Use of antibiotics in the preservation of prawn

Effect of incorporating chlorotetracycline (CTC) in ice up to 5 ppm level on the keeping quality of prawns has been studied. A shelf life extension by nearly six days is obtained for the CTC-iced sample over the control. The headless prawns absorbed greater amounts of CTC than whole prawns during storage in CTC-ice. Traces of the antibiotic are found in the muscle of the CTC-iced prawns even after cooking for one hour. The cause of destruction of CTC when used for prawn preservation is discussed.

Behaviour of marine microflora towards antibiotics 1. Sensitivity towards chlortetracycline (CTC)

The sensitivity of 61 cultures of bacteria isolated from fish towards chlortetracycline (CTC) at 5 ppm and 20 ppm levels has been determined on two solid media: sea water agar (SWA) and a distilled water based medium consisting of peptone, beef extract, glucose and NaCI (PBGA). The cultures employed consisted of (i) gram-negative rods of marine origin (Achromobacter, Pseudomonas, Vibrio and Flavobacterium) and (ii) gram positive organisms (Micrococci and Corynebacterium). Depending on the inhibition zone diameter, the order of CTC sensitivity was found to be Pseudomonasmobacter as determined on PBGA at CTC levels of 5 and 20 ppm. The SWA enhanced the development of resistance in majority of the cultures as compared to PBGA. PBG broth containing CTC at less than 10 ppm exerted a bacteriostatic effect only while that containing 10 ppm or more exerted a bactericidal effect on Achromobacter, Pseudomonas and Micrococci cultures. The significance of these results in fish preservation by CTC is discussed.

Experimental preservation of sardines (Sardinella longiceps), using Chlortetracycline

Effects of dips in 10 and 50 ppm Chlortetracycline solution on the storage-life of sardines in ice have been studied. 5 ppm-CTC-ice in conjunction with dip in 10 ppm CTC-solution has also been tried. CTC-treatment considerably reduced bacterial number and gave a better appearance to the fish. The high fat content in sardines resulted in rapid development of rancidity. CTC has been found to have no control on the development of rancidity. Hence, CTC treatment of sardines is found to be of limited advantage in extending the storage-life of the fish in ice.

Chitosan as a water clarifying agent

Chitosan may be used to reduce the bacterial load of water. Material prepared according to the method of Radhakrishnan & Prahu described in Res. & Ind., 16(4), pp. 265, used in 1% solution in 1% acetic acid was added at 10 ppm level to contaminated water and allowed to stand for 45 min. Cultures of E. coli, Staphylococci and a mixture of the 2 were inoculated into ordinary and muddy water. Bacterial load was determined, and it is shown that chitosan has excellent qualities as a coagulant/water clarifying agent, especially for muddy waters or those contaminated with suspended matter or bacteria.

Bacterial flora of fresh and iced Indian mackerel (Rastrelliger kanagurta) and its response to chlortetracycline (CTC) treatment

Quantitative and qualitative studies on the bacterial flora of fresh Indian mackerel (Rastrelliger kanagurta) have been made. The total native flora as well as 5 ppm CTC insensitive flora of the fish showed variations with season. About 90% of the fresh fish flora was sensitive to 5 ppm CTC. The natural flora of the fresh fish consisted of Vibrios, Pseudomonas, Achromobacter, Flavobacterium, Corynebacteria, Micrococci, Bacillus and yeasts. In the CTC insensitive flora, Vibrios predominated followed by yeasts. The selection of bacterial genera during storage of the fish in ice and in 5 ppm CTC incorporated ice has also been investigated. At the time of spoilage, Pseudomonas was found to be the dominant flora of the fish stored in both types of ice.

Effect of furanace on the development of larval stages of Penaeus monodon Fabricius

Zoea 2(Z SUB-2 ) Mysis 1 (M SUB-1 ) and Postlarva 1 (P SUB-1 ) of P. monodon artificially spawned in closed-system concrete hatchery tanks were bioassayed for their tolerance to the antibiotic furanace. The setup consisted of four 20-liter capacity plastic basins previously conditioned for 15 days with freshwater in full sunlight. During the experiment, each basin was filled with 5 liters of seawater to which was added filtered Chaetoceros and Brachionus to give densities of 5 . 0-7 . 5 x 10 SUP-4 cells/ml and 10-20 individuals/ml, respectively. The following are the properties of the water used throughout the experiments: salinity, 26-32%; pH, 7 . 3-8 . 4; temperature, 25-30 degree C; dissolved oxygen, 4 . 5-8 . 4 ppm; nitrite, 0 . 36-0 . 99 ppm; and ammonia, 0 . 10-0 . 30 ppm. To each basin were added 50 healthy larvae of specific stages of P. monodon. After an initial acclimation of one hour in the medium, preweighed amounts of the antibiotic were added and thoroughly dissolved. The concentrations tested were 1 . 0, 2 . 0 and 3 . 0 ppm. One basin always served as control. After 24 hours of exposure, the surviving population in each basin was counted. The survivors were then examined thoroughly under the microscope for unusual behavior and morphological defects brought about by the exposure. To minimize wide variations in the medium as a result of feeding and other manipulations, the systems were all prepared at 9:00 a.m. each time, and the feeds on two instances, one at 5:00 p.m. and another at 5:00 a.m. Fifteen trials conducted with Z SUB-2 showed survival ranges of 68% to 98% with a mean of 77 . 6% in the controls; 32% to 94% with a mean of 65 . 7% at 1 ppm, and 0% to 56% with a mean of 36 . 5% at 2 ppm. There were no survivors at 3 ppm. Interpolation from the survival-dose curve gave a 24-hr LC SUB-50 of approximately 1 . 6 ppm.

