Behavioral response of larval Atlantic menhaden Brevoortia tyrannus (Latrobe) and spot Leiostomus xanthurus (Lacépède) to rates of salinity change

Abstract

Larval Atlantic menhaden [ Brevoortia tyrannus (Latrobe)] and spot [ Leiostomus xanthurus) (Lacépède)] are transported from the western Gulf Stream edge to bays and estuaries of the southeastern United States. Direction of water flow on the continental shelf varies with depth, allowing the potential for enhancement of shoreward transport by behavioral depth regulation, possibly in response to vertical salinity gradients. Salinity profiles from the North and South Carolina continental shelf were analyzed to quantify the salinity gradients in the field. Behavioral response to salinity was investigated by exposing laboratory spawned and reared menhaden and spot larvae of different ages to rates of salinity change in an experimental apparatus in the laboratory. At each rate of salinity change tested, five replicate runs were recorded each with groups of 30 to 50 larvae. Both species and age groups showed a significant ascent response upon salinity increases. Rates of increase of 0.28 and 0.13 × 10 −1 ppt · min −1 with absolute salinity increases of 0.8 and 0.3 ppt were thresholds for response by young (4–13 days post-hatching) and old (2–4 wk post hatching) menhaden larvae, respectively. For spot, rates of increase of 1.15 and 0.85 × 10 −1 ppt · min −1 with absolute salinity increases of 1.4 and 2.0 ppt were thresholds for response by young and old larvae, respectively. Response to decreasing salinity was absent in both species, thus decreasing salinity with depth would not cause larvae to descend. In nature, salinity gradients of sufficient magnitude for depth regulation, and possibly facilitating cross-shelf transport appear common for menhaden, but not for spot.