The aim of this project is to identify and quantify the physiological and oxidative stress associated with exposure to chronic hypercapnia in eels grown under intensive conditions in closed-cycle fish farms, and the effects this stress has on performance in culture. Hypercapnia occurs because carbon dioxide produced as a metabolic end product accumulates in the recirculating water. It is well-established that hypercapnia is a physiological stressor because it causes impaired blood oxygen transport and tissue acid-base imbalances, and there is evidence that it could lead to oxidative stress in eel flesh, with consequent destruction of nutritionally valuable polyunsaturated fatty acids and production of free-radicals. Stress compromises growth and hence the economic viability of the fish farm, and oxidative stress influences the nutritional value of the product. The project will provide guidelines for the degree of hypercapnia that does not influence the performance and nutritional value of eels grown under intensive closed-cycle conditions.
STATE OF PROGRESS
A significant amount of progress has been made during the second 12 months of the project. As described in the first annual report to the Commission, a decision was made to perform growth studies on small animals for shorter periods and then separately to adapt large animals to hypercapnia for physiological studies (the original work-plan foresaw maintaining small animals within the growth study until they reached a size suitable for physiological experimentation). Within the new approach, during the second 12 months, populations of small eels have been chronically exposed both to fixed levels (PWC02S: 0, 15±1, 30-+1 and 45±1 ~g) of hypercapnia and to fluctuating levels (PWC02S: 0, 15±10, 25±10 and 35±10 mmHg) of hypercapnia (Task 1), and their growth and mortality rates measured (Task 4). For reasons linked to internal restructuring at the La Casella facility where the eels are maintained, the growth study on small eels exposed to fluctuating levels of hypercapnia was initiated and completed immediately after the growth study on small eels exposed to fixed levels. Investigation of oxidative stress (Task 3) and feed acceptance (Task 4) was completed on the small eels chronically exposed to the fixed levels of hypercapnia. Tissues were collected from the small eels chronically exposed to fluctuating levels of hypercapnia and are presently being analysed to describe the presence of oxidative stress (Task 3).
Following completion of the growth studies, large eels were chronically exposed to fixed levels of hypercapnia (Task 1) and the physiological effects of this described (Task 2, analysis of results only partial at present). Furthermore, an additional series of experiments on the physiological effects of acute hypercapnia exposure were performed (within Task 2).
Although the sequence of our Tasks has been modified (for the reasons stated above) and hence some of the deliverables have not been achieved according to the timetable described in the Technical Annex, very significant progress has been made. The studies on the effects of chronic exposure to fixed hypercapnia have been almost entirely completed, with the exception of one subtask (4c) and some data analysis within subtasks of Task 2. These latter measurements and analyses will be completed by spring 1999. The results to date indicate that eels can compensate for the physiological effects of mild hypercapnia (PWC02 = 15±1 mmHg) without any negative effects on growth. More severe hypercapnia
(PwCO2's = 30+-1 or 45±1 mmHg) is tolerated but causes significant physiological stress and inhibition of growth. There appears to be little oxidative stress associated with chronic fixed levels of hypercapnia.
The effects of exposure to fluctuating hypercapnia on growth have also been investigated, and revealed similar trends to those seen with fixed hypercapnia.
In the initial portion of the third year of the project, the participants will focus on completing the data collection and analyses within Tasks 2 and 4 for eels exposed to fixed hypercapnia. The remaining portion of the year, up to Month 36, will focus on completion of Task 2 (Physiological effects of hypercapnia), Task 3 (Oxidative stress associated with hypercapnia) and Task 4 (Effects of hypercapnia on performance) on large eels chronically exposed to four fluctuating CO2 levels. These will be completed and the results reported at Month 36.
DESCRIPTION OF WORK
This project uses an automated computerised system to expose eels to a series of accurately controlled levels of chronic aquatic hypercapnia (Task 1). Experiments will then be performed to identify and quantify the physiological stress associated with this chronic exposure (Task 2), and to evaluate the oxidative stress in eel flesh through measures of lipid composition and various antioxidant parameters (Task 3). These analyses of physiological and oxidative stress will be correlated with specific indicators of performance such as growth and feed conversion efficiency (Task 4). These investigations will not only be performed on eels exposed to a constant degree of hypercapnia but also to diurnal cycles of fluctuating hypereapnia such as may result from feeding regimens. Close collaboration amongst Participants and effective integration of research activities is assured by the fact that almost all of the experimental work will be conducted at one site, "La Casella" Fluvial Hydrobiology Research Station (formerly Experimental Thermal Aquaculture Plant) run by ENEL-Ricerca in Northern Italy.
Funding SchemeCSC - Cost-sharing contracts
B15 2TT Birmingham