Tilapia diseases and management in river-based cage aquaculture in northern Thailand

JOURNAL OF APPLIED AQUACULTURE 2016, VOL. 28, NO. 1, 9–16 http://dx.doi.org/10.1080/10454438.2015.1104950

Tilapia diseases and management in river-based cage aquaculture in northern Thailand Chanagun Chitmanata, Phimphakan Lebela,b, Niwooti Whangchaia, Jongkon Promyaa and Louis Lebelb Faculty of Fisheries Technology and Aquatic Resources, Maejo University, Chiang Mai, Thailand; bUnit for Social and Environmental Research, Faculty of Social Sciences, Chiang Mai University, Chiang Mai, Thailand

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ABSTRACT

KEYWORDS

A total of 662 farmers who rear tilapia in river-based cages in Northern Thailand were interviewed on their knowledge and perception on disease constraints and their control measures. Most farms (84%) had disease problems in the last two years. Exophthalmia ranked higher than other clinical signs. Most farmers noticed that the risk of disease problems was similar every month. Most (95%) believed that fish diseases were caused by bacterial pathogens. To treat perceived disease outbreaks, most farmers (96%) removed infected and dead fish and applied, usually inappropriately, antibiotics. As disease prevention through good management is better than treatment, farmers and fish disease experts could use these research findings as a tool to work together to develop better control strategies.

Cage culture constraints; disease outbreak; fish disease management; tilapia diseases

Introduction Tilapia cage culture in Thailand has rapidly expanded, as it is economically attractive, benefits landless people, and avoids the off-flavor problems that commonly impact pond-based systems. A serious obstacle to the sustainability and development of tilapia cage culture is the frequent occurrences of mass mortality. These events are probably caused by fish diseases, including bacterial infection (Yuasa et al. 2013), ectoparasite infestation, or water-quality problems, but have not been carefully studied. Several diseases are anecdotally related to high stocking density and improper feeding (Georgiadis et al. 2001) and climate change (Amal et al. 2013; Harvell et al. 2002; Karvonen et al. 2010). During 2005, tilapia cage culture all over Thailand suffered from acute fish mortality (Srisapoome & Areechon 2013). Moribund fish were sent to a diagnostic laboratory. Parasites, bacteria, and fungi were diagnosed, but it seems likely the underlying cause of this devastating loss was the deteriorated aquatic environment (Chitmanat 2009). In some locations, such as the Upper Ping River, severe floods also caused significant losses (Lebel et al. 2013).

Again, in 2013, massive deaths of cage cultured tilapia occurred in major rivers, while the exact cause remained unknown (Nation Channel 2013; Manager Online 2013; Bangkokbiznews 2013; Phitsanulokhotnews 2013). Fish disease diagnosis and treatment should have on-the-spot investigation by veterinarians, but in practice, it is not feasible because of the unavailability of expertise and remote farm sites (Li et al. 2002). As a result, it is necessary to find out how fish farmers diagnose, treat, and prevent fish diseases to further improve the disease control system. Our article summarizes the current knowledge of fish cage farmers in Northern Thailand with regard to fish diseases and their management.

CONTACT Chanagun Chitmanat chanagun1@hotmail.com Faculty of Fisheries Technology and Aquatic Resources, Maejo University, Chiang Mai, 50290, Thailand.

Color versions of one or more of the figures in the article can be found online at www.tandfonline.com/wjaa. Š 2016 Taylor & Francis

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Figure 1. Map of the study area.

Materials and methods Study sites were grouped by provinces into three growing regions for analysis: Ping (Chiang Mai and Lamphun), Nan (Uttaradit and Phitsanalok), and Lower (Kamphengphet, Tak, Nakon-sawan, and Pichit) region (Figure 1). The lower region included both reaches in the Ping River or in the case of Pichit province, Yom River. In these rivers fish are grown in open-top, floating, mesh cages, typically 4 × 4 × 2 m (Lebel et al. 2013). Fish are released into river cages at densities of around 50 per m3 and reared for 3–5 months, with average yields around 27 kg m−3 (Lebel et al. 2013).

An effort was made to interview all fish farmers who had recently reared tilapia in cages in the rivers of Northern Thailand. A total of 662 fish farmers were interviewed using a structured questionnaire that covered individual-, farm-, and site-level characteristics, as well as more detailed sections about risks to the profitability of their fish farm enterprise and ways such risks could be managed.

