Browsing by Author "Shelton, Peter Anthony"

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    Fish spawning strategies in the variable southern Benguela Current region
    (1986) Shelton, Peter Anthony; Field, John G
    A 120 station grid was surveyed at monthly intervals in the southern Benguela. Current system between August 1977 and August 1978. Plankton was collected at each station by means of a double oblique tow behind the ship at 2 knots down to 100m or 5m off the bottom. Plankton displaced volume was determined and all fish eggs and larvae were removed and identified to the lowest possible taxon. Water samples were collected at each·station by means of 5 litre bottles from 5 depths down to a maximum depth of 75m. Microplankton (37-100 μm fraction) particle concentration, chlorophyll concentration and salinity were determined from the samples. A vertical temperature profile was obtained at each station using a bathythermograph. Twenty plastic drift cards were released at each station and recoveries plotted. Wind measurements were made on board the ship and also obtained after the cruise from land sites at Cape Point and Cape Columbine. After August 1978, the CELP grid was resurveyed in November 1978, November 1981 and November 1982. In November 1979 and November 1980 a number of stations were sampled in the vicinity of the Cape Peninsula. to determine the vertical abundance pattern of anchovy eggs. In November 1983, 1984 and 1985 a much larger portion of the southern Benguela Current system was surveyed to determine the spawner biomass of anchovy using the egg production and acoustic methods. In these surveys anchovy eggs were sampled by means of a vertical tow with a CalVET net and at some stations on the Agulhas Bank the vertical abundance of anchovy eggs was sampled using a multiple opening and closing RMT sampler. The ichthyoplankton and environmental data were interpreted to reveal any similarity in pattern. Anchovy, hake and laternfish were selected for special attention because of their different spawning strategies, and were shown to have seasonal and spatial patterns of egg and larval abundance which could be related to the environment. The major environmental process influencing ichthyoplankton abundance patterns appeared to be the seasonal advection of warm western boundary current water into the system from late spring to early autumn, causing the development of a temperature front along the west coast and a thermocline over the Agulhas Bank. Plankton standing stocks were elevated between the front and the coast and in the thermocline. In order to place their spawning strategies into perspective, a simulation model was used to explore the performance of anchovy, hake and lanternfish life history strategies in filtering out environmental variability. Results suggested that anchovy and hake are adapted to cope with variability whereas lanternfish are less well adpated, and therefore spawn in winter when the system is most homogeneous. Conclusions are drawn as to the kinds of environmental anomalies likely to cause poor year class strength in the three species.