Land-based chemicals in the waters of the Great Barrier Reef threaten the health and survival of vulnerable dolphin species, including the rare snubfin dolphin – so named for the small blunt fin on their backs.
The team found that 68 percent of the dolphins they sampled had contamination levels that could affect their health. In one instance, a female Australian humpback dolphin sampled in 2015 from the Fitzroy River estuary had PCB concentrations “among the highest found in published literature and exceeded all available threshold levels for PCBs including those associated with carcinoma in California sea lions,” write the team in Ecological Indicators.
PCB chemicals were banned in Australia in 1975. Despite this, PCBs, DDTs, and to a lesser degree HCBs are still widespread in the Great Barrier Reef. The concoction of chemicals building up in the dolphins is a symptom of greater environmental pollution.
PCBs were widely used in building materials such as coolants and additives in paint until the late 1970s. DDTs were used as an insecticide in Australia and banned in the 1980s. However, their effects are still being felt to this day; they have a long half-life and are highly toxic. HCBs were used as a fungicide and banned under the Stockholm Convention on Persistent Organic Pollutants.
"When I saw the results I was very surprised to see an increase of PCBs, DDTs and HCBs up to seven times higher than what we recorded in previous years," study author Daniele Cagnazzi, a marine ecologist at Southern Cross University, told IFLScience. "More importantly a large proportion of the sampled population accumulated organochlorines contaminants [like PCBs and DDTs] above thresholds over which immunosuppression and reproductive anomalies are known to occur."
The team from Southern Cross University and Flinders University used a small biopsy dart system called PAXARMS to take samples from humpback and snubfin dolphins in the Fitzroy River and Port Curtis region of Queensland between 2014 and 2016. These results were compared to those taken between 2009 and 2010. Concentrations of all three chemicals increased by between two and seven times.
The Fitzroy River catchment is one of the largest in Queensland at nearly 142,665 square kilometers (55,083 square miles). Over the years, it has been modified for agriculture, becoming a major source of contamination to the Great Barrier Reef. A flood in 2011 was one of the largest on record in Queensland, likely increasing contaminants into coastal waters through freshwater runoff.
"Dolphins absorb contaminants from their food, which is mostly fish, and these contaminants are then mainly stored in their blubber," said Cagnazzi. "Dolphins can eat between 4-6 percent of their body mass in fish per day, which means an adult humpback dolphin will eat about 10 kilograms [22 pounds] of fish per day. Therefore, the levels of PCBs in a dolphin’s blubber can build up very quickly."
PCBs and DDTs in their blubber can be 100 times higher than that found in their prey. Female dolphins can, in turn, transfer up to 80 percent of those contaminants to their first-born calf.
"In times of lack of food, for example during intense flooding, dolphins tend to break down their stores of blubber for energy, releasing at the same time the contaminants accumulated in the blubber layer," said Cagnazzi.
The health implications of many of these contaminants on dolphins are still unknown. However, there has been a link to a significantly higher risk of fetal mortality, reproduction issues, and carcinoma. It's also possible the contaminants weaken the dolphin's immune system, making it a challenge to fight off parasites, like toxoplasmosis, as well as bacteria, fungi, and viruses.
"The accumulations of contaminants combined with multiple other threats have led to the declines of various dolphin populations around the world, including the extinction of the Yangtze River dolphins," said Cagnazzi. "The contaminants investigated in this study are only a small fraction of the wide range of contaminants assimilated by dolphins."
Several new ports and mining developments are in progress along the Queensland coast and inland. The authors recommend more specific water quality targets should be introduced and to ensure more efficient assessments of the toxicological status of marine species.
Cagnazzi added: "We cannot underestimate the potential implication of this threat to the conservation status of these species in Australia."