1. Introduction
The majority of the operational costs in a bivalve nursery are associated with algae culture and the maintenance feeding and water quality of seed stock
Castagna and Kraeuter, 1977; Coutteau et al., 1994. These costs can be reduced by increasing production per unit area or volume, minimizing labor requirements,
and utilizing cultured microalgae more efficiently. The technologies of computer- controlled operation, water recirculation, and fluidization offer the potential to
meet these objectives. These technologies offer the potential to reduce labor requirements, improve utilization efficiency of the algae produced, improve control
and monitoring of system operation, reduce the solids and metabolic wastes in the seed culture system, sustain water quality for optimum seed growth, and reduced
fouling of the culture units.
A recent development of shellfish aquaculture is the high-density nursery culture of clutchless oysters 2 – 25 mm by the use of water flowing upward at fluidization
velocities Ver and Wang, 1995. Under fluidized conditions, the bed of oysters is expanded due to the increased fluid flow. The oysters are suspended in the fluid
rather than lying on each other as in a packed bed under low flow conditions. The fluidization allows for a more uniform distribution of the food supply and better
transport of fecal material and other particulates out of the seed bed by the flowing water.
Typical land-based clam nursery systems utilize upwellers for seed culture in which ambient seawater or seawater with cultured algae is pumped upward through
the culture unit. The water flow through the upweller is too low for fluidization of seed to occur and as a result the density of seed is usually a single layer spread over
the bottom of the upweller unit.
In the last decade, the practice of culturing seed in a nursery system has flourished and the application of fluidized-bed technology coupled with computer-
control and recirculation technologies can potentially allow for the high-density culture of clam seed in a land-based nursery environment. An integrated system for
the production of algae and culture of northern quahog seed clams, Mercenaria mercenaria, was developed utilizing computer-control, fluidization, and recircula-
tion technologies. This paper presents 1 a description of the integrated system; 2 the computer-control strategy for the control and monitoring of the integrated
system; and 3 system performance of culturing northern quahog seed clams.
2. Materials and methods
The integrated system consists of three components a algal production unit; b seed clam culture unit; and c the computer control unit. A schematic outline of
the integrated culture system is presented in Fig. 1 and a description of the individual components is provided below.
2
.
1
. Algal culture system The algae are grown in conical 550 l fiberglass tanks and covered with clear lexan
to minimize the entrance of airborne contaminants. The microalgae, Chaetoceros muelleri CHAET 10, was selected because it grows well at high temperatures and
over a wide range of salinities Johansen et al., 1990; Nelson et al., 1992. Feeding studies have also indicated CHAET 10 to be a suitable food source for Mercenaria
seed Wikfors et al., 1992; Walker et al., 1997. The air provided for culture aeration and mixing is supplied by a diaphragm air pump Sweetwater, model L29. The air
is filtered through an in-line filter capsule 0.2 microns before entering the cultures. On a daily basis CO
2
is continuously injected into the airline at a rate of 0.5 cm
3
min
− 1
for 6 h 1000 – 1600. Seawater for algal culture was obtained from the adjacent brackish river Skidaway River. Treatment processes of the incoming
seawater included bag filtration 20 and 5 micron, chlorination, active carbon filtration, UV sterilization, and ozonation. A one micron cartridge filter at the
central inflow point of the culture tanks was the final water treatment step. Level sensors were used to control the water level in each tank. A float valve was placed
into each tank to serve as the backup mechanism to the level sensors and prevent tank overflow.
The millivolt output from a commercial turbidimeter HACH 1720°C was used to estimate the algal biomass mg dry weight algae l
− 1
, in the culture tanks. During the computer-controlled sampling procedures miniature centrifugal pumps trans-
Fig. 1. Schematic diagram of the integrated algal production and recirculating seed clam nursery system at the Shellfish Aquaculture Laboratory, University of Georgia, Savannah, Georgia, USA.
Fig. 2. General diagram of the upweller used in the recirculating seed clam nursery system for the fluidized-bed culture of Mercenaria seed clams.
ferred algal culture from the tanks to the turbidimeter at a rate of approximately 2 lpm. On the outside bottom of each tank was a 2.5 cm PVC tee, with 2.5
cm PVC ball valves fitted on both ends. One ball valve was connected to an actuator for controlling algal harvesting operations and the other ball valve was
used to manually drain the tanks. A 0.076 kW 0.1 hp centrifugal pump was computer-controlled to harvest the algal cultures and transfer the algae to a 500 l
harvest reservoir. After the harvest procedure a peristaltic pump connected to the nutrient solutions Fritz’s F2 A and B was activated to provided the dosing of
nutrients into each tank.
