Why do dolphins and porpoises fall victims to driftnets in such large numbers?
Dolphins and porpoises may become entangled in driftnets because they cannot detect the monofilament fibers with their sonar. Although the ability of dolphins to detect echoes from small objects is very good, they may not be able to detect the thin strands of plastic that make up the driftnet, partly because the acoustic properties of monofilament plastics are so similar to those of water; therefore, monofilament gives a very weak echo. Even if dolphins and porpoises do receive a signal, they may not see anything with which to connect it or may not even be echolocating at the moment before entanglement (dolphins do not echolocate all of the time). They may well detect the plastic floats at the top of the nets. But these would be no cause for concern to the dolphin and certainly give no indication to an inexperienced animal of the impending danger below. Listen to Roger Payne’s Voice from the Sea piece entitled: Getting caught in a net off Sri Lanka.
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