How Do Sperm Whales Catch Squids?

Abstract

Vision may play a central role in sperm whale predation. Two complementary hypotheses regarding the detection and capture of prey items are presented, based on a review of mesopelagic ecology. The first hypothesis postulates that sperm whales locate their prey visually, either silhouetted against the midwater "sky," or by searching for bioluminescence produced by the movements of their prey. The second hypothesis postulates that sperm whales create a zone of stimulated bioluminescence around the mouth, which attracts squids and other visual predators. Studies of midwater fishes and invertebrates document the importance of vision in mesopelagic communities. If sperm whales search for silhouetted prey, they should be oriented upside-down to improve visual coverage and to facilitate the transition from search to prey capture. Prey capture events should be marked by excursions toward the surface. If they lure their prey, they should swim at a steady pace, with little rapid acceleration, and spend most of their time foraging at depths with the greatest potential for stimulated bioluminescence.Key words: Pbyseter macrorepbalm, foraging, vision, bioluminescence.The sperm whale, Pbyseter macrocepbalus, regularly dives to depths between 200 m and 1,000 m (Lockyer 1977, Watkins et al. 1993, where it feeds on a variety of squids and fishes (Clarke and MacLeod 1980, Martin and Clarke 1986, Rice 1989. In most parts of the world, squids constitute the major part of their diet (Clarke 1980, Kawakami 1980, Clarke et al. 1988). Clarke et al. (1993 examined specimens in the Azores, and estimated that only 23% of the diet consists of fast-swimming, larger cephalopods; the bulk of the diet consists of smaller, neutrally buoyant, luminescent species. They estimated that about 1,000 prey items are consumed per day. This implies that sperm whales locate and capture their prey efficiently. 42 FRISTRUP AND HARBISON: FORAGING BY PHYSETER 43In the 19th century it was hypothesized that sperm whales might passively lure their prey. Beale (1835) suggested that,". . . when the whale is inclined to feed, he descends a certain depth below the surface of the ocean, and there remains in as quiet a state as possible, opening his enormous mouth, until the lower jaw hangs down perpendicularly, or at right angles, with the body. The roof of his mouth, the tongue, and especially the teeth, being of a bright glistening white colour, must of course present a remarkable appearance, and which seems to be the incitement by which his prey are attracted; and when a sufficient number, I suppose, are within the mouth, he rapidly closes the jaw and swallows the contents." Gaskin (1967), believing that the white lining of the mouth could not attract squids at the great depths to which they dived, proposed that the luminous mucus from previous prey items might coat the mouth and serve to lure more prey. The problem with Gaskin's hypothesis is that living squids are not covered with luminous slime, and that the generalized bioluminescence he reported was d...

Clicking for calamari: toothed whales can echolocate squid Loligo pealeii

“…Echosounders are used routinely to survey for squid of commercial interest (Jefferts et al 1987, Goss et al 2001), but it has been debated whether toothed whales are able to detect squid (Fristrup & Harbison 2002) using echolocation clicks of shorter duration and broader bandwidth (Au 1993. We report the first TS measurements of a cephalopod using a toothed whale click and receiver model akin to that of the toothed whale auditory system.…”

“…Our finding that the TS E is the same for the 2 clicks shows, as expected, that a longer click duration and thereby an increase in energy content for the same peak sound pressure will give rise to a similar increase in the energy content of the returning echo, which is consistent with the energy hypothesis for longer echolocation clicks generated by some toothed whales. A critical parameter for understanding interactions between echolocating predators and their prey, and for testing the Fristrup & Harbison (2002) hypothesis that echolocation is not useful in hunting squid, is the range at which a toothed whale can detect its prey by echolocation. Little is known about the ranges at which toothed whales detect prey in the wild (Au et al 2004, but the extensive studies on trained delphinids in target detection experiments may offer a useful framework to estimate the range at which a squid of known TS can be detected.…”

“…Except for the chitinous beak, squid tissue has acoustic properties that are too similar to seawater to effectively reflect sound' (p. 44) and cite several authors for low target strength (TS) values of -55 dB for squid. On that basis, they conclude that sperm whales most likely do not use echolocation for location and capture of squid, and they hypothesize that sperm whales may instead use vision for prey location and capture (Fristrup & Harbison 2002). Thus, there is conflict between recent behavioral and acoustic data, strongly supporting the fact that echolocation is the primary means by which deep-diving toothed whales find their predominantly cephalopod prey, and the assertion by Fristrup & Harbison (2002) that squid have TS too weak to allow for effective biosonar detection.…”

“…On that basis, they conclude that sperm whales most likely do not use echolocation for location and capture of squid, and they hypothesize that sperm whales may instead use vision for prey location and capture (Fristrup & Harbison 2002). Thus, there is conflict between recent behavioral and acoustic data, strongly supporting the fact that echolocation is the primary means by which deep-diving toothed whales find their predominantly cephalopod prey, and the assertion by Fristrup & Harbison (2002) that squid have TS too weak to allow for effective biosonar detection. Man-made echosounders are increasingly used to survey squid populations (Jefferts et al 1987, Goss et al 2001) and even their gelatinous egg capsule masses on the sea floor (Foote et al 2006), and empirical measurements of TS in different squid species have been made using different ultrasonic echosounder pulse types (Arnaya et al 1989a,b, Benoit-Bird & Au 2001, Goss et al 2001, Kang et al 2005.…”

“…In so doing, we tested the 2 hypotheses of Fristrup & Harbison (2002): (1) that squid have very low target strengths and so are unsuitable for echolocation by toothed whales, and (2) that hard parts in the squid body dominate the acoustic backscatter. Applying the sonar equation (Fig.…”