Nutrient digestibility and growth in rainbow trout (Oncorhynchus mykiss) are impaired by short term exposure to moderate supersaturation in total gas pressure

Excess levels of dissolved nitrogen gas (N-2) may occur in recirculating aquaculture systems, as a result of aeration efforts, localized occurrences of denitrification, or from insufficient degassing of makeup water. If levels of dissolved N-2 are sufficiently high, or if oxygen (O-2) is also maintained at or above saturation, this leads to a supersaturation in total gas pressure (TGP). Depending on severity, total gas pressures above saturation may lead to gas bubble trauma, evident by visual inspection of the fish. Physiological effects of subclinical levels of TGP are not well known and have not been investigated for rainbow trout. The present study examined the effects of N-2 supersaturation, with or without simultaneous excess TGP. Supersaturation with N-2 (Delta P 22 mm Hg) without total gas supersaturation (Delta TGP - 6 mm Hg) did not have any significant effects on feed intake, feed conversion or growth. Short term (16 days) exposure to N-2 supersaturation (Delta P 36 mm Hg) in combination with a Delta TGP of 23 mm Hg did not affect feed intake, nor did it cause GBT or any apparent changes in behaviour. Excess TGP did, however, significantly reduce apparent lipid digestibility, feed conversion, and the thermal growth coefficient, compared to control treatments in which N-2 and O-2 were maintained below saturation levels. In addition to a significant decrease in available metabolizable energy (energy intake corrected for faecal loss), this group also had significantly higher cost of growth. These results suggest that even moderate TGP supersaturation negatively affect aquaculture production by a dual effect on energy uptake and energy expenditure, possibly caused by a general stress response to dissolved gases. Continuing the experiment over 25 days eliminated any significant differences on production variables, suggesting that rainbow trout exposed to moderate excess levels of TGP for longer periods were able to adapt to some degree. (C) 2013 Elsevier B.V. All rights reserved.