Influence of larval sea lamprey density on transformer life history characteristics in Lewis Creek, Vermont

Density-dependent larval life history characteristics of sea lampreys Petromyzon marinus may have important management implications for sea lamprey control if life history attributes directly respond to conditions created by control strategies. Our objective was to examine the effects of larval sea lamprey density on sex ratio, size at metamorphosis, and age at metamorphosis in Lewis Creek, Vermont (one reach with naturally high larval density and one low-density reach), and to determine the effects of lampricide treatments on these characteristics. Lewis Creek has a natural falls that divides the stream into two distinct larval sea lamprey populations; this barrier created areas of high and low densities in the stream before treatment. Lewis Creek was treated with lampricide in 1990 and 1994, and all visible dead sea lampreys were collected, identified, and counted. Sea lamprey transformers (metamorphosing larvae) were measured for length and weight and sex was determined. Larval habitat was quantified above and below the falls in 1999 and 2000, and larval density was quantified by electroshocking below the falls. During the first treatment, larval density above the falls was 64 times greater than density below the falls. Transformers above the falls were similar in weight, were 11% longer, and had a significantly higher proportion of females (75% versus 53%) than those collected below the falls. Density of sea lampreys below the falls increased after treatment. Transformers decreased in average length and weight between 1990 and 1994 and increased in length and weight between 1999 and 2000. The percentage of females fluctuated from 65% after the first treatment (1994) to 44% after the second treatment (1999) to 52% a year later (2000). Overall, higher larval density (below versus above the falls, after versus before treatment) was associated with smaller transformers and lower proportions of females.