Culture of turbot (Scophthalmus maximus) juveniles

using shallow raceways tanks and recirculation

Rodrigo A. Labatuta,*, Jorge F. Olivaresb

aDepartment of Biological and Environmental Engineering, Cornell University,

Riley Robb Hall, Ithaca, NY 14853-5701, USAbFacultad de Ciencias del Mar, Universidad Catolica del Norte,

Larrondo 1281, Coquimbo, Chile

Received 14 December 2003; accepted 27 May 2004

Abstract

Traditionally, turbot (Scophthalmus maximus) have been cultured using square or circular tanks

and flow-through systems. The use of an alternative culture system that combines RAS technology

and shallow raceways tanks could produce similar growth rates at higher densities than conventional

systems. In this study, growth and survival rates of turbot juveniles cultured in shallow raceway tanks

under recirculating conditions have been evaluated. Two raceways with a 4 cm water column height

were set up in series and loaded each with 3654 turbot juveniles of 5.6 g mean weight. Over a study

period of 24 days, the average specific growth rate (SGR) observed was 1.4% and the feed conversion

(FC) was 0.84 g feed/g fish weight. Daily mortality rate observed was 0.09%, with an overall survival

of 98%. At the end of the study, average bottom coverage reached 262% (8 kg/m2). This study

demonstrated that RAS technology in combination with shallow raceways allows high fish densities,

improves overall productivity, decreases water pump requirements, and enhances reliability by

having better control over environmental and operational parameters. Additional, longer period

studies must be conducted to fully validate and approach this combined-technology to the

commercial stage.

# 2004 Elsevier B.V. All rights reserved.

Keywords: Shallow raceways; RAS; Turbot; Growth; Survival; Density

www.elsevier.com/locate/aqua-online

Aquacultural Engineering 32 (2004) 113–127

* Corresponding author. Tel.: +1 607 253 6476.

E-mail address: ral32@cornell.edu (R.A. Labatut).

0144-8609/$ – see front matter # 2004 Elsevier B.V. All rights reserved.

doi:10.1016/j.aquaeng.2004.05.008

1. Introduction

Turbot culture (Scophthalmus maximus) has been conducted in several types of tanks,

either square, rectangular or circular form. However, the direct effect of these structures has

not been quantified properly. In fact, even though turbot is a benthic species, deep fish tanks

are still normally used for its culture.

The most popular tanks used in aquaculture facilities are raceways, primarily because of

their construction simplicity, ease for harvesting and husbandry, and high surface area per

volume of water (Watten and Johnson, 1990). Raceways are rectangular tanks generally

designed to operate as plug-flow reactors. New water enters one end of the vessel and flows

lengthwise, which ideally creates a fairly constant water velocity from inlet to outlet

(Watten and Beck, 1987). Unfortunately, velocities in raceways are lower than those

R.A. Labatut, J.F. Olivares / Aquacultural Engineering 32 (2004) 113–127114

Nomenclature

A raceway cross-sectional area (m2)

AF shadow area of fish (cm2)

AR utilizable bottom area of the raceway (m2)

B fish biomass in the raceway (kg)

C percentage of raceway’s bottom coverage by fish (%)

DOin dissolved oxygen in the raceway influent (mg/L)

DOout dissolved oxygen in the raceway effluent (mg/L)

FC feed conversion (g feed/g fish weight)

FI feed intake (g)

HRT hydraulic residence time (min)

L mean fish length (cm)

No initial number of fish in the raceway

Nt final number of fish in the raceway

Oc specific oxygen consumption rate (mg O2/kg fish/h)

Pw wetted perimeter (m)

Ql water flow rate (L/min)

rh hydraulic radius (m)

Re Reynolds number (dimensionless)

SGR specific growth rate (%)

t time (day)

va actual water velocity (m/s)

vt theoretical water velocity (m/s)

W mean fish weight (wet basis, g)

Wf final mean fish weight (wet basis, g)

Wi initial mean fish weight (wet basis, g)

WG weight gained by fish (g)

z daily mortality rate (%)

y kinematic viscosity (1.139E�6 m2/s at 15 8C)

8 raceway volume (L)

required to produce efficient growth and feed conversion, fish stamina (Totland et al.,

1987), or self-cleaning (Westers and Pratt, 1977). Also, gradients of dissolved oxygen and

metabolites along the longitudinal axis of the tank often produce disparity in the

distribution and quality of fish, which can result in high mortality and poor growth rates

(Watten and Beck, 1987).

