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Species Synopses BROADBILL SWORDFISH (Xiphias gladius) Broadbill swordfish are a truly cosmopolitan species, found in tropical, temperate and sometimes cold waters of all oceans. Like blue and striped marlin, they are restricted to blue, oceanic water. They are rarely, if ever, found in waters less than 200m deep. There are three major areas of high catch-rates of swordfish in the Pacific, in the northwest, the east and the southwest. The area of high catch-rates in the southwest Pacific off eastern Australia appears to have expanded latitudinally since the early 1960s. This probably reflects changes in targeting by the Japanese longline fishery rather than any change in the abundance of fish. In 1960, high catch-rates were restricted to the area between 20°S and 25°S. By 1980, this area stretched from 15°S to 45°S. Very little work has been carried out on the stock structure of swordfish in the Pacific Ocean. Two major hypotheses have been proposed: first that three separate stocks may occur, based on the above-mentioned high catch areas, and second that there may be a single Pacific-wide stock. Broadbill swordfish have a much broader temperature tolerance than other billfish and have been recorded in waters ranging from 5°C to 27°C. Their horizontal and vertical distributions are correspondingly broader than other billfishes. They are usually found in waters with surface temperatures greater than 13°C, and at least in the northwest Pacific, have a preferred sea-surface temperature of 18°-22°C. Detailed observations of both vertical and horizontal movements of swordfish have been carried out in a series of acoustic tagging experiments by Frank Carey and his colleagues. Swordfish typically followed a daily cycle of movements in which fish were close to the surface at night but went to depths as great as 600m during the day. During these vertical movements fish can experience changes in water temperature of up to 19°C. When near submerged banks, such as the Georges Bank in the northwest Atlantic, fish would typically spend most of the day close to the bottom at 200-300m depth and move offshore and to the surface at night. In the open ocean, swordfish swim close to the surface at night and deep during the day. Based on distributions of larvae, spawning of swordfish in the western and central Pacific is widespread in waters of 24°C or more. Thus, spawning takes place in warmer waters than those that yield the highest catch-rates. In equatorial waters, spawning may take place year-round, but at higher latitudes it is restricted to spring-summer months when water temperatures are 24°C or higher. The highest concentrations of larval swordfish collected have been in the northwest Pacific and south-east of Papua New Guinea through the Solomons, Vanuatu and Fiji. On the east coast of Australia, larvae have been collected in oceanic waters off the Cairns-Lizard Island region. Swordfish with mature ovaries have been caught in the northern Coral Sea in October. It has been suggested that young swordfish remain in the tropical and subtropical areas where they were spawned for at least their first year and then migrate to higher latitudes (cooler waters) as their size increases. While there has been for many years a recreational fishery for swordfish off New Zealand, a consistent and productive game-fishery has yet to be established elsewhere in the southwest Pacific. Swordfish are usually fished in the vicinity of canyons on continental slopes or around seamounts. Fishing is usually carried out at night, with a chemical light-stick attached to the line near the bait. Recent trials fishing with short longlines in this way, above seamounts off New Caledonia, have proved highly productive. Age and growth rates of broadbill swordfish are poorly understood, particularly in the Pacific and Indian Oceans. After about 2 years of age, females grow faster than males, reach a larger maximum size (about 540kg) and may live longer than males. In the Atlantic, males mature at 2-3 years (about 20kg) and females at 4-5 years (around 75kg). web@aims.gov.au
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