Variation in abundance of blacklip pearl oyster (Pinctada margaritifera Linne.) spat from inshore and offshore reefs in Solomon Islands

This study was designed to test two predictions. The first was that collect ions of spat of the blacklip pearl oyster, Pinctada margaritifera, should b e greater from offshore than inshore zones in 'open' reef systems adjacent to high islands. The second was that the lack of heavy run-off from land-ma sses will result in greater abundance of spat being caught from inshore zon es during the dry season as compared to the wet season, whereas offshore zo nes less influenced by run-off should have catches that are comparable betw een seasons. Sampling was done at three sites within inshore and offshore z ones at two regions (Gizo and Noro) in Solomon Islands, during the dry and wet seasons of 1996-1997. During both seasons, abundances of spat from offs hore zones were significantly greater than from inshore zones in both regio ns. However, the two patterns of distribution in the second prediction were upheld at only one region: there was a dramatic decline in abundance of sp at between the dry and wet season at Noro, and there was no difference in t he number of spat caught inshore during the dry season and wet season at Gi zo. In addition to natural fluctuations in settlement of spat over space an d time, patterns of distribution between inshore and offshore zones corresp onded to variations in salinity and water clarity, and current speed. Howev er, the exact mechanism(s) determining the distribution of spat between ins hore and offshore zones cannot be identified until experiments on the effec ts of land run-off on collections of spat are completed, and until plankton surveys are done to quantify variation in supply of larvae to inshore and offshore zones. (C) 1999 Elsevier Science B.V. All rights reserved.