Introduction
Marine larval feed technology is a fast changing field with great progress in manufacture technology. Many of the fish larval feed industries provide small enough feed particles that cover the nutritional needs of newly hatched larvae and have high acceptability and palatability, water stability and low nutrient leaching (Hardy and Barrows, 2002). In the hatchery practice of red seabream Pagrus major, dry feed is progressively introduced to fish larvae not earlier than 20-22 days post hatching (dph) with the concomitant use of live feeds (rotifers, Artemia) until weaning. However, compared to live feed, dry feeds appropriately manufactured (i.e. physical and nutritional properties) are expected to better provide for all nutritional needs of altricial fish species, such as red seabream. Although the exclusive use of dry feeds is still inefficient, it is widely accepted that the longer the co-feeding period of live and dry feeds, the better the larvae performance at weaning (e.g. Cañavate and Fernández-Díaz, 1999; Khoa et al., 2020). The aim of the present study was to investigate the introduction of dry feed as early as on mouth opening (3 dph) on growth and functional development of the digestive system of red seabream larvae, as well as to monitor post-larvae growth performance and deformities.
Materials and methods
The experimental trial was conducted in two stages: Stage 1, Hatchery rearing (Hr), was performed in a commercial marine fish hatchery. Four tanks of 9 m3 were stocked with eggs of the same broodstock. In two of the tanks a commercial dry feed (Larviva Prostart, Biomar) was introduced on 3 dph (DF3), while in the other two tanks the dry feed was introduced on 22 dph (DF22) according to a common hatchery protocol. In all experimental tanks, the larvae were fed with rotifers from 3 dph up to 21 dph and with Artemia nauplii/metanauplii from 16 dph up to 35 dph. From 36 dph larvae were fed dry feed only. Larvae samples were observed under microscope to confirm food consumption. During larval rearing, water quality was monitored daily and larvae were sampled daily from 2 to 10 dph, and at 6-days intervals up to 40 dph to estimate larvae length and digestive function (i.e. lipase, amylase, trypsin, chymotrypsin, pepsin specific activities). On 53 dph larvae of each duplicated tank were graded to two size classes (Big, Small). Data obtained were used to calculate survival and performance. Stage 2, Laboratory rearing (Lr), was performed in a recirculating seawater system. On 43 dph ungraded larvae of each duplicated tank were transferred to laboratory installations. On 50 dph, six hundred fish from each duplicated Hr tank were group weighed and randomly distributed in pentaplicated tanks (120 fish per tank). Fish growth (i.e. body mass, survival, specific growth rate-SGR, thermal growth coefficient-TGC, mass variation) was monitored for six (6) weeks. All fish were fed the same commercial diet ad libitum. Water quality was monitored daily and fish were group weighed (app. 15-20 fish per group) every 15 days, while at the end of rearing fish were individually weighed. Phenotypic deformities were also individually recorded. During the last 15 days of rearing, food consumption was recorded to estimate feed efficiency (food conversion ratio-FCR).
Results
Stage 1, Hatchery rearing (Hr): After 10 dph and up to 40 dph, larvae length was significantly higher in DF3 larvae. Survival and length variability were not affected by experimental treatments, while DF3 produced a much greater percentage of Big fish (61%) than DF22 (9%). Furthermore, both the Big and Small fish of DF3 were significantly larger than those of DF22. Trypsin and chymptrypsin specific activities were higher in 3-5 dph and 16-28 dph DF3 larvae. Pepsin was firstly detected in both treatments on 22 dph, peaked on 28 dph and remained at higher levels in DF3 larvae. Amylase activity was higher on 4 dph DF3 larvae while lipase pattern was similar in both DF3 and DF22 larvae. Stage 2, Laboratory rearing (Lr): After 6 weeks of pre-growing (up to 95 dph) fish of the DF3 treatment were significantly larger than DF22 (6.7 vs 6.0 g respectively) with higher daily weight gain. Body weight frequency distribution were different between treatments; fish larger than 6 g consisted the 63% of DF3 fish and the 48% of DF22 fish. No differences were observed for coefficient of weight variation and survival. Furthermore, spinal cord malformations of DF3 fish were significantly lower than those observed for DF22 fish (3.8 vs 11.7 % respectively).
Discussion and conclusion
Present results showed that the introduction of dry feed from mouth opening (on 3 dph), was more efficient for larvae and post-larvae performance compared to a more commonly used protocol (dry feed on 22 dph). The functional development of the digestive system was not compromised and it was similar to previously reported results for red seabream (e.g. Waqalevu et al., 2019; Khoa et al., 2020). Besides, digestive enzymes differences observed indicate an increasing number of actively feeding larvae in DF3 treatment. Present beneficial effects gained from the early introduction of dry feed are probably related to a better acclimation of larvae to dry feed particles, which once ingested offer a diet of higher nutritional value able to support the higher demands of faster larvae growth. Growth benefits obtained during larviculture were maintained during further rearing up to 95 dph. In accordance to our previous similar work on gilthead seabream larvae (Karakatsouli et al., 2019), the present markedly lower spinal cord deformities recorded at the end of the pre-growing period support the hypothesis that the ingestion and digestion of an appropriate dry feed may provide larvae with the necessary nutrients to form a healthy skeletal system at the sensitive stages of skeleton ontogenesis. Overall, the introduction of dry feed as early as on mouth opening is considered safe and feasible for red seabream providing advantages for both larval and post-larval production stages.
References
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