Life History and Abundance of Young Adult Lampetra ayresi in the Fraser River and Their Possible Impact on Salmon and Herring Stocks in the Strait of Georgia

Abstract

Metamorphosis in Lampetra ayresi begins in July but is not completed until approximately April of the following year when the oesophagus opens. The prolonged period of metamorphosis differs from that of other lamprey species and may have evolved in response to the pattern of discharge of the Fraser River. Prior to the opening of the oesophagus, some metamorphosing L. ayresi congregate just upstream of the salt water that moved into the river from the ocean. Soon after the oesophagus opens, lampreys are able to osmoregulate in salt water and enter the Strait of Georgia from May to July. Maximum numbers go to sea in early June, correlated with the maximum discharge from the Fraser River. Laboratory studies indicated that there was a relatively short period during which it was possible for the lampreys to go to sea. The mortality of L. ayresi held in fresh water in the laboratory greatly increased after mid-June, with few surviving beyond mid-July. These mortalities indicate that most L. ayresi must go to sea. In the laboratory a small number grew and reproduced normally in fresh water. A population estimate of the number of downstream migrants in 1979 indicated that L. ayresi were very abundant in the Fraser River. Three additional estimates of density and abundance were used to corroborate this observation. Even though L. ayresi feed for only about 10 wk in the Strait of Georgia, they are an important source of direct or indirect mortality to herring and young salmon. If L. ayresi was uncommon in British Columbia in the past, as previously described, then this present level of predation is an important new source of mortality to herring and salmon.