Instream size segregation and density of Geotria australis ammocoetes in two New Zealand streams

Geotria australis occurred almost to the upstream limit of detectable discharge in two New Zealand streams. The length, and consequently age, of larvae differed over the course of the stream and smallest animals - presumed to be age 0 - predominated upstream. Metamorphosing larvae (mean length 99.5 mm) typically occurred in downstream reaches, where flows were higher and substrate was coarser. We estimated that annual larval survival was 47% for age 0 to 1 and 77% for age 1 to 2. Differences in the relation between length and weight occurred both between years within a stream and between streams; thus, we assume growth is generally variable. The density of ammocoetes in New Zealand streams ranged from 0 to 104 m^?2 and was significantly related to substrate type and depth in a multiple linear regression model. Because of the predictable nature of lamprey distribution and abundance, we believe it should be possible to estimate the absolute abundance of ammocoetes in a catchment with only a limited assessment of substrate properties.