Homeward bound

Some fish return to the reefs where they spawn. The idea is to maintain genetic distinction

Published: Wednesday 31 January 2001

new evidence suggests that larvae of fish that inhabit coral reefs do not get scattered and join other communities, as previously believed. They return to the reefs where they were born. Earlier, marine biologists believed that larvae got scattered by oceanic currents to other shores, settling and mating in other communities. This was assumed to result in mixing of the gene pool and widespread presence of genetically similar populations. Scientist term this an 'open system'. But the new findings could alter our understanding of fish population dynamics. They definitely have implications for protecting marine biodiversity and managing fisheries.

One group of researchers examined the dna of three geographically separated populations of the rabbit fish (Siganus fuscescenes), which lives close to the reefs. They found that each population had distinct genetic pattern. Although seasonal currents affect the outcome, they cannot take the larvae to suitable new habitats. This leaves very few individuals to inhabit new reefs.

This pattern of local 'closed systems' was confirmed by another study of bluehead wrasse, a reef fish. The study showed that a majority of the fish under observation had never ventured far from the reef where they had spawned. Another study on three species of mantis shrimps, carried out in the Indonesian waters, showed that in all the three species, individuals from specific regions shared distinctive genetic markings not found in other regions.

If reef fish do live in closed communities that help maintain their genetic distinctiveness, a different strategy maybe required for protecting them. Limited movement of the larvae will mean that one large protected population will not be able to protect the biodiversity of different regions. Instead, each region will require its own protected area. But experts say that there is a need to define the scale of the 'region' within which a school of fish remains bound.

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