Toward design criteria in constructed oyster reefs: Oyster recruitment as a function of substrate type and tidal height

Restoration of degraded oyster reef habitat generally begins with the addit ion of substrate that serves as a reef base and site for oyster spat attach ment. Remarkably, little is known about how substrate type and reef morphol ogy affect the development of oyster populations on restored reefs. Three-d imensional, intertidal reefs were constructed near Fisherman's Island, Virg inia: two reefs in 1995 using surfclam (Spisula solidissima) shell and six reefs in 1996 using surfclam shell, oyster shell, and stabilized coal ash. We have monitored oyster recruitment and growth quarterly at three tidal he ights (intertidal, mean low water, and subtidal) on each reef type since th eir construction Oyster recruitment in 1995 exceeded that observed in the t wo subsequent years. High initial densities on the 1995 reefs decreased and stabilized at a mean of 418 oyster/m(2). Oyster settlement occurred on all reef types and tidal heights in 1996; however, postsettlement mortality on the surfclam shell and coal ash reefs exceeded that on the oyster shell re efs, which remained relatively constant throughout the year (mean = 935 oys ters/m(2)). Field observations suggest that predation accounts for most of the observed mortality and that the clam shell and coal ash reefs, which ha ve little interstitial space, suffer greater predation. Oyster abundance wa s consistently greatest higher in the intertidal zone on all reefs in each year studied. The patterns observed here lead to the preliminary conclusion that the provision of spatial refugia (both intertidal and interstitial) f rom predation is an essential feature of successful oyster reef restoration in this region. In addition, high levels of recruitment can provide a nume rical refuge, whereby the oysters themselves will provide structure and inc rease the probability of an oyster population establishing successfully on the reef.