Journal of Fisheries science and Technology - No.4/2018

reatments, as well as between 
the MD and the LD treatments (Table 1). 
Sediment pH was similar among treatments, 
and relatively stable throughout the experiment. 
Sediment Chl a concentration was signifi cantly 
higher in the HD treatment than those in the 
MD and the LD treatments. Mean value of pore 
water TAN was signifi cantly higher in the HD 
treatment than that in the LD treatment. There 
was no signifi cant difference in mean pore 
water SRP among treatments (Table 2).
The signifi cant differences in some major 
Table 1: Water parameters in the experimental treatments of rabbitfi sh culture at different stocking 
densities. Values are means ± SD.
Mean values in a same row with different superscript letters are signifi cantly different (P<0.05).
Table 2: Sediment parameters in the experimental treatments of rabbitfi sh culture at different stocking 
densities. Values are means ± SD.
Mean values in a same row with different superscript letters are signifi cantly different (P<0.05).
environmental parameters between the HD 
and the LD treatments, (Table 1&2) indicated 
the effects of rabbitfi sh stocking density on 
environmental variation in the culture tanks. 
These effects were possibly derived from the 
amount of food feeding daily and rabbitfi sh 
activities. Boyd and Tucker (1998) stated that 
most of the feed were eaten directly by fi sh, but 
usually only 10 – 30% of phosphorus (P) and 
20 – 40% of nitrogen (N) applied in feed were 
retained by cultured animals. The remainder of 
the N and P entered pond ecosystems in faeces 
or other metabolic products. Depending on 
the species and culture techniques, up to 85% 
of P and 52 – 95% of N input into a marine 
fish culture system as feed might be lost into 
the environment through feed wastage, fish 
excretion, faeces production and respiration, 
and some of 21% of N and 53% of P of feed 
input accumulated in the bottom sediments 
(Wu, 1995). N in sediment organic matter may 
be mineralized to ammonia and recycled to 
the pond water. P released by decomposition 
of organic matter in pond bottoms is rapidly 
adsorbed by sediment and little of it enters the 
water (Boyd et al., 2002). As the experiment 
was carried out in the closed tanks, all 
released waste and nutrients were retained 
and accumulated in the water columns and 
sediments over the course of the experiment. 
The accumulation of waste and nutrients led 
to increasing and variation of some of the 
environmental parameters in the culture tanks, 
especially in the HD treatment. The high 
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Journal of Fisheries science and Technology No. 4 - 2018
increases of TAN and SRP in the HD treatment 
were probably derived from larger quantity of 
waste, fi sh excretion, nutrients loading from 
larger amount of feed used in comparison 
with the lower quantities in the MD and the 
LD treatments. High concentrations of TAN 
and SRP might bring about well development 
of phytoplankton and microphytobenthos in 
water column and sediment (Table 1&2). In 
aquaculture ponds, N and P are the two most 
important nutrients because they are often 
present in short supply and limit phytoplankton 
growth (Boyd, 1998). The nutrient 
concentrations likely increased following 
the stocking density, and thus got the highest 
values and wide ranges of variations in the HD 
treatment (Table 1&2). However, these values 
still lied in acceptable ranges for ammonia, 
NH+4 0.2 - 2 mg.L
-1 (14.3 – 143.0 µM), NH3< 
0.1 mg.L-1 (7.1 µM), and phosphorus, 0.005 – 
0.2 mg.L-1 (0.2 – 6.5 µM) in pond aquaculture 
water (Boyd, 1998). Notably, the present 
experiment was conducted in a closed system 
without water exchange, so nutrients released 
by feed loading and metabolic products would 
be accumulated within the tanks that probably 
led to degradation of water quality and then 
effects on rabbitfi sh growth and survival. 
2. Rabbitfi sh growth performance
There was no signifi cant difference 
in rabbitfi sh growth performance among 
treatments. Fish SR was 100% in the LD 
and MD treatments, while fi sh mortality 
strongly occurred in one of replicate of the HD 
treatment. Rabbitfi sh yield was signifi cantly 
greater in the MD and the HD treatments 
than that in the LD treatment, but it was not 
signifi cantly different between the MD and the 
HD treatments. Food conversion ration (FCR) 
was not signifi cantly different between the MD 
and the LD treatments (Table 3).
