ICES Journal of Marine Science, 63: 326e334 (2006)
doi:10.1016/j.icesjms.2005.10.010
Effects of water quality and stocking density on growth
performance of juvenile cod (Gadus morhua L.)
Björn Björnsson and Sólveig R. Ólafsdóttir
Björnsson, B., and Ólafsdóttir, S. R. 2006. Effects of water quality and stocking density
on growth performance of juvenile cod (Gadus morhua L.). e ICES Journal of Marine
Science, 63: 326e334.
In a 5-month experiment where groups of juvenile cod were reared in a flow-through system
at low density (Group 1) and in a recirculating system at low (Group 2) and high densities
(Group 3), the recirculated water had negative effects on growth rate, nutritional condition,
and mortality. After the first month, mean weight was significantly larger in Group 1 than in
the other two groups. The effect of stocking density on mean weight was not significant until the end of the experiment, when densities were 9.0 and 48.3 kg m3 in Groups 2 and 3,
respectively. Initial mean weight of fish was 37.3 g and at the termination of the experiment
mean weights were 225.2, 181.8, and 167.9 g in Groups 1, 2, and 3, respectively. After 5
months, mean condition factors were 1.074, 0.965, and 0.946, mean liver indices 9.5,
9.0, and 7.6, and mean mortalities over the course of the experiment were 1.0%, 5.1%,
and 2.4% in Groups 1, 2, and 3, respectively. Water temperature (10.4e10.6(C), pH
(7.2e7.4) and oxygen concentration (8e10 mg l1) were similar among groups. Concentration of total ammonia nitrogen (TAN) was similar among groups for the first half of the
experiment (0.3e0.4), but during the last month it was 0.6, 1.3, and 1.5 mg l1 in Groups
1, 2, and 3, respectively. There was a negative correlation (r2 ¼ 0.48, n ¼ 36) between relative growth rate and TAN, suggesting that ammonia may have been a limiting factor in the
recirculating system. The apparent threshold limit of TAN for reduced growth was approximately 1 mg l1.
Ó 2005 International Council for the Exploration of the Sea. Published by Elsevier Ltd. All rights reserved.
Keywords: ammonia, CO2, nitrate, nitrite, oxygen, pH.
Received 13 June 2004; accepted 6 October 2005.
B. Björnsson and S. R. Ólafsdóttir: Marine Research Institute, Skúlagata 4, PO Box 1390,
121 Reykjavı́k, Iceland. Correspondence to B. Björnsson: tel: þ354 575 2000; fax: þ354
575 2001; e-mail: [email protected].
Introduction
In recent years, interest in intensive cod farming (Gadus
morhua L.) has been growing in Norway, Scotland, Canada,
and Iceland. Often, the farming has been based on the ongrowing of wild-caught fish (Jobling, 1988; dos Santos
et al., 1993; Björnsson, 1999). Wild cod, fed on whole or
chopped fish, have shown excellent growth rates (Braaten,
1984; Björnsson et al., 2001; Björnsson and Steinarsson,
2002). One of the obstacles to developing commercial
farming of wild-caught cod has been the difficulty in obtaining large quantities of viable fish. As a result, these
trials have been relatively small. Therefore, most aquaculturists agree that large-scale intensive cod farming must
be based on hatchery-produced cod juveniles.
During the last few years, substantial progress has been
made in developing methods of intense juvenile cod production (Svåsand et al., 2004). Currently, the cost of producing cod from larvae to market-sized fish is too high to
make cod farming profitable. It is expected, however, that
1054-3139/$30.00
the production cost of juveniles will decrease rapidly in
the next few years and, with selective breeding and development of rearing technology, it will be possible to increase
growth rate and growth efficiency sufficiently during
the on-growing phase to make cod farming profitable
(Moksness et al., 2004).
The on-growing part must be carried out in seapens
owing to the high cost of land-based facilities (Knútsson,
1997). The optimal size of juveniles to be stocked in
seapens depends on local conditions. In Norway and Scotland, winter temperatures are much higher (5e6(C) than in
Iceland and Canada (0e2(C). Optimal temperature for
growth is highest for small cod juveniles at about 17(C
for 2 g fish, but it decreases to about 7(C for 2 kg fish
(Björnsson et al., 2001) as the size of cod increases.
Because cod juveniles grow very slowly at temperatures
below 2(C (Björnsson and Steinarsson, 2002), it may be
necessary to grow them in land-based facilities, where temperature can be regulated, for longer periods in Iceland and
Canada than in Norway and Scotland.
Ó 2005 International Council for the Exploration of the Sea. Published by Elsevier Ltd. All rights reserved.
