Pelagic fish population biology was studied in the large Swedish lakes Vänern, Vättern, Mälaren and Hjälmaren. It is crucial for fish fry in temperate regions to hatch early in the growth season to survive, and achieve large size before winter, and it is suggested that the key factors are to match the spring development of phyto- and zooplankton, but to avoid predation. This is more easily accomplished by the studied spring spawners smelt (Osmerus eperlanus) and pike-perch (Stizostedion lucioperca) than autumn spawners, such as vendace (Coregonus albula). It is shown that hatching of vendace fry shortly after ice-break-up is beneficial for year-class strength. In oligotrophic large lakes with few predatory species a rapid increase in water temperature after ice-break is also promoting recruitment, whereas this is not the case in eutrophic lakes where predation pressure from other species may become too high. The results indicate that autumn spawners will have difficulties in adapting to global warming and it is also suggested that the life history can explain the large variations observed in year-class strength between years.
Abstract— Age determinations of whitefish (Coregonus lavaretus (L.)) were compared in two different tests. In the first test, the readers determined the age of each individual from one calcified structure (scale, otolith, or opercular bone) at a time. The samples from three populations, 50 specimens in each, were mixed so that the readers did not know which population each calcified structure was from. A sample of known‐age whitefish was used in the second age determination test, where information such as time of catch, length, weight, and sex was available to the readers. In each of the 50 envelopes the reader got scale impressions and two otoliths, one of which had been burned and ground. In the first test, the precision of the readers was low both between readers and between different structures. In the samples of slow‐growing populations, the determinations made from the otoliths showed older ages than the determinations from the scales. In the second age determination test the results were better; 73‐90% (average 82%) of the determinations were correct. The use of two calcified structures and the knowledge of the material were considered to improve the accuracy. Age determination bias may occur that affects the age distribution: even though 80% of the fish were aged correctly, an exceptionally strong or weak year class could remain unidentified. The estimation of growth rate seemed less sensitive to incorrect age determination than age distribution.
This study showed that it is possible to model a bimodal gillnet selectivity curve without knowing how individual fish have been caught. A bimodal model based on a modified normal density distribution was fitted over the length ranger of smelt Osmerus eperlanus from 8·9 to 17·0 cm. The model had eight parameters, and the height of the second peak was allowed to change with fish length. The coefficient of determination (r 2 ) was 0·915 and the residuals were distributed symmetrically against variable axis. Corrected relative length distribution did not differ statistically from the relative length distribution of the trawl catch. In earlier studies of other species, pooled relative efficiency of multimesh gillnets composed by a geometric series mesh-size combination has been constant on the average. Because the model increased along fish length, the pooled relative efficiency was increasing also with fish length. Therefore, it is suggested that the gillnet catches of smelt always have to be corrected for gillnet selectivity. 1998 The Fisheries Society of the British Isles
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