Most people will have seen captive dolphins being fed fish, but in hunting mode in the open ocean they have to operate very differently. Pacific bottlenose dolphins equipped with video cameras have now been observed capturing and eating live fish for the first time, by the US National Marine Mammal Foundation (NMMF).
The scientists were making use of six of the US Navy’s team of conscripted dolphins which, along with sea-lions, are trained to carry out military underwater tasks such as mine-clearing, force protection and recovery missions.
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The Navy has been deploying dolphins since the days of the Vietnam War but, according to the NMMF, the cetaceans “work in open water almost every day” so are free to swim away if they choose to do so. In fact almost all of them return to base to supplement their wild diet with Navy-issue frozen fish.
Although not wild, the scientists believe that the military dolphins’ open-ocean hunting behaviour remains instinctive and broadly similar to that of wholly wild dolphins. The video and audio equipment was angled from the dolphins’ backs or sides to establish what was going on with their mouths and eyes as they fed.
Six months’ worth of recordings were captured as the dolphins swam freely in San Diego Bay, California. One was seen catching 69 fish, all but five of them on or near the seabed, while another caught 40, again mostly at low level. Prey included bass, croakers, halibut, smelt and pipefish.
Two other dolphins were observed capturing 135 live fish in a seawater pool, while two others fed opportunistically, with one of them surprising the researchers by consuming eight venomous yellow-bellied sea snakes (Hydrophis platurus) with no apparent ill-effects.
Such a dietary item had not been noted before, and the scientists said it was possible that a purely wild dolphin might have been more conservative.
‘Terminal buzz & squeal’
Of most interest were the dolphins’ methods of tracking and consuming the fish. Echolocation was used to pinpoint fish at a distance, with the technique supplemented by vision as the dolphins homed in on their prey. Their increasing heartbeats were recorded as they exerted themselves in the chase.
In the initial stages of hunting, a dolphin would click every 20-50m, but as it approached its prey the click intervals shortened until they turned into a “terminal buzz and then a squeal” – consisting of bursts of clicks varying in duration, peak frequency and amplitude.
These squeals continued as the dolphin seized, manipulated and swallowed its prey. If the fish escaped the dolphin continued in pursuit, with the terminal buzz and squeal heard more often than sonar clicks.
The dolphin would track every move made by the fish and, if necessary, swim inverted to get a clearer view with its swivelling eyes.
On capturing a fish, the dolphin’s lips flared to reveal nearly all of its teeth, while the throat expanded outward. As fish entered the dolphin’s open mouth, still swimming in a bid to escape, the predator to manoeuvre it to the side of its mouth before sucking it straight down, moving its tongue out of the way and expanding its powerful throat muscles to create negative pressure.
The study was led by NMMF veterinarian Sam Ridgway, who died recently aged 86, and can be read in the journal PLOS ONE.
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