Largest baleen whale mass mortality during strong El Niño event is likely related to harmful toxic algal bloom.

Verena Häussermann,Ana M Valenzuela-Toro,Michael Beddington,Carolina S Gutstein,Hector H Sepúlveda,Fanny E Horwitz,David Cassis,Kaitlin M Mcconnell,Maria Jose Perez-Alvarez,Carlos Olavarria,Andrew C Dale,Günter Försterra

doi:10.7717/peerj.3123

Abstract

While large mass mortality events (MMEs) are well known for toothed whales, they have been rare in baleen whales due to their less gregarious behavior. Although in most cases the cause of mortality has not been conclusively identified, some baleen whale mortality events have been linked to bio-oceanographic conditions, such as harmful algal blooms (HABs). In Southern Chile, HABs can be triggered by the ocean–atmosphere phenomenon El Niño. The frequency of the strongest El Niño events is increasing due to climate change. In March 2015, by far the largest reported mass mortality of baleen whales took place in a gulf in Southern Chile. Here, we show that the synchronous death of at least 343, primarily sei whales can be attributed to HABs during a building El Niño. Although considered an oceanic species, the sei whales died while feeding near to shore in previously unknown large aggregations. This provides evidence of new feeding grounds for the species. The combination of older and newer remains of whales in the same area indicate that MMEs have occurred more than once in recent years. Large HABs and reports of marine mammal MMEs along the Northeast Pacific coast may indicate similar processes in both hemispheres. Increasing MMEs through HABs may become a serious concern in the conservation of endangered whale species.

Highlights

Most populations of whales have been fully protected from industrial hunting for half a century, some were reduced to such low levels that recovery is still very slow (Baker & Clapham, 2004)
About 90% of the whales died during one mass mortality events (MMEs) (94.7% for time since death and 87% for time at sea analysis), most probably between Feb and Apr 2015
Since it is likely that all or most of the affected whales were sei whales, the documented mortality may represent a significant increase over the usual death rate of Southern Hemisphere sei whales (Reilly et al, 2008)

Summary

Introduction

Most populations of whales have been fully protected from industrial hunting for half a century, some were reduced to such low levels that recovery is still very slow (Baker & Clapham, 2004). Less frequent and much smaller in magnitude are sudden and locally restricted baleen whale mortalities The largest of those involved 14 humpback whales, which died around Cape Cod during five weeks in Nov 1987 (Geraci et al, 1989) (Table 1). The causes of most MMEs have not been conclusively identified (Anonymous, 2015; Coughran, Gales & Smith, 2013; Gulland et al, 2005); paralytic shellfish poisoning (PSP) during harmful algal blooms (HABs) has been argued as one of the main likely causes (and this is the case for other marine vertebrate mass mortalities; Geraci et al, 1989; Durbin et al, 2002; Doucette et al, 2006; Rowntree et al, 2013; Cook et al, 2015; D’Agostino et al, 2015; Wilson et al, 2015; Lefebvre et al, 2016)

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