News - October 21, 2009

Fishermen's practical knowledge at variance with researchers' models

Fishing researchers often produce information that does not fit in with the perceptions of fishermen. Basic information about fish population levels is hidden in complex models that fishermen do not understand. This leads to constant disputes about how many fish are in the sea and what measures might improve population levels.

That is the conclusion of Marieke Verweij of the Environmental Policy chair group. She studied the perceptions of researchers, fishermen, policymakers and environmental groups of sole and plaice population levels in the North Sea. Her article on the subject has been accepted by the journal Marine Policy.
It is impossible to count the number of fish in the North Sea because the fish are difficult to see and always on the move. That is why researchers make estimates every year of the number of fish in the sea that have reached reproductive maturity using virtual population analysis. Key information for this model is the catch rate for fishermen and research ships. The rule is: if the catch rate increases, there must be more fish in the sea. Assumptions about natural mortality and fish growth rates also affect the outcomes. Recent estimates of the fish population levels have a large margin of uncertainty and measurements in 2009 have led to adjustments being made retrospectively to the estimated population levels for the last ten years. Vervweij, who trained as a biologist, says: 'I am a scientist but it takes me a lot of time to figure out this model.'
Fishermen are best able to interpret the data on catch rates because that fits in with their own experiences. But Verweij found it very difficult to find a graph with the catch rate for plaice and sole over the past ten years. 'That information is hidden within the model. Researchers should share this kind of basic information more and discuss it with the fishermen. If you don't have unambiguous basic information, you will keep having disputes about the number of fish in the sea and whether population levels are increasing or decreasing.'
Another factor preventing the two sides from understanding each other is the fact that the researchers have to simplify the real situation. When researchers make policy recommendations, they use their model to predict the impact of one measure in isolation. 'They assume all the other variables in the model remain unchanged. Fishermen don't think like that', says Verweij. 'They see all kinds of complex interactions and emphasize the unpredictability of nature. The assumptions researchers often make, such as 'the number of juveniles remains constant', conflicts with their perception of the complex reality. For example, fishermen often point to water pollution by oestrogens, or the fall in phosphates in sea water, which is said to affect the growth and survival of flatfish. Such factors are not independent variables in the model but their effects are seen in the number of juveniles, flatfish growth and natural mortality.'
Verweij thinks researchers and fishermen should carry out joint research projects more often so that they can understand how the other side thinks and works. 'For example, they could test hypotheses together as to why population levels are increasing or decreasing', says Verweij. 'On condition that they use the same basic information.'