Fisheries research overestimates fish stocks

The state of fish stocks in the world's ocean is worse than previously thought. While overfishing has long been blamed on fisheries policies that set catch limits higher than scientific recommendations, a new study by four Australian research institutions reveals that even these scientific recommendations were often too optimistic. The result? Far more global fish stocks are overfished or have collapsed than we thought. Dr Rainer Froese from the GEOMAR Helmholtz Centre for Ocean Research Kiel and Dr Daniel Pauly from the University of British Columbia have provided their insights on the study. In their Perspective Paper, published today in the journal Science alongside the new study, the two fisheries experts call for simpler yet more accurate models and, when in doubt, a more conservative approach to stock assessments.

Many fish stocks around the world are either threatened by overfishing or have already collapsed. One of the main reasons for this devastating trend is that policymakers have often ignored the catch limits calculated by scientists, which were intended to be strict thresholds to protect stocks. But it has now become clear that even these scientific recommendations were often too high.

In the European Union (EU), for example, fisheries are primarily managed through allowable catch limits, known as quotas, which are set by the European Council of Agriculture Ministers on the basis of scientific advice and recommendations from the European Commission. A new study by Australian scientists (Edgar et al.) shows that already the scientific advice has been recommending catch limits that were too high.

The journal Science, where the study is published today, asked two of the world's most cited fisheries experts, Dr Rainer Froese from the GEOMAR Helmholtz Centre for Ocean Research Kiel and Dr Daniel Pauly from the University of British Columbia, to interpret the findings. In their Perspective Paper, they advocate for simpler, yet more realistic models based on ecological principles, and call for more conservative stock assessments and management when uncertainties arise.

For the study, Edgar et al. analysed data from 230 fish stocks worldwide and found that stock assessments have often been overly optimistic. They overestimated the abundance of fish and how quickly stocks could recover. Particularly affected are stocks that have already shrunk due to overfishing. The overestimates led to so-called phantom recoveries, where stocks were classified as recovered while, in reality, they continued to decline. "This resulted in insufficient reductions in catch limits when they were most urgently needed," explains Dr Rainer Froese. "Unfortunately, this is not just a problem of the past. Known overestimates of stock sizes in recent years are still not used to corrected this error in current stock assessments."

The research by Edgar et al. also shows that almost a third of stocks classified by the Food and Agriculture Organization (FAO) as "maximally sustainably fished" have instead crossed the threshold into the "overfished" category. Moreover, the number of collapsed stocks (those with less than ten per cent of their original biomass) within the overfished category is likely to be 85 per cent higher than previously estimated.

But what causes these distortions in stock assessments? Standard stock assessments use models that can include more than 40 different parameters, such as fish life history, catch details, and fishing effort. This large number of parameters makes the assessments unnecessarily complex, write Froese and Pauly. The results can only be reproduced by a few experts with access to the original models, data and settings. Moreover, many of the required input parameters are unknown or difficult to estimate, leading modelers to use less reliable values that have worked in the past. Froese notes: "Such practices can skew the results towards the modelers' expectations."

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Coastal river deltas threatened by more than climate change

Worldwide, coastal river deltas are home to more than half a billion people, supporting fisheries, agriculture, cities, and fertile ecosystems. In a unique study covering 49 deltas globally, researchers from Lund University and Utrecht University have identified the most critical risks to deltas in the future. The research shows that deltas face multiple risks, and that population growth and poor environmental governance might pose bigger threats than climate change to the sustainability of Asian and African deltas, in particular.

"We can clearly show that many risks are not linked to climate. While climate change is a global problem, other important risk factors like land subsidence, population density and ineffective governance are local problems. Risks to deltas will only increase over time, so now is the time for governments to take action," says Murray Scown, associate senior lecturer, Lund University Centre for Sustainability Studies, and lead author.

Collapse of delta environments could have huge consequences for global sustainable development.

In the worst-case scenario, deltas could be lost to the sea; other consequences are flooding, salinization of water, which affects agriculture, coastal squeeze, and loss of ecosystems.

The study, published in Global Environmental Change, looked at five different IPCC scenarios for global development in 49 deltas all over the world, including famous deltas such as the Nile, Mekong, and Mississippi, but also more understudied deltas such as the Volta, Zambezi and Irrawaddy deltas.

The research identifies possible risks to deltas stretching 80 years into the future.

The researchers based their analysis on 13 well-known factors affecting risk in deltas and drew upon unique models to identify which of these risks are most likely to endanger different deltas in the future.

Risk factors include increasing population density, urban development, irrigated agriculture, changes to river discharge, land subsidence and relative sea-level rise, limited economic capacity, poor government effectiveness, and low adaptation readiness.

Population density, land subsidence and ineffective governance are high risk factors

The analysis shows that there are some risks that are more critical to deltas than others -- in all of the five future scenarios.

These include land subsidence and relative sea-level rise, population density, ineffective governance, economic capacity, and crop land use.

For some deltas, physical risks are especially pronounced. Land subsidence is, for example, the highest risk factor for the Mekong delta in Vietnam.

Extreme sea levels are among the most concerning risk factors for deltas in China, on the Korean peninsula, and in the Colorado (Mexico) and Rhine (Netherlands) deltas.

In the Nile (Egypt), Niger (Nigeria), and the Ganges (Bangladesh) deltas, it is increasing population density that is of most concern under certain scenarios.

For other deltas, it is the lack of economic capacity and government effectiveness to manage risks, for example in the Irrawaddy (Myanmar) and Congo (Angola and Democratic Republic of the Congo) deltas.

"Analysed all together, we can see that the Asian mega-deltas are at greatest risk, with potentially devastating consequences for millions of people, and for the environment. They are under pressure from population growth, intense agricultural land use, relative sea-level rise, and lagging adaptation readiness," says Murray Scown.

Local and global approaches and a mixture of hard and soft adaptation can mitigate risks

"Instead of sitting back, governments need to think long-term, and put plans in place to reduce or mitigate risks. In the Mekong delta, for example, the Vietnamese government are making strong efforts to restrict future groundwater extraction in the delta to reduce land subsidence and salinization," says Philip Minderhoud, assistant professor at Wageningen University and Research.

The researchers highlight that a mixture of hard ("grey") and soft ("green") adaptation approaches will be required to manage and mitigate delta risks.

They include both hard infrastructures, like sea walls to stop the sea inundating the delta, and soft approaches using nature-based solutions.

One example is the Dutch experience of creating room for the river in the Rhine delta, by lowering floodplains, relocating levees, and using spaces that are allowed to flood for grazing.

Initiatives to build up delta surfaces by allowing rivers to flood and deposit sediment on the delta to maintain elevation above sea level are also promising, notes Frances Dunn, assistant professor at Utrecht University.

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