As the majority of marine organisms are water-breathing ectotherms, temperature and dissolved oxygen are key environmental variables that influence their fitness and geographic distribution. In line with the gill-oxygen limitation theory (GOLT), the maximum asymptotic body size of water-breathing ectotherms is limited by an insufficient amount of oxygen that is supplied to meet metabolic demand once a threshold of gill surface area to body weight ratio is surpassed. Here we employed generalised additive models (GAMs) to investigate the relative influence of temperature, regional variation in dissolved oxygen, and geographic location (that encompasses multiple latent variables) on the maximum body length of ten teleost fish species, as well to predict their maximum body length, across a large temperature, depth and latitudinal gradient throughout Norwegian waters. As dissolved oxygen levels were near saturation across the study area, we conclude that the predicted maximum lengths of our study species were not limited by oxygen availability. Conversely, the majority of study species display a clear relationship in that their predicted maximum length is either decreasing, asymptotic or parabolic across their observed temperature range. We also observed smaller maximum body lengths for multiple species within the coldest extent of their temperature range, which may be explained by increases in basal metabolism via cold denaturation. Overall, our results suggest that the maximum lengths of our study species are influenced by temperature, thus supporting the tenants of the GOLT.

See manuscript for methods. The marine organism length data employed in analyses were derived from the Norwegian Institute of Marine Research's long-term bottom trawl surveys between 1990 and 2020 (Djupevåg 2020). A pre-filtered version of this dataset can be found in the 'length_location' zip file. Djupevåg O (2020) IMR bottom trawl data 1980-2020. https://doi.org/10.21335/NMDC-328259372 The ocean temperature and dissolved oxygen concentration data used in analyses were extracted from the online database Bio-Oracle. Assis J, Tyberghein L, Bosch S, et al (2018) Bio-ORACLE v2.0: Extending marine data layers for bioclimatic modelling. Glob Ecol Biogeogr 27:277–284. https://doi.org/https://doi.org/10.1111/geb.12693