Water In Aquaculture Systems
To a Great Extent Water Quality Determines the
Success or Failure of a Fish Farming Operation
Water In Aquaculture Systems
Fish Perform All Their
Bodily Functions in Water
Eat
Breathe
Grow
Take In & Lose Salts
Reproduce
Pond Organisms
Can Alter Water Chemistry
1. Nitrogen Cycle – Controlled by Bacterioplankton
2. Oxygen Cycle - Photosynthesis Replenishes
Respiration Consumes
3. Water Chemistry Subject to:
Both Diurnal and Seasonal Changes
As Are Organisms
Review – Components of Water Quality
1. Temperature
2. pH
3. Hardness
4. Alkalinity
5. Nitrogenous Compounds
6. Oxygen
7. Light
8. Particulates & BOD
9. Salinity
10. Feed
11. Fertilizer
Water Quality Management
1. Large-Scale Solids Separation
2. Preliminary Considerations
3. Typical Pond Water Quality Data
4. Water Quality Sampling
5. Oxygen Management
6. Fertilization Policy
7. Water Exchange Policy
8. Calculating Water Exchange
Large-Scale Solids Separation
Sedimentation: Process Where Suspended Materials
are Separated by Gravity – Used by Aquaculture
Farms via Settling Ponds
Goal: Remove 90% of Suspended Solids
Design Parameters: Pond Cross-Sectional Area
Detention Time, Depth, Overflow Rate
Efficiency: Determined by Characteristics of the
Water, Flow Variations – etc
Avoiding Build-Up In Ponds
Solids – Originating Due to Events in the Pond
Sources: Unused Feed, Feces, Decaying Organisms
Method 1:
Method 2:
Method 3:
Method 4:
Adequate Inflow, Effluent Thru Gates
Adequate Longitudinal Slope
Channeling
Circulation – Reduce Dead Areas
Preliminary Considerations
Attempts to Improve Water Quality will Not Bear
Fruit Immediately
Pond Dynamics Change Only as Fast as You Can
Input Changes
You Must Remain Current Regarding Water Quality
Must Maintain Data Base on Pond Parameters
If Operating in Marine Environment, You Must
Maintain Current Tide Tables
Typical Pond Water Quality Data
for Temperature
Time Sampled: 05:30-06:00, 15:30-16:30
Frequency: At Temp Max/Min (2x Daily)
Locations: Ponds, Canal Sections, Water Source
Equipment: Thermometer, DO Meter, Multiprobes
Depth: Bottom, Midway, Surface at Deep End
Note: Pond Temps Vary Tremendously
Typical Pond Water Quality Data
for Salinity
Time: Anytime for Ponds, But be Consistent
Frequency: Daily, Estuary at High/Low Tide
Location: All Ponds, Canal Sections, Estuary
Equipment: Refractometer, Conductivity Meter
Depth: Surface (Ponds), Various (Estuary, Canal)
Note: High Tide – Higher Salinity, Low Tide –
Lower Salinity
Typical Pond Water Quality Data
for pH
Time: O2 Max/Min, 05:30-06:00 – 15:30/16:30
Frequency: 2x Daily – or If Color Changes in Pond
Location: All Ponds
Equipment: Any pH Meter w/ Compensating Probe
Depth: Surface, Middle, Bottom (Ponds)
Note: Should Increase in Afternoon, Decrease in
Morning, Used to Calculate Ammonia Toxicity
Typical Pond Water Quality Data
for Secchi Disc - Color
Time: Between 11:00 & 13:00 – be Consistent
Frequency: Daily
Location: All Ponds, Canals, Estuary
Equipment: Secchi disk, Turbidimeter, Color Wheel
Spectrophotometer
Depth: Secchi (NA) Turbidometer / Spectrophotometer
(Middle) – Color Wheel (Near Harvest Gate)
Notes: Secchi at 2 depths (disappearing – reappearing)
Secchi Correlated with Phytoplankton Counts – Color
Coding Standardized
Typical Pond Water Quality Data
for TAN – Total Ammonia Nitrogen
Time: 06:00 (low), 15:30/16:30 (High)
Frequency: Weekly
Location: All Ponds
Equipment: Hach Drel 2-3000, Any Spectrophotometer
Depth: Middle of Water Column
Note: Use Same Samples as pH, Undertaken to Confirm
Toxicity Potential of Fertilization with Urea or
Di-Ammonium Phosphate – Unnecessary for Ponds
Fertilized with Nitrates – Can be Used to Gauge Source
