Sustainable exploitation and management of autogenic ecosystem engineers: application to oysters in Chesapeake Bay

Abstract

Autogenic ecosystem engineers are critically important parts of many marine and estuarine systems because of their substantial effect on ecosystem services. Oysters are of particular importance because of their capacity to modify coastal and estuarine habitats and the highly degraded status of their habitats worldwide. However, models to predict dynamics of ecosystem engineers have not previously included the effects of exploitation. We developed a linked population and habitat model for autogenic ecosystem engineers undergoing exploitation. We parameterized the model to represent eastern oyster (Crassostrea virginica) in upper Chesapeake Bay by selecting sets of parameter values that matched observed rates of change in abundance and habitat. We used the model to evaluate the effects of a range of management and restoration options including sustainability of historical fishing pressure, effectiveness of a newly enacted sanctuary program, and relative performance of two restoration approaches. In general, autogenic ecosystem engineers are expected to be substantially less resilient to fishing than an equivalent species that does not rely on itself for habitat. Historical fishing mortality rates in upper Chesapeake Bay for oysters were above the levels that would lead to extirpation. Reductions in fishing or closure of the fishery were projected to lead to long-term increases in abundance and habitat. For fisheries to become sustainable outside of sanctuaries, a substantial larval subsidy would be required from oysters within sanctuaries. Restoration efforts using high-relief reefs were predicted to allow recovery within a shorter period of time than low-relief reefs. Models such as ours, that allow for feedbacks between population and habitat dynamics, can be effective tools for guiding management and restoration of autogenic ecosystem engineers.

Reef height drives threshold dynamics of restored oyster reefs

“…Population models suggest that sediment deposition mediated by initial reef height is capable of producing alternative stable equi libria similar to reef outcomes observed in the field (JordanCooley et al 2011, Wilberg et al 2013, Housego & Rosman 2016. In these models, a stable, non-zero equilibrium is achieved when live oyster and shell volume growth outpaces sediment de position, which varies with reef height and reaches a maximum at the seafloor (Jordan-Cooley et al 2011, Housego & Rosman 2016.…”

“…The threshold dynamics observed in this study are a likely explanation for both the initial loss of reef habitats along the Atlantic coast (Scheffer et al 2001, Kirby 2004) and for the failure of many repletion and restoration efforts that utilized low-relief reefs (Schulte et al 2009, Kennedy et al 2011, Wilberg et al 2013. Destructive harvesting techniques like dredging and hydraulic tonging reduce the height of reefs by disaggregating the reef structure and removing accreted oyster shells along with live oysters (Lenihan & Peterson 1998).…”

“…1). This range of reef heights encompasses those typically used in oyster restoration or repletion activities in Chesapeake Bay (0.05 to 0.1 m; Wilberg et al 2013) and heights shown to produce 'successful' reefs (Schulte et al 2009. Reefs were circular (1.3 m diameter) and plateau-shaped to maximize the area of intended height (Fig.…”

“…While this bioengineering function provides habitat heterogeneity and promotes biodiversity (Lucken bach et al 2005), it also renders species less resilient to intensive harvesting and severe habitat degradation (Herman et al 1999, Scheffer et al 2001, Dame et al 2002, Wilberg et al 2013. The collapse of oyster populations around the world has often in -© The authors 2017.…”

“…Modeling studies suggest that these outcomes represent 2 alternative stable states of oyster reef ecosystems (Jordan-Cooley et al 2011, Wilberg et al 2013, Housego & Rosman 2016) that result from biophysical feedbacks re sponding to initial reef height. Elevation of the reef above the seafloor restricts the height of the water column as it passes over the reef, resulting in faster flow rates relative to non-constricted flow (Lenihan 1999).…”