Gut phenotype is associated with superior growth performance in Atlantic salmon (<i>Salmo salar</i>) following exposure and recovery from warming and hypoxia

Atlantic salmon (Salmo salar) in Tasmania are cultured in a region recognised as a global hotspot for ocean warming, where summer water temperatures can at times exceed the species' optimal thermal range of 12–18 °C. These conditions will often coincide with intermittent hypoxia, further challenging the physiological capacities of cultured fish. Salmon in aquaculture respond to warming and hypoxia through a reduction or cessation of feeding, resulting in reduced growth, condition, product quality and sustainability. While the physiological effects of these stressors have been widely studied, the involvement of the gastrointestinal system to growth outcomes remains poorly understood. Here, we investigate the association between gut phenotype and growth performance in 83 Tasmanian Atlantic salmon following 86 days of experimentally induced summer conditions and a 38-day recovery period. As expected, feed intake declined from 13.0 ± 0.2 to 7.1 ± 0.1 g fish⁻¹ day⁻¹ during the transition from control conditions (∼15 °C, 100 % DO) to summer treatment conditions (∼19 °C, 80 % DO). Despite shared rearing conditions, we observed substantial inter-individual variation in growth performance. Indeed, only half of the population maintained positive specific growth rates (SGR > 0) during exposure and recovery. Using an in vitro gut sacing technique, measures of visceral fat and gross gut morphology (Zihlers index, relative gut length, relative gut mass), we determined that fish with relatively longer guts, and higher mid-intestinal transport capacity for Cl⁻, K⁻ and Na⁺ were associated to a higher SGR throughout the experimental period. No associations were found between growth, relative visceral fat, relative gut mass or the transport capacity of glucose in any region of the intestine. We discuss these results in the context of Na⁺ nutrient co-transport mechanisms and a possible trade-off between gut mass and gut length during periods of environmental stress and recovery. As fish were sampled after recovery, the observed traits likely reflect integrated responses to both stress and compensatory growth. These results point to a role for the gastrointestinal system in supporting resilience and provide a basis for future research with relevance to climate impacted aquaculture systems.<p></p>