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Aquaculture Biology Knowledge Base

PhD-Level Reference: Growth, Water Chemistry, Feeding, Disease, Welfare & Species Data

Fish Growth Models & Equations
Dissolved Oxygen Biology
Nitrogen Cycle & Ammonia Toxicity
pH, CO2 & Water Buffering
Feeding Science & FCR
Temperature & Metabolism
Disease Triggers & Environmental Factors
Behavioral Stress Indicators
Stocking Density & Carrying Capacity
Harvest Timing & Economics
Recirculating Aquaculture Systems (RAS)
Species-Specific Biology
Biofloc Technology
Welfare Science & Cortisol
Algae Blooms & Eutrophication
Master Water Quality Parameter Table

1. Fish Growth Models & Equations

1.1 Specific Growth Rate (SGR)

The most widely used relative growth index in aquaculture.

Equation:

SGR = [ln(W2) - ln(W1)] / dt * 100

Units: % body weight per day (% day^-1)
Typical range: 0.9 - 5.0 % day^-1 (for cultured fish, mean weights 3.5-270 g)

Instantaneous form (IGR):

IGR = (1/W) * (dW/dt)

Key property: SGR decreases with body weight following a power law:

SGR ~ A * (mean body weight)^(-B),  where A, B > 0

1.2 Thermal-Unit Growth Coefficient (TGC)

Accounts for temperature accumulation, making it superior to SGR for comparing growth across temperature regimes.

Classical equation:

TGC = (W2^(1/3) - W1^(1/3)) / SUM(T)

where SUM(T) = sum of daily temperatures over the growth period.

Constant-temperature version:

TGC = (W2^(1/3) - W1^(1/3)) / (T * dt)

Units: g^(1/3) * (degree-C * day)^-1
Typical range: 0.1 - 3.2 g^(1/3) (degree-C * day)^-1

Instantaneous form:

ITGC = (T / W^(2/3)) * (dW/dt)

SGR-TGC relationship:

ITGC = T * W^(1/3) * IGR
TGC ~ (SGR / 100) * (W1^(1/3) / T)     [finite-interval approximation]

1.3 Von Bertalanffy Growth Function (VBGF)

The most widely used growth model in fisheries biology (5,000+ parameter sets for 1,300+ species in FishBase).

Length form:

L(t) = L_inf * [1 - exp(-K * (t - t0))]

Weight form (derived via W = a * L^b):

W(t) = W_inf * [1 - exp(-K * (t - t0))]^b

Parameters:

Parameter	Meaning	Typical Range
L_inf	Asymptotic length (theoretical max avg length)	Species-specific (cm)
K	Growth coefficient (rate of approach to L_inf)	0.05 - 2.0 year^-1
t0	Hypothetical age at zero length (modeling artifact)	Usually negative
b	Weight-length exponent	2.0 - 4.0 (isometric = 3.0)
a	Weight-length scaling coefficient	Species-specific

Critical note: L_inf is NOT the maximum length -- it is the asymptotic average. Some individuals exceed L_inf.

1.4 Weight-Length Relationship

W = a * L^b

Isometric growth: b = 3 (weight proportional to cube of length)
Positive allometric: b > 3 (fish gets relatively heavier as it grows)
Negative allometric: b < 3 (fish gets relatively lighter)
Typical b range: 2.0 - 4.0 (usually near 3.0)

Log-linear form for regression:

log(W) = log(a) + b * log(L)

1.5 Fulton's Condition Factor

K = (W / L^3) * 100     [when W in g, L in cm, scaling factor = 100]

Relative condition factor (for allometric growth):

K' = W / (a * L^b)

K' > 1: fish is in better condition than average
K' < 1: fish is in worse condition than average

1.6 Feed Conversion Ratio (FCR) as Growth Metric

FCR = Feed consumed (kg) / Weight gained (kg)

Lower FCR = more efficient conversion. See Section 5 for species-specific values.

2. Dissolved Oxygen Biology

2.1 Oxygen Solubility vs. Temperature

Temperature (deg C)	DO Saturation (mg/L) at 760 mmHg, 0 ppt salinity
0	14.60
5	12.77
10	11.29
15	10.08
20	9.08
25	8.26
30	7.54
35	6.95
40	6.41

Key principle: Warm water holds dramatically less oxygen. A pond at 30 deg C holds only 52% of the oxygen that a pond at 0 deg C can hold.

Depth effect at 20 deg C:

Surface (0 m): 9.08 mg/L
1.0 m: 9.98 mg/L
4.0 m: 12.67 mg/L

2.2 Critical DO Thresholds

Species Category	Optimal (mg/L)	Stress (mg/L)	Lethal (mg/L)
General recommendation	>= 5.0	2.0 - 4.0	< 2.0
Coldwater (salmonids)	>= 6.5	< 5.0	< 2.5 - 3.5
Warmwater (catfish)	>= 5.0	< 3.5	< 2.0
Tilapia	>= 5.0	< 3.0	1.0 - 2.0
Shrimp	>= 5.0	< 2.0	< 1.2

Saturation-based thresholds:

Coldwater species: minimum 60% saturation (6.48 mg/L at 15 deg C)
Warmwater species: minimum 50% saturation (4.13 mg/L at 25 deg C)

2.3 Oxygen Consumption Rates

Average adult fish: 200 - 500 mg O2 / kg / hour

Species-specific data:

Species / Size	Resting (mg O2/kg/hr)	After feeding	Active max
Channel catfish, 10 g	1,050	--	--
Channel catfish, 500 g	480	680	--
Channel catfish, 500 g (fasted overnight)	380	--	--
Southern catfish, 25 deg C	160	--	--
Southern catfish, 10 deg C	65	--	--
Salmon/trout (active)	--	--	~1,000 (+/- 200)
Shrimp	Similar to fish	--	--

Size effect: Larger fish consume LESS O2 per kg. 10g catfish use 2.2x more O2/kg than 500g catfish. Temperature effect: O2 consumption roughly doubles per 10 deg C increase (Q10 ~ 2). Feeding effect: Post-feeding O2 consumption is ~1.8x resting rate (specific dynamic action / SDA).