2
.
2
. Seed clam nursery system The seed clam nursery system consisted of seven components 1 six 5-cm
diameter clear PVC pipes, 76 cm in height, for seed clam culture Fig. 2; 2 a 400-l system reservoir; 3 a 0.076 kW 0.1 hp centrifugal pump for system water
recirculation; 4 an in-line solids separator for removal of large particulate waste material; 5 a 0.028 m
3
1.0 ft
3
bead filter for additional particle entrapment and nitrification; 6; a 0.37 kW 0.50 hp chiller unit for water temperature control in
the summer and 7; two header tubes. One tube served as the air escape vent from the bead filter after backflushing operations. The other tube provided static head to
allow adjustments of water flow to each upweller to be made without affecting the adjusted flow in the other units.
Algae for the clam system was obtained from the harvest reservoir of the algal culture system. Water loss in the clam system by evaporation or after filter
backflushing was replaced by water from the water treatment system reservoir. Cleaning of the clam seed bed in the upweller units was done by the control
program that closed the actuator valve at the end of the upweller line each hour for 30 s. Closing of this valve resulted in increase water flow through each of the
upweller units thereby dislodging the seed mass clumps and flushing settled waste matter out of the seed bed. The control program also opened and closed valves for
purging of wastes from the solids separator and for backflushing the bead filter.
Each upweller contained a flow distribution plate 0.64 cm PVC with a radial pattern of 0.32 cm holes to provide uniform water flow into the culture tubes.
Placed slightly above the flow distribution plate was a mesh screen 1.0 mm Nytex for retaining the seed mass. The water flow rate through the seed mass of each
upweller was maintained and adjusted by an in-line flow meter placed at the inflow of each upweller. Flow through each upweller was manually adjusted with a PVC
gate valve 1.9 cm placed below the flow meter.
2
.
3
. Process control The computer-control system included a laptop computer Zenith Supersport
e286, a multiport controller, three analog to digital converters Remote Measure- ments Systems, Seattle, WA, and a 10-volt solid-state relay switch for each
controlled output. The control program, was written in Turbo Pascal 6.0 Borland International Inc., 1990. The program is menu driven to facilitate use by system
users unfamiliar with the Turbo Pascal programming language. The program contained over 40 commands that are used to monitor or control all components of
the integrated system.
There were three program input parameters associated with the control and monitoring of the algal culture system. Two parameters were used to set the daily
start 08:00 h and end time 20:00 h of algal culture sampling and harvesting operations. These times covered the range of observed algal growth under green-
house conditions, where lighting for algal growth depends on the available sunlight. The third parameter was the biomass set point for algal harvest.
Between the set hours of operation the chambers were sampled every 2 h to estimate the standing algal biomass. If the estimated algal biomass was lower than
the harvest set point, the sampling procedure was complete and repeated in 2 h. If the estimated biomass was greater than the harvest set point, command actions
were issued to the priority queue to initiate the harvest cycle. After harvesting a volume of 450 l, the chamber was refilled with the treated seawater and reinocu-
lated with nutrients. The sampling procedure resumed 2 h after harvest.
The recirculating nursery clam culture system had ten input parameters 1 initial clam biomass g whole wet weight; 2 feeding rate of clam biomass per day;
3. estimated clam biomass growth rate per day; 4 feeding per day; 5. pump flow rate of algae from algal reservoir to clam reservoir lpm; 6 algal reservoir
concentration mg l
− 1
dry weight; 7 number of feedings between bead filter backflushing; 8 purge rate from solids separator lpm; 9 frequency min in
closing of the header valve; and 10 duration of header valve closure s. The first eight parameters are used to calculate the daily dry weight of algae
required for feeding the seed clams and to adjust the ration based on the estimated clam growth. The daily ration, adjusted at the end of each day at 2345 h
according to the estimated growth rate, is calculated as a percent dry weight of algae per g whole wet weight of clams dry weight per wet weight per day as
follows:
1. Clam biomass g, day n = [clam biomass day per n] × [estimated growth rate]. 2. Dry weight algae g, day n = [clam biomass day n] × [ feed ration100]. The
feed ration delivered to the clam algae reservoir was then calculated as follows: 3. Ration amount, g = [Dry weight algae g, day n]number of feeding per day.
The ration amount must be expressed volumetrically in order to determine the duration of the pump activation time to transfer algae from the algal harvest
reservoir to the clam algal reservoir the algal concentration in the calculation is the value of the harvest set point.
