Archive for October 20th, 2008

October 20, 2008

Rising Concerns For Lagoon Dolphins

Chloe Wells /

The Indian River Lagoon (IRL) system is home to a semi-residential population of roughly five hundred Atlantic bottlenose dolphin.   Well, roughly four-hundred and fifty after this summer.  Researchers from Fish and Wildlife and the Hubbs-SeaWorld Research Institute have noted fifty dolphin deaths within the lagoon from May to October 11th when a female was reported pushing a dead calf near Titusville. 

While there are certainly natural fluctuations in any population’s size, the mortality rate for summer 2008 is nearly twice the typical rate seen for bottlenose dolphin in the IRL.  What’s going on?  No one has a ready answer.

Lagoon dolphins – like other marine mammals – are impacted by boat traffic, entanglement in fishing gear, and changes in habitat quality that alters their food sources’ location and population.  But it seems they’re squaring off against other troubles too.  Mercury contamination from necropsies of dead dolphin tissues was reportedly very high. 

Bioaccumulation is another potential threat in the form of toxic algal species like Pyrodinium bahamense which produces saxitoxin.  Saxitoxin enters, and accumulates, in the lagoon food chain as it becomes concentrated in the tissues of filter feeding shellfish, then shellfish-loving pufferfish and burrfish, which are in turn consumed by the dolphins.  If saxitoxin seems familiar, its because I’ve written about its impacts in the IRL before: Left for Dead, Burrfish Bycatch in the Lagoon

Harmful algal blooms are suspected in the sudden increase in dolphin mortality in the IRL since a Pyrodinium bloom was recorded in August.  The deaths also declined suddenly following Tropical Storm Fay when conditions for Pyrodinium would have deteriorated lowering its count in lagoon waters and, hopefully, within the food chain. 

But, there could be more at work on the bottlenose then we are currently aware.  Marine disease – especially viral disease – is an emerging field of study and current projects have revealed intriguing evidence for increased rates of disease in several marine mammals living in coastal areas heavily impacted by human use.  Whether or not there is a strong relationship between disease and pollution (or mercury or algal bloom toxicity) is a question that needs to be answered not only to assess the health of the dolphins but the Lagoon as a whole.