9 GLOBEC INTERNATIONAL NEWSLETTER APRIL 2001 The Southern Benguela Anchovy population reached an unpredicted record level of abundance in 2000: another failure for fisheries oceanography? Claude Roy 1 , Carl Van der Lingen 2 , Scarla Weeks 3 , Mathieu Rouault 4 , Janet Coetzee 2 , Greville Nelson 2 and Ray Barlow 2 . 1-IRD and University of Cape Town, South Africa (croy@uctvms.uct.ac.za) 2- M&CM, Cape Town, South Africa 3-OceanSpace cc and University of Cape Town, South Africa 4-University of Cape Town, South Africa. The annual pelagic recruitment survey carried out during the austral winter (May/June) of 2000 by Marine and Coastal Management (M&CM) recorded an extremely high abundance of young anchovy off the West Coast of South Africa (Fig. 1 - back page), which is the main anchovy nursery ground in the Southern Benguela. Although a survey in this region in March 2000 (M&CM pre-recruit survey) had detected high densities of pelagic targets over a substantial portion of the continental shelf, their identity was not established, and the recruitment survey was the first concrete indication that anchovy reproduction had been particularly successful during the spring-summer 1999-2000 spawning season. This was confirmed a few months later by the results of the spawner biomass survey carried out in early summer (November/December) of 2000. During this cruise, anchovies were found to be widespread in relatively high densities over the Western Agulhas Bank (Fig. 1 - back page), and their distribution was also found to be less patchy than during previous years. The proportion of older fish in the population was relatively small, and the very strong year class spawned at the end of 1999 dominated the anchovy biomass. The estimated anchovy abundance early in summer 2000 was 4.03MT, which is more than 2 times the previous highest level observed since the start of the time-series in 1984. Previous investigations had shown that anchovy recruitment is inversely related to the intensity of the Southeast, upwelling-favourable wind recorded during the spawning season (Boyd et al., 1998). Increased south-easterly wind, and consequently intensified upwelling, appear to be detrimental to anchovy recruitment because of increased offshore advection and losses of eggs and larvae, whilst weak upwelling would favour successful transport of the reproductive products from the Agulhas Bank spawning ground to the nursery ground off the West Coast. The summer of 1999-2000 had the strongest positive Southeast wind anomaly over the past 40 years (Fig. 2), indicating that upwelling intensity along the West Coast during this period was stronger than usual. Surprisingly, and in contradiction with previous findings, the rather intense 1999-2000 upwelling season appears to have favoured anchovy recruitment in 2000. Does this apparently contradictory result represent yet another failure of recruitment vs. environment relationships, or can we reconcile the 1999-2000 events with previous knowledge that underlined our understanding of the detrimental effect of enhanced upwelling on recruitment ? Using weekly SST anomalies data recorded from three locations off the West Coast, it appears that short-term environmental variability was important during the 1999- 2000 upwelling season (Fig. 3). During the last two weeks of December, a pronounced positive SST anomaly (+2.0°C) resulting from a cessation of the upwelling- favourable wind was recorded. As a result, holidaymakers on Cape Town beaches enjoyed coastal water as warm as 20°C,when they are usually greeted with icy waters as low as 10°C. In early January, the upwelling-favourable Southeast wind intensified and the SST dropped to more normal values. Later in the season, upwelling increased still further and SST anomalies dropped to values as low as -2.0°C from early March to May (Fig. 3). Figure 2: Cumulative October-to March Southeast wind anomaly (compared to the 1960-1991 long-term mean) measured at Cape Point, 1960-2000 (updated from Boyd et al. 1998). Figure 3: Weekly SST anomalies at three locations off the West Coast of South Africa from November 1999 to mid-May 2000. Source OI-SST (Reynolds and Smith,1994), available at: http://ingrid.ldeo.columbia.edu/SOURCES/.IGOSS/.nmc/.weekly/ . 3/11/99 17/11/99 1/12/99 15/12/99 29/12/99 12/1/00 28/1/00 9/2/00 23/2/00 9/3/00 22/3/00 5/4/00 19/4/00 3/5/00 SST anomaly (°C) Cape Town (33° 30’S) Cape Colombine (32° 30’S) Hondeklip Bay (30° 30’S)