Keystone-SDA
This content was published on 23 August 2024 – 06:05
(Keystone-SDA) Many fish stocks worldwide are threatened by overfishing or have already collapsed. According to a study, this is not only because scientifically calculated maximum catch levels have not been adhered to.
In fact, these recommendations are already too high, report researchers in the journal “Science”. The size of stocks and the dynamics of their recovery have so far been estimated far too optimistically.
Fisheries models provide the basis for regulating global and regional fisheries and are considered an important tool against overfishing. The main cause of this overfishing was previously considered to be that fisheries policy set catch levels higher than those recommended on the basis of the models.
Significant overestimations
A team led by Graham Edgar from the University of Tasmania (Australia) examined data from 230 fishing grounds worldwide and compared the values with those from models. According to the findings, the recommendations often significantly overestimated how many fish of a species there were left and how quickly a stock could recover. The deviation of the models used was particularly serious for populations that were already overfished.
But even stocks classified as recovered often continued to shrink in reality, according to the analysis. “This meant that catches were not reduced sufficiently, although this was urgently necessary,” explained Rainer Froese from the Geomar Helmholtz Centre for Ocean Research in Kiel, co-author of a commentary on the study published in the journal “Science”.
The problem does not only affect the past. “The known overestimations of the population sizes from recent years are not being used to correct the current population sizes,” says Froese.
Overfished instead of sustainably fished
According to the study, almost a third of the stocks classified by the Food and Agriculture Organization (FAO) as being “maximally sustainably fished” are actually overfished. Fishing is considered sustainable when no more fish are taken than can be replaced.
In addition, far more populations have already collapsed than previously thought, it continues: 85 percent more populations than previously estimated have collapsed, i.e. shrunk to less than 10 percent of their historical maximum.
Froese and his co-author Daniel Pauly from the University of British Columbia (Canada) explain in their commentary that the models sometimes use more than 40 parameters. These include characteristics of the species’ lifestyle, catch details and the fishing effort required in each case. This large number of variables makes the estimates unnecessarily complex, say Froese and Pauly. In addition, modelers use values that are hardly reliable for some parameters.
Highly complex models
“The big question is why the sometimes very improbable predictions of the official models were and are accepted,” said Froese. Fisheries science has been giving politicians incorrect advice for years and thus bears part of the responsibility for the overfished and collapsed stocks, including in Europe.
“Overfishing is a problem especially in the Mediterranean, West Africa and South Asia,” explained Boris Worm of Dalhousie University (Canada), who was not involved in the study. Worldwide, many coastal fisheries have long since collapsed and are no longer recorded. The current study is a warning “that many stocks that are still considered well managed – including in Europe – may actually be worse off than expected.”
Baltic Sea example: cod
The phenomenon has been shown, for example, for cod in the western Baltic Sea, explained Christian Möllmann from the University of Hamburg. “In my opinion, the often overly positive estimate of the biomass has also contributed to the overfishing of the stock.”
Froese and Pauly conclude that the assessment procedures need to be revised to include simpler, more realistic models. In addition, the precautionary principle should be applied more strongly – in cases of uncertainty, more conservative estimates should be used.
Möllmann, on the other hand, does not consider the quality of the models to be crucial. “In my opinion, the will of the fishing industry not to take every fish out of the sea is more important.” The will and understanding to fish carefully and cautiously is often not evident.
This can be clearly seen in the Baltic Sea, where all once important cod and herring stocks have been so overfished, despite years of warnings from scientists and environmental protection associations, that recovery is largely uncertain or even unlikely.