2.4 Oxygen Budget in Ponds

Sources:

Photosynthesis by phytoplankton (most important in ponds -- can produce 2-3x saturation in afternoon)
Wind/wave action (diffusion)
Artificial aeration

Sinks (example: shrimp pond, Shigueno 1975):

Water column (bacteria, organic matter decomposition): 69.4%
Bottom sediment: 14.8%
Target species (Penaeus japonicus): 8.6%
Fish (bycatch): 6.7%
Other shrimp: 0.5%

Critical principle: In most aquaculture ponds, the fish themselves consume only ~10% of total oxygen. Microbial decomposition in water and sediment dominates.

2.5 Oxygen Demand per kg Feed (RAS)

System Type	O2 consumed per kg feed
Efficient (non-submerged biofilter, fast solids removal)	~0.3 kg O2/kg feed
Moderate RAS	~0.5 kg O2/kg feed
High-load (submerged biofilter, retained solids)	~1.0 kg O2/kg feed

Feed Oxygen Demand equation:

FOD (kg O2/kg feed) = [(% C in feed/100) - FCE * (% C in fish/100)] * 2.67
                     + [(% N in feed/100) - FCE * (% N in fish/100)] * 4.57

where:

2.67 = 32/12 = stoichiometric O2 per C oxidized
4.57 = 64/14 = stoichiometric O2 per N oxidized (nitrification)
FCE = feed conversion efficiency

2.6 Diel Oxygen Cycle

In tropical ponds, DO follows an extreme daily pattern:

Dawn (6-7 AM): Minimum -- near zero possible (all night respiration, no photosynthesis)
Afternoon (2-4 PM): Maximum -- can reach 200-300% saturation (photosynthetic supersaturation)
Difference: Daily max can be 2-3x saturation level

Monitoring protocol: Measure at late afternoon (5-6 PM) and late evening (8-10 PM). Aerate from 10 PM through 7-8 AM.

3. Nitrogen Cycle & Ammonia Toxicity

3.1 The Nitrogen Cycle

Feed protein --> Fish excretion --> NH3/NH4+ (TAN)
                                       |
                                       v
                              Nitrosomonas/Nitrosospira (AOB)
                              [ammonia-oxidizing bacteria]
                                       |
                                       v
                                    NO2- (nitrite)
                                       |
                                       v
                              Nitrospira/Nitrobacter (NOB)
                              [nitrite-oxidizing bacteria]
                                       |
                                       v
                                    NO3- (nitrate)
                                    [relatively non-toxic]

Key facts:

Both AOB and NOB are autotrophic, aerobic bacteria
New biofilter requires 6-8 weeks to establish sufficient bacterial colonies
Nitrification is aerobic and produces CO2 + H+, reducing pH
Each kg of NH3-N oxidized requires 4.57 kg of molecular oxygen
Each gram of TAN oxidized consumes 7.14 g of alkalinity (as CaCO3)
Each gram of TAN oxidized produces ~0.17 g bacterial biomass

3.2 Ammonia Chemistry

Total Ammonia Nitrogen (TAN) = NH3 (unionized, toxic) + NH4+ (ionized, ~100x less toxic)

The critical equation:

Fraction NH3 = 1 / (1 + 10^(pKa - pH))

Temperature-dependent pKa:

pKa = 0.09018 + (2729.92 / T_kelvin)

where T_kelvin = temperature in Celsius + 273.15

Practical examples of NH3 fraction:

pH	Temperature	% NH3 of TAN
7.0	20 deg C	~0.4%
7.0	25 deg C	~0.6%
8.0	20 deg C	~4%
8.0	25 deg C	~6%
8.5	25 deg C	~15%
9.0	25 deg C	~36%
9.25	25 deg C	~50%

Key rule: Each 1.0 pH unit increase multiplies unionized NH3 by approximately 10x.

3.3 Ammonia Toxicity Thresholds

UIA-N Level (mg/L)	Effect
0.00 - 0.02	Safe zone
0.02 - 0.05	Chronic stress begins; sublethal effects possible
0.05 - 0.20	Tissue damage (gills, liver, kidney)
0.20 - 0.50	Severe damage; growth impairment
0.50 - 1.00	Acute toxicity; significant mortality risk
>= 2.0	Death for most species

Species variation: Coldwater fish (salmonids) are generally more sensitive than warmwater fish (catfish, tilapia).

3.4 Nitrite Toxicity

Toxic threshold: As low as 0.10 mg/L NO2-N for sensitive species
Safe level: < 0.25 mg/L NO2-N general recommendation
Mechanism: Nitrite oxidizes hemoglobin to methemoglobin ("brown blood disease"), reducing oxygen-carrying capacity
Chloride protection: Chloride ions competitively inhibit nitrite uptake at fish gills; maintain Cl:NO2 ratio > 6:1

3.5 Nitrate Levels

Traditional safe threshold: Up to 200 mg/L NO3-N
Recent research: May be more detrimental than previously believed
Accumulation risk: In closed RAS systems, nitrate can exceed 250 mg/L without water exchange
Management: Partial water exchange or denitrification reactors

3.6 TAN Production from Feed

TAN produced (kg/day) = Feed (kg/day) * Protein_fraction * 0.16 * 0.50 * 1.2

where:

0.16 = g nitrogen per g protein (N content of amino acids)
0.50 = fraction of feed nitrogen wasted (not assimilated)
1.2 = conversion factor g N to g TAN

Rule of thumb: ~4% of feed weight becomes TAN in the system (range: 3-5%).

