Assessment of stress and growth of the eel "Anguilla anguilla" in a closed recirculating aquaculture system

Abstract

1) Closed recirculating intensive aquaculture potentially offers major advantages over existing technologies including reduction in normal production time, reduced water input and output and beneficial environmental effects. 2) The major aim of this study was to produce a basic scientific understanding of the factors that affect intensive recirculating culture of the European eel Anguilla anguilla in order to increase efficiency and economic viability of eel aquaculture in the E.U. 3) Unlike some intensively farmed fish such as salmonids little is known of the stress factors affecting optimal growth rates in intensive eel culture. The primary effects of stress are mediated by corticosteroids and catecholamines which may have profound effects on growth, appetite and ion and water balance. 4) Growth rates of the eel Anguilla anguilla were investigated in closed water recirculating systems utilising fresh water or saline water (12 ppt)at 23°C. Eels were initially graded into two similar populations consisting of three categories, small (12g), medium (24g) and large (48g) based on initial growth rates. 5) During a 300 day period the medium and large group's growth rates were significantly greater in 12 ppt saline water than in fresh water, although for the small fish group there was no such difference. Stocking densities were maintained at commercial levels of approximately 30-100 kg/m3. 6) Plasma cortisol concentrations increased throughout the growth period in both fresh and saline water, although there were no significant differences between the two groups during the experiment. Metabolic clearance rates of cortisol were however consistently higher in saline water fish. 7) Both groups showed an increase in plasma glucose concentration throughout the experiment. However there were no significant differences between fresh water and saline water fish for plasma concentrations of glucose, free fatty acids or lactate. 8) Eels held at stocking densities of 130 kg/m3 continued to grow in the saline water whereas the control fish in fresh water ceased growing. The results suggest that maintaining water salinity at 12 ppt in closed recirculating aquaculture systems produces increased growth rates and possibly increased efficiency of food conversion. 9) In response to acute grading stress, plasma osmolality and glucose concentrations were elevated in both fresh and salt water groups 20 minutes after grading but returned to pre-grading values within 90 minutes. Plasma cortisol concentrations were elevated after 20 and 40 minutes in saline water but returned to control values after 90 minutes. In fresh water fish, plasma cortisol concentrations were elevated after 20 minutes and remained elevated throughout the experiment. 10) Acute netting stress (tank transfer) resulted in a transient increase in plasma osmolality within 20 minutes after net transfer. Plasma cortisol concentrations were significantly elevated after 20 minutes in saline water but returned to control values after 60'minutes. In fresh water fish, plasma cortisol concentrations were elevated throughout the 90 minute period monitored after net transfer. 11) In both cases of acute stress (netting and grading), plasma catecholamines were elevated within a five minute period after the stressor was applied. This study has developed techniques to assess both long-term and short-term stress in eels and has optimised the environmental conditions leading to improved growth rates. Improvements in the performance of recirculating aquaculture for on-growing eels have been demonstrated and suggestions for future possible improvements as a way forward in commercial aquaculture have been suggested. These factors will, hopefully, lead to increased economic efficiency and increased profits in eel aquaculture within the E.U.