BY CAROLYN LUCAS-ZENK | WEST HAWAII TODAY
Thirty-three years ago, a mass death involving at least 50 critically-endangered Hawaiian monk seals occurred at Laysan Island. While the cause wasn't conclusively determined, officials suspected ciguatera, said Charles Littnan, lead scientist with the National Oceanic and Atmospheric Administration's Pacific Islands Fisheries Science Center.
Ciguatera is an illness caused by eating certain reef fish whose flesh is contaminated with potent and highly-debilitating ciguatoxins produced by dinoflagellates. Affecting thousands of people annually worldwide, ciguatera causes acute gastrointestinal and neurological illness with symptoms resembling chronic fatigue syndrome, Littnan said.
High levels of ciguatoxins were detected in the livers of two monk seals in 1978 using mouse bioassay, a method that has since been criticized as ineffective in detecting algal toxins. Still officials considered ciguatoxins as a potential threat to monk seals, whose population is estimated to be 1,100 to 1,200, Littnan said.
NOAA researchers have now finally confirmed the presence of ciguatoxins in monk seals using a better detection method called the Neuro-2A cytotoxicity assay, which shows greater sensitivity to toxins, Littnan said.
"This is the first evidence of a transfer of ciguatoxins to marine mammals," he added.
Marine toxin experts at the National Ocean Service collaborated with veterinarians and ecologists at the National Marine Fisheries Service on a recent study that revealed monk seals are exposed to significant levels of ciguatoxins.
"Based upon this study, we believe that ciguatoxin exposure is common in the monk seal population," said Littnan, who coauthored the study. "This study is an important first step."
The threat could pose management challenges for this species, which has been dwindling at 4 percent annually because of poor foraging success, as well as additional environmental and human factors. Experts have already predicted the population will dip below 1,000 in the next three to four years, making the Hawaiian monk seal one of the world's rarest species.
For the study, more than 50 monk seals were sampled throughout the Hawaiian Islands, including in the protected waters of Papahanaumokuakea Marine National Monument where large numbers of juveniles are starving to death. Samples were then shipped to NOAA's National Centers for Coastal Ocean Science laboratory in Charleston, S.C., for toxin analyses.
About 19 percent of the monk seals tested positive for ciguatoxins. Of the affected monk seals, approximately 50 percent were weaned individuals, 25 percent were juveniles and 25 percent were adults, said Littnan.
"We already know juvenile survival is poor. Less than 1 in 5 pups that are born live to adulthood," he said. "Besides food limitations and competition with other predators, this study confirms ciguatoxin is another factor we should take into consideration."
Still questions linger and more research is needed to better understand how ciguatoxins affect monks seals, how widespread the exposure is, the prevalence in male and female seals, as well as their age and location, and what role the ciguatoxins may be playing in the decline. This information will help officials develop strategies that will hopefully make an impact and improve the survival odds for monk seals, Littnan said.
He claimed ciguatoxin exposure is not likely something officials will be able to fix. But if they can determine a sick juvenile has the toxin, intervene and give it supportive care, there's a chance the monk seal can become healthy again and possibly cope with another exposure.
Once monk seals make it past 4 or 5 years of age, they tend to be pretty much in the clear and have learned the skills needed to survive, he added.
The study was published this month by the American Chemical Society online at pubs.acs.org/doi/abs/10.1021/es2002887. It has prompted similar investigations of other marine mammals, such as dolphins and whales.