The world’s fish are shrinking as the climate warms. We’re trying to figure out why

Marius Masalar / Unsplash Timothy Clark, Deakin University

Fish are the most diverse group of vertebrates, ranging from tiny gobies and zebrafish to gigantic tunas and whale sharks. They provide vital sustenance to billions of people worldwide via fisheries and aquaculture, and are critical parts of aquatic ecosystems.

But fish around the world are getting smaller as their habitats get warmer. For example, important commercial fish species in the North Sea have declined in size by around 16% in the 40 years to 2008, while the water temperature increased by 1–2℃. This “shrinking” trend is forecasted to significantly exacerbate the impacts of global warming on marine ecosystems.

The link between warmer water and smaller size is well known, but poorly understood. Our experiments keeping fish in warmer water offer some crucial clues – and may help us learn how to prepare for a warmer future with smaller fish.

The temperature–size rule

Fisheries are a potential confounding factor when studying the effect of warmer waters on fish, because fisheries often target large fish. Removing these larger fish from the population benefits the survival of fish that mature quickly and reproduce at a younger age, when they are smaller.

This trait of maturing early can be passed through fish generations. Indeed, it can lead to a phenomenon known as “fisheries-induced evolution”, where the exploited species tends to decrease in size over time.

How do we tell the difference between the impacts of climate warming and those of fisheries?

One way is to examine the body size trends in fish species that are not targeted by fisheries. Several fish species in French rivers, for example, are not exploited by fisheries but have decreased in size over several decades while their environment has grown warmer. Fishing can reduce fish sizes, but even fish populations largely unaffected by fisheries appear to be shrinking. Sebastian Pena Lambarri / Unsplash

Another way is to examine fish under controlled conditions, by manipulating water temperature and studying the impact on fish size. Such experiments have shown that fish do indeed end up smaller in body size when kept under warm conditions, and the trend is so common it has been given a name: the “temperature–size rule”.

We also know that smaller fish produce proportionally fewer offspring. And if fish are shrinking, fisheries that base their catch quotas on weight will be taking a larger number of individual fish.

So shrinking fish means each fish will have fewer offspring, and more fish being caught. This is likely to have substantial ecological and commercial ramifications.

Supply and demand

Warmer water means smaller fish, but why?

The most popular current theories suggest the cause is due to a mismatch between how much oxygen a fish needs (to sustain its body’s metabolism) and how much it can get (via its gills).

The argument is that fish gills do not grow at the same pace as the rest of their bodies. Once a fish reaches a certain body size, its gills can only supply enough oxygen to keep its body running – there is no oxygen left over for growth.

What does this have to do with warming? The next step of the argument says fish use more oxygen in warmer water – but their gills don’t get any bigger. So fish reach the limit of their growth at a smaller size, leading to the temperature–size rule.

This “oxygen mismatch” theory has sparked heated debate among global scientists, largely because insufficient data exist to confirm or refute it.

Oxygen supply can keep up with demand

To get some data, we have carried out long-term experiments keeping fish under warmer water conditions than normal. We also tried providing extra oxygen, to see if it benefited their growth.

We have regularly taken metabolic measurements, and quantified the gill surface area of the fish to understand how well they can transport oxygen from the water into the body. Fish need more oxygen when they live in warmer waters – but research shows their gills are capable of keeping up with the increase in demand. Paco Joss / Unsplash

Our results show the “oxygen mismatch” theory doesn’t hold up. While the metabolism of fish does increase with warming of the water, we found the gills grow sufficiently to keep up with the increased oxygen demand as fish increase in size.

So, why then are fish shrinking as the climate warms?

Is reproduction the key?

We know that fish tend to grow faster in warmer conditions and reach reproductive maturity at an earlier age and smaller size. It is possible that once fish start reproducing, energy is channelled into reproduction rather than further growth.

Evidence for this comes from a population of fish living in a Swedish lagoon that gives us an eye to a warmer future, as the lagoon receives warm (non-contaminated) water from a nearby nuclear power plant.

Fish in the warm lagoon grow faster and reach reproductive maturity earlier, then they tend to die at a younger age and at a smaller body size than their counterparts living in adjacent, cooler waterways. “Live fast, die young”, as the saying goes.

While this idea seems to be broadly applicable, some conflicting findings point to the need for more focused research attention.

Fish can’t keep shrinking forever

As our understanding of the relationship between temperature and fish size increases, we would also like to know whether we can do anything about it.

In our latest research, we explored differences in growth rates between individual fish of the same species.

One thing we wanted to know was whether particular physiological traits may allow some individuals to get around the temperature–size rule and be impacted less by climate warming. We found there is significant variability across individual fish, but we don’t know how this variability could be harnessed to future-proof fish populations.

As our work continues, we also look to the future and think about the ramifications to fish and the industries that rely on them.

Fish cannot keep shrinking forever. There is a minimum size that each species must reach in order to maintain a viable population.

If species reach their specific thermal limits in particular locations, they will not be able to reproduce and they will cease to exist in those locations. If their entire habitat range becomes too warm, the species will become extinct.

These considerations of smaller fish and shifting thermal habitats will be critical for the sustainability of fisheries and aquaculture industries as we continue into a future with a warmer, more extreme climate. Our efforts to quantify and forecast the impacts will help resource managers and industries prepare for climate-linked disruption.