Effects of water quality and stocking density on growth performance of juvenile cod
Because of the high cost of each rearing unit in a landbased facility, it is important to maximize the production
of each unit. It is necessary to heat the ambient seawater to
operate at optimal temperatures, and to use a recirculating
system instead of a flow-through system to reduce heating
costs. Many growth experiments have been carried out in
flow-through systems (e.g. Lambert and Dutil, 2001; Björnsson and Steinarsson, 2002). However, no literature has been
published on the possible negative effects of recirculated
water on the growth rate and well-being of juvenile cod.
To maximize productivity, it is necessary to operate with
the highest possible stocking densities in each rearing unit.
For some species, stocking density appears to be a limiting
factor, with no change in growth rate below a certain
threshold value (Brett, 1979; Björnsson, 1994). The growth
rate of cod weighing about 0.7 kg decreases as stocking
density increases from 2 to 40 kg m3 (Lambert and Dutil,
2001).
In recirculating systems at high stocking densities, ammonia, the main end product of nitrogen metabolism in teleost fish, may reach toxic levels (acute or chronic), unless
the system is equipped with efficient biofilters with bacteria
that break down ammonia to less toxic nitrite (NO
2 ) and
nitrate (NO
3 ). In fish culture systems, ammonia appears
in two forms, un-ionized (NH3) and ionized (NHþ
4 ). The
toxicity of ammonia in aquatic organisms is attributed
mainly to the un-ionized form (Thurston et al., 1981).
The fraction of NH3 of total ammonia nitrogen (TAN) depends largely on pH and, to a lesser extent, on temperature
and salinity (Tomasso, 1994).
In studies of chronic toxicity of ammonia in warm-water
marine fish (Dover sole Solea solea (L.), turbot Scophthalmus maximus (L.), gilthead sea bream Sparus aurata (L.),
European sea bass Dicentrarchus labrax (L.)), the lower
limit for reduced growth rating has been shown to be
between 0.1 and 0.4 mg NH3-N l1 and between 2.5 and
30 mg TAN-N l1 (Alderson, 1979; Wajsbrot et al., 1993;
Person-Le Ruyet et al., 1997; Lemarié et al., 2004). Safe levels
of ammonia for Atlantic salmon (Salmo salar L.) postsmolts in
8e9(C seawater (pH 7.8e7.9) have been reported as
0.01 mg NH3-N l1 and 1 mg TAN-N l1 (Fivelstad et al.,
1995).
The main purpose of the present study was to determine
if juvenile cod, reared in a modern recirculating system,
were growing as fast as juveniles reared in a flow-through
system. A further aim was to examine the effects of stocking density and water quality on the growth rate and wellbeing of juvenile cod.
Methods
The experiment was a cooperative effort of the Icelandic
fishing company, Útgerdarfélag Akureyringa Ltd, and the
Marine Research Institute with the purpose of developing
cod farming in Iceland. The fish and the facility were
327
owned by the company, but the research was supervised
by this study’s first author. The experimental facility, where
the fish were grown, is located on the west coast of a large
fjord, Eyjafjördur, in northern Iceland. The facility consisted of nine circular rearing tanks each 3 m in diameter
and volume 9 m3. The seawater intake was located 800 m
from the shore at a depth of 50 m. In three of the tanks
(2, 5, 8), a flow-through system (0.33 l s1 in each tank)
was used, whereas a recirculating system was employed
in the six remaining tanks (4.2 l s1 in each tank). The seawater leaving the bottom drain of all nine rearing tanks was
collected in a central drainpipe and run through a drum filter (40 mm) before entering two 10-m3 biofilters with a 30cm thick, 7-m2 layer of pumice to break down ammonia. Finally, the filtered water was run through a large aerator to
remove CO2 before entering the rearing tanks (1, 3, 4, 6,
7, 9). Near the top of the 175-cm-tall aerator, filled with
plastic packings, the water was spread with a 2.8-m2-perforated plastic sheet. Fresh air (200 l s1) was pumped
through the aerator. The biofilters were cleaned by vigorous
air bubbling and drainage once or twice a month during the
first half, and once or twice a week during the second half
of the experiment. For a detailed description of the seawater
system see Björnsson (2004).
The experiment lasted from 8 January to 12 June 2003.
Cod juveniles hatched from eggs and obtained from wild
cod near the southwest coast of Iceland in April 2002
were used in the experiment. They were reared on rotifers
and Artemia until weaning on commercial dry feed, in
July 2002 at the Marine Research Laboratory, Grindavı́k.
About 12 000 juveniles were transported to the experimental site on 16 December 2002, and randomly assigned to the
rearing tanks: 500 in Tanks 2, 5, and 8 (Group 1: low density, flow-through), 500 in Tanks 1, 4, and 7 (Group 2: low
density, recirculation), and 2900 in Tanks 3, 6, and 9
(Group 3: high density, recirculation). The initial stocking
densities were 2.0, 2.0 and 11.2 kg m3 (54 and 300 fish m3),
and final stocking densities were 12, 9, and 48 kg m3 in
Groups 1, 2, and 3, respectively.
