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Marine Ecology Progress Series

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MEPS 328:161-170 (2006)  -  doi:10.3354/meps328161

Invariant size selection of blue mussels by roach despite variable prey size distributions

Mats Westerbom1,2,3,*, Antti Lappalainen4, Olli Mustonen2,3

1Aronia Research Centre, Åbo Akademi University and Sydväst Polytechnic, Raseborgsvägen 9, 10600 Ekenäs, Finland
2Tvärminne Zoological Station, J. A. Palménsväg 260, 10900 Hangö, Finland
3Department of Biological and Environmental Sciences, Division of Ecology and Evolutionary Biology, PB 65, 00014 University of Helsinki, Finland
4Finnish Game and Fisheries Research Institute, PB 2, 00791 Helsinki, Finland

ABSTRACT: Predation is a key trophic component with a potentially large influence on rocky shore community organization. We studied prey size selection by roach Rutilus rutilus, feeding on blue mussels Mytilus edulis in the northeastern Baltic Sea. In this region, roach feed extensively on abundant populations of blue mussels living at the very edge of their range primarily set by low salinity conditions. The study area is characterised by a marked decline in mean mussel size from the saltier west to the less salty east. We predicted that average prey size of roach would decrease with decreasing availability of larger prey but increase with increasing size of the predator. The size of mussels ingested by roach ranged from 2 to 28 mm, largely covering the size distribution of blue mussels in the area. In accordance with our prediction, body size of the predator was the foremost factor determining patterns of prey selection and mean prey size increased steadily with increasing size of the predator. Roach were size selective, preferring median and large mussels in proportions different from their accessibility in the habitat. Contrary to our expectations, no differences between the areas were seen regarding prey size selection. These results contrast with optimal foraging theory predicting that predators under higher prey densities decrease the proportion of less profitable prey. Our results suggest that roach have the potential to significantly affect the blue mussel dynamics in areas with poor prey availability and we predict that ongoing environmental change in the Baltic will likely increase its impact.

KEY WORDS: Prey size selection · Predator–prey · Mytilus · Edge species · Optimal foraging · Body size · Rocky shores · Top-down effects

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