Tuna’s sweet home ?

When I was studying in the U.S.A., I joined a research team surveying Pacific salmon in their spawning grounds in Alaska. I was absolutely impressed and overwhelmed by the mass of spawning sockeye salmon that had returned from the middle of the Pacific Ocean for over a thousand miles, having migrated up the river for several hundred miles more, destined to their small branch streams where they had hatched and had spent their early lives.

Such homing instincts are considered also present in tuna. Spawning grounds of tropical tunas are widely spread over the tropical waters and spawning seasons are well extended throughout the year. Therefore, it is difficult to prove the existence of their homing instincts. However, some matured fish which were tagged in the spawning area have been recovered in the same area and season, a year later.

For temperate tunas, their homing instincts are even clearer, as their spawning grounds and seasons are very much limited. For example, it has been known for a long time that Pacific bluefin tuna spawn off east of Taiwan and nearby waters of southern Okinawa Islands. More recently, spawning has also been identified in the Sea of Japan. It is still too early to determine whether this is the result of an extension of conventional spawning area towards north or whether it is the result of our lack in knowledge concerning northern spawning until recently. It will be the task of future research efforts to understand if those spawning groups represent two distinct (sub-)stocks or both belong to the same stock.

In the Atlantic Ocean, which is far smaller in area than the Pacific, there are two well-known spawning areas for the Atlantic bluefin tuna; one in the Gulf of Mexico and another in the west Mediterranean Sea. Recent research through tagging with electronic memory and through genetic studies indicates those two groups are independent stocks and fish have fairly good fidelity to each spawning ground. This means that spawners return to the same area where they originated from for reproduction activities. However the strength of fidelity or homing instinct are not as well understood as those of Pacific salmon. Also it is well known that both stocks, one from Gulf of Mexico and another from the Mediterranean intermingle well in the Atlantic but exactly where and what proportions are still to be investigated. The knowledge of mixing is very important but to understand it is a difficult task for us, as effective stock management has to be made per stock basis and the status of mixing is quite dynamic depending on the relative stock size of east and west.

In the Mediterranean Sea, spawning has been known for a long time, in the western part, near the Baleares. However, recent research indicates much wider spawning activities throughout the Mediterranean Sea, including its eastern part. As is the case with the recent expansion of spawning grounds for Pacific bluefin tuna, we cannot decide whether this extension in the Mediterranean Sea is due to an actual change in spawning behavior, or whether it is our change in perspective through recent new research efforts. If the former were the case, it could have been related to general warming of the earth?