Decodificador 44

By and large the relationships between sea surface temperatures and recruitment success found in this chapter confirm previously observed trends for pilchard in the northern Benguela but not for anchovy.

That pilchard recruitment success generally has a positive relationship with SSTs, as illustrated by figures 5.15c and 5.18, is in agreement with Shannon et al.'s (1988) findings. Assuming that the VPA estimates of year class strength are reasonably accurate, the wide spread of temperature conditions between 1961 and 1987, including two strong Benguela Niño warm events (1963 and 1984), mean that the results in figure 5.15 can be treated with confidence.

The lack of any domed optimal environmental window relationship between SSTs and pilchard recruitment suggests that even during Benguela Niño years there are always sufficient levels of enrichment in the northern Benguela to promote the successful feeding and survival of the early life-history stages. The omnipresence of the Luderitz upwelling cell may provide a reason for this, given that even during the Benguela Niño event in 1984 some upwelling activity was maintained, as illustrated by the monthly SST maps in the appendix (appendix I). Consequently, there is good reason to believe that pilchard recruitment in the northern Benguela is not so much enrichment limited, but rather is limited by levels of retention and concentration.

Anchovy in contrast appear to have a less clear cut relationship with SST. Figure 5.19 shows a negative relationship between the number of SST events above various temperatures and recruitment success, whilst figures 5.16 and 5.17 indicate the existence of an optimal environmental window with recruitment success being maximised at intermediate SSTs. Either way, one can conclude that the influence of environmental conditions on anchovy recruitment is different to pilchard, insofar as the relationship between recruitment success and SST is definitely not positive. Moreover, these findings are in some agreement in indicating that anchovy recruitment in the region, unlike pilchard, could be limited by levels of enrichment, insofar as warmer conditions indicate less upwelling activity.

These results, however, directly contradict Shannon et al.'s (1988) finding of a positive relationship between SST and anchovy recruitment from 1972 to 1983. However, all can be accounted for by the optimal environmental window model. The years from 1972 to 1983 were generally very much cooler than the following four years (e.g. see figure 5.13). Hence they tend to lie on the left hand side of the domed transformation plots in figures 5.16c and 5.17c where there is indeed a local positive relationship between SST and recruitment success.

The optimal environmental window model cannot, however, explain the difference between the dome shaped optimal transformation results and the negative relationship between recruitment and the number of coastal SST events above various temperatures (figure 5.19). The years from 1981 to 1987 covered a wide range of temperature conditions and so do not lie on one side or the other of the domed relationships in figures 5.16c and 5.17c. Nonetheless there are two possibilities as to why the results in figure 5.19 do not even show a semblance of a domed relationship. The first is that six years is simply not long enough to resolve any underlying trends.

The second possibility concerns the apparent disparity between the 1986/87 spawning season having intermediate temperatures from the COADS dataset and having low temperatures from the coastal CORSA data (see figure 3.13). If this spawning season had an intermediate number of SST events above the cut-off temperatures then some kind of an optimal environmental window might be resolvable from the plots in figure 5.19.

The reason for this disparity between the two datasets is that the area from which the COADS mean monthly SSTs were extracted stretches further offshore than the 50 km outer boundary of the 0.5° latitude coastal areas used to construct figure 3.13 (see figures 3.3b and 3.3c). A comparison of the mean monthly SST maps from November to May for 1986/87 with 1981/82 and 1982/83 (appendix I) shows that although the 1986/87 reproductive season had similar mean monthly inshore SST values to these other two cool years, sea temperatures further offshore were often higher. Hence given that the COADS SST values were extracted from an area which extended further offshore, it follows that they would be higher too. Why it was that offshore temperatures during the 1986/87 spawning season were higher than during the either the 1981/82 or 1982/83 seasons is a separate question, but is likely to be linked with the anomalous intrusions of Agulhas water into the Benguela system during the later part of 1986 (Shannon & Agenbag 1987).