4. pH, CO2 & Water Buffering

4.1 pH Ranges

Category	pH Range
Optimal for most freshwater fish	6.5 - 8.5
Optimal for Nile tilapia	6.0 - 8.0
Optimal for marine shrimp	7.5 - 8.5
Stress threshold (acid)	< 5.0
Stress threshold (alkaline)	> 10.0
Lethal (acid)	~4.0
Lethal (alkaline)	~11.0
Optimal for nitrifying bacteria	7.0 - 8.6

4.2 CO2 Levels

CO2 Concentration	Status
< 10 mg/L	Adequate for most aquaculture
10 - 20 mg/L	Monitor closely
> 20 mg/L	Requires intervention (aeration/degassing)

CO2-pH relationship: CO2 dissolution in water forms carbonic acid (H2CO3), which dissociates:

CO2 + H2O <--> H2CO3 <--> H+ + HCO3- <--> 2H+ + CO3(2-)

More CO2 = lower pH. This is why dawn pH is lowest (overnight respiration accumulates CO2) and afternoon pH is highest (photosynthesis removes CO2).

4.3 Alkalinity & Buffering

Alkalinity recommendations:

System Type	Minimum Alkalinity (mg CaCO3/L)
Freshwater ponds	40
Conventional marine ponds	70
Intensive biofloc/RAS	100 - 150
Optimal general range	50 - 300

Alkalinity consumption:

Nitrification consumes 7.14 g CaCO3 per g TAN-N oxidized
Practical rule: ~0.25 kg baking soda supplementation per kg feed
Liming rate for pH correction: 100-150 kg/ha/day hydrated lime (split doses, applied early morning)

Hardness:

Freshwater: typically < 100 mg CaCO3/L
Seawater (35 ppt): 6,000 - 7,000 mg CaCO3/L
Optimal calcium hardness to total alkalinity ratio: ~1:1
Minimum recommended: > 50 mg CaCO3/L

4.4 The Water Buffering System (WBS)

The carbonate equilibrium system buffers against pH swings:

Nighttime: CO3(2-) dissolves --> releases OH- --> buffers pH drop from respiration CO2
Daytime:   CO3(2-) precipitates as CaCO3/MgCO3 --> moderates pH rise from photosynthesis

The higher the total hardness (TH) and total alkalinity (TA), the stronger the buffering capacity. This is why low-alkalinity ponds experience dangerous pH swings (below 5 at dawn, above 10 at peak photosynthesis).

5. Feeding Science & FCR

5.1 FCR by Species

Species	Typical FCR	Notes
Atlantic salmon	1.0 - 1.2	Best-in-class among farmed fish
Rainbow trout	1.0 - 1.5
Nile tilapia	1.4 - 2.5	Edible (fillet) FCR ~4.6
Channel catfish	1.5 - 2.0
Common carp	1.5 - 2.5	Chinese carp edible FCR ~4.9
Pangasius	1.5 - 1.8
Shrimp (P. vannamei)	1.2 - 2.0
Sea bass	1.5 - 2.5

Key insight: Feed costs = 30-70% of total production costs. A 0.1 improvement in FCR can represent enormous economic gain.

5.2 Feeding Rates (% body weight / day)

By fish size (Common carp, 20-23 deg C):

Fish Weight (g)	Feeding Rate (% BW/day)
< 5	9%
5 - 20	7%
20 - 50	6%
50 - 100	5%
100 - 300	4%
300 - 1,000	3%

General rules:

Juveniles: 5 - 10% body weight / day
Grow-out: 2 - 5% body weight / day
Adults near harvest: 1 - 3% body weight / day
Tilapia: ~3% BW/day
Catfish: ~4-5% BW/day (season-dependent)

Each feed event should ideally be ~1% of body weight. So if feeding 5%/day, deliver 5 separate feedings.

5.3 Feeding Frequency

Species / Life Stage	Feeds per Day
Salmon/trout fry	20 - 24
Salmon/trout fingerlings	6 - 8
Salmon/trout juveniles	3 - 4
Salmon/trout adults	1 - 3
Channel catfish fry	8 - 10
Channel catfish 7.6 cm+	3
Channel catfish juvenile-adult	2
Tilapia fry	4 - 8
Tilapia fingerlings	4 - 5
Tilapia adults	2 - 3
Common carp (optimal at 40 g)	9
Shrimp (intensive)	4 - 6

5.4 Overfeeding Consequences

Uneaten feed decomposes --> ammonia spikes, oxygen depletion
Increased BOD (biological oxygen demand) in water column and sediment
Higher FCR (wasted feed counted, not converted to growth)
Digestive stress in fish --> reduced immune function
Eutrophication from excess nitrogen and phosphorus
Disease outbreaks -- bacterial pathogens thrive on organic matter from decomposing feed

5.5 Temperature Effects on Feeding

Channel catfish: 13-29 deg C = full feeding (6-7 days/week); outside this range = reduce to 4-5 days/week
Below species-specific lower threshold: feeding stops entirely
Korean rockfish optimal feeding rates: 3.41% at 16 deg C, 3.75% at 20 deg C, 3.34% at 24 deg C (peak near species optimum)

