Exploratory analysis of the relationship between swordfish capture and environmental features in the southwest Indian Ocean

Oceanic circulation plays a major in the distribution of nutrients and oceanic structures such as fronts and eddies may become hotspots of biological activity through concentration and enrichment processes. Oceanic structures generally attract forage fish and cephalopods and may therefore be targeted by marine top-predators. The link between swordfish (Xiphias gladius) captures and environmental structures is poorly documented in the southwest Indian Ocean despite the growing need of the local fishery from Reunion Island for such information. In this study we used a set of temporal (date, moon phase), geographic (longitude, latitude, distance to coast, bathymetry) and environmental covariates (sea surface temperature, chlorophyll-a concentration, sea level anomalies, index of convergence) to explain variations in swordfish catch per unit of effort (CPUE) throughout 2012. Univariate analyses show strong seasonal and latitudinal patterns in swordfish occurence. We also found that higher swordfish CPUE are associated with shelf-break areas and sea mounts, as well as dynamic environmental structures, even though these relationships explain only a small fraction of swordfish CPUE variations. Chlorophyll-a fronts and dynamic convergent structures as identified by finite-size Lyapunov exponents might attract swordfish.