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We studied the influences of female pikeperch (Sander lucioperca) length and growth (i.e., maternal effects) on the length of hatched and starved larvae as well as on larval starvation resistance in two different temperature conditions (mean temperatures 13.2 and 17.9 degrees C). The data included fertilised eggs and reared larvae from 21 individuals caught during the spawning season from eutrophic Lake Pyhajarvi in Finland. The length of the hatching larvae was dependent on female size but also on water temperature and on the potential energy the female had invested on reproduction. Maternal effects seemed to also influence the timing of larvae growth, and larvae that were large at hatching grew less on mere yolk sac reserves, and vice versa. Female length had a positive effect on the larval starvation resistance but only under cold conditions. The results suggest that higher temperature might reduce the advantage the larvae from large females get for starvation resistance.

Hypolimnetic withdrawal (HW) is a restoration method for eutrophied lakes that aims to remove phosphorus (P) and other nutrients from the system. It is conventionally carried out by pumping or siphoning nutrient-rich bottom water to the discharge of the lake during periods of thermal stratification. However, there is growing interest in developing closed circuit modifications of HW in which nutrients could be captured and the purified water returned to the same lake. This would tackle some problematic aspects of conventional HW, and additionally enable the capture and recycling of P stored in lakes. A pilot closed circuit HW system has been constructed at a eutrophic dimictic lake located in Southern Finland. This hypolimnetic withdrawal and purification circuit (HWPC) consists of a withdrawal pipe installed at the lake deep, a treatment and filtering unit on shore, and a wetland. In the treatment unit, P is first precipitated and then captured by sand filters, while the purified water flows subsequently through a wetland and finally back into the lake. In the current study, we investigated the pool of potentially removable P in the study lake, the optimal timing of HW within the annual cycle, and the functioning of the HWPC. The P retention capacity of the purification unit and the composition of the precipitate trapped in the filters were both examined. The results showed that P accumulation in the near-bottom water of the study lake during thermal stratification is substantial, allowing significant amounts of P to be removed from the lake via HW. The concentration of total P of the water entering the HWPC was over 300 µg/L, of which the system captured more than 80%. The P content of the precipitate trapped in the filters varied between 6-12 g P/kg, and consisted of both iron and calcium-bound P phases. These results imply that it is possible to recover P accumulated in eutrophied lakes for potential recycling purposes.

In pike E. lucius L., evidence on maternal effect on reproductive output is mixed. We studied whether older and larger pike females produce eggs and larvae of higher quality (weight, starvation resistance) in three forest lakes in southern Finland. Later, the study lakes were subjected to intensive experimental pike fishing, which we assumed would increase resource availability and lead to higher maternal investment (larger egg size). Length of female pike was positively correlated with the dry weight of eggs and larvae but this relation was dependent on female age. In old females, the effect of female length on egg weight was lower or even negative. Survival analysis showed a positive effect of female length on larval survival time indicating that larvae from larger females are less vulnerable to starvation during the early stage of life. After the intensive pike fishing, the positive effect of female length on egg weight was stronger in all age classes probably due to the released resources. Based on the high quality and amount of reproductive products in large (but not very old) females, they are important for the reproduction of pike populations. This should be considered in fisheries management.