What happens to fish during a severe heat stress event, that is, when the water temperature rises, when the corals have not yet bleached and died?
Despite the many challenges facing the world's oceans today, coral reefs remain strongholds of marine biodiversity. Thousands of species of fish of all shapes and sizes call these colorful, complex and economically important ecosystems home. However, the impending warming of the ocean spells trouble for these fish.
Since the first global coral bleaching event that devastated reefs in the late 1990s, scientists have worked to document the effects of these catastrophic phenomena on coral reef fish. In the wake of severe bleaching, coral mortality often leads to changes in the community of fish living on the reef: fish that feed on corals decrease, while those that feed on algae increase as the latter proliferates.
But what happens to fish during a severe heat stress event, that is, when the water temperature rises, when the corals have not yet bleached and died? Very few scientists seem to have tried to find out.
Our new study, published in Ecological Applications , surveyed reef fish communities before, during and after El Niño 2015-16 in Kiritimati, a coral atoll in the Pacific Ocean, which is part of the country of Kiribati. Our research suggests that short-term increases in water temperature can have devastating effects on reef fish populations and the local communities that depend on them.
Warming the world's largest atoll
Kiritimati, or Christmas Island, is the world's largest coral atoll, ring-shaped reef, by land mass. The nearest major airport is more than 2,000 miles away, in Hawaii. The people living in Kiritimati rely heavily on reef fish for food and income.
While the 2015–16 El Niño wreaked havoc on reefs around the world, its effects were especially catastrophic in Kiritimati. Unprecedented levels of heat stress persisting for 10 consecutive months led to over 80% of coral mortality on the island, but not before triggering a change in the local fish community.
After just two months of heat stress, reef fish populations around the atoll had plummeted by half. The number of fish species also decreased, and some species disappeared entirely. Five species have not been seen since, including the Chevron butterflyfish (Chaetodon trifascialis ), which feeds exclusively on living coral.
However, one year after the heat wave, we found, somewhat surprisingly, that the biomass and total abundance of reef fish had recovered, rebounding to levels similar to what we had observed in years prior to the heat wave hot. This begs the question: What exactly happened during those long hot months?
While intense heat stress can lead to decreased fitness and even mortality in reef fish, we believe that most of the missing fish sought refuge on the island's deeper and cooler reefs during the wave. of heat. Once the heat had subsided, they could have easily returned to the shallow waters.
However, the recovery of the reef fish community was not the same across the board. Sites on the atoll closest to villages, where reefs have been heavily affected by dredging, fishing and pollution, had made recovery difficult relative to areas of the atoll far from villages where reefs were near virgin before the heat wave.
This suggests that local environmental protection could help make reefs more resistant to the ravages of severe ocean warming. While it may not be enough to entice fish to sit still during a severe warming event, high-quality reefs may be more attractive to these fish upon their return.
A window to the future
If reef fish do return after heat stress ends, is their disappearance in the short term really important? Considering that the survival of millions of people around the world depends on tropical reef fish, we believe the answer to this question is a resounding yes.
The impacts of climate change on coral reefs are predicted to worsen in the coming decades. Studying the effects of severe heat stress in the present can serve as a window into the future, heralding the consequences of gradual ocean warming and the more frequent and severe marine heat waves that are forecast.
By understanding how fish populations react to elevated water temperatures, we can also attempt to predict and mitigate the effects of ocean warming in highly reef-dependent communities, such as those in Kiritimati.
Within the realm of coral reef research, most heat stress studies to date have focused on the link between heat stress and coral bleaching, and the collateral effects of catastrophic bleaching in reef fish. However, corals are not the only animals affected by heat stress per se. Unless we intervene to limit global climate change, we may risk losing not only corals but also critically important reef fish.