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Many small fishes do not disperse far from their birthplace.

Bottom-dwelling coral reef fishes live in patchy environments that limit their migration but spawn their offspring into the ocean where currents may disperse them far from their birthplace. The amount of larval exchange and the spatial scale at which it occurs has major implications for the management for fish stocks, particularly the design and distribution of Marine Protected Areas (MPAs) and harvest strategies.

Trace elements, which are present in every environment, provide one means to assess the degree of connectivity among populations of reef fishes. These elements are absorbed by fishes across the gills or gut and accumulated throughout their life span in calcified structures including their otoliths (ear bones). A collaboration between AIMS, Charles Darwin University and the University of Perpignan investigated trace elements in fish from French Polynesia to examine connectivity patterns. The project, supported by the Total Fina Foundation, collected fish from numerous sites on the islands of Tahiti and Moorea and showed that the trace elements in their otoliths were unique to the site of collection.

The most distinctive signature was found in fish living in Papeete Harbour because of the presence of anthropogenic pollutants, which provided a natural tag to distinguish between fish born in the Harbour and those born elsewhere. Using this tag, it was found that 40% of the young fish that colonised reefs in Papeete Harbour were recolonising their birthplace; thus indicating limited dispersal, in the order of tens of kilometres. Although the spatial scale of dispersal is an active area of debate in scientific circles, this distance is much less than commonly assumed by most reef fish ecologists and natural resource managers to occur in reef systems. These findings raise an interesting question over the required density of MPA networks, which currently assume that average larval dispersal occurs over scales of hundreds of kilometres.