When is it too crowded?

Transcription

1 When is it too crowded? Effect of stocking density on the stress response and energy metabolism of European sea bass Dr. Ingrid Lupatsch, Centre for Sustainable Aquaculture, Swansea University, UK

2 High Stocking Density when is it stress? Increase in cortisol and glucose levels? Increase in overall energy demand? Decrease in growth rate? Decrease in feed efficiency?

3 Objectives Is there a reduction in feed intake at high stocking density? Is there an increase in metabolic energy costs under stressful conditions? Is there a reduction in the efficiency with which feed is converted into growth?

4 Experimental system European sea bass, Source France 12 tanks of 200L, recirculation system Artificial sea-water 33 ppt, 22 0 C Continuous oxygen measurements inflow - outflow Wageningen University, the Netherlands

5 Methodology: Growth trial (70 days) Fish of 72g stocked at two densities: Low density (LD) - 15 fish/tank ~ 5-10 kg/m 3 High density (HD) fish/tank ~ kg/m 3 Fish at each density fed manually at three feeding levels: maintenance, medium and to satiation. Energy and protein gain measured by comparative body composition.

6 Methodology: Acute Stress Test Blood samples collected from 10 or 7 fish from each tank at the end of trial. Additional 10 or 7 fish subjected to additional (acute) stressor, consisting of 1 hour confinement in submerged nets. Cortisol and glucose determined before and after acute stress test.

7 Results: Growth trial - Sea bass Feeding level Final Weight g Gain g/day/fish Feed intake % FCR Maintenance LD HD Medium LD HD Satiation LD A 1.11 a 1.44 HD A 1.05 a 1.43

8 Cortisol levels before and after acute stress Cortisol (ng / ml) A B Low density High density a b Control Stressed Control Stressed Control Stressed Maintenance A A a b Medium A B a a Satiation

9 Results : Glucose levels 20 Low density High density Glucose ( mol / ml) a b a a a a 0 Control Stressed Control Stressed Control Stressed Maintenance Medium Satiation

10 Results: Blood parameters Glucose No effect of stocking density upon basal values Response to acute stress depends upon feeding level Cortisol: Fish kept at higher density had increased basal values Response to acute stress depends upon feeding level

11 ? Due to increased cortisol levels: Is there an increase in the metabolic costs? Is there a reduction in the efficiency with which feed is converted into growth?

12 Results: Oxygen Demand Oxygen demand (mg / kg biomass / h) A B Maintenance Low density A B Medium High density A B Satiation

13 Results: Maintenance Requirement Efficiency of Energy Utilization Energy gain (kj / kg 0.80 / day) High density 43.2 kj/kg 0.80 Low density 50.9 kj / kg 0.80 Efficiency k = Digestible Energy fed (kj / kg 0.80 / day)

14 Results: Body composition Lipid content of fish (%) Low density High density Maintenance Medium Satiation

15 Results: Energy and Oxygen demand Oxygen demand Increased oxygen demand of fish kept at low density compared to fish kept at high density Energy demand and feed efficiency Increased energy demand for maintenance requirement of fish kept at low density. No difference in efficiency of energy utilization for growth above maintenance

16 Are the fish stressed? No reduction in feed intake, growth nor feed efficiency with increasing stocking density up to 60 kg/m 3. No increased metabolic costs (energy, oxygen) due to increased cortisol levels. Increased metabolic costs (energy, oxygen) in groups kept at low density due to Aggression - Fighting for dominance - Hierarchy Negative effects of any additional acute stressor are reduced in well fed fish.

17 Growth and feed efficiency of white grouper raised at increasing stocking densities

18 Method: Growth trial - Grouper Duplicate groups of white grouper of 5.2g were reared for 63 days at stocking densities of 20, 40, 60, 80 and 100 fish/ 200L tanks. Grouper were fed manually twice a day to apparent satiation and feed intake recorded Attention was paid that water quality parameters did not decline with increasing biomass.

19 Results: Growth trial - Grouper Number of fish per tank Feed intake % day -1 Final weight g fish -1 FCR feed fed/ gain 20 fish fish fish fish fish

20 Results: Growth trial - Grouper 1.0 Feed intake Weight gain 1.0 Feed intake (g fish -1 day -1 ) Weight gain (g fish -1 day -1 ) fish 40 fish 60 fish 80 fish 100 fish Number of grouper per tank 0.0

21 Conclusions Rearing fish at higher stocking densities results in improved feeding response, growth rate and feed efficiency, as long as optimal water quality parameters are maintained. Higher stocking densities prevent harmful interactions, fighting for dominance, increased aggression and thus lowers overall energy demand.