An assessment of the reliability of fossil pilchard and anchovy scales as fish population indicators off Namibia

Master Thesis


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University of Cape Town

Fossil fish scales hold potential for ellucidating past fish population fluctuations. A system of classification for scales from the pilchard, Sardinops ocellata, and the anchovy, Engraulis capensis, is presented. Both species show an unexpected range of scale types. The classifications reduce errors in distinguishing between the scales of the two species to ±2,5%. Scale loss from these fish is quantified under laboratory conditions. Pilchard lose 1,56 scales/fish/day due to death and 1,50 scales/fish/day due to deciduousness over their expected lifetimes. For anchovy the figures are 2,42 scales/fish/day (death) and 0,48 scales/fish/day (deciduousness). Application of these scale-loss studies to scales preserved in the anaerobic sediments off Walvis Bay, Namibia, shows that deciduousness is the dominant process contributing scales to the sediments. This basic information on pilchard and anchovy scale loss is used to interpret counts of scales in the laminated interval of a core taken from the diatomaceous muds off Walvis Bay. This pilot study shows that: i) the Namibian Fishery was dominated by pilchard in the past as it was prior to the collapse in the early 1970's and, because of this, is distinctly different from the anchovy/anchovetta-dominated east Pacific systems; and ii) major pre-fishery stock fluctuations do appear to be reflected in the sedimentary record. Further scale studies on larger sediment samples are recommended to ellucidate the Namibian fish stock fluctuations.