Effects of size-selective mortality on estimates of the growth rate of zander, Sander lucioperca (L.) from the Jeziorak Lake

Fish growth rate is usually determined based on a single sample or a number of sub-samples considered jointly. A study on cisco, Coregonus artedi, successively collected within 6 years in Canada, had demonstrated a possibility of determining size-selective mortality in relation to larger or smaller fish. In three recent papers on the growth of zander, Sander lucioperca, from the Odra River (Poland) estuary, a higher mortality of smaller fish (a negative Lee's phenomenon in inclusive sense) was suggested. On the other hand zander from the Jeziorak Lake (Poland), collected by the present Polish authors, exhibited a size-selective mortality in relation to the larger fish (a positive Lee's phenomenon in inclusive sense). The aim of this paper is to discuss this issue. The zander were collected from the Jeziorak Lake with a summer seine in 1970 (328 specimens), 1971 (72 spec.), 1972 (73 spec.), and in 1973 (143 spec.). The fish were aged 2- through 5-years, representing three year-classes (i.e. hatched in 1967, 1968, and 1969). The age and growth rates of the fish were determined based on scales and verified based on cross sections of fin rays. Growth rate indices were based on empirical relationship between the standard length and the oral radius of scale, which assured elimination of the phenomenon of apparent change of growth rate. Despite the above assumption, the lengths attained at the moment of establishing sequential annuli decreased with age of fish studied. The differences between individual year-classes, however, were decreasingly smaller. Those regularities were tested on individual year-classes as well as on individual age groups caught within a single season. It reflects a higher mortality of faster-growing fish. The length frequency distribution curves of both empirical and back-calculated values for sequentially collected samples were plotted, separately for each of three year-classes. The intention was to verify whether the higher mortality of larger fish could contribute to a skewness of length distribution curves. Skewness was not observed, but it turned out that each year-class had its specific frequency distribution of length, persisting from their second-through fourth year of life. The occurrence of positive- and negative Lee's phenomenon in inclusive sense in fish stocks of the same species, inhabiting different bodies of water, is discussed. The following research problem: What are the reasons of different size-selective mortality of di