This study met the ethical standards prescribed by the Animal Experimentation Committee of the Aquaculture Research Institute of Kindai University (ARIKU-AEC-2022-67).
Experimental fish used in this study were artificially hatched PBF juveniles collected 24 days post-hatching [mean total length (TL) was 3.4 cm; mean body weight (BW) was 0.36 g, n = 10] reared in a 60 kl-volume circular concrete tank. A total of 472 PBF juveniles were randomly distributed and stocked in eight 1 kl-volume tanks (54–65 individuals per tank). The appropriate intensity (147.8 ± 6.3 lx; Honryo et al., 2013) of nighttime lighting was provided to each experimental tank during nighttime (18:00–6:00). An automatic feeder was deployed in each tank during the day (06:00–18:00), through which the fish were fed an appropriate-size (0.91–1.41 mm) artificial diet (Magokoro Diet Size C, Marubeni Nisshin Feed). In addition, fish were fed live prey, such as that used in practical fingerling production (e.g., Artemia, yolk-sac larvae of Oplegnathus fasciatus, and fertilized eggs of O. fasciatus), from 09:00–16:00 during the experiment.
The experimental treatment group was given nighttime feed, in which fish were fed such live prey at 19:00 and 22:00 (positive treatment; Night-feed, n = 4). The control group (negative treatment; Control, n = 4) were not fed during the night. Experimental duration was 1 week. Because a high incidence of mortality while transferring fish from nursery tanks to sea cages has been reported (Okada et al. 2014), the day of transportation was considered as day 1 and the feeding experiment was terminated on day 7. Rearing conditions such as water temperature (℃), dissolved oxygen level (%), and salinity (mg L− 1) were constant, as given in Table 1. Filtered and UV-treated sea water was provided to each tank at a rate of 2.0 L min− 1. Dead fish were counted every day when the tank bottom was siphoned for cleaning.
Table 1 Rearing conditions of rearing experiment | |
Treatment | Replications | Temperature (℃) | Dissolved oxygen (%) | pH | Salinity (mg L− 1) |
Night-feed Control | 4 4 | 27.0 ± 0.1 27.0 ± 0.1 | 114.8 ± 6.8 118.9 ± 5.5 | 8.16 ± 0.04 8.20 ± 0.02 | 32.2 ± 0.2 32.1 ± 0.3 |
In this study, growth was compared between the two treatments. Ten individuals were randomly selected from among the surviving fish in each tank and measured for TL and BW at the end of experiment. In addition, gut contents were anatomically examined during the night (22:00–23:00) on days 1, 3, and 5 in both treatments. PBF juveniles were captured from each tank (n = 3–5, respectively) using a hand net and then immediately euthanized using ice-cold sea water, similar to previous methods (Honryo et al. 2013). The TL and BW of captured individuals were measured, and then the abdomen was cut open using scissors and the gut was removed. The stomach and intestine were cut open and their contents were inspected. When the experiment was terminated on day 7, all the surviving fish were captured and counted to calculate the survival rate. Survival rate was calculated using the number of stocked individuals after subtracting the number of sampled fish that were taken to investigate gut contents and the daily dead fish count.
All the date were expressed by mean ± standard deviation. Significant differences between the treatments were compared using an independent t-test at p < 0.05 by using software (SPSS 23, IBM, Tokyo).