Tanner crab (Chionoecetes bairdi), AFSC
Yochum et al. 2015 evaluated the flexibility of RAMP 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 authors found that: “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.”
Conditions for holding crab after exposure to stressors can control delayed mortality and should be considered in experiments. “There were differential mortality rates by holding type. Higher mortality rates occurred in the on-board tanks (where the crab were held for the first few days) and in the laboratory tank. Moreover, Score-zero crab died in the holding tanks, but not in the at-sea cages. These results indicate that holding tanks contribute additional stressors, either due to transport, additional handling, or stress from being held in an unnatural setting or at temperatures greater than what was experienced in their natural environment.
Our holding duration of two weeks was sufficient to determine mortality for all Scores. Given that it can take longer for Score-zero animals to die than those with higher Scores, our holding period allowed us to sufficiently capture Score-zero mortalities. However, the death of a Score-zero crab at day 12 may indicate that holding for more than a week confuses mortality attributed to fishing stressors with that from captivity.”
Evaluation of RAMP flexibility was made by comparing results from different studies. “To evaluate the divergence between the RAMPs we analyzed the differences between the studies. The primary difference was in experimental methods, namely the treatment of the crab before assessment. Crab from the Discard-mortality study were exposed to air for 90min on average (range from 9 to 230min) without any “recovery” in water. In contrast, crab from the Unobserved-mortality study had only brief air exposure and were held in water while awaiting assessment (generally less than 15 min), which may have allowed some recovery.
These differences in air exposure and recovery in water probably affected the relationship between observed reflex impairments and delayed mortality and hence accounted for the discrepancy between RAMPs. Prolonged air exposure and experiencing cold temperatures was linked with increased delayed and instant mortality, number of autonomies for crab, as well as reduced vigor, juvenile growth, and feeding rates (Carls and O’Clair, 1995; Giomi et al., 2008; Grant, 2003; Stoner, 2009; Warrenchuk and Shirley, 2002). Stoner (2009) found that reflex impairment score and exposure to freezing temperatures were nearly linearly related for Tanner crab. Moreover, he found that the different RAMP reflexes had variable sensitivity to freezing temperatures, namely that the chela closure reflex was the most sensitive reflex, and mouth closure was least. Similarly, Van Tamelen (2005) found that the legs and eyes of snow crab cooled faster than the body, perhaps making them more susceptible to cold air exposure. We hypothesize that the prolonged air exposure for the Discard-mortality study likely impaired the crabs’ reflexes and resulted in higher Scores.”
Recommendations made by the authors. “Results from this study indicate that bias can be introduced in mortality rate estimates when using a RAMP created for one study to estimate mortality rates for a different study where the experimental methods differ, especially with respect to air exposure and recovery in water before assessment. However, when RAMP is used only to approximate mortality rates or to make comparisons between gear types or uses, a previously established RAMP could be used with caution, especially if animals with intermediate Scores are not predominant. For more accurate bycatch mortality rate estimates, our results indicate the importance of using a RAMP that was created by assessing animals that experienced similar stressors to those which the RAMP will be applied. Namely, the procedure for assessing the animals should be similar. We feel that the amount of time the animal spends out of water before assessment be standardized within a time range, along with whether or not the animal is allowed to recover in water before assessment, unless these variables are the treatments being studied.”
“Our results indicate that consistency in methodology and relevance with respect to mimicking actual fishing stresses for the RAMP approach increases the flexibility of RAMP. It is therefore important, when creating a RAMP, to create repeatable methods that are well documented when publishing. RAMP reflexes should be assessed in a specified order to prevent bias from reflexes that are physiologically linked. If there is a reflex that influences the determination of other reflexes it should be assessed last or not at all. Reflexes that are difficult to determine presence or absence should not be used, and it should be clear in the methods what constitutes an “absent” reflex and how immediate mortalities are treated (are they given a Score or classified separately?). In addition, when a RAMP is being created, data should be recorded on all possible stressors, including injury, and evaluated for their contribution to mortality. Moreover, effort should be made (within the logistical constraints of field and laboratory research) to minimize additional stressors that are unrelated to the fishing stressors of interest.”
