Sex Ratio, Fecundity, and Models Predicting Length at Sexual Maturity of Redband Trout in Idaho Desert Streams

Abstract

Factors affecting length at maturity and other important reproductive characteristics have not been investigated for Idaho redband trout Oncorhynchus mykiss gairdneri residing in desert streams. Prespawning redband trout were collected from nine streams, and estimates of length at sexual maturity, median age at maturity, sex ratio, and fecundity were developed along with models predicting length at maturity from physical stream conditions. Males first matured 1–2 years prior to females in seven of nine streams, while in the remaining two populations a few fish of both sexes first matured at the same age. Total length (TL) was strongly related (P < 0.0004; n = 49) to fecundity (F) via the curvilinear function F = 0.0002TL^2.5989. Redband trout residing in Idaho desert streams appeared to be less fecund than several other stream-dwelling populations of Pacific salmon Oncorhynchus spp. The best estimate of sex ratio for all fish, including immature fish, was 1:1. The best two-variable logit model for predicting length at maturity in males included stream order and conductivity, whereas the best model for females included drainage area above the sample site and stream gradient. These models can be used by fishery managers to estimate the total number of mature redband trout present in Idaho streams and, in combination with other data, can be used to approximate effective population sizes across southwestern Idaho desert stocks.