Results According to field trip observations and lab diagnosis, farmed tilapia in Thailand suffer mainly from Trichodina, Streptococcus, Flavobacterium columnare, and Aeromonas hydrophila infections (Table 1). Trichodina is a protozoan parasite. Infected fish display flashing, rapid breathing, and weakness. The heavy infestation of the gill and body surface cause extremely high mortality,

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Table 1. Aetiological agents of the economically important fish diseases affecting tilapia cultures in Thailand.

Agent Aeromonas hydrophila Flavobacterium columnare Streptococcus spp.

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Trichodina spp.

Disease Clinical signs Motile Aeromonas Off-feed, abnormal swimming, pale gills, abdominal distention, skin Septicemia ulceration Columnaris, mouth fungus Gill rot, fin rot, the appearance of grayish white or yellow areas of erosion Streptococcosis Swirling swimming, lethargy, exophthalmia, opaque eyes, skin haemorrhage Trichodiniasis Excessive mucus, clubbing of the gill filaments

particularly in young fish. Crowding condition in small fish causes a secondary infection of Trichodina sp., leading to massive mortality (Mohamed et al. 2012). Madsen et al. (2000) suggested that the infection pressure from trichodiniasis in farms with a relatively high load of organic dry matter (>15–20 mg/L), may be reduced by reducing the organic matter in the water. However, for public reservoirs and rivers, improved watershed management is needed to prevent this problem.

The dominant clinical signs of Streptococcus infection observed during this study included hemorrhagic septicemia, lethargy, hemorrhagic eyes, corneal opacity in one or more eyes, exophthalmia (protruding eyes), swollen kidney, and an erratic spiral swimming motion. The most suitable temperature for tilapia is 29°C–31°C; stress-associated mortalities occur when the temperature is >37°C–38°C (Lim and Webster 2006). Kayansamruaj et al. (2014) found that experimentally elevated temperatures (35°C–37°C), increased pathogenicity of Streptococcus agalactiae to Nile tilapia.

Like Streptococcus, Aeromonas results in the clinical signs of hemorrhagic septicemia, lethargy, and loss of appetite. Aeromonas generally is a secondary infection following poor handling, overcrowding, or poor water quality.

Another serious bacterial disease is caused by Flavobacterium columnare, which is distributed worldwide in the aquatic environment. Its unique clinical signs are gill rot and yellow erosions in the tegument. Gill damage causes the fish to begin breathing rapidly and “gasping” at the surface. These long rod-shaped bacteria can be observed in a wet mount of infected tissues under light microscopy. The outbreaks usually occur during rapid changes in weather, increased water temperature, low dissolved oxygen, and high stocking densities. Most farmers and news reporters mistakenly believe that F. columnare-induced gill clogging is caused by silt, leading to asphyxiation (Nong Khai Inland Fisheries Research and Development Center 2013; Krobkruakao 2013).

However, high silt in water during rainfall possibly results in the clogging of cage nets and low dissolved oxygen and water quality within cages, thus perhaps increasing the risks of gill rot infection. Fish kept in stagnant water have been shown to be more susceptible to columnaris disease compared to fish kept in running water (Decostere, Haesebrouck, Turnbull et al. 2009).

Ichthyophthirius multifiliis parasitism of tilapia enhanced F. columnare invasion and resulted in higher fish mortality (Xu et al. 2014). However, in our study, we are uncertain whether Trichodina spp. or F. columnare is the primary infection. Plumb (1997) stated that the presence of Trichodina spp. presumably caused epidermal injuries, leading to streptococcal infection in a recirculation tilapia culture. Rintamäki-Kinnunen and Valtonen (1997) reported that 30% of fishes with parasitic infection had a coincident Flavobacterial infection.

Most farmers (83.8%) reported they had faced disease problems in the last 2 years. All clinical signs for disease were commonly observed by farmers (Table 2): Over 90% reported seven or eight of the listed eight clinical signs. Other, more rarely reported clinical signs were clearly distinct from those listed in Table 2, including (number of cases): mottling skin and flashing swimming due to Trichodina infection (17), loss of fish scales (13), torn tail (12), and an appearance of cotton woollike tufts on the body caused by fungi infection (8).