In spite of these discrepancies between the COADS and CORSA SST indices, however, anchovy and pilchard recruitment success clearly have very different relationships with temperature conditions in the northern Benguela. Moreover, it means that these stocks conform to the general pattern that has been observed both in the eastern Pacific and the eastern North Atlantic; namely that pilchard/sardine stocks flourish under warm conditions, and that anchovy populations flourish during cooler conditions (Sharp & McLain 1993, Cushing 1996).

Why it is that anchovy generally appear to thrive under cool conditions, whereas pilchard do better during warmer periods, may in turn be linked to differences in the larval feeding preferences of both species, in terms of prey type and prey size. In upwelling systems it has been well established that different levels of upwelling activity are associated with differences in abundance, species composition and size distribution of phytoplankton and zooplankton (e.g. Hutchings et al. 1995; see also section 2.2.5 and section 2.2.6). During cool upwelling conditions productivity levels are high, phytoplankton production is dominated by large diatoms, and zooplankton production is dominated by macro & mega-zooplankton. During warm stable conditions productivity is much lower, the phytoplankton become dominated by micro & dino- flagellates, and smaller zooplankton are favoured.

Sadly there is no information on clupeoid larvae feeding habits in either the northern or southern Benguela (van der Lingen pers. comm.). Nonetheless the results from two studies conducted elsewhere, one in the Californian system on Engraulis mordax and Sardinops sagax larvae (Arthur 1976) and one in the Peruvian system on E. ringens and S. sagax (Muck et al. 1989), give us some idea of what we might expect to find. In both systems the first- & early-feeding larvae (< 7 mm) of anchovy/anchoveta and sardine show evidence of some niche differentiation, insofar as the anchovy/anchoveta larvae have a mainly phytoplanktophagous or mixed diet whereas the sardine larvae are mainly zooplanktophagous.

As regards the preferred prey size of the early feeding larvae, Arthur's (1986) results (as reproduced by Blaxter & Hunter 1982) show similar size preferences for anchovy and pilchard; whereas Muck et al.'s (1989) indicate that the early feeding anchoveta larvae prefer smaller prey than equivalent sized sardine larvae. Nonetheless, both studies are in agreement in showing that at lengths greater than about 9 mm the anchovy/anchoveta larvae start to prefer very much larger prey to the sardine larvae.

If these trends hold true for anchovy and pilchard larvae in the northern Benguela, then based on the assumption that adults will tend to spawn in areas and times which

maximise the feeding success of their early-feeding larvae, they are in tune with two observations. The first is that anchovy prefer to spawn further inshore than pilchard (see section 2.3.4). If anchovy larvae in the region are mainly phytoplanktophagous and pilchard larvae mainly zooplanktophagous these inshore-offshore spawning preferences would make sense, given that the belts of maximum phytoplankton concentration tend to occur inshore of the areas of maximum zooplankton concentration off central and northern Namibia (see section 2.2.6).

The second observation is that although similar oceanographic conditions appear to promote peak spawning in both stocks (see figures 5.10 and 5.11) eventual recruitment success is favoured by different conditions. The elevated inshore SSTs (17° - 23° C) associated with peak anchovy and pilchard spawning seem to promote recruitment success in pilchard only, as illustrated by figure 5.17. In contrast, figure 5.18 demonstrates how successful anchovy recruitment appears to require the relative absence of the same inshore SSTs which promote peak spawning.

A dramatic increase in the preferred prey size of anchovy larvae, relative to pilchard larvae, after reaching lengths of about 9 mm might just explain this paradox. If the early feeding anchovy larvae have similar preferred prey sizes to the pilchard larvae then it makes sense that peak anchovy spawning occurs during similar conditions to peak pilchard spawning. After growing to 9 mm, however, the feeding success and survival of anchovy larvae would then be maximised by cooler upwelling conditions when larger prey would be more prevalent. Moreover, if this were true then it would be consistent with the increasingly held view, as reported in Cushing (1996), that if there is a critical larval period influencing eventual recruitment success it occurs at larger larval sizes rather than at the first feeding stages proposed by Hjort (1914).