John Fleng Steffensen' and Anders Drud Jordan

Aquaculture 2010 - San Diego - Physiological Insights Towards Improving Fish Culture.

Hypoxia is an increasing problem in coastal near areas and estuaries. Hypoxia can also be a problem in aquaculture systems with a high degree of recirculating water and fish kept at high densities.
Feeding is followed by an increase in the metabolic rate. termed the specific dynamic action (SDA). SDA integrates all the energetic expenditures involved in feeding. It is generally agreed that the increased metabolic rate is caused by biochemical transformation of food and protein synthesis leading to the deposition and turnover of tissue components.
Oxygen consumption of juvenile cod was measured with computerized intennittent respirometry at 10 0 C. A specially designed Plexiglas respirometer with a "chimney" in either end allowed feeding without disturbing and stressing the fi sh. The fish were fed single rations of fillets of herring, equivalent to 5 % wet body mass. Hypoxia was controlled by an oxygen regulator injecting compressed nitrogen in the water via a solenoid valve.
In addition the maximum metabolic rate of starved fish was measured in normoxia and 4 levels of hypoxia compared. Maximum metabolic rate compared to standard metabolic rate is termed the Scope for Activity.
The results showed that hypoxia at 6.3 kPa does influence the SDA. The effect on the SDA curve on cod fed 5 % of the wet body mass was that the peak metabolic rate was significantly depressed (44 %) and the peak appeared significantly later (29 h) compared to what was observed in normoxia at 19.8 kPa (10 h). In addition the SDA lasted 212 h in hypoxia compared to only 95 h in normoxia.
Comparing the peak metabolic rate during SDA with maximum metabolic rate showed that a food ration of 5 % body mass reduced the Scope for Activity by 55 % in nonnoxia. In hypoxia the effect
was more pronounced with a 69 % reduction of the scope for activity.
In conclusion hypoxia prolong the postabsorptive state of fi sh by limiting the peak metabolic rate, causing that less food is assimilated over a certain period, with reduced growth as a consequence.