In studies conducted by Cripps and Poxton (1993), the hydrodynamic performance of

the raceways was correlated with the influence of water depth, flow rate and fish density.

They demonstrated that as the depth of water was reduced or the flow rate was increased,

the dead volumes of the tanks were reduced significantly, which in turn increased the

mixing and the water utilization efficiency. From these ideas, the concept of shallow

raceways was born. Øiestad (1998) describes the basics of these tanks and shows a

summary of some of the main works conducted to date.

A shallow raceway is similar to a standard raceway, with some small, but crucial

differences that must be fully accomplished: (1) low water level, ranging from 0.7 to

25 cm, depending on the fish size; (2) high density, where fish use 200–300% of the

available bottom area; and (3) the existence of a turbulent and plug-flow pattern.

Consequently, a steady and uninterrupted flow of water is critical for the performance

and safety of the culture. However, this is often difficult to achieve in classic flow-

through systems, where the water supply is fully dependent on generally limited external

sources. Water flow may periodically and unexpectedly undergoes drops in pressure, or

even complete outages as a result of several reasons, from pumping problems or water

shortages, to noxious algal bloom events. Recirculating aquaculture systems (RAS) are

less susceptible to sudden environmental changes or generalized failures since most of

the water flow is recirculated, which considerably minimizes any potential impact on

the production operation. Furthermore, RAS is a more environmentally sound technol-

ogy, reduces water consumption, conserves energy, allows better control over environ-

mental parameters, eliminates escapement of cultured animals, and is basically site-

independent.

So far, all shallow raceway studies have been conducted in flow-through conditions.

Although they have succeeded at pilot scale, the implicit risks of this low-water-level

technology are still too high to become an alternative at commercial scale. Proved

reliability of RAS not only could reduce these risks, but also its efficiency and flexibility

could diminish its costs and enhance performance. The feasibility of combining shallow

raceways and recirculating system technologies is examined in the present study by

evaluating the growth and survival rates of turbot juveniles under these conditions.

2. Materials and methods

The present study was conducted at the turbot hatchery of Tecnofish S.A., located in Las

Cruces, Chile. During a 24-day trial period under recirculating conditions, growth and

survival of turbot juveniles cultured in shallow raceways were evaluated. Two raceways

were set up in-series so that reuse of water was achieved in the second tank. Raceway 1

(upstream) and raceway 2 (downstream) were loaded each with 3654 turbot juveniles

weighing an average of 5.6 g. The water level was fixed at 4 cm and the recirculating rate

R.A. Labatut, J.F. Olivares / Aquacultural Engineering 32 (2004) 113–127 115

was kept at 90% of the total pumped volume per day with a mean water flow rate of 75 L/

min throughout the trial. Feeding was delivered ad libitum as frequently as possible during

a daytime 9-h period, depending on the water quality and the fish behavior.

2.1. Tank design

Two traditional raceways were used in this study. Constructed in fiberglass, the

dimensions of each were 5 m length, 0.8 m width, and 0.4 m height (Fig. 1). In the inlet

of each raceway, a perforated 30-cm height PVC plate was placed. With the intention of

homogenizing the influent water, 3.81-cm Bio-Barrels1 were located inside of each

influent chamber (A). In order to facilitate solids driving toward the drainage, V-shape PVC

baffles were installed at the end of the tank (B), where a 110-mm diameter PVC stand-pipe

was used to maintain the desired water level (C). The utilizable bottom area of the tank was

roughly 3.4 m2.

