Sardine and Anchovy Recruitment

Sardine and Anchovy Recruitment Prediction--South Africa

by Suzanne J. Painting

The pelagic fishery, predominantly anchovy, in the southern Benguela upwelling region is of great commercial and socio-economic value. Annual landings average 400,000 mT and have a value near U.S. $70 million. The life-history of the anchovy, Engraulis capensis, is relatively well understood (Fig. 1). Adults spawn on the south coast throughout summer; and eggs and larvae are transported by a jet current up the west coast. Late larvae and/or juveniles are either transported or migrate into the nearshore nursery area. These nearshore regions are extremely productive because the prevailing southerly winds cause episodic upwelling of cool, nutrient-rich water throughout much of the year. The commercial fishery for anchovy is based on fish (recruits) which are 6-9 months old and have attained 9 cm. In spring the recruits begin their return migration to the Agulhas Bank on the south coast to spawn. Fishing quotas for the following season are provisionally determined on the basis of estimates of the total biomass of the spawner stock in November, but quotas are adjusted during the fishing season using acoustic surveys. Poor correlations between spawner biomass and recruitment the following year complicate management of these resources.

The Benguela Ecology Program (BEP) in South Africa recently initiated a new Sardine and Anchovy Recruitment Prediction (SARP) program involving scientists at the Sea Fisheries Research Institute (SFRI) and the University of Cape Town (UCT). Cruises are done every month during the summer spawning season of the anchovy to determine within-season variability in spawner stock size, fish condition, egg production rates and environmental factors (e.g., food concentration, water column stratification). Recruits captured six months later during a winter survey are aged to determine birth-date distributions, which are then related back to the variability observed during the spawning season. The overall objectives of the SARP program are:

To identify the key biological and environmental factors regulating the recruitment of anchovy and sardine.
To describe their variability and quantify their potential and realised impact on recruitment.
To develop the ability to predict the spawning success of anchovy and sardine from year-to-year and so allow more rigorous management of these resources.

During its initial phases, the SARP program will focus only on spawner stocks found on the western Agulhas Bank (i.e., west of Cape Agulhas) although recent studies suggest that a large spawner biomass may concentrate on the eastern side of the Bank as well. Thirteen different projects are incorporated into the SARP program. The specific objectives of these projects include:

To determine the spatial-temporal distribution and length-frequency distributions of adult anchovy and sardine on the western Agulhas Bank and west coast.
To determine egg production rates and gonad states of anchovies and sardines.
To collect biological data on anchovies, sardines and round herring.
To estimate the availability of planktonic food in relation to hydrography and the food requirements of pelagic fish.
To determine rates of invertebrate predation on pelagic fish eggs and larvae.
To examine within-season variability in anchovy spawning in relation to environmental factors.
To assess the nutritional condition of anchovy larvae in relation to environmental factors.

Details of Summer SARP Cruises

SARP cruises take place every month from September to March (austral summer) on the F.R.S. Algoa. The total number of days per cruise alternates between 6 and 8 days. The area covered on the shorter surveys is between Cape Agulhas and the Olifants River. On longer cruises the survey grid includes a two-day acoustic survey of pilchard stocks east of Cape Agulhas, and extends only as far north as Cape Columbine.

A standard grid with 7 sampling lines has been designed for the cruises (Figure 2). Transects (lines 1-7) start as close as possible to the coast and extend to the 200 m or 500 m depth contour, with stations spaced every 10 nautical miles (nm). Acoustic surveys for pelagic fish are conducted while underway, and midwater trawls are done on an ad hoc basis. An acoustic Doppler current profiler (ADCP) is also operated while underway in order to obtain current profiles. Pelagic fish eggs and larvae are collected from the upper 70 m at each station using a vertical Calvet net haul and an oblique Bongo tow. CTD's at each station provide water column hydrography and vertical profiles of fluorescence. Size-fractionated chlorophyll analysis is done on water from the surface and the subsurface fluorescence maximum. Five of the transects (Lines 1,2,5,6 and 7) are "Full Biological Lines" and are sampled more intensively to determine zooplankton biomass and copepod secondary production rates (from egg production and molting rate experiments).

Greater emphasis is placed on acoustic surveying during the 8 day cruises. A standard acoustic grid with 6 transects (A to F in Fig. 2) has been designed for the area between Cape Agulhas and Cape Barracouta in the east. These lines are 50 nm long, with stations at 10 nm intervals. At each of these stations an electronic bathythermograph (BT) is deployed to obtain a profile of water column temperature. There is no other sampling at these stations. After the two day acoustic survey of lines A to F, the ship does a standard survey of lines 1 to 6, as described above. During the 6 day cruises, lines 3, 4 and A to F are omitted, and line 7 is sampled.

SARP--South Africa, in its present format is planned to continue over the next two to five years. Future research plans may include greater emphasis on larval research, and the dynamic linkages between the eastern and western Agulhas Bank regions. (Dr. Suzanne Painting is a research scientist with the Sea Fisheries Research Institute in Rogge Bay, South Africa)