Abstract
This study analysed the temporal pattern of Atlantic bluefin tuna (ABFT) spawning in the Balearic spawning ground and examined its reproductive performance after years in captivity. Furthermore, ABFT hatching time at different on-site temperatures was determined for the first time. Spawning surveys were carried out in 4 spawning seasons (2009–2012) aboard tuna transport vessels. Three groups of spawners were monitored: a captive group transported to the spawning region and monitored throughout the four spawning seasons and two wild groups caught in 2009 and 2010 which were transferred to a monitoring transport cage immediately after being caught. Surface plankton samples were collected nightly, beginning immediately after the first purse seine catches were made and concluding after spawning was observed to have ended. All groups displayed the same spawning hours, restricted between 2:00–5:00 a.m. The captive group, as they got older, shifted towards the earliest hour, suggesting an age influence on reproductive time. The onset of spawning varied annually from the end of May to the beginning of June at temperatures around 19°C–20°C, ending by the second week of July. The peak of spawning was consistently around the summer solstice, June 15th–30th. The results showed the negative effect of unstable oceanographic conditions in the spawning process which might influence the annual reproductive success of ABFT. The influence of temperature on hatching time was higher than that observed in other tuna species, twice as fast at 26°C (23 h) as at 19.5°C (49 h). Overall, this study shows the strength of the internal mechanism in ABFT that controls spawning traits. Spawning in ABFT is cyclical and highly synchronised on diel and annual scales. We consider that the timing of spawning is rather influenced by day length and its adaptive significance is discussed.
Highlights
The Atlantic bluefin tuna (ABFT) is an emblematic species from every perspective: economic, biological, environmental and social
This study reports the first direct observations on the temporal spawning patterns and hatching time of Atlantic bluefin tuna and reveals the capability of tuna transport cages as exceptional monitoring observatories
The three groups monitored in this study, two wild groups (WG) and the captive group (CG), displayed the same spawning hours, restricted between 2:00 and 5:00 a.m
Summary
The Atlantic bluefin tuna (ABFT) is an emblematic species from every perspective: economic, biological, environmental and social. This is due to its high commercial value, its particular physiology and behavior and its wide spatial distribution over the entire North Atlantic and the Mediterranean Sea, where its fishery has roots that date back to 7000 BC [1]. The spawning migration towards the Mediterranean is seasonal and progressive: for five decades it has been known that fish enter through the Strait of Gibraltar in MayJune and leave in July-August [18], with a staggered arrival at and exit from the spawning grounds It is unknown whether spawning is likewise staggered, starting upon the arrival of the first spawners, or whether there is a waiting period until a particular set of required environmental conditions, such as temperature, become optimum, triggering spawning
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