Metabolic trade-off between locomotion and detoxification for maintenance of blood chemistry and growth parameters by rainbow trout ( Oncorhynchus mykiss) during chronic dietary exposure to copper

Abstract

The aim of this study was to obtain a holistic view of the sublethal toxic responses, and compensations, of fish exposed to Cu in the diet. Rainbow trout were fed to appetite on either a Cu-loaded diet (500 mg Cu kg −1 dw) or a control diet (11.9 mg Cu kg −1 dw) for 3 months. Nutritional performance, blood and muscle chemistry, histology, respiration, and swimming behaviour were measured. Copper concentration in intestine and liver of exposed fish was 16 and 2.5 times greater than in controls respectively, confirming oral Cu exposure. [Cu] remained low in the gills of fish irrespective of Cu treatment (<0.096 μmol g −1 dw). Hepatic and intestinal metallothionien levels increased 8- and 1.5-fold, respectively, in Cu-treated fish compared to controls. Growth rate, appetite, food conversion ratios, carcass composition, oxygen consumption and ventilation rate in exposed fish were not different from controls. There were no treatment related changes in serum chemistry (glucose, triglycerides, protein, Na, K, Hb) or the mineral composition of skeletal muscle (Ca, Mg, Zn, Na, or K). However, quantitative histology showed a 9% increase in gill secondary lamellae length, together with lipid depletion in the livers of copper treated fish compared to controls. Routine swimming activity was affected significantly by oral Cu exposure. Time spent swimming was 35% lower in Cu-exposed fish after 3 months, while total distance moved was reduced by only 21% because mean swimming speed was 12% higher in contaminated fish. Maximum speeds attained by exposed fish were not different compared to controls. Exposed fish lowered activity overall by decreasing time spent swimming at low (1–17 cm s −1) and medium speeds (17–40 cm s −1), whilst increasing proportionately time spent fast swimming (40–58 cm s −1). These results are discussed in terms of sub-lethal oral Cu affecting the partitioning of energy utilisation between locomotion and maintenance metabolism by trout in a physiological drive towards invariant growth rates.