Abstract
The environmental factors influencing the abundance of fluvial larvae of the lamprey Lethenteron japonicum was studied in a tributary of the Ishikari River, central Hokkaido, Japan. We also conducted their food requirements by laboratory rearing experiment. The abundance of larvae and environmental parameters (Water temperature, Water depth, Water velocity, pH, Dissolved oxygen, Conductivity, Turbidity, Substrate hardness) were measured in one-meter-square quadrats. The larvae were captured using an electro-fisher and separated into two size classes (1-5 cm and 5-15 cm). The relationships between abundance and environmental factors were analyzed using generalized linear mixed models for each size class. The environmental factors that predicted the patterns of abundance of the two sizes of larvae differed for each size class. The abundance of larvae in the large size class was positively correlated with substrate hardness and dissolved organic matter (DOM), while that of the small size class was positively associated with substrate hardness, substrate composition (Silt), and current velocity. Sub-yearling larvae were also collected and reared for two months in the laboratory to observe their feeding preference by adding different foods. Their food uptake ratio was determined using stable isotope analysis. By the rearing experiment, the larvae which were given fallen leaves had significantly greater increases in wet mass and body length, as compared to the larvae which were given algae. The stable isotope analysis indicated that the larvae can take up nutrients from fallen leaves. We concluded that lamprey larvae change their habitat requirements with growth stage, and fine substrate containing fine organic matter is a significant microhabitat for fluvial larvae lamprey.
