Behavioural response of larval Atlantic menhaden to different rates of temperature change

Larval Atlantic menhaden Brevoortia tyrannus, spawned off North Carolina (U.S.A.) during the winter, undergo cross-shelf transport from the western Gulf Stream edge to coastal bays and estuaries. Variation in water flow direction with depth provides larvae the opportunity to enhance shoreward transport, if they can regulate their depth behaviourally. Temperature, which normally decreases with depth on the continental shelf, is one possible cue for depth regulation. Laboratory-reared larval menhaden of two different ages were exposed to varying relative rates of temperature increases and decreases, which were presented from both above and below the larvae. Temperature decreases from below caused an ascent response in both young and old larvae, but neither responded to this cue from above. The minimum (threshold) relative rates of decrease for initiating ascents were similar (7.9 × 10^?2, 10.7 × 10^?2° C min^?1) for both age larvae as were the minimum absolute amounts of decrease that must occur before a response (0.1, 0.05°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 × 10^?2°C min^?1 from below and 14.79 × 10^?2° C min^?1 from above. The threshold rates and range of larval speeds during vertical movements were used to calculate vertical temperature gradients that could be perceived. These values were compared to measured gradients in areas inhabited by menhaden larvae. On the continental shelf, detectable temperature gradients appear common for temperature decreases that would occur upon descending and temperature increases upon ascending. However, it is uncommon for larvae to encounter temperature increases upon descending that would initiate an ascent response. These results support the hypothesis that menhaden larvae are capable of using temperature gradients for depth regulation.