Center for Macroecology, Evolution and Climate, National Institute for Aquatic Resources (DTU Aqua), Technical University of Denmark, Charlottenlund, Denmark.
PLoS One. 2012;7(7):e39998. doi: 10.1371/journal.pone.0039998. Epub 2012 Jul 24.
The lucrative and highly migratory Atlantic bluefin tuna, Thunnus thynnus (Linnaeus 1758; Scombridae), used to be distributed widely throughout the north Atlantic Ocean, Mediterranean Sea and Black Sea. Its migrations have supported sustainable fisheries and impacted local cultures since antiquity, but its biogeographic range has contracted since the 1950s. Most recently, the species disappeared from the Black Sea in the late 1980s and has not yet recovered. Reasons for the Black Sea disappearance, and the species-wide range contraction, are unclear. However bluefin tuna formerly foraged and possibly spawned in the Black Sea. Loss of a locally-reproducing population would represent a decline in population richness, and an increase in species vulnerability to perturbations such as exploitation and environmental change. Here we identify the main genetic and phenotypic adaptations that the population must have (had) in order to reproduce successfully in the specific hydrographic (estuarine) conditions of the Black Sea. By comparing hydrographic conditions in spawning areas of the three species of bluefin tunas, and applying a mechanistic model of egg buoyancy and sinking rate, we show that reproduction in the Black Sea must have required specific adaptations of egg buoyancy, fertilisation and development for reproductive success. Such adaptations by local populations of marine fish species spawning in estuarine areas are common as is evident from a meta-analysis of egg buoyancy data from 16 species of fish. We conclude that these adaptations would have been necessary for successful local reproduction by bluefin tuna in the Black Sea, and that a locally-adapted reproducing population may have disappeared. Recovery of bluefin tuna in the Black Sea, either for spawning or foraging, will occur fastest if any remaining locally adapted individuals are allowed to survive, and by conservation and recovery of depleted Mediterranean populations which could through time re-establish local Black Sea spawning and foraging.
盈利丰厚且高度洄游的大西洋蓝鳍金枪鱼（Thunnus thynnus）（Linnaeus 1758；鲭科）曾广泛分布于北大西洋、地中海和黑海。自古以来，其洄游就支撑着可持续的渔业，并影响着当地的文化，但自 20 世纪 50 年代以来，其生物地理范围已经缩小。最近，该物种于 20 世纪 80 年代末从黑海消失，至今尚未恢复。黑海物种消失和物种范围收缩的原因尚不清楚。然而，蓝鳍金枪鱼以前曾在黑海觅食，甚至可能在那里产卵。如果失去了当地繁殖的种群，就意味着种群丰富度下降，物种对捕捞和环境变化等干扰的脆弱性增加。在这里，我们确定了该种群为了在黑海特定的水文（河口）条件下成功繁殖必须具备（曾经具备）的主要遗传和表型适应性。通过比较三种蓝鳍金枪鱼的产卵区的水文条件，并应用一个关于卵浮力和下沉速度的机械模型，我们表明，在黑海繁殖必须具有特定的卵浮力、受精和发育适应性，以实现繁殖成功。在河口产卵的海洋鱼类种群中，这种适应是很常见的，从对 16 种鱼类的卵浮力数据的荟萃分析中可以明显看出这一点。我们得出的结论是，这些适应性对于蓝鳍金枪鱼在黑海成功进行本地繁殖是必要的，而一个本地适应的繁殖种群可能已经消失。如果允许任何剩余的本地适应个体存活，并且通过保护和恢复地中海已经枯竭的种群，蓝鳍金枪鱼在黑海恢复产卵或觅食的速度将最快，这些种群经过一段时间后可以重新建立黑海的本地产卵和觅食。
