Climate Change

It’s getting hot: are fish getting bigger?

Australian estuaries are warming up because of climate change and this might be making its fish bigger.

Nicolle P, Hughes J, Fowler A, Schilling HT (2022) Long-term increase in growth of an estuarine predator, mulloway Argyrosomus japonicus, predicted to continue under future warming scenarios. Mar Ecol Prog Ser 688:1-17. https://doi.org/10.3354/meps14048

What’s the situation?

Climate change is affecting the world at large. Temperatures rise, and this affects living organisms and habitats in different ways. Specifically, in south-eastern Australia, estuaries are warming up at a faster speed than the oceans nearby. This place is a global warming hotspot with a temperature rise 3-4 times faster than the global average!

Estuaries are coastal ecosystems that are highly sensitive to changes in terrestrial and oceanic conditions. They are environmentally important, as they support a high level of biodiversity, act as nurseries, and even function as carbon storage and nutrient filter. It is thus crucial to find out how these impacts are affecting fish biology in estuaries for good management and resources.

To learn more about this phenomenon, an economically key species of estuarine fish was studied: the mulloway Argyrosomus japonicus. Environmental characteristics like temperature influence the degree at which organisms can transform food into energy, the metabolic rate (metabolism per unit time especially as estimated by food consumption, energy released as heat, or oxygen used in metabolic processes).

Asking fish their age

Thanks to a chronology spanning between 1980 and 2018, changes in growth were recorded. It is needed to know how much fish grow between years. For this, we check their otoliths (Figure 1). These are calcified structures existing in their ear which, similar to tree rings, present increments that can be counted to find out about their age. The distance between them is used to know their growth. As they grow old, their growth rate decreases, and their growth spur is before they reach maturity. It can vary between individuals, sexes, and is influenced by environmental factors like temperature and food availability. For this study, otoliths from 3112 individuals of A. japonicus were taken.

Figure 1. Otoliths of Argyrosomus japonicus show decades of increasing growth rates, suggesting positive growth caused by warming temperatures. Photo: D. van der Meulen. Graphic: P. Nicolle

The studied species stay within the same estuary systems, so it was safe to assume that the individuals stayed there their whole life, which permitted the link between their growth rate and historical environmental data.

What was found?

Most of the growth variability was due to age. Besides this, there was an important variance between late 1980s−early 1990s through 2018. Analysis showed that the temperature between November and February has the most crucial effect on growth in a high emission scenario, RCP 8.5.  On the other hand, in the low emission scenario, growth is flat but varies according to the forecasted temperature changes. Even so, in both scenarios by 2099 growth rates increase, especially in Sydney. It was estimated it will increase by up to 8.9% by this year.

However, as the temperature rises, the optimal temperature for the fish will be reached. A further increase will lead to a drop in growth rates and even mortality. In this case, these projections won’t be valid.

Argyrosomus japonicus isn’t the only species to be affected. Other estuarine and marine fish in the same zone have presented responses to the increase in temperature, like the rock flathead Platycephalus laevigatus. Not all marine species presented a positive reaction: over 600 marine fish species had their growth negatively affected. How species respond to temperature increase depends on individual tolerance to alterations in the environment. This variability could lead to changes in ecosystems.

Despite these results and the connection between temperature and growth, temperature by itself doesn’t explain the trend in the last three decades. This means there are more factors other than temperature that could lead to the long-term rise in growth rates observed. Some of those factors are drivers of estuarine temperature, how space effects, and fishing impacts.

Conclusion

Climate change and its warming have consequences, such as a long-term rise in growth rate to temperature for a predator fish in south-eastern Australia, as it was observed over 39 years. This will continue to happen according to predictions of the future, so it is crucial to continue further study, as growth is a key factor in population dynamics and therefore will affect fisheries management.

As time passes and the temperature continues to influence organisms, estuaries could end up having individuals that have a higher reproductive rate and lower mortality rates, leading to a population with fast-growing predator fish. This could naturally end up having ecological consequences on the ecosystem. Despite that, the population of A. japonicus is presently classified as depleted.

In this study, it wasn’t possible to determine the precise ways in which temperature impacts growth. For this reason, the forecasted growth variations based only on temperature should be read carefully. It is vital to continue studying the effects on organisms and this relationship between growth and temperature to further understand what we are facing because of climate change.

 

Cover photo: Rick Stuart-Smith / Reef Life Survey. License: CC BY Attribution

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