Introduction
The cleaner fish Ballan wrasse have long been established as an effective measure to control salmon lice. However, the commercial production of this species is compromised by welfare related disorders such as fin erosion (biting/aggression) and jaw deformities. This eventually leads to a "poor quality" cleaner fish, resulting in high mortalities in salmon cages (National Inspection Campaign 2018/2019). The prevalence of jaw deformities has been reported to be as high as 33 %, however the cause is elusive (Cavrois-Rogacki et al., 2021). Ballan wrasses undergo a distinct ontogenetic shift in feeding strategy, transitioning from zooplankton in early larval stages using pharyngal teeth to ensure that prey doesn't escape, to benthic foraging using maxillary and dentary teeth to ingest pray from a surface. This is associated with ossification of the skull and maturation of the dental apparatus alongside the developing digestive system (Norland et al., 2022). However, in captivity, wrasse is fed with conventional pelagic feed delivery from weaning and onwards. We have therefore hypnotized that depriving the species natural foraging is disrupting normal bone development of the jaw (Ding et al., 2024). This grazing feeding habit of larvae supports the hypothesis that substrate feeding is more appropriate for this species during critical ossification windows in early development, thereby reducing the deformities during production. Thus, this study aimed to investigate whether providing feed on a substrate (feed pasted on to a pipe) rather than pelagic feed (feed dispersed onto the water using a belt) could reduce biting behavior or jaw deformities, and stress thereby improving the welfare in Ballan wrasse.
Materials and Methods
The newly hatched larvae were distributed into six 500 L tanks (B5, B6, B7, B8, B9 and B10) with seawater (35.0 ± 0.5 ppt) and maintained under constant light. The larvae were start fed with rotifers from day 3 post-hatch, followed by artemia from day 25 post-hatch. Weaning started at day 52 post-hatch. Feeding substrate/pipe feeders were introduced into three tanks (B6, B8 and B10) as the treatment group and the control tanks (B5, B9 and B9) larvae were fed using an automatic belt feeder. The Experimental feeding substrates were prepared by mixing a known quantity of the dry feed (Aglonorse Berggylt Start – 0.2-0.3 mm – TROFI) with an agar-based solution until a consistent texture was achieved as previously described by Leclercq et al., 2015 with slight modifications (Leclercq et al., 2015). This feed mixture was then pasted onto a pipe (52 mm x 15 cm) covered with a cotton fabric for a better adherence of feed particles. The pipe was stored for 24-48 h at 5 °C prior to immersion. For each experimental treatment group, three pipes were placed during the start of the experiment and later replaced accordingly with fresh feeders based on the feed left on to the pipe. The net weight of the pipe feeders was measured before and after its deployment to the tank to record the amount of feed leached or eaten. Adjustments were made to the amount of feed in the substrate feeders to match the feed dispersed using belt feeders to keep the feed quantity consistent across these groups. Further, underwater video recordings were also performed to monitor the feed intake and behaviour of larvae/juveniles. The water temperature ranged between 12-15°C for 1-4 weeks days post hatch (dph), and 16-18°C from 5-14 weeks dph. The water flow was maintained at 50 L h-1 during the start of the feeding trial and increased to 300 L h-1 at the end of the trial. At the end of the experiment, larval growth and mortality were recorded. The water quality parameters such as turbidity, and microbial community were also monitored. Hormones such as cortisol, melatonin and serotonin were measured using Liquid Chromatography-Mass Spectrometry (LC-MS) to monitor the general wellbeing of larvae. At the end of the experiment, larvae/juveniles were scored manually and imaged using micro-CT for recording jaw deformities.
Results
A high prevalence of jaw deformities was observed in the belt fed group which were characterised by short or curved upper/lower jaw. The most common deformity observed was short or askew snout. The recorded deformities were significantly higher (P<0.05) in the belt fed group with 33%, than the substrate fed group with 9%. There was no significant difference in turbidity measurements among the different groups, however the substrate fed group had relatively less turbidity than the belt fed group. The microbial community in the water was not significantly (P>0.05) different between the two feed treatments, and the main change in microbial community composition observed was associated with the switch from artemia to the dry feed. Hormone analysis on the whole larvae indicated no significant differences among the group. However, the cortisol levels (ng mg-1 of sample) corresponded with the mortality pattern in the trial, reaching peak level at 67 dph with substrate fed group (9.9±5.02) had relatively higher level than belt fed group (6.4±3.60). The cumulative mortality (%) were significantly higher (P<0.05) in the substrate fed group than belt fed groups. This difference was also reflected in the final growth were belt fed group had a significantly higher growth (mm) (43.86±7.00) than the substrate fed group (34.19±7.38).
Discussion
A functional jaw is critical for delousing efficiency, and fish with jaw deformities can cause significant economic and welfare losses for hatcheries. Previous studies have reported a high percent of jaw deformities indicating production and husbandry practices as a cause for higher deformities (Cavrois-Rogacki et al., 2021). This study showed lower incidences of jaw deformities in substrate fed group compared to belt fed group. This could be because, the feed presented on a solid surface mimics the grazing behaviour observed in wild ballan wrasse which is critical for their dentary ossification process. However, the current study reports a lower survival rate in substrate fed group. Such low survival rates are also reported from commercial rearing of Ballan wrasse hatcheries with rates as low as 3-5% (SINTEF Ocean, 2023). Thus, low survivability in the current study indicates a reduced feed accessibility or feed quantity or feeding efficiency in the substrate fed group during the trial. This was also reflected in the lower weight in this group compared to belt fed group. Reports have suggested that larval survival is better when they are fed continuously (Helland et al., 2014). Therefore, the amount of feed introduced on to the pipe needs to be adjusted and optimised for sufficient feed intake. This reduced feed availability might have caused stress, elevating the cortisol levels, reducing the survival across the trial. Collectively, these findings suggest a trade-off between improved growth and survival under belt feeding and improved morphological welfare under substrate feeding.
Acknowledgment
This work was funded by FHF (FHF# 901800), ROBUST berggyltlarve
References
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