A risk-based forecast of extreme mortality events in small cetaceans: Assessing trends and changes over time

Abstract

Numerous conservation programs are now targeting the mitigation or reduction of anthropogenic pressures to restore biodiversity. The aspirational and ambitious objectives underlying these programs need to translate into timely management actions for maximum impact, a process whose first steps assume efficient monitoring and appropriate indicators. Capitalizing on a well-coordinated stranding network, we developed further a risk-based indicator informing indirectly on pressures resulting in at-sea-mortality events of small cetaceans. This indicator uses Extreme Value Theory to forecast the expected maximum number of cetacean strandings on the French seaboard adjacent to three sub-regions defined by the ’Marine Strategy Framework Directive’ of the European Union. A comparison of forecasts and observations allows (i) a risk assessment, and (ii) inference on the trajectory towards ’Good Environmental Status’ on a time-scale aligning with the 6-years cycle required by the Directive, thereby allowing stakeholders to make informed and timely decisions. We applied the use of the indicators on 3 species illustrating different case studies: the harbour porpoise (Phocoena phocoena) in the Channel, the common dolphin (Delphinus delphis) in the Bay of Biscay, and the striped dolphin (Stenella coeruleoalba) in the Western Mediterranean Sea. Our results showed notably how by-catch in fisheries remains a large threat on short-beaked common dolphins, with no sign of abating.

Keywords

• Strandings
• Extreme value theory
• Good environmental status
• Ecological forecasting
• Die-off
• Mass mortality event