Comparative Fulton’s Condition and Relative Weight of American Brook Lamprey (Lethenteron appendix) Larvae and Adults in Streams in Southeastern Minnesota, USA

To reproduce successfully, non-parasitic brook lamprey must accumulate all nutrients needed for growth and reproductive development during a multi-year larval stage while feeding on low-quality detritus. We used total length and wet mass data of American brook lamprey (Lethenteron appendix) in 14 streams across four watersheds in southeastern Minnesota, USA, to examine Fulton’s condition factors (Fulton K = [g wet mass/mm TL3] × 106) of both lamprey ammocoetes (or larvae, n = 717) and spawning adults (n = 154) and developed preliminary standard mass equations for both life stages to allow for calculations of relative weights, a first attempt for any lamprey species. Condition factors and relative weights were most variable through the first year or two of the larval stage, with both condition factors and relative weights rising slightly through the remainder of the larval phase. Relative weights of most late-stage larvae ranged from 90 to 110% with condition factors at or slightly above 1.5. The standard mass equation for American brook lamprey larvae based on the top 25% heaviest individuals across the length range was: log10 wet mass (g) = 2.7078 log10 total length (mm)—5.115. Adult male American brook lampreys were slightly but not significantly longer than females at most of the sites examined, and condition factors and relative weights differed between the sexes only at one site. Overall, adult condition factors averaged 2.0, and relative weights averaged 86.9% and did not change significantly across the total length range (137 to 214 mm). The standard mass equation for American brook lamprey adults based on the top 25% healthiest or fittest individuals across the length range was: log10 wet mass (g) = 2.7411 log10 total length (mm)—5.047. American brook lamprey adults and ammocoetes approaching metamorphosis generally exhibited good condition factors and relative weights. Both adult and ammocoete condition factors and relative weights, along with adult lengths, differed significantly among streams. Length-wet mass data are needed from more populations of American brook lamprey across its range to build a more robust relative weight model for the species.