The cod were handfed to satiation three times a day
(8:00e9:00, 13:00e14:00, and 16:00e17:00) on commercial dry feed [the first 5 months on DANEX 1562 (15%
fat and 62% protein), and the last month on a turbot feed
produced in Iceland (a diet from Fódurblandan Ltd with
similar proportions of macronutrients)]. Almost equal temperatures were maintained in all tanks, slightly lower than
the optimal temperature for growth rate of cod (Björnsson
et al., 2001). For most of the experiment the temperature
remained 11e12(C, but during the final 4-week period, it
was 8e9(C. Pure oxygen was bubbled in each tank to
maintain approximately 100% saturation. From 31 March,
pH was maintained at approximately 7.2 by continuous
dripping of an NaOH solution (167 mg l1) into the recirculating system at the bottom of the large aerator. Temperature and oxygen concentrations were measured daily
between 8:00 and 9:00. Oxygen concentration and pH
B. Björnsson and S. R. Ólafsdóttir
were measured three times a week in late afternoon between 16:00 and 17:00. If the daily measurements showed
that the oxygen concentration in any tank was too low or
too high, the flow of oxygen was immediately readjusted.
Lights remained on 24 h a day (40e80 lux). The mean
salinity in the incoming water was 34.5.
Water samples from the surface of each tank were taken
in the morning (8:00e10:00) five times during the experiment: 25 February, 20 March, 22 April, 15 May, and 3
June 2003. They were chilled and transported the same
day to the Marine Research Institute in Reykjavı́k where
they were analysed the next day. Standard analytical methods were used to measure total ammonia nitrogen (TAN),
nitrite, nitrate, total carbonate, and total suspended matter
(TSM) (Grasshoff et al., 1983). Total carbonate was measured only in samples taken in February, March, April,
and June, and TSM in samples taken in February, April,
and June. Concentration of dissolved CO2 was calculated
from measurements of total carbonate, temperature, pH,
and salinity (Lewis and Wallace, 1998).
The fish were weighed once a month after fasting for 1 day.
They were gathered in the tank using a flexible frame with net
walls, and 100 fish were randomly sampled from each tank
with a large handnet, drained on a moist towel, and individually weighed (W, in g). During the final weighing, the
lengths (L, in cm) of all the sampled fish were also measured
and, of these, ten fish were collected randomly from each
tank to measure gutted weight (W#, in g) and liver weight
(Li, in g). Fish from Tanks 1, 2, and 3 were measured on 10
June; fish from Tanks 4, 5, and 6 on 11 June; and fish from
Tanks 7, 8, and 9 on 12 June. The final feeding day was 8
June. Two condition factors were calculated, one based on
whole weight (KW), and the other on gutted weight (KW#):
KW ¼ 100W/L3 and KW# ¼ 100W#/L3. Liver index (LI) was
calculated as: LI ¼ 100Li/W#. The fish were inspected for injuries and abnormalities, such as ‘‘popeye’’ and cataract (see
Björnsson, 2004). Specific growth rate (SGR) was calculated
as follows: SGR ¼ 100(Wn W1)/d where Wn was the final
and W1 the initial mean weight of the fish, and d the number
of days in the growth period. No correction was made for
weight reduction of unfed fish measured on 11 and 12 June
(equal effect for all groups). Relative growth rate in each of
the nine tanks was calculated as a percentage of maximum
daily weight gain (g fish1 day1) in each of the five growth
periods to remove the time- and size-related changes in
growth rate. ANOVA (nested and Tukey’s honest test) was
used to compare body weight, condition factor, and liver index within and between groups, chi-square (c2) to compare
natural mortality between groups, and regression analysis
to study the possible effects of TAN and NO2 on relative
growth rate.
both at low density (Group 2) and high density (Group 3)
(Figure 1, Table 1). There was never a significant difference
in mean weight between the three replicates in each group
(nested ANOVA). At the start of the experiment, there were
no significant differences in mean weight among the three
groups but, by the second weighing, the mean weight was
significantly larger in Group 1 than in the other two groups,
and by the fourth weighing, the gap had increased further.
There was no significant difference ( p < 0.05) between
Groups 2 and 3 until the final weighing (ANOVA Tukey’s
honest test). Thus, it is clear that water quality had a much
greater effect than stocking density on growth rate. The
stocking densities were initially 2.0, 2.0 and 11.2 kg m3
and, at the end of the experiment, 11.8, 9.0, and
48.3 kg m3 in Groups 1, 2, and 3, respectively.
There was a decrease in specific growth rate (SGR) with
time, from 1.3e1.7 in growth period 1 to 0.5e0.8% d1 in
growth period 5 (Figure 2a), and an increase in weight gain
with time, from 0.6e0.8 in growth period 1 to
0.8e1.5 g fish1 d1 in growth period 5 (Figure 2b). During
the final three growth periods, the growth difference between the fish in the flow-through system (Group 1) and
the recirculating system (Groups 2 and 3) was more pronounced when weight gain was used instead of SGR.
