Long-term occurrence of Atlantic bluefin tuna Thunnus thynnus in the North Sea: contributions of non-fishery data to population studies

The sudden disappearance of Atlantic bluefin tuna in northern Europe in the 1960's, after its frequent occurrence in the preceding period, has often been discussed in the scientific literature. Remarkably enough, the question whether the figures from the beginning of the 20th century have always been at the same high level in earlier periods, has so far only been poorly investigated. The answer to this question is crucial to define realistic targets for conservation measures aiming at the return of bluefin tuna to this feeding area. Taking account of texts from 16th to 19th century authored by naturalists and zoologists, and scientific papers by fishery biologists in the early 20th century, it is concluded that tuna has frequently been present over the last centuries but their abundance has not been consistently high. This conclusion is supported by national newspaper databases which help to learn more about age group distribution, fishery, and local differences in consumer acceptance with regard to tuna in the 1910's. These sources revealed two new ecological insights to bluefin tuna ecology in the region. First, the documents showed an earlier phenology than can be seen from most of the 20th century commercial catch statistics, with occasional sightings and strandings during March-June. Second, schools of bluefin tuna appeared in the Rhine-Scheldt estuary in the first decades of the 20th century.