Evaluating the impact of predation by fish on the assemblage structure of fishes associated with seagrass (Heterozostera tasmanica) (Martens ex Ascherson) den Hartog, and unvegetated sand habitats

The role of fish predation in structuring assemblages of fish over unvegeta ted sand and seagrass was examined using enclosure and exclusion cages to m anipulate the abundance of predatory fish from November 1998 to January 199 9. In our exclusion experiment, piscivorous fish were excluded from patches of unvegetated sand and seagrass to measure how they altered abundances of small fishes, i.e., fish < 10 cm in length. Habitats from which piscivorou s fish were excluded contained more small fish than those with partial cage s, which in turn contained more fish than uncaged areas. These patterns wer e consistent between unvegetated sand and seagrass areas, although the rela tive differences between predator treatments varied with habitat. Overall, small fish were more abundant in unvegetated sand than seagrass. Atherinids and syngnathids were the numerically dominant families of small fish and v aried in complex ways amongst habitats and cage treatments. The abundance o f atherinids varied inconsistently between cage treatments through time. On ly during the final two sampling times did the abundance of atherinids vary significantly across cage treatments. Syngnathids were strongly associated with seagrass and were significantly more abundant in caged than uncaged h abitats. In our enclosure experiment, five individuals of a single species of transient piscivorous fish, Western Australian salmon (Arripidae: Arripi s truttacea Cuvier), were enclosed in cages to provide an estimate of the p otential for this species to impact on small fish, The abundance of small f ish varied significantly between cage treatments. Small fish were more abun dant in enclosure cages and exclusion cages than uncaged areas; however, th ere was no difference in the abundance of small fish in enclosure cages and partial cages, and no difference between exclusion cages and partial cages . These patterns were consistent amongst habitats. Atherinids and syngnathi ds were again the numerically dominant families of small fish: atherinids v aried more with cage structure while syngnathids did not vary statistically between cages, blocks (locations within which a single replicate of each c age treatment was applied) or habitats. Dietary analysis of caged A. trutta cea demonstrated the potential for this species to influence the assemblage structure of small fish through predation atherinids were consumed more fr equently in unvegetated sand than seagrass, and syngnathids were consumed o nly in seagrass. where they are most abundant. Observations of significant cage or predation effects depended strongly on the time at which sampling w as undertaken. In the case of the atherinids. no predation or cage effects were observed during the first two sampling times, hut cage effects and pre dation effects strongly influenced abundances of fish during the third and fourth sampling times, respectively. Our study suggests that transient pisc ivorous fish may be important in structuring assemblages of small fish in s eagrass and unvegetated sand, and seagrass beds may provide a refuge to fis hes. But the importance of habitat complexity and predation, in relation to the potentially confounding effects of cage structure, depends strongly on the time at which treatments are sampled, and the periodicity and multipli city of sampling should be considered in future predation studies. (C) 2000 Elsevier Science B.V. All rights reserved.