Sea bed disturbance increases flat oyster recruitment for low to moderate stock densities

1. It has long been suggested by commercial fishing interests that the sea bed benefits from being trawled or disturbed. Evidence to support increased benthic food web productivity in areas disturbed by trawling has suggested that this is the case, and that some mobile consumers can benefit from this increased productivity. 2. The same hypothesis has been put forward for shellfish recruitment, that disturbance of the sea bed, e.g. ‘harrowing’, increases the exposure of suitable settlement substrates for shellfish larvae. This is an approach often taken in shellfish mariculture in private fisheries, and has led to calls for support of expanding such activities into publicly managed areas to promote shellfish recovery and restoration. 3. Increased seabed disturbance, however, may not align with conservation policy or societal objectives for natural recovery of the seabed landscape. Furthermore, evidence for increased shellfish recruitment from seabed disturbance is mixed, and many attempts to elucidate whether relationships exist receive criticism for operating at small spatial and temporal scales. 4. An analysis is presented from 3 years of data (2016–2018) from a stock survey of a private European flat oyster, Ostrea edulis, fishery operating in the Blackwater estuary, Essex, UK. Using data for adult and recruit oyster abundance and distribution in 2018, with ‘harrowing’ effort from 2016–2018, it is asked whether oyster recruitment was related to disturbance effort. 5.It was found that oyster recruitment is positively related to increased seabed disturbance, but only up to intermediate adult oyster abundance equivalent to 60 oysters/100 m dredge, beyond which harrowing results in recruitment declines. This has implications for approaches to oyster fishery recovery, but also for restoration projects seeking evidence-led guidance on which ways may be appropriate to kick-start natural recovery in historical oyster areas that are habitat limited.