AN EXPERIMENTAL-STUDY OF GILL NET AND TRAMMEL-NET GHOST FISHING OFF THE ALGARVE (SOUTHERN PORTUGAL)

Four 100 m lengths of both monofilament gill nets and trammel nets wer e deployed at depths between 15 and 18 m off the coast of the Algarve (south of Portugal) between April 1995 and June 1996. The nets were se t on a natural rocky bottom with one end cut loose to simulate lost ne ts. Changes in net structure (net height, effective fishing area, move ment, colonisation, wear and tear) and their catches (species, sizes, numbers, and biomass) were monitored by divers. Similar patterns were observed in all the nets, with a sharp decrease in net height and effe ctive fishing area, and an increase in visibility within the first few weeks. Net movement was negligible except in the case of interference from other fishing gears. Catch rates were initially comparable to no rmally fished gill nets and trammel nets in this area, but decreased s teadily over time. No sea birds, reptiles or mammals were caught in an y of the 8 nets. Catches were dominated by fish (89 % by number, at le ast 27 species), in particular by sea breams (Sparidae) and wrasses (L abridae). Under the conditions experienced throughout the study the fi shing Lifetime of a 'lost' net is between 15 and 20 wk. Based on an ex ponential model, we estimated that 100 m lengths of gill net and tramm el net will catch 314 and 221 fish respectively over a 17 wk period. H owever, we consider this to be an underestimate due to high rates of p redation and scavenging by octopuses, cuttlefish, moray eels, conger e els, and other fish such as the wrasse Coris julis. When the nets were surveyed in the following spring, 8 to 11 mo after being deployed, th ey were found to be completely destroyed or heavily colonised by algae and had become incorporated into the reef.