2.2. System configuration

A packed column aerator (PCA) was designed to accomplish biological filtration as well

as degasification and aeration processes. Air was supplied at the maximum flow rate of

50 L/min, measured by the use of an air flow meter (Key InstrumentsTM, model MFR4200).

The design of the PCA was based on criteria described by Wheaton (1977), Wortman

(1990), Hackney and Colt (1982), and Colt and Bouck (1984) for the designing of each of

the mentioned functions. Technical feasibility of this three-function device and the design

procedures are described in Labatut (2001).

Mechanical filtration was achieved by a rapid sand filter that was designed to remove

particles larger than 180 mm. A venturi-type foam fractionator with a constant air flow rate

of 28 L/min was used for fine-particle removal. Through this same device, but for only

R.A. Labatut, J.F. Olivares / Aquacultural Engineering 32 (2004) 113–127116

Fig. 1. Side and plan view of the shallow raceways (not to scale, dimensions in meters): (A) inlet water chamber;

(B) flow baffles (3-mm thickness PVC); (C) outlet (110-mm Ø PVC).

4.5 h/day, ozone gas was injected using an UV-type ozone generator (Sweetwater1, model

OZS12) with pure oxygen as a feed gas at a flow rate of 1–3 L/min. The design procedure

of the foam fractionator is detailed in Labatut (2001) and is based on the modeling and

criteria described by Timmons et al. (1995), Weeks et al. (1992), and Chen et al. (1994a).

The flowchart of the system is depicted in Fig. 2. Further detail of each component design,

dimension and operating parameters is found in Labatut (2001).

2.3. Biological parameters

On a weekly basis, fish weight sampling was conducted in each raceway and the amount

of feed delivered was recorded. From the mean values of the initial weight of fish (Wi), final

weight of fish (Wf), and feed intake (FI), the following indexes were calculated for each

raceway: weight gained (WG) (Eq. (1)), specific growth rate (SGR) (Eq. (2)) (Ricker, 1975)

and feed conversion (FC) (Eq. (3)) (Martınez, 1987):

WG ¼ Wf � Wi (1)

SGR ¼ lnðWf � WiÞt

� 100 (2)

FC ¼ FI

WG Nt(3)

Possible differences in growth and survival between the raceways were determined by a

two-way analysis of variance, ANOVA (Sokal and Rohlf, 1995). In addition, the number of

dead fish was recorded daily, and daily mortality rate (z) was determined through the

following equation:

z ¼ No � Nt

No t� 100 (4)

On a daily basis, once feeding period was finished, dissolved oxygen and temperature

were measured with a digital DO meter (YSI1, model 55) at the inlet and outlet of each

R.A. Labatut, J.F. Olivares / Aquacultural Engineering 32 (2004) 113–127 117

Fig. 2. System configuration.

raceway. Additionally, oxygen consumption was determined as a measure of fish meta-

bolism. In each raceway, the specific oxygen consumption rate (Oc) of the turbot juveniles

was determined based on the dissolved oxygen concentrations recorded during a 24-h

monitoring trial and the mathematical relationship described by Imsland et al. (1995):

Oc ¼QlðDOout � DOinÞ

B(5)

2.4. Operational parameters

Maintaining a turbulent water regime as well as adequate flow velocities is essential for

shallow raceways to perform effectively. Such conditions allow good mixing, efficient

solids removal, minimum dead volumes, and adequate distribution of oxygen and feed

throughout the tank. In this study, the following hydraulic parameters were determined:

actual (measured) and theoretical velocity, theoretical residence time, and Reynolds

number.

The actual water velocity (va) was determined through the float method for open

channels (Salgado and Valenzuela, 1992), using the same floating-type feed employed for

feeding in this study. Three vectors along the width of the raceway were designated to

measure the surface velocity in triplicate basis (n = 9).

