Historical fishing regimes uncover deep-sea productivity hotspots in the SW Atlantic Ocean

Abstract

In the deep-sea, abundant and diverse biological communities tend to occur in areas where combinations of geological, physical and biological processes locally enhance trophic-wide productivity, sustaining aggregations of consumers and top predators. These areas are ‘productivity hotspots’ and their delimitation should improve the effectiveness of area-based strategies designed to manage human activities and protect the deep-sea. We explored the premise that fishing operations in association with geomorphological features are effective surrogates for delimiting productivity hotspots distribution in the Brazilian Meridional Margin (BMM; 18°S - 35°S; 200 – 3300 m). We described along-slope and across-slope geoforms and related them with catch patterns of fishes, deep-sea shrimps, deep-sea crabs and squids in 23565 fishing hauls conducted between 2000 and 2007. Over 97% of the total catch was obtained in Santos (42.7%) and Pelotas (54.8%) sedimentary basins. In the former, 55.7% of all fishing hauls and 38.6% of the catch were obtained from one of 12 slope geoforms, which covered 22% of the available fishing area. In the Pelotas basin, a shelf break geoform covering 6% of the fishing area concentrated 29.5% of the fishing hauls and 30.2% of the catch. Best fit delta-lognormal Generalized Linear Models (explaining >50% of the total variance) highlighted the positive effect of shelf break embayments of Santos Basin in occurrence and abundance of the pool of species targeted by fisheries. Within these embayments, sectors incurvated and excavated by erosive action of the Brazil Current positively affected abundance and occurrences of fishes and deep-sea shrimps, respectively. The upper slope (300 – 500 m depths) were particularly favorable for fishes and squid concentrations, whereas the lower slope (600 – 800 m depths) increased the probability of deep-sea shrimp catches. We propose that the Santos Basin shelf break embayments host mesoscale and sub-mesoscale productivity hotspots, sustained by biophysical processes promoted by the Brazil Current flow, coupled with biological active transport of pelagic and demersal vertical migrators, some of them targeted by bottom fisheries. We advocate that these are unique features, in the otherwise oligotrophic SW Atlantic subtropical gyre, that should be taking into consideration in regional systematic conservation plans.