Hatchdate distributions and growth rates of anchovy (Engraulis encrasicolus) in the Southern Benguela ecosystem
Master Thesis
2002
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University of Cape Town
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Abstract
Larval and juvenile Cape anchovy (Engraulis encrasicolus) of the particularly strong 1999/2000 year-class were collected during two consecutive surveys in March and May 2000 in the southern Bengueia region off South Africa. Otoliths from sub-samples were examined to calculate hatchdate distributions and growth rates to estimate the main apparent spawning peaks of the 2000 year-class from both surveys, and to look for possible correlations between spawning, survival and growth of anchovy on die one hand. and the prevailing environmental conditions on the other. Sub-samples were chosen proportional to the length-frequency distributions at each stratum or station at each survey. Otoliths of 193 specimens (14-70 mm standard length, SL) from the March survey and 80 specimens (52-114 mm SL) from the May survey were successfully prepared for light microscopy and electron microscopy respectively. Daily increments were counted and otolith radii were measured. The age-structure of the anchovy population at the time of each survey was calculated from length-frequency distributions using age-length-keys. Mortality rates, hatch date distributions and growth rates were estimated. Instantaneous mortality rates calculated from the catch curves were 0.0353 day-1 for larvae caught in March 2000 and 0.0202 day-1 for juveniles caught in May 2000. These compared with previous laboratory studies and mathematical models in the literature and were, thus, used to back-calculate hatchdate distributions. Resulting hatchdate distributions of larvae and pre-recruits caught in March 2000 showed two apparent hatching peaks, late September - late October 1999 and late December - early January 2000. Thus. either 1) spawning peaked in those two periods or 2) continuous spawning took place between August 1999 and March 2000, but survival of eggs and young larvae peaked in September/October and January/February. The hatchdate distribution of juveniles caught in May 2000, showed peaks in early September - October and late November - December. Therefore, apparent hatching peaks were dissimilar between the two samples of the same year-class, taken three months apart. Thus, higher age-selective mortality of larvae younger than 75 days old in March 2000 (hatched after 17 December 1999) is likely to have taken place in 2000. Large areas of > 19 °C sea surface temperatures (SST) throughout the period from August to March indicated that spawning is likely to have been continuous. Conditions, especially in December-January, when upwelling was reduced because of a cessation in south-easterly winds, may have been conducive to survival of eggs and yolk-sac larvae, due to reduced offshore losses. The period of weak upwelling was subsequently followed by a period of strong upwelling from March to May 2000. This is likely to have increased availability of planktonic food for post-larvae and juveniles. On the other hand, upwelling may have caused offshore dispersal of food patches needed by younger larvae, which could lead to age-selective mortality caused by starvation. The critical period (Hjort 1914) thus seemed to be at a later stage than the first-feeding stage. Mean larval growth rates were estimated between 0.33 and 0.36 mm SL.day-1 using three different growth functions (linear r2 = 0.90, Laird-Gompertz r2= 0.90, and von Bertalanffy r2 = 0.89 growth functions). Juvenile growth rates were estimated at 0.22-0.24 mm SL.day-1l (r2= 0.45, 0.48. 0.47 for the three functions respectively). These rates were slower than previously reported rates, possibly due to the high density of larvae present and hence less per capita food availability. However, reduced food availability did not affect survival in 2000 and, therefore, does not support the Growth-mortality hypothesis (Ware 1925).
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Bibliography: leaves 77-96.
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Reference:
Wilhelm, M. 2002. Hatchdate distributions and growth rates of anchovy (Engraulis encrasicolus) in the Southern Benguela ecosystem. University of Cape Town.