Assessment of spawning of Atlantic bluefin tuna farmed in the western Mediterranean Sea

Mediterranean tuna farms account for >60% of the eastern Atlantic bluefin tuna (ABFT) catch quota. Besides the direct impact of purse seining on wild stocks, ABFT farming practices may have environmental implications that are still poorly known. An unexplored potential source of interactions of ABFT farms with wildlife is the release of eggs into the environment in places other than spawning grounds. Purse seine-caught ABFT schools are known to spawn in towing cages as they are transported to farms. We show here that farmed ABFT are also capable of spawning during at least 2 subsequent reproductive seasons following their capture. The reproductive potential of ABFT commercial stocks was investigated in a farm located in the western Mediterranean Sea from 2012 through 2014, using occurrence and number of postovulatory follicles as proxies of spawning fraction and realised batch fecundity, respectively. Although the spawning fraction among farmed fish was lower than that in the wild, the mean fecundity of captive spawners was similar to that of wild fish; consequently, the number of fertile eggs released from grow-out cages is thought to be significant. Larvae hatched from eggs spawned in farms are likely to grow and join wild-born ABFT juveniles that use nearshore areas of the western Mediterranean as foraging grounds. Depending on the volume of fish ranched for >1 yr and the larval survival rate in the region, the escape through spawning may have a significant impact on the ecosystem and could affect recruitment, thus influencing the population dynamics of ABFT in the Mediterranean Sea.