Novel Shell Stock–Recruitment Models for Crassostrea virginica as a Function of Regional Shell Effective Surface Area, A Missing Link for Sustainable Management

Stock–recruitment relationships (SRR) are used by many resource managers as a metric to evaluate stock reproductive potential and rebuilding capacity when harvested. This relationship is more evident in traditional finfish stock assessment models where larvae are frequently limited by the abundance of spawning adults, whereas invertebrate resources are much more confounding and often do not express a clear spawning SRR. Oysters in particular appear to be limited more strongly by post-settlement mortality as larvae are regularly demonstrated to be present in high abundance in response to timely shell plants; yet unexpectedly, a brood SRR appears to be present. Because of the unique life history of oysters which provide their own habitat through reef accretion, a posit is that the quantity and quality of such habitat are crucial determinants of recruitment to the stock, and habitat quality acts as a surrogate for brood stock in the stock–recruit relationship. An extensive dataset from Delaware Bay with a 65-y time series was analyzed to assess the relationship between available Crassostrea virginica natural reef materials and subsequent spat (<20 mm) settlement. Live oyster, box (dead and articulated valves), and cultch (disarticulated whole valves and shell fragments) surface areas were calculated for oyster reefs found in each of four regions oriented up-bay to down-bay within the survey area. Surface areas were standardized using RMA regressions which compared available surface area per shell type (live, box, and cultch) and the number of spat recruiting to each shell type. Standardized shell surface areas (i.e., effective surface areas) supported previous accounts that recruitment occurs with higher frequency on live oysters and boxes than cultch, and that excessive cultch availability does not always lead to successively high recruitment. Recruit density was found to not only vary by substrate type but also along a salinity gradient. In high salinity reefs, boxes function as high-quality recruitment substrate and cultch as low-quality substrate, whereas in low-salinity regions cultch is only slightly inferior to live and outcompetes box surfaces as a dominant recruitment substrate. Beverton–Holt and Ricker models were used to express the relationship between total effective surface area and recruitment; these models revealed that medium- and high-salinity regions produced higher recruitment rates than low-salinity reefs. Model steepness also increased at higher salinity, indicating that down-bay reefs responded more rapidly to a change in effective surface area than did up-bay reefs. Steepness was highest at the confluence of the medium-salinity and high-salinity reaches where larval supply provided by the down-bay drift of larvae from up-bay reefs was high, but salinity remained low enough to reduce biofouling and predation compared with the high-salinity region. Unlike the low-salinity region, the relationship between effective surface area and recruitment for the three down-bay regions exhibited compensation; recruitment did not rise linearly with effective surface area. In all four regions, however, recruitment was initiated on a reef at a nonzero effective surface area, an analogy to an Allee effect which existed in each case. By inference, reefs can die before effective surface area disappears.