4. V
A
= [1algal conc. l mg
− 1
] × [ration amount g algae] × [1000 mg g
− 1
] 5. Pump time, Pt min = [V
A
L] × [1algal pump rate lpm]. Before this volume of algae can be pumped to the clam algal reservoir an equal volume must be
removed from the clam system. Using the parameter value for the system purge rate outflow from the swirl separator, the time interval required to displace an
equal volume of clam system seawater via the separator purge valve was calculated:
6. Drain time min = [Pt min] × [algal pump rate per system purge rate]. The last calculation is the time interval between feeding. The input is in days and the
program converts it to seconds to follow program control structure. This is calculated as follows:
7. Feed time interval day = [24 h per day per feeding per day × 3600 s h
− 1
] 86,400 s per day
All of these calculations were performed as a single procedure before the feed command was initiated.
2
.
4
. Analytical procedures and techniques Control charts were maintained for the turbidimeter process of estimating the
standing algal biomass and for the process of delivering algae from the algal harvest reservoir to the clam algal reservoir. A control chart is a graphical display of a
specific characteristic that has been measured from a sample versus the sample number or time Montgomery, 1997. The chart contains a centerline that repre-
sents the average value of the specific characteristic corresponding to the in-control state. The specific characteristic for the control chart of the recirculating seed clam
nursery system was the amount of algae to be delivered g dry weight. The characteristic for monitoring the turbidimeter was the harvest set point value. Two
other horizontal lines on the chart, the upper control limit UCL and the lower control limit LCL are shown and are typically called the ‘3-sigma’ control limits.
Sigma refers to the standard deviation S.D. of the statistic plotted on the chart not the S.D. of the quality characteristic. The processes were considered stable or
in statistical control when the points plot within the control limits X 9 3s
n and no action is necessary. A point that plots outside the control limits is corrective
action are required to find and eliminate the cause or causes responsible for this behavior.
Samples for monitoring feeding operations and the turbidimeter were collected three times each week. Dry weight mg l
− 1
algal estimates were obtained from triplicate 100 ml samples. An ammonium formate solution 0.5 M was used to
prerinse the filters and the filtered algal sample to remove residual salts prior to oven drying at 100°C.
The specific growth rate SGR of the seed clams in the culture units was calculated using the equation
SGR = ln N
t
N dt
= lnN
A
N dt
where N , initial biomass of the seed clams in each upweller unit, g whole wet
weight; N
t
, final biomass of the seed clams in each upweller unit, g whole wet weight; dt, culture period, days.
Percent seed bed expansion was obtained by the following equation: Percent seed bed expansion =
S
Ve
S
Vp
100, where, S
Vp
, volume displacement of seed clam mass ml; S
Ve
, total volume displacement of expanded seed clam mass ml.
2
.
5
. Seed culture with the integrated system Hatchery reared Mook Sea Farms, Damariscotta, Maine northern quahog clam
seed, M. mercenaria, of a single cohort were used in the 83-day trial period. The introductory trial was conducted during the fall and winter of 1996 – 1997. Initially,
40 ml 60 g whole wet weight of seed n = 12, 180 with a mean shell length of 2.5
mm S.D. 9 0.5, n = 300 was distributed into each upweller. During the trial period the seed clams in the culture upwellers units were thinned or sorted three times day
14, 52, and 73. The initial stocking density was 3.0 g whole wet weight of seed cm
-2
and was reduced to 2.0 – 3.0 g m
− 2
after each sorting or thinning procedure. The feed ration and feeding frequency during the trial period is provided in Table 1.
Based on the amount of available algae the daily ration ranged from 1 to 4 dry algae per wet weight clam during the trial period.
A flow rate of 3.5 9 0.3 lpm was maintained in each upweller during the trial period. A flow rate above 5.0 lpm moved the seed mass up the upweller column and
out of the upweller unit. Water quality parameters of temperature, salinity, dissolved oxygen, and pH were measured in situ each morning. Levels of total
ammonia-N, nitrite-N, nitrate-N, and total alkalinity were monitored weekly. pH measurements were obtained with a benchtop pH meter Orion model 620, salinity
with a hand-held, temperature compensated refractometer, and dissolved oxygen with a YSI oxygen meter model Y58. Temperature probes in the clam system
reservoir and algal chambers provided data acquired via the computer. Water samples for total ammonia-N, nitrite-N, nitrate-N, and alkalinity were analyzed
using the HACH DREL2000 portable laboratory equipped with a DR 2000 direct reading spectrophotometer. Survival data was approximated for each individual
upweller unit from the wet weight count of 1 g of clams.
3. Results
Kategori