Effects of dietary water content on meal size, daily food intake, digestion and growth in turbot, Scophthalmus maximus (L.)

When fed once daily with wet squid, turbot (30-50 g) accustomed to dry pellets require many days to increase intake to meet their feed requirement (approximate to 10 mg dry matter g(-1) bw meal(-1)). Adaptation takes 1-2 days if several daily feedings are given. With dried squid, they ingest about 20% of the wet squid bulk because the stomach contents expand when moisturised. In contrast, turbot eat enough wet squid to fill most of the available stomach volume (approximate to 7.6 mL 100 g(-1) bw). When presented in gelatine capsules, food water content is masked and does not affect the volume ingested. Moistening the contents shortens the delay before gastric emptying starts to one-third (0.6 h) compared with dry food (1.9 h). Daily dry-matter intake increased when dry contents were moistened but only if two or more meals were offered per day. Turbot adapt their digestion to supply water for dry diets but this may add extra metabolic costs. When offered 20 mg dry matter g bw(-1) day(-1), divided into four equal meals, turbot grew faster and more efficiently with moist than with dry squid. Protein, energy and dry-matter digestibilities were also enhanced. The increased daily protein absorption did not increase ammonia release, indicating that the extra protein was used for somatic growth.