Using reflex action mortality predictors (RAMP) to evaluate if trawl gear modifications reduce the unobserved mortality of Tanner crab (Chionoecetes bairdi) and snow crab (C. opilio)

摘要

Management of Bering Sea crab and groundfish fisheries must account for the delayed (and hence unobserved) mortality of crab that encounter bottom trawls, but are not captured. A new approach to predicting the delayed mortality of crab uses a reflex action mortality predictor (RAMP) model to establish a relationship between mortality and reflex impairments. A 2007 pilot study of 649 crab established RAMP curves for both Tanner crab (Chionoecetes bairdi) and snow crab (C opilio). Additional data (1775 crab) collected in 2008 allowed us to update the existing RAMP curves to more fully examine the effect of injury scores on the RAMP models and to determine the best method for estimating overall mortality. Results confirmed that the additional measurements did not significantly alter the original relationship between mortality and reflex impairment score. Additionally, the RAMP curves were used to predict unobserved mortality from observed reflex impairment scores of Chionoecetes spp. captured after encounters with different parts of bottom trawl gear (footrope and sweep) and alternative types of footrope and sweep. In addition to estimating mortality rates caused by each gear part, we tested whether the alternative footrope and sweep designs reduced the unobserved mortality rates of crab. Results showed that the alternative footrope (58 cm disk footrope) reduced mortality from 11.4 to 7.2% for Tanner crab and from 9.7 to 5.0% for snow crab. The alternative (off-bottom) sweep reduced mortality from 4.1 to 1.0% for Tanner crab and from 4.9 to 0.0% for snow crab. Thus, the use of the reflex impairment score through the RAMP model is a cost effective way to estimate delayed mortality and to assess the effect of gear types on delayed mortality for management purposes.