How we count fish

MPI measures fisheries sustainability by estimating the size of our fish populations. Scientists estimate the size and productivity of our most important fisheries to ensure they're at sustainable levels.

Getting the balance right

Maintaining healthy fish populations means catching what we need now but leaving enough for the future. We need a lot of data to get this balance right.

Fisheries scientists from MPI and NIWA (the National Institute of Water and Atmospheric Research) work to estimate the size of fish populations (stocks) and compare the results with what we consider ideal sizes. We're then able to work out how catch levels impact stocks and where changes are required.

Monitoring catches

Commercial fishers must report their catches. We can use this information to work out trends in catch rates – important for getting an idea of fish abundance.

When there are more fish in an area, it's usually easier for commercial fishers to catch the same amount from one year to the next. When there are less fish, they'll need to work harder to catch the same amount. However, this information sometimes gives us a false sense of overall abundance. If fishers were to start using new catch technology, we'd have to be careful how we interpreted the data.

In many important fisheries, we also gather information about the length and age of fish caught. Some of this comes from MPI observers on board fishing vessels. Knowing the catch rates and the size and age of fish being caught are often enough to tell us how sustainable a particular catch level is.

Research studies used in valuable fisheries

In high-value fisheries, there's often a desire to catch more fish. To give us better information, we use research vessels to carry out surveys or tagging studies. These are expensive, but give us independent estimates of fish abundance. We repeat some of these at regular intervals so we can track trends in abundance over time.

Sharing and analysing information

Throughout the year, several Fishery Assessment Working Groups meet to discuss the latest research and fisheries catch information. These groups then update the status of different fish stocks and work to estimate the likely abundance of each stock. By doing so, they can compare the abundance to management targets.

Later, the working groups meet as a plenary to discuss any major changes in stock status or methods to estimate stock status. The groups are made up of fisheries scientists and researchers from across government, research institutes, industry bodies, universities, and non-government organisations. The plenary meetings are open to the public.

Sustainable management

The more information we have, the more closely we can manage a fish stock at an ideal size. More information also allows us to more easily adjust our catches. Where there’s little information, it is hard to gauge how close a fish stock is to an ideal size – and so we must act cautiously, setting the catch limit at what we think is a safe level.

However, information comes at a cost. With the fourth largest fishing zone in the world, we have to prioritise the science needed to manage our fisheries.

Ongoing research in deepwater fisheries shows that we are maintaining these at sustainable levels. However, we know less about some inshore species, and MPI aims to improve our understanding of these in the immediate future.

Who to contact

If you have questions about how we count fish, email

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