3. Results and discussion
The average water velocity in raceways was 0.026 m/s
(0.15 m3/s) which equates to a water flow of 9.3 m3/min. This flow
rate resulted in one water exchange every 4.9 min or ≈12 times
per hour (45.9 m3 tank volume/9.3 m3/min). Brune et al. (2003)
observed water exchange rates ranging from 0.226 to 0.988 times
per hour within individual culture units of the partitioned aquaculture
system, while Hawcroft (1994) reported water exchange rates
as frequent as once every 3.8 min, Bernardez (1995) once every
3.2 min, Martin (1997) once every 6.4 min, and Wilcox (1998) once
every 6.2 min in an in-pond, floating raceway. High flow rate and
water exchange are beneficial in maintaining adequate DO concentration
at high fish densities and avoiding fish stress, especially at
heavy feed rates during the warmer part of the production season
(Boyd and Tucker, 1998). Soderberg (1995) recommended a minimal
water velocity of approximately 0.033 m/s to eliminate solid
waste settling at the rear of raceways. The six raceways of the inpond
raceway system of the present study had a combined water
flow of 55.7 m3/min. Thus, the entire water volume in the pond theoretically
flowed through the raceways every 12.1 h [(40,594 m3
pond volume/55.7 m3/min)/60 min/h].
The recommended operating air flow rate from the manufacture
of the diffuser tubing was approximately 0.047 m3/min/m
length of hose. A standard pressure loss from the diffuser tubing
of 2.98 kPa plus the additional pressure loss of 10.43 kPa from
the diffuser water depth (1.07 m) yielded a total pressure loss of
13.41 kPa. The standard oxygen transfer rate and standard aeration
efficiency were determined to be 1.88 and 12.96 kg O2/h and 1.26
and 2.87 kg O2/kW h for the diffuser aeration device and paddlewheel
aerator, respectively.
The feed delivery system was inefficient at delivering feed to
individual raceways, so a new design was adopted. The main problem
with the initial delivery system was that the feed delivery
lines were not sealed, and rain water flowed in to contaminate the
feed after a rainfall event. This was corrected by re-plumbing the
delivery lines and sealing them with silicon.
There initially was an uneven distribution of feed per raceway
because feed fines accumulated at the funnel section of the hop-76 T.W. Brown et al. / Aquacultural Engineering 44 (2011) 72–79
Table 2
Water quality variables measured in pond water samples collected from an in-pond raceway system (IPRS) in Browns, AL, as compared to optimum and tolerated levels for
growth of channel catfish. Values of the IPRS represent the mean and standard deviation (SD).
Water quality variable IPRS Optimal level* Tolerated level*
Min Max Mean ± SD
Dissolved oxygen (mg/L) 0.98 19.83 6.38 ± 3.62 5–15 2.00
Temperature (◦C) 12.30 33.60 25.15 ± 5.12 27–29 0-40
pH 6.99 9.33 7.74 ± 0.48 6–9 5-10
TAN (mg/L) 0.23 4.75 1.45 ± 1.15 0 as un-ionized ammonia <0.2 as N
Nitrite-N (mg/L) 0.00 0.84 0.17 ± 0.19 0 as nitrite Depends on chloride concentration
Total alkalinity (mg/L) 117.70 187.20 164.60 ± 16.40 20–400 <1 to >400
Total hardness (mg/L) 128.00 152.20 140.10 ± 8.60 20–400 <1 to >400
Chloride (mg/L) 598.00 933.00 705.30 ± 75.60 – –
Salinity (g/L) 1.00 1.40 1.24 ± 0.12 0.5–3.0 <0.1–8.0
* Tucker and Robinson (1990).
per and water splashed by the feeding action of the fish created
a paste that clogged the delivery funnel of the feed hopper and
restricted feed delivery. This problem was corrected by cleaning
and calibrating the feed hoppers on a weekly basis. A 20-L bucket
was attached to the bottom of each hopper, power was applied to
the feed broadcaster, and the amount of time was recorded for each
feeding event. After weighing the amount of feed from each hopper
in a given amount of time, a calibration factor was calculated for
each feed hopper.
Water quality variables were within acceptable limits for catfish
production throughout the experiment (Table 2). There were few
instances when DO concentration fell below 3 mg/L, and fish were
thought to have suffered little stress from water quality impairment
from high total ammonia nitrogen (TAN) or nitrite nitrogen
(Nitrite-N) concentrations. The elevated total alkalinity and total
hardness were likely related to the water supply and pond bottom.
The main water supply consisted of high concentrations of alkalinity
and hardness and the pond bottom was mainly calcareous clay
based limestone. Prior to the study additions of sodium chloride
were added to the pond to improve osmoregulation capabilities of
the fish and to prevent nitrite toxicosis or methaemoglobinaemia,
commonly known as “brown blood disease”. The combination of
elevated total alkalinity, total hardness, and chloride concentrations
was thought to have been beneficial for fish production and
equated to a more balanced and stress-free rearing environment.
This system demonstrated a high buffering capacity against diurnal
fluxes in DO and pH which was more than likely associated with the
use of mechanical aeration and elevated total alkalinity (Boyd and
Tucker, 1998). Water inflow had a higher concentration of DO and
pH as compared to the water outflow (Table 3). TAN and Nitrite-N
were lower in concentration in the water inflow while water out-
flow was higher in concentration which would be expected due
to fish respiration and metabolite production. Concentrations of
TAN and Nitrite-N were lowest at the water inflow point indicating
nitrification.
