Does Moon Phase Affect Indices of Kokanee Size and Abundance as Characterized by Gill‐Net Catch?

Abstract

Many pelagic fish species such as kokanee Oncorhynchus nerka undertake diel vertical migration in response to dynamic interactions between ambient light, foraging opportunity, and predation risk. Consequently, kokanee populations are almost universally sampled during the dark phase of the moon (i.e., the new moon), presumably to optimize capture efficiency. However, it is unclear if this sampling precaution is necessary to avoid bias in kokanee catch data related to the moon phase. We used experimental gill nets to sample kokanee populations in two thermally stratified reservoirs during three distinct moon phases (i.e., new, first quarter, and full) to understand the relative effects of moon phase and other ambient light variables on total catch and average size of captured kokanee. The total catch of kokanee differed significantly between populations but was not significantly affected by moon phase, secchi depth, or net depth. The average size of kokanee sampled from both populations increased significantly with moon illuminance and likely reflects behavior associated with predatorprey dynamics. Results from this case study suggest that the effect of moon phase and other ambient light variables on gill-net catch composition of kokanee is likely population-specific and is governed in part by population parameters such as abundance, growth rate, and size-structure. As such, investigators should be cognizant of-or perhaps standardize gill-net samples to-ambient light variables when indexing populations of kokanee and other pelagic fishes that undertake diel vertical migrations, especially when size indices are examined.