The theoretical water velocity (vt) represents the mean velocity of any cross-sectional

area, and it is obtained by the following equation (Haestad Methods, 1999):

vt ¼Ql

A(6)

The hydraulic residence time (HRT) stands for the theoretical total time that the liquid

phase remains inside a reactor at a specified flow rate (Metcalf and Eddy, 2003). It depends

on the volume and the flow rate:

HRT ¼ 8Ql

The HRT was calculated for the shallow raceways and each of the treatment units (sand

filter, PCA/biofilter, foam fractionator and header tank1). The water volume in each

raceway was 136 L, while in the treatment units was 830 L, resulting in a total system

volume of 1102 L.

Flow regime in shallow raceways must be turbulent for maximum effectiveness, i.e.,

Reynolds number >�4000. The mixing action of this regime produces an exchange in the

amount of movement through the transversal section, which in turn produces uniform

pattern of velocities (Nelson, 1989) and more homogeneous water mixing. With such a

high fish density and low water level, the existence of turbulent flows in shallow raceways

is almost certain, although it must be theoretically verified by the Reynolds number. The

R.A. Labatut, J.F. Olivares / Aquacultural Engineering 32 (2004) 113–127118

1 The header tank was considered as a treatment unit, since it provided contact time for ozone reaction and

residual destruction.

Reynolds number (Re) is the relationship between the inertial forces and the viscosity

forces, and it is determined by the following equation (Streeter and Wylie, 1988):

Re ¼ 4vt rh

y

where rh is the hydraulic radius

rh ¼ A

Pw(9)

Pw is the wetted perimeter, which corresponds to the cross-sectional length of the tank that

is in contact with water.

Fish density is a key parameter in shallow raceways. It is primarily measured in terms of

percentage of bottom coverage (C) by using the following equation:

C ¼ AF Nt

AR� 0:01 (10)

where 0.01 is the conversion factor from cm2 to m2 and from fraction to percentage, and AF

is the flat side shadow body area of a fish: AF = 0.46 L2 (Øiestad, personal communication).

L was determined through the following empirical relationship2:

ln W ¼ �4:396 þ 3:221 ln L (11)

At the low water levels used in shallow raceways, the effective cross-sectional area (A)

and volume ( 8 ) of the tanks is affected by fish presence, and so does the calculation of vt,

HRT, and Re. Therefore, the cross-sectional area and the volume of the shallow raceways

were corrected based on fish’s mean body width (3 mm for a 5–10 g turbot), shadow body

area (AF), length (Eq. (11)), and total number at the beginning of the experiment.

3. Results

Average temperature and salinity recorded throughout the study were 16.9 0.7 8C and

35 1%, respectively. Mean dissolved oxygen was 7.58 0.2 mg/L in the inlet of

raceway 1 and 5.56 0.5 mg/L in the outlet of raceway 2. Frequent problems occurring in

the sand filter during the study resulted in a progressive deterioration of the water quality,

which consequently delayed the setting up of nitrifying bacteria in the biofilter (Labatut,

2001). As a result, high concentrations of ammonia–N were observed throughout the study,

with an average of 11.95 3.93 mg TAN/L (Labatut, 2001). Results of growth and

survival indexes for raceways 1 and 2 are shown in Tables 1 and 2, respectively. Differences

between raceways for mean weight, biomass, feed intake, weight gained, specific growth

rate, feed conversion, and mortality rate were not significant over the study (P > 0.05).

At the beginning of the study, the percentage of bottom coverage was 220% for

both raceways, while by the end of the experiment, raceways 1 and 2 reached 265 and

R.A. Labatut, J.F. Olivares / Aquacultural Engineering 32 (2004) 113–127 119

2 Based on data correlations of 5–100 g turbot cultured in shallow raceways (Øiestad, personal communica-

tion).

260%, respectively. At these densities a uniform distribution of the fish throughout the

tanks was observed (Fig. 3). The average volumetric and area densities at the end of the

study reached 200.8 kg/m3 and 8.03 kg/m2, respectively, and the system loading was

0.73 kg/L/min.

