Abstract
Simple SummaryCurrently plant products are used to partially replace fish meals and oils in fish diets. However, the excessive use of these products can cause nutritional imbalances and environmental problems in their production. For this reason, microalgae appear as an alternative, since they have a high nutritional value and improve the immune status of fish. In the present work Nannochloropsis gaditana was included at 2.5% and 5% in substitution of plant products to observe its influence on the growth and morphology of gilthead seabream at low inclusion levels. Furthermore, cellulases were used to degrade cell walls and to increase the bioavailability of the intracellular bioactive compounds. The results showed that the inclusion of N. gaditana at low levels in the raw state was sufficient to obtain optimum growth, so it can be used as a partial substitute of vegetables in gilthead seabream diets, without substantially increasing the cost of the feed.A 90-d feeding trial was conducted in which five groups of gilthead seabream (11.96 g initial body weight) were fed with a microalgae-free diet (control group, C) or four diets containing the microalgae Nannochloropsis gaditana at two inclusion levels (2.5% or 5%), either raw (R2.5 and R5 batches) or cellulose-hydrolyzed (H2.5 and H5 batches), to study their effect on the body and muscle growth. At 40 days, the highest values of body length and weight were reached in R5 group, but at 64 and 90 days, these were reached in R2.5. However, feed conversion rate, specific growth, daily intake, and survival (100%) were similar in all the groups. The acquisition of a discoid body shape was accelerated depending on the inclusion level of N. gaditana in the diets. Moreover, H5 diet affected the fish geometric morphology compared to R5 diet. The white muscle transverse area was similar in all groups at 40 days, with the exception of H2.5 group, which showed the lowest area. At day 90, C and R2.5 displayed the highest muscle growth, attributable to increased hyperplasia in C, and higher hypertrophy in R2.5. However, the highest proportion of small and medium fibers was observed in R5 and H5.
Highlights
The use of plant proteins has allowed the aquaculture industry to grow without increasing the pressure on wild fisheries
We studied the influence of diets with low inclusion level of Nannochloropsis gaditana on the biometric parameters and muscle growth of gilthead seabream (Sparus aurata) juveniles throughout a 90-d feeding trial
We studied the influence of N. gaditana either raw or hydrolyzed on the muscle cellularity of gilthead seabream, in order to determine its possible influence on this specific aspect, even at low inclusion levels
Summary
The use of plant proteins has allowed the aquaculture industry to grow without increasing the pressure on wild fisheries. The sustainability of land-based protein sources is debatable as these compete with land area for human food production and depend on the use of fresh water [1]. Complete replacement of fishmeal with plant proteins has proven difficult for many species, for marine fish [2,3]. Compared to fishmeal and fish oil, terrestrial plant ingredients show imbalanced amino acid composition, high levels of carbohydrates, and potential presence of anti-nutritional compounds that may lead adverse effects on gut health, digestion, and utilization of nutrients [4,5,6]. Soybean derivatives are regularly used as feedstuffs in aquafeed manufacturing for partial replacement of fishmeal and fish oil [2,3]. Dehulled solvent-extracted soybean meal is a high-quality protein source with steady supply and competitive costs. The soybean farming areas have become a major driver for worldwide deforestation and loss of biodiversity in developing countries, along with other environmental and social concerns [7]
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