Introduction
Swim bladder inflation (SBI) is a fundamental step in larval development for both physostome and physoclist fish species (Balon, 1975 ; Price and Mager, 2020). Failure of the SBI triggers higher energy expenditures of the individual, which negatively affects swimming ability, growth rate, feeding efficiency, susceptibility to predation, and survival rat e (McElman and Balon, 1979 ; Summerfelt, 1996) . In aquaculture, the SBI failure is caused by different factors including: (a) water contamination with oil increasing the surface water tension and preventing the larvae from gulping air bubbles for inflation, and (b) bacterial aerocystitis of the swim bladder caused by gulped organic debris (Summerfelt, 2013). Hence, novel and effective water treatment technologies such as nanobubbles can be employed in aquaculture to reduce the risk of low SBI. This study aimed to apply the air nanobubbles alone and in combination with conventional water treatment technologies, i.e . spray and water skimmers to promote swim bladder inflation success in European perch larvae under aquaculture rearing conditions.
Materials and methods
Four experimental treatments were conducted in a triplicate design to investigate the effect on SBI of European perch larvae: (a) surface spray covering one-third of the tank diameter and skimmers (SS); (b) air nanobubbles (ANBs); (c) surface spray covering one-third of the tank diameter, skimmers and air nanobubbles (ANBsSS ); and (d) control with no water treatment technology. To prevent cross-contamination of NBs between treatments and control groups, the experiment was done in two recirculating aquaculture systems (RASs). Larvae from all four experimental conditions were sampled at the 6th, 9th, 12th, 15th, 18th, and 21st dph. For each sampling point, 30 individuals per tank (90 ind. per treatment) were killed by a lethal dose of clove oil (Hamackova et al. 2006) and weighed to the nearest 0.10 mg. Measuring the individual standard length (SL), counting larvae with inflated and non-inflated swim bladder and determining the swim bladder length (SBL) were done. Swim bladder inflation efficiency (SBIE) was calculated as the number of larvae with inflated swim bladder/total larvae examined x 100. Survival rate (SR) was determined as the percentage of surviving larvae in relation to the total number of stocked larvae in each sample. Viability was calculated as the SR × SBI /100 = (S × GBI )/100, where SR is the percent survival rate, and SBI is the percentage of the survivors that had an inflated swim bladder (Clayton and Summerfelt, 2010). The yield presents the number of harvested juveniles with inflated swim bladder in the total water volume of the rearing tank (Ljubobratović , et al., 2019). the nanobubble generator was connected to the RAS system with the respective experimental treatments, i.e. ANBs and ANBsSS . Water from the retention tank was pumped into the nanobubble generator and then to tanks once per hour for 15 min.
Results
At the end of the experiment, the final SBL and SL of the larvae were not significantly different (p ≤ 0.05) across all treatments. T he final BW was approximately 1.5-fold higher in all treatment groups compared to the control groups. The low BW of the control groups seems to be in line with the final SBIE that is 2.2- to 2.4-fold higher in SS- and ANBsSS larvae with SBI compared to the control groups. In contrast, the SR was 2-fold higher in individuals from SS and ANBsSS groups compared to the control groups. A very low final SR (19.57%) was also recorded in the ANBs treatment group, although final SBIE and BW were in comparable ranges. The viability and yield were significantly higher in ANBsSS group, followed by SS group, while ANBs and control groups did not differ.
Discussion and conclusion
All treatments showed higher SBI efficiency compared to the control , where no water treatment technology was employed. Higher SR and SBIE and consequebtly higher viability and yield were found in individuals from SS and ANBsSS groups. This may be explained by the beneficial combination of SS and ANBsSS to remove the oil film layer on the water surface. The resulting enhanced SBIE observed in our study, invariably achieved better larvae growth gain, what was also shown for ANBs but with lower SR. A higher SR in the ANBsSS group could be achieved by the Brownian motion of the air NBs, which produce an “up-welling” effect and stirs up small and passively swimming larvae in the water suspension, thus minimizing their swimming requirements(Kolkovski et al.,2004). However, applying ANBs alone resulted in an unexcepted low SR especially in early larval stage. Hence, appling NBs later in the larval stage can result in an additive effect to already used technology such as surface sprays, skimmers and upwelling. Moreover, different types of cannibalism should be also explored in future studies to get an overall picture of the NBs effect.This puzzling observation remains to be investigated in the future because there are still gaps in the effects of ANBs on larvae’s SB inflammation processes.The current study offers the first investigation of the nanobubbles’ effect on SBI promotion in larvae of European perch and gives motivation for future optimizations.
References
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