The cod reared in the flow-through system were heavier
for a given length than the cod in the recirculating system.
At the end of the experiment, the mean condition factor
(KW) was much higher for the cod in Group 1 (1.074) than
in Group 2 (0.965) and Group 3 (0.946) (Table 2). The difference between Groups 2 and 3 was only marginally significant (ANOVA Tukey’s honest test, p ¼ 0.04). A similar
trend was found for the condition factor based on gutted
weight (KW#) (Table 2). There was a significant difference
in liver index between Groups 1 and 3 (Tukey’s honest
test, p ¼ 0.004) and a nearly significant difference between
Groups 2 and 3 (Tukey’s honest test, p ¼ 0.06) (Table 2).
Specific growth rate for the whole experimental period
was highest in Group 1 and lowest in Group 3 (Table 2).
250
200
Mean weight (g)
328
150
100
50
0
0
Results
Cod reared in the flow-through system (Group 1) grew
much faster than cod reared in the recirculating system,
50
100
150
200
Time (days in exp.)
Figure 1. Mean weight of juvenile cod as a function of time in the
period from 8 January to 10 June 2003 for Groups 1 (>), 2 (-),
and 3 (:). Each data point is a mean of 300 measurements.
Effects of water quality and stocking density on growth performance of juvenile cod
329
Table 1. Mean weight and standard deviation of cod in Group 1 (flow-through system, low density), Group 2 (recirculating system, low
density), and Group 3 (recirculating system, high density).
Group 1
Date
Tank 2
(A) Mean weight
8.01.2003
6.02.2003
6.03.2003
10.04.2003
8.05.2003
10.06.2003
of cod (g)
39.0
61.0
86.6
135.3
166.1
211.7
Group 2
Group 3
Tank 5
Tank 8
Tank 1
Tank 4
Tank 7
Tank 3
Tank 6
Tank 9
39.0
63.6
94.1
143.3
182.1
234.0
35.8
63.3
89.2
128.2
176.2
230.0
37.6
56.3
83.4
125.2
150.4
177.6
35.8
54.9
81.5
122.9
147.4
184.5
36.2
56.0
86.5
125.9
149.8
183.3
38.8
55.8
83.2
118.9
137.4
156.2
36.8
55.8
82.8
126.2
149.7
175.4
36.7
52.5
84.5
115.4
136.8
172.2
(B) Standard deviation of weight (g)
8.01.2003
14.7
12.1
6.02.2003
18.5
15.1
6.03.2003
24.6
25.3
10.04.2003
38.9
38.7
8.05.2003
41.9
51.9
10.06.2003
66.4
57.8
11.8
18.1
27.1
38.5
53.2
72.1
10.7
17.7
25.5
37.4
43.3
51.0
11.3
15.6
25.6
33.8
44.5
47.1
11.4
17.8
23.3
31.3
50.0
56.5
12.0
17.4
21.3
32.8
34.7
44.5
11.3
17.3
23.6
39.5
42.5
52.8
9.8
15.8
22.6
36.8
44.9
49.1
Mortality in the experiment was much lower in Group 1 (15/
1498, i.e. 1.0%) than in Group 2 (75/1474, i.e. 5.1%) and
Group 3 (202/8554, i.e. 2.4%) (Table 2), a highly significant
difference in all cases (chi-square test, c2 ¼ 11.2e42.3).
2.0
1.8
a
Group 1
Group 2
Group 3
SGR (% day-1)
1.6
1.4
1.2
1.0
0.8
0.6
0.4
0.2
0.0
2.0
Weight gain (g day-1)
1.8
b
1.6
1.4
1.2
1.0
0.8
0.6
0.4
0.2
0.0
1
2
3
4
5
Growth periods
Figure 2. Growth rates in the five different growth periods (Jan
8eFeb 6, Feb 6eMarch 6, March 6eApril 10, April 10eMay 8,
May 8eJune 10) for Groups 1, 2, and 3; specific growth rate
(SGR) (a) and weight gain (b). Each column is a mean of three replicates with coefficient of variation as 10.9% for SGR and 11.7%
for weight gain.
The mean temperature was only slightly lower in Group
1 (10.4(C) than in the other two groups (10.6(C) (Table 3).
The mean oxygen concentration measured in the morning
was slightly higher in Group 1 (9.7 mg l1, 108% saturation) than in Group 2 (8.9 mg l1, 99% saturation) and in
Group 3 (8.3 mg l1, 93% saturation). Owing to the slow
flow rate of seawater in Group 1, it was difficult to regulate
the oxygen concentration in those tanks, resulting in a more
variable concentration than in the tanks that were part of the
recirculating system. The mean oxygen values from late afternoon were slightly lower than the morning values:
9.0 (100% saturation), 8.4 (94% saturation), and 7.9 mg l1
(88% saturation) for Groups 1, 2, and 3, respectively (Table 3).