Table 3: Growth performance of rabbitfi sh cultured at different stocking densities.
Mean values in a same row with different superscript letters are signifi cantly different (P<0.05).
(*): FCR could not be calculated for the high density treatment because of negative weight gain in a replicate where high mortality occurred.
There was no signifi cant difference in 
rabbitfi sh survival and growth performance 
among all treatments, indicating that stocking 
densities at tested levels had no negative effect 
on rabbitfi sh survival and growth. Similar results 
were recorded by other authors (Yousif et al. 
2005; Saoud et al. 2008). Stocking density may or 
may not cause adverse effects on fish survival and 
growth, depending on the species of fish being 
reared and their development stages (Jorgensen 
et al. 1993, El-Sayed 2002). Since rabbitfi sh are 
schooling fish (Lam, 1974) and have tolerance 
of overcrowding (Ben-Tuvia et al., 1973), little 
competitive behaviour is expected among 
individuals reared at high densities. 
Rabbitfi sh mortality occurred in one of four 
Journal of Fisheries science and Technology No. 4 - 2018
NHA TRANG UNIVERSITY • 107
replicates of the HD treatment without known 
apparent reason. This phenomenon happened 
near the end of the experimental period when 
phytoplankton was blooming in the tank as Chl 
a concentration reached 179.2 µL-1. The toxic 
gas, such as NH3, was lower than lethal level 
for fi sh (TAN 0.5 – 1.33 mg.L-1, and NH3 0.02 
– 0.09 mg.L-1, which was probably not a reason 
of rabbitfi sh mortality. But this concentration of 
ammonia could damage gills and reduce growth 
of fi sh (Lazur, 2007). 
An increase in stocking density is desirable 
since generally reduce production costs per 
culture area (Huguenin, 1997). However, as 
biomass increases, so does the quantity of feed 
offered, resulting in potential eutrophication and 
oxygen concentration depletion. The results of 
this study showed that stocking density had no 
directly negative effect on growth and survival 
of Siganus lineatus by competing among 
individuals. High stocking density (in this 
experiment, 21 fi sh.m-2), however, might cause 
high environmental variability, as a consequence 
that adversely affects on fi sh performance. At 
low density (7 fi sh.m-2), the environment was 
well maintained, but low yield was produced. 
Stocking density at 14 fi sh.m-2 seemed to be more 
suitable for rabbitfi sh rearing in a closed system, 
produced a relative high yield without widely 
environmental variations. However, further 
researches need to be carried out for longer 
period of culture with different stocking densities 
at various size groups of rabbitfi sh to determine 
optimal stocking density and size to optimize high 
production versus low environmental changes in 
a closed system. 
IV. CONCLUSION
The results showed that goldlined rabbitfi sh 
S. lineatus can well adapt and grow in a closed 
culture system. The fi sh has little competitive 
behavior among individuals when stocked at 
size and density of 5.7 g, 7 – 21 fi sh.m-2. The 
density has no effect on growth performance of S. 
lineatus, but when increase stocking density from 
7 to 14 fi sh.m-2 can elevate harvested yield. The 
environmental quality can be adversely affected 
as increasing stocking density (7 – 21 fi sh.m-2), 
leading to environmental deterioration by 
potential eutrophication, high water and sediment 
nutrient concentrations and phytoplankton bloom. 
The factors associated with hyper - eutrophication 
could cause fi sh mortality and reduce growth. 
ACKNOWLEDGEMENTS
We are very grateful to the laboratory technical 
staff at IFREMER, IRD (LAMA) and New 
Caledonia University for their help in sample 
analysis. This study was supported by grant from 
the South Province of New Caledonia and carried 
out at the IFREMER Saint-Vincent Aquaculture 
Research Station and the New Caledonia 
University. I would like to especially thank Pr. 
Yves Letourneur, Dr. Hugues Lemonnier and 
Dr. Sebastien Hochard, who provided me many 
helps to implement the experiment and valuable 
comments during working.
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