Predicting Oyster Harvests at Maximum Sustained Yield: Application of Cultch and Stock Benchmarks to Depleted Public Oyster Reefs in the Northern Gulf of Mexico

Abstract

A sustainable fisheries model for the estimation of harvests of oysters, maintenance of cultch, and restoration of reefs has been applied across the northern Gulf of Mexico. Oyster density and size, and cultch density are provided from surveys by State agencies. The model simulates oyster growth and mortality, and cultch loss. Reef shell mass is increased when oysters die in place and diminished when oysters are removed by fishing. Harvest is estimated as sacks of live oysters that can be removed while retaining reef quality (i.e., reef cultch mass, oyster density). Application of oyster density and cultch density standards (OCS) for fishing at maximum sustained yield shows little sustainable harvest on Public Grounds in the northern Gulf of Mexico. Model heuristics focus discovery of negative and positive feedbacks to reef demise or persistence. Oyster reef dynamics are viewed as persistent processes within negative and positive feedback loops, and occasional shifts between them. A negative feedback loop is set up on poor-quality reefs. Lack of cultch and a paucity of adult oysters diminish recruitment and shell replenishment. In the absence of recruitment, natural shell loss ultimately drives the reef to extinction. A positive feedback loop is set up on reefs with adequate cultch and an abundance of oysters. There, oysters set, survive critical early stages in boxes and reef interstices, reproduce, grow to a large size, and die in place. Harvests on high-quality reefs beyond that which maintains reef cultch and spawning stock, and consequential recruitment failure cause a shift from a positive to a negative feedback loop. Likewise, restricting harvests and planting cultch on poor-quality reefs, together with successful recruitment, promotes a shift from a negative to a positive state. Results of no-net loss of cultch (NNL) and OCS simulations suggest two distinct management approaches. The NNL promotes the sustainability of reef shell and assumes that a stable shell bed promotes a stable living resource, whereas OCS explicitly promotes the sustainability of cultch and stock at msy (maximum-sustainable-yield). Furthermore, OCS, which incorporates optimal reef cultch and oyster density standards, provides restoration benchmarks. Achieving sustainability at msy will simultaneously promote habitat restoration, restoration of ecosystem services, and a sustainable oyster fishery.