Local replenishment of coral reef populations

There is a very interesting paper in Science magazine published May 04:

Local Replenishment of Coral Reef Fish Populations in a Marine Reserve. G.R. Alamny et al, Science 316:742

In this really quite clever experiment, the authors were able to tag larval fish by injecting a heavy isotope of barium into females. They used Amphipiron percula (AKA Nemos :glare: ...) and Chaetodon vagabundus (Vagabond Butterflyfish) as their target species, these being two species with different levels of parental care and different durations of pelagic larval phase. Juveniles of these two species were captured a couple of months after the mothers were injected with the isotopic barium, and by measuring the barium isotope in the bones of the ear, they were able to identify which of the larval fish were progeny of the original females.

They did this on Kimbe Island's reefs, in Kimbe Bay, PNG. Kimbe Island's reef is quite small (0.3 square km), and it is a part of a marine protected area.

So why do this, you may ask?

Well, one of the big questions surrounding preservation of marine areas is "How much area needs to be preserved?". One of the assumptions of preserving these areas is that they can act as an "incubator" to provide larval fish for other reefs downcurrent. Obviously another assumption of all of this is that there is a significant loss of larval fish during the pelagic period, and that some of these larval fish will end up washing on to other reefs. If most of the larvae hang out on their natal reef, then the numbers of larval fish that are available to other reefs will be lower.

The authors found really quite astonishing rates of retention of the larval fish. They went back and randomly captured juveniles of both species, then looked to see if they had been tagged or not. Sixty percent of the clownfish (9 of 15 juveniles) were tagged. This suggests that a bit over half of the reef's fish are arising from larvae that were spawned on the reef, rather than coming in on currents.

Interestingly, they saw a very similar rate of return of the Vagabond butterfly fish larvae, these larvae have no parental care and long pelagic phase, thus you'd expect them to be "washed away" more. Yet even here about 60% of the juveniles caught had grown from larvae spawned over this particular reef.

This has some interesting implications for marine reserve management. By seeing that a very small reef is able to provide enough larvae to sustain itself (ie the 60% of the juveniles that were on the island are a replenishment for the reef's own stocks), while also having significant larval transfer with other reefs (about 40% of the juveniles came from other reefs, which are at least 10 km distant from Kimbe), this suggests that small reefs may actually be able to achieve both these goals (self replenishment and larval source). Yet if larval fish are actually good at not being washed away from reefs, then the ability of a reef to act as a replenishment source for other areas may not be as great as originally thought, and perhaps larger numbers of small MPA's may be necessary.

Hi Craig,

Thanks for posting, this is fascinating! I've heard many amazing stories about larvae finding a place to live. How do they do it???? Say you're a coral (or fish) larva drifting 100 feet over a reef - how do you know NOW is the time to settle??? Very interesting indeed.

Cheers

James

I'm intrigued by this too. Some studies have indicated that larvae can hear the sound of reefs, others let larvae choose between water collected from there ancestral reef and other sites and a majority picked the home-reef water suggesting the "smell" of the reef is imprinted (or the pheromones or some other chemical secreted by their own species). Research from Barbados showed that larvae use different currents at different depths to control their movement. I'm not sure home much of this is fact and how much still needs to be confirmed but there is probably more smarts in these 3mm translucent specks than we give them credit for.

I wonder if this also has a psychological impact. If the majority of the fish larvae drift away to the benefit of down stream shores on different islands/countries there could be less incentive to limit the local fisheries. However, if a lot of the fish stocks depends on self-replenishment it becomes a quite different story.

Bart