Marine birds and harmful algal blooms: sporadic victims or under-reported events?

Abstract

From the late Pliocene to now, blooms of toxic algae are associated with mortalities of marine birds. Given the long historical presence of harmful algal blooms (HABs) worldwide and the numbers of seabirds that feed on filter-feeding fish and shellfish, it is surprising that relatively few incidents of seabird deaths as a result of toxic algae have been reported. The limited information available tends to come from major events, whereas the rare events are missed and hence not reported. Much is anecdotal and still more probably is not published. We suspect that factors working in concert may lead to deaths and wrecks that might not occur as a result of anyone factor working independently, e.g. starvation tends to render birds more vulnerable to stress. “Seabird wrecks”, very much larger than usual concentration of seabird corpses washed ashore over a short period of time, often provide evidence of deleterious conditions in offshore populations, e.g. weather, food, pollution, fishing activities, and parasites. It is noted in the literature that wrecks caused by natural toxins such as botulism and algal toxins are apparently less common; however, this perception may be due to a combination of factors including the bird species involved, size of populations, location, and chance of discovery. Wrecks involving near-shore species probably provide a more accurate estimate of total mortality for any given event than offshore species. A survey of available data on the impacts of toxic algae on seabirds revealed an array of responses ranging from reduced feeding activity, inability to lay eggs, and loss of motor coordination to death. Severe impacts on recruitment have been noted in some populations. There are few experimental studies; however, evidence has been provided for the ability of some species to ‘learn’ to avoid toxic food sources. We present a summary of available data on seabird/toxic algal interactions and suggestions of how impacts on seabirds during future blooms of harmful algae be recorded.