Hirotaka Ijima | Searcharxiv

arXiv2024

Tuna and billfish larval distributions in a warming ocean

Tuna and billfish are charismatic pelagic fishes attracting considerable scientific attention due to their ecophysiological and socioeconomic importance. However, the knowledge of their basin-wide spawning and larval habitats, especially in a warming ocean, is limited. This knowledge gap undermines effective fishery management by introducing recruitment uncertainty, which makes population dynamics unpredictable. We fill the gap with a parsimonious geostatistical species-distribution model trained on the largest available dataset on tuna and billfish larvae in the Pacific Ocean. The model reveals (i) the basin-wide seasonal larval distributions over the reference period 1960-85, (ii) the expected impact of ongoing ocean warming on these distributions, and (iii) the biogeochemical factors, such as pH, phosphate concentration, and sea-surface height, that shape the larval habitat. Our findings make a quantum leap in understanding the ecophysiology of tuna and billfish, providing valuable information for future conservation efforts.

physics.ao-ph↗

arXiv2022

Inferring the ecology of north-Pacific albacore tuna from catch-and-effort data

Catch-and-effort data are among the primary sources of information for assessing the status of terrestrial wildlife and fish. In fishery science, elaborate stock-assessment models are fitted to such data in order to estimate fish-population sizes and guide management decisions. Given the importance of catch-and-effort data, we scoured a comprehensive dataset pertaining to albacore tuna (Thunnus alalunga) in the north Pacific ocean for novel ecological information content about this commercially valuable species. Specifically, we used unsupervised learning based on finite mixture modelling to reveal that the north Pacific albacore-tuna stock can be divided into four pseudo-cohorts ranging in age from approximately 3 to 12 years old. We discovered that smaller size pseudo-cohorts inhabit relatively high -- subtropical to temperate -- latitudes, with hotspots off the coast of Japan. Larger size pseudo-cohorts inhabit lower -- tropical to subtropical -- latitudes, with hotspots in the western and central north Pacific. These results offer evidence that albacore tuna prefer different habitats depending on their size and age, and point to long-term migratory routes for the species that the current tagging technology is unlikely to capture in full. We discuss the implications of the results for data-driven modelling of albacore tuna in the north Pacific, as well as the management of the north Pacific albacore-tuna fishery.

q-bio.PE↗