Identifying meso-scale spatial patterns from fishing vessel trajectories

BOGAZZI, E.; PARMA, A.M.; ORENSANZ, J.M.

Sarasota, USA

Simposio; The Seventh William R. and Lenore Mote International Symposium in Fisheries Ecology. The Spatial Dimensions of Fisheries: Putting It All in Place; 2008

Florida State University and Mote Marine Laboratory

The growing availability of positional data through vessel monitoring systems and observer programs creates opportunities to capture the fishing process at an appropriate spatial scale. This is essential for the analysis of exploitation and depletion of sedentary resources, where vessel movements may result in hyperstable catch rates, and depletion may not be apparent unless one zooms in. Decisions about where to fish involve the identification of promising fishing destinations or ‘fishing opportunities’ (FO) from two kinds of fishing operations: exploration and effective fishing. In highly aggregated stocks typical of sedentary species, FOs appear as relatively small and intensely fished areas. We explored alternative methodologies for the identification and mapping of FOs from the analysis of individual vessel trajectories using the Patagonian scallop off-shore trawl fishery (Argentina) as a study case. Distance between tows (nearest neighbor methods), cluster analysis and tortuosity of vessel trajectories allowed discrimination of exploratory tows from effective fishing events. The intensity function of effective fishing tows was then used to map the spatial distribution of FOs. Analyses of catch rates within individual FOs indicated that even at that small scale depletion was not always detectable. A  stitching” pattern of hardly overlapping tows was observed when zooming into the FOs. Modern positioning technology used by industrial vessels is imposing ever more serious limitations on the use of catch rates for the analysis of abundance of sedentary stocks.