Compositional convergence of island demersal fish assemblages in the East Sea: a long-term trammel-net comparison between Dokdo and Ulleungdo

Marine fish assemblages are reorganizing under rapid ocean warming, yet long-term evidence for local compositional convergence between adjacent island systems remains scarce. Here we test whether neighboring island demersal fish assemblages in the East Sea have become more similar over two decades and whether this change is associated with a regional warming background. We analyzed shallow demersal fish assemblages around Dokdo and Ulleungdo using standardized, fishery-independent trammel-net surveys conducted at comparable bottom depths of approximately 50 m. Dokdo was monitored from 2006 to 2025, providing longer-term context, and Ulleungdo from 2013 to 2025; direct inter-island comparisons were restricted to the shared 2013–2025 period. Compositional change was quantified with Bray-Curtis dissimilarity, turnover-nestedness partitioning, ordination, and permutational analyses, with trends tested using rank correlation and autocorrelation-corrected Mann-Kendall tests. Both assemblages showed directional temporal reorganization. During the shared period, inter-island Bray-Curtis dissimilarity declined from 0.478 in 2013–2017 to 0.210 in 2021-2025, a decline driven by species turnover and robust to alternative dissimilarity metrics, raw biomass, dominant-species exclusion, leave-one-species-out tests, and temporal autocorrelation. Convergence coincided with a narrowing of the Ulleungdo-Dokdo gap in subtropical-affinity biomass, but species-level contributions showed that the pattern involved broad changes in taxa that formerly differentiated the two islands, not only warm-affinity species. Sea-surface temperature rose at similar rates around both islands and was a significant predictor of overall assemblage composition in generalized additive models, although direct causal attribution remained limited. Our results identify local biotic homogenization of adjacent island demersal fish assemblages under a regional warming background and highlight the value of standardized long-term surveys for detecting beta-diversity change in marine ecosystems.