Whirling disease found only in S.I.

Press, 10 January 1983, Page 4

Whirling disease found only in S.I.

Wellington reporter

More than 8500 wild or hatchery-reared baby rainbow trout have been examined throughout New Zealand to determine the origin and extent of the parasite. Myxosoma cerebralis, which causes whirling disease in fish.

The provisional results of the study have been released by the Fisheries Research Division of the Ministry of Agriculture and Fisheries in Wellington. The parasite has been found in the Silverstream hatchery and at Peacock Springs, both north of Christchurch. It was also found in the Waitati hatchery just north of Dunedin, and in Post Office stream in West Otago. A partial positive test was returned from Longslip Creek in the Upper Waitaki Valley, but the disease was not found at all in the North Island. The diease was found in fish from the Kaiapoi River at Silverstream. but not in fish held in water from the

bore there. The parasite might have been present in the river for some time and have become apparent only when rainbow trout were raised in the hatchery in 1980. the first time for many years. Ten rainbow trout examined from Peacock Springs Wildlife Park in 1980 were found to be infected, but the examination of 104 fish in 1981 did not reveal the presense of the parasite. One wild fish from 88 collected at Longslip Creek had cartilage damage probably caused by whirling disease.

No evidence of whirling disease was found in the North Island. Indeed, during the survey, the disease was found only in rainbow trout in the South Island. Examination of quinnat and sockeye salmon at Silverstream. and brown trout at both Silverstream and Post Office stream, showed no evidence of the parasite or the disease.

This is particularly interesting, considering’ the

presence of the parasite in rainbow trout in those areas, according to a fisheries scientist, Mr Nelson Boustead. Overseas studies have showm other salmonids can be infected, but to a lesser degree than rainbow trout. The absence of spores in other salmonids in New Zealand may be caused by a lower intensity of the parasite in this country, he says. This is supported by the smaller number of spores recovered in fish examined, compared with reported numbers of spores recovered in fish from overseas studies.

The knowledge that whirling disease exists in New Zealand in some populations of wild rainbow trout makes it seem surprising that more positive identifications were not made, Mr Boustead says.

Identification of the areas where the disease exists can help those working in fisheries management avoid unwittingly spreading it. However, whirling disease may be spread by the natural movement of fish, or by birds, he says.