Night-to-day zooplankton volume ratios showed no clear change at 57 CalCOFI stations from 1978–1999 to 2000–2021

We tested whether the contrast between night and day zooplankton catches changed in the California Current. We used 7,704 bongo net tows at 57 fixed CalCOFI stations in 1978–2021. A log-volume model accounted for station, nominal cruise quarter, cruise, and tow depth. The adjusted night-to-day volume ratio was 1.577 in 1978–1999 and 1.583 in 2000–2021. The later ratio divided by the earlier ratio was 1.004. Its nominal 95% interval from 3,999 calendar-year-block resamples was 0.916 to 1.139. All eight planned checks also gave uncertain period changes. Added checks with an independent solar model and separate spatial-seasonal means by period gave the same conclusion. The data establish higher night catch volume in both periods. They do not establish a period change or prove a constant ratio. Volume is wet displacement volume, not carbon mass. The contrast can reflect both vertical movement and daytime net avoidance. It does not directly measure either process. The package preserves raw records, source hashes, all selections, model results, resampling draws, figures, and offline code.

CalCOFI · zooplankton · day and night catches · displacement volume · California Current · long-term survey · block bootstrap · net avoidance