Dj. Jellyman et Pr. Todd, WHY ARE MIGRATING MALE SHORTFINNED EELS (ANGUILLA-AUSTRALIS) IN LAKE ELLESMERE, NEW-ZEALAND, GETTING SMALLER BUT NOT YOUNGER, Bulletin francais de la peche et de la pisciculture, (349), 1998, pp. 141-152
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
.