Vertical migrations may control maturation in migrating female Anguilla dieffenbachii

The marine spawning area of the New Zealand longfin eel Anguilla dieffenbachii is unknown. The present study used 3 popup tags to obtain information on the offshore swimming behaviour and possible swimming directions and destination of migrating eels. All 3 eels showed very consistent behaviour and underwent daytime dives to depths of 600 to 900 m, where they experienced temperatures of 6 to 7 degrees C. These dives were followed by nighttime ascents to depths of 200 to 300 m. From the consistency of these dives, we propose that eels are using regular exposure to cool temperatures to regulate the rate of maturity. While swimming at depth could also reduce predation, the increased pressure would stimulate development of the ovaries, something mediated by ascent to shallower water. Comparisons of ascent locations and swimming speeds are consistent with spawning in the tropics, possibly in the South Fiji basin.