6. Temperature & Metabolism

6.1 Temperature Classification

Category	Optimal Range	Species Examples
Coldwater	10 - 18 deg C	Atlantic salmon, rainbow trout, brown trout
Coolwater	15 - 25 deg C	Walleye, perch, striped bass
Warmwater	25 - 32 deg C	Tilapia, catfish, carp, shrimp

6.2 Species-Specific Temperature Data

Atlantic Salmon (Salmo salar)

Parameter	Temperature
Egg incubation optimal	~10 deg C
First-feeding fry optimal	16 - 20 deg C
Parr optimal growth	18 - 19 deg C
Post-smolt optimal growth (70-150 g)	12.8 deg C
Post-smolt optimal growth (150-300 g)	14.0 deg C
Post-smolt optimal FCR	~11 - 13 deg C
Appetite decline onset	> 20 deg C
Upper stress threshold	> 19 deg C
Critical survival (parr/smolt)	30 - 33 deg C
Practical farming range	10 - 14 deg C

Nile Tilapia (Oreochromis niloticus)

Parameter	Temperature
Optimal growth	27 - 32 deg C
Best FCR observed	32 deg C (FCR 2.43)
Growth ceases	< 17 deg C
Juvenile mortality begins	< 17 deg C and > 35 deg C
Lower lethal (adults, wild)	11 - 12 deg C
Upper lethal (adults, wild)	42 deg C

Pacific White Shrimp (Penaeus vannamei)

Parameter	Temperature
Optimal growth (small, 1 g)	30 deg C
Optimal growth (large, 12-18 g)	27 deg C
General optimal range	23 - 30 deg C
Salinity range	0.5 - 45 ppt (optimal 10-15 ppt)

Channel Catfish

Parameter	Temperature
Optimal growth	25 - 30 deg C
Feeding stops	< 13 deg C
Survival range	near-freezing to ~32 deg C
Disease (Edwardsiella) virulence peak	22 - 28 deg C

6.3 Q10 Temperature Coefficient

The Q10 describes the factor by which a biological rate changes per 10 deg C temperature increase.

Equation:

Q10 = (R2 / R1) ^ (10 / (T2 - T1))

where R1, R2 = rates at temperatures T1, T2.

Typical Q10 values in fish:

Resting metabolic rate, warmwater fish: Q10 < 2
Resting metabolic rate, coldwater fish (salmonids): Q10 > 2
Shrimp growth: Q10 ~ 2 (e.g., growth increases from 1.20 to 1.44 g/week with 2 deg C rise from 27 to 29 deg C)
General oxygen consumption: Q10 ~ 2 (doubles per 10 deg C)

6.4 Metabolic Rate Equations

Standard metabolic rate (SMR): Minimum aerobic metabolic rate (measured by respirometry). Represents baseline energy cost for survival.

Active metabolic rate (AMR): Maximum sustained oxygen consumption during activity. Can be 5-10x SMR.

Aerobic scope: AMR - SMR. Represents the energy available for growth, feeding, reproduction, and stress response. Aerobic scope is maximized at the species' optimal temperature and declines toward thermal limits.

Body mass scaling:

SMR = a * W^b     where b ~ 0.7 - 0.8 (allometric scaling)

This is why larger fish have lower mass-specific metabolic rates.

7. Disease Triggers & Environmental Factors

7.1 The Disease Triangle

Disease outbreak = intersection of:

Susceptible host (stressed, immunocompromised fish)
Virulent pathogen (sufficient dose/concentration)
Conducive environment (poor water quality, temperature extremes)

All three must align. Good environmental management prevents outbreaks even when pathogens are present.

7.2 Environmental Triggers

Trigger	Mechanism	Examples
High temperature	Pathogen multiplication accelerates; fish immune suppression	Bacterial gill rot optimal 28-35 deg C; Hemorrhagic septicemia peaks > 27 deg C
Low DO	Stress-induced immunosuppression; fish crowding at surface	Below 3 mg/L triggers cortisol cascade
High ammonia	Gill damage creates pathogen entry points	UIA > 0.05 mg/L causes tissue damage
High stocking density	Pathogen transmission rate increases; chronic stress	Overwintering pond disease outbreaks
Sudden temperature change	Thermal shock suppresses immune function	Spring/fall transition disease peaks
Overfeeding	Organic matter fuels pathogen growth; water quality degradation	Enteritis in grass carp from overfeeding
Mechanical injury (handling)	Wounds allow pathogen entry	Saprolegniasis from netting injuries
Low pH	Increased ammonia toxicity, stress	< 5.0 causes acute stress
Decreased air pressure	Increases pathogen virulence	Enteritis outbreak trigger

7.3 Major Fish Diseases by Environmental Trigger

Temperature-Triggered Diseases

Disease	Pathogen	Optimal Temp	Season	Mortality
Hemorrhagic septicemia	Reovirus	> 27 deg C	June-Sept	Mass mortality in fingerlings
Enteritis	Aeromonas punctata	20-25 deg C	May-June, Aug-Sept	50-90%
Bacterial gill rot	Myxococcus piscicolus	28-35 deg C	Spring/summer	Variable
Ichthyophthiriasis ("white spot")	Ichthyophthirius multifilis	15-25 deg C	Winter/spring	Mass mortality
Dactylogyrosis	Dactylogyrus spp.	20-25 deg C	Late spring	Variable
Lernaesis (anchor worm)	Lernaea spp.	15-33 deg C	April-Oct	High in juveniles

