Counting fish: effective tools to monitor juvenile abundance
Unbiased, precise and cost-effective methods for monitoring fish abundance are crucial for stock assessment and science-based fisheries management. Juvenile abundance is a key component needed for understanding the status of a stock. It provides an early warning of low numbers of young fish in the population that can affect rebuilding of adults in the stock.
Conventional tagging programs that use physical tags to mark fish can provide informative data, including estimates of juvenile abundance; however, there are often problems with tag loss and non-reporting of tags from fishers that are difficult to overcome.
Innovative monitoring using DNA
We have developed a genetic-tagging program for juvenile SBT, funded by the Commission for the Conservation of Southern Bluefin Tuna (CCSBT), to resolve serious problems with non-reporting of tags from fishers that undermined historical conventional SBT tagging programs.
This innovative method uses the unique genetic 'fingerprint' of individual fish as an invisible and life-long tag. Large numbers of fish are 'tagged' by taking a tissue sample to determine the fish's DNA fingerprint, before being released back to sea.
Approximately a year later, large numbers of fish are sampled again, this time from the commercial harvest with co-operation from the Australian SBT fishing industry. Abundance is estimated by determining how many fish from the release and harvest samples have matching DNA fingerprints, with more matches indicating a smaller population size.
We have successfully demonstrated the logistics and feasibility of efficiently collecting uncontaminated DNA samples PDF (1 MB) both at sea and during commercial harvesting, and provided abundance estimates for use in management of the species.
Scientific knowledge for better management of global stocks
The first estimate of juvenile abundance was calculated to be approximately 2.3 million age 2 fish in 2016, based on finding 20 matching DNA fingerprints between 3,000 tagged and 15,000 harvested fish.
An ongoing juvenile abundance monitoring program has been established, with funding from the CCSBT and European Union, to produce an annual estimate of the abundance of juvenile SBT. This time series of abundance estimates will be used in a fisheries management tool for setting the global catch of SBT.