Evaluating the flexibility of a reflex action mortality predictor to determine bycatch mortality rates: A case study of Tanner crab (Chionoecetes bairdi) bycaught in Alaska bottom trawls

Abstract

To quantify total fishing mortality it is necessary to incorporate mortality rates attributed to bycatch, including animals that are discarded and that interact with the gear without being caught. The Reflex Action Mortality Predictor (RAMP) approach has been increasingly used to determine bycatch mortality rates in fisheries. This methodology creates a RAMP that relates reflex impairment to probability of mortality. As the RAMP approach becomes more prevalent it becomes important to evaluate the efficacy of its application. We evaluated the flexibility of this methodology by creating a RAMP for Tanner crab (Chionoecetes bairdi) discarded from the groundfish bottom trawl fishery in the Gulf of Alaska and comparing it to a previously established RAMP for unobserved Tanner crab bycatch (encountered gear and remained on the seafloor) from the bottom trawl fishery in the Bering Sea. The two RAMPs and the overall mortality rates calculated using these predictors were comparable. However, we detected significant differences between RAMPs. While probabilities of mortality were similar between the two studies for crab with all or no reflexes missing, discarded crab with intermediate reflex impairment had lower mortality probabilities than those from the unobserved-bycatch study. Our results indicate that a RAMP may produce more accurate mortality estimates when applied to animals experiencing similar stressors as those evaluated to create the RAMP, through similar methodology.