A single, extreme heat wave in Shark Bay, Australia spanning two months in 2011 drove down the local bottlenose dolphin population by 12% over the six years that followed, leading to a decline in dolphin calf births and suggesting “that the ecological consequences of extreme weather events may be too sudden or disruptive for even highly adaptable animals to respond,” concludes a new study in the journal Current Biology.
“The extent of the negative influence of the heat wave surprised us,” said lead author Sonja Wild of the University of Zurich. “It is particularly unusual that the reproductive success of females appears to have not returned to normal levels, even after six years.”
Carbon Brief notes that similar marine heat waves could become up to 41 times more likely by the end of the century if global greenhouse gas emissions aren’t brought under control.
“In 2011, Shark Bay—a world heritage area in Western Australia famous for its seagrass meadows and unique wildlife—faced an unprecedented marine heat wave,” the publication recounts. “For more than two months, coastal water temperatures soared to 2.0 to 4.0°C above average, damaging around 36% of the region’s seagrass meadows. In some regions, almost 90% of seagrasses died,” and “the extreme heat also killed vast numbers of fish, including commercial populations of scallops and crabs.”
The new study shows how this “ecological turmoil” filtered up to the top of the marine food chain.
“The researchers hypothesize that long-term decreases in dolphin survival are a result of the ‘catastrophic reduction in seagrass coverage, which shows little sign of recovery,’” writes science reporter Daisy Dunne. “The loss of seagrass ‘appears to be responsible for preventing fish stock recovery’, the authors add, reducing the food available to the dolphins.”
The loss of fish prey may have triggered the sharp decline in dolphin birth rates, leading to “increased rates of abortion during pregnancy or increased neonatal pregnancy,” the study stated, while lack of food could have led to “suppressed ovulation or delayed sexual maturity” for some dolphins.
The study did track a more resilient subset of dolphins, about 12% of the total population, that adapted to the drastic change in their habitat by carrying marine sponges on their rostrums, or beaks, when they dove to the deep sea for fish. “Researchers suspect that sponges protect the animals’ beaks from sharp coral and stingray barbs, much like a glove protects a human hand,” Dunne notes. The “spongers” showed a 6% drop in population in the years following the heat wave, compared to 12% among “non-spongers”.
The technique meant the subset of dolphins was less affected by the loss of fish near seagrasses, but it won’t be enough to protect them from climate change. “In a sense, these sponging dolphins just got lucky in that their foraging niche [the deep sea] wasn’t as badly affected,” Wild said. “Therefore, we do not believe that sponge tool use protects against climatic events in general.”
Carbon Brief says ocean warming has led to an average 4% drop in global fish stocks since the 1930s.