encouraged efforts to culture this species as a cheap protein source in this region which Ž
. Ž
. includes China Menzel, 1988 , Singapore Cheong and Lee, 1981 , the Philippines
Ž .
Ž .
Ž Walter, 1982 , Thailand Chalermwat and Lutz, 1989 and India Rajagopal et al.,
. Ž
1998 . In Singapore, P. Õiridis accounted for 70.4 of aquaculture production Chou .
and Lee, 1997 . After China, Thailand is the second largest producer of mussels in Asia with nearly 35,000 ha of coastal area that are suitable for mollusc culture, and P. Õiridis
Ž yields the highest net profit of any bivalves cultured in the country Chalermwat and
. Lutz, 1989 . In Hong Kong, P. Õiridis is distributed widely from oceanic waters to
estuarine waters. It is a dominant intertidal and subtidal species and highest densities Ž
2
. Ž
have been recorded from Victoria Harbour 246 per m and Tolo Harbour 1000 per
2
. Ž .
m Huang et al., 1985 . Although this species is not intentionally cultured in Hong
Kong, their abundance on fish cages is so great that they are collected by fish farmers as Ž
. a byproduct. Its high density and fast growth rate in Hong Kong Cheung, 1991
demonstrates the potential of P. Õiridis as an aquaculture species. Although this species plays an important economic role in southeast Asia, information on the relationship
Ž .
between feeding physiology and food availability is limited Hawkins et al., 1998a . A recent approach to developing a sustainable aquaculture industry is to use ecosystem
Ž modelling to determine the carrying capacity of the environments Grant et al., 1993;
. Heral, 1993; Dame, 1996; Bacher et al., 1998 . The first step in this approach is to
´
understand the inter-relationship between food availability and feeding response of the Ž
aquaculture species Stenton-Dozey and Brown, 1994; Bacon et al., 1998; MacDonald et .
al., 1998 . In this experiment, the physiological parameters of feeding in P. Õiridis Ž
. Ž
. which included clearance rate CR , absorption rate AR and absorption efficiency
Ž .
AE , and their relationships with food availability in the experimental water column were studied at Kat O, the site with the fastest growth recorded for P. Õiridis in Hong
Kong. The results may help in the calculation of the carrying capacity for this potential Ž
culture site and in the selection of culture sites in different waters of Hong Kong Lee, .
1986; Cheung, 1991 .
2. Materials and methods
Ž .
Green mussels, P. Õiridis, were collected from a fish farm in Kat O Fig. 1 at the northeast of Hong Kong and cultured in nylon bags at the same site from December
1997 to November 1998. The site was visited once a month. During each visit, feeding rates including CRs, filtration rates, ingestion and organic ingestion rates, and ARs were
determined. Hydrographic parameters including water temperature, salinity, dissolved oxygen level and seston characteristics were also measured monthly for 12 months.
Ž
y1
. Seston characteristics which included total particulate matter TPM: mg l
, particu- Ž
y1
. Ž
y1
. late organic matter POM: mg l
, particulate inorganic matter PIM: mg l and
organic fraction of seston in the seawater were determined. This was done by collecting a water sample of 300 ml at the time interval of about 30 min from the outflow of the
Ž . Ž
. empty container control
see the description of physiological measurements . Each Ž
. water sample was filtered onto ashed and pre-weighed 25 mm GFrC filters Whatman ,
Fig. 1. The map of Hong Kong showing Kat O, the experimental site.
Ž .
rinsed with distilled water, dried in an oven 1108C for 24 h , weighed, and then ashed Ž
. in a muffle furnace 4508C for 6 h before final weighing. Thus, concentrations of the
Ž
y1
. Ž
y1
. Ž
y1
. TPM mg l
and PIM mg l were measured. The POM concentration mg l
was estimated by subtracting PIM from TPM. The organic content of suspended matter was
computed as f s POMrTPM. To determine feeding rates, each time 13–42 individuals of P. Õiridis of sizes
Ž .
ranging from 25 to 85 cm were placed into separate containers 250 ml into which Ž
. natural seawater 10 cm below the sea surface
was pumped from the sea via a peristaltic pump. One empty container without an animal was used as a control to
determine the seston characteristics of the natural seawater. The pump provided a water flow through each chamber of about 100 ml min
y1
which, as determined in a preliminary study with an electronic coulter counter, allowed the mussels by their
filtering activity to deplete the particle concentration by less than 40. The animals were maintained in the circulated natural seawater for about 1 h prior to experimentation
so as to evacuate the gut. After that, all the pseudofaeces and faeces produced in the first hour were removed. The animals were then maintained in the containers for about 2–2.5
h, and faeces and pseudofaeces produced were collected. The faeces and pseudofaeces
Ž .
collected were filtered onto preashed GFrC filters Whatman . The total, inorganic and organic weight of faeces and pseudofaeces were determined by the same methods as
Ž those described for seawater samples. Then, egestion rate faeces produced per h, ER:
y1
. Ž
y1
. mg h
and rejection rate pseudofaeces produced per h, RR: mg h were calculated.
Ž .
Ž .
ER included organic egestion rate OER and inorganic egestion rate IER whereas RR Ž
. Ž
. included organic rejection rate ORR and inorganic rejection rate IRR . AE was
Ž .
