ABSTRACT The sustainability of global tuna fisheries exhibits regional divergence, with some stocks showing significant recovery while Indian Ocean bigeye tuna ( Thunnus obesus ) remains overfished. Particularly against the backdrop of intensifying global climate change, both large and small size classes of bigeye tuna are experiencing shifts in distribution and catch rate patterns influenced by trans‐basin climate events. This study focuses on the effects of trans‐basin climate change on the resource dynamics of bigeye tuna in the Indian Ocean, comprehensively considering the differing responses of large and small size classes to environmental changes. We systematically examined the causal relationship between trans‐basin climate events and tuna resource fluctuations by integrating fishery and climate data using the Convergent Cross‐Mapping (CCM) method. Concurrently, the Climate Events Synergy Model (CESM) was employed to analyze the interactions between these climate factors and the Indian Ocean Dipole (IOD) in shaping tuna habitat suitability. Results indicate that different trans‐basin climate events exert heterogeneous impacts on large and small size classes of bigeye tuna by altering water temperature, nutrient transport, and circulation patterns, and these events affect the resource dynamics in conjunction with the IOD. This study provides scientific evidence for elucidating the mechanisms underlying tuna resource fluctuations under multi‐scale climate drivers in the Indian Ocean, holding significant implications for formulating climate‐specific fisheries management strategies and addressing climate change challenges.
ABSTRACT Small pelagic fish are highly sensitive to environmental variability, yet sex‐disaggregated assessments of condition and reproductive investment remain rare in Mediterranean fisheries science and absent for the Southern Alboran Sea (SAS). Here, we report a decadal reversal in sex‐specific somatic condition in Sardina pilchardus and show that this reversal is mechanistically explained by the asymmetric reproductive costs imposed by warming‐induced changes in spawning phenology. Using 20,383 individuals sampled across two contrasting periods (1999–2003 and 2012–2023) and 10 environmental predictors, we applied generalized additive models (GAMs) to quantify environmental drivers of relative condition factor (Kn) and gonadosomatic index (GSI) and to project future biological states under SSP245 and SSP585 for 2050 and 2100. Females exceeded males in Kn during 1999–2003 but fell consistently below them by 2012–2023 (sex × period interaction: p < 0.001, ΔAIC = 228). Concurrently, the explanatory power of environmental drivers over Kn eroded substantially between periods (seasonality: −40%; SST: −39%), and this erosion was larger in females than males, constituting a sex‐differentiated form of ecological non‐stationarity not previously quantified for this species or region. SST and seasonality dominated both indicators but with opposing effects: Kn increased monotonically with SST, whereas GSI declined sharply above 16°C–20°C, revealing a physiological trade‐off invisible to biomass‐based monitoring. NPP exhibited a negative threshold effect on GSI, consistent with a phenological mismatch between peak productivity and the thermal spawning window. Projections indicate Kn declining 18%–59% and male GSI falling up to 46% under SSP585 by 2100, whereas female GSI remains comparatively stable. These findings expose multi‐level hidden stress that standard stock assessments cannot detect and establish a formal case for sex‐disaggregated monitoring as a prerequisite for climate‐resilient management of small pelagic fisheries.
ABSTRACT Stock assessments provide key scientific input for managing fisheries. Age‐structure information is important for these assessments because it provides the foundation for estimating recruitment, which impacts estimates of future spawning biomass and other assessment parameters that inform management. For some species, like petrale sole ( Eopsetta jordani Lockington, 1878) along the US West Coast, the younger cohorts are poorly sampled, which leads to uncertainty in estimates of recent recruitment. We used recruitment deviations from the most recent petrale sole stock assessment and oceanographic indicators derived from the GLORYS oceanographic model to develop an environmental index of recruitment. We then tested the impact of including the index in the assessment using Stock Synthesis. Quadratic relationships for degree days during the larval stage and alongshore transport during the pelagic juvenile stage explained 66% of the variation in recruitment deviations for 1993–2018 (the training period). We then included the index predicting the 2019–2022 (forecast period) recruitment deviations in the assessment model and compared the output to the 2023 base model without this index. Including the index provided information to capture recent recruitment variability and improved near‐term forecasting, leading to more accurate estimates of recruitment, spawning biomass, and projected catch—assessment estimated quantities that inform management decisions.
ABSTRACT Oceanographic variability plays a critical role in shaping the migratory behavior of the Atlantic bluefin tuna ( Thunnus thynnus ). This study investigates the thermal preferences of individuals migrating through the Strait of Gibraltar and the Gulf of Cádiz during pre‐ and post‐spawning movements. Comparing data measured from electronic tags and satellite‐derived sea surface temperature (SST) fields, we identified distinct SST preferred ranges associated with each migratory phase: 17.4°C–19.3°C inbound to the Mediterranean and 18.4°C–22°C outbound to the Atlantic. Based on these preferences, we developed a SST adequacy index reflecting thermal suitability, which we applied to 13 years of high‐resolution SST satellite data (2010–2022). Spatial and temporal patterns of SST adequacy were then classified using Self‐Organizing Maps, revealing dynamic hotspots of thermal suitability along migratory corridors. Our findings highlight the nonrandom, environmentally modulated nature of ABFT migrations and offer a framework to advance towards climate informed fisheries assessment, a current request of the International Commission for the Conservation of Atlantic Tunas.
ABSTRACT Episodic high‐latitude occurrences of subarctic fish due to Arctic warming present significant challenges for managing high‐latitude ecosystems. In this study, we present four consecutive years (2021–2024) of integrated environmental DNA (eDNA) monitoring and larval surveys for walleye pollock ( Gadus chalcogrammus ) conducted aboard the Korean icebreaker IBR/V Araon in the Chukchi Sea (CS) and Central Arctic Ocean (CAO). Of the 342 eDNA samples from 180 stations (55°N–76°N, spanning the Bering Sea, CS, and CAO, including underway surface‐pump stations), walleye pollock eDNA was detected in 25 samples (7.3%), with annual detection rates ranging from 2.8% to 13.2%. In 2024, eDNA was detected at 75°58′N, representing the northernmost molecular record for this species and extending beyond the previous northernmost trawl record (75.44°N). Larvae were collected in 2021 ( n = 5; standard length [SL]: 3.4–5.1 mm) and 2022 ( n = 3; SL: 9.7–14.6 mm) but were absent in 2023 and 2024 despite peak eDNA detection, indicating that northward pollock occurrence has not yet resulted in successful local reproduction. eDNA‐positive stations exhibited significantly warmer bottom temperatures than negative stations ( U = 2845, p = 0.0012; odds ratio [OR] = 1.74/°C [95% confidence interval (CI): 1.25–2.44]). Detection rates and the spatial distribution of positive stations differed markedly between years: In 2023, two stations yielded positive detections over a 1.2° latitudinal span, whereas in 2024, seven stations were positive spanning 10.8° (65°N–76°N), although interannual variation in detection rates was not statistically significant. Overall, these findings provide the first multi‐year molecular evidence that walleye pollock occur episodically at higher latitudes than previously documented, with detections spatially correlated with warmer subsurface bottom temperatures. This pattern is consistent with ongoing atlantification, which may promote the occurrence of pollock at higher latitudes. However, a causal relationship cannot be established because hydrodynamic transport data were not available.
