In 1997, the first large-scale Atlantic bluefin tuna farming (in the sense of tuna fattening) started in the Mediterranean Sea, near the Spanish coast of Murcia. Although there were some small-scale tuna fattening operations for post-spawned bluefin tuna at the Strait of Gibraltar in the late 1970s, the Murcian operation was the start of a new tuna farming era and had a very strong impact on the Atlantic bluefin tuna fisheries and its managements.
Until the Murcian operation, juvenile bluefin tuna had been caught and consumed only by the local people throughout the Mediterranean coasts. The price for fish was not that high. However, the farming converted young bluefin tuna into golden eggs. Consequently, in the late 1990s to the beginning of the 2000s, there was a sudden increase in demands for live juvenile bluefin tuna to be used as the stock for farming. This was then followed by: the soaring price of fish; new entries of many fishing boats to this fishery; and the rapid increase of catches. Naturally, such a sudden increase in catches of juvenile, as well as the spawning adult bluefin tuna in the Mediterranean Sea, had a serious adverse impact on tuna stocks.
The International Commission for the Conservation of Atlantic Tunas (ICCAT) adopted a total allowable catch and country catch quota. However the increasing catch trends were not curtailed by the catch quota alone. One of the major reasons was that the farming provided a loophole in the tuna fisheries management measures. This problem still remains even at present, but to a lesser extent, as various supplemental regulations have been adopted since then. The fish caught are not landed but moved from a fishing net to a carrying or farming cage directly under the water. This means the accurate catch quantities are not known.
The only occasion that the fish can be seen above the water and weighed is when they are harvested (landed) for shipping to the market; a few months or sometimes, a couple of years after they were caught. The weight taken at the harvest, of course, includes a growth during the fattening process. Therefore, it is very difficult, if not impossible, to estimate the weight of fish at the time of capture backwards from its harvest weight. The key for such a backward-calculation is to have an accurate rate of growth during the farming.
Much research has been conducted on this subject but the results vary from 15% to 150% increase in weight of fish. These variations are related to the period of farming, density of fish in the cage, water temperature, forage, environment, size of the fish and many other factors. The best option is to estimate the number and weight of fish when they are being transferred from the fishing nets to the cages underwater. A stereo-underwater video camera has been developed for this purpose and known to be useful but there still appears to be some difficulty in implementing it fully.
A further difficulty is that fish from various sources (often from different countries) are mixed through a selection process during farming. The selections are made according to the size of fish in order to keep the similar-sized fish in the same cage. As is well known, if fish of different sizes are kept in the same cage, small fish can not compete with big fish and therefore these small fish show very slow or no growth. This mixing of fish from different sources makes it impossible to identify which fish are caught by which vessels or flag countries. Recent management regulation prohibits mixing of fish from different sources in one cage but the complete implementation of this measure is sometimes difficult. Therefore, even now, there are many uncertainties when estimating catch in weight of each vessel, once the fish are transferred into the farming cage. Having said this, great efforts have been made to close these loopholes (e.g. catch document system or observer system at the farming ground) of management in order to ensure a greatly improved compliance. Further research and advancement in technology would eventually solve the remaining problems.