Physico-chemical parameters of water
The physicochemical parameters of water such as temperature (°C), pH, dissolved Oxygen (mg L
-1), Total hardness (mg L
-1), total alkalinity (mg L
-1), ammonia (mg L
-1), Nitrite-N (mg L
-1), Nitrate-N (mg L
-1), were recorded. The observed water quality parameters were as follows: temperature (28.5°C - 30°C), pH (7.3-7.8), dissolved oxygen (5.02-6.35 mg L
-1), alkalinity (104 -136 mg L
-1 as CaCo
3), total hardness (102-114 mg L
-1 as CaCo
3), Water conductivity (0.194-0.36 S/m), Phosphate level (0.0075-0.85 mg L
-1), nitrite-N (0.21-0.87 mg L
-1), nitrate-N (0.007-0.87 mg L
-1) and ammonia (0.04-0.08 mg L
-1) during the experimental period of 120 days.
Egg size and fish maturity
The mean egg sizes of the broodstock fishes from different treatment groups were presented in Table 1. The average egg size of treatment T1 (Provided with only reservoir water without any soil base and macrophyte) was found smallest among all treatments, measured about 0.68±0.03 mm. The average diameters of eggs were measured around 0.80±0.03 mm and 0.95±0.71 mm in T2 and T3, respectively. Egg size in T4 was observed to be the largest among all the treatments and recorded to be 1.29±.21 mm. The pictures of immature and mature egg are represented in Plate 1.
Histological observations of male and female brood fish
The gonad histological sections of the brooders raised in different habitats are presented in Fig 1 and 2. The ovarian stages of fishes from different treatment groups were varied considerably (Fig 1). In the treatment T1 (without any soil base or macrophyte), the ovary was found at maturing stage characterized by the presence of chromatin nucleolar oocyte and early peri nucleolar stages. Most oocytes in T2 were of the late peri nucleolar stage, but the fishes were still maturing. Yolk vesicle stage is clearly visible in the matured oocytes of ovary of the brooder fishes from treatment T3 and Granular stage was also visible in many matured oocytes of the same sets of ovary. The fishes from the T4 were almost in the spawning stage, where most of the cells were in late granular stage. Apart from the ovary, the histology of testes of the experimental fishes was also showed significant differences among different treatment groups (Fig 2). In T1, testis cells were mostly found in the spermatogonia stage, whereas in T2, the spermatocytes were visible along with the spermatogonia. In the T3 and T4, the testis cells had both spermatids and spermatozoa in abundance and especially the histo-morphology of T4 indicates the spermeation stage.
Habitat plays a vital role in the successful spawning of fishes and the captive environment, often resulting in inhibition of reproductive maturation. Thus, the availability of quality and desirable spawning habitat is the key to reproduction success in fishes
(Taylor et al., 2019). Striped murrel brooders usually prefer to stay in a muddy area with heavy macrophytes
(Kumar et al., 2021a). However, rearing striped murrel brooders in the weed-infested earthen ponds also causes hardships during the collection due to their hiding behavior under the mud. This increased handling stress on striped murrel brooders often results in poor breeding response upon successive induced breeding trials. Thus, mimicking natural conditions in the confined tank environment is an effective strategy for speeding up the reproductive maturation of the fishes in the captive seed production process. Bottom substrate and aquatic vegetation are proven to be affecting the maturation synchronization of the brooder in captive breeding (
Zohar, 1989;
Yaron, 1995;
Bohlen, 2003;
Migaud et al., 2013 and
2018). Considering the effect of habitat, the various manipulations on the tank environment were tried to achieve early maturation in striped murrel. The maturating stages were analysed through external morphological characters, egg size and histological studies.
During the present study, the striped murrel brooders were fed with a nutritious diet to favour the early maturation process in captivity (
Kumar and Mohanty, 2018) and the bulk of the dietary protein share comes from fishmeal (
Siddaih et al., 2022 and
2023). Small indigenous fishes were packed with healthful nutrients like omega -3 and omega-6 fatty acid (12.2 to 27.2%), fat (>4%), protein (>16%), minerals (calcium, iron and manganese, phosphorous) and vitamin A, D, E and K
(Mohanty et al., 2019). In addition to their nutritive value, feeding the brooders with live fishes often helps maintain the predatory and chasing behaviour of the striped murrel brooders. Sardines are a rich source of protein, omega 3 fatty acid, vitamin B12, minerals (
Nisa and Asadullah, 2011;
Shaji and Hindumathy, 2013;
Mohanty et al., 2019;). Squids are a rich source of protein and contain a good amount of omega-3 polyunsaturated fatty acid (
Steffens, 1997;
Atayeter and Ercoşkun, 2011).
In this study, different habitats were tested for early maturation. Habitat, which resembles their natural environment (soil base majorly having clay loam soil type with macrophytes), had a more positive impact on the gonadal maturation of striped murrel brooders. The more or less similar effects of the environment were suggested by (
Zohar and Mylonas, 2001). The size of an egg is a key tool for assessing the gonadal maturity of the fishes. In striped murrels, the ova of above 1000 µm diameter is considered mature and indicates the readiness of spawning (
Kilambi, 1986;
Kumar et al., 2021b). Accordingly, in the present study, the eggs size of fully matured striped murrel brooder fish was ranged from 1.29±0.21 mm to 1.38±0.006 mm. Apart from that, the size of the egg also has a strong impact on the hatchability and vitality of the offspring produced, as studied by
(Furuita et al., 2003; Biswas et al., 2005; Furuita et al., 2009; Sarmento et al., 2018; Kumar et al., 2021b).
Dopamine a potential neurotransmitter produced in the brain that inhibits the action of GnRH in the pituitary of fish
(Peter et al., 1988). The increased breeding performance in the soil-based and macrophytes provided tanks of the present experiment is probably because of the elimination or reduction of that maturation inhibiting compounds in fishes. Apart from that, the environmental microbiome of the rearing tank is enriched when provided with the bottom soil layer. These naturally correlated microbiomes influence the gut microbiome of the fishes, which is suspected of having a connection with the brain and might eventually alter the behavior pattern of fish (
Gershon and Margolis, 2021). The consecutive positive changes in the reproductive endocrinology and physiology of the experimental fishes might be the reason for the early maturity in the fishes reared in captivity.
Histological studies of T3 and T4 indicated that the male and female fishes were fully mature and in the spawning stage. In the mature ovary, the cells were blended with the yolk vesicle and yolk granules, whereas yolk granules and late granules were observed in spawning ovaries. In the case of matured testes, the spermatid and spermatozoan cells were abundant. A similar observation has also been recorded in striped murrel by various authors
(Irmawati et al., 2019; Milton et al., 2018; Boonkusol et al., 2020). In the present study, the brooders from the T1 and T2 were in maturing stage, T3 and T4 were in matured and spermeation stage, respectively.