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
3
.
1
. Algal culture During system operation the harvest set points for the algal cultures were
between 30 and 60 mg l
− 1
dry weight. In this range the sampling error of the HACH turbidimeter is approximately 25 – 13, respectively. The control chart
presented in Fig. 3 illustrates the performance of the HACH turbidimeter for measuring the algal tank culture biomass. A harvest set point of 40 mg l
− 1
dry weight was used as the value of the center line in the control chart. The upper and
lower limits of the control chart were 24.3 and 55.7 mg l
− 1
dry weight, respectively, where s
y
= 7.4 mg l
− 1
and n = 2. Values were plotted in their sequence of measurement. Sample data points outside the control limits were a result of a
biofilm on the sensor lens or algal floc in the sample rather than the turbidimeter being out of calibration. Consequently, part of the system daily maintenance was to
clean the turbidimeter sensor lens and rinse the sample chamber.
Fig. 4 presents a summer and winter algal production profile for Chaetoceros muelleri from the algal system under greenhouse conditions. The harvest levels were
set at 60 mg l
− 1
dry weight during spring and late summer seasons March – Octo- ber and 30 mg l
− 1
dry weight during the winter seasons November – February. The harvest volume was 450 l, approximately 90 of the culture volume. A large
harvest volume was employed since it was observed to minimize the growth of biofilm on the tank walls thereby extending the culture periods between tank
23 T
.J .
Pfeiffer ,
K .A
. Rusch
Aquacultural
Engineering
24 2000
15 –
31
Table 1 Trial results for Mercenaria seed clams grown in a land-based nursery system utilizing computer-control, recirculation, and fluidized-bed technology
Bed porosity Feeding
Ending shell Stocking
Days Mean daily
Initial shell Ending
Number of Daily feed
strategy
b
ration
a
units growth rate
density length mm
density length mm
g cm
− 2
g cm
− 2
day
− 1
3.7 2.7 9 0.6
6 0.49 9 0.06
2 0.015
0.5 per 6 h 1–14
3.0 2.5 9 0.5
2.0 2.7 9 0.6
2.7 3.1 9 0.8
0.49 9 0.05 0.011
2 14–42
1.0 per 12 h 6
6 3.1 9 0.8
2.7 3.1 9 0.8
0.49 9 0.05 0.000
2 1.0 per 12 h
42–52 2.7
2.7 3.8 9 0.5
0.67 9 0.02 0.010
3.6 9 0.5 0.5 per 12 h
2.5 52–59
6 1
3.8 9 0.5 6
4.0 4.4 9 0.8
0.62 9 0.02 0.029
2 1.0 per 12 h
2.7 59–73
5.6 73–83
5.5 9 0.7 4
0.64 9 0.01 0.059
4 1.0 per 6 h
3.0 4.2 9 0.5
5.4 7.9 9 0.8
0.67 9 0.01 0.063
5.7 9 0.5 3.0
1.0 per 6 h 2
4
a
Percent of dry weight algae per wet weight clam.
b
Percent of daily ration per feeding frequency.
cleanings and reinoculation. In addition, the large harvest volumes helped control the protozoan and bacterial contamination of the algal culture as the harvested
culture volume was replaced with the relatively contaminant-free filtered seawater. Sunlight availability and photoperiod, as well as the ambient temperature in the
greenhouse principally governed the harvest set point during system operation. In the summer months, the high ambient air temperature inside the greenhouse
resulted in the water temperature in the closed culture chambers to reach levels \ 35°C considered sub-optimum for growth Chen, 1991. To minimize algal
cultures collapsing from the high temperatures, the chambers were batch-harvested approximately every 3 days following the initial harvest. In the summer, refilling the
chambers after harvesting with the chilled reservoir seawater 20°C of the water treatment unit was a helpful passive approach for maintaining the culture tempera-
tures below 35°C during the 3-day culture period. With this harvest frequency 3 days, a 90 harvest of the culture, and avoidance of high temperatures, it was
difficult to maintain a harvest biomass level above 60 mg l
− 1
dry weight. In the winter, the reduced sunlight and the low ambient temperatures in the greenhouse
limited biomass growth. By lengthening the harvest intervals to 4 days a culture biomass level of 30 mg l
− 1
dry weight was attainable. Beyond a 4-day culture period minimal additional growth was observed.
