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Seasonal and annual changes in availability of the adult crustacean plankters of Shuswap Lake Ward, F.J. (Frederick James)

Abstract

Samples of adult crustacean plankters were obtained from Shuswap Lake, B.C. during 1954, 1955 and 1956. The validity of quantitative plankton sampling techniques used during the investigation were examined. Data were analysed for evidence of seasonal and annual variations in availability of adult crustacean plankters. Rapid, random changes in efficiency of Wisconsin large-type nets when used as vertical samplers did not obscure differences in availability of plankton at different stations. Comparison of catches of a new net to those made by a well-used net showed that the nets maintained constant and equal efficiency for periods of at least three months. The means of groups of hauls made at three different rates of haul did not vary significantly and it was concluded that variability in rate of haul was not a serious source of error. A series of thirty consecutive hauls was divided into five groups. The means of the groups were tested by analysis of variance and it was found that, although there were significant differences between the means, there was no evidence that a progressive decline in the efficiency of the net occurred. Increased accuracy in measurement of relative abundance of adult crustacean zooplankters was obtained by washing samples on a screen before centrifuging. By this process most of the phytoplankton and small zooplankters were removed. Short-term changes in availability of the adult crustacean component of the plankton occurred on all stations during all three years; however, these rapid changes in availability did not hide seasonal trends or annual differences in availability. Examination of average catches representative of the same time-interval in each year showed that availability of plankton in 1955 was lower than either 1954 or 1956 for all stations. These data also showed that consistent differences in availability occurred at the various stations. Conclusions regarding the effect of the dominant year-class of sockeye on availability of plankton cannot be reached at present, although it can be concluded that low plankton availability does not limit the abundance of sub-dominant and "off year" runs of sockeye. Average annual differences in water temperatures apparently do not cause annual differences in plankton availability. Differences in productivity of different parts of the lake were probably caused by differences in dissolved mineral content of the water, which in turn were probably caused by differences in geology of the watersheds of the parts of the lake.

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