BEHAVIORAL-RESPONSE OF LARVAL ATLANTIC MENHADEN TO DIFFERENT RATES OFTEMPERATURE-CHANGE

Larval Atlantic menhaden Brevoortia tyrannus, spawned off North Caroli na (U.S.A.) during the winter, undergo cross-shelf transport from the western Gulf Stream edge to coastal bays and estuaries. Variation in w ater flow direction with depth provides larvae the opportunity to enha nce shoreward transport, if they can regulate their depth behaviourall y. Temperature, which normally decreases with depth on the continental shelf, is one possible cue for depth regulation. Laboratory-reared la rval menhaden of two different ages were exposed to varying relative r ates of temperature increases and decreases, which were presented from both above and below the larvae. Temperature decreases from below cau sed an ascent response in both young and old larvae, but neither respo nded to this cue from above. The minimum (threshold) relative rates of decrease fdr initiating ascents were similar (7.9 x 10(-2), 10.7 x 10 (-2 degrees)C min(-1)) for both age larvae as were the minimum absolut e amounts of decrease that must occur before a response (0.1, 0.05 deg rees C). Young larvae did not respond to a temperature increase, while old larvae ascended regardless of whether the increase was presented from above or below. Threshold relative rates of increase were 8.59 x 10(-2 degrees)C min(-1) from below and 14.79 x 10(-2 degrees)C min(-1) from above. The threshold rates and range of larval speeds during ver tical movements were used to calculate vertical temperature gradients that could be perceived. These values were compared to measured gradie nts in areas inhabited by menhaden larvae. On the continental shelf, d etectable temperature gradients appear common for temperature decrease s that would occur upon descending and temperature increases upon asce nding. However, it is uncommon for larvae to encounter temperature inc reases upon descending that would initiate an ascent response. These r esults support the hypothesis that menhaden larvae are capable of usin g temperature gradients for depth regulation. (C) 1995 The Fisheries S ociety of the British Isles