There was only a minor difference in mean pH between the
groups: 7.4, 7.3, and 7.2 for Groups 1, 2, and 3, respectively
(Table 3). Total suspended matter was low in all tanks
(2e7 mg l1) (Table 3).
On the first two sampling dates, total ammonia nitrogen
(TAN) was low and similar for all three groups,
0.3 mg l1, but the difference between groups increased in
the three final sampling dates (Figure 3a). The highest
mean values measured on the fourth sampling date (15
May) were 0.7, 1.4, and 1.6 mg l1 in Groups 1, 2, and 3, respectively. The TAN concentrations were slightly lower on
the final sampling date (June 3). The average of the measurements from 15 May and 3 June was used to represent the
TAN during the final growth period (8 May to 11 June):
0.6, 1.3, and 1.5 mg l1 in Groups 1, 2, and 3, respectively.
The concentration of nitrite was always much lower in
Group 1 than in Groups 2 and 3, but no difference existed
between Groups 2 and 3 (Figure 3b). In all the groups, there
was a rapid increase in nitrite concentration with time,
reaching a peak on the third sampling date, followed by
330
B. Björnsson and S. R. Ólafsdóttir
Table 2. Condition factor based on whole weight (KW) and gutted
weight (KW#), liver index (LI) at the end of the experiment, and
specific growth rate (SGR) and mortality (M ) in the whole experiment (% of initial number of fish) in Group 1 (flow-through system, low density), Group 2 (recirculating system, low density),
and Group 3 (recirculating system, high density).
Group 1
Group 2
Group 3
KW
n
Mean
s.d.
Minimum
Maximum
330
1.074
0.124
0.742
1.666
330
0.965
0.093
0.745
1.261
330
0.946
0.081
0.724
1.191
KW#
n
Mean
s.d.
Minimum
Maximum
30
0.889
0.106
0.735
1.049
30
0.835
0.075
0.730
0.972
30
0.821
0.064
0.677
0.993
LI
n
Mean
s.d.
Minimum
Maximum
30
9.5
2.9
3.5
14.6
30
9.0
2.1
4.8
12.2
30
7.6
1.5
4.8
10.9
SGR
n
Mean
s.d.
Minimum
Maximum
3
1.164
0.056
1.105
1.216
3
1.049
0.030
1.015
1.072
3
0.967
0.055
0.910
1.021
M
n
Mean
s.d.
Minimum
Maximum
3
1.0
0.0
1.0
1.0
3
5.1
2.2
2.6
6.5
3
2.4
0.2
2.2
2.6
a decline. The concentration of nitrate always remained low
in Group 1 (<0.2 mg N l1), but was high in Groups 2 and 3
on the first three sampling dates (4.7 mg N l1), followed by
a gradual decline until the end of the experiment (Figure 3c).
The high concentration of nitrate in the recirculation system
relative to TAN and nitrite suggests that the two biofilters
were functioning reasonably well. In the flow-through system, almost no nitrate was produced, but there was a substantial amount of nitrite produced, perhaps by bacteria living on
the surface of the rearing tanks, aided by the long residence
time of the water (471 min) (Björnsson, 2004). The large increase in TAN and the large decrease in the concentration of
nitrite and nitrate during the latter part of the study (Figure 3)
may have been caused by a reduction in the efficiency of the
biofilters (perhaps due to excessive cleaning). Concentration
of CO2 increased with time in all three groups (Figure 3d),
mainly because of an increase in fish biomass. CO2 was
lowest in Group 1 (from 3.6 to 8.7 mg l1) and highest in
Group 3 (from 7.5 to 13.4 mg l1).
There was a highly significant negative correlation
(r2 ¼ 0.48, n ¼ 36, p < 0.001) between relative growth
rate and TAN, ranging from 0.2 to 1.6 mg l1 (Figure 4).
The relationship suggests that juvenile cod are sensitive
to ammonia concentrations above 0.7 mg N l1. However,
the threshold limit for reduced growth must be somewhat
higher, since these were morning values, which must be
lower than evening values (Burel et al., 1996). In the regression analysis, only values from growth periods 2e5
were used (Table 2b), since no water samples were taken
during the first growth period. There was also a significant
negative correlation between relative growth rate and nitrite
concentration (r2 ¼ 0.27, n ¼ 36, p < 0.01).
