Investigation on Larvicidal Efficacy of Two Native Ornamental Murrels of Assam under Controlled Condition

Mosquito-borne diseases persist as one of the most prevalent threats to human health all over the world. According to World Health Organization 2017 report, there were 219 million cases of malaria across 90 countries. Control of mosquitoes using insecticides is expensive, harmful to the environment and can lead to pesticide resistance in mosquitoes (Chandra et al., 2008). There are some other alternative approaches with organic pesticides and biological control (Howard et al., 2007). Use of fishes for control of mosquito larvae has been practiced in many countries of the world (Neng et al., 1987; Morton et al., 1988; Kim et al., 1994 and Hurst et al., 2004). Fish as biocontrol agents is a safe, cheap and effective alternative strategy to chemical control (Kusumawathie et al., 2008) yet proof for their outcome is very scarce (Walshe et al., 2017). Also, the use of some non-native larvivorous fish for mosquito control leads to serious ecological concerns (Azevedo-Santos et al., 2016; El-Sabaawi et al., 2016). Widely used larvicidal fishes like Gambusia affinis and Poecilia reticulata are invasive and highly competitive with the native fishes (Hurlbert and Mulla, 1981; Bence, 1988).

Bhattacharjee et al. (2009) assessed indigenous air-breathing fishes as suitable predators of mosquito larvae. Phukon and Biswas (2011 and 2013) worked on larvicidal potential of certain locally available fishes of Assam. Rao et al., (2015) also studied the larvicidal efficacy of four indigenous ornamental fish species. Therefore, the present study was designed to evaluate the larvicidal potential of Channa bleheri and Channa stewartii under laboratory condition.

Collection of sample and acclimatization
 
Two prominent indigenous ornamental murrels (Biswas et al., 2007), C. stewartii (Fig 1A) and C. bleheri (Fig 1B) were collected from wetlands of Tinsukia district (27°53' N, 95°65' E) of Assam, India during March-April, 2019. Live specimens were brought to the Department of Zoology, Digboi College, Assam, identified as per Talwar and Jhingran (1991) and acclimatized them properly in separate enclosures. The period of experiments was from June to August 2019, as availability of mosquito larvae is maximum during monsoon. Both Channa bleheri and Channa stewartii were sorted out into two size groups- large and small. Weight of the experimental fishes were taken by a digital balance. Group A Channa bleheri ranged from 11.0 to 15.5 cm (10.9-30.8gm) having mean length 13.27±2.25 cm and weight 21.0±9.95 gm and that of Group B was 8.5 - 10.0cm (7.7-8.9gm) with mean length 9.17± 0.76 cm and weight 8.2±0.62 gm. For Channa stewartii, size of Group A ranged from 10.0 to 13.0 cm (9.3-14.9 gm) having mean length 11.63±1.52 cm and weight 12.43±2.85 gm; Group B 8.0 - 9.2 cm (6.8-12.0g) having mean length 8.67±0.61 cm and weight 9.27±2.61 gm. Collected fishes were stocked in 1% KMnO4 treated water at 25 - 26°C and fed them commercial food ‘Tokyu’ (Sanyal and Ghosh, 2014).



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Feeding experiments
 
The target species were introduced in three 8 litre glass aquaria of equal size (12² x 12² x12²) to observe the consumption efficacy. The experiments were conducted in two size groups of each fish species both at individual (9 fishes) and group (27 fishes) level as well as during daytime in natural light and evening time with artificial light (tube light). During daytime experiments, the fishes were kept in glass aquaria with indirect sunlight. In case of group, three fishes of each batch were introduced into three different aquaria. The fishes were kept in starvation for 12 and 24 hrs. prior to the actual feeding experiments. Known numbers of larvae (n=100) were offered (Rao et al., 2015; Bano and Serajuddin, 2016) to the experimental fishes both in the case of individual and group fish triplicates.  Predation rate/ hour of test fishes was calculated by taking the difference of initial and final no. of larvae. Water temperature of rearing enclosures was also recorded twice a week and pH of water was recorded with a digital pH meter. Average temperature of water was maintained at 25 - 26°C and pH at around 8.3. All the experiments were performed following the ethical guidelines for fishes.

Culture of mosquito larvae was done in a separate tank. Cut pieces of potato were placed and about 250 gm of cow dung was mixed to attract mosquitoes to lay eggs. Mosquito larvae were harvested with a fine net with small mesh size, stored in glass container and then 4th instar larvae were sorted out to feed the experimental fishes (Das, 2012). Each trial was repeated three times with three sets of fishes for individual and group and the mean consumption rate/min and standard deviation were calculated using MS Excel 2007.

The results on the predation rate of the selected murrels after different intervals of starvation and also under daylight and artificial light in evening hours were given in Table (1-4) and elaborated as follows:-

Predation rate of Channa stewartii at different experimental set-up
 
During daytime, mean consumption of mosquito larvae by individual Channa stewartii (Group A) (10-13cm) was 19.0±2.88/min and in group 28.9±4.16/min at 12 hr of starvation whereas it was 30.9±4.21/min and 38.4±5.10/min by individual and group respectively at 24 hr starvation. In the evening, however, the mean consumption was recorded as 16.3±0.95/min and 27.1±1.85/min after 12 hr of starvation at individual and group level respectively. After 24 hr starvation, the mean consumption by individual C. stewartii was 28.7±2.50/min and 34.4±4.05/min by the group during evening hours (Table 1).



