Larval fish samples became more alike in two California sectors in 2017-2019 than in 2014-2016 despite cooler spring water

We tested whether warmer spring water predicts lower spatial differences among fish larva samples. We used CalCOFI spring bongo tows in two separate sectors from 1978 to 2022. Station-pair offsets, repeat-site rules, season and effort controls reduced observation bias. Pre-2014 temperature slopes for spatial turnover had mixed signs across two sectors and two periods. All four 95% intervals included zero. In 2017-2019, mean pair-adjusted Sorensen dissimilarity was 0.035 lower in the north and 0.065 lower in the south than in 2014-2016, while regional spring water temperature was 0.62 C lower. The dissimilarity changes were lower than all 13 northern and 23 southern earlier complete six-year window changes. Those windows overlap and do not form an independent null test. Matched station pairs and equal-individual samples retained the direction. The equal-individual check used only six northern stations at ten larvae per tow. Turnover also fell in both sectors, but two earlier windows in each sector had larger falls. These descriptive results show spatial convergence after the warm event. They do not show that the heatwave caused it. Shared environmental forcing, detection, sorting, taxon labels, and short event windows limit inference. Air pollution was not tested.

CalCOFI · marine heatwave · spatial dissimilarity · turnover · observation bias · rarefaction · replication