Fig. 3. Example of the control chart maintained for the HACH 1720C turbidimeter measuring process of estimating the culture chamber algal biomass.
Fig. 4. Profile of the growth for CHAET 10 in the algal chambers under greenhouse conditions during the summer and winter months.
3
.
2
. Control operations of algal deli6ery and nursery system There were several concerns with regards to delivering algae from the algae
production unit to the seed clam nursery unit. The major concern was the error associated with estimating the alga culture biomass. Underestimating the algal
biomass can result in the clams being overfed and inefficient use of the algae produced. An overestimate can result in the underfeeding of the seed clams thus
lowering growth rates and potentially increasing the mortality rates. Thus, a cost-effective approach was required for estimating the algal biomass. The low
range turbidimeter provided acceptable performance and stability, as indicated by the control chart Fig. 3, for estimating the algal biomass. Utilizing an in-line
fluorimeter approximately 10 × more expensive than the turbidimeter was not considered cost-effective for the minimal gain in performance approximately 5.
Utilization of an algal biomass sensor comprised of a photovoltaic solarcell and light source, and much less expensive than the turbidimeter 100 × less expensive
was rejected as the unit required constant calibration and performance reliability was inconsistent.
Another concern was with the computer-controlled volume exchanges between the algal and seed clam system. The volume of water discharged from the seed clam
unit by backflushing the beadfilter or purging the solids separator was replaced by
delivering an equal volume of algae from the algal system. The computer-controlled volume exchanges were within the control chart UCL and LCL limits and changes
to the delivery setup were not necessary i.e. adjustment of the algal pump activation time period or reservoir purge time period.
There was also a concern with the cell integrity and viability from the shear stress in using centrifugal pumps for harvesting, transferring and recirculating the mi-
croalgae. Microscope observation indicated cell walls remained intact and no difference in whole cell counts was observed before and after harvesting algae from
the culture chambers or after transfer from the algal harvest reservoir to the clam system reservoir. Additionally, when recirculating the algae in the nursery system
without seed clams present and bypassing the beadfilter, cell counts and suspended solids analysis did not indicate a reduction in cell numbers or biomass.
Another reason for the concern with regards to accurate estimates of algal biomass and volume exchanges was the need to avoid a high algal concentration in
the seed clam system. A high algal concentration in a seed clam system can result in pseudofeces production and inefficient algal utilization Tenore and Dunstan,
1973. Pseudofeces production represents a loss of potentially utilizable algal cells and the production of organic matter in the form of mucous. Consequently, fouling
of the seed bed increases which minimizes uniform water flow and food distribution through the seed bed. All of these conditions result in the decrease of seed clam
growth. Therefore, a pulsed feeding strategy was utilized to avoid high cell concentrations in the system and minimize psuedofeces production. The transfer of
algae from the algal system harvest reservoir to the clam feed reservoir was 2 – 4 times a day, depending on the alga concentration and availability. The resulting
algal concentration in the seed clam system after each feed transfer ranged from 50 000 to 200 000 cells ml
− 1
. As stated above, before each feeding, a volume of water equal to the volume of
algae being transferred was removed from the seed clam system. This volume of water, removed by backflushing the bead filter or by purging the solids separator,
resulted in a water volume exchange per feeding from 11 to 40. The resulting total daily volume exchange ranged from 20 to 160. With such high daily system
volume exchange, incorporating the biofilter into the recirculation process is not necessary and eliminating the biofilter would reduce any algal ration loss due to
filter entrapment.
3
.
