Apparent survival of North Atlantic right whales after entanglement in fishing gear

The impacts of human activities on cryptic marine species can be difficult to assess. The North Atlantic right whale is an endangered species numbering just over 500 individuals. Entanglement in fishing gear is one documented source of injury and mortality, but population-level effects have been difficult to quantify. We used documented entanglements, long-term population studies and mark-recapture statistical techniques to evaluate the effect of these events on North Atlantic right whale survival. Estimates were based on 50 individuals observed carrying entangling gear between 1995 and 2008, and compared to 459 others that were never observed with gear during the same period. Entangled adults had low initial apparent survival (0.749, 95% CI: 0.601-0.855), but those that survived the first year achieved a survival rate (0.952, 95% CI: 0.907-0.977) that was more comparable to unaffected adult females (0.961, 95% CI: 0.941-0.974) and males (0.986, 95% CI: 0.975-0.993). Juveniles had a post-entanglement survival rate that was comparable to the initial survival of entangled adults (0.733, 95% CI: 0.532-0.869) and lower than un-impacted juveniles (0.978, 95% CI: 0.969-0.985). Of three entanglement characteristics examined, health impacts were most predictive of subsequent survival, but the entanglement configuration and the resulting injuries also appeared to affect outcome. When the entanglement configuration was assessed as high risk, human intervention (disentanglement) improved the survival outcome. This is the first mark-recapture estimate of entanglement survival for any whale species. The results indicate the need for continued mitigation efforts for this species, as well as for a better understanding of entanglement impacts in other baleen whale populations.