Water, New or Old Pond Water for N2 Fertilization
Typical Pond Water Quality Data
for Orthophosphate
Time: 06:00 (low), 15:30/16:30 (High)
Frequency: Weekly
Location: All Ponds
Equipment: Hach Drel 2-3000
Depth: Middle of Water Column
Note: Use Same Samples as pH, Undertaken to Confirm
Toxicity Potential of Fertilization with Urea or
Di-Ammonium Phosphate – Unnecessary for Ponds
Fertilized with Nitrates
General Comments: Water Quality Sampling
Key to Getting Relevant Data is Proper Timing &
Accuracy
Sample each Water Source at the Same Time for Each
Parameter – eg Temp Reading at Same Time Daily
To Compare Data from One Pond with Another –
Sampling Must Occur Within a Short Time Frame
This Requires Established Infrastructure (Equipment
Personnel, Transportation) to Allow This to Happen
Areas of Concern: Oxygen Policy
To Control Night-Time O2 Levels, Must Monitor O2
Trends over Several Days or a Week
Problems: Overfeeding, Algal Die-Offs, Cloudy
Days, No Wind
Identify: Increases & Decreases
Actions: Increase Water Exchange, Not Helpful if
Source Water has Low O2 Levels
Institute a Low O2 Policy Written and Followed by
Managers
Diel Oxygen Fluctuations
Typical Pattern with O2 Max
During Late Afternoon
Problem: Differs from Pond
to Pond & by Season
Vertical Mixing & Aeration
Can Also Impact
During Dry Season – Faster
Heating at Surface – Less
Variation
Example of a Low O2 Policy
3.0-2.5 ppm O2: No Late Night Feeding, Open
Inflow Gates 25%
2.5-2.0 ppm O2: No Night Feeding, Open Inflow
Gates 50%
2.0-1.5 ppm O2: Only Day Feeding, Open Inflow
Gates 75%
1.5-1.0 ppm O2: No Feeding, Open Inflow Gates
100%, Remove 1 Exit Gate Board
< 1.0 ppm O2: No Feeding, Open Inflow Gates
150%, Remove 2 Exit Gate Boards
Aeration
Most Aquatic Systems Employ Some Means of
Oxygenating Holding Tanks
Source Can be Purified O2 or Ambient Air
Depending Upon Needs
Typically This is a Three-Step Process:
Transfer of Gaseous O2 to Gas-Liquid Interface
Transfer Across the Gas-Liquid Interface
Transfer from Interface into the Liquid
Aerator Types
Three Basic Types:
Gravity, Surface, Diffuser
Aerators use Energy to Increase Liquid
Surface Area for Oxygen Transfer or
Ensures Water with low DO Contacts O2
Mixing Increases Surface Area and
Concentration Gradient Transfer
Aeration also Helps Keep Suspended
Particles in Suspension
Gravity Aerators
Use Energy Released when Water Loses
Altitude to Increase Air-Water Surface
Area, Increasing DO Concentration
Turbulent Motion of Streams – Waterfalls
Achieve This Effect
Used in Large Applications, Tanks at
Different Levels, Weirs Situated on
Ponds or Raceways
Gravity/Step/
Cascade
Aerators
Surface Aerators
Agitate Water Surface
Resulting in Larger O2
Transfer Rates
Example: Pump Spraying
Water into Air, Nozzle
Aerators
Surface Aerator
Transfer Rate Depends On
1. Depth of Submergence
2. Rotor Speed
3. Rotor Diameter
4. Power Input per Unit Area
5. Liquid Being Aerated
6. Liquid Tank Dimensions & Shape
7. Oxygen Concentration Gradient
8. Aerator Design
Diffuser Aerators
Inject Air or O2 into
Water as Bubbles
O2 Transfer by
Diffusion
Bubbles Rise in Water
Column
Smaller Diameter
Bubbles Better
Diffuser Determines
Bubble Size
Diffuser Aerators
Oxygen Transfer Depends on Steepness of
Concentration Gradient Between Bubble
and Surrounding Water, Percentage of O2
Saturation of Water Around Bubble
Retention Time of Bubbles Rising in Water
Column – Bubble Size, Gas Flow Rate
Large Scale Diffuser Aerator
Oxygen & Model Ecosystems I
Most Aquatic Organisms Require Oxygenated Environment
Some Submerged Aquatic Plants can Store O2 in Tissues
Bacteria Remove O2 from H2O Directly Through Cell Wall
