Temperature and age effects on latitudinal growth dynamics of the commercially valuable gadoid Northeast Arctic saithe (Pollachius virens)

It is well known that marine fish dynamics relate with climate variability, and predicting future population developments have become increasingly urgent with on-going climate change. One approach in widespread populations is to study how individuals from different parts of the distribution area respond to climate, thereby testing for potential resilience in terms of local adaptions against environmental change. In this study, we collated extensive survey data on size-at-age for Northeast Arctic saithe (Pollachius virens) (1992–2013; 62–70 °N; ages 0–3 years) and detailed hydrographic information from three fixed, coastal stations along the Norwegian coast (at ≈63, 68 and 71 °N). Dedicated ad libitum feeding experiments were additionally performed to further strengthen the insight in saithe growth dynamics, also including adults (≥4 years). Likely temperature-mediated effects on whole body weight (W), total length, Fulton’s K, and specific growth rate (SGR) of wild saithe were explored using generalized linear models with age and region as fixed factors. A positive relationship with temperature was found, as evidenced by the before-mentioned body metrics of saithe age 2–3 years all becoming higher at positive local annual temperature anomalies, i.e. the deviation from the long-term mean temperature. The direct use of corresponding, absolute temperature instead of temperature anomalies gave comparable results. Hence, acclimatization to local temperatures per se was likely minor. Furthermore, examination of GLM interaction terms indicated that dissimilar feeding conditions in part of the study area, using K as a proxy, were not a confounding factor. Taken together, the present material provided an excellent opportunity to scrutinize temperature-mediated effects on growth dynamics across age classes, regions, and years. However, data sets pooled over years turned out to be more informative: SGR from age 0 to 1 year was clearly highest in the warmer, southern region but individuals in the colder, northern region gradually caught up and thereby showed higher W at age 3 years. This pattern mechanistically speaks for a gradual displacement of response curves for growth towards lower temperatures as saithe become larger or older. Although the present feeding experiments evidenced steady declines in SGR with W for younger individuals while this size effect faded away in the adults, the results also implied that saithe in the field show far from optimal growth performance, according to standard physiological principles. Thus, the term “optimal growth” should be treated with caution. Nevertheless, further studies are merited in this research area, potentially aiding body growth predictions and thereby management routines of this productive stock.