While trawling off a sport fishing boat in mid October we hooked and released five small bluefin tuna, of 500g-1kg weight. This was 10-15 miles due West of Cadiz and the boat master told me that from his experience, catching juvenile Bluefin is quite common at this time of year.
However, according to the scientific literature they should not be here! The literature tells us that mature tuna dont spawn in these waters. Tuna only transit through these waters while on their spawning migration from the Atlantic through the straits of Gibraltar and into the Mediterranean. They spawn in June and July in regular spawning regions of the Mediterranean that are the subject of on-going research.
At 500-1,000g these tuna had developed from eggs spawned in June/July and are much too small to have swam back to here from the Mediterranean. My only conclusion is that Bluefin tuna are spawning somewhere in the Gulf of Cadiz.
Over the 9 years that I have been studying and working on developing tuna farming I have been aware of a number of major constraints to making this a viable fish farming sector.
Probably the most significant bottle neck at present is the transfer of the fish from the hatchery to the farms. From a land based tank to a floating sea cage. There is a serious physical problem associated with this sea transfer (for which, by the way, I have a proven solution: http://www.fish-hatchery-consulting.com/tuna-hatchery-2015/) but, on top of this there is the problem of growth rate with relation to temperature. Basically, the fish are introduced into the cage at 1-3g weight and need to reach a weight of around 1kg before sea temperatures fall below 18oC. In the Western Mediterranean the rapid temperature drop in autumn is a killer. But in the Gulf of Cadiz the temperature drop at the end of summer is much slower, and in fact, the temperature here in winter is higher than that of the western Mediterranean.
The significance of this observation for developing offshore tuna farms in the gulf of Cadiz is that in these waters juvenile tuna reach the size that enables them to survive through their first winter. Further studies are now needed to understand the oceanography as related to offshore farms. After which it will be time to tackle the authorities, and that is probably the most serious impediment to the development of fish farming.
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