R.A. Labatut, J.F. Olivares / Aquacultural Engineering 32 (2004) 113–127120

Table 1

Data and indexes recorded in shallow raceway 1 during the study

Variable Days Total

1 9 17 24

Number of fish, Nt 3654 3645 3612 3612 3612

Mean fish weight, W (g) 5.56 5.77 6.99 7.63 7.63

Mean fish length, L (cm) 6.67 6.75 7.16 7.36 7.36

Biomass, B (kg) 20.32 21.03 25.25 27.56 27.56

Feed intake, FI (g) – 1250 2565 2615 6430

Weight gained, WG (g) – 0.21 1.22 0.64 2.07

Specific growth rate, SGR (%) – 0.46 2.40 1.25 1.38

Feed conversion, FC (g feed/g fish weight) – 1.63 0.58 1.13 0.86

Daily mortality rate, z (%) – 0.03 0.11 0 0.05

Table 2

Data and indexes recorded in shallow raceway 2 during the study

Variable Days Total

1 9 17 24

Number of fish, Nt 3654 3652 3550 3550 3550

Mean fish weight, W (g) 5.55 5.76 6.87 7.62 7.62

Mean fish length, L (cm) 6.66 6.74 7.12 7.35 7.35

Biomass, B (kg) 20.28 21.04 24.39 27.05 27.05

Feed intake, FI (g) – 1250 2525 2320 6095

Weight gained, WG (g) – 0.21 1.11 0.75 2.07

Specific growth rate, SGR (%) – 0.46 2.20 1.48 1.38

Feed conversion, FC (g feed/g fish weight) – 1.63 0.62 0.87 0.83

Daily mortality rate, z (%) – 0.01 0.35 0 0.12

Fig. 3. Fish distribution in the tanks: (left) outlet view of raceway 1; (right) inlet view of raceway 2.

Mean volume of fish calculated from the two raceways at the start of the trial was

0.0267 m3. With a raceway volume of 0.136 m3 the percent of fish present in the tank was

roughly 20% of the total water volume. After correcting the raceway dimensions with these

values, the theoretical velocity calculated in the raceways was 4.9 cm/s, which is only

0.2 cm/s above the measured velocity, 4.7 cm/s. Similarly, Reynolds number denoted the

development of a turbulent flow regime in the raceways. A summary of the hydraulic

parameters found in the system is shown in Table 3.

During the 24-h monitoring period, the average DO drop from inlet to outlet was 1.55 0.15 mg/L (21.5 0.9%). Also, the specific oxygen consumption rate (Oc) found was

254.7 24.6 mg O2/kg fish h (Fig. 4). No significant differences were observed between

raceways (P > 0.05).

R.A. Labatut, J.F. Olivares / Aquacultural Engineering 32 (2004) 113–127 121

Table 3

Hydraulic parameters of the system

Variable Value

Actual water flow velocity, va (cm/s) 4.7 0.1a

Theoretical water flow velocity, vt (cm/s) 4.9

Reynolds number, Re (dimensionless) 5079b

Water flow rate, Q (L/min) 75

Hydraulic residence time, HRT (min) 2.9 (raceways)

11.1 (treatment units)

a It corresponds to the average S.D. (n = 9) of the velocities measured in the surface lateral vectors (4.6 and

4.7) and the surface central vector (4.8).b Based on y = 1.139E�6 at 15 8C.

Fig. 4. Specific oxygen consumption rate during the 24-h monitoring period.

4. Discussion

4.1. Growth and survival

At the end of the present study, the overall SGR was 1.4%, ranging from 0.5 to 2.5%.

The FC was 0.84 g feed per g fish weight, and the bottom coverage reached 262%.

Previous studies conducted with turbot juveniles in traditional tanks show mixed results.

