by Melanie Abecassis and Thierry Work
Between June and October 2010, beach-goers all around the Hawaiian Islands were discovering dead pufferfish on the beach or distressed in the ocean, puffed up and floating. In some cases, puffers were found in pairs with one fish biting and holding another. Occasional reports of puffers acting aggressively towards each other were received, and in one instance, a puffer was observed actively attacking snorkelers in the water. Thus started a five-year long episode of CSI-Fish-Hawai‘i.
Pufferfish are poisonous fish and are mostly solitary animals that rarely interact with other fish. Researchers collected sick puffers, held them temporarily in fish tanks and noticed they struggled to remain submerged, ending up “bobbing” at the surface. This phenomenon affected three species of pufferfish, mainly stripebelly puffers (Arothron hispidus) with a minority of spotted puffers (A. meleagris) and porcupine fish (Diodon hysteria). Populations of spotted puffers that are routinely monitored by scientists on the west coast of Hawai‘i Island declined precipitously starting in 2009, and remained low from 2010 onwards, whereas no evident population change was seen for porcupine fish or stripebelly puffers.
Because pufferfish are highly toxic, they are not typically targeted for human consumption (except in Japan, where the meat of some pufferfish is considered a delicacy. Called fugu, it is extremely expensive and only prepared by trained, licensed chefs who know that one bad cut means almost certain death for a customer). However, fish die-offs are important to investigate in tropical marine ecosystems because they are honest signals of environmental perturbations that could damage populations of native species or, in some cases, impact public health.
Using various laboratory tools, veterinarians and chemists with the U.S. Geological Survey’s National Wildlife Health Center, Honolulu Field Station and NOAA’s National Ocean Service spent five years trying to identify the cause of death and discovered a series of marine toxins comprised of very small molecules that had never been identified before as the likely cause of this mass mortality. The toxins led to liver failure, acid base imbalance in the blood, and subsequent inability of fish to remain submerged. Their findings are striking in that 1) a marine toxin killed a species of fish that is, itself, toxic, 2) they identified a plausible mechanism (liver failure) to explain clinical signs of affected fish, and 3) this epidemic likely depleted some pufferfish populations. Scientists suspect the marine toxin was ingested, but the source remains a mystery.
PIFSC scientists collaborated on this project to investigate whether there were any abnormal ocean conditions that could have explained the timing of the fish die-off, but no clear link between ocean conditions and fish mortality was identified.
This study provides a template for marine fish kill investigations associated with marine toxins and highlights the need for more rapid and cost-effective methods to identify new marine toxins, particularly small molecules.