Scientists: Juvenile Tuna Can Be Fished

Decades of scientific fishery management advice and volumes of simulation studies may not have been effective at all and might even have damaged stocks and productivity. That is the conclusion of the fishery biologist professor Paul van Zwieten of the Dutch Wageningen University, together with his Danish colleague Jeppe Kolding of the University of Bergen, in a recently published scientific paper. According to Kolding and Van Zwieten the existing models that result in bans on fishing juveniles and quota regulations don’t fit reality and are ‘ecologically vacuous’. Kolding and Van Zwieten base their conclusion on a study of inland small scale fisheries in Africa. “But you can apply the basic conclusion to al fisheries, including tuna”, says Mr. Van Zwieten. “The catch of juvenile tuna in Indonesia might be not as harmful as has been thought. And even the policy of avoiding bycatch might be contra productive.” The conclusions add to the heated debate in circles of marine biologists that question the existing wisdom from single species management models that have been applied in the last 50 years. According to Kolding and Van Zwieten generations of fisheries biologist have been taught the Yield-per-Recruit models to the point that ‘indiscriminate’ fishing methods are by default synonymous with destructive fishing practices. Killing juveniles, as happens in small scale fisheries, has been condemned as a form of depleting the stocks , “so dogmatic that it doesn’t even warrant verification”. “With the increasing focus on discarded bycatch problems in single species industrial fisheries the issue of selectivity has been further highlighted, and much research is devoted to develop increasingly selective fishing methods and exclusion devices”, write Kolding and Van Zwieten. The result of all this is that modern objective for industrial fisheries has become a highly selective kill on targeted species and sizes. But data prove a totally different and more complex reality. Populations experimentally harvested on small sizes produce after only four generations nearly twice as much yield as the populations where only large specimens were harvested. This is clearly in contrast to what the existing model predicts. “Fisheries scientific advice to management, however, is largely oblivious to these evolutionary and ecological studies and continues to reiterate the standard recipe from Yield-per-Recruit models”, write Kolding and Van Zwieten. According to several data, indiscriminate fishing methods might not always be bad from an ecologically point of view as long as it forms part of a fishery that fishes all different age levels in proportion to their natural production. The scientist point out that Lake Kainji - one of the most productive lakes in Africa - since 1996 experienced a 60 % reduction in effort due to banning beach seines and introducing small mesh sizes and mandatory licensing. The only visible result was a corresponding 60 % decrease in yield and no positive response in the individual catch rates as the models assume. It turned a high biologically productive into a less productive system. Instead the most productive inland fisheries like Lake Victoria, are also the most intensely exploited. Open access fisheries prove to result in a certain stable average amount of individual catch. This situation is not unique for African inland fisheries, also investigations in Newfoundland inshore cod fisheries did find similar results. According to both scientists the existing fishery management is based on the long existing misbelieve among ecologists that ecosystems are closed entities in a process towards equilibrium. Human interventions such as fishing are therefore regarded as an external disturbance that affect the productivity of the system. But new dynamic ecology questions this view. Instead it regards ecosystems in a constant and ever changing state of disequilibrium with chaotic fluctuations due climatic variation or human interventions. Another serious misconception is that fish is treated like live stock. But there has been the consistent lack of relationship between adults and recruitment in fisheries science. That indicates that the life history of fish is probably closer to insects and plants. “There is increasing evidence that only show negative ecological effects of adult size selectivity. Everything else being equal, we can safely deduce that the less we select on species and sizes, the more the original composition and structure of a fish community will remain the same”, conclude the biologists. According to Van Zwieten these kind of new fishery management views can also be applied in fisheries like yellowfin tuna and skipjack. For heavily overfished stocks like bigeye tuna, the fishing pressure should nonetheless substantially be reduced regardless the new insights.

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