The impacts of increasing temperature and moderate hypoxia on the production characteristics, cardiac morphology and haematology of Atlantic Salmon (Salmo salar)

Abstract

Given the predicted effects of climate change on seawater temperatures and water oxygen levels (i.e., the frequency and severity of hypoxia), it is important to understand the impact of these two environmental challenges on salmon used in the Atlantic Canada aquaculture industry (i.e., of Saint John River origin). Thus, we exposed salmon to: 1) control conditions (12 °C, 100% air sat.); 2) an incremental temperature increase (1 °C per week from 12 °C to 20 °C) and then held the fish at 20 °C for an additional 3 weeks (Experiment #1); and 3) an incremental increase in temperature until mortalities reached ~ 30% (Experiment #2). Further, we conducted the two temperature challenges under both normoxic and moderately hypoxic (~ 70% air sat.) conditions. Our experiments show that these salmon can be held at temperatures up to 20 °C, and 20 °C plus moderate hypoxia, for periods of up to a month without any mortalities. However, exposure to the incremental temperature regime resulted in a ~ 10% increase in feed conversion ratio, and the addition of hypoxia decreased feed intake, and thus, growth (15–20% depending on metric). In Experiment #2, specific growth rate and feed intake dropped dramatically after 19 - 20 °C, and mortalities were ~ 15% at 22 °C and ~ 30% by 23 °C. Interestingly, hypoxia did not affect survival at these temperatures, and high temperature + hypoxia enhanced blood oxygen carrying capacity (by 15%) whereas high temperature alone increased relative ventricular mass (by 21%). This research suggests that further increases (i.e., beyond 18 - 20 °C) in coastal temperatures in Atlantic Canada may negatively impact salmon production, especially if combined with hypoxia. In addition, it highlights the need for a better understanding of how these two environmental stressors interact to affect salmon biology/physiology (especially as increases in sea-cage water temperatures are incremental world-wide, and poorly investigated).