The North Atlantic right whale (Eubalaena glacialis, hereafter NARW) has faced a millennium of hunting pressure; all that remains of this long-lived, endangered species is a small and vulnerable population. Protected from hunting by international regulation, NARWs continue to face multiple stressors as a result of their highly industrialized coastal habitat. This population has shown very limited recovery compared to Southern right whales (Eubalaena australis), which inhabit less industrialized waters. Progress has been made to reduce vessel strikes on NARWs, but entanglements in fixed fishing gear have become more prevalent and are now the leading cause of serious injury and mortality. In total, 1,194 individual entanglement events were detected from 1980-2011. To quantify the impacts of entanglement on NARWs, we first fit a hierarchical Bayesian (HB) model to photographs of individual NARW in order to estimate latent health states. Next, we combined these health estimates with photographic evidence of entanglement injury, which we classified into six categories of entanglement severity. We then quantified the impact of entanglement on: 1) health; 2) reproductive success; and 3) survival.
We found that as the severity of entanglements increased, right whales were in significantly worse health. In particular, severe entanglements resulted in worse health outcomes for all whales; reproductive females were in significantly poorer health during both moderate and severe entanglements. In terms of the impact on reproduction, females with severe injuries gave birth at significantly lower rates; indeed no calves were born to females following a severe entanglement. In terms of the impact on survival, males and females with severe injuries were eight times more likely to die than males with minor injuries. In summary, NARWs, particularly reproductive females, are experiencing significant sublethal health impacts from fishery entanglement. Considering that 83% of NARWs show evidence of entanglements, with nearly 60% of those whales experiencing two or more entanglements, these results indicate that the current level of effects of fishery entanglements are a significant factor in impeding recovery of this population. Entanglement is a conservation concern for many large whales and these findings underscore the importance of addressing this issue on a global scale.