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above results and discussion lead to question the hypothesis of “natal homing” for BFT

(Cury, 1994; Cury et al., 1998). This hypothesis postulates that the newly hatched

individuals retain environmental cues, which determine their future spawning

environment. In the case of BFT, natal homing is first based on the millennial

observations of BFT migrations into the Mediterranean (Doumenge, 1998), but this

does not imply that the size of the migrating population remained constant over

centuries. The second argument for natal homing is that only two BFT spawning sites

(i.e. sites where both fully ripe individuals with hydrated oocytes and eggs are found)

have been identified. However, Mather et al., (1995) underlined that estimating BFT

spawning sites from BFT gonad conditions are subject to many limitations: (i) fully ripe

BFT individuals are difficult to catch on hook as they are reluctant to feed (Sella, 1929),

(ii) BFT can travel a great distance in short periods (Gunn and Block, 2001; Lutcavage

et al., 2000), and (iii) BFT is a multiple spawner (Medina et al., 2002). Finally, recent

archival tagging of BFT has shown a large spectrum of migration patterns (including

western residency and trans-Atlantic movements, with or without annual visitation of a

known spawning site) and the possibility of new breeding areas (Block et al., 1998;

2001; Gunn and Block, 2001; Lutcavage et al., 1999; 2000), a set of observations that

do not support the “natal homing” hypothesis.

The alternative hypothesis to “natal homing” is that individuals track the optimal

environmental conditions that will maximise their total reproductive output (Balchen,

1979; Harden Jones, 1968; Potts and Wootton, 1984). This hypothesis of “opportunistic

spawning” is generally accepted for tropical tuna (Cayré, 1990). Their remarkable

potential of migration allow them to explore a large range of environmental conditions

and to select the most suitable ones for their physiological and biological needs.

However, unlike tropical tuna, temperate tuna feed and spawn in different areas

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