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

Abstract

Effective 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.

Stranding patterns and feeding ecology of striped dolphins, Stenella coeruleoalba, in Western Iberia (1981–2014)

“…No significant seasonal pattern in number of strandings was found among overall records, in line with Bouchard et al (2019), who also showed low seasonality of strandings in this species, interpreted as an indicator of constant pressures across the seasons. However, along the CS coast, which is the only area with no local stranding team operating, there were significant seasonal differences.…”

Statistical Analysis of the Possible Association Between Geomagnetic Storms and Cetacean Mass Strandings

“…The seasonality is seen as a bimodal distribution with peaks around March and August. Seasonality is also seen for the New Zealand mass strandings as shown in Figure 3c (see also Bouchard et al, 2019, for seasonality in a French stranding data set not analyzed here). Seasonal dependence of mass stranding and geomagnetic data will give rise to a seasonally varying statistical association, even in the absence of causal connection.…”

“…During major solar storm events, interplanetary disturbances can generate major changes in the Earth's near‐space electric current systems and those changes cause geomagnetic field variations at times scales varying from seconds to days. We refer to these magnetic field variations as “geomagnetic storms.” Interaction with fisheries through bycatch (e.g., Bouchard et al, 2019). Active sonars utilized in military activities (e.g., Southall et al, 2013).…”