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The Fish That Should Have Got Away

Credit: Piotr Wawrzyniuk/Adobe

Credit: Piotr Wawrzyniuk/Adobe

By Diego Barneche

Attempts to catch the biggest fish may have unwittingly caused the fishing industry to crash in many parts of the world. To make things more worrying, new research indicates that climate change will reduce the capacity of fish to reproduce.

Have you ever wondered why adult animals are so much bigger than juveniles? Many decades ago, evolutionary biologists proposed an explanation: there must be an advantage to growing large. But what advantage? Relative to what? If an animal’s genes are going to be passed on to the next generation, they need to ensure somehow that they can reproduce better than their competitors.

In the particular case of fish, reproductive success is achieved by producing as many good-quality eggs as possible. The great majority of fish eggs die within the first couple of days after being released by their mothers. They are either eaten by a predator or simply do not survive harsh environmental conditions.

Just as we see with chickens, fish eggs have a yolk – a stash of food that keeps them going for a few days until they mature and can feed themselves. Female fish reproductive strategies range between two extremes: they can make a few large eggs with lots of yolk, or spawn many tiny eggs.

When we look at fish mothers of different sizes we see that larger females produce more eggs. This would seem to explain why young fish grow to become large adults: so that they can produce more eggs and increase their chance of spreading their genes to future generations.

Scientists were not satisfied by this simple observation and explanation. They wanted to quantify exactly how much bigger mother fish should be. How much better at making eggs is a large fish?

Some argued for decades that larger mothers always make the same number of eggs relative to their size. If that were the case, a mother fish weighing 10 kg would make as many eggs as ten mothers that each weigh 1 kg. This is referred to as an isometric relationship.

Others, though, have argued that larger mothers make disproportionately more eggs as they grow. So, for example, a 10 kg female fish would produce many more young than ten 1 kg mothers combined. This is known as a hyper-allometric relationship.

This question remained largely unanswered until recently, when my colleagues and I decided to compile data for as many different marine fish species as we could find in the scientific literature. In particular, we were interested in obtaining data about the number of eggs that mothers make as they grow, how big these eggs get, and how much energy they pack. So we went on a 2-year journey looking for data, compiling scientific papers and fisheries reports all the way back to the late 1880s, and in English, Portuguese and Spanish.

If you are wondering whether all this effort paid off, it did! We found very strong support for the hyper-allometric hypothesis: larger mothers do produce many more, larger eggs as they grow, which gives them an even bigger advantage over smaller mothers. This is likely to yield more robust future generations.

For example, our analysis indicates that if the ability to produce more eggs increased proportionally with mother size (the isometric scenario), then 15 2 kg female Atlantic cod (Gadus morhua) would produce the same number of eggs as one 30 kg female. Instead, we find that a single 30 kg female produces more eggs than approximately 28 2 kg females (weighing a total of 56 kg).

Furthermore, if we add the fact that the size and energy of eggs also increase with mother size, a 30 kg female actually makes a batch of eggs with a total energy content that is approximately 37 times as high as a batch of eggs from a single 2 kg female. An isometric relationship would underestimate this difference by 147%. This is a very large difference from what many scientists and fisheries managers previously assumed.

Biologists who have made assumptions based on isometric relationships will need to revise their predictions to recognise the importance of allowing mother fish to grow, because there is a disproportionate increase in how one female contributes to the next generation.

These findings also have much more practical implications. Unfortunately, our pursuit of the largest trophy fish has resulted in the removal of most of these big mothers from the sea. It is now difficult to find any of these very large fish compared with only a few decades ago for most species of marine fish. What we have to understand is that by removing these big fish we are reducing the capacity of the population to replenish because the larger fish contribute so much more to the reproductive capacity of the population. This explains why so many fisheries crash soon after we observe declines in the average size of the adults in the population. We would not be able to explain these crashes if we were still assuming an isometric relationship for the reproductive output of fish.

Climate change, in particular the warming of our oceans, raises the possibility of another alarming consequence. Warmer sea temperatures cause adult fish to grow less and to mature earlier. If fish mothers do not grow as large, they will have fewer eggs.

For example, a study has shown that a 1.5°C increase in sea surface temperature will decrease fish lengths by approximately 15% in the Mediterranean Sea. Based on our estimates, this size decrease would result in a 50% reduction in fecundity for each female Atlantic mackerel (Scomber scombrus).

These effects would exaggerate the predicted impacts of climate change on the productivity of fisheries. Previous studies have already indicated that the number of fish in the ocean will decrease with warming, but our results indicate that warmer oceans will likely have fewer fish with much lower reproductive output.

But we can do something about it. Marine Protected Areas (MPA) are a very useful management tool that limit or even completely forbid fisheries in a given area of the ocean, enabling fish to reproduce and be bountiful without our interference. Studies show that fish grow by 28% in length inside MPAs. This means that fish inside MPAs have a much higher reproductive capacity than smaller fish outside the MPAs, so populations inside MPAs have a much higher chance to replenish and persist over time. For example, our estimates indicate that an MPA could enhance population replenishment of the widow rockfish (Sebastes entomelas) by 74%.

What does this mean to all of us? If we are to make sure that fish populations stay healthy and abundant for future generations, we need to stop chasing the big fish. Many places in Australia already impose maximum-size restrictions for fish, and this helps to increase the abundance of our awesome big fish mothers. However, most places around the globe do not have such rules.

It is time to make some noise, and demand that our fisheries regulators and policymakers do something about it.

Diego Barneche is a Postdoctoral Research Fellow in the Evolutionary & Ecological Physiology Laboratory at The University of Sydney’s School of Life and Environmental Sciences.