Stimulating investment in pearl farming in ... - World Fish Center

Abundance, Size Structure and Quality of Silver-lip Pearl Oysters in the Solomon Islands Final, June 2008heavily exploited broadcast spawners, recruitment may rely on either natural or humaninduced refuge populations that act as reservoirs for larval production (Clareboudt 1999).Prolonged recruitment failure in populations of silver-lip pearl oysters is likely to affect thesex ratio. Pearl oysters are protandrous hermaphrodites (i.e. they change sex from male tofemale as they mature) and most of the oysters found during the current and previoussurveys undertaken in the Solomon Islands are likely to have been female. In WesternAustralia, animals develop into males at a length of 110-120 mm, but by the time they reach170 mm approximately 50% of individuals are female and by the time they reach 190 mmthey are >9 years old and all female (Fletcher et al. 2006). The temporal/size aspects of sexchange in animals at Solomon Islands is not known, and gonads samples were not taken inthis or earlier surveys, but the dominance of old, large individuals in all of the surveysundertaken to date suggest that sex ratio may be a potential barrier to population growthand is worthy of further investigation. The susceptibility of pearl oyster stocks to suchrecruitment over-fishing has been noted previously (Sims 1992).If over-exploitation, resulting in low density and a biased sex ratio, has caused a markeddecline in the number of recruits, recovery of the fished population is likely to either be veryslow or non-existent (Jennings et al. 2003). The three phases of exploitation to which stocksin the Waghina region have been subjected implies that some degree of recovery of fishedstocks has occurred in the past. The phasing could also simply reflect a change in fishingmethod, with divers in the latter phase using hookah equipment to access stocks in deeperwater. If the silver-lip pearl oyster stocks in the Solomon Islands eventually recover fromthe last phase of commercial exploitation, fisheries management strategies will need to beimplemented to protect the recovered populations to prevent recruitment overfishing priorto any further form of exploitation. One option for doing this would be to establish anetwork of spawner sanctuaries within defined fishing zones or fully-protected, permanentmarine reserves (Tegner 1993; Peterson 2002). If healthy stocks can be located in otherregions of the Solomon Islands, the possibility of facilitating the recovery of depletedpopulations by means of stock enhancement programmes should be investigated. Stockscould be enhanced by either releasing juveniles raised in hatcheries, transplanting juvenilesfrom naturally occurring stocks into areas where recruitment is low but conditions forgrowth and survival are good or by using translocated broodstock to enhance larvalproduction (Bell et al. 2005).However, it is possible to reduce populations to points at which recovery is barely possible.In some of the atolls in the Cook Islands and French Polynesia, where black-lip pearl oysterswere subjected to intense fishing during the late nineteenth and early twentieth centuries,stocks had not recovered from over-exploitation by the 1980s (Intes 1984; Sims 1990). Thesituation at Suwarrow in the Cook Islands, in fact, became so bad in the early 1990s thatoysters were imported from Penrhyn to assist in the recovery of this population (Dalzell andAdams 1996). Sims (1992) suggested that rapid water exchange and loss of planktoniclarvae through passages in the reef probably contributed to the lack or recovery of black-lippearl oyster populations in the open atoll systems of the Cook Islands. In more “open” coralreef systems, such as those of Solomon Islands, dispersal of larvae from unexploited stocksand refuge stocks beyond the depths in which most divers operate, could lead to recovery ofsome stocks, albeit at a slow rate.The present survey indicated that silver-lip pearl oysters occurred on a variety of benthicsubstrata, including mud, sand, gravel, coral rubble and various mixtures thereof, but weregenerally more abundant in areas characterised by coral rubble, gravel/rubble, sand/rubbleand sand/shell/coral and with a fast flow of water over them. Silver-lip pearl oyster spatselect a hard surface for initial attachment (Gervis and Sims 1992; Wells and Jernakoff 2006)The Ecology Lab Pty Ltd – Marine and Freshwater Studies Page 13