IMPROVING MICRODIET FORMULATIONS FOR LARVAE OF TWO FLATFISH SPECIES: SENEGALESE SOLE AND TURBOT

Introduction

Flatfish species, such as Senegalese sole (Solea senegalensis) and turbot (Scophthalmus maximus) are successful niche species in European aquaculture, due to their high quality white flesh and high market value. However, these species face different scenarios in husbandry during the early developmental stages: while in Senegalese sole a significant R&D effort performed in weaning procedures during the last two decades has translated into an evolution in feeding regimes, microdiet quality and optimization of larval zootechnical procedures (Pinto et al., 2018), published effort on turbot early life-stages feeding and nutrition is very limited. Currently, under normal rearing conditions, Senegalese sole larval survival at the end of two months of development may reach 60-70 %, whereas in turbot lower values are commonly reached (20–30%). One of the hypotheses for this limitation in turbot survival during the early development stages is the lack of microdiets tailored to its nutritional requirements and feeding behaviour. To this end, this study aims at demonstrating the impact of microdiet formulation evolution in Senegalese sole larval husbandry, as well as demonstrating the first steps in the creation of tailored diets for turbot larvae.   

Materials and methods

In Senegalese sole, a trial for specific microdiet development aimed at establishing its dietary preference for a main protein source. To this end, four microdiets were formulated and produced by cold-extrusion, comprising of the following main protein sources: fishmeal (Fish), squidmeal (Squid), mixture of plant-proteins (Veg) and mixture of marine and plant-based ingredients (Mix). Sole were reared under standard procedures in triplicate tanks and fed on these experimental diets from 28 to 53 DAH. Diets were supplied to the tanks using automatic feeders, set to supply 8 meals in a 24 hour period (two hour feeding and one hour of stoppage time). In turbot, a trial aiming at determining its dietary preference for a main protein source was also conducted. In this case, four microdiets were also produced by cold-extrusion and the following dietary treatments were considered: Fish, Squid, krillmeal (Krill) and Mix. Turbot post-larvae were reared under similar zootechnical procedures described for Senegalese sole from 23 to 49 DAH. At the end of both trials fish were analysed for dry weight, survival, relative growth rate (RGR) and feed conversion ratio (turbot only). 

Results

At the end of the sole experiment, no significant differences were observed in larval dry weight, RGR (varying from 6.3 to 8.6 % per day) and survival (Figure 1) when post-larvae were fed diets Fish, Squid, Veg and Mix. Conversely, diet Squid negatively affected turbot dry weight, survival (Figure 1) and feed conversion ratio (0.7-1.1), with post-larvae dealing positively with diets Fish, Krill and Mix. No significant differences were found between treatments for larval relative growth rate during the course of the experiment (varying between treatments between 14.5 and 15.1 % day-1).

Discussion

In general terms, Senegalese sole and turbot post-larvae displayed different responses to microdiets containing different protein sources. Senegalese sole post-larvae were more eclectic, not showing a clear preference between diets containing marine or plant-based proteins. The relative growth rate (RGR; up to 8.6 % day-1) reached for the different treatments was satisfactory, but continuous research focusing on the impact of different nutritional compositions, including protein complexity (Pinto et al., 2016), have boosted Senegalese sole growth performance, nowadays reaching RGR values of 12-13 % day-1 during weaning. On the other hand, turbot larvae performed best on diets containing fishmeal, krill and a mixture of marine and plant-based proteins, with a clear detrimental effect on larval performance being observed in fish fed a high quality squid-meal based diet.

Interestingly, both flatfish species also present opposite feeding behaviours: while Senegalese sole is more passive and feeds in the bottom of the tanks, turbot is more active and feeding occurs at water surface (Bruno et al., 2018). Such comparison of feeding behaviour, together with diet preference results herein obtained, suggest that microdiets suitable for Senegalese sole may not be optimised for turbot, both in terms of nutrition and, particularly, in terms of microdiet physical properties in water (dispersion at surface and sinking behaviour). Overall, results support that diets should be optimised in terms of nutritional composition and physical behaviour in water to boost turbot growth and survival at early developmental stages. Moreover, this work supports the concept that dietary customisation for marine fish larvae of different species will contribute to the production of high quality juveniles in aquaculture.

References

Bruno, E., Højgaard, J.K., Hansen, B.W., Munk, P., Støttrup, J.G., 2018. Influence of swimming behavior of copepod nauplii on feeding of larval turbot (Scophthalmus maximus). Aquaculture International 26, 225-236.

Canada, P., Engrola, S., Richard, N., Lopes, A.F., Pinto, W., Valente, L.M.P., Conceição, L.E.C., 2016. Dietary indispensable amino acids profile affects protein utilization and growth of Senegalese sole larvae. Fish Physiology and Biochemistry 42, 1493-1508.

Canada, P., Conceição, L.E.C., Mira, S., Teodósio, R., Fernandes, J.M.O., Barrios, C., Millán, F., Pedroche, J., Valente, L.M.P., Engrola, S., 2017. Dietary protein complexity modulates growth, protein utilisation and the expression of protein digestion-related genes in Senegalese sole larvae. Aquaculture 479, 273-284.

Pinto, W., Engrola, S., Santos, A., Bandarra, N.M., Dias, J., Conceição, L.E.C., 2016. Can Senegalese sole post-larvae effectively grow on low dietary DHA and lipid levels during weaning? Aquaculture 463, 234-240.

Pinto, W., Engrola, S., Conceição, L.E.C., 2018. Towards an early weaning in Senegalese sole: A historical review. Aquaculture 496, 1-9.

Acknowledgements

This project has received funding from the Ministry of the Sea, Portuguese Republic, through the program Blue Growth, Innovation and SME, under reference PT-INNOVATION-0025.