J. Horppila et al., EFFECTS OF 5 YEARS OF INTENSIVE TRAWLING ON A PREVIOUSLY UNEXPLOITED SMELT STOCK, Journal of Fish Biology, 49(2), 1996, pp. 329-340
In the years 1989-1993, over 1000 metric tons of fish (392 kg ha(-1))
was removed by trawls from the Enonselka basin in Lake Vesijarvi in or
der to enhance the water quality. Roach(52%) and smelt (28%) were the
most abundant species in the catches. In this study, the effects of th
e mass removal on the previously unexploited smelt stuck was assessed.
The total smelt catch was c. 282 metric tons. Due to the mass removal
, the density of the stock collapsed from c. 46 000 fish ha(-1) in 198
9 to c. 14 000 fish ha(-1) in 1993 (M=1.0). The biomass of the stock d
ecreased from 75 kg ha(-1) in 1989 to 12 kg ha(-1) in 1993. The age gr
oup composition of the stock changed clearly towards the dominance of
younger age groups. The male/female relationship in the samples change
d from clear dominance of females to the dominance of males and the in
fection rate by the sporozoan parasite Glugea hertwigi declined. The g
rowth rate of the smelt did not change considerably during the years o
f mass removal. The results suggest that in biomanipulated lakes sever
al mechanisms may prevent the exploited stock from responding to the i
ncreased mortality. (C) 1996 The Fisheries Society of the British Isle
s