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What drove tuna catches between 1525 and 1756 in southern Europe

Long-term changes in temperature can influence the productivity and composition of ecosystems since many biochemical processes are temperature-driven. For ecosystems encompassing large geographical areas this influence is however difficult to prove empirically since recent human influence may have a much stronger effect than environmental conditions. The analysis of the variations of ecological data over periods in which this human influence was not as pressing as today can, therefore, more clearly reveal linkages between climatic or environmental conditions and ecological productivity.

Bluefin Tuna (Thunnus thynnus)

This study attempted to disentangle the relationship between temperature variations and populations of Mediterranean tuna (Thunnus thynnus) between the 16th and 18th centuries by analyzing historical records of fish captures, temperature reconstructions based on proxy-data and simulation of past climates performed with a global climate climate model.

The period between the 16th and 18th centuries in Europe were characterized by a very prominent drop in temperatures. The maximum cooling was attained around 1700 during a period of very low solar activity known as the Late Maunder Minimum, in which temperatures in Europe could have been around 1 degree on average colder than today - although this number is still uncertain. These cold temperatures had a strong negative influence in the population of Mediterranean tuna. This population ca be estimated from well documented historical records of catches hauled at ports in South Western Spain. During this period, the fishing methods did not change much, so that the changes in catches can be shown to reflect changes in the political environment (e.g. wars and social turmoil) but also due to temperature changes.

The statistical analysis of all these data allows to determine that climatic variations over this period in the past can explain about 10% of the variations of the Mediterranean tuna population. This effect can be also understood in terms of recruitment and population survival. A large portion of variation, however, is due to internal population dynamics.

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