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(Fromentin and Fonteneau, 2001; Mather et al., 1995). Opportunistic spawning is, thus,

unlikely for BFT.

Finally, we suggest that BFT reproductive strategy could be in between the two

above hypotheses, i.e. a kind of “opportunistic homing”. A recent physiological study

pointed out that the last stages of BFT gonad maturation (intense production of yolked

oocytes of stages 3 and 4) occurred within a short period of time and probably under the

control of temperature (Medina et al., 2002). If suitable temperatures induce the last

stages of gametogenis and thus, reproduction, one may hypothesise that more locations

in the North Atlantic (encountered by BFT along their migration route) could be

suitable for spawning as water temperature increases. In other words, the Atlantic BFT

population could also reproduce elsewhere than in its traditional grounds (i.e., the

Western Mediterranean Sea and the Gulf of Mexico) during warm periods. This

hypothesis is in agreement with some pop-up tag records (Lutcavage et al., 1999) and

several past works, which have mentioned the occurrence of other hypothetical

spawning areas, such as the Ibero-Moroccan bay and the Bay of Biscay (e.g. Buen (de),

1927; 1935; Mather et al., 1995). During cold periods, reproductive activities would be,

in contrast, restricted to the traditional and permanent spawning grounds (which would

further remain suitable during warm ones). This hypothesis of an “opportunistic

homing”, which combines the “natal homing” hypothesis with the opportunistic

reproductive strategy of tuna, is consistent with our results and current knowledge on

tuna. This hypothesis could be tested by gathering new in situ information on

migrations and environmental preferences of BFT, e.g. by combining pop-up archival

tags with satellite oceanography and ocean modelling.

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