Relationships between catch per unit effort, catchability, and abundance based on actual measurements of salmonids in a mountain stream

The relationships between catch per unit effort (CPUE), catchability, and abundance were examined in a mountain stream (Ara Stream, Japan) using actual measurement data from two sympatric salmonids, the white-spotted char Salvelinus leucomaenis and red-spotted masu salmon Oncorhynchus masou ishikawae. We counted the number of individuals of each species by underwater observations after effort-regulated baitfishing in 92 pools that were considered to be short-term closed systems. Catch per unit effort increased with increasing abundance for both species, but the relationships were different. For white-spotted char, a linear relationship (i.e., proportional increase) was a better fit than a power curve relationship, whereas for red-spotted masu salmon a power curve relationship was a better fit. The shape parameter (P) was estimated to be less than 1.0 for red-spotted masu salmon, suggesting that CPUE declines more slowly than abundance (i.e., hyperstability). The relationship between catchability and abundance also differed between the species. For white-spotted char, catchability was constant and independent of abundance, whereas red-spotted masu salmon catchability increased with decreasing abundance. This corresponds to the relationship between CPUE and abundance for each species. We conclude that the CPUE-abundance relationship differs between species, even when the same fishing method is used in the same habitat. The relationship may be fundamentally species specific, determined by the vulnerability of each species to fishing gear and fishing methods.