No need to fear the N.Z. lamprey

Forest and Bird, Volume 14, Issue 6, 1 May 1983, Page 44

No need to fear the N.Z. lamprey

By

Fisheries Research Division, Ministry of Agriculture and Fisheries, Christchurch

R. M. McDowall,

LAMPREYS are strange, very primitive, fish-like creatures, slimy, lacking jaws, with no paired fins, and looking snake- or eel-like. They are not true fish, but are related, most nearly, to the marine hagfishes, though even this relationship may be both very distant and ancient.

IN THE GREAT LAKES of North America in the 1940s, when the St Lawrence seaway opened access for sea-living fish, sea lampreys invaded the lakes and established landlocked populations. Numbers exploded and within a very short time the lampreys had attacked and almost eliminated the populations of lake trout or ‘‘mackinaw’’ (Salvelinus namaycush). They feed by attaching themselves to larger fish, rasping away the flesh and eventually killing their prey. A valuable and thriving commercial and recreational fishery for mackinaw (which grows to almost 50 kg) was destroyed. Not only did lampreys invade the lakes, but other marine species did also. And not only was the mackinaw population destroyed, but the entire ecology of the lakes suffered massive disruption.

New Zealand has a lamprey (Geotria australis), a species which we share with Australia, Tasmania, and South America. Occasionally there are fears expressed that New Zealand’s lamprey may have harmful effects on our trout populations. Fortunately it has so_ far shown no indication of adopting the destructive habits of its North American relative. I suppose it is natural to wonder whether the lamprey will attack man. There is nothing to suggest so. There is no evidence to indicate they feed at all or attack anything in fresh water. The few lam-

preys that are seen in rivers and streams are adults migrating from the sea to spawn. As with the North American sea lamprey, a part of the life of the New Zealand species-probably more than half-is spent in the sea. Resemble eels Lampreys leaving the sea are very slender, more so for their length than eels, which they superficially resemble. They are about 500-600 mm long and are very handsome — silvery on the belly, the back deep blue, with a pair of brilliant turquoise bands along each side of the midline. The mouth is a circular sucker armed with spiral rows of horny, orange teeth, and at the centre of the sucker is a pistonlike tongue, also armed with teeth. During its several years in the sea the lamprey has fed by attaching itself to marine fishes, rasping away the flesh and consuming the juices of its prey. Essentially it is a parasite. During winter and _ spring the adults begin to leave the sea and move up rivers and streams. They are sometimes seen by whitebaiters, but seem to move mostly at night. Occasionally accumulations of lampreys may be found below ovstructions such as falls and dams. They usually lie deep in the water, attached by the sucker to rocks on the river bed. As they migrate their once-beautiful colours fade to a drab, muddy, grey-brown, and the male develops a baggy

pouch beneath the head; what for, no one knows. They seem to move well up stream into small rocky tributaries in the bush, but how long it is before they spawn and when and where they do so is not known. They are probably in fresh water for many months. Tiny larvae Tiny larvae hatch from the eggs and may later be found inhabiting burrows in gently flowing, sandy or silty backwaters. They are about 11 mm when first found and over several years they grow to a length of about 100mm, feeding by filtering microorganisms from the _ water.

This larval stage, or ammocoete, is decidely worm-like in appearance, is a pinkish-brown in colour, and lacks eyes. During autumn, several years after hatching, a dramatic transformation takes place. The drab brown and eyeless ammocoete becomes a silver and blue macrophthalmia, so called because it has large eyes. The macrophthalmia is a miniature of the adult, in both form and colour, and it heads down stream to the sea to begin growth to adulthood in just the same way as its parents did-by parasitising marine fish. The lamprey once formed a significant food resource for

the Maoris, who used to construct elaborate weirs across rivers to catch the up-stream migrating adults. A lamprey weir, or utu-piharau, is still used on the upper Wanganui River by the Maori population to catch lampreys as food. Today lampreys seem little known and their migrations are largely unmolested by man. In a few places they are recognised and may be taken for food, especially in Southland. In a few rivers large runs are known and are harvested semi-commercially. Maoris and European immigrants compete for them as a soughtafter, though rather esoteric, delicacy. st