Impacts of algal toxins on marine mammals

Abstract

Diatoms and dinoflagellates are microalgae that make up the base of the marine food webs that support marine mammals. However, certain species produce potent toxins that are responsible for extensive fish kills and human illnesses. Acute morbidity or mortality events occur almost annually among marine mammals that also appear to correlate with the presence of toxic algal blooms. Four classes of algal toxin have been associated to date with marine mammal morbidity and mortality events (Figure 10.1, Table 10.1). All are neurotoxins, but are structurally diverse and act on a variety of different cellular receptors, leading to expected differences in symptomology, pathology and epidemiology in exposed animals. Exposure to algal toxins may be direct, through respiratory exposure, or indirect via food-web transfer. Thus, the potential for exposure of marine mammals to these toxin classes is dependent upon the occurrence of toxin-producing microalgae within the habitat of the mammals in question and, in the case of food-web transfer, the co-occurrence of appropriate prey species with the toxin producer. This chapter reviews our current understanding of the impacts of algal toxins on marine mammals. Historically, the identification of an algal toxin as the causative agent in marine mammal events has been difficult due to inadequate detection methods for these toxins, the lack of hallmark symptoms in marine mammals associated with intoxication by a particular algal toxin class, and difficulty in obtaining fresh tissues from stranded animals for pathological and toxicological analyses. However, with the improvement in toxin detection methods over the past decade, evidence is mounting to support the hypothesis that marine algal toxins may play a significant role in previously unexplained episodic mass mortalities of marine mammals. The global occurrence of harmful algal blooms has expanded over the past half century, in terms of both frequency and geographic distribution (Smayda, 1990; Hallegraeff, 1993; Van Dolah, 2000), due, at least in part, to anthropogenic impacts on the coastal environment, including the effects of global warming. This suggests that the impacts of algal toxins on marine mammals may likewise increase.