Many Eukaryotes Have Specialized Blood Pigments for
Carrying Oxygen
Molds, Protozoans & Fungi can be Adapted to Anaerobic
Environments
Hence, Need for Aeration or Oxygenation in Aquatic
Systems
Oxygen & Model Ecosystems II
In Microcosms & Mesocosms – All Aspects of Environment
Simulated & Adequate Light for Photosynthesis Provided
to Generate Baseline O2 Levels
Aquarium Managers often Attempt to Mimic Conditions in
The Environment
Difficulties: Scaling & Ratio of Water Surface to Water
Volume Do Not Allow This
In Aquaria, Display Optimized—Volume Small & Biomass
Density High
Feeding Typically in Excess of Wild Equivalents
Oxygen & Model Ecosystems III
Hard to Simulate Adequate
O2 Environment by Aeration
Can be Achieved Using Liquid
or Bottled O2
This Approach Expensive &
Potentially Dangerous
Model Ecosystems Typically
Designed to Optimize
Photosynthesis
Areas of Concern: Pond Fertilization
Total Algal Count Should be ~ 300,000 cells/ml
If Less: Fertilization Required – If More:
Reduce Feed or Flush System to Lower Count
Secchi Disc Should be < 60 cm – If Less:
Reduce Feeding – Flush System – If More:
Fertilization Required
Typical Fertilization Rate: 13 lbs Nutrilake / acre
Plus 4.5 lbs Na3PO4 (TSP) / acre Response
Will Take 2-3 days
Areas of Concern – Water Exchange
There are Two Ways to Manage
Water Exchange:
1) Monitor Water Quality & Make
Changes as Parameters go Bad or
2) Follow Guideline Where Water
Exchange Increases with Biomass
versus
Method 1: Lag Time – Large-Scale
Cyclic Patterns (valleys & peaks)
Method 2: Less Lag Time – Smaller
Peaks & Valleys
Time
Water Exchange
Most Follow Guidelines for Water Exchange
Where No Exchange Occurs Until Biomass
Density is > 270 lbs / acre
Often a Small Volume Added Routinely to
Compensate for Evaporation and/or High
Salinity
Rate of Water Exchange Needs to Increase
with Biomass Density – This Requires
Population Sampling
Cross Section Typical Inflow Gate
CANAL
SIDE
TOP OF DIKE
ATTACHMENT SLOT
POND SIDE
↓
↙
CULVERT PIPE
←
PRIMARY
FILTER
FLASHBOARDS
FILTER SLOT
BAG FILTER
Water Exchange Rate Changes with Density
Daily
Water Exchange
as % of Total
Pond Volume
Biomass Density (Kg/m2)
Water Exchange
How does One Achieve Exchange?
Answer: Depends on How Inflow &
Exit Gates are Managed
Two Approaches:
Lower Flashboards in Exit Gates
Increasing Output – Decreasing Volume
Vary Overflow Rate
Increasing Input – Increasing Volume
Water Exchange
Evaluate Volume of Water Flowing Over
the Exit Gate
The Larger this Volume, the Greater the
Exchange Rate
Volume can Vary Substantially for
Various Reasons One Needs to Take
the Average
This Average Volume is the Flow Rate
Water Exchange Example
A Pond 8.0 hectares in Surface Area
That has a Water Depth of 1.25 m
Has a Volume of 100,000,000 L
At an Average Flow Rate of 60 L/sec
Over 24 hours the Total Volume of
Water Exchanged is 5,184,000 liters
The Water Exchange Rate is 5.18%
Problem: Flow meters are Expensive –
Need One to Obtain Reproducible Results
Water Exchange Example II
Question: What do You Do Without a Flow Meter?
Answer: Estimate Using Discharge Equation
Q = KLH1.5
Q: Discharge Volume L/sec – K: 1,838 a Unit Constant
L: Width of Outflow – H: Outflow Height (depth)
Imaginary Pond Q = (1,838) (1) (0.1)1.5 = 58.12 L/sec
Or: 5,021,568 L/day ~5% Water Exchange – Very Close
to Measured Volume of 5,184,000 or 5.18%
Calculated Volume 97% of Measured Value
Aquaculture Water Summary
Maintenance of Water Quality Depends On:
Knowledge of the Water Chemistry
Interpretation of the Water Chemistry
Proper Facility Design Including:
Adequate Water Supply
Suitable Quality of Source Water
Your Ability (Skill & Resources) to
Modify Conditions