In late July 1994 MBRC officials were informed by the management of the Aquaculture Institute in Khoms that an unknown sickness was affecting shrimps at the Ain Kaam Aquaculture Farm, and that shrimp mortality was occurring at a serious rate. Members of the Project LIBFISH Aquaculture Working Group were requested to visit the farm and provide technical advice on the matter. Between the end of July and early November 1994 participants in the investigation visited Ain Kaam a total of eight times (see table, Annex I). In the course of these visits shrimp specimens were collected from the farm for microbiological examination in co-operation with the Project's Fish Technology group and the Faculty of Veterinary Medicine. Al Fateh University, (Tripoli).
It was immediately obvious that a significant proportion of shrimps at Ain Kaam were suffering from something very serious. The eyes were affected and a few dark spots, especially at the level of the gills, could be observed on the body.
Most of the samples examined appeared to be very light grey in colour, as opposed to the darker colour normally displayed by healthy specimens. Shrimps with one infected eye were seen swimming around in circles and without balance.
3.1. Water Quality
Seawater is pumped from a point just offshore, very close to the beach. It is sent through a sand/gravel filter before being circulated into the fish tanks. Several measurements taken around midday during visits in August confirmed that water temperature was around 29°C in shade-protected tanks and 30 °C in unprotected (direct sun exposure) tanks.
Dissolved oxygen was around saturation point, i.e. about 6 mg/l at these ambient temperatures.1 This concentration of oxygen is quite low for shrimps. Artificial aeration is provided to the tanks inside the hatchery hall by compressed air diffusers, but no such facilities are provided to the outdoor tanks.
1 At 20°C the saturation level of oxygen is 7.25 mg/l.
3.2. Hygiene
Cleanliness of the hatchery hall is compromised by a number of obvious faults. Birds and insects can fly inside the building through a broken window, and none of the windows are protected by mosquito netting. Dust is transported inside the building through space around doors and windows and does not seem to be removed regularly.
The floor and drainage channel of the hatchery do not seem to be washed very often and thus may represent additional sources of hazard. The laboratory, plankton seed room, and live feed production unit which are also part of the hatchery building do not appear to be very well maintained and have accumulated a considerable amount of dust.
Tanks within the hatchery are made of fibreglass whereas the outside tanks are concrete and contain a central bottom layer of sand some 2 to 4 cm in depth. Although fibreglass tanks are in principle easier to maintain, neither they nor the concrete tanks were being kept sufficiently clean at the time of the team visits. Tanks without animals had accumulations of dirty water and fermented sediment, which constitute possible points of microbial infection.
3.3. Feed and Feeding Practices
Various types of feed were being used during the period of team visits, including crushed crab, minced fish (gurnard, shark, ray), and frozen shrimps However, the freshness of these items did not always seem adequate. Another concern is the way crabs are caught along the beach and subsequently prepared as feed. Crabs are trapped during the night using spoilt fish as bait. Thus pathogens from the bait are stocked inside crab stomachs and intestines and later pose risks to the shrimps, since the crabs are crushed alive (without previous starvation) before being distributed to the tanks.
When fish is prepared as feed, it often seems to be gutted and filleted on the cement floor inside the hatchery hall, near the tanks. As farm staff walk about the area they thus are transporting wastes with their shoes from place to place.
Feed distributed to the shrimps is usually quickly eaten and little or no waste is left inside the tanks. On several occasions, though, uneaten feed was observed lying on tank bottoms during morning time. It was already rotten and therefore constituted a hazard for the animals kept inside.
It was also observed that wastes of feed are dumped at the back of the hatchery building. This practice is bound to attract insects, rodents, etc. and therefore constitutes a further focal point of microbial infection.
3.4 Stocking Densities
The density of adult shrimps appeared to be sufficiently low, inasmuch as the animals had enough space to lay on the bottom of the tanks without crowding. The juvenile tanks seemed to be definitely overcrowded, however.
4.1 Possible Causes
On the basis of initial observations several possible reasons or combinations of reasons for the disease outbreak were identified. These included:
Presence of infection in original broodstock (imported from an aquaculture farm in Egypt).
Microbial contamination through feed and bad hygiene.
High water temperatures allowing fast multiplication of micro-organisms (29–30°C).
Effect of high temperature and low oxygen concentration of water (optimum for Penaeus japonicus is around 18 – 22°C and saturated level of oxygen).
Pollution of water by heavy metal(s).
Transmission of micro-organisms throughout the farm due to limited hygiene control.
Overcrowding and other stress factors.
4.2 Preliminary Recommendations and Actions
4.2.1 Immediate measures
The team recommended that a number of measures be implemented immediately by the Ain Kaam management so as to control and contain the situation. These measures include the following:-
Isolate sick shrimps as soon as symptoms are visible.
Improve the sea water intake to reduce the temperature inside the tanks (while surface water temperature is 30°C, it is around 25°C at a depth of 6 metres).
Cover the PVC pipe from the pumps to the 61ter with sand in order to protect it from direct sunshine and avoid unnecessary heating of water.
