Experts call for better fishery management as stock figures go underreported

27 Aug 2024 --- The sustainability estimates of global fisheries is “overstated” and overfished populations are in worse condition than has been reported, according to an international study of 230 fisheries worldwide. The findings underscore the need for effective fishery management at a time when a decline in fish stocks is growing due to the impact of climate change on fisheries.

Scientists at the University of Tasmania in Australia and University of Victoria in Canada used publicly available data on the “best-known fisheries” species to test whether stock biomass estimates made in the year of the assessment were as precise as specifically calculated hindcast models, which determine probable past conditions.

The findings, published in Science, found that for fish populations that were “overfished, low value or located in regions with rising temperatures, historical biomass estimates were generally overstated compared with updated assessments.” Moreover, rising trends reported for overfished stocks were often “inaccurate.”

“With consideration of bias identified retrospectively, 85% more stocks than currently recognized have likely collapsed below 10% of maximum historical biomass.”

Analyzing depleting stocks

The team’s retrospective analysis of “spawning stock biomass”(B) — the combined biomass in metric tons of all mature females — included 230 stocks with 986 assessments encompassing 128 species or species complexes. Annual modeled estimates of B extended over an average of 47 years in time series.

The study found that stock biomass estimates for any year were “often far from the (unknown) true value.”“We compared 756 values of B reported in the final year of older stock assessments with the equivalent estimates of B (i.e., in the same year) provided as hindcast values in the most recent assessment.”

“Modeled estimates of stock biomass for assessments released in different years showed even greater variability.”

The authors mention the Pacific cod in the Gulf of Alaska, for which estimates of B for the same year varied by 1.49 times between consecutive assessments. 

“Pacific cod variability was similar to mean change between assessments averaged across all 230 stocks (1.40 times) and is thus typical of the variability in stocks considered in this work,” they explain. 

Such marked differences suggest that stock biomass estimates for any year were “often far from the (unknown) true value.”

Constraints in fishing operations

Marine fish populations are subject to fishing pressure along the world’s coastlines by local fishers and further offshore by industrial fleets, whose huge capacity is capable of decimating the fish that they target, state Rainer Froese and Daniel Pauly (researchers from Germany and Canada, respectively) in a related perspective.

“Fishing operations are therefore typically constrained bycatch limits derived from stock assessments — i.e., reports on the condition of a fish stock and its estimated future abundance.”

They flag that while theoretically, catch limits should ensure sustainability by not exceeding the productive ability of fish populations and by allowing depleted stocks to rebuild, in reality, the fraction of overfished stocks “continues to increase.” Additionally, reported global catches stagnate despite increasing fishing efforts, implying that fish abundance is declining.

Their results indicate that 29% of the stocks classified by the FAO as “maximally sustainably fished” instead cross the threshold for categorization as “overfished.”

Experts say that catch limits should ensure sustainability by not exceeding the productive ability of fish population.The study’s findings also underscore, what Froese and Pauly deduce as, “how systematic bias in stock estimates can lead to management advice that is not sufficiently conservative to sustain productive fish populations.”

“Efforts on several fronts are needed to improve the accuracy of stock assessment models that serve as the backbone of effective fisheries management.”

Effective fishery management

The authors recommend some primary conditions to improve fisheries stock assessment models’ accuracy and sustainability.

These include precautions because of the high uncertainty in stock assessments, consideration of cognitive biases, modeling multiple scenarios, adjusting for known biases and including climate change effects in stock assessment models.

The authors also advise openness of data for reproducibility of stock assessments and describing the sources of uncertainty. 

Finally, they suggest implementing fishery-independent population surveys and obtaining data from “no-fishing” reserves.

By Insha Naureen