After 12 hr of starvation, C. stewartii (Group B) (8.0-9.2 cm) predated more larvae than the adult individual during daytime (Table 2). Mean consumption by individual C. stewartii was 36.2±2.63/min and that of larger (Group A) was 19.0±2.88/min. Mean consumption by smaller specimens was recorded as 36.2±2.63/min by individual and that of in group as 44.2±3.07/min. After an interval of 24hr starvation, individual C. stewartii (8.0-9.2cm) showed the daytime mean consumption of 51.2±5.20/min and in group 68.2±2.77/min.

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Experiment carried out with Group B during evening hours revealed mean consumption by individual  fish was 27.5±3.26/min and in group was 37.6±2.88/min after 12hr starvation; whereas the mean consumption by individual fish was 46.3±3.33/min and in group 56.9±5.08/min after 24 hr starvation in the evening (Table 2).
 
Predation rate of Channa bleheri at different experimental set up
 
C. bleheri (Group A) showed mean predation rate of 37.4±3.49/min by individual fish after 12 hr of starvation during daytime and that of 42.8±1.09/min when in group. After 12 hr of starvation, predation in the evening was 33.4±2.25/min and 38.0±2.17/min by the same individual fish and group respectively. Again after 24 hr starvation, C. bleheri exhibited mean consumption of 40.6±6.22/min by individual fish and 51.8±3.17/min in group during daytime. But at the same interval of starvation during night the species showed mean consumption of 33.3±4.36/min at individual level and 42.3±2.02/min by group (Table 3).

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During daytime, Group B C. bleheri (8.5-10.0cm) after 12 hr starvation mean consumption was 45.4±6.50 /min by individual fish and 54.7±4.44/min by group whereas the same size group had mean consumption of 53.8±4.52/min and 71.6±5.15/min by individual fish and group respectively after 24 hr of starvation (Table 4). In contrast, when larvae were provided to the fasting group after 12 hr interval at night hours, the mean consumption by individual fish was 45.2±5.00/min and that of group was 49.7±6.00/min (Table 4).

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During evening, after 24 hr starvation, C. bleheri (Group B) had mean consumption of 47.9±4.55/min and 58.2±11.18/min at individual and group level respectively (Table 4). In the evening, the consumption was found to be less than daytime predation both at individual as well as group level of both the species.

Feeding intensity in both the murrels under natural and artificial light varied depending on their size. Consumption of larvae during daytime was observed to be higher than that of evening time. It was observed that during the first 10-15 minutes of feeding experiment, the fishes consumed maximum numbers of mosquito larvae and then it gradually slowed down. Fishes when in groups, consumed large number of larvae than the individual fishes. Also, variation in predation rate of mosquito larvae at 12 and 24 hrs of starvation was quite prominent in both the species. Feeding behaviour indicated that both the fish species are diurnal feeders.

Group A (larger size fishes) of both the experimental species consumed mosquito larvae fairly well. However, smaller size (Group B) C. bleheri had the least consumption time after 24 hrs starvation both individual and group level.

Among the larvicidal fish used across the globe, the native fishes were found to be better performer for biological control (Howard et al., 2007; Chandra et al., 2008; Kendie, 2020). Studies were also made to assess the larvicidal efficacy of some native fish species of Assam – Channa gachua, Puntius sophore and Trichogaster fasciata and the murrel, Channa gachua showed the highest larvicidal efficacy (Phukon and Biswas, 2013). Similar study was also carried out on indigenous fish with exotic Gambusia affinis by Bano and Serajuddin (2016). The present study showed that efficacy of smaller specimens of C. bleheri were better than those of the larger specimens at individual and group levels. Phukon and Biswas (2011) however, found that among three size groups of Channa gachua, the largest size group consumed highest number of mosquito larvae. The present study also revealed C. stewartii as an efficient predator was in accordance with that of Das (2012), who also reported it as an efficient consumer of mosquito larvae among five species of potential ornamental fishes.

It is well known that the abiotic parameters, particularly water temperature and dissolved oxygen have a great role in feeding as well as survival of fish. Both the selected species are air-breathing murrels, well adapted to live and propagate in ditches, ponds and wetlands; hence dissolved oxygen is not a very important factor for them. However, all Channa species hibernate and as such feeding intensity of murrels is very low in natural condition during winter. In the present study, no significant difference in feeding intensity was noticed as water temperature was always maintained around 25^oC in the rearing enclosures.

Both C. bleheri and C. stewartii during 1 hour observation, revealed as efficient predators of mosquito larvae as they consumed a considerable number of 4th instar larvae in two different starvation periods both at daytime and evening. It may be concluded that fry and fingerlings of the selected murrels are highly potential agents for effective controlling of mosquito larvae. Further, both the experimental fishes are insectivorous in feeding habit and breed naturally in wetlands (Nayak et al., 2020). There is alarming degradation in the freshwater biodiversity due to entry of several exotic fishes in India (Kumar, 2000). Use of potential native fishes as a part of vector management programme hitherto been unexploited and both Channa bleheri and Channa stewartii  have proved their potential as larvicidal fishes.