Discussion
The results indicate that water quality had greater effects
than stocking density on growth rate, nutritional condition,
and natural mortality. Poor water quality may have resulted
in lower mean weight in the recirculating system (Groups 2
and 3), than in the flow-through system (Group 1). This was
true even in the first growth period, when stocking density
in the high density group was still low (15 kg m3). Ammonia concentration was not measured during the first growth
period, but it may have been high in Groups 2 and 3, while
the biofilters were starting up. Furthermore, there was not
a significantly lower mean body weight in the high density
group than in the low density group in the recirculating system until the conclusion of the experiment, when stocking
density in Group 3 had reached 48 kg m3. During the
length of the experiment, the mean weight gain was only
10.2% lower in Group 3 than in Group 2. As total ammonia
nitrogen (TAN) in the final growth period was 11.9% lower
in Group 2 than in Group 3, it is possible that this slight difference in growth rate was caused by a difference in water
quality rather than stocking density.
Over the course of the experiment, the gain in mean
weight was 22.3% lower in Group 2 than in Group 1. Presumably, this was caused by a difference in water quality,
perhaps as a result of the difference in TAN which, in the
final growth period, were 0.6, 1.3, and 1.5 mg l1 in Groups
1, 2, and 3, respectively. All water samples were taken in
the morning, just before the first-feeding, and as a result
of the daily feeding routines, all measurements of nutrients
must be minimum values (Burel et al., 1996). Diurnal
changes in TAN were not measured in this experiment.
However, in a similar experiment with juvenile cod in
high-quality seawater (W ¼ 160 g, T ¼ 7.1(C, pH ¼ 7.2,
feeding times: 9:30e10:30 and 15:30e16:30), TAN was
lowest (0.406 mg l1) at 12:00 and highest (0.587 mg l1)
at 24:00 (unpublished results). Thus, it is likely that the
midnight TAN values in the final growth period may
have been approximately 0.9, 1.9, and 2.2 mg l1 in Groups
1, 2, and 3, respectively.
The negative correlation between relative growth rate
and TAN indicates that ammonia concentration may have
been a limiting factor of growth in the study. As the diurnal
Effects of water quality and stocking density on growth performance of juvenile cod
331
Table 3. Temperature (T, (C), oxygen (O2, mg l1), pH, and total suspended matter (TSM, mg l1) in Group 1 (flow-through system, low
density), Group 2 (recirculating system, low density), and Group 3 (recirculating system, high density). Some measurements were made in
the morning and some in the late afternoon.
Group 1
Group 2
Group 3
Tank 2
Tank 5
Tank 8
Tank 1
Tank 4
Tank 7
Tank 3
Tank 6
Tank 9
T (8:00e9:00)
n
Mean
s.d.
126
10.4
1.5
126
10.4
1.6
126
10.4
1.6
126
10.5
1.3
126
10.6
1.3
126
10.5
1.3
126
10.6
1.3
126
10.6
1.3
126
10.6
1.4
O2 (8:00e9:00)
n
Mean
s.d.
133
10.3
2.7
133
9.1
2.0
133
9.7
2.7
133
8.9
0.8
133
8.9
0.8
133
8.8
0.8
133
7.9
0.9
133
8.5
1.1
133
8.5
1.2
O2 (16:00e17:00)
n
Mean
s.d.
37
10.4
3.4
37
8.3
1.4
37
8.4
1.8
37
8.5
1.0
37
8.6
0.7
37
8.2
0.8
37
7.4
1.0
37
8.2
1.4
37
8.0
1.3
pH (16:00e17:00)
n
Mean
s.d.
40
7.3
0.1
40
7.5
0.1
40
7.4
0.1
40
7.3
0.1
40
7.4
0.1
40
7.3
0.1
40
7.2
0.1
40
7.2
0.1
40
7.1
0.1
TSM (8:00e10:00)
n
Mean
s.d.
3
3.8
1.1
3
3.9
0.1
3
4.5
1.0
3
3.4
1.1
3
3.6
1.4
3
4.5
2.0
3
4.2
0.9
3
4.1
1.4
3
4.8
1.0
changes in TAN were not measured, it is not possible to estimate with great accuracy the threshold limit for reduced
growth from the present results, but it may have been
close to 1 mg l1 TAN, which corresponds to about
0.003 mg NH3-N l1 at 10.5(C and pH 7.2 (Bower and
Bidwell, 1978; Spotte, 1979; Johansson and Wedborg,
1980). A truly safe, maximum acceptable concentration
of un-ionized, or of total ammonia, for fish culture systems
is not known (Meade, 1985). However, as a general rule,
warm-water fish are more tolerant of ammonia toxicity
than cold-water fish, and freshwater fish are more tolerant
than saltwater fish (Timmons et al., 2002). For trout, it
has been recommended that TAN be kept below 1 mg l1
in recirculating systems (Timmons et al., 2002). Thus,
cod might be expected to be relatively sensitive to ammonia
compared with many other species of fish.
