World Aquaculture - September 2023

WWW.WAS.ORG • WORLD AQUACULTURE • SEPTEMBER 2023 23 work in a basic environment, because the stomach was not ready. We call this endogenous feeding, and the embryo is taking all its nutritional requirements from the yolk sac (Figure 4). This occurs at the same period the embryo maintains the mouth and the anus closed; it is not ready for exogenous feeding. However, at the beginning of the larval stage the embryo will be able to, hopefully, catch and digest prey. At 2 DAH, eyes were pigmented and the mouth and anus opened. Although embryos were capable of exogenous feeding it was not yet convenient to administer a microparticulate diet, because the digestive tract was still in development. Pigmentation of the eyes and opening of the mouth and anus is relevant at this stage because this means larvae could begin a combination of exogenous and endogenous feeding. This period lasts while the yolk sac gradually decreases in size and by 11 DAH we observed the last remnants of yolk goblets (Figure 5). This condition should be favorable to larval survival because they have two sources of food for a few days. It could also be advantageous for periods when there is no food available in the required particular size range. The quality of the yolk sac and goblet is known to vary according to various factors in the progenitor’s life cycle, so the observed age and size range might differ from hatch to hatch. Microparticulate diet at 8 DAH combined with live food improved nutritional conditions for larvae (Roselund et al. 1997). In this phase, the liver occupied more space and there is a clear differentiation between the stomach and the intestine (Figure 6). Perhaps, the administration and combination of live food with inert food works to complement the live food with minerals, vitamins and other components needed to improve the health of the larvae. This stage of development is not yet the adequate moment to substitute inert food for live food, but it is very convenient to reduce the amount of live food provided. The fact that the larvae already presented morphologically well-differentiated livers, stomachs, and intestines was considered as indicative of the acceptability of administering inert food. At this age the larvae demonstrated better movement in the tank and better hunting capabilities, ingesting the live food administered in this period. We decided that the time for complete substitution of live food had not arrived because the larvae did not necessarily meet the requirement for visual attraction to the inert food. For this reason we recommend maintaining a combination of both live and inert foods for a longer period. By 25 DAH, when the larvae began to transform to juveniles, swimming behavior changes and the young fish adopt an assentation in the bottom of the tank. It is in this period that we recommend the complete substitution of live food with inert food, with a gradual change. We identified various structures at different times in barred sand bass development, resulting in a good reference for adjusting the larvae food protocol used in the experiment. For example, the proposed time to administer microparticulate diet in combination (co-feeding) with live food and the time recommended for the complete substitution of live food for inert food was related to the morphological changes of the digestive system, instead of the mobility of the larvae in the culture tanks or the size of their mouth opening. At this time there are no farms using this species for aquaculture, but upon completion of our efforts for designing an aquaculture protocol for barred sand bass, this work will provide the foundations for future research focusing on growth and survival in relation to dietary considerations. FIGURE 4. Histoligical cut, embryo 1 DAH, showing incipient tube (IT), liver (L), notochord (NC) and neural tube (NT). Photo Oscar Rosales, CICIMAR. FIGURE 5. Histological cut, 11 DAH; still showing residual oil globule (OG). Photo Oscar Rosales, CICIMAR. FIGURE 6. Histological cut, 8 DAH, showing clearly the liver, stomach (ST) and foregut (FG) well differentiated, and the rest of the oil globule (OG). Photo Oscar Rosales, CICIMAR (CONTINUED ON PAGE 24)