Table 1: Explanatory factors initially included in the linear model. Use of continuous and discrete (category) variables follows suggestions of [14], where category variables are categorised in as few as possible meaningful categories, and ordinal variables are assumed to be continuous. Logistic variables are categorised as discrete with levels.

Factor

Variable type

Notes

Date

Continuous

From day 1 to day 158

Boat

Category ()

Differences in height of sighting platform; possible differences in acoustics of boat engines