Characterizing and comparing the spawning habitats of anchovy (Engraulis capensis) and sardine (Sardinops sagax) in the Southern Benguela upwelling ecosystem
Master Thesis
2002
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University of Cape Town
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Abstract
The spawmng habitats of anchovy (Engraulis capensis) and sardine (Sardinops sagax) in the southern Benguela upwelling ecosystem were characterized by comparing the data on abundance and distribution of eggs of the two species with environmental variables. Data were collected from two different survey programs: (a) 14 SARP (Sardine and Anchovy Recruitment Programme) cruises, conducted monthly for 2 spawning seasons between August 1993 and March 1994, and September 1994 and March 1995, off the west coast of southern Africa from Cape Columbine to Cape Agulhas; (b) annual November/December spawner biomass surveys conducted from 1984-1999 along the South African coast and covering the continental shelf between Hondeklip Bay on the West Coast and Port Alfred on the South Coast. A CalVET net was used to collect fish eggs at stations on a survey grid, and physical and biological data were collected concurrently with egg samples. Physical variables measured included sea surface temperature, nitrate concentration, water depth, salinity, current speed, wind speed and mixed layer depth, whereas biological variables included phytoplankton biomass (as 50m integrated chlorophyll a) and zooplankton biomass and production. The spawning habitats selected by anchovy and sardine were identified by constructing quotient curves derived from egg abundance data and individual environmental variables. Anchovy and sardine spawning probability with reference to the distribution of environmental variables was examined using overlay operation analysis. Relationships between eggs of the two species and the environmental variables were verified using multivariate co inertia analysis. Relationships among the environmental variables were examined through cluster analysis. During SARP surveys, single parameter quotient analysis indicated that the spawning habitats of these two species were most dissimilar in terms of water depth, sea surface temperature, current speed and zooplankton biomass, and most similar in terms of salinity and phytoplankton biomass. Coinertia analysis using all environmental variables as inputs showed a positive association between anchovy eggs and salinity and sea surface temperature, and a negative association between anchovy eggs and secondary production and phytoplankton biomass. Sardine eggs were strongly positively associated with phytoplankton biomass. During spawner biomass surveys, anchovy and sardine spawning habitats appeared to differ with respect to sea surface temperature, wind speed and current speed, and overlapped in terms of water depth, phytoplankton biomass, zooplankton biomass and production. Anchovy eggs were strongly positively associated with SST, salinity, mixed layer depth and zooplanktop production. Sardine eggs were strongly positively associated with current speed and zooplankton biomass.
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Bibliography: leaves 85- 97.
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Reference:
Twatwa, N. 2002. Characterizing and comparing the spawning habitats of anchovy (Engraulis capensis) and sardine (Sardinops sagax) in the Southern Benguela upwelling ecosystem. University of Cape Town.