Spawning fish and embryos most vulnerable to climate’s warming waters

(CN) — Increasingly warm water temperatures brought on by climate change are likely to hit spawning fish and embryos harder than during other times in their life cycle, leaving them more vulnerable to extinction in less than a hundred years.

A study published Thursday in the journal Nature shows that many ecological and economically valuable fish are now being threatened by climate change.

By the year 2100, 60% of fish species evaluated may not be able to survive in their current geographic range, based on the Shared Socioeconomic Pathway Scenarios, a group of projections used by climate scientists and others to model future environmental impacts. Even if global warming is limited to 2.7 degrees in line with the Paris Accord, 10-15% of fish species will remain under threat.

A team of researchers, led by Flemming Dahlke of the Alfred Wegener Institute Helmholtz Centre for Polar and Marine Research in Germany, studied observational and experimental data to gauge the heat tolerance of 694 species of fish across multiple climate zones during their various life stages.

“Narrow thermal safety margins of spawners and embryos indicate that the temperature requirements for reproduction define the climate change vulnerability of fish. For many species, the highest warming trajectory represents a major threat, as water temperature may exceed their current tolerance limit for reproduction,” the researchers noted.

Oxygen levels in the oceans have decreased by about 2% since 1960 due to warming temperatures. Bodies of water absorb oxygen through gas exchange at the surface, which gets mixed around like a giant blender by sea life, wind and in the case of oceans, by the tides. Oxygen escapes from warm water more quickly because hot molecules move faster, which also lessens water’s ability to absorb more oxygen from the air.

As temperatures increase, oxygen levels become depleted, leading to dead zones where aquatic life can’t survive — such as certain spots off the coasts of California, Peru, Namibia and the Arabian Sea. Scientists believe these low oxygen regions are currently expanding.

Aside from oxygen depletion, temperature stress can also impact gonadal development by affecting the production and release of sex hormones, further restricting a species’ ability to reproduce.

Heat tolerance varies throughout the lifetime of most plants and animals. The ultimate impact of global warming for every organism depends on its most vulnerable stage of life — a seedling will wither and die in an otherwise thriving greenhouse.

The danger posed to fish embryos averages 24% higher than in larvae and adults. Most studies thus far have concentrated on adult fish, ignoring their most sensitive life stages and underestimating the impact of warming oceans, lakes and rivers.

Fish from regions where the temperature remains mostly stable, such as at the poles and in the tropics, show an even lower tolerance to temperature fluctuations relative to fish from more temperate waters.

“Available data suggest that tolerance ranges are as narrow as possible to ensure survival under local conditions while minimizing costs for maintaining homeostasis over wide temperature ranges,” notes the study. “Consequently, tolerance ranges are expected to reflect the magnitude of local temperature variability.”

Species with lower thermal safety margins are already experiencing hardship. Some are no longer able to reproduce in their preferred seasons and locations, forcing them to venture into unknown waters to avoid extinction. Species also differ in the ability of their heart and gills to supply oxygen to their tissues and dispel carbon dioxide, which explains why certain varieties are more susceptible than others.

Freshwater fish are even more vulnerable to warming waters than their sea-dwelling cousins because of geographic barriers between habitats and human-induced degradation such as dams, hydroelectric power plants and pollution. Fish require specific substrates in which to deposit their eggs, meaning that even if the temperature at another location meets their needs, it may lack a suitable nursery habitat.

The authors say that such shifts in spawning times and locations may be impossible in some cases, depending on the reproductive strategy of a species and its location.

“Coping with climate change would be achieved through changes in thermal tolerance (through acclimatization of individuals, or through evolutionary adaptation across generations) and by shifting the timing and/or location of spawning to cooler seasons or regions. However, adaptation over generations is probably too slow to cope with major anthropogenic change,” say the authors.

“Very clearly, many fish species and people who depend on healthy fish stocks would benefit from intensified efforts to stabilize global warming at 2.7°F or even less.”