Influence of different volumes and types of detached macrophytes on fish community structure in surf zones of sandy beaches

Detached macrophytes (seagrass and macroalgae) are transported from more offshore areas and accumulate in substantial volumes in surf zones, where they are commonly called wrack. Fishes were sampled using seine nets in 4 volume categories of detached macrophytes (bare sand, low, medium and high volumes) in the surf zone at 2 sandy beaches in southwestern Australia to determine how increasing volumes of surf-zone wrack influences fish community and size composition. Species composition and densities of fish, which were dominated by juveniles, differed between areas where wrack was present or absent, and also among volumes of wrack in the surf zone. Total fish abundance and biomass increased as the volume of wrack increased.Cnidoglanis macrocephalusandPelsartia humeraliswere the dominant species and were most abundant in medium and high wrack volumes. Fish gut contents were analysed forC. macrocephalusandP. humeralis, and verified thatAllorchestes compressais a major prey item for juveniles of these species. A series of habitat preference trials conducted in outdoor aquaria tested whether juvenileC. macrocephalusandP. humeralisshowed a preference for different types of detached macrophytes as a habitat, i.e. seagrass, brown algae, or a mixture of both macrophyte types. Non-parametric goodness-of-fit binomial tests for differences in the number of fish between each habitat type showed no clear pattern in habitat preference for either species of fish. Field and laboratory results suggest that the amount, rather than type, of detached macrophytes is more important in providing a habitat for juvenileC. macrocephalusandP. humeralis.