During daytime shrimps prefer to stay inside the sediment. The bottom of the outside tanks should therefore be covered with a layer of sand and drainage pipes should be installed inside this layer so that it remains oxygenated.
All the outside tanks should be shaded from direct sunshine to avoid heating of water and to protect the shrimps against such bright light.
Keep all tanks in a thoroughly clean condition, including the ones without animals.
4.2.2 Reference research
Extensive documentation on shrimp diseases exists at the MBRC Library, and a variety of sources were consulted by members of the investigating team. Relevant material is listed in Annex II.
4.2.3 Microbiological Examination
Microbiological investigation was initiated in order to confirm suspicions about the cause of the disease that arose in the course of the library reference work. Material selected for analysis included infected shrimps, feed, water, and sediment from rearing tanks. Some examination was carried out at the MBRC, but because the laboratories there are not fully equipped for such work, the assistance of staff at the Faculty of Veterinary Medicine, El Fateh University, Tripoli was also enlisted.
4.2.4 International cooperation
Requests for further information on shrimp diseases were sent off to the following institutions:
Department of Microbiology, University of Hawaii;
FAO Fisheries Library, Rome; and
the National Aquaculture Centre, Malta.
Valuable documents and other information were provided by all of these sources.
5.1 Symptoms
Based on the symptoms observed and reference to relevant literature a preliminary diagnosis was made that the Ain Kaam shrimps were suffering from Black gill disease.
5.2. Microscopic examination
This diagnosis was confirmed through further inspection and studies conducted at MBRC on the affected Penaeus japonicus samples. Microscopic examination revealed that the pathogenic agent was of the genus Fusarium (cause of Fusarium or black gill disease).
The diagnosis was further confirmed by the laboratory of the Faculty of Veterinary Medicine, Al Fateh University as per the results of Analysis No. F.V.M/545, dated 12/11/1994 (Annex III). Slides for histopathological examination were also prepared at the University to be used for additional confirmation. Results of this examination are presented in Annex IV.
5.3 Media culture
In vitro cultivation of pathological lesions using various media at both the MBRC and the University provided still further confirmation of the presence of Fusarium spp.
The Aquaculture investigating team concludes that the outbreak of the shrimp disease at Ain Kaam is caused by uncontrolled infection of fungi of the Fusarium genus. The very fast development of fungal infection is related to the combined effects of high water temperature prevailing in the months of July and August and deficient hygiene practices. It is doubtful that the infection could have been introduced through any other channel, since Fusarium spp. are ubiquitous and multiply rapidly when conditions are favourable. It has been documented in the literature that Fusarium spp. are commonly isolated in specimens of cultured and captive wild decapod crustaceans such as those being used as shrimp feed at Ain Kaam farm (Sindermann and Lightner 1988). Although the disease appears only on adults, all the shrimps of the farm, including juveniles, were infected. The disease only manifests itself after an incubation period, and thus is not readily apparent in young shrimp.
The final recommendations offered by the Aqua team are basically elaborations of the immediate measures suggested for implementation earlier.
Install a new sea water intake in order to avoid warm surface water during summertime, and/or dig a well near the beach as this can provide a suitable and clean water and avoid several problems arising from direct pumping out of the sea.
Regularly maintain the seawater filter so that it does become a focus of infection.
Maintain the hatchery hall in a high state of cleanliness.
Ensure that hatchery hall doors and windows are properly sealed and install mosquito netting on windows.
Improve general hygiene within the form. As there is no cure for Fusarium-infected shrimps, prevention is essential.
Install a ‘foot disinfectant bath’ at each entrance of the hatchery hall and at various places of the farm compound so that the staff can avoid transporting contaminants from place to place.
Quarantine and disinfect any new animal supplied to the farm.
Keep the entire farm clean; avoid dumping garbage at any hidden place; clean the drains.
Store pellet and fresh feed in suitable places.
The store for pelleted feed should be made secure against dust and pests (rodents and insects), and it should be kept dry and cool (air conditioned) during hot months.
Pelleted feed is perishable. Even under proper storage conditions it must be used within three months. Orders of feed should be placed as often as necessary to avoid wastes due to excessive storage time, as well as shortage.
Stores and cold rooms must be kept absolutely clean and they should be disinfected on a regularly recorded basis.
Fresh feed should be supplied regularly (fresh trash fish from trawlers). The quality must be verified at the time of purchase by farm staff responsible for disease monitoring. Fishes must be immediately kept with ice, then washed with clean cold water and gutted.
Fresh feed should be prepared in a special room, which must be washed and disinfected after each use.
Farm staff in charge of disease monitoring should work with the understanding that their first responsibility is prevention. This requires:
- careful control of farm hygiene standards;
- careful record-keeping and full awareness of any previous disease outbreaks in order to avoid repetition;
- quick action in the case of disease occurrence so further propagation is prevented; and
- proper monitoring of imported/newly introduced stocks of animals (fish / shrimp / mollusc broodstock or juveniles).