Water Quality-Triggered Diseases

Condition	Resulting Disease Risk
Low DO + high density	Saprolegniasis (fungal)
High organic load	Bacterial gill rot
Poor circulation + shallow water	Trichodinasis (protozoan)
Post-handling wounds	Erythroderma, saprolegniasis
Continuous rain	Trichodinelliasis
Contaminated equipment	Secondary bacterial infections

7.4 Disease Prevention Protocol

Pond disinfection before stocking (quicklime 100 kg/mu or bleaching powder)
Fingerling disinfection (8 ppm copper sulphate bath, 20-30 min; or 10 ppm bleaching powder)
"Four Fix" feeding procedure (fixed time, place, quality, quantity)
Daily inspection especially mornings during epidemic season (May-September)
Feed disinfection (aquatic grasses: 6 ppm bleaching powder, 20-30 min)
Equipment disinfection (sunlight exposure 1-2 days, or chemical immersion)
Monthly pond treatment (bleaching powder 1 ppm, or quicklime 20-25 kg/mu)
Quarantine of new stock (strict prohibition on transporting diseased fish)
Rotation farming (prevents host-specific parasites from accumulating)

8. Behavioral Stress Indicators

8.1 Swimming Behavior Changes

Indicator	Stress Type	Species Documented
Reduced swimming speed	Hypoxia	Atlantic cod, white sturgeon
Reduced tail beat frequency	Hyperoxia	Atlantic salmon
Elevated swimming speed	Underfeeding	Multiple species
Erratic "tornado" or "hourglass" patterns	Acute stress	Atlantic salmon (cages)
Stereotypic circular/triangular loops (10-240 sec)	Chronic confinement	African catfish
Vertical swimming loops	High stocking density	Atlantic halibut
Sharper turns during feeding	Predictable ration frustration	Atlantic salmon
Reduced critical swimming speed (U_crit)	Disease, parasites, pollution	Multiple species

8.2 Feeding Behavior Changes

Indicator	Meaning
Increased latency to start feeding	Acute stress or illness
Reduced feeding rate and daily feeding times	Grading stress, handling
Reduced self-feeder activation	High stocking density
Strong anticipatory response (crowding near feeder before feeding)	Good welfare indicator
Loss of feed anticipatory activity	Post-stress state
Recovery of feeding behavior SLOWER than cortisol recovery	Important: behavioral recovery lags hormonal recovery

8.3 Ventilation Rate (Opercular Beat Frequency)

Response time: Seconds (fastest behavioral indicator)
Baseline: Acclimated unstressed fish have minimum ventilatory frequency
Hypoxia: Ventilation increases up to a critical low O2 level, then collapses
Hyperoxia: Ventilation decreases when O2 is in excess
Stressor-induced increases: Lighting changes, loud sound, unsuitable temperature, restricted movement, reduced retreat space, handling, air exposure, chemical presence, disease, ammonia/nitrate excess

8.4 Other Behavioral Indicators

Category	Observations
Aggression	Skin lesion count correlates with aggressive acts; dominance hierarchies form when resources are limited; subordinates show behavioral inhibition, color changes
Body color	Subordinates show color changes under social stress
Fin condition	Fixed feeding regimes increase fin damage vs. demand feeding
Vacuum behaviors	Tilapia: vacuum pit digging in substrate-free tanks
Surface rubbing	Ichthyophthiriasis: fish rub against objects or jump out of water
Solitary swimming	Enteritis: fish separates from group, slow solitary movement
Spatial distribution	Post-acute stress: fish concentrate near bottom of tanks/cages

8.5 Welfare Assessment Framework

Operational welfare indicators at individual level:

Skin condition (lesions, erosion, ulcers)
Gill health (color, mucus, parasite presence)
Fin condition (erosion score)
Eye condition (exophthalmia, cataracts)
Body condition factor

Operational welfare indicators at group level:

Feeding behavior (anticipatory response, intake rate)
Swimming patterns (speed, distribution, stereotypies)
Mortality rate (daily and cumulative)
Disease prevalence

9. Stocking Density & Carrying Capacity

9.1 Optimal Densities by Species

Species	System	Optimal Density (kg/m3)	Notes
Atlantic salmon (parr, 0-28 g)	RAS	1.76 - 14.55	Size-dependent
Atlantic salmon (juvenile, 7-98 g)	RAS	14.55 - 38.38	Size-dependent
Atlantic salmon (post-smolt)	RAS	< 30 (max recommended)	Higher causes stress
Rainbow trout	Raceways	10 - 40	Fin erosion above 40
Nile tilapia	Intensive tanks	Up to 136	With good water management
Channel catfish	Ponds	70 - 200+	Hybrid catfish can handle higher
Common carp	Concrete ponds	70 - 80
P. vannamei (shrimp)	Intensive ponds	70 - 150/m2 stocking	12-24 tonnes/ha production
P. vannamei (shrimp)	Super-intensive tanks	Up to 400/m2	Requires full RAS

9.2 Density Effects on Biology

Growth: Specific growth rate (SGR), final weight, and weight gain decrease significantly at high densities. Effect is body-mass dependent (different optimal densities for different life stages).

