ATTAINS HIGHEST RECORDED LEVEL
By Carl van der Lingen, Janet Coetzee, Mark Prowse, Rob Crawford and José De Oliveira
Results from the recently-completed November 2000 Pelagic Spawner Biomass survey have confirmed the exceptionally good recruitment of anchovy Engraulis capensisobserved earlier last year. Anchovy recruitment strength in May/June 2000 was estimated at 415 billion recruits, nearly four times higher than the previous maximum of 116 billion recruits observed in 1988. This exceptional recruitment has given rise to an adult population of just over 4 million tons, more than double the highest previously recorded (1.75 million tons in 1986). Since anchovy has been the main contributor to South Africa’s purse-seine fishery in most years since 1964, during which period more than 8½ million tons has been caught, the current high population level is good news for the pelagic fishing industry. An initial anchovy TAC of 378 000 tons for the 2001 fishing season has been allocated, compared to an initial TAC of 123 000 tons for 2000.
Because anchovy have a short life span and a high natural mortality rate, and because recruitment in Clupeiformes (the Order to which anchovy, sardine and herring belong) is generally highly variable, large interannual fluctuations in population sizes of these fish are frequent. Predicting recruitment strength is one of the goals of contemporary fisheries science, and several studies towards this are underway at M&CM. However, the exceptional anchovy recruitment of 2000 is not easily explained. Anchovy recruitment strength appeared to be inversely related to the intensity and duration of southeast winds at Cape Point over the spawning season, with stronger SE winds appearing to curtail recruitment because of advective losses whereas weak SE winds appear to favour recruitment. Nevertheless, the summer of 1999/2000 had the strongest positive SE windrun anomaly over the past 30 years, but also the highest recruitment yet observed. This discrepancy may be in part due to an eastward shift in the principal anchovy spawning grounds observed during recent years, or may be due to some factor other than SE wind being the dominant force driving recruitment strength last year. Further research, including an examination of high-resolution satellite data and a determination of the birthdate distribution of anchovy recruits, is currently underway in order to determine the cause of the exceptionally high anchovy recruitment observed in 2000.
