Global warming is an increasing threat for many of earth’s ecosystems. Climate change is rapidly and drastically altering the natural world, but what effect is this having on fish and is there anything we can do about it?

We know that global warming is causing rapid increases in the temperature of aquatic ecosystems on a massive scale (Alfonso et al. 2020). Additionally, the early ages of fish are particularly sensitive to their environment where increases in temperature impact egg and larval stages more than adult fish (Pörtner and Farrell, 2008). What this means is that global warming could shape fish populations into the future by increasing egg and larval mortality, altering juvenile development rates, causing changes in reproductive investment and ultimately, reducing recruitment success (Pankhurst and Munday, 2011).

Climate change is not the only challenge that fish populations face. Fishing can also impose strong mortality on populations, and often preferentially removes the largest individuals (Tu et al. 2018). This can also impact on reproduction in fish populations, as large individuals often are the most fecund (Barneche et al. 2018).

What this means is that fishing could be increasing the sensitivity of fish populations to global warming impacts. Despite this, we do not currently have a good understanding of whether warming and fishing might interact to cause increased impacts on fish populations.

A 2021 study by Wootton et al. aimed to identify the mechanisms underpinning fish population responses to the combined impacts of fishing and warming. The study found some interesting multigenerational impacts of warming climate on the tropical freshwater zebrafish (Danio rerio).

During the experiment, 18 different populations of fish were exposed to temperature treatments (26°C (control) and 30 °C (warmed)), and three size-based fisheries selection regimes over six consecutive generations. A range of early life characteristics were measured, which allowed the authors to study whether any impacts of fishing and warming would build up over generations, and also how fish populations might respond to multiple stressors over the long term.

The warming temperature imposed was designed to be reflective of a realistic warming scenario. Using this design, the researchers tested whether there is a significant interaction of warming and size-selective fishing on reproductive output (egg size and egg size variation), early life history (survival and development rate), and survival to subadult stage, which represents the “recruitment” of wild stocks.

Results

The study found strong evidence that the recruitment capacity of populations is extremely sensitive to the factors of warming and harvesting but often these effects take several generations to appear. Here, warming reduced recruitment by 30-50% but only after three generations. This warming-induced decline was exacerbated by size-selective fishery harvest that removed the largest individuals, where one population even went extinct. More balanced fishing practices that did not remove the largest fish did not increase the impacts of warming. The results suggest that synergistic impacts of fishing and warming can have delayed effects on stock resilience.

Importantly, when the researchers stopped fishing and brought temperature back to ambient levels at the end of the experiment, recruitment in all of the surviving warmed populations recovered to normal levels. This suggests that periods of benign environmental conditions could still facilitate some successful recruitment even in the face of global warming.

This study shows that preserving the size diversity of fish stocks, through changes in fisheries selectivity or marine protected areas, could be crucial to improve fisheries resilience in the face of global warming. Interestingly, the findings also show that short-term studies may underestimate the effects of warming and fishing on critical demographic traits.

What could this mean for Australian freshwater fish?

Although this study was focused on the tropical zebrafish, the findings are significant for other freshwater fish species. Researchers often use model organisms because they can be easily housed, have shorter generation times and conducting research does not remove individuals from important or threatened wild populations. Here, zebrafish represented an ideal species to study as they are readily bred in the lab and have a short generation time of only three months. The same experiment conducted on Murray Cod would have taken over 40 years by which time it could be too late! In Australia, our native fish are likely just as vulnerable to the effects of warming and fishing. This study is a potential indicator of fishing and warming pressures acting together to impact the early stages of future generations of native fish, meaning that decreased recruitment could lie ahead.

The findings here should be taken into account when managing native populations, and should direct future research on our iconic freshwater fish. It is vital to continue studying the impacts of a warming climate and fishing to aid in future mitigation and management.

Resources

Original study

Additional studies:

Effects of climate change on fish reproduction and early life history stages

Temperature increase and its effects on fish stress physiology in the context of global warming

Fishing and temperature effects on the size structure of exploited fish stocks

Fish reproductive-energy output increases disproportionately with body size

Featured image: Grayling eggs.

Credit: Arthur Rylah Institute

‍