How are coral reef monitoring programmes helping fisheries management in Seychelles? Jan Robinson of the Seychelles Fishing Authority and Tim Daw of the University of Newcastle, UK, make some suggestions.
Healthy reef are vital to the stocks of many commercially exploited fish species © Big Blue Divers
Scientific study of coral reef ecology has come a long way in the last 30 years. To understand these complex ecosystems, we can nowadays call on a large body of knowledge. Although many questions remain unanswered and scientists will probably never be able to model it all, we have a pretty good idea of what can happen to reefs when we poison them, smother them in sediment or remove too many fish. Compared to the slow changes to reefs brought about by natural geological and ecological processes, these subtle human impacts relatively easy to detect and measure - if you look in the right place.
Coral reef monitoring has developed rapidly from a few local/national programmes into regional/global initiatives in response to the rapid degradation and loss of reefs. Programmes are in place to look at the impacts of coral bleaching, coastal development, fisheries and other pressures. Often, however, a single programme seeks to address all these issues and fall short on all of them.
Looking in the right place
In Seychelles, recent monitoring programmes have developed out of a need to assess changes brought about by coral bleaching, which is undoubtedly the biggest threat to coral reefs of these islands. For the coral reef manager, these programmes have provided many answers. For the fisheries manager, it is tempting to try and use SCUBA diver counts of fish numbers. Unfortunately, in Seychelles, coral reef monitoring programmes are neither monitoring the fishing grounds where most fishers fish nor the species that fishers catch and that we eat.
Looking too deep?
Unlike many reef systems, where the reef slope drops precipitously to the deeper ocean, the fringing reefs of the inner granitic islands bottom-out at depths generally shallower than 15 metres, grading into the rather flat expanses of the Mahé Plateau. While most coral reef fisheries are confined to the shallow and narrow reef slope, beyond which most artisanal fishers cannot fish, Seychellois fishers have an enormous bank beyond the reef slope. This is not a featureless expanse of sand and mud, but a highly productive environment of rubble fields, patch reefs, granite 'pate', seagrass and Sargassum beds. The latter form some of the largest habitats of their kind on Earth. These habitats are not mapped, nor monitored, yet they represent a vast fishing ground. While the scientists count fish on the slopes of exposed fringing reefs, the fishers are setting traps or lines with an unerring accuracy at 30 metres, on fishing grounds known only to them. More snorkeling, less diving?
Coral reef monitoring programmes also miss far more obvious and important fishing grounds. Are we perhaps fixated with diving to count fish and corals on reef slopes? Meanwhile, reef crests, reef flats with associated habitats are simply not monitored, when all that is needed to do so is snorkeling gear.
Where trap fisheries are concerned, far more fishing effort is concentrated on the reef flat than on the reef slope, particularly in the SE monsoon season. Divers monitoring fish on the reef slope hardly see fish like Kordonnyen Blan, perhaps the most important commercial trap species, because this herbivore prefers the shallow reef flats. Unlike in East African waters, even when gathering to spawn, Kordonnyen in Seychelles prefer to migrate down the reef slope on to the plateau to submerged granite 'pate'. Measuring fishing effort
There are other ways to monitor changes in fish numbers. Collecting catch and effort data allows catch per unit effort (CPUE) to be calculated. CPUE, or catch rate, is assumed to be proportional to numbers of fish, with declines in CPUE reflecting population decreases, and vice-versa. However, CPUE is not always directly proportional to numbers because of conditions known as hyperstability and hyperdepletion, which may result from changes in fisher or fish behaviour.
In tropical fisheries management, it is vital to combine CPUE monitoring with fisheries-independent approaches, of which underwater visual census surveys are the most common. As we have seen, however, existing coral reef monitoring programmes in Seychelles cannot provide the answers fisheries managers need. The reinvigorated Seychelles National Coral Reef Network (SNCRN) is seeking to create long-term plans for coral reef monitoring with its partners, it is vital that more emphasis is placed on collecting data that are helpful to fisheries management in the longer term.
Back to basics
To get there, we need to go back to the snorkel and mask, establishing monitoring sites in lagoons and on reef flats. Surveying at depths greater than 20 metres can be a risky business, so the use of remote visual survey techniques, such as towed cameras, could be adopted. This has been successfully used on a recent sea cucumber survey.
Using local knowledge
Fishers often report changes to certain species and habitats. As yet the scientists and managers do not have the means to investigate and validate this local knowledge. A case-by-case approach to coral reef monitoring needs to be adopted, designing sampling programmes that can really address particular issues of concern. This is where we can learn from terrestrial ecology, developing much more sophisticated techniques. By focusing all our efforts on a few habitats which make up only a tiny portion of Seychelles' marine area, we run the risk of overlooking important changes in our marine environment.