A sailfish is a fish of the genus Istiophorus of billfish living in colder areas of all the seas of the earth. They are predominantly blue to gray in colour and have a characteristic erectile dorsal fin known as a sail, which often stretches the entire length of the back. Another notable characteristic is the elongated bill, resembling that of the swordfish and other marlins. They are therefore described as billfish in sport-fishing circles.
Species
Two sailfish species have been recognized. No differences have been found in mtDNA, morphometrics or meristics between the two supposed species and most authorities now only recognized a single species, (Istiophorus platypterus), found in warmer oceans around the world. FishBase continues to recognize two species:
- Atlantic sailfish (Istiophorus albicans).
- Indo-Pacific sailfish (Istiophorus platypterus).
Description
Sailfish grow quickly, reaching 1.2â€"1.5 m (3 ft 11 inâ€"4 ft 11 in) in length in a single year, and feed on the surface or at middle depths on smaller pelagic forage fish and squid. Sailfish can supposedly reach very high swimming speeds of over 100 km/h (Lane 1941). Recent studies, however, do not support these claims and suggests that sailfish do not exceed swimming speeds of 36 km/h (22 mph). Generally, sailfish do not grow to more than 3 m (9.8 ft) in length and rarely weigh over 90 kg (200 lb). Sailfish have been reported to use their bill for hitting schooling fish by tapping (short-range movement) or slashing (horizontal large range movement) at them.
The sail is normally kept folded down when swimming and raised only when the sailfish attack their prey. It has been shown that the raised sail reduces sideways oscillations of the head which is likely to make the bill less detectable by prey fish. This strategy allows sailfish to put their bill close to fish schools or even into them without being noticed by the prey before hitting them.
Sailfish usually attack one at a time and the small teeth on their bills inflict injuries on their prey fish in terms of scale and tissue removal. On average about two prey fish are injured during a sailfish attack but only 24% of attacks result in capture. As a result injured fish increase in number over time in a fish school under attack. Given that more injured fish are easier to catch, sailfish benefit from the attacks of their conspecifics but only up to a particular group size. A mathematical model showed that that sailfish in groups of about to 70 individuals should gain benefits in this way. The underlying mechanism was termed proto-cooperation because it doesn’t require any spatial coordination of attacks and could be a pre-cursor to more complex forms of group-hunting.
The bill movement of sailfish during attacks on fish is usually either to the left or to the right side. Identification of individual sailfish based on the shape of their dorsal fin identified individual preferences for hitting to the right or left side. The strength of this side preference was positively correlated with capture success. It is believed that these side-preferences are a form of behavioural specialization that improves performance. There is, however, a possibility that sailfish with strong side preferences could become predictable to their prey because fish could learn after repeated interactions in which direction the predator will hit. Given that individuals with right and left-sided preferences are about equally frequent in sailfish populations, it is possible that living in groups offers a way out of this predictability. The larger the sailfish group the greater the possibility that individuals with right- and left-sided preferences are about equally frequent. Therefore prey fish should find it hard to predict in which direction the next attack will take place. Taken together these result suggest a potential novel benefit of group hunting which allows individual predators to specialize in their hunting strategy without becoming predictable to their prey.
The injuries that sailfish inflict on their prey appear to reduce their swimming speeds with injured fish being more frequently found in the back (compared with the front) of the school than non-injured ones. When a sardine school is approached by a sailfish, the sardines usually turn away and flee in the opposite direction. As a result the sailfish usually attacks sardine schools from behind putting at risk those fish that are the rear of the school because their reduced swimming speeds.
Timeline
References
- Schultz, Ken (2003) Ken Schultz's Field Guide to Saltwater Fish pp. 162â€"163, John Wiley & Sons. ISBN 9780471449959.
External links
- Images from National Geographic
