Direct versus indirect effects of wave exposure as a structuring force on temperate cryptobenthic fish assemblages
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The structure of cryptic reef fish assemblages was assessed on sheltered and exposed aspects of coastal breakwaters at two locations in the northwestern Adriatic Sea. There were distinct differences between the two levels of exposure, which were consistent between locations. Habitat characteristics, measured on scales of tens of centimetres, explained 50% of the variability in assemblage structure between exposures, whereas ‘exposure’ alone (implying direct effects of wave energy on the fish) explained <5% of the variation. The most important explanatory variables were the presence of macroalgae, sandy habitat and oyster shell, the last of which increased the degree of small-scale complexity and provided nesting sites for blennies. We found little evidence to suggest that wave action had large direct effects on the fish assemblages, although this may be in part due to the relatively small degree of difference between ‘exposed’ and ‘sheltered’ samples under the calm conditions of a sea with a relatively short fetch. These results suggest that wave action acts mainly indirectly as a structuring force on cryptic reef fish communities, by altering the composition and/or the relative density of epibiota that influence the distribution of fish. Thus, relative wave energy may provide a useful means of predicting fish assemblage structure only at large spatial scales. Microhabitat, composed of a combination of physical complexity and biological elements, always explained the greater part of variability at small (<1 m) spatial scales.
KeywordsBreakwater Fish Assemblage Reef Fish Oyster Shell Wave Exposure
Thanks to C. Borsini and R. Ribeiro for field assistance, and to A. Sundelöf and L. Airoldi for allowing us to use their clod card data. We are grateful to M.J. Anderson, as ever, for statistical advice and a thorough reading of the manuscript, and M. Abbiati, F. Colosio, L. Airoldi, S.P. Griffiths, N. Tolimieri, Y. Triossi and the two anonymous referees for discussion and further comments on the manuscript. Further inspiration was provided by F. Cannavaro, M. Materazzi and M.P. Smile. T.J.W. was supported by grants from ENI/AGIP Italia to M. Abbiati. S.S. was supported by a grant from the Italian Ministry of Environment and Territory (Artificial Marine Structures: Multifunctional Tools for Research and Environmental Management in the Mediterranean and Red Sea), and T.J.W. by grants from ENI/AGIP Italia to M. Abbiati.
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