Trawling or Visual Censuses? Methodological Bias in the Assessment of Fish Populations in Seagrass Beds |
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| Authors: | Mireille L Harmelin-Vivien Patrice Francour |
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| Institution: | Centre d'Océanologie de Marseille, Station Marine d'Endoume, C.N.R.S. UA 41;F-13007 Marseille, France;Laboratoire d'Ecologie du Benthos, Facultédes Sciences de Luminy, F-13009 UniversitéAix-Marseille II, France |
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| Abstract: | Abstract. Fishes associated with the seagrass Posidonia oceanica were censused both by skid trawl and visual counts around Ischia Island, Italy. Both sampling procedures were performed during the day in summer and in winter at two sites. Significant differences in the number of species, number of individuals, biomass, and trophic structure of the fish assemblage were observed between sampling methods. Fewer fish species were recorded by visual counts than by trawling. More individuals and a greater biomass, however, were recorded from visual counts. The population of macrocarnivores (Scorpaenidae, Serranidae) were better estimated by trawling, as were canopy-dwellers (Syngnathidae; Symphodus rostratus) and benthic species (Gobiidae; Blenniidae; Bothidae). Conversely, good swimmers ( Sparidae; Coris julis, Symphodus spp.), and planktivorous fishes (Centracanthidae, Pomacentridae) mostly escaped the trawl and were better assessed by visual counts. The importance of methodological biases differed from one season to the other and was higher in summer than in winter. Particular attention should be paid to the biases induced by sampling techniques when interpreting data, and different sampling methods should be used to accurately study the fish assemblages of seagrass meadows. |
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| Keywords: | Fishes methodology visual counts trawling Mediterranean Sea seagrass beds |
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