Swimming depths of offshore migrating longfin eels Anguilla dieffenbachii

Pop-up tags were attached to 10 migratory female longfin eels Anguilla dieffenbachii, ranging in size from 6000 to 10 550 g. Tags were scheduled to ascend at monthly intervals, commencing 3 mo after release. A combination of eel mortality, tag failure, and premature detachment meant that limited data were retrieved. The 3 eels where tags remained attached for 25 d or more generally swam too deep for light reception and hence geolocation estimates, meaning that no tracks of their swimming routes could be determined; all 3 eels showed diel vertical movements with 1 fish always returning to near surface depths (1 to 10 m), while the other 2 tended to ascend to 150-200 m but frequently dived to >600 m; maximum recorded depth was 980 m. It was assumed that such extensive diel movement was in response to predator avoidance and also to thermoregulation as eels often spent time in 5 to 6degreesC water. The tag from one of these eels commenced transmissions at a location 700 km east of New Caledonia. This is the first evidence that this species moves to the tropics during spawning migrations