Association between suffering from diseases and a small set of plausible predictors was explored using logistic regression (Hosmer & Lemeshow 2000) (Table 3). Small farms were less likely to have experienced diseases than very large farms. Farms in the Ping were more likely to have had disease

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Table 2. Prevalence of common clinical signs of fish diseases among those farms (n = 554) during 2011–2012.

Clinical signs

% farms

Protruding eyes (exopthamia) Wounded skin Bruises Slow swimming (lethargy) Abdominal swelling Pale gills Eroded fins Excess mucus on body

98.9 98.2 98.0 97.1 96.9 94.9 93.0 86.3

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Table 3. Association between disease problems with flooding, farm size, and river system resulting from a logistic regression analysis with multiple predictors (***P < .001, **P < .01, *P < .05).

Predictor Flood impact Farm size Very large Large Medium Small Region Lower Ping Nan

Odds ratio (95% confidence interval) 2.96 (1.89, 4.62)*** 1 1.25 (0.46, 3.38) 0.73 (0.30, 1.80) 0.38 (0.15, 0.96)* 1 2.46 (1.22, 4.95)* 0.60 (0.35, 1.03)

problems than in the other two regions (Nan, Lower). Farms that had been impacted by floods in the last 2 years were also more likely to have disease problems. Surprisingly, there was no association between diseases and stocking density of the last crop. Most farmers had the impression that disease occurrence was similar over the course of the year, with a possible increase around April.

Farmers believe that a range of factors contribute to diseases on their farms (Table 4). Most (95.1%) believed that fish diseases were caused by bacterial infection. Stocking at too high densities was much less frequently seen as a causal factor for disease outbreak. Water quality and climaterelated variables were other frequently perceived causes of diseases.

Apart from removing diseased fish and reducing feed, most methods of managing disease involved use of medication or chemicals (Table 5). Antibiotic use was very widespread but so was using potassium permanganate and salt. Traditional herbs were also reported to be used for treatment. When asked about the type of the herbs, the farmers mentioned pineapples, lemon grass, banana, and tamarind.

Table 4. Commonly attributed causes of fish disease during 2011â&#x20AC;&#x201C;2012 among farmers (n = 554).

Causes of diseases Bacterial infection Poor water quality Agricultural chemical contamination Low DO Climate variability Low-quality fry Low water flow High water temperatures Residential wastewater Farm wastewater Heavy rainfall Too-high stocking density

% farms 95.1 89.2 82.3 78.5 76.9 75.0 74.0 68.5 65.1 60.0 56.1 49.2

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Table 5. Common methods for managing disease on fish farms among those farms (n = 554) during 2011–2012.

Disease treatment Separate infected fish Reduce feeding Apply antibiotics Treat with KMnO4 Apply salt Reduce stocking density Provide aeration Apply herbs Treat with formalin

% farms 96.4 92.1 92.1 80.4 77.9 65.5 48.9 43.1 32.5

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Discussion Diseases are one of the most significant risks for river-based cage culture of tilapia due to uncontrolled water quality and the spread of disease from infected fish upstream. Most farms have experienced disease problems in the last 2 years, but the exact causes of mortality are often not well understood. According to the interview results, most farmers can recognize common clinical signs of fish diseases, with the general exception of Trichodina, a microscopic ciliate protozoa, normally found on the gills and skin, which is hard to diagnose without the aid of a microscope.

Referring to farmers’ self-reports, most clinical signs including exophthalmia, wounded skin, and bruises appeared to be caused by bacterial infection. Most farmers focus on finding ways to treat diseased fish—using antibiotics or herbs—rather than identifying what exactly caused fish to be sick.

One of the most common methods for managing disease on fish farms is the application of antibiotics. Farmers usually deal with disease outbreaks with a high dosage of drugs and chemicals.

Most farmers do not know exactly what kind of antibiotics they should use during a disease outbreak (Khoi et al. 2008). Only a few have a chance to send diseased fish, together with water samples, to a diagnostic laboratory prior to applying the treatment.

Because of the increased incidence of drug-resistant bacteria, antibiotic residues in fish and in the environment pose significant risks; an alternative approach to solve disease problems is the use of immunostimulants, including vitamins and some herbs to enhance the nonspecific immunity of fish. It was seen that some farmers had applied herbs for disease prevention and treatment. Although immunostimulants may be used for treatment of some infectious diseases, they are not as effective as many chemotheraputics (Sakai 1999). Furthermore, overdoses and long-term administration may reduce the efficacy of immunostimulates and generate unwanted side effects (Bricknell & Dalmo 2005).

