Oysters, crabs, and burrowing shrimp: Review of an environmental conflict over aquatic resources and pesticide use in Washington State’s (USA) coastal estuaries

Abstract

Washington State’s coastal estuaries are productive shallow water environments that support commercial fisheries for Dungeness crabs (Cancer magister) and English sole (Parophrys vetulus) by providing 0+(settlement to age l) populations with critical refuge and foraging habitats until subadults migrate to the nearshore coast. Intertidalmudflats also constitute prime areas for commercial oyster (Crassostrea gigas) culture, an, important industry for the coastal communities of Willapa Bay and Grays Harbor that supply much of the nation's oysters. Conflicts over natural resources and estuarine utilization have arisen over the last 37 yr due to the use of carbaryl (an organo carbamate pesticide) by oyster growers on their grounds to control populations of burrowing thalassinidean shrimp (Neotrypaea californiensis and Upogebia pugettensis). Burrowing shrimp, which have an indirect negative effect on oyster survival and growth through bioturbation and sediment destabilization, are killed by carbaryl, as are 0+ and subadult Dungeness crabs, 0+English sole, and other non-target species prsent on the tideflats at the time of application. The pesticide is delivered at 9 kg ha⁻¹ directly to the mudflat as a wetable powder during low tides in July and August. Commercial crabbers and other groups who have economic, recreational, and environmental interests in the estuaries have generally opposed use of the chemical that oyster growers maintain is essential to sustain production levels. For years, government natural resource agencies that regulate the use of carbaryl lacked critical information needed to effectively manage the program. An Environmental Impact Statement (EIS) and Supplemental EIS have provided much of that data and helped shape management decisions with regard to establishing carbaryl concentration rates and total allowable spray area. Additional research is needed to develop more economically and environmentally sound policies for shrimp control based on burrowing shrimp-oyster interactions on an estuarine-wide scale. In this paper we review issues pertaining to oyster culture, the use of carbaryl to control burrowing shrimp populations, and effects on non-target species, drawing upon research from, published articles as well as unpublished data collected by the authors. We also discuss what is known of burrowing shrimp life history and ecology and emphasize the importance of integrating information on shrimp, such as timing of recruitment, variability in year class strength, and patterns of habitat use, into carbaryl control policies or alternative strategies that may be developed in the future. We recommend controlled experimentation be done to examine the ecological effects of delaying carbaryl application to some ghost shrimp beds until October after peak recruitment of 0+ ghost shrimp has occurred, allowing the number of hectares treated each year to vary based on fluctuations in pest population densities, and modifying the substrate by applying a dense layer of oyster shell to the mudflat (shell pavement) to reduce recruitment of ghost shrimp.