Solar zenith angles for biological research and an expected catch model for diel vertical migration patterns that affect stock size estimates for longfin inshore squid (Doryteuthis pealeii)

Solar zenith angles are useful in diel studies because they are directly related to potential solar irradiance at the point of sampling and can be calculated from location, date, and time of day. We used zenith angles to quantify diel vertical migration effects on both the probability of a positive tow and catch size for longfin squid (Doryteuthis pealeii) using two-stage generalized additive models (GAMs). Zenith angles were better than time of day for modeling diel effects on inshore longfin squid bottom trawl survey catches and were particularly suitable for data collected over large areas and extended time periods. Diel effects were size-specific in most cases. Our expected catch method can be used to account for diel effects when estimating swept-area stock size from research survey data. Differences in observed day–night catches and model results show the potential for bias in swept-area stock size estimates that ignore diel migration effects. Zenith angles may be useful in specifying prior distributions for survey catchability parameters in stock assessment models.