Winter mortality of pikeperch (Stizostedion lucioperca) at age 0 is dependent on size, based on samples collected with trawls during 1961-1994 in Pärnu Bay, Estonia. Mortality increased as mean length of pikeperch in the first autumn decreased. The size-dependent mortality was analysed as correlation between mean lengths at age 0 in autumn and changes in mean lengths between age 1 and age 0. When the varying lengths at age 0 were taken into account, the duration of winter also affected mortality; longer durations of ice cover resulted in less size-dependent winter mortality. Since mean length at age 0 in autumn correlated positively with summer water temperature, winter mortality can be expected to decrease the warmer the summer.

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SynopsisThe burbot population off the River Kyronjoki has suffered from acidification in its reproduction area, causing a reduction of the population throughout the 1980s . During the same period of time the mean length-atage of burbot was found to increase, with a significant increase in length-at-age for age groups 3-8 from 1979 to 1993 . The length-weight relationship however was not found to have changed during this same period . The decreasing burbot population was found to influence the changed length-at-age . An increase in potential prey species population was also documented during this period of increasing length-at-age . The increased lengths were associated to the changed availability of food caused by the simultaneous fluctuations in both burbot and potential prey species populations .

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