A risk‐based forecast of extreme mortality events in small cetaceans: Using stranding data to inform conservation practice

Jérôme Spitz,Colin Bouchard,Olivier Van Canneyt,Vincent Ridoux,Willy Dabin,Cameron Bracken,Matthieu Authier

doi:10.1111/conl.12639

Abstract

AbstractEffective conservation requires monitoring and pro‐active risk assessments. We studied the effects of at‐sea mortality events (ASMEs) in marine mammals over two decades (1990–2012) and built a risk‐based indicator for the European Union's Marine Strategy Framework Directive. Strandings of harbor porpoises (Phocoena phocoena), short‐beaked common dolphins (Delphinus delphis), and striped dolphins (Stenella coeruleoalba) along French coastlines were analyzed using Extreme Value Theory (EVT). EVT operationalizes what is an extreme ASME, and allows the probabilistic forecasting of the expected maximum number of dead animals assuming constant pressures. For the period 2013–2018, we forecast the strandings of 80 harbor porpoises, 860 common dolphins, and 57 striped dolphins in extreme ASMEs. Comparison of these forecasts with observed strandings informs whether pressures are increasing, decreasing, or stable. Applying probabilistic methods to stranding data facilitates the building of risk‐based indicators, required under the Marine Strategy Framework Directive, to monitor the effect of pressures on marine mammals.

Highlights

In the Anthropocene, anthropogenic pressures (Maxwell, Fuller, Brooks, & Watson, 2016; Steffen, Broadgate, Deutsch, Gaffney, & Ludwig, 2015) and extreme environmental events (Ummenhofer & Meehl, 2017) erode biodiversity
This study aimed to develop a risk-based indicator for small cetaceans by building a statistical model using Extreme Value Theory (EVT) to identify extreme at-sea mortality events (ASMEs) based on stranding data and forecast their probable magnitude in the future
Fresh deaths were largely attributed to bycatch for common dolphins and harbor porpoises; both sexes seemed impacted (Figure 3)

Summary

Introduction

In the Anthropocene, anthropogenic pressures (Maxwell, Fuller, Brooks, & Watson, 2016; Steffen, Broadgate, Deutsch, Gaffney, & Ludwig, 2015) and extreme environmental events (Ummenhofer & Meehl, 2017) erode biodiversity. Previous works on extreme events have focused on their retrospective identification and underlying causes (Barbraud, Delord, & Weimerskirch, 2015; Solow, 2017). The latter can be very difficult to investigate, especially in ecology where collecting the necessary long-term data is challenging (Anonymous 2017). Studying extreme environmental events through their consequences, rather than their causes, is relevant for biodiversity conservation to emphasize proactive rather than reactive policies.

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Conclusion