Daily and seasonal activity rhythms, swimming speed, and
modes of swimming were studied in a school of spring-spawned age-0 bluefish (Pomatomus saltatrix) for nine months in a 121-kL research aquarium. Temperature was lowered from 20° to 15°C, then returned to 20°C to match the seasonal cycle. The fish grew from a mean 198 mm to 320 mm (n= 67). Bluefish swam faster and in a more organized school during day (overall mean 47 cm/s) than at night (31 cm/s). Swimming speed declined in fall as temperature
declined and accelerated in spring in response to change in photoperiod.
Besides powered swimming, bluefish used a gliding-upswimming
mode, which has not been previously described for this species. To glide, a bluefish rolled onto its side, ceased
body and tail beating, and coasted diagonally downward. Bluefish glided in all months of the study, usually in the dark, and most intensely in winter. Energy savings while the fish is gliding and upswimming may be as much as 20% of the energy used in powered swimming. Additional savings
accrue from increased lift due to the hydrofoil created by the horizontal body orientation and slightly concave shape. Energy-saving swimming would be advantageous during
migration and overwintering
