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6.1 What are the mechanisms involved in delivery of fishery benefits?Of the two mechanisms for transfer of animals from reserves to fisheries – larval export, and spillover of adultsand juveniles – there is most evidence for spillover. Spillover can take place through several mechanisms: (1)random movements of animals taking them across reserve boundaries, (2) density-dependent movements takinganimals from high density populations in reserves, to lower density fishing grounds, (3) directed movements,such as daily or seasonal migrations, and (4) ontogenetic habitat shifts which take animals out of reserves intodifferent habitats. Sometimes it is hard to separate these processes, especially mechanisms 1-3, but other casesare clear. For example, fishery closures of nursery grounds take advantage of ontogenetic habitat shifts. In theCyprus trawl fishery, areas close to shore were closed to fishing to protect juvenile fish that later moved offshoreand into the trawl fishery as they grew (Garcia and Demetropoulos 1986).Tagging data and the development of fishing-the-line supported the view that spillover of Zuwai crabwas occurring from the fishery closure in the Sea of Japan (Yamasaki and Kuwahara 1990). Similarly, taggingand tracking of protected lobster populations in Newfoundland and New Zealand reserves indicates movementfrom reserve to fishery (Kelly 2001, Rowe 2001), with a pronounced seasonal component as lobsters movebetween nearshore and offshore habitats. Reserves in New Zealand also export the bream Pagrus auratus on aseasonal basis as fish move from shallow to deeper areas (Willis et al. 2001). In East Africa, catch rates offishers around the Mombasa Marine National Park were clearly augmented by fish moving from the reserve(McClanahan and Mangi 2000). Reef fish frequently undergo daily foraging migrations, particularly from restingsites on reefs to nighttime foraging areas over sandy or seagrass habitats (e.g. Meyer et al. 1983). Suchmovements commonly span 100s of metres to a kilometre and can take fish in and out of reserves (Holland et al.1993, 1996).In the Philippines, Russ and Alcala (1996b) present evidence of spillover from the Apo Island Reservebased on annual fish counts. Fish densities began to increase in fishing grounds close to the reserve boundaryafter 9 years of protection from fishing. However, spillover had probably been occurring for much longer. It isonly after the rate of spillover exceeds the rate of its removal by fishers that fish populations will build up closeto reserves.Catches of world-record sized fish around the Merritt Island National Wildlife Refuge in Florida areundoubtedly caused by spillover of large, old fish (Johnson et al. 1999, Roberts et al. 2001a). Tagging studies atthe reserve also show export of smaller gamefish to the nearby recreational fishery (Stevens and Sulak 2001).Forty years of protection should be long enough for population densities in the reserves to have reached naturallevels and it may be that spillover is now density-dependent, although studies have not yet addressed thisquestion. Lizaso et al. (2000) note that while density-dependent spillover is frequently assumed, there has beenlittle research to test whether this is the case. Attwood (2002) analysed detailed tagging data for galjoen in SouthAfrica and found that movement rates from reserves to fishing grounds were independent of fish densities inreserves. Here spillover is principally via both random and directed movements of fish that are probably insearch of higher quality habitats (Attwood 2002).Not all marine reserves have led to spillover. Davidson (2001) conducted experimental fishing tomeasure effects of the Long Island-Kokomohua Marine Reserve on populations of blue cod. While catch rates inthe reserve rose by 300% over the seven year study, they remained constant in control sites located 1.5–5.6kmfrom the reserve boundary. It may be that densities had not yet built up high enough to trigger movement andspillover from the reserve, or that the species is so sedentary, that any fishery enhancement will depend on larvalexport rather than spillover.There is far less evidence as yet for larval export from reserves to fishing grounds. This is not becauselarval export is rare, but because of the great difficulty of determining the sources of animals recruiting to afishery (Palumbi 2001). Wherever spawning stocks increase in reserves above those in fishing grounds, then areafor area, reserves can be expected to outperform fishing grounds in their contribution to future recruitment. Thatcontribution will depend on the scaling of reproductive output from reserves compared to fishing grounds. Forexample, the Edmunds Underwater Park in Washington State covers just 550m of coastline, but with 100 timesgreater reproductive output, copper rockfish in the reserve could supply eggs equivalent to production from55km of exploited shoreline in Puget Sound (Palsson and Pacunski 1995). Reserve networks at Nabq, Egypt, andin St. Lucia were designed to promote spillover, with small reserve units interspersed with fishing grounds alongthe coast (Roberts et al. 2001a, Galal et al. 2002). However, the magnitudes of improvements in stocks outsidereserves in these locations, and also at Apo Island (Maypa et al. 2002) make it likely that fisheries are beingsupplied by significant larval export too.The easiest situations in which to detect larval export are those where stocks have been severelydepleted prior to reserve establishment. At an experimental scale, Tegner (1993) increased recruitment ofjuvenile green abalone, Haliotis fulgens, by transplanting adult broodstock to a location on the Californian coastwhere they were rare. In Chile a 3 year closure of the squat lobster (Pleuroncodes monodon) fishery ending in1991 led to increases in biomass exceeding all previous estimates since the population was first monitored in13