Regional exploitation signals inferred from a data-limited per-recruit assessment of Trichiurus japonicus in Taiwanese waters

Abstract

Trichiurus japonicus is a heavily exploited coastal fishery species in the Northwest Pacific Ocean. Although Taiwan contributes less than 2% of regional landings, localized assessment is important because exploitation pressure reflects cumulative fishing mortality across jurisdictions and biological parameters may vary geographically. The lack of species-specific catch data and long-term abundance indices has prevented formal stock assessment in Taiwanese waters. To evaluate relative exploitation status under severe data constraints, agestructured and length-structured per-recruit analyses were conducted using length-frequency data collected from Taiwanese coastal fisheries between 2019 and 2023. Natural mortality was estimated using six empirical methods, and total mortality was estimated using seven catch-curve approaches, yielding forty-two fishing mortality scenarios. Length compositions were consistently dominated by small to medium individuals, and reconstructed age compositions were strongly truncated, with age-0 and age-1 fish comprising most of the catch. Current fishing mortality ranged from 0.749 to 1.483 year(-1) and exceeded conservative biological reference points, including F0.1 (0.203-0.731 year(-1)), in most scenarios. It also frequently approached or exceeded FMAX (0.385-1.887 year(-1)) and spawning potential ratio (SPR)-based thresholds. Current SPR ranged from 0.128 to 0.456 and was generally below the 40% target threshold and often near or below the 25% limit threshold. Although available data preclude a complete stock assessment, consistent signals across model structures and selectivity assumptions indicate elevated exploitation that reduces survival to older ages and limits reproductive output. These findings demonstrate that per-recruit analyses can provide useful indicators of relative exploitation under data-limited conditions and highlight the importance of incorporating localized biological indicators into future regional assessments of this transboundary resource.