Daily patterns in the activities of swordfish, Xiphias gladius, observed by acoustic telemetry

Horizontal and vertical movements of swordfish were studied using acoustic telemetry. Five swordfish in the Pacific and one in the Atlantic were tagged with transmitters which provided information on location. depth. and surrounding water temperature. Two of the Pacific fish showed a clear daily cycle of movement between an inshore bank during the day and deep water offshore at night. All of the swordfish responded to light, swimming deep during the day and coming near the surface at night. In the Pacific depth during daylight appeared to be limited to about 100 m by the oxygen-minimum layer, but in well-oxygenated waters of the Atlantic, a midday depth of greater than 600 m was recorded and tbe fish appeared to follow an isolume. Depth of the Atlantic fish in daylight was related to changes in ligbt caused by variation in water transparency. The vertical movements were associated with temperature changes ofas much as 19" C within 2 hours. The implications of rapid vertical movements on buoyancy and swim bladder volume are discussed. Despite their high commercial value and their significance as one of the large predators of the open ocean, relatively little is known about the habits and behavior of the swordfish, Xiphias gladius. The information which is available comes chiefly from observations made at the sea surface by commercial fishermen and from a few scientific studies based on commercial captures. Swordfish are large, fast-swimming fish that offer little opportunity for direct observation; however, some aspects of their behavior can be readily examined by telemetry from attached sensors. X iphias gladius is found throughout tropical and temperate waters. Its appearance in higher latitudes usually occurs in warm-season aggre­ gations along the edge ofthe continental shelf and on offshore banks <Rich 1947; Wise and Davis 1973). The seasonal appearance of swordfish in both the western North Atlantic and Hawaiian fisheries (Uchiyama and Shomura 1974; Caddy3) coupled with the known distribution of their larvae and of adults with ripe gonads indicate a spawning migration to waters warmer than 23°­ 24° C (Arata 1954; Taning 1955; Gorbunova 1969; Markle 1974; Nishikawa and Ueyanagi 1974),