EFFECTS OF HABITAT CHANGE IN THE ST-MARYS RIVER AND NORTHERN LAKE-HURON ON SEA LAMPREY (PETROMYZON-MARINUS) POPULATIONS

The abundance of parasitic and spawning-phase sea lamprey (Petromyzon marinus) in northern Lake Huron increased by more than fourfold from 1 978 to 1985, remaining high through 1993. We determined whether (i) in stream rehabilitation in the St. Marys River, (ii) improvements in lar val habitat quality in the St. Marys River, or (iii) increased forage fish biomass and salmonid stocking in northern Lake Huron could accoun t for this population increase. We found no significant changes in lar val sea lamprey abundance in the St. Marys River from 1971 to 1992. Fu rthermore, habitat rehabilitation in the St. Marys River occurred 3-5 yr following the increases in adult sea lamprey. Therefore, improvemen ts in spawning and larval habitat quality and quantity could not expla in the increase in parasitic-phase sea lamprey. However, bloater chub (Coregonus hoyi), one of the initial targets for recently transformed sea lamprey, increased in biomass between 1978 and 1982. We observed s ignificant, positive correlations between parasitic-phase sea lamprey abundance and bloater chub biomass and lake trout (Salvelinus namaycus h) and chinook salmon (Oncorhynchus tshawytscha) stocking rates. We co nclude that the increase in parasitic-phase sea lamprey can be attribu ted to improved survival of recently transformed sea lamprey resulting from greater prey availability.