Restoration of eastern oyster populations with positive density dependence

Abstract

Positive density dependence (i.e., Allee effects) can create a threshold of population states below which extinction of the population occurs. The existence of this threshold, which can often be a complex, multi-dimensional surface, rather than a single point, is of particular importance in degraded populations for which there is a desire for successful restoration. Here, we incorporated positive density dependence into a closed, size- and age-structured integral projection model parameterized with empirical data from an eastern oyster, Crassostrea virginica, population in Pamlico Sound, North Carolina, USA. To understand the properties of the threshold surface, and implications for restoration, we introduced a general method based on a linearization of the threshold surface at its unique, unstable equilibrium. We estimated the number of oysters of a particular age (i.e., stock enhancement), or the surface area of dead shell substrate required (i.e., habitat enhancement) to move a population from an extinction trajectory to a persistence trajectory. The location of the threshold surface was strongly affected by changes in the amount of local larval retention. Traditional stock enhancement with oysters <1 yr old (i.e., spat) required three times as many oysters relative to stock enhancement with oysters between ages 3 and 7 yr old, while the success of habitat enhancement depended upon the initial size distribution of the population. The methodology described here demonstrates the importance of considering positive density dependence in oyster populations, and also provides insights into effective management and restoration strategies when dealing with a high dimensional threshold separating extinction and persistence.

Success of concrete and crab traps in facilitating Eastern oyster recruitment and reef development

“…In general, the sustained, high levels of oyster settlement and growth on both structures indicate that oyster larval loads were quite high during the experiment and are consistent with long-standing knowledge that oysters settle gregariously on conspecifics (Crisp, 1967; O’Beirn et al, 2000). These results also suggest that there are positive, density-dependent feedbacks whereby strong recruitment and growth in one year can promote and sustain higher oyster densities in subsequent years, as has been documented in longer-term datasets (Powell et al, 2009) and proposed in a recent modeling study (Moore, Puckett & Schreiber, 2018).…”

Can relic shells be an effective settlement substrate for oyster reef restoration?

“…Intertidal reefs are subjected to tidal fluxes, impacting larval encounter patterns, but subtidal reefs are continuously submerged allowing for continual and behavioral‐driven settlement (Dame & Patten 1981; North et al 2008; Johnson & Smee 2014). Given oyster settlement can be driven by Allee effects and conspecific cues (Moore et al 2018), there was no significant correlation between mean oyster abundance and spat settlement to each reef, suggesting that current oyster abundance was not driving settlement within the cages and further indicating settlement was a function of shell type. Thus, the results suggest, despite tidal dynamics, restoration efforts within subtidal environments should also be highly dependent on substrate quality.…”

From individual vital rates to population dynamics: An integral projection model for European native oysters in a marine protected area

“…When these rings are visible, O. edulis may occasionally lay down multiple rings in a single growing season (Orton & Amirthalingam, 1927;Richardson, Collis, Ekaratnc, Dare, & Key, 1993). This means that IPMs incorporating age, as have been created with Eastern oysters (Moore, Lipcius, Puckett, & Schreiber, 2016;Moore, Puckett, & Schreiber, 2018), are not currently appropriate for this species.…”