Recently, it has been reported that the threshold limits
for reduced growth of juvenile cod (17e70 g) are
0.06 mg NH3-N l1 and 3.4 mg l1 TAN (Foss et al.,
2004), which is much higher than those in the present study
(0.003 mg NH3-N l1 and 1 mg l1 TAN), a 3-fold difference in TAN and a 20-fold difference in un-ionized ammonia, the most toxic form of ammonia. There are a few
possible explanations for the discrepancy between the two
studies.
(i) NH3 becomes more toxic to fish as the pH of the water
decreases (Thurston et al., 1981; Tomasso, 1994);
the pH was 8.0 in the study by Foss et al. (2004)
and 7.2 in the present study. Also, it has been suggested that the excretion of CO2 may depress pH at
the gill surface to cause the actual NH3 exposure
concentration in high pH water to be much lower
than the NH3 concentration of the bulk water (Meade,
1985).
(ii) Fish may be more sensitive to a given ammonia value,
which fluctuates within the day (diel change) or from
day to day compared with a constant value (Handy
and Poxton, 1993; Tomasso, 1994). In the experiment
by Foss et al. (2004), the ammonia concentration
must have remained nearly constant, because of the
high water flow relative to the biomass
(>0.9 l min1 kg1), and because most of the ammonia was of exogenous origin. In the present study,
the fish themselves produced all the ammonia in the
system and, because the feeding time only lasted
from morning until afternoon, the ammonia concentration must have been higher in the evening than in
the morning when water samples were taken (Brett
and Zala, 1975; Bergheim et al., 1991; Steinarsson
and Moksness, 1996; Burel et al., 1996; Forsberg,
1996; Hargreaves and Kucuk, 2001). Also, there
must have been large daily variations in ammonia excretion as a result of daily changes in food intake.
(iii) Large juveniles may be more sensitive to ammonia
than small juveniles. Person-Le Ruyet et al. (1997) reported that the threshold limits for reduced growth of
14 and 23 g turbot were 0.41 and 0.21 mg NH3-N l1,
respectively, compared with 0.10 mg NH3-N l1
(2.5 mg l1 TAN) in 104-g turbot. Foss et al. (2004)
used juveniles with mean weights of 17e70 g, compared with 38e225 g in the present study.
B. Björnsson and S. R. Ólafsdóttir
1.8
1.6
1.4
1.2
1
0.8
0.6
0.4
0.2
0
a
0.3
Nitrite (mg N l-1)
Relative growth rate (% of max)
TAN (mg N l-1)
332
b
0.25
120
y = 95.6 - 33.0x
r 2 = 0.48
100
80
60
40
20
0
0.0
0.2
0.5
1.0
1.5
2.0
TAN (mg Nl-1)
0.15
Figure 4. Relative growth rate of juvenile cod in each tank (% of
maximum weight gain in each growth period) as a function of total
ammonia nitrogen (TAN) in the rearing water.
0.1
0.05
0
Nitrite (mg Nl-1)
6
co-vary with ammonia in a recirculation system at
high stocking density, since oxygen consumption
tends to increase with ammonia concentrations
(Adams et al., 2001).
c
5
4
3
2
1
CO2 (mgl-1)
0
16
14
12
10
8
6
4
2
0
d
0
50
100
150
200
Time (days in exp.)
Figure 3. Time related changes in total ammonia nitrogen (TAN)
(a), nitrite (b), nitrate (c), and carbon dioxide (CO2) (d) concentration from 25 February to 3 June 2003 in Groups 1 (>), 2 (-), and
3 (:). Each data point is a mean of three replicates with coefficient
of variation as 8.0% for TAN, 12.0% for nitrite, 4.0% for nitrate,
and 11.4% for CO2.
(iv) Ammonia may be more toxic at lower than at higher
temperatures. Knoph (1992) found for Atlantic salmon
parr that the 96-h-LC50 (lethal concentration) of unionized ammonia (in mg NH3-N l1) ranged from
0.031 at 2(C to 0.111 at 17(C at pH 6.0, and from
0.030 at 2(C to 0.146 at 12(C at pH 6.4. Foss et al.
(2004) reared cod juveniles at 13.1(C, but the fish
in the present study were reared at 8.8(C during the
final growth period (Groups 2 and 3), when ammonia
concentration was highest.