Water quality at high density:

DO depleted faster
Ammonia accumulates faster
pH swings become more extreme
Disease transmission increases

Behavioral effects:

Atlantic salmon: increased aggression OR polarized schooling (collision avoidance)
Rainbow trout: reduced self-feeding
Sea bass: decreased swimming speed

Health effects:

Reduced immune function
Higher cortisol levels
Increased fin erosion
Greater disease prevalence and mortality

9.3 Carrying Capacity Concept

Carrying capacity = maximum biomass a system can sustain without crisis. Determined by:

Oxygen supply rate (aeration + photosynthesis - microbial demand)
Ammonia removal rate (nitrification + water exchange)
CO2 removal rate (degassing + aeration)
Heat exchange capacity (temperature control)

In RAS: all components must be sized to handle the same feed loading. The weakest link determines carrying capacity.

10. Harvest Timing & Economics

10.1 Growth Curve Shape

Fish growth follows a sigmoid (S-shaped) curve:

Lag phase: Slow initial growth (small fish, developing digestive system)
Exponential phase: Rapid growth (juveniles, high feeding rate, optimal conditions)
Plateau phase: Slowing growth as fish approaches asymptotic size (increasing maintenance costs)

10.2 Optimal Harvest Principle

Marginal condition for profit maximization:

Marginal increase in value by delaying harvest
= Sum of (opportunity cost + mortality cost + feed cost + energy cost + maintenance cost)

Harvest when the cost of one more day of growth exceeds the additional revenue from that growth.

Key insight from economic models:

A 1% decline in interest rate can induce a 70% increase in optimal harvest weight
As interest rates decrease, optimal harvest weight and time increase in a stepwise, nonlinear fashion
Fish markets provide premiums for larger fish (often piecewise linear price-weight functions)

10.3 Production Cycle Duration

Species	Seed to Harvest	Market Size
Atlantic salmon	24-42 months (12-18 mo freshwater + 12-24 mo sea)	3-6 kg
Rainbow trout	12-18 months	250g - 3 kg
Nile tilapia	3-5 months (grow-out)	400g - 1 kg
Channel catfish	6-10 months	0.5 - 1.5 kg
P. vannamei shrimp	~90 days	18-25 g
Common carp	12-24 months	1-3 kg

10.4 Feed Cost Optimization

Total feed cost = Daily feed rate * Feed price * Culture duration
Marginal revenue from growth = (dW/dt) * Price per kg at harvest weight

Feed represents 30-70% of total operating costs. Therefore:

Optimizing feeding rate (not overfeeding) is the single most important cost lever
FCR improvement of 0.1 on a 1000-tonne operation saves ~100 tonnes of feed

11. Recirculating Aquaculture Systems (RAS)

11.1 Core Components

Every RAS must address five processes:

Circulation (pumps, pipe sizing)
Clarification (mechanical filtration -- drum filters, settling basins)
Biofiltration (nitrification -- MBBR, trickling filters, fluidized beds)
Aeration (oxygen supplementation)
CO2 stripping (degassing columns)

11.2 Water Exchange & Flow Rates

Parameter	Coldwater (salmonids)	Warmwater
Tank turnover rate	<= 30 min	~60 min
Water recirculation rate	90 - 99% of total volume retained	Same
Make-up water	1 - 10% per day	Same

Flow rate estimation (TAN-based):

Flow rate (m3/day) = TAN production (g/day) / (desired TAN conc * biofilter efficiency)

Typical desired tank TAN: 1.5 mg/L (can reach 3 mg/L)
Biofilter removal efficiency per pass: ~50%
Passive nitrification (tank walls, pipes): 20-30% of TAN conversion

11.3 Biofilter Design

TAN production estimation:

TAN produced (g/day) = Feed (g/day) * protein% * 0.16 * 0.50 * 1.2

Rule of thumb: ~4% of feed weight = TAN.

Volumetric TAN Conversion Rate (VTR):

Biofilter Type	VTR (g TAN/m3/day)	Notes
Trickling filter (200 m2/m3 SSA)	~90	Low but reliable
Moving bed biofilm reactor (MBBR)	~350	Most popular modern choice

Biofilter sizing equation:

Media volume (m3) = TAN production (g/day) / VTR (g TAN/m3/day)

Example: 2,300 g TAN/day / 350 VTR = 6.57 m3 MBBR media.

Nitrification requirements:

Dissolved oxygen: > 4 mg/L in biofilter
pH: 7.0 - 8.6 (optimal for nitrifying bacteria)
Temperature: most efficient at 27-28 deg C (operates 7-35 deg C)
Alkalinity: maintain > 100 mg/L CaCO3

Alkalinity supplementation:

7.14 g CaCO3 consumed per g TAN oxidized
Practical: ~0.25 kg baking soda per kg feed
Monitor alkalinity daily in intensive RAS

11.4 RAS Water Quality Targets

Parameter	Target	Critical Limit
DO	> 6 mg/L	> 4 mg/L minimum
TAN	< 1.5 mg/L	< 3.0 mg/L
UIA-N	< 0.02 mg/L	< 0.05 mg/L
Nitrite (NO2-N)	< 0.1 mg/L	< 0.25 mg/L
Nitrate (NO3-N)	< 100 mg/L	< 200 mg/L
pH	7.0 - 7.8	6.5 - 8.5
CO2	< 10 mg/L	< 20 mg/L
Alkalinity	100 - 200 mg CaCO3/L	> 50 mg/L
TSS	< 15 mg/L	< 25 mg/L

12. Species-Specific Biology

12.1 Atlantic Salmon (Salmo salar)

Parameter	Value
Market size	3-6 kg
Production cycle	24-42 months total
Freshwater phase	12-18 months
Seawater phase	12-24 months
Optimal growth temp	10-14 deg C (farming)
Upper stress limit	> 20 deg C
Critical thermal max	30-33 deg C
FCR	1.0-1.2
TGC (typical)	2.0-3.0
SGR (post-smolt)	0.8-1.5 %/day
Optimal DO	> 6.5 mg/L
Smoltification	Photoperiod + size triggered

Life stages: Egg --> Alevin --> Fry --> Parr --> Smolt --> Post-smolt --> Adult Key biology: Anadromous (freshwater birth, marine growth). Smoltification involves silver coloration, salt-tolerance development, behavioral changes. Farmed fish are selected for fast growth and late maturation.

