Effect of heat‐treated feed and exogenous zymogen on survival and growth of grey mullet, Liza ramada (Risso), larvae in Egypt

A preliminary trial and two experiments were carried out to study the effect of heat‐treated feed and exogenous zymogen on the survival and growth of grey mullet, Liza ramada (Risso), larvae. The fish were held in glass aquaria supplied with continuous aeration and salt water, which was changed daily. Two test diets (40% crude protein) formulated from commercial ingredients were used. In the preliminary trial, diet A was either heat treated in an autoclave using a maximum pressure of 1.2 kg cm–2 for 15 min, or supplemented with exogenous zymogen at rates of 0%, 2%, 4%, 6% or 8% of the diet. In experiments 2 and 3, four heat treatment times (0, 10, 20 or 30 min) using the previous pressure and three exogenous zymogen supplementation rates (0%, 2% or 4%) were evaluated in a 4 × 3 factorial arrangement for effects on the survival and growth of grey mullet using diet B. Zymogen and vitamins were added to the diets after heat treatment. In addition to survival and growth, dissolved oxygen, temperature and salinity were measured periodically through the study. The grey mullet larvae were introduced to the treatments 24 h after transportation to the laboratory in experiment 2 and after 3 weeks acclimatization in experiment 3. In the preliminary trial, significant differences in survival rates were observed after 7 days for larvae maintained at heat‐treated diet A and diet A supplemented with 4% exogenous zymogen. In experiment 2, significant differences in weight gain corresponded to increasing dietary zymogen in diet B. Likewise, the percentage of survival rate increased significantly starting from week 2 as zymogen increased in diet B. Heat treatments in diet B had no significant effect on weight gain of grey mullet after 2 weeks. However, significant differences in survival rate corresponding to heat treatments were observed for grey mullet larvae after 4 weeks in experiment 2. Diet B heat‐treated for 20 min with 4% zymogen gave the best results for larval survival rate and growth. Although the larvae were acclimatized for 3 weeks to salinity using the best diet of the second experiment before starting the third experiment, the same results were observed after 3 weeks. Further experiments must be performed to confirm the best conditions for acclimatization.