Fishing up and down the marine food web with hydroacoustics

We have been using scientific echo-sounders to map and quantify Engraulis anchoita in the Southern coast of Brazil. This is a low trophic level species and a link from primary producers to top predators. Echo-records collected along thousands of nautical miles contain information about migratory habits and abundance of this fish. Fishing down the food web is a late 90s concept that called international attention to the fact that large predator fishes were gradually less common and also that smaller fishes, which have been caught for many years to the reducing fish meal industry, should now contribute directly to human diet. Along the Anchovy hydroacoustical assessment cruises we mapped adult as well as juvenile fish. Recently, small, ten to twenty meters long coastal purse seiners, equipped with echo-sounders and multibeam sonars are catching young Anchovy which is used as live bait by an oceanic Pole and Line six boats fleet to catch Skipjack (Katsuwonus pelamis), a top predator. Catch rate from live anchovy to Bonito is about 1:25, much better than fresh fish to fishmeal. Hydroacustics is an integral tool along the whole up and down the food web catching processes and without it such activity would not be economically feasible, just considering for example, fuel costs. However, looking down the sea surface with ultrasound is not all these days in modern fishing fleets. Boats are receiving very high resolution satellite data that describe sea surface characteristics. Also, sub surface data from large oceanographic modeling programs predict water movements at depth and daily maps are sent to the fishing fleet as well. Fishermen are becoming as dependent on satellite data to find fish as they already are with hycroacoustics. Furthermore, drifting echo-sounders move free around fishing grounds and digital echograms can be checked on line from the boats or the fleet headquarters. This paper intends to review the hydroacoutics and satellite systems involved within the catching process of low and high trophic level fishes along the southernmost coast of Brazil.