BEHAVIORAL-RESPONSE OF LARVAL ATLANTIC MENHADEN BREVOORTIA-TYRANNUS (LATROBE) AND SPOT LEIOSTOMUS-XANTHURUS (LACEPEDE) TO RATES OF SALINITYCHANGE

Larval Atlantic menhaden [Brevoortia tyrannus (Latrobe)] and spot [Lei ostomus xanthurus) (Lacepede)] are transported from the western Gulf S tream edge to bays and estuaries of the southeastern United States. Di rection of water flow on the continental shelf Varies with depth, allo wing the potential for enhancement of shoreward transport by behaviora l depth regulation, possibly in response to vertical salinity gradient s. Salinity profiles from the North and South Carolina continental she lf were analyzed to quantify the salinity gradients in the field. Beha vioral response to salinity was investigated by exposing laboratory sp awned and reared menhaden and spot larvae of different ages to rates o f salinity change in an experimental apparatus in the laboratory, At e ach rate of salinity change tested, five replicate runs were recorded each with groups of 30 to 50 larvae, Both species and age groups showe d a significant ascent response upon salinity increases, Rates of incr ease of 0.28 and 0.13 x 10(-1) ppt.min(-1) with absolute salinity incr eases of 0.8 and 0.3 ppt were thresholds for response by young (4-13 d ays post-hatching) and old (2-4 wk post hatching) menhaden larvae, res pectively. For spot, rates of increase of 1.15 and 0.85 x 10(-1) ppt.m in(-1) with absolute salinity increases of 1.4 and 2.0 ppt were thresh olds for response by young and old larvae, respectively. Response to d ecreasing salinity was absent in both species, thus decreasing salinit y with depth would not cause larvae to descend. In nature, salinity gr adients of sufficient magnitude for depth regulation, and possibly fac ilitating cross-shelf transport appear common for menhaden, but not fo r spot.