Physiological stress responses in spotted wolffish (Anarhichas minor) subjected to acute disturbance and progressive hypoxia

The spotted wolffish (Anarhichas minor) is considered a promising species for coldwater aquaculture. It is a sedentary, bottom-dwelling fish which exhibits a calm and “non-stressed” behaviour in captivity. There are, however, no reports on the physiological responses to stressors in this species. In the present study we investigated primary (cortisol secretion) and secondary (glucose mobilization) responses to common aquaculture stressors like disturbance and hypoxia. Pre-stress plasma cortisol levels were within those generally considered representative for unstressed fish (~10ng ml⁻¹), whereas basal glucose levels were unusually low (0.3-0.4mM). After exposure to a short, but severe disturbance challenge (emptying the tank of water for 10min), the increase in plasma cortisol level was slow and relatively weak, reaching a peak level of 25ng ml⁻¹ 4 to 8h after disturbance. When the fish were exposed to a gradually decreasing oxygen level in the tank, a significantly elevated plasma cortisol level (35ng ml⁻¹) was seen in the fish that remained in the tank until oxygen saturation had decreased to 20% oxygen saturation after 2.5h. However, a two-fold higher plasma cortisol level (~70ng ml⁻¹) was seen in all fish exposed to reduced oxygen levels (60, 40 and 20% oxygen saturation) after 3.5h recovery in normoxic water. Plasma glucose levels showed only moderate increases (~70%) following disturbance and hypoxia challenges. An in vivo injection of ACTH caused a strong elevation of plasma cortisol (peak level ~170ng ml⁻¹), demonstrating a high capacity for interrenal steroidogenesis in the spotted wolffish. The slow and relatively weak cortisol response to stressors, and low plasma glucose levels, may relate to the sedentary lifestyle of the spotted wolffish. The stress-response is characterised by a passive (reactive) coping style, which is considered adaptive for farming of this species.