SPATIAL STRUCTURE OF COOCCURRING ANCHOVY AND SARDINE POPULATIONS FROMACOUSTIC DATA - IMPLICATIONS FOR SURVEY DESIGN

A geostatistical analysis has been undertaken on the spatial structure of co-occurring adult and recruit populations of anchovy, Engraulis c apensis, and sardine, Sardinops sagax, in the southern Benguela upwell ing region, using information from two acoustic surveys. The study was prompted by the need for a more efficient design for surveying sardin e abundance, which is increasing in relation to that of anchovy; the c urrent acoustic survey design is based on the distribution of anchovy. Variograms of fish density and density indicator variables were compu ted, as well as cross-variograms between the indicator variables. The sardine variograms were less structured than the anchovy variograms, w ith slightly greater nugget effects, indicating greater randomness in space at small scales. The indicator variograms showed progressive los s of structure with increasing density. Anchovy formed high-density sc hools during the day, breaking into larger, low-density aggregations a t night. Sardine, on the other hand, remained in relatively high-densi ty schools throughout the day. The cross variograms revealed some spat ial continuity between low- and high-density areas for anchovy, but no such transitional structures were evident for sardine. It was conclud ed that sardine are more patchily distributed than anchovy and, unlike anchovy, they may not have a single common way of occupying space at the population level, perhaps partly because of the broader age struct ure of the population. It is concluded that, while the current acousti c survey design is well suited to the spatial distribution of anchovy, it is not as well suited to that of sardine. Future survey designs sh ould he more tailored to the spacial distribution of sardine, possibly by using sampling methods to cope with the highly patchy distribution s expected.