Tolerance of Penaeus monodon larvae to cupric sulfate added in bath

Copper is used to deter the growth of bacterial, fungal and protozoan disease organism in fishes. Zoeae (Z SUB-1 ), myses (M SUB-1 ) and postlarvae (P SUB-1 ) were exposed to copper sulfate at concentrations of 0 . 025, 0 . 05, 0 . 75, 0 . 1 and 0 . 2 ppm from 24 to 96 hours. The number of surviving larvae were counted at the end of each 24-hour period and the percentage of survival is determined for each dose level. The LC SUB-50 for each of the larval stages was interpolated from the data whenever possible. Three trials with 2 replicates per trial were conducted. The physico-chemical characteristics of the bath taken before and at the end of the experimental period show insignificant differences between initial and final values in each trial. Results indicate that mortality rates of all larval stages increased with exposure time and that mortality rates of the experimental group is higher than the control. Interpolation of the LC SUB-50 is possible only for the 48-h and 72-h exposure times for both zoeae and myses and for the 48-h exposure time for the postlarvae. This is due to the high survival percentage of the 24-h group and the low survival percentage (below 50%) of the larvae exposed for 96 hours. The 48-hour LC SUB-50 for Z SUB-1 , M SUB-1 and P SUB-1 are 0 . 225, 0 . 350 and 0 . 125 ppm respectively. Postlarvae seem to be more sensitive than either of the 2 larval stages having a lower 48-h LC SUB-50 and a low survival rate after 72 hours. The larvae were observed to lose their balance and were lethargic, producing few swimming movements so that they were mostly confined to the bottom of the aquaria. Moribund larvae observed under the microscope had a faster but weak heartbeat compared to healthy larvae. Slight or complete loss of feeding ability indicated by empty guts and delayed molting of Z SUB-1 to Z SUB-2 were also noted.

Reduction in Chaetoceros populations by furanace

One of the most promising prophylactic agents being tested to control Penaeus monodon larval diseases is furanace (6-hydroxymethyl-2 2(5-nitro-2-furyl) vinyl pyridine). To evaluate further its suitability as a chemotherapeutic agent, its effects on the population growth of Chaetoceros calcitrans, the diatom used as feed for the zoeal stages, was examined. Chaetoceros populations of uniform density (initial density in all runs: 130-141x10 SUP-3 cells /ml) were placed in nine white, circular (382 sq cm), plastic basins. The physio-chemical characteristics of the culture water were as follows: salinity, 28 . 5-30 . 0 ppt; pH, 8 . 62-8 . 72; temperature, 23-25 . 5 degree C; dissolved oxygen, 7 . 1-9 . 3 ppm; nitrate, 0 . 03-0 . 07 ppm; and ammonia, 0 . 005-0 . 03 ppm. Preweighed furanace granules were dissolved in the culture water, with resulting concentrations of 1 and 2 mg/l (3 replicates each). A set of replicates without furanace served as the control. Population counts of the diatom were taken after 2, 4, 6, 8, and 10 hr exposures. After 4 hr, the population decreased in all three levels. The population in 2 mg/l furanace showed the lowest count and that in control the highest. The population means are not statistically different from one another. The results of the study show that the furanace causes reductions in Chaetoceros population at all durations of exposure.

The biology and control of Caligus sp., an ectoparasite of the adult milkfish Chanos chanos Forskal

One unidentified species of copepod belonging to the genus Caliqus of the family Caigidae was found to infest the adult milkfish broodstock. To control the parasites infesting the adult milkfish, tests were made using the chemical (2,2,2-trichloro-1-hydroxyl)-phosphonic acid-dimethylethol (Neguvon) at a concentration of 0 . 25 ppm. It is noted that a concentration of 0 . 25 ppm of Neguvon maintained for 12-24 hours in the sabalo-containing tanks in a closed water system but with aeration is effective in controlling the parasites. Fish mortality during the experiment was due to inadequate aeration in the tanks.

