Seasonal variation in abundance and stock composition of Atlantic cod (Gadus morhua L.) in Placentia Bay, Newfoundland, in relation to fisheries

Atlantic cod (Gadus morhua) in Placentia Bay, Newfoundland, exhibit marked seasonal variation in abundance, age composition and size distribution. A theory is developed to account for these variations. To test it, surveys of distribution, abundance and biological traits of cod were conducted seasonally from fall 1998 to spring 2000 and commercial fishery data were gathered. For the main year of this study in 1999, acoustic estimates of abundance were 4.5 million fish in April, mostly spawning and older cod (ages 7-9). Abundance increased four-fold in July, comprised mostly of migrant smaller and younger cod (ages 4-6). By October abundance decreased (5.6 million fish) as summer migrants left the bay, but increased again in November to 6.5 million fish when mostly larger resident fish that moved southward during summer returned to the inner bay where part of the population overwinter. The observed seasonal variability in stock abundance, age, size composition and condition indices (K, HSI and GSI) were consistent with the expectations of the theory. Information on commercial catch rates and landings suggested that resident and non-resident cod were targeted by the fishery in different proportions. Estimated harvest rate reached 33% in November 1999 for resident cod. Results of this study emphasize the need for smaller-scale management strategies that take into consideration seasonal changes in the availability of various stock components.