Progressive changes in fish larval gastrointestinal tract are similar in all teleosts and are important in defining proper larval feeding and weaning strategy. However, variations exist between species’ developmental stage and are influenced by quality and quantity of feed fed to the fish. These variations affect the digestibility and absorption of nutrients in larval fish.
Globally, the culture of Clarias gariepinus is growing both in quantity and value. However, its growth has not attained its full exploitation potential because of low larval quality, unavailability of quality, and inexpensive larval feed. In addition, the species has slow gut maturation with radical shifts in dietary demands in relation to morphology and functioning.
The larval stage of C. gariepinus remains undefined. Though agreeable that the stage starts with exogenous feeding, there are differences on when it ends. The stage is suggested to end at different larval lengths when fin-fold is fully developed (Balon, 1975), or with the development of a functional stomach (Verreth et al., 1992). In both cases, the larvae are characterized by an incomplete digestive system, hence Artemia nauplius is the preferred live feed because of high survival rates and faster growth of nauplii fed larvae. However, hatching and handling of Artemia cysts require special equipment and skill, its nutritional value varies with batch, strain, and developmental stage (Van Stappen, 1996). Decapsulated Artemia is an alternative. However, it gives a low survival rate, is more expensive, and mostly unavailable in Africa. Further, quality starter dry diets are expensive and unavailable. As such, efforts have been directed to combined diets to optimize growth.
Different feeding strategies have different effects on the ontogeny of larval digestive capacity and have been described by different techniques (Rønnestad et al., 2013). However, literature remains scarce on C. gariepinus larvae intestinal ontogeny despite the dependence of digestive development on diet. Thus, effects of Artemia nauplii and decapsulated Artemia as single diets or combined with dry diets were investigated.
Materials and methods
Feeding trials were conducted at Fleuren and Nooijen B.V., Someren, the Netherlands, in 150 L glass aquaria connected in a Recirculation Aquaculture System (RAS). Ripe females were stripped, and the eggs were fertilized and incubated at 290C, as described in De Graaf and Janssen (1996). On the 2 dph, each aquarium was stocked with 26 larvae l– and randomly assigned one of the 5 diets ((A) Artemia nauplii +dry diet, (B) shell-free Artemia + dry diet, (C) Artemia nauplii, (D) shell-free Artemia, and (E) dry diet) in triplicate at 27.5 ± 0.5 °C, pH of 7.0–7.1, and a flow rate of 2.6 L min– for 11 days. Feeding started in the evening of 2dp at 25 % fish WBWd– reducing to 20%. Dry diet increased by 20% daily in daily adjusted feeding ration until 6 dph. Thereafter, larvae were fed on 100% dry diet on a dry weight basis. Larvae were fed 6 times a day and excess food was siphoned out before the start of next feeding. Specific growth rate (SGR) and food conversion ratio (FCR) were calculated.
Randomly, 3 live larvae on 2 dph, 1 larva on 4 dph, and 6 dph were sampled, quickly fixed in Bouin solution for 12 h before preservation in 70% ethanol. Preserved samples were dehydrated in a series of graded ethanol in a tissue processor, embedded in paraffin and cooled to blocks. Relative positions of proximal, middle and distal intestines were defined according to Holdon et al., (2013). On each section, 5 slices of 5 µm were sectioned; Haematoxyln and Eosin (H & E), Periodic Acid Schiff (PAS) stained and 3 best field views observed under a light microscope fitted to a camera. All measurements and counts were analyzed by digital imaging.
Results and discussion
Final mean wet weight and specific growth rate (SGR), were significantly (p<0.05) higher in diet A (17.90±0.38g and 24.12±0.30%/day respectively) compared to other diets. This could be attributed to the combined advantage of protein in the two diets and contribution of exogenous enzymes from the nauplii that increased digestion and absorption of nutrients. This was supported by the significantly (p<0.05) low FCR (0.70±0.30) observed with the diet A. Diets C and D are not different nutritionally, however, diet D and its combination recorded significantly low mean values in wet weight (8.90±0.44), SGR (17.18±0.28) and high FCR (1.66±0.21) compared to other diets.
The observations could be attributed to the fast sedimentation that may make it unavailable for ingestion, processing and drying procures that could have affected protein structure. In addition, its small size may not have ensured energy optimization for larvae growth (Prokešová et al., 2017). Dry diet intermediate results on growth suggested quality and feed digestible.
At 2 dph, mucosal parameters were not significantly differentiated. However, they increased over time and decreased from proximal to distal intestine in all diets. Diets A and C had significantly (p<0.05) more folds (31.37±0.34 and 30.45±0.40 respectively) and those of diet A significantly longer (84.12±1.40) compared to all diets. This observation did suggest an increased surface area for digestion and absorption. Diet D had the lowest fold (25.78±0.60 – 5.87±0.60) counts and height (68.23±1.61), an indication of partial starvation and nutrient deprivation which might have resulted to tissue degeneration due to disuse (Verreth and Den Bieman,1987). Further, diets D or its combination (B) had significantly (p<0.05) thick mucosa (64.68±0.36 and 70.54±0.87 respectively), a probable restructuring adaptation to reduced food uptake. Proximal intestine receives large quantities of food that requires more energy to move it along the intestine. This requires dense muscle development so as to provide necessary energy hence, thick proximal intestine in all diets (Rao et al., 2010).
Diets A and D posted significantly higher (4.32±0.08-1.87±0.13) and low (2.28±0.22 – 1.57±0.25) goblet cell counts per 100µm on 4 dph and 6 dph respectively compared to other diets. Variations in goblet cell count in diets A and D or in their combination with dry diet could be attributed to differences in protein structure and lipid levels in the diets and diet availability in the intestine after ingestion. In all diets, goblet cell counts increase from 2 dph to 4 dph before decreasing at 6 dph, suggesting digestive capacity inefficiency. However, this observation could also suggest staining variability or differentiation of goblet cells (Uc, 2014). The later could be a possible explanation since only PAS staining only stains neutral goblets cell observed on the apical mucosa.
Conclusion and recommendations
Reduction of Artemia nauplii in larval feeding improved larval intestinal morphology and growth of C. gariepinus larvae with no compensatory growth when larvae were fed on a dry diet. Thus, starter diets may affect further rearing C. gariepinus after larval stage. The way forward is a cost-benefit analysis to determine the most economical diet for optimal growth of C. gariepinus. Further, an intestinal microbial abundance and their influence on digestive capacity could be investigated.
These findings are described in the article entitled Growth performance and intestinal morphology of African catfish (Clarias gariepinus, Burchell, 1822) larvae fed on live and dry feeds, recently published in the journal Aquaculture. This work was conducted by Callen Nyang’ate Onura, Wim Van den Broeck, Nancy Nevejan, and Gilbert Van Stappen from Ghent University, and Patricia Muendo from Machakos University.
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