Marine Mammal Strandings and Environmental Changes: A 15-Year Study in the St. Lawrence Ecosystem

Marie-Hélène Truchon,Sylvie Lessard,Lena Measures,Jean-Claude Brêthes,Michel Starr,Vincent L’Hérault,Nicolas Lecomte,Peter S Galbraith,Michel Harvey,Howard Browman

doi:10.1371/journal.pone.0059311

Marie-Hélène Truchon, Sylvie Lessard + Show 8 more

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https://doi.org/10.1371/journal.pone.0059311

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Abstract

Understanding the effects of climatic variability on marine mammals is challenging due to the complexity of ecological interactions. We used general linear models to analyze a 15-year database documenting marine mammal strandings (1994–2008; n = 1,193) and nine environmental parameters known to affect marine mammal survival, from regional (sea ice) to continental scales (North Atlantic Oscillation, NAO). Stranding events were more frequent during summer and fall than other seasons, and have increased since 1994. Poor ice conditions observed during the same period may have affected marine mammals either directly, by modulating the availability of habitat for feeding and breeding activities, or indirectly, through changes in water conditions and marine productivity (krill abundance). For most species (75%, n = 6 species), a low volume of ice was correlated with increasing frequency of stranding events (e.g. R2 adj = 0.59, hooded seal, Cystophora cristata). This likely led to an increase in seal mortality during the breeding period, but also to increase habitat availability for seasonal migratory cetaceans using ice-free areas during winter. We also detected a high frequency of stranding events for mysticete species (minke whale, Balaenoptera acutorostrata) and resident species (beluga, Delphinapterus leucas), correlated with low krill abundance since 1994. Positive NAO indices were positively correlated with high frequencies of stranding events for resident and seasonal migratory cetaceans, as well as rare species (R2 adj = 0.53, 0.81 and 0.34, respectively). This contrasts with seal mass stranding numbers, which were negatively correlated with a positive NAO index. In addition, an unusual multiple species mortality event (n = 114, 62% of total annual mortality) in 2008 was caused by a harmful algal bloom. Our findings provide an empirical baseline in understanding marine mammal survival when faced with climatic variability. This is a promising step in integrating stranding records to monitor the consequences of environmental changes in marine ecosystems over long time scales.

Highlights

Environmental changes are occurring worldwide and the consequences of recent climatic variability are acknowledged as a global perturbation with geographic differences in intensity [1]
Our multiple regression models suggest that the interannual variability of marine mammal stranding events over a 15year period is associated with regional and continental (e.g. North Atlantic Oscillation index values (NAO) index) changes in the environment for both resident and migrant species (Table 2)
The marine mammal community was likely affected by low ice conditions observed in recent years through two pathways; 1- directly with changes in ice conditions modulating the availability of habitat for feeding and breeding activities or 2- indirectly with changes in water conditions and marine productivity

Summary

Introduction

Environmental changes are occurring worldwide and the consequences of recent climatic variability are acknowledged as a global perturbation with geographic differences in intensity [1]. To understand complex (and variable) ecological responses of marine mammals to environmental change, we need to consider environmental parameters over multiple scales (regional and continental) and to address data from multiple species [17,18]. Such a hierarchical approach provides a promising avenue to evaluate effects of climatic variability; this is especially true when dealing with species with great variability in habitat use, e.g. with resident vs migratory species (Fig. 1) [19,20]

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