70 years of helping people to care for our ocean

So how do we know how many fish are in the sea?

Dr Bruce Mann
Senior Scientist, ORI

 Many fishermen question how scientists come up with estimates of fish population sizes. Dr Bruce Mann, Senior Scientist in charge of fish research, gives us some answers:

The Fisheries Branch of the Department of Agriculture, Forestry and Fisheries (DAFF) is the national department responsible for the management of South Africa’s marine fish resources. They have a Chief Directorate for Research (previously called the Sea Fisheries Research Institute) where scientists are employed to undertake fish stock assessments, among other tasks. From time to time they may contract out this work to other research organisations such as universities or research institutes.  

The Oceanographic Research Institute (ORI) is a division of the South African Association for Marine Biological Research (SAAMBR), a non-government, not-for-profit organisation. ORI’s mission is basically to undertake applied marine research to help assist in the wise and sustainable use of our marine resources. ORI does therefore undertake some fisheries stock assessment work and runs a number of long-term monitoring projects to help assess trends in marine resource use.

This being said, there are basically two general methods used for undertaking fish stock assessments namely: fisheries independent and fisheries dependent methods. Fisheries independent methods usually involve scientists collecting biological data on the species being investigated. Most importantly this involves determining the age and growth of the species under investigation (e.g. in fish this involves removing the otoliths to count annual growth rings). This information is then used with measurements of the length/age distribution of the catch to assess the stock. Per-recruit models such as spawner biomass per recruit have been the most commonly used models to undertake such assessments for SA linefish.

Fisheries dependent methods involve monitoring angler’s catches over time to determine trends in fish population size. Knowing the long-term trends in catch and effort for a specific species allows scientists to use what are known as surplus production models to assess the status of that stock. These methods are easier to use in commercial fisheries where the number of boats, crew and fishing effort is known and catches have to be recorded by law. DAFF scientists undertake regular stock assessments of all the major commercial species harvested around South Africa. For linefish this includes Cape snoek, Cape yellowtail, carpenter, silver kob, geelbek, hottentot, slinger and santer. Assessing catch and effort in recreational fisheries is much more difficult as there are a large number of participants and a variety of different sectors. ORI does run a number of long-term monitoring projects such as the National Marine Linefish System (KZN recreational data), the KZN Boat Launch Site Monitoring System, the KZN Invertebrate Catch Statistics Project, the ORI Cooperative Fish Tagging Project and others. These projects help to feed in data which allow scientists to make better informed decisions when undertaking stock assessments.   

Once a stock assessment is completed it is normally sent out for peer-review before it can be published. At this stage the models used and the assumptions made are thoroughly tested. Once published, the results of the stock assessment feed into the management decision-making process and it is here where the various and most appropriate regulations are determined. Once legislated (which can take many years!) the regulations are supposed to be enforced but unfortunately this is an area which is sorely lacking in South Africa’s recreational fisheries.


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