Warming oceans may affect the reproductive success of many fish species

Original article: Thermal bottlenecks in the life cycle define climate vulnerability of fish. DOI: 10.1126/science.aaz3658

Fish are an essential source of food and livelihood for many communities, including humans. A new report published in Science by experts from the Alfred Wegener Institute, Helmholtz Centre for Polar and Marine Research (AWI) reveals that many more fish species are at greater risk from global warming than previously thought. 

What we didn’t know about fish

Scientists have observed increased ocean temperatures due to climate change. However, only a few studies have collected experimental data on how fish respond to temperature changes over their lifetimes. This leaves future predictions as to whether targets for reduced ocean warming are sufficient to maintain healthy fish populations incomplete. To tackle such uncertainty, scientists performed analysis on a huge collection of past observational, experimental, and phylogenetic data. They collected information on 694 marine and freshwater fish species from all climate zones.

Using this information, they identified the embryo and spawning stages in which fish were especially susceptible to temperature increases and hypothesized why these stages are more sensitive to temperature flux. They also mapped the temperature tolerances of these fish against water temperature scenarios through 2100, demonstrating that anywhere from 10-60% of all fish species may be forced to migrate to cooler waters to continue to reproduce successfully.

Temperature plays an important role in spawning habitats

Fish normally reproduce at certain times (spawning seasons) and in certain locations (spawning habitats) that provide conditions necessary for offspring survival. Temperature plays a large role in what makes a spawning habitat ideal. And temperature tolerance levels are normally pretty narrow. A small tolerance range ensures survival at a specific temperature without the energy cost it would take to maintain homeostasis over a wide temperature range. But is this tolerance the same at all life stages? According to this new data, the answer appears to be no.

Collecting data from fish all over the world

Scientists collected thermal tolerance data of four life stages: embryos, larvae, adults, and spawners. They then organized the data to reflect phylogenetic and life stage patterns. To do this, they created a 4-ring phylogenetic tree that shows the evolutionary relationships among biological species based on their physical or genetic characteristics. Each ring represents a life stage. A colored scale described the maximum temperature, the minimum temperature, and the temperature range (maximum – minimum) tolerance of each fish species. This design allowed scientists to easily look for patterns among many species. 

An interesting observation that came out of all this data was that freshwater species had a larger temperature range tolerance than marine species. The authors believe greater temperature variations in lakes and rivers compared with the ocean contributed to this pattern. They also observed that embryos and spawners appeared to be the most temperature-sensitive stages in the life cycle of fish, compared with both larvae or adults.

Oxygen plays a key role in how tolerant fish can be to temperature changes

The results in this study strongly support the concept of oxygen-limited thermal constraints on many fish species. 

Embryos are in need of more oxygen than other life stages, primarily due to the lack of gills for taking in more oxygen. When coupled with temperature changes, embryos may be unable to sustain adequate oxygen supplies. Upon formation of the cardiorespiratory system (heart and gills) the fish can more effectively distribute oxygen throughout its body during the larval and adult life stages. Therefore, temperature tolerance is wider in these stages. When fish enter the spawning stage, higher energy and oxygen requirements are required to sustain the added body mass. Once again, temperature places an enormous strain on the cardiorespiratory system and thermal tolerance may further decrease during this life stage. 

What our current climate predictions mean for fish

Temperature requirements for reproduction will ultimately determine the fate of fish during climate change. If rising water temperatures can be kept in check, less fish will be forced to migrate to more hospitable waters. If not, up to 60% of the world’s fish species will have to find different spawning habitats or risk extinction due to poor reproduction. Adaptability is not an available option for many of these species. Adaptation through biological means normally occurs over a long time period, much longer than the onset of inhospitable temperatures. Additionally fish in rivers and streams may be unable to migrate due to geographical constraints.

In this paper, the authors assessed vulnerability of each fish species by taking the difference of their upper thermal tolerance limit and the mean temperature of the coldest month during the species-specific spawning season. They then investigated which areas would undergo rising temperatures in the future due to climate change using new climate scenarios (Shared Socioeconomic Pathways – SSPs), which will also be used in the IPCC’s next Assessment Report. The SPPs are classified as either high (SPP 1-1.9), intermediate (SPP 4-6.0), or high (SPP 5-8.5) pathways. Higher SPPs are equivalent to greater global warming. 

If current projections hold (SSP 4-6.0), the spawning habitats in over one-third of the marine and freshwater species investigated in this study will be threatened. If global warming were reduced so that the SPP 1-1.9 pathway (equivalent to 1.5 °C of warming) occurred, the percentage of species at risk would drop to 10-15%.

Ultimately this large, and previously unexplored, analysis of fish life stages in response to water temperatures indicates that climate change will affect the ability of species to reproduce at their current times and locations. To what extent depends on how humans respond to the growing climate crisis.