The variability in size of pelagic and demersal marine and freshwater fish eggs is examined.The difference between the smallest and largest volumes, based on published figures for the diameters, is large in many species. In marine species with planktonic eggs, the median percentage difference is just over loo%, and this is similar in species with demersal eggs and in freshwater fish.The available evidence suggests that geographical differences in egg size are small, hut in marine fish there is a well-known seasonal decline in egg size. In herring it has previously been shown that egg size in different spawning groups can be correlated with the timing of the production cycle. A similar correlation can be seen in the seasonal shift in time and locality of spawning, and egg size, of the plaice. Sufficient data on seasonal freshwater fish egg variations are not available, but the time of spawning does appear to be linked with the availability of food for the larvae in both lake and stream species.
Factors regulating year-class strength in the percid genera Stizostedion and Perca are summarized. Some index of water temperature regime correlates significantly with year-class strength of percids in many water bodies. Moderate synchrony of year-class strength is noted for walleye (Stizostedion vitreum vitreum) in several lakes in North America. A probablistic model is proposed to explain the basis of temperature dependence of year-class strength in percids, but tests of the model using Lake Erie data indicated that observed correlations between temperature and year-class strength of yellow perch (Perca flavescens) and walleye may not be the result of direct effects of the temperature regime on survivorship of early life-history phases. Key words: Percidae, year-class strength, temperature, probalistic model, early life history
During the last 30 years the fecundity of the plaice, Pleuronectes platessa L., has been widely studied over its complete geographical range. In this paper all this previous work is brought together and a distinct pattern (with two exceptions) emerges. The fecundity is lowest in the Southern Bight of the North Sea, and radiating from there in all directions the fecundity increases, while in Faxa Bay (Iceland) and in the Barents Sea it is lower again. The possible factors producing this pattern are examined in detail and it is concluded that the amount of food available, which in turn is related to population density, is the most important. Only in Trondheim Fjord and in the Baltic is the fecundity so different that it is necessary to postulate racially distinct populations. Finally the importance of fecundity variations in natural population regulation is stressed.
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