The relative by-catches of taxa associated with vulnerable marine ecosystems by autolines and Spanish longlines

We reanalyzed data collected by the longline fishery in Subarea 88.1 to compare the bycatches of VME taxa made by vessels using Autolines and Spanish longlines. The fractions of sets with positive bycatch, as well as the weights of bycatch when bycatches were observed, were higher for Autolines than for Spanish longlines. Using a generalized linear approach with a Tweedie distribution to model the large number of zero-bycatch sets, we found that both the amounts of positive bycatches and the probabilities of observing positive bycatches decreased with depth for both gears. We also found that the rate at which positive bycatches decreased with depth was slightly greater for Autolines than for Spanish longlines. Since we only analyzed data from areas where there was substantial spatial overlap of sets made with each gear type, we attributed this difference to a difference in the rate at which VME taxa drop off Autolines relative to Spanish longlines while the gear is being hauled. We used a Bayesian approach and estimated that bycatch of VME taxa by Autolines is about 9 times greater per hook (about 5 times greater per set) than bycatch by Spanish longlines at 600 m. While there is considerable uncertainty about this ratio (the 95% credible interval for this ratio ranges from about 3 to 22 times greater per hook or about 2 to 12 times greater per set), there is near certainty that Autoline bycatch is greater than Spanish bycatch at all depths represented in our data. Limiting the use of Autolines, which are in more intimate contact with the sea floor than Spanish longlines, may decrease bycatches of VME taxa and thereby provide a precautionary approach to mitigating impacts on VMEs.