Effect of some physico-chemical factors on the survival and growth of Penaeus monodon postlarvae

Growth and survival rates of P. monodon postlarvae were examined at different temperatures, salinities, and nitrite and ammonia concentrations, using one feed level. Condition of postlarvae greatly affected the experimental results shown in some instances where very low survival rates were obtained, even for the controls. Results indicated that postlarvae from PSUB-10 and up can tolerate salinity changes of 10 to 20 ppt without prior acclimation. Survival generally appears the same for temperatures between 24 and 36 C. It appears that P. monodon postlarvae have higher temperature tolerance. Tolerance of postlarvae at the early postlarval stage is between 30 and 50 ppm of nitrate. They were more tolerant from PSUB-10 upwards. Although survival was high in runs containing nitrite, growing appears to have been affected. Postlarvae could tolerate ammonia concentrations up to about 50 ppm. At 100 ppm higher mortality rates were observed. Whether or not there was any permanent effect by nitrate and ammonia at high but apparently tolerable levels is not known.

Effect of nitrite, ammonia, and temperature on P. monodon larvae

P. monodon larvae were studied for the effects of temperature, ammonia, and nitrite on survival. Toxicity levels of nitrite were found to vary with larval stage. Larvae could tolerate ammonia up to about 10 ppm, with the effect more clearly shown by the zoea stage. Survival and growth were not significantly affected by temperature, although moulting was enhanced at temperatures higher than 29 C. Larvae of P. monodon have lower tolerance toward nitrite and ammonia compared to postlarvae. Although high survival was obtained at low levels of nitrite and ammonia, it is still necessary to know their effects on metabolism, in order to examine possible biochemical parameters for diagnosing sublethal toxicity or stress.

Levels of cadmium in seafood products

The concentrations of total cadmium were determined in 448 samples of processed fishery products comprising crustaceans, molluscs, marine fish and brackish water fish. Shrimp product, the major export item, contained on average 0.122 ppm cadmium and 20% of crustaceans analysed did not contain cadmium on detectable level. Average content of cadmium in 68 marine fish of 9 species was 0.153 ppm and 35% of the samples did not contain the element. Of the total number of crustaceans analysed only 2% showed a level > 0.5 ppm. All the 174 samples of molluscan products, except 3 canned oysters, showed cadmium concentration far below the limit allowed. The average cadmium content in molluscan products was 0.552 ppm.

Minimum dissolved oxygen tolerance of four different sizes of milkfish

Four size groups of milkfish were tested, 4-18 g, 20-34 g, 35-95 g and 200-300 g. A number of fish from each group were placed separately in identical 1.2 m2 wooden tanks containing seawater filled up to 30 cm depth. The aggregate weight of fish per size group was approximately 1 kg. The fish were held for 72 h, fed with lab-lab and provided with continuous aeration to allow recovery from stress during transport and handling. After the recovery period, aeration was stopped and 200 g of the fine rice bran was spread over the water in each tank creating a film of bran particles on the water surface. This was designed to speed up depletion of dissolved oxygen considering the combined effects of the screening-off of sunlight, the reduction of air-water interface and the breakdown of the bran particles. It is probable that stress on milkfish in brackishwater ponds could start when oxygen level drops to about 1.4 ppm. A further decrease to 0.04 ppm could produce a total kill of all specimens above 4 grams with marketable size and bigger size fish dying first.

Effects of 2-phenoxy ethanol and MS-222 on milkfish fingerlings (Chanos chanos) as anaesthetic agents

An experiment was undertaken in order to determine an adequate anaesthetic and optimum concentrations for use in the handling of fingerling milkfish (Chanos chanos). The compounds 2-phenoxy ethanol and MS-222 were investigated. Results show the latter to be adequate with optimum concentrations between 100 and 200 ppm.

Acute toxicity of unionized ammonia to milkfish (Chanos chanos Forsskal) fry

The study was conducted to determine the effects of varying concentrations of ammonia to milkfish fry. Two runs of static 96h bioassays were conducted to determine the median lethal concentration (LC 50) of unionized ammonia (NH₃) to milkfish fry. Test concentrations were based on exploratory 24h and 48h bioassays and were made in three replicates. Reagent grade ammonium chloride (NH₄Cl) was used to adjust the level of unionized ammonia. The 96h median lethal concentration, determined by the Reed Muench method was calculated at 28.029 ppm NH₃ 29.69 ppm. Even at high concentrations of unionized ammonia, most of the fry mortality occurred after 48 to 96 hours exposure. Severe gill damage occurs only at concentrations above 20 ppm, especially above the LC 50. The high LC 50 value obtain shows that milkfish fry has great tolerance to ammonia, that even fry with severely-damaged gills can still recover days after it is returned to favorable culture condition. The result suggest that observed mortalities of milkfish fry under culture conditions are not due to ammonia toxicity.