(v) In recirculating systems, ammonia could interact with
several other environmental factors, e.g. dissolved oxygen (Wajsbrot et al., 1991; Foss et al., 2003b), salinity (Alabaster et al., 1979; Sampaio et al., 2002),
nitrite (Lemarié et al., 2004), and carbon dioxide
(Randall and Wright, 1989). Carbon dioxide may
Apparently, the slight difference in temperature, pH, oxygen concentration, and total suspended matter (TSM) between the flow-through and recirculation system cannot
explain the observed difference in growth rate. The mean
temperature was 0.2(C, closer to optimal temperature
(Björnsson et al., 2001) in Group 3, where reduced growth
rate was seen (in the final growth period the difference
was 0.6(C). The mean pH was 7.2 in Group 3 and 7.4 in
Group 1, a difference resulting in lower un-ionized ammonia in Group 3. The mean oxygen concentration was above
7.4 mg l1 (82% saturation) in all the tanks, which is well
above the threshold limiting growth of fish (Brett, 1979;
Pedersen, 1987; Pichavant et al., 2001). The low growth
rate in the recirculating system cannot be explained by
a large variation in oxygen concentration, since the largest
fluctuations and lowest oxygen values were found in the
flow-through system where the fish exhibited the highest
growth rates; 20% higher than predicted by the growth
model of Björnsson and Steinarsson (2002). TSM was
similar in all the tanks, and much lower than the
10 mg l1 recommended by Timmons et al. (2002).
The maximum nitrite concentration in the experiment,
0.27 mg N l1, surpassed the recommended target value for
trout, 0.1 mg N l1, in low salinity water (Timmons et al.,
2002). The amount of nitrite entering the blood depends on
the ratio of nitrite to chloride in the water, in that increased
levels of chloride reduce the amount of nitrite absorption
(Tomasso, 1994; Timmons et al., 2002). Because the present
experiment was carried out in seawater with high concentrations of chloride, it is unlikely that the nitrite concentrations
had any effect on the growth rate and well-being of cod. For
fish in seawater, the 48e96-h-LC50 values range between 86
and 812 mg NO2-N l1 (Tomasso, 1994), i.e. 2e3 orders of
magnitude higher than in the present experiment. Nitrate is
the least toxic of the nitrogen compounds, with 96-h-LC50
Effects of water quality and stocking density on growth performance of juvenile cod
values of 6000 mg NO3-N l1 for channel catfish, Ictalurus
punctatus (Rafinesque) (Colt and Tchobanoglous, 1976),
i.e. 3 orders of magnitude higher than in the present
experiment.
In the present study, CO2 concentration increased with
time as a result of increased biomass of fish, but never exceeded 13 mg l1, and therefore, it is unlikely that this water quality parameter was a factor limiting growth. No
reduction in the growth rate of rainbow trout (Salmo gairdneri Richardson) was found in fish exposed to 24 mg l1
compared with 12 mg l1 of carbon dioxide (Smart et al.,
1979). For Atlantic salmon postsmolts in seawater (at
15e16(C and 6e7 mg l1 of oxygen), a significant reduction in growth rate was not found between 11 and 26 mg l1
of carbon dioxide (Fivelstad et al., 1998). No differences in
growth rates of spotted wolffish (Anarhichas minor Olafsen) were seen in the range 1.1e33.5 mg CO2 l1 (Foss
et al., 2003a). An upper limit of 15e20 mg l1 of carbon
dioxide as a steady state maximum for finfish has been recommended, although this recommendation is poorly supported by research (Timmons et al., 2002). Thus, it seems
unlikely that the observed growth reduction in cod reared
in the recirculating system was caused by elevation in CO2.
Cataract was more common in the recirculating system
(Groups 2 and 3) than in the flow-through system (Group
1), owing to UV treatment of the water (Björnsson,
2004). The tank with the water intake closest to the UV
light was most affected: 39% of the fish developed cataract
compared with 5% in the tank located farthest away from
the UV light. The frequency of cataract decreased exponentially with increased post UV-treatment time. Perhaps
ozone or other short-lived photoproducts formed by the
UV radiation caused the development of cataract in cod.
No clear relationship between frequency of cataract and
growth rate or mortality (within each group) was found
(Björnsson, 2004).
The present results suggest that juvenile cod can be
reared at high stocking densities (>40 kg m3) without reduction in growth rate, provided that water quality is acceptable. However, Lambert and Dutil (2001) found that
growth rate of cod weighing about 0.7 kg decreased with
stocking density from 2 to 40 kg m3. In their study, there
was a constant flow of 20 l min1 in each tank through
a semi-recirculating system, where 100% of the water
was renewed every 5e6 h. The concentrations of ammonia
or carbon dioxide in their study were not published, but it
must have increased with stocking density and, perhaps,
surpassed a limiting value at the highest densities.
Acknowledgements
The rearing of the fish was funded by the fishing company
Útgerdarfélag Akureyringa Ltd. Dr Ragnar Jóhannsson,
Technological Institute of Iceland, Óttar Már Ingvason,
Útgerdarfélag Akureyringa Ltd, and Asgeir
Gudnason,
333
Stofnfiskur Ltd, designed the recirculating system and crit
ically read the manuscript. Asgeir
supervised daily feeding
of the fish and routine measurements of temperature, pH,
and oxygen. Useful suggestions concerning the manuscript
were made by Valdimar I. Gunnarsson, Marine Research
Institute, and two anonymous reviewers.
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