Image of southern bluefin tuna hooked

Rapid developments in DNA and genetic research, once considered only in the realms of science fiction is now a reality, and making a very real difference to sustainable fisheries management.

Fisheries scientists are starting to use DNA to work out how big a fish population is, to determine if fish from different areas are from the same populations and discover from what part of the ocean fish come from. Researchers are even developing methods that can take a sample of ocean water and based on fragments of DNA in that water, determine what organisms live in that area!

The Australian Fisheries Management Authority (AFMA), working the Commonwealth Scientific and Industrial Research Organisation (CSIRO), is already applying this technology in some of its fisheries, including in the Eastern Tuna and Billfish Fishery off eastern Australia. The resulting benefits of better management will be felt across all fishing sectors, including gamefishing.

So - how does genetic research help ensure healthy fish stocks?

Well, for fisheries managers like those at AFMA to be able to determine what are sustainable catch levels for target species in fishery, we need to know how big a fish population is. In general, larger populations can sustain larger catches and smaller populations can sustain only smaller catches.

One way scientists used to work out population size was by tagging and releasing a whole lot of fish, and then seeing how many were recaptured after those released fish had mixed back into the population. Basically, if you tag 1000 fish and the population is large (e.g. a million fish), your chances of recapturing a released fish later are low. You’d expect few recaptures. However if the population is small (e.g. 10000 fish) your chances of recapturing a tagged fish is much higher so you would expect lots of recaptures. The number of recaptures of tagged fish gives you information about population size and also about the movement of fish.

But, this method has some limitations such as assuming that fish mix evenly, that fishers report recaptured tags to scientists, that fish don’t die post tag-release and that tags don’t fall off the fish etc. Plus, it’s costly.

Close Kin Mark Recapture

Now scientists have come up with a new type of genetic “tagging” called “Close Kin Mark Recapture” This very clever method works on the idea that each individual juvenile fish has two parents, each of which contribute half of that juvenile’s genetic material. Each gene has two copies, one from each parent, and because some of the “code” in some genes is unique to each individual fish, effectively, the juvenile fish has “tagged” its parents. These parent-offspring pairs are referred to as “POPs”.

How does this help scientists estimate population size? Well, like with conventional tagging described above, if the population is large, the probability of finding a POP is low. If population is small, the probability of finding a POP is higher. In this way, the number of POPs you expect to find in a sample of fish caught in the fishery is (inversely) related to the size of the population.  

Another really important piece of information fisheries managers need to know is if fish from two different areas are from the same population. Why? Because if catches are high in one area and low in the other, fisheries managers might think things are ok overall, if they believe the fish are from one population. But this is a poor assumption if in fact they are from separate populations. It might mean one of the populations is being overfished while the other is quite healthy.

However, scientists can use the fact that some DNA in fishes genes change (mutates) over generations and those changes get inherited by the offspring. If fish in an area only breed with each other, those mutations only get passed on (inherited) between fish from that area, and not to fish from other areas. So, if there is a difference in the genes of fish from one location compared to fish from another location, this suggests fish from each area are not mixing and interbreeding. This is invaluable information for scientists and fisheries managers when assessing fish stock sizes and how much catch is sustainable.

Traceability through genes

Another use of DNA relates to scientists’ ability to create maps of how particular genes differ in different populations of a species from different parts of the ocean.

How does this help?

Well, if you buy fish from a fish shop, you might want to know if it’s from a sustainably managed fishery. Different fish stocks of a species around the world have different management and some might be more sustainably managed than others. Scientists are developing methods so that the DNA in your fish shop fish can be analysed to determine which area of the world (and thus which fishery) your fish came from, because it will have genes with specific DNA that occurs only in specific areas. If scientists have identified how that DNA differs in each location, they can identify which area/location your fish came from. This ability to “trace” the origins of fish is becoming increasingly important as more buyers and consumers of seafood ask ‘where did my fish come from?’

Environmental DNA

Finally, fisheries management is not just about individual species and stocks. It’s about ensuring that fisheries impacts on marine ecosystems are sustainable too. Sometimes, it is very difficult to collect information on what species of organism occur in different areas that might be being fished. They might be rare or hard to find or costly to find. However, scientists are now developing techniques where they take a sample of seawater and analyse the microscopic bits of DNA that are found in cells and tissues that animals have shed as they swim through the water. Because different species have differences in their DNA, these differences can be detected and tell scientists and managers what species might occur in a particular area.

Scientists in Tasmania are also analysing poo from albatross breeding colonies to work out what species of fish and squid they are eating. Feathers from birds accidently killed by fishing are also being analysed to confirm what species they are and even what breeding colony they are from.

New horizons

AFMA staff recently attended a workshop in New Caledonia where fisheries scientists from around the Pacific converged to talk about some of these new or developing techniques and how they can be applied to management of tuna and billfish populations in the Pacific Ocean, including the recreational and commercial tuna and billfish fisheries off eastern Australia. The Fisheries Research Development Corporation (FRDC) has already funded a CSIRO led project using genetic analysis techniques to determine whether the populations of tuna and billfish off eastern Australia are mixing and interbreeding with those in adjacent and further afield Pacific Ocean regions.

This is all pretty mind blowing stuff and while these methods are still being developed and tested, our hope is they will revolutionise how fisheries are assessed and monitored into the future.

One day, tagging at gamefishing tournaments may be done with genetics, rather than physical tags.

View the article printed in issue 135 of BlueWater Magazine