Abstract
Five experiments were carried out to investigate the biology and culture of mola and darkina with carps in pond
and rice-fields for economic and nutritional benefits in rural areas. Experiments 1 and 2 were carried out in
Fisheries Field Laboratory of Faculty of Fisheries, Bangladesh Agricultural University and 3, 4 and 5 in Dinajpur
and Rangpur Districts during 2012-2014. The first trial was conducted to evaluate the breeding biology of mola in
semi natural condition. It had three treatments of varied hapa size: T1 (2m³), T2 (10m³) and T3 (20m³) with two
replicates each and the stocking density were 50g, 250g and 500g, respectively. Considering the biological
characteristics, i.e. gonad weight, gonado-somatic index (GSI), condition factor (K) of both sexes and the
fecundity, mola was found to have inherent regulating mechanism of oocyte development i.e. in spite of their
single peak spawning habit, they do not discharge all oocytes at the same time but in different batches throughout
the spawning season. The second experiment was undertaken to know the breeding biology of darkina in thirty
six small hapas (0.25 m³ each) and one big hapa (1 m³). The stocking density was 10 fish.hapa-1 and 40 fish. hapa-1
in small (H,) and big hapa (Hb), respectively. The fishes of three small hapa out of thirty six were emptied, fishes
were sacrificed and their length (cm) and weight (g) were measured in each month. The fishes of big hapa were
counted and weighed in every month released and reared up to the end of trial. Monthly morpho-histological
examination of the gonad revealed the existence of five maturity stages (immature, maturing, mature, ripe and
spent and resting) in darkina. Considering microscopic and histological assessment, the peak breeding season of
darkina was in June and September. The fish was found to be a multi spawner. The third experiment was
conducted to increase the mola production within carp polyculture system under different management regimes.
Household ponds were stocked with the fingerlings of rui (Labeo rohita), catla (Catla catla), mrigal (Cirrhinus
mrigala), silver carp (Hypophthalmichthys molitrix), grass carp (Ctenopharyngodon idela) and bata (Labeo bata) at a
standard stocking rate, and mola (Amblypharyngodon mola) in weight of 150g 40 m-2. There were three treatments,
early stocking, T1 (April mola stocked), seasonal stocking, T2 (July mola stocked) and traditional stocking, T3
(April-December no mola stocked). The total fish production was the highest (3,783 kg ha-¹) in T1 and the lowest
(2,837 kg ha-¹) in T3. Small fish contributed 55% to total fish production and was higher in T3 than T2 and T₁
treatments. Economic analysis showed the highest BCR (3.29) in treatment T₁. The fourth trial was carried out to
examine small fish production with rice concurrently. Rui, catla, mrigal, silver carp, silver barb, bata mola and
darkina were stocked. In rice plots, different carps and mola were stocked in T1- (RCM), carps and darkina in T2-
(RCD) and both mola and darkina with carps in T3- (RCMD). The stocking rates of carps were same (0.5 m-2) in all
treatments. Either mola or darkina or both were stocked 150g 40 m-2. The gross production of fishes was the
highest (1, 023 kg ha-1) in T3 and the lowest (994 kg ha-1) in T2. Between the treatments (T1 and T2), darkina
production was 4% higher than mola. In cash earning, mola contributed just double of darkina between RCM and
RCD treatments. Among the treatments, carp production was more or less similar, but the production of rice and
straw were higher in T3. Both mola and darkina reproduced two to three times during the experiment. Therefore,
the total fish production was enhanced due to their self recruiting characteristics. Among all fishes, mola played a
major part for increasing BCR because of their high market value. Among the three treatments, the highest BCR
(2.70) was recorded in T₁, where 232.56 kg ha-¹ mola was produced. Carp-SIS polyculture in rice-field provided
the farmers with nutrient-rich mola and darkina and cash income by selling carps. From the concurrent culture,
the fish excreta accumulated in the soil, which would increase rice production in the following year. The fifth trial
was carried out where fish and rice were cultivated alternatively. The two treatments were conducted in ricefields with carps-mola (T1 - RCM) and only carps (T3 - RC) and in ponds with carps-mola (T3 - PCM). The stocking
rate of carps was (0.5 m-2) in all the rice plots but 1 m-2 in ponds. Mola was stocked at the same rate (150g 40 m-2)
in all the treatments. The gross production of fishes was the highest (3158 kg ha-¹) in T2. Among all, carps species
had the highest harvesting weight in rice-fields. Subsequently, other SIS production was the highest in T3, where
small fish was not stocked. Higher production and high market value of mola boosted up BCR (3.43) in T1 among
all the treatments. Hence, it may be summarized that alternate rice plots are richer in natural food which may
have contributed to enhance production of both carps and mola.