Using Environmental Dynamics to Create Spatial Structure within a Single Area Stock Assessment

The recent stock assessment of north Atlantic swordfish was faced with the issue of conflicting indices of abundance within the various commercial longline fisheries catch per unit effort (CPUE) time series. In an effort to resolve these conflicts, the assessment integrated spatial partitioning in fishing effort and environmental dynamics, coupled with time varying catchability, to create psudo-spatial structure within the one area stock assessment model. Rather than by fleet, as is usually done, CPUEs were estimated by fishing area. Residuals of the fit to the CPUEs were regressed against the AMO and spatial patterns emerged. The model was used to test the hypothesis that swordfish distribution within the northern Atlantic Ocean is not random, but rather correlated to the phase of the Atlantic Multidecadal Oscillation (AMO). Inclusion of the AMO as an environmental driver of area specific CPUEs resulted in a significantly better fit to the conflicting data while supporting the hypothesis that swordfish densities (and thus catchability) can undergo area specific changes that are synchronous with the phase of the AMO. Given the observed temperature tolerance limits of swordfish, it is possible that their either their preferred habitat has recently moved north, a preferred prey species, or both.