12.2 Nile Tilapia (Oreochromis niloticus)

Parameter	Value
Market size	400 g - 1 kg
Production cycle (grow-out)	3-5 months
Optimal growth temp	27-32 deg C
Best SGR observed	2.93 %/day at 32 deg C
Lethal low temp (adults)	11-12 deg C
Lethal high temp (adults)	42 deg C
FCR	1.4-2.5
Optimal pH	6.0-8.0
Min DO	> 3.0 mg/L (tolerant species)
Stocking density (intensive)	Up to 136 kg/m3

Key biology: Warm freshwater. Extremely hardy and tolerant of poor water quality. Mouth-brooding reproduction. Male monosex culture preferred (XX males or hormonal sex reversal) to prevent uncontrolled reproduction. Can filter-feed on phytoplankton/biofloc.

12.3 Pacific White Shrimp (Penaeus vannamei)

Parameter	Value
Market size	18-25 g (90-day culture)
Production cycle	~90 days
Optimal growth temp (small)	30 deg C
Optimal growth temp (large)	27 deg C
Salinity range	0.5-45 ppt
Optimal salinity	10-15 ppt (isosmotic)
FCR	1.2-2.0
Stocking density (intensive)	70-150/m2
Stocking density (super-intensive)	Up to 400/m2
Production rate	12-24 tonnes/ha
Survival rate	70-90%
Min DO	> 5.0 mg/L
Optimal pH	8.0-8.5

Key biology: Euryhaline (wide salinity tolerance). Grows well in low salinity (isosmotic at 10-15 ppt minimizes osmoregulatory energy cost). Molting cycle creates periodic vulnerability. Nocturnal feeder. Susceptible to White Spot Syndrome Virus (WSSV) and Early Mortality Syndrome (EMS/AHPND).

12.4 Channel Catfish (Ictalurus punctatus)

Parameter	Value
Market size	0.5-1.5 kg
Production cycle	6-10 months
Optimal growth temp	25-30 deg C
Feeding stops	< 13 deg C
Survival range	Near-freezing to ~32 deg C
FCR	1.5-2.0
Feeding rate	3-5% BW/day
DO threshold	> 3.0 mg/L for good growth
Stocking density	Highly variable (pond-dependent)

Key biology: Warmwater. Bottom feeder. Can tolerate low DO briefly. Susceptible to Enteric Septicemia (Edwardsiella ictaluri) at 22-28 deg C. Overwinters without feeding. Spawns in cavities.

12.5 Rainbow Trout (Oncorhynchus mykiss)

Parameter	Value
Market size	250 g - 3 kg
Production cycle	12-18 months
Optimal growth temp	12-16 deg C
Upper lethal	~25 deg C
FCR	1.0-1.5
DO requirement	> 6 mg/L
Stocking density (raceways)	10-40 kg/m3

Key biology: Coldwater. Resident form of steelhead. Sensitive to water quality (ammonia, nitrite). Fin erosion is a major welfare concern at high densities. High-quality flesh commands premium prices.

13. Biofloc Technology

13.1 Principles

Biofloc technology (BFT) is a zero or minimal water exchange system that leverages heterotrophic bacterial communities to convert waste nitrogen into microbial protein.

Core concept: By adding a carbon source to maintain a high C:N ratio, heterotrophic bacteria outcompete autotrophic nitrifiers and immobilize ammonia directly into bacterial biomass (biofloc), which the cultured animals can consume as supplemental food.

13.2 Carbon-to-Nitrogen Ratio

C:N Ratio	Dominant Pathway
< 10:1	Autotrophic nitrification (NH3 --> NO2 --> NO3)
10-15:1	Mixed autotrophic + heterotrophic
15-20:1	Heterotrophic dominance (optimal for BFT)
> 20:1	Carbon excess (not beneficial)

Key advantage: Heterotrophic bacteria grow 10x faster than nitrifying bacteria and produce 10x more biomass per unit substrate. Ammonia immobilization is therefore much more rapid.

13.3 Biofloc Composition

Heterotrophic bacteria (Bacillus, Pseudomonas -- primary floc builders)
Autotrophic nitrifiers (Nitrosomonas, Nitrobacter)
Microalgae / phytoplankton
Zooplankton (rotifers, protozoa)
Dead organic matter
Extracellular polymeric substances (EPS -- the "glue")

13.4 BFT Requirements

Continuous aeration (24/7 -- keeps floc suspended and aerobic)
Carbon source addition (molasses, sugar, wheat flour, rice bran)
Monitoring: TSS (target 300-500 mg/L floc volume), TAN, DO, pH, alkalinity
Best species: Tilapia (filter-feeds on floc), shrimp (detritivore), catfish

13.5 Benefits

Reduced or eliminated water exchange
Reduced feed costs (biofloc provides ~25-50% of nutritional needs)
Improved FCR
Improved biosecurity (closed system)
Probiotic effect (competitive exclusion of pathogens)

14. Welfare Science & Cortisol

14.1 Cortisol as Stress Biomarker

Cortisol levels in salmonids (plasma):

State	Cortisol (ng/mL)
Unstressed baseline	0-5
Chronic stress (confinement, crowding)	~10 (sustained weeks)
Acute stress (handling, 1 hr confinement)	40-200
Recovery to baseline after acute stress	24-48 hours

Rainbow trout response magnitude: 12-29 fold increase across matrices (plasma, mucus, muscle, fins).

