Introduction
Production of fish larvae is dependent on the use of live feeds for initial feeding. However, live food is costly, and may cause nutritional deficiency and low growth rates. A successful weaning strategy must ensure the production of well-fed larvae with fast growth and high survival, thus reducing production costs (Imentai et al., 2020). The histological study of the liver and intestine might support the evaluation of the nutritional status of larvae during the rearing process.
The flathead grey mullet Mugil cephalus is an emergent species for Mediterranean aquaculture diversification. However, little information on its larval nutritional requirements is available, and weaning still represents a bottleneck in this species (Loi et al., 2020). The aim of this work was to test three different co-feeding strategies on M. cephalus larvae, to find out whether reducing the level of Artemia sp. in the feeding protocol affected specific indicators of nutritional status, such as growth performances, hepatocyte vacuolization and intestine morphology.
Materials and methods
Larvae (22 dph), previously fed enriched rotifers, were divided into 3 experimental triplicated groups, at initial density of 4 larvae L-1 (1090 larvae in each 300 L tank). A 12h L: 12h D artificial photoperiod was set. Main water parameters were regularly monitored. Three weaning protocols: A100, A50 and A0 (2, 1, and 0 Artemia sp. ml-1 day-1, respectively) were applied. For A100 (standard protocol), 5 enriched rotifers ml-1 day-1 were provided until 26 dph, together with 2 enriched Artemia nauplii ml-1 day-1 until 25 dph, gradually reducing the dose until 32 dph. In A50, the dose of Artemia was halved, while in A0 larvae received only rotifers until 32 dph (end of live prey period). In addition, a commercial microdiet (Gemma micro 0.1, Skretting) was offered in all treatments for the whole experiment, mixed 1:1 with Gemma wean 0.2 (Skretting) from 31 dph to the end of the experiment (36 dph).
Mullet growth performances were assessed at each of 3 sampling points, measuring total length (TL, mm), myotome height (MH, mm), Condition index (CI) and eye diameter (ED, mm). Five fish per tank were also preserved for histological analysis. Liver lipid area was assessed as described in Loi et al. (2020). At the end of the experiment, 10 intestinal fold height (Fh) and 10 enterocyte height (Eh) were measured in the histological sections of each fish and the mean values were used for the analysis. Finally, at the end of the experiment, mortality was also estimated.
Results
All the growth parameters increased over time. TL and MH were significantly (p<0.05) higher in the A100 treatment. CI and ED showed no statistical difference between A100 and A50, but in the A100 they were significantly higher than in A0 (p<0.05). The liver area covered by lipids grew significantly in the last experimental week, with no difference between treatments. In the intestine, Fh was significantly higher in fish fed Artemia, while Eh in A100 was statistically higher than in the other treatments. The lowest mortality was observed in A0 (35%), while A100 presented the highest mortality percentage (68%), followed by A50 (57%).
Discussion
ED resulted bigger in A100 larvae, implying a larger retinal image and an improved visual acuity (Papadakis et al., 2018). This suggests that fish were better able to catch preys than fish from other treatments and this may have led to a higher growth rate in the larvae fed standard Artemia dosage. The myotome development and the CI are sensitive markers for assessing inappropriate feeding conditions in fish larvae (Yúfera et al., 1993). The lower MH and CI measured in A0 may be a consequence of poor feeding conditions of the larvae fed rotifers only, as previously observed by Imentai et al. (2020).
Histological observations of liver and intestine are considered indicative of the nutritional status of fish larvae (Przybyl et al., 2006). In this study, no significant effect of the dietary protocol was observed at the hepatic level; however, lipid coverage was slightly higher in fish fed Artemia and especially in those from the A100 treatment. This is possibly related to a higher amount of lipids in Artemia compared to rotifers (Imentai et al., 2020). The higher lipid coverage is consistent with better growth performances observed in the Artemia treatment. Significant differences were furthermore observed in the size of intestinal villi and in the height of the enterocytes, which were longer in fish from A100 than in fish from A0. Similar results were observed in pikeperch by Imentai et al. (2020), where larvae fed exclusively rotifers had significantly lower enterocyte height. It is thus plausible that fish from the A100 treatment were better-fed and grew faster as a consequence of a higher surface area for nutrients absorption.
In conclusion, fish fed standard dose of Artemia shown the best growth performances and hepatic and intestinal conditions. This may have been driven by the lowest final density observed in A100 due to the mortality rate, or by the highest nutritional value brought by the Artemia intake. Nonetheless, the data observed in the A50 treatment suggest that halving the Artemia dosage may still result in good larval performances and survival, and lower the production costs of live food.
References
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