Trade-offs and synergies in the management of environmental pressures: a case study on ship noise mitigation

Underwater noise from shipping is increasingly recognized as a significant pollutant that can have a range of detrimental effects on marine organisms. However, ships impact marine life in more than one way. From a management perspective, a holistic approach could provide a more successful way to minimize the impact of ship traffic than sequential, single-pressure mitigation. In this paper, we assess how other shipping pressures are affected by six noise mitigation measures: ship speed restriction, rerouting, convoying, frequent hull/propeller cleaning, ship-quieting technologies, and incentivising fewer, larger ships. Here, we present and apply a framework to evaluate the synergies and trade-offs in the implementation of mitigation measures to better consider cumulative effects and advance effective, and holistic management. Using expert judgement and peer-reviewed literature, we evaluate each of the proposed mitigation measures to determine whether they are likely to have synergistic or trade-off effects on the impacts from other shipping pressures, the scale of the effect, and the strength of the evidence. Overall, speed reduction has mostly synergies with only weak trade-offs in the other shipping pressures. Frequent hull and propeller cleaning has fewer synergies, but also very few trade-offs, whereas convoying is expected to be the measure with the most trade-offs with other pressures. Re-routing and the incentivization of fewer larger ships have mostly unclear outcomes, because this will depend on the circumstances of implementation. We conclude that carefully considered and thoughtfully implemented mitigation measures can lead to multiple benefits across shipping pressures.