14.2 Stress Response Cascade

Primary response (seconds to minutes):

Catecholamine release (adrenaline, noradrenaline)
Cortisol release via HPA/HPI axis

Secondary response (minutes to hours):

Blood glucose elevation
Blood lactate increase
Osmotic disturbance
Immune cell redistribution

Tertiary response (hours to weeks):

Growth suppression
Reproductive impairment
Immune suppression --> increased disease susceptibility
Behavioral changes (see Section 8)
Chronic cortisol elevation can downregulate cortisol receptors

14.3 Stress Sources in Aquaculture

Stressor Category	Specific Stressors
Physical	Handling, grading, transport, crowding, netting
Water quality	Low DO, high ammonia, temperature extremes, pH shifts
Social	Hierarchies, aggression, territorial disputes
Husbandry	Vaccination, light manipulation, feed deprivation
Sensory	Noise, vibration, sudden lighting changes

14.4 Coping Styles

Fish exhibit individual variation in stress responses:

Proactive (bold): Lower cortisol production, higher aggression, active coping strategies
Reactive (shy): Higher cortisol production, lower aggression, passive coping strategies

These are heritable traits with implications for selective breeding in aquaculture.

15. Algae Blooms & Eutrophication

15.1 Harmful Algal Blooms (HABs)

Causes in aquaculture:

Excess nitrogen and phosphorus from uneaten feed and fish waste
Warm temperatures + high light + nutrient loading
Reduced grazing pressure (overfishing alters food webs)

Key organisms: Cyanobacteria (blue-green algae: Microcystis, Anabaena, Cylindrospermopsis)

15.2 Mechanisms of Fish Kill

Oxygen depletion: Dense bloom dies --> massive BOD from decomposition --> DO crashes to zero --> mass suffocation
Toxin production: Cyanotoxins (microcystins, cylindrospermopsin, saxitoxin) cause liver damage, neurological effects, gill damage
pH extremes: Dense blooms drive pH above 10 during peak photosynthesis
Physical gill damage: Some algae species (Heterosigma, Chattonella) directly damage gill epithelium

15.3 Prevention

Reduce nutrient input: Improve FCR, reduce overfeeding, optimize stocking
Water exchange: Dilutes nutrients and algal inocula
Clay flocculation: Sprinkle clay particles during bloom to flocculate and settle algal cells
Maintain alkalinity: Buffers against pH spikes from photosynthesis
Aeration: Prevents stratification and nocturnal DO crashes
Monitoring: Secchi disk transparency (> 30 cm), chlorophyll-a levels, phytoplankton cell counts

15.4 Eutrophication Link

Aquaculture-derived nutrients (N, P) have significantly enhanced eutrophication in areas of heavy aquaculture activity. Reducing feed consumption per mass of fish produced and introducing nutrient recycling are the primary mitigation strategies.

16. Master Water Quality Parameter Table

Comprehensive Thresholds for Aquaculture Management

Parameter	Unit	Optimal	Acceptable	Stress	Lethal	Notes
Dissolved Oxygen	mg/L	> 5.0	3.5-5.0	2.0-3.5	< 2.0	Coldwater species need > 6.5
Temperature	deg C	Species-specific	+/- 5 of optimal	Near CTmin/CTmax	Beyond CTmin/CTmax	See Section 6.2
pH	--	6.5-8.5	6.0-9.0	< 5.0 or > 10.0	< 4.0 or > 11.0	Affects ammonia toxicity
TAN	mg/L	< 1.0	1.0-3.0	> 3.0	Species/pH dependent	Must calculate UIA fraction
UIA-N (NH3)	mg/L	< 0.02	0.02-0.05	0.05-0.20	> 2.0	pH and temp dependent
Nitrite (NO2-N)	mg/L	< 0.10	0.10-0.25	0.25-1.0	> 1.0	Use Cl:NO2 > 6:1 protection
Nitrate (NO3-N)	mg/L	< 50	50-150	150-250	> 400	Accumulates in RAS
CO2	mg/L	< 10	10-20	20-40	> 60	Suffocation + pH depression
Alkalinity	mg CaCO3/L	100-200	50-300	< 40	< 20 (no buffering)	Consumed by nitrification
Hardness	mg CaCO3/L	50-300	30-500	< 20	--	Affects osmoregulation
Salinity	ppt	Species-specific	--	--	--	See Section 12
TSS	mg/L	< 15	15-25	> 25	> 80	Gill clogging, light reduction
Turbidity	NTU	< 25	25-50	> 80	--	Affects feeding behavior

Key Interactions Between Parameters

pH x Temperature --> Ammonia toxicity: Higher pH + higher temp = exponentially more toxic NH3
Temperature x DO --> Fish stress: Higher temp = lower DO saturation + higher metabolic demand = double jeopardy
CO2 x pH --> Acidification: More CO2 = lower pH = more ionized (less toxic) ammonia but direct CO2 toxicity
Alkalinity x Nitrification --> pH stability: Each gram TAN oxidized consumes 7.14 g alkalinity, potentially crashing pH
Stocking density x All parameters: Higher density amplifies every water quality challenge
Feeding rate x O2/NH3/CO2: Each kg feed produces ~0.3-1.0 kg O2 demand, ~40g TAN, and proportional CO2