A total fish loading rate of 888 kg fish/m3/min was calculated
by dividing the mean fish yield by the water exchange
rate [8320 kg fish/(45.9 m3/4.9 min)]. The observed loading rate
was slightly higher than previous in-pond raceway studies with
Table 3
Water quality variables measured in pond water samples from various collection
points of an in-pond raceway system in Browns, AL. Values represent mean
concentrations.
Water quality variable Water inflow Water outflow Opposite end
of pond
Dissolved oxygen (mg/L) 7.24 6.35 –
pH 7.83 7.75 –
TAN (mg/L) 1.34 1.43 1.45
Nitrite-N (mg/L) 0.17 0.18 0.17
catfish; 166 kg fish/m3/min (Hawcroft, 1994), 545 kg fish/m3/min
(Bernardez, 1995), and 517 kg fish/m3/min (Wilcox, 1998). Trout
have been cultured at loading rates ranging from as 1152
to 1920 kg fish/m3/min in rectangular plug flow, circular, and
cylindrical cross flow tanks (Ross et al., 1995) and tilapia
at 2339 kg fish/m3/min (Watten and Johnson, 1990). Roque
et al. (2009) produced rainbow trout at loading rates of
551 kg fish/m3/min and 276 kg fish/m3/min in a flow through and
recirculating aquaculture system, respectively.
This system was designed to maximize the use of DO naturally
produced by the productivity of the pond biota by minimizing
the total biomass at any given point in time. Total catfish respiration
was calculated by an equation presented by Boyd et al.
(1978). For example, six different size classes of fish (100 g, 200 g,
300 g, 450 g, 600 g, and 700 g) distributed based on size across the
raceway units would have a total oxygen requirement for respiration
of approximately 290 kg O2/day at a biomass of 36,266 kg
at 25 ◦C water temperature. If similar size fish were stocked into
the raceways and allowed to reach market size (0.75 kg) at a
biomass of 68,000 kg there would be a total oxygen requirement
for respiration of approximately 448 kg O2/day at 25 ◦C water
temperature.
DO concentrations can be maintained without expending
unnecessary energy, by estimating the total DO requirement by
the fish based primarily on the total biomass and daily feed rate
in a raceway. Experience with trout and other species cultured
in flow-through systems indicates that for each pound of feed
applied, approximately 0.2 lb of DO will be needed (Boyd et al.,
2003). DO concentrations should not decline below 3.0 mg/L in
catfish raceways. Thus, if the incoming water contains a mean
DO concentration of 6.38 mg/L, only 3.38 mg/L DO is available
(6.38 mg/L inflow − 3.0 mg/L outflow). The daily weight of the
feed that can be applied can then be calculated. For example,
in our study, a mean water flow rate of 9.3 m3/min per raceway
and a mean DO of 6.38 mg/L was reported. In this case,
the maximum daily feed input would be approximately 220 kg
[9.3 m3/min × (6.38 g/m3 − 3.0 g/m3) × 1400 min/day × 0.001 g/kg
× 5 kg feed/kg oxygen]. The maximum feed rate can then be used to
calculate the total biomass of fish that can be grown in the raceways
without reducing growth due to restricted feed amount. From the
above calculation and an estimated feed rate of approximately 3.0%
total body weight (biomass) per day yields a maximum biomass
of 7333 kg (220 kg feed/3.0% body weight/day). Thus, if the mean
biomass is 8320 kg and the maximum biomass that can be grown
is 7333 kg, there will be a DO requirement from the additional
987 kg of fish (8320 kg − 7333 kg). The additional feed amount and
DO required will be approximately 30 kg (987 kg fish × 3.0% body
weight/day) and 6 kg (30 kg feed × 0.2 kg DO/1.0 kg feed), respectively.
The diffuser aeration device would have to be operated
for approximately 3.2 h (6.0 kg O2/1.88 kg O2/h), use approxi-T.W. Brown et al. / Aquacultural Engineering 44 (2011) 72–79 77
Table 4
Summary of channel catfish and hybrid catfish production in an in-pond raceway system during an 8-month period at Browns, AL.
Fish species and rearing
location
Average
stocking
weight (g)
Total weight
stocked (kg)
Number of fish
stocked
Average
harvest
weight (g)
Total weight
harvested (kg)
Total weight
harvested
(kg)
Production
(g/fish/day)
Survival
(%)
FCR
Channel catfish
Raceway 1b 59.09 812 13,742 277.27 2515 147 1.3 66.0 1.21
Raceway 2 177.27 2082 11,745 590.91 6600 220 1.8 95.1 1.54
Raceway 5 140.91 4230 30,019 395.45 9125 193 1.1 76.9 2.11
7124 18,240
Hybrid catfish
Raceway 1a 418.18 4845 11,587 640.91 7270 96 2.2 97.9 1.78
Raceway 3 (split into 3 & 4)a 240.91 4943 20,517 768.18 14,559 239 2.1 92.4 1.43
Raceway 6 61.58 1594 25,500
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