Temporal variation in stranding numbers of marine mammals: how individual populations are impacted and their repercussions.
Ashley Kidd (1), Mike Belanger(2), Carin Wittnich(1,2)
1 Department of Ecology & Evolutionary Biology, University of Toronto, 25 Willcocks Street, Toronto, Ontario, Canada
2 Oceanographic Environmental Research Society, 12 Burton Avenue, Barrie, Ontario, Canada, L4N 2R2
Abstract. Marine mammal strandings are a worldwide phenomenon that can be attributed to a variety of factors. Strandings may be live or dead marine species caught in debris, fishing gear, or found beached on shore. It was hypothesized that, assuming there were no external factors affecting the probability of an individual to strand, that strandings rates would be positively associated to population size and growth rates, and any deviation above this would be attributed to unique physiological, behavioural and environmental factors increasing the rate of stranding.
Stranding numbers have been increasing over time, therefore population specific stranding rates were analyzed on a temporal scale. This was assessed based on multi-year data on reported strandings and population estimates and also accounted for anecdotal information i.e. current protection status to aid in the interpretation of the severity of the observed stranding rates.
Historical data and recent stranding records were compared to corresponding population estimates reported or generated from scientific literature. To assess the association between population and stranding events, population data was compiled on three continents for 6 cetaceans, 1 pinniped and 1 dugong. Four populations displayed and increase in the proportion of stranding rates during the past decade, and 4 populations showed a decrease in relative population stranding rates.
Within certain populations, different implications on the rate of strandings depended on their population (1) recruitment rates: California Sea lion Zalophus californianus, stranding rates are declining but are reported to have exceeded their carrying capacity in 1997, (2) conservation status: the dugong, Dugong dugong, populations are protected along the Queensland coast, which showed no change in the proportion of strandings over time, (3) the degree of direct or indirect effects of human presence: United Kingdom Common dolphin, Tursitops truncatus, and Harbour porpoise, Phocoena phocoena, showed increases in relative stranding rates over time.
The data supports the theory that observed stranding rates are a result of changes in the fitness and environment of the species. In the future, the proportion of stranding rates will be expected to increase due to new climate models predicting permanent changes to ocean systems, affecting marine mammal survivorship, from increased anthropogenic interactions and susceptibility to novel environments, as some cases suggest.