Gibbons et al : Habitat use by demersal nel<ton on the continental shelf in the Benguela ecosystem 



481 



showed distinct preferences for environments with sandy 

 substrata (Fig. 4, B and C) and were absent from rockier ar- 

 eas. Dragonets and cuttlefish (Sepia sp. ) appeared to select 

 opposite environments along the PC2 axis, with the former 

 favoring areas with echinoderms and sponges and the lat- 

 ter avoiding them. Hake again appeared to avoid extremes 

 of environment along the PC3 axis, whereas false jacopever 

 selected areas of high relief substrata (Fig. 4, D and E). The 

 results suggest that hake selected areas without cerian- 

 thids (and other biota, PC4), whereas dragonet preferred 

 areas with cerianthids (Table 7). 



Assemblages 



Assemblage composition was a function of substratum 

 type. Cluster analysis of the similarity matrices generated 

 between samples among all the data indicated a similarity 

 in assemblage composition that was generally based on 

 major substratum type (data not shown). These patterns 

 were mirrored in separate analyses of the samples (Fig. 5, 

 A and Bl. The communities were not always distinct, how- 

 ever, and samples from hard substrata were sometimes 

 grouped with those from mixed, and even soft, substrata. 

 A noteworthy feature of these data was the generally low 

 level of similarity between samples, and this undoubtedly 

 reflected their small individual size (area) and the low 

 density of nekton. 



The dendrogram produced by analysis of the species 

 (and not sample) matrix revealed common patterns of as- 

 sociation in both the northern and southern areas (Fig. 6, 

 A and B). In the northern area, there were two clusters at 

 the 30% level of similarity (with sole as an out-group) (Fig. 

 6A). This result is difficult to interpret clearly because the 

 one group comprised species of both soft (hake and cuttle- 

 fish) and hard (false jacopever) substrata. These species 

 were also the most abundant and thus the likelihood of 

 their co-occurrence in samples was high. In the south. 



two groups of associated species were also observed at the 

 30% level of similarity. The association between hake and 

 cuttlefish was again tight, and both were more distantly 

 linked to false jacopever (Fig. 6B). The other group com- 

 prised two clusters at the 40% level of similarity: one of 

 gobies and grenadiers, Caelorinchus simorhynchus, and 

 the other of species from soft (dragonets and sole) and 

 mixed substrata. 



Activities 



None of the dominant species appeared to be either attracted 

 to, or repelled by, the presence of the submersible and its 

 lights, and avoidance behavior was seen only at the moment 

 when the submersible was directly over the fishes. 



Few of the fishes seen were actually swimming (behav- 

 ior pattern 3), and most were resting on the substratum 

 itself (Fig. 7). Indeed, all gobies and soles were seen lying 

 on the substratum. Grenadiers seemed to hover above the 

 substratum, and false jacopever and kingklip appeared to 

 be situated in some sort of shelter. Although a few indi- 

 vidual hake, kingklip, and dragonets buried themselves in 

 sand, this behavior was otherwise uncommon. Most of the 

 fish seemed to behave in a similar way over hard and soft 

 substrata. The exceptions to this were kingklip and grena- 

 diers, which tended to hide and rest, respectively, more 

 often over hard substrata rather than soft (Fig. 7). 



Discussion 



The demersal nekton assemblages over the continental 

 shelf along the west coast of southern Africa are clearly 

 structured in a way that reflects the substratum type. This 

 assemblage composition is in part due to the responses of 

 individual species to the environment. These are novel 

 observations for the region, and they provide a baseline 



