, age and size composition, impacted fishing levels and harvest rates of putative stock components and affected precision of abundance index estimates. Simulation results suggest that stock performance and productivity are impacted by the way fishing mortality is distributed across age groups and that stock growth and catch yield are driven by the survival of younger fish and by allowing age diversity in the stock, which appears to facilitate good recruitment, particularly when abundance is high. The simulations show that a weight-based fishery in the fall when cod are heavier and in good physiological condition would harvest fewer fish and result in better yield and product quality. However, a fall fishery would concentrate exploitation on the resident component of the stock. The largest sustainable catches were observed in summer when the most abundant non-resident fish are found in the bay. Overall, the results and conclusions of this thesis suggest that seasonal biological patterns in cod may be used to develop fishing and management strategies that minimize the impact of harvesting on productivity while optimizing economic benefits and conservation of stock components. The results from this study are likely relevant to other cod stocks and perhaps to other species (e.g., invertebrates, marine mammals) as seasonality is a common feature of reproduction and growth of many temperate and high latitude aquatic species, which are normally synchronised with periods when organisms benefit from favourable thermal conditions and high forage status.

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