Modulation of fish growth hormone levels by salinity, temperature, pollutants and aquaculture related stress: a review

The focus of this review is on the importance and regulation of fish growth hormone (GH), during exposure to stress. Alterations in environmental salinity impose osmoregulatory stress on fish and upon exposure to increased salinities GH has been shown to be important in maintaining hypoosmoregulatory function. Whilst studies mainly on salmonids, demonstrate that GH essentially performs a role as a seawater adapting hormone a clear correlation of elevated GH with growth and isoosmotic salinity exposure has been identified from studies on sparids. Variations in water temperature have been shown to modulate fish GH with the overall consensus of highest levels of GH during the warmer seasons of the year, suggesting an important role for GH during the temperature acclimatization process, but whether this relates to growth is unclear. Environmentally important pollutants, including xenoestrogens and heavy metals have been shown to affect GH mediated mechanisms, in fish, possibly via interference with the GH receptor and/or GH transcription, whereas aquacultural related stressors such as handling, confinement/overcrowding and nutritional stress have also been shown to affect GH levels. In addition the impact of aquacultural related stressors can also pre-dispose fish to disease leading to chronic suppression of GH. Finally, GH has been recently demonstrated to exert an anti-apoptotic effect in fish cells, when exposed to chemical stress, providing evidence that GH can also serve as a protective agent.