Effects of seasonal change on activity rhythms and swimming behavior of age-O bluefish (Pomatomus saltatrix) and a description of gliding behavior.(Report)

Abstract--Daily and seasonal activity rhythms, swimming speed, and modes of swimming were studied in a school of spring-spawned age0 bluefish (Pomatomus saltatrix) for nine months in a 121-kL research aquarium. Temperature was lowered from 20[degrees] to 15[degrees]C, then returned to 20[degrees]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.

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Bluefish (Pomatomus saltatrix) are temperate-zone fish with seasonal cycles of activity that revolve around lengthy coastal migrations. Age-1 and older bluefish migrate northward from April through July along the continental shelf of the United States from Florida to as far north as Maine. They spawn in southern and middle-Atlantic waters. These bluefish are collected on the shelf in spring at a temperature range of 8-23[degrees]C, mainly 10-19[degrees]C (Shepherd and Packer, 2006). Spring-and summer-spawned cohorts of age0 bluefish arrive in coastal waters and estuaries beginning in June and remain there throughout summer at temperatures of 14-25[degrees]C (Nyman and Conover, 1988; Able et al., 2003). Age-0 fish reside in estuaries and coastal waters until late October, whereas older age classes remain until late November (Scharf et al., 2004; Shepherd and Packer, 2006). Bluefish migrate southward along the Atlantic continental shelf from along the beaches to well offshore. Temperatures on the shelf where they are captured in fall are 10-27[degrees]C, mainly 17-25[degrees]C (Shepherd and Packer, 2006). Adults and the age-0 spring-spawned cohort spend winter on the outer continental shelf and slope from Virginia south to Florida (Shepherd and Packer, 2006; Shepherd et al., 2006). These lengthy migrations involve risks and are energetically costly.

With increasing temperature, the rates of metabolic processes of ectotherms increase (the bioenergetic response) (Fry, 1971; Brown et al., 2004). Within the thermal tolerance ranges of most fishes, as temperature increases, activity, food consumption, and growth increase (Beamish, 1978). Bluefish have a higher rate of increase in metabolism with temperature than their main competitors in the temperate zone, weakfish (Cynoscion regalis) and striped bass (Morone saxatilis) (Hartman and Brandt, 1995). The bioenergetic response in bluefish was observed experimentally in a school of 550-650 mm adults held in a 121kL research aquarium under a summer photoperiod (Olla and Studholme, 1971). At 19.5[degrees]C, they swam at 40-60 cm/s. As the temperature increased to 30[degrees]C over a month, their swimming speed increased to 80-100 cm/s. The high food consumption rates of bluefish rival those of tropical species (Juanes and Conover, 1994; Buckel et al., 1995). Increased food consumption at higher temperatures is accompanied by increased growth rates in bluefish (Buckel et al., 1995; Hartman and Brant, 1995).

Diel activity cycles or rhythms in bluefish are known. Olla and Studholme (1972) examined effects of photoperiod upon the activity of six age-1+ bluefish in the aforementioned 121-kL aquarium. The mean swimming speed of the school increased after transition to daylight, peaked at midday, and slowed in afternoon. In darkness, the school was more dispersed, interfish distance increased, and speeds were more variable (Olla and Studholme, 1971, 1972). Less is known about seasonal rhythms in bluefish. Under a winter photoperiod, as water temperature decreased, swimming speed correspondingly decreased (Olla and Studholme, 1971), but a longer study of behavior over a seasonal cycle has not been undertaken.

Bluefish use sustained or powered swimming for daily activity and long distance travel. They propel themselves by flexing the rear part of the body and tail and use their other fins mainly for stability and maneuvering. Powered swimming can be used for long periods without fatigue, although it has energetic costs (Beamish, 1978). For prey capture, bluefish use burst swimming at speeds recorded at up to 800-1000 cm/s (Olla et al., 1970). A novel and unexpected locomotory behavior was witnessed by Studholme and others during an unpublished study of a school of juvenile bluefish in the 12- kL research aquarium in 1984-85. At night, individual fish rolled onto their sides and, with their bodies and fins held still and slightly curved, glided downward diagonally, and then ascended. This behavior is remarkable in teleost fishes with laterally compressed bodies, because these fish are assumed to swim in a vertically oriented manner.