Egg cannibalism by anchovy in the Southern Benguela Current Region

Master Thesis


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University of Cape Town

Samples of adult anchovy and plankton were collected on Sea Fisheries Research Institute R.S. Africana 1984-1985 anchovy spawner biomass estimation cruises. A total of 40 sets of anchovy samples of 30 fish each and 40 plankton samples were taken roughly at the same time within an area of intensive spawning over the Agulhas Bank, using an Engels 308 midwater trawl and a CalVET net respectively. Fish samples were frozen in a deep freeze at 20° C shortly after capture. Plankton samples were stored in formaldehyde solution. Fish were measured (total length), weighed (total and ovary mass) and de-stomached. Weight of the stomach contents were determined and anchovy eggs in the stomach were counted. Anchovy eggs in CalVET net samples were counted and staged. Frequency distributions of densities of eggs in the plankton on the Agulhas Bank and off the West Coast were plotted to compare egg density in the two areas. Approximately 53% of the eggs caught over Agulhas Bank occurred in only 25% of samples, indicating a patchy distribution. A frequency distribution was plotted of abundance of eggs in the stomachs of fish. Egg patchiness caused a skewed frequency distribution of egg abundance in anchovy stomachs. Feeding time was estimated from an examination of the relationship between weight of stomach contents versus time of day, taking into account time for gut evacuation. Based upon a developmental stage/temperature/age key, mortality rates of eggs in the sea were calculated, and it was estimated that 44% of anchovy eggs were lost daily. Taking into account estimated rates of egg mortality, egg production, gastric evacuation rate, number of eggs eaten and feeding time, cannibalism was estimated to account for about 62%-70% of the egg mortality. The rate of cannibalism was shown to be consistent with a density-dependent functional response.