Efficacy, non-target impacts, and costs of mechanical control options against a bioturbator in bivalve aquaculture

Abstract

Bivalve aquaculture benefits from non-chemical, operational-scale pest management tools to sustain or increase production. Mechanical methods to control a bioturbating pest (burrowing shrimp Neotrypaea californiensis) have been tested in Washington State (USA), providing a roadmap for other regions affected by native or widespread pests. These shrimp smother and bury ground-cultured oysters on tidal flats even when the shrimp are at low densities. Therefore, control methods with high efficacy are required, also considering non-target effects and costs to implement. From 2002 to 2023, 55 mechanical control trials were carried out, plus two shell-addition trials from earlier research. Methods included: 1) surface barriers, 2) shrimp removal, 3) sediment disruption, and 4) physical conditions intended to cause direct mortality (e.g. electricity, heat). Data were compiled through a meta-analysis framework, in which effect size was calculated as the log response ratio of treated relative to reference plots. Most surface barriers were not effective because they were penetrated by shrimp or insufficiently anchored; an exception was application of five inches (12.7 cm) of gravel. Shrimp removal was effective with a water-jet technique developed to collect shrimp for bait at low tide, but efforts to deploy multiple jets with a towed mechanized device were unsuccessful. Sediment disruption through surface compaction was the most common farm-scale approach but insufficiently reduced shrimp densities, whereas consolidating sediment with vibration to 1 m depth had high efficacy but has been applied only in small plots. Effect sizes were not available for any field trials of direct mortality methods, but energy required to kill shrimp was calculated from laboratory studies (1–50 kW-hr m⁻³ in water, and higher energy required in saturated sediment). Across all field methods, efficacy improved with effort (person-hours per area). Sediment showed reduced penetrability and increased muddiness following treatment. Non-target effects on infauna included both positive and negative effect sizes, consistent with a community change following a reduction in shrimp density. In the six studies measuring epibenthic species, no overall positive response to control of shrimp occurred, even though the reason for control is to protect surface-dwelling species from bioturbation by shrimp. This outcome illustrates the importance of pairing efficacy in terms of reduction of shrimp and improvement of farming. The vertical position of N. californiensis below nearly a meter of water-saturated sediment, along with an innate tolerance to pressure and anoxia, has placed them out of reach of most attempted mechanical methods. Although progress has been made in recent years, mechanical control options remain limited that can be carried out without permits and that economically reduce shrimp to densities compatible with benthic shellfish aquaculture at a farm scale.