The results of this study were similar to those reported by Poxton et al. (1982) under

recirculating conditions (SGR = 1–1.8%), and Imsland et al. (1996) in flow-through

systems (SGR = 1.4–2.7%), but lower than those obtained by Labatut (1997) (SGR =

4.7–6.2%), also under flow-through conditions. In the case of turbot juveniles cultured

in shallow raceways, the results of this study were comparable to those obtained by

Lyngstad (1994) (overall SGR = 1.7%), but quite different to those found by Klokseth

(1996) (overall SGR = 4.5%) and Øiestad (1998) (overall SGR = 4%). In these last three

studies, flow-through systems were used, and bottom coverages near 200% were

reached. Finally, a 22-day preliminary trial conducted in shallow raceways with a

similar system configuration to that used in the present study, revealed an overall SGR

of 3.8% and a daily mortality rate of 0.003% (99% overall survival) when recircula-

tion rate was maintained at 60% and the final bottom coverage reached 269%

(Labatut, 2001). Unlike the present study, in this trial, two 30-mm cartridge filters

used instead of a single 180-mm sand filter helped to maintain a high water quality

throughout the trial.

Person-Le Ruyet et al. (1997) reported depressing effects on growth of S. maximus

caused by high concentrations of un-ionized ammonia nitrogen (UIA-N) starting at

0.21 mg/L. During the present study the concentrations of UIA-N were as high as

0.28 mg/L caused by the delay in the biofilter startup attributable to water quality

problems. Hence, it is possible that the differences of SGRs respect to other studies were

the consequence of both, the high levels of ammonia observed and the necessary reduction

in the amount and periodicity of feeding as a result of these ammonia levels. In fact, when

comparing the results of this study with the ones reported by Labatut (1997) in a flow-

through system, it is observed that even though in both cases the feeding regime was ad

libitum, the daily feed intake achieved by Labatut (1997) was at least 3.3 times greater

(30.5 g versus 9.6 g feed/kg fish day), which consequently resulted in a much larger SGR.

Furthermore, although survival and bottom coverage reached in the present study was

almost the same as those reported by Labatut (2001), growth rates differed greatly.

Probably good solids filtration and a lower recirculation rate in the study of Labatut

(2001) made possible to keep ammonia concentrations near zero and thus not limiting the

growth. The depressing effects of increasing density on turbot growth (Irwin et al., 1999)

also need to be taken into account, because while Labatut (1997, 2001) reached 0.7

and 5.4 kg fish/m2, respectively, in the present study the density reached as much as 8 kg

fish/m2.

The low water level used in shallow raceways in addition to the fact that turbot locate the

feed visually (Fernandez and Blasco, 1993) allowed to minimize the amount of feed loss,

and to obtain feed conversions near 0.84, which are better than those found by Poxton et al.

(1982) under RAS conditions (1.4–2.3).

R.A. Labatut, J.F. Olivares / Aquacultural Engineering 32 (2004) 113–127122

In both raceways the daily mortality rate was below 0.5%. The main cause of

mortality was due to daily fish pickings that removed those fish that looked apparently

weak and those with injuries in the pectoral fins presumably as a result of the high

density (Labatut, 2001). No mortalities would be attributed to ammonia levels con-

sidering that Person-Le Ruyet et al. (1997) found up to 100% of survival at UIA-N

concentrations over 0.36 mg/L. Constituting a rather natural cause of mortality were

those fish found outside the raceway as a consequence of spontaneous jumps occurred

near to the baffles of the tank. It cannot be discarded however, that those episodes had

been the result of sporadic rises of ozone levels, as this kind of behavior has been

reported in fish exposed to toxic concentrations of dissolved ozone (Bullock et al.,

1997). However, it is quite improbable that at the average ozone concentrations found

during the study in the influent of raceway 1 (0.04 0.02 mg/L), ozonation had caused

long term effects on growth or fish survival, as microscopic observations of the gills did

not reveal any kind of damage, excess of mucus or simple irritation in the area (Labatut,

2001). Moreover, the estimated mean ozone loading rate during this study was 0.019 kg

ozone/kg feed, which was lower than that used by Bullock et al. (1997) inducing no

mortalities in Rainbow trout (0.025 kg ozone/kg feed).

