Three exact rules assign 36–73% of a northward center shift to catch density loss for 31 Bering Sea fish taxa, 1982–1991 to 2017–2026

We tested whether catch density gains or losses give the larger part of a northward fish center shift. We used open NOAA trawl data for 31 fish taxa at 271 fixed eastern Bering Sea stations. We compared 1982–1991 with 2017–2026; no survey occurred in 2020. Each taxon had equal weight. The mean biomass center shifted 27.6 km north (95% shared year-block interval 21.2–34.6 km). Under a symmetric exact calculation, gains gave 12.8 km and losses gave 14.9 km. Their difference was -2.1 km (interval -6.3 to 4.3 km). Neither part clearly dominated. The loss share was 54% (43–63%). Two additional exact rules gave loss shares of 36% and 73% from the same data. All three rules reproduce the full shift, so exact agreement does not select a causal explanation. Counts, longer periods, haul-performance filters and added northwest stations retained both net parts. A held-out-year sign check gave similar parts. Pooling all selected biomass before calculation gave a smaller shift of 8.2 km (interval -0.2 to 18.1 km). These results describe common, consistently recorded taxa in a fixed survey domain. Catch changes do not identify migration, births, deaths, colonization or extinction. The package preserves frozen source extracts, source hashes, code, all results, figures and repeat instructions. It matches 1,364 parent-study annual centers and passes separate CSV and exact-sum checks.

Bering Sea · fish distribution · catch density · centroid · spatial change · gain and loss · accounting choice · bottom trawl survey · reproducibility