Survey finds world fisheries are in much worse shape than expected, with many already bankrupt

When fish are caught in our oceans faster than they can reproduce, their populations decline. This overfishing disrupts marine ecosystems. It is also harmful to human populations who depend on fish for food.

To manage fisheries sustainably, we need accurate data on the number of fish present and their future abundance. Fisheries scientists use complex mathematical models to determine this data.

But a survey conducted by my colleagues and myself, published in the journal Sciencecasts serious doubt on the accuracy of these models.

We studied 230 fisheries around the world. We found that populations of many overfished species are in much worse shape than reported, and that the sustainability of fisheries has been overestimated. Urgent action is needed to ensure our oceans are not fished beyond their capacity to recover.

Alarming results

Sustainable fishing would ensure that the number of fish caught does not exceed the population’s reproductive capacity. In cases where an area has been overfished, stocks should be allowed time to recover.

To determine appropriate catch rates, computer models are used to assess fish stocks. The models are fed data such as fish biology, catch history, and fish reproduction, growth, and mortality rates.

Our investigation tested the accuracy of fish stock estimates. It looked at data from 230 of the world’s largest fisheries, covering 128 fish species. These fisheries are located off Australia, New Zealand, the United States, Europe, the United Kingdom, Canada, Argentina and South Africa.

We focused on the depletion of the “biomass,” or total weight, of fish stocks. When fish catches fall below 10% of their biomass at the start of fishing, the fish stock is generally said to have collapsed.

For each fish stock, we collected data that provided the best estimate of stock depletion in a given year in the past. The data were generated by scientists and shared with fisheries managers and databases.

We compared these historical data to an updated model produced years later. The updated data were the most recent assessment of this fish stock, but they were also “back-cast” to the same year as the historical data. The most recent estimates should be more accurate because they are based on data collected over a longer period of time and after improvements to the modeling process.

So what did we find? Early stock assessments were often overly optimistic about the number of fish in the ocean.

For sustainably fished stocks, previous estimates were generally accurate. But for overfished stocks, most previous data have been found to be grossly overestimated. In many cases, fish stocks were considered to be recovering at the time when they were actually declining.

Among overexploited stocks, we estimated that the number of collapsed stocks was probably 85% higher than currently recognized.

How did this divergence arise? The models used to assess stocks are complex and involve a lot of data. This can lead to uncertain or inaccurate results, a problem that accumulates each time a value is entered into the model.

As I highlight below, the consequences can be devastating.

The Case of Jackass Morwong

The jackass morwong (also known as the deep-sea perch) lives off southern Australia and New Zealand. In 2009, models estimated the total size of the south-eastern Australian stock at 4,680 tonnes, or 22% of the 21,200 tonnes that existed when the fishery began. This estimate has informed fisheries managers’ decisions about how many fish can be sustainably caught in future years.

But the 2014 modeling showed that the stock size in 2009 was likely 3,330 metric tons, and the initial stock size was likely about 28,800 metric tons. That means that by 2009, stocks were likely to have declined by 12 percent from their initial level, not 22 percent.

These inaccurate estimates mean that the Australian Fisheries Management Authority’s “total allowable catch” for jackass morwong was likely not sustainable. Fishing continued with few constraints and the morwong population continued to decline for a decade.

But by 2022, the decline in fish numbers was evident. That year, authorities announced that five areas of the ocean would be closed to trawlers, in an effort to protect jackass morwong and other fish species. The federal government also allocated about A$24 million to buy back fishing permits.

This could probably have been avoided if accurate stock models had been applied and the extent of the depletion had been recognised a decade earlier.

Fundamental change is needed

Our research shows that the global overfishing problem is much more serious than currently thought. What should be done?

It is clear that scientists should try to improve the accuracy of the models used to assess fish stocks.

Fisheries management should be much more careful to protect fish stocks around the world. This is essential for sustainable fisheries, healthy oceans and our own food security.

This article is republished from The Conversation under a Creative Commons license. Read the original article.