Effects of suspended sediment on cortisol levels in wild and cultured strains of ayu Plecoglossus altivelis

It is well known that excessive suspended sediment (SS) negatively affects the fitness of fish by causing gill damage, lowered feeding efficiency, and physiological stress, all of which may impair growth, survival, and reproduction. Despite numerous researches on the effects of SS on fishes, few studies have examined its impact on stress hormone response. We conducted an experiment to test whether increased levels of SS elevate stress hormone levels in wild and low- and high-inbred hatchery strains of ayu Plecoglossus altivelis (Plecoglossidae) inhabiting clear rivers. We measured serum cortisol concentrations in fish exposed for 3 h to water with 200 mg/l of kaolin (SS treatment), fish exposed for 3 h to clear water (control treatment), and fish in stock tanks (initial control). Fish exposed to SS exhibited greater elevations in cortisol levels than control fish and initial controls. Control fish showed a small increase in cortisol levels compared with initial controls; this was probably because of the stress of handling and transfer to a novel environment. Stress response to SS in high-inbred strains was significantly lower than those in wild and low-inbred strains. These results suggest that SS is a significant environmental stressor that may increase the susceptibility of ayu to disease. Further, highly domesticated ayu may be maladaptive in nature because of lower reactivity of their stress axes as compared with those of wild or low-inbred strains.