In summary, the high survival found at the end of the study (>98%) is on the range of

what was previously found by Øiestad (1998), and it could be explained in part by the

minimal human intervention into the fish environment due to the less frequent cleaning and

husbandry tasks required in the shallow raceways. Daily tasks in conventional tanks are

quite stressful, as they include sweeping up the bottom and walls of the tank, brush air

diffusers, and flush almost the entire volume of the tank, while at the same time dead fish

are removed from the central drainage grid of the tank. Conversely, shallow raceways

demonstrated to be self-cleaning and thus minimal human intervention into fish environ-

ment was required. Also, no air diffusers were necessaries and dead fish were cumulated at

the end of the tank, therefore they could be easily and quietly removed avoiding disturbing

the whole tank.

4.2. Hydraulic parameters and oxygen consumption

Reynolds number relates the steady-state flow velocity and geometry of the tank with

the viscosity of the fluid. However, in a fish-loaded tank, velocity and geometry are not

steady, but they change constantly as they are affected by characteristics such as density,

movement, and respiration of fish, among others. Therefore, Reynolds number would serve

only as a theoretical reference of the flow behavior in most aquaculture tanks. The actual

Reynolds number in a shallow raceway with such a high density and low water depth would

be much higher than that calculated (Re = 5079 in this study), and so turbulent flows in this

kind of systems would be almost a certainty. As turbulence increases, viscosity effects

become less significant and the velocity path lines tend to be more erratic but at the same

time more even in the longitudinal profile. This can explain the good agreement between

the actual velocity (4.7 cm/s) measured in the surface and the theoretical velocity (4.9 cm/

s), which represents the velocity of the entire profile. Turbulence also promotes flow

mixing, which allows a more homogeneous distribution of feed and diffusion of meta-

bolites throughout the water column. These conditions, along with the high fish density

R.A. Labatut, J.F. Olivares / Aquacultural Engineering 32 (2004) 113–127 123

were probably the main factors that allowed observing a uniform fish distribution

lengthwise and breadthwise of the raceways.

Although the flow velocity was quite high, it was observed that most of the feed

delivered was consumed before reaching the outlet, maybe because of the high fish density

and the low water level of the tank. Most of the remaining fraction of uneaten feed settled

down in the bottom of the tanks, but fish movement and the water flow moved any

remaining feed and feces to the outlet end of the tank, just as reported by Øiestad (1998) at

velocities three times lower than those reached in our tanks. Similarly, the actual velocity

achieved in the shallow raceways (4.7 cm/s) was 0.64 times the fish body length per

second, which is within the range of 0.5–2.0 body length per second reported by,

Summerfelt et al. (2001) and Timmons et al. (2002) to be safe (i.e., non-fatiguing) and

also optimum for promoting fish health, muscle tone, and respiration in salmonids.

Considering that turbot unlike salmonids are most of the time resting in the bottom of

the tanks, this would be a conservative approach.

Conventional turbot tanks exchange their volume in approximately 0.5 times per hour.

In this study shallow raceways exchanged their volume 17 times in an hour. For a given

flow rate the small water volume of shallow raceways can result in high HRT ratios

between treatment units and culture tanks, low DO drops between inlet and outlet, and less

accumulation of metabolites throughout the tank. Also, the ability of shallow raceways to

rapidly flush solids allows reducing degradation of organic matter inside the culture unit,

which in turn lowers BOD, reduces ammonia production, and ultimately decreases the

proliferation of opportunistic bacteria. Such characteristics could allow shallow raceways

to offer higher loading capacities than conventional, large HRT tanks.

