6. Travel time model with individual covariates
6.1. Introduction

The biotic trait that I incorporate into the model is fish length. The lengths of all PIT tagged fish are measured at the time of release, and the effects of this covariate can be determined for single release groups. The abiotic factors I examine at the individual level are average river flow, dates of release, and river temperature. Since fish released at the same time encounter similar levels of these factors, series of releases from the same point are required to examine the effects of these covariates.

In the next section of this chapter, I develop the models that include individual covariates and statistical procedures to analyze them. In the following sections, I apply the technique to several data sets. I first apply the model with only the length covariate, since this covariate is commonly available and varies within single release groups. This model is applied to several releases of spring and fall chinook and steelhead. I then expand the model to incorporate date of release, river flow, and river temperature and apply this to a series of releases of fall chinook in the Snake River in the years 1991-1993 and sockeye released in the mid-Columbia in the years 1992 and 1993.