A hypothesis of a redistribution of North Atlantic swordfish based on changing ocean conditions

• Published in: Deep Sea Research Part II: Topical Studies in Oceanography
• Main Authors: Schirripa, M.J., Abascal, F.J. (Francisco Javier), Andrushchenko, I. (Irene), Díaz, G., Mejuto-García, J. (Jaime), Ortiz, M., Santos, M.N. (Miguel Neves), Walter, J.
• Format: Article in Journal/Newspaper
• Language: English
• Published: Centro Oceanográfico de Canarias 2017
• Subjects: Abundance indices; Environmental factors; Stock distribution; Swordfish; fish; deep water; marine fisheries; fisheries; research; North Atlantic
• Online Access: http://hdl.handle.net/10508/11117; https://doi.org/10.1016/j.dsr2.2016.08.002

Conflicting trends in indices of abundance for North Atlantic swordfish starting in the mid-to late 1990s, in the form of fleet specific catch-per-unit-effort (CPUE), suggest the possibility of a spatial shift in abundance to follow areas of preferred temperature. The observed changes in the direction of the CPUEs correspond with changes in trends in the summer Atlantic Multidecadal Oscillation (AMO), a long term mode of variability of North Atlantic sea surface temperature. To test the hypothesis of a relation between the CPUE and the AMO, the CPUEs were made spatially explicit by re-estimating using an “areas-as-fleets” approach. These new CPUEs were then used to create alternative stock histories. The residuals of the fit were then regressed against the summer AMO. Significant, and opposite, relations were found in the regressions between eastern and western Atlantic areas. When the AMO was in a warm phase, the CPUEs in the western (eastern) areas were higher (lower) than predicted by the assessment model fit. Given the observed temperature tolerance limits of swordfish, it is possible that either their preferred habitat, prey species, or both have shifted spatial distributions resulting in conflicting CPUE indices. Because the available CPUE time series only overlaps with one change in the sign of the AMO (~1995), it is not clear whether this is a directional or cyclical trend. Given the relatively localized nature of many of the fishing fleets, and the difficulty of separating fleet effects from changes in oceanography we feel that it is critical to create CPUE indices by combining data across similar fleets that fish in similar areas. This approach allowed us to evaluate area-specific catch rates which provided the power to detect basin-wide responses to changing oceanography, a critical step for providing robust management advice in a changing climate.