Tethering as a method for evaluating shrimp predation by San Juan Island rockfish (Sebastes spp.)

Abstract

Trophic cascades have been shown to completely alter marine communities, especially after the collapse of an important fishery. It is therefore important to understand trophic relationships between marine organisms in order to prevent the drastic change of ecosystem structures due to anthropogenic impacts such as overfishing. In the Salish Sea, rockfish, Sebastes caurinus and Sebastes maliger, are an important predatory fish that have been in decline in past years. In order to understand the predator-prey interactions of rockfish in the San Juan Islands, we used prey tethering techniques, to assess relative rates of predation on shrimp and how much predation is occurring from rockfish. Coon-stripe shrimp, Pandalus danae, were tethered and attached to a frame that was deployed at two sites with different rockfish abundance. A camera was also deployed in order to record video footage of predation. Our method of tethering was successful, showing 100% predation in deployments longer than 18 hours and capturing video footage during the day without the use of an artificial light. Various predators were observed in video data from each site. We recommend that this method be used to assess the rockfish and shrimp relationship in the San Juan Islands and suggest expanding this study to include multiple trials throughout various seasons.