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Internationale Vereinigung für Theoretische und Angewandte Limnologie: Verhandlungen

Another insight gained from the present analysis that quantitative changes in the food base of whitefish affect egg weight to a minor degree was unexpected and deserves further consideration. It was therefore tempting to suppose that egg weight might respond in a similar way to changes in the food base of whitefish, increasing during the s and s, and decreasing thereafter.

In the case of ULC pelagic whitefish, egg quality i.

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This result agrees with the observations of Zijlstra , who compared herring eggs produced in years with varying food availability and did not find any influence of food supply on egg weight. When larvae arrive at the surface after their ascent from the lake bottom, the temperature is generally low and food is scarce as spring turnover does not usually take place until late February. These peculiar limnological conditions of ULC have probably prevailed during most of the time since whitefish first colonized the lake after the last glaciation, and they undoubtedly impose a strong selective pressure.

It is reasonable to assume that these harsh conditions favour investment in egg weight over egg number in order to increase the starvation resistance and thus the survival chances of newly hatched larvae. A reduced food supply for adult female whitefish is thereby translated into reduced fecundity rather than lower egg weight, whereas an improved food supply results in increased fecundity with egg weights remaining largely unchanged for females of similar age.

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We did not find any comparable trend in our data. Additionally, delayed hatching does not guarantee sufficient prey abundance when larvae are released into the lake. A late beginning of spring in some years will still select for larvae with a higher starvation resistance. We might, however, expect a relaxation of the selection for a high egg weight in the long run, if global climate change leads to an earlier onset of spring, and if the intensive stocking programme is continued.

The increased food supply for zooplanktivorous whitefish resulted in accelerated somatic growth. In addition to somatic growth, whitefish were also able to invest more energy in reproduction, in the form of higher fecundity. The most remarkable result of this study is that the overall increase in fecundity over time is greater than can be accounted for by lake trophy alone.

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Although fishing effort clearly declined during the study period with legal mesh size remaining unchanged , the selection for size is most probably as strong today as it was in earlier years. This notion is supported by results from the monthly test fishery, which hardly catches any fish at all in nets of mesh sizes larger than the legal size. The still strong size selection despite lower fishing effort is probably due to the reduced growth rate of whitefish. Under these conditions, females which allocate a greater fraction of available energy into reproduction early in life produce more eggs overall and enhance their reproductive success.

Selection for higher fecundity takes place on the phenotypic level, but fecundity can be assumed to have a genetic basis. In addition to intensive fishery exploitation, there is probably a second mechanism driving selection for higher fecundity, namely the progressive intensification of stocking.

Eggs are obtained from spawners, which are caught with gill nets of legal mesh size, in which females with a larger body girth have a higher catch probability. Higher fecundity probably, but not necessarily, in combination with larger egg size results in larger body girth, so that females with higher fecundity are more likely to be caught and to contribute offspring to the stocking cohort for the following spring. Eggs obtained in the spawning fishery are incubated in hatcheries under optimized conditions.

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Stocking policy could be a powerful mechanism for a sustained genetic selection towards high fecundity, achieved at the cost of somatic growth. Group 2 white bars , mature fish, which remained in the lake and reproduced naturally. The solid line indicates the amount of larvae stocked into the lake from hatcheries.

The gill net fishery represents a selective force towards slower growth, earlier maturation not previously verified for ULC whitefish so far , and increased fecundity. As higher fecundity can only be obtained at the cost of reduced somatic growth with egg weight remaining unchanged , there is a twofold selective pressure towards slower growth. Furthermore, in the spawning fishery, which is conducted to obtain eggs for incubation in the hatcheries, females with the largest body girth, i. A high NAO index during holomixis in late winter leads to higher hypolimnetic temperature, which shortens the development time of whitefish eggs spawned in the next winter, and consequently the loss of eggs to infection and predation.

This suggests that natural reproduction is still highly relevant despite intensive stocking.

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The gonadosomatic index increased until the early s in parallel with fecundity. This effect can be attributed to changes in the age structure of whitefish stock during these decades. As egg weight increases with female age, heavier eggs produced by older females largely compensated for reduced fecundity, and GSI, which is strongly related to egg weight and fecundity, decreased only marginally. The finding that the majority of whitefish offspring are derived from rather few individuals should be the reason for concern. Genetic diversity will probably be impoverished, and this might have consequences for population structure and production, the extent of which are not yet fully conceivable.

Taking these likelihoods into account is a challenge for the future development of fisheries management at Upper Lake Constance and elsewhere, where similar, as yet undetected problems may exist. Sampling and processing of material for this analysis comply with the current law in the EU. Volume 22 , Issue 1. If you do not receive an email within 10 minutes, your email address may not be registered, and you may need to create a new Wiley Online Library account.

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Please review our Terms and Conditions of Use and check box below to share full-text version of article. Introduction The potential for fishery practice to induce phenotypic changes in fish stocks has received increasing attention in recent years. Term d. MS, mean squares; P , error probability; , adjusted coefficient of determination; DSS, days between sampling and spawning. Statistical analysis Multiple linear regression models were fitted in jmp 7.

Figure 1 Open in figure viewer PowerPoint. Figure 2 Open in figure viewer PowerPoint. Figure 3 Open in figure viewer PowerPoint. Figure 4 Open in figure viewer PowerPoint. Egg weight Egg weight was strongly related to female age and to much less degree to phosphorus concentration, whereas standing stock biomass cf. Figure 5 Open in figure viewer PowerPoint. Gonadosomatic index The gonadosomatic index increased until the early s in parallel with fecundity.

Berkeley, S. Maternal age as a determinant of larval growth and survival in a marine fish, Sebastes melanops. Ecology 85 : — Google Scholar. Crossref Google Scholar. You have the right to access the personal data that CHRG holds on you.

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