Density-dependent Changes in the Spatial Distributions of Atlantic Cod (Gadus morhua), American Plaice (Hippoglossoides platessoides), and Greenland Halibut (Reinhardtius hippoglossoides) on the Flemish Cap during 1988–2002

Abstract

The Flemish Cap is an isolated bank located east of the Grand Banks in the Northwest Atlantic Ocean. Research bottom trawl surveys of the Flemish Cap have been conducted in July by the European Union since 1988. Data from these surveys were used to assess changes in the spatial distributions of Atlantic cod (Gadus morhua), American plaice (Hippoglossoides platessoides), and Greenland halibut (Reinhardtius hippoglossoides) during 1988–2002. Kriging and indices of collocation and co-occurrence indicated that cod and plaice distributions have a high degree of overlap and that density-dependent range contraction of both stocks occurred during a time when Greenland halibut, a more common inhabitant of deep water, exhibited range expansion into shallow water on the Cap. However, the timing and locations of these distributional changes were such that little overlap occurred between the distributions of Greenland halibut and either cod or plaice. The range expansion of Greenland halibut was also density-dependent and primarily attributable to young fish (ages 1 and 2 from the 1994 and 1995 year-classes) which prefer shallow water. The range contractions of cod and plaice have persisted since the mid-1990s, and despite fishing moratoria implemented for plaice in 1994 and for cod in 1999, neither stock has shown signs of recovery. Biological and stock assessment implications of these distribution changes are discussed in relation to the lack of recovery.