Timothy Clark, Associate Professor - Animal Ecophysiology, Deakin University

This article is republished from The Conversation under a Creative Commons license. Read the original article.

Read More........→July 06, 2023

We are poised to pass 1.5℃ of global warming – world leaders offer 4 ways to manage this dangerous time

Marcus E Jones, Shutterstock Jonathan Symons, Macquarie University

For three decades, the goal of international climate negotiations has been to avoid “dangerous” warming above 1.5℃. With warming to date standing at around 1.2℃, we haven’t quite reached the zone we labelled dangerous and pledged to avoid.

But recent scientific assessments suggest we’re on the brink of passing that milestone. Within this decade, global annual temperatures will likely exceed 1.5°C above the pre-industrial average for at least one year. This threshold was already briefly passed for the month of July 2023 during the Northern summer. The question is, how do we manage this period of “overshoot” and bring temperatures back down? The goal will be to restore a more habitable climate, as fast as possible. Today an independent group of global leaders released a major report. The Climate Overshoot Commission offers guidance at this crucial time. So far the report’s call for an immediate moratorium on “solar radiation management” (deflecting the sun’s rays to reduce warming) has attracted the most attention. But the details of other recommendations deserve closer inspection.  How can we respond to climate overshoot? Historically, climate policies have focused on mitigation (reducing greenhouse gas emissions). More recently, adaptation has gained prominence. But the climate overshoot report identifies at least four different kinds of responses to warming above 1.5℃, 

• 1. cut emissions to mitigate warming
• 2. adapt to the changing climate 
• 4. remove carbon that is already in the atmosphere or ocean, 
• 5. explore intervening to limit warming by intentionally reflecting a fraction of sunlight into space. 

The commission’s task was to examine how all possible responses might best be combined. Their report was written by 12 global leaders – including former presidents of Niger, Kiribati and Mexico – who worked alongside a youth panel and a team of scientific advisers. The four-step plan to reining in warming Not surprisingly, the commission argues our central task is mitigation. Transitioning away from fossil fuels remains the first priority. But reaching net zero emissions is just the first step. The commission argues developed countries like Australia should go further and aim for net-negative emissions. Why net-negative? In the short term, drawing down carbon can create space for the least industrialised countries to fight poverty while transitioning to clean energy. In the longer term, the whole global economy must achieve net-negative emissions if the planet is to return to our current “safe” climatic zone. The second step is adaptation. Only a few decades ago former United States Vice President Al Gore branded adapting to climate change a “lazy cop-out”. Today we have no choice but to adapt to changing conditions. However, adaptation is expensive – whether it is developing new crop varieties or rebuilding coastal infrastructure. Since the poorest communities who are most vulnerable to climate harms have the least capacity to adapt, the commission recommends international assistance for locally controlled, context-specific strategies. As a third step, the commission agrees with scientific assessments that carbon dioxide “will need to be removed from the air on a significant scale and stored securely” if we are to avoid permanent overshoot beyond 1.5℃ warming. But how to achieve large-scale permanent, carbon removal? Some environmental activists support natural solutions such as planting trees but oppose industrial methods that seek to store carbon in inorganic form such as carbon capture and storage underground. The commission agrees the organic/inorganic distinction is important. However, it points out while forests bring many benefits, carbon stored in ecosystems is often re-released – for example, in forest fires. The commission worries many carbon removal approaches are phoney, impermanent or have adverse social and environmental impacts. However, instead of ruling out technologies on ideological grounds, it recommends research and regulation to ensure only socially beneficial and high-integrity forms of carbon removal are scaled up. The fourth step – “solar radiation management” – refers to techniques that aim to reduce climate harms caused by reflecting some of the Sun’s energy into space. No-one likes the idea of solar radiation management. But no-one likes getting vaccinated either – our gut reactions don’t provide a fool-proof guide to whether an intervention is a worth considering. Should we trust our guts on this one? While climate models suggest solar radiation management could reduce climate harms, we don’t yet properly understand associated risks. The commission approaches this topic with caution. On the one hand, it recommends an immediate “moratorium on the deployment of solar radiation modification and large-scale outdoor experiments” and rejects the idea that deployment is now inevitable. On the other hand, it recommends increased support for research, international dialogue on governance, and periodic global scientific reviews. Time to examine intervention in the climate system? The idea we can avoid dangerous warming completely seems increasingly quaint. Like baggy jeans, the boy band NSYNC and the iPod shuffle, it reminds us of a more innocent era. Yet, Australia’s climate debate often seems stuck in this era. The widespread hope we “still have time” means we are not yet discussing the merits of more interventionist responses to the climate crisis. However, there’s increasing reason to be sceptical incremental measures will be sufficient. We may soon be forced to move beyond the non-interventionist, conservation paradigm. Whether or not its recommendations are taken up, the Climate Overshoot Commission’s work shows how the international community has failed to avert dangerous climate change. Reckoning with the consequences of this failure will dominate public policy for decades to come. This new report takes us a step forward. Jonathan Symons, Senior Lecturer, Macquarie School of Social Sciences, Macquarie University This article is republished from The Conversation under a Creative Commons license. Read the original article.

Read More........→September 18, 2023