engleski

Fish abundance and diversity depends more on small-scale spatial heterogeneity than on habitat type and structure

Seagrass meadows are often considered the most important animal habitat of the shallow marine benthos. While Croatia has the highest linear extent of shallow benthic habitats per hectare of marine territory of any Mediterranean country, with unusually extensive beds of the seagrass Posidonia oceanica, there is yet no published study of the macrofaunal communities of Posidonia and neighboring habitats in the Croatian Adriatic. Our objective was to document the habitat value of all shallow benthic habitats in Croatia to adult and juvenile fish species. We used lure-assisted visual census in 3650 10-m belt transects at 74 locations spanning the entire Croatian coast from spring to fall 2009. More than 47000 fish of 72 species were observed at six basic habitats – rock, algae, sand, Posidonia oceanica, Cymodocea nodosa, and Zostera noltii – which also occurred in 19 distinct combinations. Highest adult fish abundance and diversity were observed on rock and algae and lowest within Cymodocea, with intermediate values within Posidonia. Mixtures of >2 habitats at the scale of meters showed a higher abundance and diversity of adult fish than <= 2 habitats, regardless of the identity of those habitats. Our results suggest that areas with high spatial heterogeneity due either to small scale patchiness or to transition between larger habitat patches are more attractive to fish than homogeneous areas even if they offer high structural complexity. Juvenile fish abundances at the edges of Cymodocea meadows were 3-4x the abundance observed in their interior. Furthermore, abundance of juvenile fish at algae and rock was 2x the abundance within Posidonia patches. We conclude that seagrass meadows, including those of P. oceanica, do not provide a uniformly superior habitat for adult or juvenile fishes in the Croatian Adriatic, and that other factors related to the scale of habitat heterogeneity are more important, species-specific, and deserving of further study.

Posidonia meadows; lure-assisted visual census; Adriatic Sea

nije evidentirano