Typical fish stocking densities in commercial juvenile turbot production tanks are

around 25–30 kg/m3 (Iglesias et al., 1978). Based on the results of this study, in shallow

raceways fish biomass per volume could be up to eight times larger. Also, if footprint area is

considered, shallow raceways can become more advantageous. As an example, four

circular tanks of 3 m diameter require an area near to 36 m2, while for the same area,

a total of 14 shallow raceways of 5 � 0.8 m can be set up when arranged in parallel two-

level racks. In summary, for a similar space area, shallow raceways could load roughly 10

times more biomass than a typical 30-cm depth circular tank

Throughout the current study, dissolved oxygen levels were above the normal para-

meters for the majority of the species (Meade, 1989), i.e., DO exceed 5 mg/L and the

oxygen saturation was over 60% at the effluent of raceway 2, which is the most critical

point of the system. Low HRTs, high water velocities, and the existence of a turbulent

regime resulted in inlet-to-outlet DO drops that were less than 22% in both raceways. The

oxygen consumption rate was quite similar in both raceways (P > 0.05) based upon the

24-h monitoring period. As usual, an increment in the oxygen consumption rate was

observed during the feeding time, from 9:00 a.m. to 6:00 p.m., to then decrease to the

lowest point at 6:00 a.m. Thus, the oxygen consumption rate varied from 207.4–299.3 mg

O2/kg fish h; results comparable to those described by Imsland et al. (1995) in turbot

juveniles, who found values from 150 to 250 mg O2/kg fish h at 16 8C. The differences

could be explained first because of Imsland et al. (1995) conducted their study in flow-

through systems, which have much lower BODs than RAS, and secondly, due to the

temperature difference (+1.5 8C in the present study).

R.A. Labatut, J.F. Olivares / Aquacultural Engineering 32 (2004) 113–127124

Shallow raceways demonstrated self-sufficiency and reliability through the course of

this study. Several water shortages occurred in the hatchery as a result of problems in the

suction pumps or simply due to scheduled maintenance tasks in the hydraulic system.

While flow-through production tanks were maintained with a mixture of air and pure

oxygen, RAS shallow raceway tanks were operated as usual, with no need of further

assistance or any additional equipment.

5. Conclusions

Although the SGR was mainly limited by the ammonia levels, it was within the range of

performance previously reported for RAS produced turbot. Due to the high fish density

capability of shallow raceways, even lows SGRs allow having greater overall productivities

(i.e., biomass produced per footprint area) than conventional tanks. Feed conversion

showed better feed utilization efficiency than previous studies conducted with turbot in

RAS. The low mortality rate suggests that survival of turbot was not limited by ammonia or

ozone at the levels managed in this study. The less frequent cleaning procedures in the

shallow raceways resulted in a minimal human intervention on the fish environment, which

apparently contributed to achieve a high survival rate.

A high and even longitudinal flow velocity as well as an adequate culture density

promote tank self-cleaning and helps to keep a uniform fish distribution throughout

the tanks. Further hydraulic studies should be conducted in order to predict the per-

formance of these reactors by means of residence time distribution (RTD) analysis to

further detail percentages of dead volumes of the shallow raceways and its actual mixing

capacity.

Ammonia concentration problems need to be corrected in future trials by having more

efficient solid removal systems, ensure full activation of the biofilter, quantify its actual

ammonia (and nitrite) removal capacity, and increase fish biomass accordingly.

In summary it can be concluded that there is a mutual benefit of using shallow raceways

in RAS conditions. Unique characteristics of shallow raceways allow improving loading

capacity of RAS, while RAS allows shallow raceways to have better control over

environmental parameters and to improve the performance and safeness of the culture.

Promising applications can be envisioned from the combination of RAS and shallow

raceways, but further and longer studies need to be conducted to approach these

technologies to the commercial stage.

Acknowledgements

The authors would like to thank Tecnofish S.A. for allowing using their installations,

and to the entire staff for their technical as well personal support. Very especial acknowl-

edgements to Dr. Michael Timmons (Cornell University), Dr. Steven Summerfelt (The

Freshwater Institute) and Dr. Victor Øiestad (University of Tromsø) for the important

contribution provided to this study. This work was done with especial dedication to the

memory of Jorge Olivares M.

R.A. Labatut, J.F. Olivares / Aquacultural Engineering 32 (2004) 113–127 125

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