Slow down but level up: to address ship strikes, we need more than speed limits

Résumé

Shipping speed reductions have been and should remain a key strategy for reducing whale‐ship strikes, but using them to justify additional shipping is questionable. Here we review what is known – and what is not – about the efficacy of speed mitigation in reducing vessel collisions and/or their lethality. We found 30 papers that present original evidence on this issue, half of which pertained to whale‐watching. Of the remaining papers, the majority reported negative, ambivalent, or highly context‐dependent evidence of avoidance or reduced ship‐strike lethality. The most compelling support was found for North Atlantic right whales and humpback whales, but even for these species, there were also studies with negative results. For the other species that have been studied (n = 4), evidence is too limited to be conclusive but suggests that avoidance is uncommon to rare. On balance, while speed reductions probably do reduce strike rates and lethality in some whale species, those rates likely remain high, even at low speeds, apparently more probable than not for ships at speeds as low as 8–9 kn. Avoidance is less understood than lethality at this juncture, but overall our knowledge of whale‐ship interactions is so sparse and mixed that speed controls alone ought not be equated with legal compliance. Instead, they should be treated within management contexts as a sensible but unverified hypothesis. Practically, this means that speed controls should always be used in tandem with other operational mitigations, and their efficacy should never be assumed. Robust monitoring should be required whenever speed mitigations are used.