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

Abstract

Restoration of degraded oyster reef habitat generally begins with the addition of substrate that serves as a reef base and site for oyster spat attachment. Remarkably, little is known about how substrate type and reef morphology affect the development of oyster populations on restored reefs. Three-dimensional, intertidal reefs were constructed near Fisherman's Island, Virginia: 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 heights (intertidal, mean low water, and subtidal) on each reef type since their construction. Oyster recruitment in 1995 exceeded that observed in the two subsequent years. High initial densities on the 1995 reefs decreased and stabilized at a mean of 418 oyster/m2. 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 reefs, which remained relatively constant throughout the year (mean = 935 oysters/m2). Field observations suggest that predation accounts for most of the observed mortality and that the clam shell and coal ash reefs, which have little interstitial space, suffer greater predation. Oyster abundance was 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) from predation is an essential feature of successful oyster reef restoration in this region. In addition, high levels of recruitment can provide a numerical refuge, whereby the oysters themselves will provide structure and increase the probability of an oyster population establishing successfully on the reef.