A fishing-ground shift concentrates the chub mackerel (Scomber japonicus) catch on small fish

The size of chub mackerel (Scomber japonicus) landed by the Korean large purse-seine fishery has raised concern as evidence of population-wide down-sizing. Using grade-resolved auction records (Busan, 2014–2025), within-grade length measurements, and block-level catch records, we show instead that the catch has been chronically dominated by small fish. There is no statistically significant trend in catch mean fork length (28.7 → 27.9 cm; p = 0.24) or in the proportion below the length at 50% maturity (Lm50 = 29.5 cm; 66 → 78%; p = 0.15); Grades 3–4 together made up 92–100% of the landed weight throughout. What changed is the composition within the small fish: Grade 4 rose from 77% to 97% of landed weight (p = 0.003), peaking in 2025. In the winter of 2025 the fishery concentrated abruptly in the southern East Sea (region SES; 83.8% of the catch, against 0–37% over the preceding decade) and fished much closer to home port (117 km, against 402 km in 2022). The SES catch is the smaller one (Grade 4 97.0% in SES versus 75.3% elsewhere). The shift coincided with an African export market for small mackerel (exports US$28 M → US$200 M) and a weakening won: Grade 4’s share of chub mackerel revenue rose from 47% to 89% without any gain in its price relative to the largest grade (0.16 → 0.15). Autumn warming in SES is a permissive background, but neither autumn indicator was extreme in the season preceding the shift, so warming alone does not account for 2025. We interpret the small landings as not requiring population-wide down-sizing, but as consistent with a substantial contribution from restructuring of the fishery. The 2025 intensification is consistent with a market-driven relocation among other possible drivers; these data cannot separate the fleet’s choice of ground from the fish’s availability there, nor exclude recruitment or reduced size-at-age. Small fish dominated even outside SES in 2025 (Grade 4 75%).