The hydrographic limit of the distribution of Nematoscelis megalops in the Northwestern
Atlantic Ocean is usually marked by the abrupt changes in water properties across the Gulf
Stream. There are, however, isolated but repeated occurrences of this species in the Sargasso
Sea. In our study, individuals in the Sargasso Sea were expatriates from the Slope Water which
had been transported to the collection site by Gulf Stream cold core rings with but two exceptions.
The exceptional cases can be indirectly linked to the presence of rings.
Expatriated populations do not persist. Extinction in a ring appears to take place in one or
two generations, and for N. megalops it is related to changes in hydrographic properties, and
in particular, the vertical temperature structure. Both in the Slope Water and in the ring 50%
or more of the population is found in a restricted temperature regime centered about 10°C. As
a ring ages, the preferred temperature regime and N. megalops along with it move deeper into
the water column. The physiological and biochemical data given by Boyd, Wiebe and Cox
(1978) combined with data given here indicate that withdrawal from the surface results in
progressive deterioration of the nutritional condition of the population, a cessation of growth,
a drastic reduction in the number of males relative to females, reproductive incapacitation,
and ultimate extinction. It is conceivable that a process similar to that occurring in rings is
responsible for the maintenance of the Gulf Stream as a hydrographic limit in the distribution
of N. megalops.