Exophthalmia and swirling swimming motions are presumptive signs of tilapia infected with Streptococcus spp. Some diseased fish had been brought to the Fisheries Disease Lab at the Faculty of Fisheries Technology and Aquatic Resources, Maejo University, isolated, and confirmed by traditional diagnostic procedures. According to farmers, the risk of this disease seems to be highest in April, when temperatures are typically very high. As this bacterial disease outbreak usually occurs in summer, fish farmers should reduce the stocking density. Shoemaker et al. (2000) reported that tilapia held at densities ≥11.2 kg/m3 (˜20 fish/m3 under local conditions) significantly increased in mortality due to S. iniae. However, less than 50% of fish farmers thought that too-high stocking density could cause diseases. Observed mean stocking density in tilapia cage culture in Northern Thailand was 49 fish/m3 (Lebel et al. 2013). The survival of caged tilapia decreased from 91% to 57%, with increased stocking densities from 30 to 70 fish/m3 (Yi et al. 1996). In addition, low dissolved oxygen and high nitrite increased mortality due to Streptococcus infection (Bunch & Bejerano 1997).

Another major bacterial disease resulting in devastating mortality rates in tilapia farms is Flavobacterium columnare (formerly Flexibacter columnaris). Outbreaks are primarily caused by the fluctuation of temperature, high stocking densities, low dissolved oxygen stress (Decostere, Haesebrouck, Charlier et al. 1999), injury associated with seining, handling, and fish transport

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(Plumb 1999). Farmers noticed that this disease occurred after sudden changes in temperature of surface water in rivers following heavy rain. Aeromonas hydrophila has also been isolated from tilapia and is considered as a facultative or an opportunistic pathogen that causes disease when the fish are under stress. There were a few incidences in A. hydrophila infection in this survey.

It was found that smaller fish farms had fewer problems from diseases. One reason may be that owners of small farms pay more attention and dedicate more time to fish care. Alternatively, the smaller number of fish cages possibly reduces risk of disease transmission. This is contrary to common claims that say resource-poor farmers have less capacity to manage risks from disease (Sidahmed 2013) because the investment cost for aeration, high-quality feed, immunostimulants, antibiotics, etc., might be too high for poor farmers to afford.

Flood and related water-quality stress possibly contribute to fish disease (Skynews.com.au 2013).

Agricultural and sewage runoff, as well as rapid changes in water parameters during flooding, are able to cause fish stress; thus they would be more prone to pathogen infection. In addition, high flows might push fish against the net cage and subsequently develop scratches, leading to wound infection. On the other hand, low flow and high temperature during the dry season result in low dissolved oxygen, so avoiding overcrowding must be taken into consideration. Good water management at the farm level might not be enough; integrated approaches to manage water quantity and quality at the level of the watershed are also needed. As prevention of disease is better than curing disease, applying good management practices is needed.

Certification of fingerling quality is another important strategy for disease management, as it allows traceability when problems arise. Certificates should show the source of brood stock, quality of fingerlings, chemicals and antibiotics used during the rearing of fingerlings, and the laboratory results of fingerlings’ health status. Unlike the marine shrimp business, there are no certificated fingerlings available in the sites studied.

Overall, most farmers do not understand precisely what causes fish diseases and so treat them inappropriately, often uselessly wasting money and ultimately making the situation worse. Some farmers’ method of disposing of the fish was to put them back into the river, greatly increasing risks of further disease spread.

To reduce crop loss from diseases, application of basic biosecurity practices are needed to improve disease resistance (Moss et al. 2012). Transporting fish properly, using suitable quantities of high-quality feeds, and vaccinating fish (Evans et al. 2004) are all parts of best management practices to control infectious diseases (Plumb 1999). Avoiding very high stocking densities, prompt removal and burial of diseased fish with quicklime away from the riverbanks, and regular cleaning of cage nets are practical measures farmers can take now to reduce risks of disease in the future.

Acknowledgments Thanks to the many field assistants, students, officials, and farmers who helped with the surveys.

Funding The work was supported by the aid of a grant from the International Development Research Centre, Ottawa, Canada, under Project Number-Component Number 107087 as a contribution to the AQUADAPT project.

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