Ž . Ž
Ž calculated by the Conover ratio method Conover, 1966 in which AE s f y e r f = 1
.. y e , where f was the organic content of suspended matter and e was the percentage
Ž
y1
. Ž
. of organic matter in faeces. CRs l h
were estimated as CR s IRR q IER rPIM Ž
. Ž
y1
. Iglesias et al., 1996 . ARs
mg h were obtained by AE = CR = POM. When
pseudofaeces was produced, AE and AR were obtained by biodeposition method Ž
. Iglesias et al., 1998 . These calculations assume no significant delay in the deposition
of filtered particles, estimating particle availability as the integrated average over Ž
. corresponding periods of faecal collection Hawkins et al., 1998b . The determination of
seston characteristics was discontinuous, however, the feeding rates were continuous. Therefore, the data of food availability obtained in this experiment were actually a pool
of estimated mean values. The CR of P. Õiridis showed significant relationship with
Ž .
tissue dry weight Table 1 . CR was then calculated for a standardized animal of 1 g to allow comparisons among different sized groups. CRs in the following text were
standardized data without special notice. The AR and AE were found to be independent of body size in the present study.
Ž
y1
. Scope for growth SFG: J h
is defined as the amount of energy available for production and it is the energy absorbed minus energy expended on respiration. SFG
was determined in February, May, July and October 1998 to represent the situation in Ž
y1
. winter, spring, summer and autumn, respectively. Respiration rates mg O
h of
2
40–42 individuals with shell lengths of between 25 and 85 cm were determined using a Ž
. YSI 51 oxygen meter. Each mussel was put into a plastic chamber 220 ml filled with
seawater and the chambers were then sealed off for 30–60 min. The time was used such that the oxygen tension in the chambers fell neither fast to make the individuals cease
aerobic pumping nor slow to influence the rate of aerobic respiration. Initial and final oxygen content of the seawater in each chamber was measured. Three chambers without
Table 1 Ž
y1
. Ž .
Relationship between CR l h and body dry tissue weight g
2
Month n
Equation R
P-value
0.22
Dec. 97 42
CR s 0.87=W 0.086
0.05
0.64
Jan. 98 42
CR s 2.25=W 0.494
- 0.01
0.26
Feb. 98 42
CR s 0.58=W 0.080
0.05
0.33
Mar. 98 42
CR s 0.32=W 0.249
- 0.01
0.33
Apr. 98 42
CR s 0.31=W 0.148
- 0.05
0.33
May. 98 42
CR s 0.57=W 0.214
- 0.01
0.34
Jun. 98 42
CR s 0.56=W 0.167
- 0.01
0.41
Jul. 98 42
CR s 0.59=W 0.107
- 0.05
0.85
Aug. 98 13
CR s 0.81=W 0.563
- 0.01
1.02
Sep. 98 41
CR s 0.78=W 0.263
- 0.01
1.01
Oct. 98 40
CR s 0.84=W 0.548
- 0.01
0.80
Nov. 98 40
CR s1.06=W 0.385
- 0.01
animals were served as controls. The respiration rate of each individual was calculated
y1
Ž .
as mg O h
. Energy absorbed was calculated from absorbed rate AR using a
2 y1
Ž .
conversion factor of 20.78 J mg POM
Crisp, 1971 and energy expended on
y1
Ž .
respiration was expressed using a conversion factor of 13.98 J mg O
Ivlev, 1934 .
2
SFG of each individual was obtained by subtracting energy expended on respiration from the correspondent AR.
Enzymatic activities in digestive glands and crystalline styles of the same group of experimental animals used in the respiration study were also determined. The amylase
Ž and cellulase activity was determined by the Nelson–Somogyi Method Nelson, 1944;
. Somogyi, 1952 . Digestive glands and crystalline styles were collected in situ at the
experimental site, put into liquid nitrogen, transported back to the laboratory and stored at y308C for in vitro enzymatic assays. Wet weights of the digestive glands and
crystalline styles were recorded just before the enzymatic assays. One digestive gland or
Ž two crystalline styles were homogenised in cold 20 mM phosphate buffer pH 6.9 and
. pH 6.5 for the digestive gland and crystalline style, respectively containing 20 mM
NaCl, then centrifuged for 15 min at 4000 = g. The clear supernatants were used to determine a-amylase and cellulase activities. Standard calibration curves were set up
with glucose. The substrates for the determination of a-amylase and cellulase were
Ž .
Ž .
starch 1 and carboxymethylcellulose 1 , respectively, and were made up with the corresponding phosphate buffer for digestive glands and crystalline styles, respectively.
Mass of gland and crystalline style were estimated as protein using the method of Lowry Ž
. et al. 1951 and the standard calibration curve was set up using bovine serum albumin
Ž .
y1 y1
BSA . The enzyme activity was expressed in mg glucose h mg
protein. Ž
. Statistical methods used include one-way analysis of variance ANOVA , multiple
comparison of Student–Newman–Keuls Test, and multiple stepwise regression analyses Ž
. of simple linear and non-linear procedures Zar, 1984 . Any regression analysis per-
formed in this paper depended on the most appropriate function to be fitted in each case, following standard least square procedures.
3. Results
