WHY ARE MIGRATING MALE SHORTFINNED EELS (ANGUILLA-AUSTRALIS) IN LAKE ELLESMERE, NEW-ZEALAND, GETTING SMALLER BUT NOT YOUNGER

Lake Ellesmere is a large coastal lake in the South Island of New Zeal and which supports important commercial fisheries for both feeding and migratory shortfinned eels Anguilla australis. Over the past 50 years , the sex distribution of migratory eels in the lake has changed from a dominance by females to a dominance by males. Age and size data of m ales have been collected intermittently since 1974. During this time, the mean age has remained relatively constant while the average length has declined by almost 20%. Despite this reduction, length distributi ons are less variable than weight, condition, or growth rate distribut ions, and length is considered to be the primary factor associated wit h the onset of migration. The reduction in average size does not appea r to be a result of selective harvest of larger individuals, but must reflect changing environmental conditions in the lake itself - such co nditions could include loss of macrophytes, and a reduction in the pro portion of larger eels of both species with resultant proliferation of the small benthic bully Gobiomorphus cotidianus. The suggested manage ment strategy is to maximise harvest of migratory male eels and thus r educe the capture of females which are large enough to prey on bullies .