3
. Culture of seed clams The growth of the seed clams in the integrated system cultured under high-den-
sity, fluidized-flow conditions is presented in Fig. 5. The culture trial was termi- nated after 83 days, due to a sustained decline in the ambient air temperature of the
greenhouse resulting in sub-optimal culture unit water temperature for seed clam growth and insufficient cultured algae for a food supply. The mean daily and
weekly water quality data are summarized in Table 2. There were no major differences in the progression of the water quality parameters with time during the
culture period except for water temperature. The water temperature for the seed
Fig. 5. Profile of the growth for Mercenaria seed clams utilizing fluidized flow in a computer-controlled integrated algal-seed clam nursery system.
clam system progressively declined due to the seasonal changes in temperature and lack of temperature control in the greenhouse.
After 2 weeks of culture from initial stocking, seed survival was 76.8 9 11.5 with an unexpectedly low growth rate 0.015 per day. Compared to previous
laboratory data for forced-flow upweller culture of Mercenaria seed, the growth rate was approximately 70 lower. The stocking density was thus thinned from 3.7
g wet weight clam cm
− 2
to 2.0 g wet weight clam cm
− 2
. After an additional four
Table 2 Water quality results of the land-based nursery system utilizing recirculation and fluidized-flow
technology Variable
Maximum value Minimum value Average
Standard deviation
28.0 16.6
19.0 Temperature °C
1.8 Dissolved oxygen mg l
− 1
6.0 9.0
0.6 7.2
7.9 0.2
7.5 7.3
pH 28.0
24.0 Salinity ppt
26.0 1.1
0.000 0.005
Ammonia-N mg l
− 1
0.017 0.090
2.30 0.01
Nitrite-N mg l
− 1
0.55 0.27
14.1 5.8
26.4 3.8
Nitrate-N mg l
− 1
70.0 103.1
10.3 121.0
Total alkalinity mg l
− 1
CaCO
3
and a half weeks of culture, minimal growth 0.4 mm shell length of the seed clams was observed and survival rates were lower, 67.3 9 6.7. During this 52-day
culture period, there was noticeable byssal thread attachment among the smaller clams in the upweller culture units. To minimize and breakup the byssal thread
attachment amongst the smaller seed, the actuator valve to the second header tube was closed for 1 min every hour. The resulting increased water flow through each
upweller approximately 25 increase helped breakup seed clumping, but was not sufficient. It is interesting to note that some of the air which was displaced after the
beadfilter was backflushed escaped into the upwellers units instead of through the header tube and was more effective at expanding and breaking up the seed mass,
and clearing settled solids from the seed bed.
As a result of the byssal thread attachment amongst the smaller seed in the culture units, seed less than 3.0 mm in shell length were removed. The mean shell
length of the seed replaced in the upwellers was 3.6 mm S.D. 9 0.5 and the resulting bed porosity increased from 0.49 to 0.67. A week after the small seed were
removed day 59, the growth rate of the seed improved from 0 to 0.01 per day. On day 73, 3 weeks after the removal of the small seed, the mean shell length increased
from 3.6 9 0.5 to 4.4 9 0.8 mm. The growth rate tripled to 0.029 per day and survival rate improved to 88.0 9 1.7. The shell length measurements indicated
a wide distribution of seed size, therefore, a second sorting was conducted to separate the larger seed from the smaller seed. The larger seed, those retained on a
5 mm sieve, were distributed into two upwellers at a density of 3.0 g wet weight cm
− 2
. The remaining seed, those retained on a 3 mm sieve, were distributed into the remaining four upwellers at the same density. The trial period was terminated
10 days later because sufficient quantities of cultured algae for the seed clams were unavailable and declining ambient temperature inside the greenhouse. During the
last 10 days of culture, the average growth rates between the two groups improved to 0.059 – 0.063 per day, respectively. Survival rates dropped to 78.1 9 6.5 for
the units with the smaller seed and 72.0 9 4.4 for those with the larger seed. The handling stress from the sorting procedures and lower water temperatures may
have potentially resulted in the lower survival rates. Once the smaller seed were removed on day 52 from the culture units, the bed porosity remained steady,
ranging from 0.62 to 0.67. The trial results indicating growth rates, bed porosity, shell length, stocking densities, feeding rations and strategies are summarized in
Table 1.
4. Discussion
