Mid Sweden University

The influence of turbidity on foraging and growth of young-of-the-year (YOY) of two percid species with relatively dissimilar sensory physiology, perch Perca fluviatilis and pikeperch Sander lucioperca was tested. A littoral mysid, Neomysis integer, was used as prey. Functional response was investigated indoors at two levels of turbidity, 3 NTU (clear) and 25 NTU (turbid). Growth and consumption rate were studied both during day and night in outdoor experiments at similar turbidity levels. Pikeperch were not affected by turbid conditions in either of the experiments. Foraging of perch, on the other hand, was significantly negatively affected by higher turbidity in the functional response experiments and during the night-time in the outdoor experiments. As opposed to pikeperch, consumption rates of perch decreased markedly during nights. Perch also grew more slowly in the treatments with turbid water. The dissimilar reactions of the two species indicate that sensory physiological adaptations and foraging behaviour are important factors that partly can explain disparate reactions of YOY fishes to the level of turbidity.

Weaning success of pond-cultured pikeperch and wild-caught perch (mean length 51 and 48 mm respectively) was evaluated using different weaning techniques and different formulated feeds. Juveniles that were fed formulated feed grew as well as or better than juveniles that were weaned successively using zooplankton or yolk. Four different formulated feeds (agglomerated marine larvae feed, marine larvae feed, trout feed and a semi-moist feed) were evaluated regarding specific growth rate (SGR), condition factor and a subjective stomach fullness estimate. The agglomerated marine larvae feed gave significantly better weaning performance than the other feeds regarding all parameters (SGR = 7.3% day(-1) and 3.4% day(-1) for pikeperch and perch respectively).

The most critical period at onset of feeding in pikeperch Stizostedion luc\ioperca is short (<5 days at 20degreesC). The larvae are sensitive to prey density during the first week of exogenous feeding. First-feeding larvae of 6.5 mm total length (L-T) needed prey densities of >585 prey l(-1) to maintain mass (C-maint), whereas 5 days older larvae of 7 mm L-T C-maint=55 prey l(-1) and for 11 mm L-T larvae C-maint <10 prey l(-1). Changes in specific growth rates for larvae of 7 and 11 mm were similar to a type-II functional response curve reaching a specific growth rate of 26 and 30% day(-1), respectively, at 1000 prey l(-1), whereas the 6.5 mm larvae showed a linear growth response reaching a specific growth rate of only 9% day(-1) at 1000 prey l(-1). The results suggest that prey density is a limiting factor, which might contribute to the high variation between year-class strengths.