Density-independent and density-dependent factors affecting spatiooral dynamics of Atlantic cod (Gadus morhua) distribution in the Gulf of Maine

Abstract

Due to strong spatial interplays between intraspecific interactions and environmental forcing, both density-dependent and densityindependent processes can affect spatiooral dynamics of fish populations in a spatially explicit fashion. To this end, this study investigated the underlying mechanisms of spatiooral dynamics of Atlantic cod (Gadus morhua) in the Gulf of Maine (GoM). Based on the data from the Northeast Fisheries Science Center (NEFSC) bottom-trawl surveys in spring and fall from 1982 to 2013, empirical cumulative distribution function (ECDF) curves and geographic distribution indices were used to examine the species-environment and abundance-occupancy relationship, respectively. Then, a variable-coefficient generalized additive model was constructed to quantify the simultaneous effects of environmental variables and population size on the spatiooral dynamics of cod distribution. Area occupied remained relatively high through the late 1990s, but underwent a pronounced contraction into the western GoM (WGoM) for the reminder of the time-series. The model results suggest that the spatiooral dynamics of GoM cod have been driven by complex interactions of density-dependent and density-independent factors over the past three decades. Better knowledge of these dynamics can improve our understanding of the causality of abundance-occupancy and species-environment relationships and help to reduce error estimates for survey-based indices.