Cooperative Fish and Wildlife Research Units Program: Montana Fishery
Education, Research and Technical Assistance for Managing Our Natural Resources

To estimate the age structure of rainbow trout and brown trout in the Missouri and Madison rivers:
Compile relevant data from Montana Fish, Wildlife and Parks (MFWP) database for rainbow trout and brown trout populations for the Madison and Missouri rivers.Age otoliths (provided by MFWP) for rainbow trout and brown trout from the Madison and Missouri rivers.
Sagittal otoliths will be provided by MFWP. Otoliths will be mounted in clear epoxy, and transversely sectioned about the nucleus using a low speed IsoMet saw (Quist et al. 2012). Cross sections (0.8–1.0 mm) will be affixed to microscope slides and polished using fine grit sandpaper until the otolith nucleus and annuli are clearly visible at 40× magnification. A length–frequency histogram will be constructed from the electrofishing samples to visualize population length structure. Somatic growth of rainbow trout and brown trout sampled will be described using the von Bertalanffy (VBF) growth model. Age-length keys will be used to assign ages to unaged fish. Age-length keys will be constructed using the FSA package in R (Ogle 2016; Ogle 2018; R Core Development Team 2021) and applied to the rainbow trout and brown trout in the Madison and Missouri rivers.