Annual changes in spatial aggregation of Atlantic cod, Gadus morhua, age classes in NAFO Divisions 2J3KL and 4RS (1978-1994)

Bennett, Elizabeth Jane (2008) Annual changes in spatial aggregation of Atlantic cod, Gadus morhua, age classes in NAFO Divisions 2J3KL and 4RS (1978-1994). Masters thesis, Memorial University of Newfoundland.

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Abstract

Geographic contraction of range can be explained by using a basin model (MacCall 1990) which predicts that density-dependent populations will tend to contract toward the most favorable habitat as the population collapses. Literature has suggested that the distribution of demersal species such as Atlantic cod, Gadus morhua (Swain 1993) may be a product of the basin model phenomena. In this thesis I examine change in Atlantic cod aggregation during stock decline as it depends on age class, in order to better understand the spatial dynamics of the collapse of the Atlantic cod stocks in both NAFO (Northwest Atlantic Fisheries Organization) divisions 2J3KL (northeast Newfoundland Shelf) and 4RS (Northern Gulf of St. Lawrence), and explore how the basin model and density-dependent habitat selection relate to the change in aggregation into restricted areas during the collapse of Atlantic cod. -- I use research vessel data which was collected by Department of Fisheries and Oceans (1978-1994) in NAFO divisions 2J3KL, and 4RS to identify common characteristics of the spatial dynamics of the cod collapse. I explore the distributional shifts and change in aggregation of Atlantic cod age classes (≤2-12+ yrs) and age groups (ages ≤2, ages 3-4 ages, 5-6 and age 7-12+) from 1978 to 1994, by using distributional maps, the Gini index, the variance, the skewness of the frequency distribution, the exponent of power law, and the shape of cumulative frequency distribution. -- My results show that the most distinct measure of aggregation was the Gini index in combination with the distribution maps. These measures show that older cod peaked in aggregation while collapsing into fewer locations in the early 1990s and that the cod distribution collapse into reduced areas was specific to older age classes of Atlantic cod, with a lesser degree of spatial contraction in younger age classes and little or no contraction in the youngest age classes and variable rather than fixed locations of aggregation. Thus I offer an alternate explanation for the collapse into restricted areas, one that does not depend on habitat selection, as in the basin model. The alternative explanation is that each year reproductively mature cod aggregate to spawn and that as the population shrinks the number of spawning locations shrinks but not necessarily at the same site each year. Patterns of aggregation in this study have important implications for these stocks and that aggregative behavior by age group should be monitored and displayed for all cod stocks. -- Spatial structure and degree of aggregation should be monitored and displayed for all demersal species, not just for management of pelagic species. In this instance, when evaluating cod aggregation during a collapse a measure of aggregation that performs well, the Gini index should be considered in science advice to management.

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