Detection distances of bottom-set gillnets by harbour porpoises (Phocoena phocoena) and bottlenose dolphins (Tursiops truncatus)

Many odontocetes die annually in gillnet fisheries. Why they become entangl ed is not yet clear. Maybe some species detect the nets too late to avoid c ollision. Therefore, the target strength of 11 types of bottom-set gillnets was measured under 0 and 45 degrees angles of incidence. From these target strengths and from knowledge on the echolocation abilities of two odontoce te species (harbour porpoises, bottlenose dolphins), the detection ranges o f the nets by these small cetaceans could be estimated. The 90% detection r ange by echolocating harbour porpoises, approaching the nets at right (perp endicular) angles under low noise level conditions, varied between 3 and 6 m depending on the net type. For bottlenose dolphins, under high noise cond itions, the 90% detection range varied between 25 and 55 m. At other angles of approach, the estimated detection ranges are shorter. The study suggest s that echolocating bottlenose dolphins can detect nets in time to avoid co llision, whereas echolocating harbour porpoises cannot in;most cases. Sugge stions for future research to reduce small cetacean bycatch by improving th e nets' detectability by echolocation are given. (C) 2000 Elsevier Science Ltd. All rights reserved.