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Fish in Hot Water

Banded marwong

Banded morwong on the extreme warm edge of their range are starting to experience negative effects from increased temperatures.

By Anna Neuheimer

Warming waters in the Tasman Sea may have exceeded the tolerance limits for fish growth. Image: Hugh Pederson

Like insects, reptiles and amphibians, most fish are cold-blooded, with temperature shaping all aspects of their biology, including their growth rate. When temperature increases a small amount, biological reactions can proceed more quickly, growth increases and fish get bigger.

However, temperatures can eventually increase past the tolerance limits of the fish. When temperatures become too high, the fish can’t keep up with energy demands, and growth and fish size decreases.

This tolerance temperature marks the point at which an increase in temperature switches from being beneficial to detrimental to growth. If temperatures continue to increase past the tolerance temperature, a critical temperature is reached after which growth ceases, enzymes are destroyed and eventually death occurs.

Normally, these extreme effects are only measured in the laboratory, where fish can be artificially heated to temperatures far higher than normal. However, as ocean temperatures in many locations are rapidly warming, we are now starting to see these effects in the wild.

For example, the Tasman Sea between Australia and New Zealand has experienced rapidly warming waters due to surface heating and movement of a major warm current, the East Australian Current, further into the area. Last year I worked with researchers at the CSIRO and the University of Tasmania to explore how these rapidly warming waters are affecting fish growth in the Tasman Sea.

We examined growth in a long-lived (up to 97 years) reef fish called the banded morwong (or red moki). Growing up to 55 cm, banded morwong live on shallow (10–50 metres) rocky reefs and are territorial, picking a spot on the reef when they are around 6 months old and staying there. This site loyalty means that we have an easier time estimating the temperature they’ve been living at through most of their life. This is normally a complicated task due to the migrations that many fish undertake.

We sampled banded morwong in the field and measured their past growth from otoliths. Also known as ear-stones, otoliths are small bones in the fish’s head used for orientation and detection of movement. They grow continuously through the life of the fish and, like trees, they grow in a ring structure, with one ring laid down every year.

Otoliths store a lot of information about the fish’s life history, including chemical information about the waters in which the fish has been living. More importantly for our work, the fish’s age can be determined by counting the number of rings, and the relative annual growth can be measured as the width of each ring.

We examined growth in banded morwong from five populations in the Tasman Sea: four from Tasmania and mainland Australia, where they live in moderately warm waters, and one from the North Island of New Zealand, where temperatures are the highest in their distribution range. We looked at temperature changes for the same areas, and found that they were increasing at all sites by up 1–2°C over the past century.

But the same wasn’t true when we looked at growth. The fish from Australian populations showed increasing growth with increasing temperatures: waters were warmer and the fish were bigger. However, fish from the warmest waters off New Zealand showed declining growth with increasing temperatures.

This negative effect of temperature on growth looks like what we see in the laboratory when tolerance temperatures are exceeded. Also, a similar tolerance temperature (~17°C) was estimated from preliminary experiments for banded morwong in the lab, with the fish found to be unable to sustain swimming speeds at these high temperatures.

It appears that banded morwong on the extreme warm edge of their range are starting to experience negative effects from increased temperatures. What this means for the future size of the fish will depend on a number of things.

First, temperature will affect all aspects of the fish’s biology, including processes other than growth. For example, the time at which a fish becomes mature marks the time when a fish starts directing some resources to reproduction, and this timing is affected by temperature. When a fish matures earlier, less energy is available for growth and fish size decreases.

Second, temperature will have indirect effects on banded morwong through changes in available food. As all prey of the banded morwong are cold-blooded, the increased temperature will affect them as well, and each of these will have their own specific tolerance limits. Effects of temperature on prey will change the amount of food available to banded morwong and the amount the fish can grow.

Third, as many exploited fish stocks have shown, the harvesting of fish of a certain size through fishing can result in size changes to the remainder of the population. While the New Zealand population is protected in a marine reserve, a commercial fishery for banded morwong was established in Australia about 20 years ago. This fishery may affect the future size of banded morwong in the Tasman Sea.

These various factors will combine to determine the future size of fish in the different populations. In turn, the population’s productivity, including population size, will vary with the size of fish. Larger females can produce many more eggs, and often higher quality eggs, than smaller females, with a doubling in female size often leading to eight times as many eggs produced. Thus, if banded morwong at the warm-edge continue to show reduced growth, we might expect their population size to decline.

Further, if temperatures continue to rise past the tolerance limits of the fish, we would eventually expect the fish’s distribution to change, with the warmest habitats being left and fish expanding their range into the cooler waters toward the pole. Similar shifts have been noted in fish populations in the Northern Hemisphere, including Atlantic cod.

In this way, direct effects of climate change on the biology of cold-blooded animals can lead to major changes in population ecology, production and distribution.

Anna Neuheimer completed this work during an Endeavour Research Fellowship at CSIRO in Hobart, and is now a postdoctoral fellow at Aarhus University in Denmark.