Cape fur seals Arctocephalus pusillus reduce catches of pelagic shoaling fish (mainly anchovy Engraulis capensis) made by purse-seine vessels off Namibia and South Africa. Seals scatter shoals, eat the fish and damage nets. Following reports of fishermen in Namibian waters shooting seals, a series of harmless acoustic seal deterrent methods were developed and trialled, undertaken by the Society for the Prevention of Cruelty to Animals Seal Deterrent Committee of the University of Cape Town.
Firecrackers: Three firecracker types that exploded underwater were used to try and scare seals from seine nets (seals can simply escape encircling nets by sliding over the float-line). Trials (carried out from purse-seine vessels) with Thunderflash and Beluga firecrackers were carried out on 23 occasions in Namibian waters near Walvis Bay (22°57'S, 14°30'E) between 1970 and 1972. Firecrackers were thrown into the area enclosed by the net; responses of seals were recorded.
Seal Deterrent firecrackers trials were carried out in 1974 and 1978, close to a seal colony at Seal Island (34°08'S, 18°35'E), near Cape Town (South Africa).
Underwater playback of killer whale Orcinus orca vocalization recordings: This was tested on five occasions: twice near Cape Town in 1974 - a 10 min silent period followed by 10 min playback. The vessel was stationary; behaviour of seals was recorded.
A third set of trials was carried out from a purse-seine vessel when catches were being made. Playback (for 2-14 min) commenced when the net had been pursed and hauled to enclose an area of 10 m diameter, and seals within were feeding.
A fourth set of trials was carried out using two vocalization recordings played simultaneously (plus models of killer whale dorsal fins). A final test was made from a side-trawler; trials commenced when most of the net had been hauled aboard but with the cod-end (containing the catch) in the water.
Rifle shots and arc-discharge transducer: The effectiveness of rifle shots (0.303 bulletsfired into the water near seals or into the air over their heads) and of an arc-discharge transducer (producing an underwater compression and sound level similar to that of Seal Deterrent firecrackers and 0.303 bullets) in frightening seals were tested from a purse-seine vessel and side-trawler in March and May 1978.
Firecracker deterrents: Seals responded to Thunderflash and Beluga firecrackers by diving with most (70%; two sets of trials combined) leaving the net after the first explosion. However they soon returned, often within 4-5 min. Seal Deterrent was also ineffective. About 100 seals were in the water when two firecrackers were exploded 30 m and 100 m from them; they dived for 10s and then resumed their previous activities; they responded to the second explosion less promptly. None moved away.
Killer whale playback: Seals responded by diving and remaining submerged typically for 15-30 s, they then looked at the ship (where the sound source was located). This response was interpreted as alarm and then inquisitiveness. Some approached within 5 m of the underwater loudspeaker during playback. In one trial, about 1 min after playback commenced, seals began returning and feeding. In the fourth trial, the presence of killer whale dorsal fin models among them did not appear to affect behaviour. Results of the fifth trial were similar.
Rifle shots: Shots fired into the water close to two groups (15 and 7) of seals taking fish at the net caused them to dive and remain submerged for 15-20 s, after which they returned to fish. There was little reaction when shots were fired above three groups (15, 7 and 4) of seals.
Arc-discharge transducer: During none of eight observation periods did seals leave a purse-seine net when the transducer was discharged. However, during side-trawling an average of 82% of seals (12 observation periods) at the cod-end moved away upon discharge, a statistically significant reduction.
It is concluded that the deterrents trialled were ineffective in reducing disturbances by fur seals at purse-seine nets.
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