Studies On Food Selection Of Macroalgae By The Rabbitfish Siganus Canaliculatus And Its Influencing Factors

The rabbitfish Siganus canaliculatus is a small inshore mainly herbivorous teleost fish which widely distribute throughout the Indo-Pacific and Eastern Mediterranean region, including in southeastern China Sea. With characters of strong disease resistance, available fry, short culture period and high nutrition value, etc, this fish is becoming a burgeoning aquacultural species along the southeast coast of China. Food choice is an effective foraging strategy that animals adopt in consideration of the palatability, preference and availability of food to meet the nutritional needs and to adapt the environmental conditions. For better understanding the mechanism of fish's food selection and its influencing factors, in this paper we compared the feeding preferences of S. canaliculatus to seven macroalgae (Enteromorpha intestinalis, Chaetomorpha linum, Ulva lactuca; Gracilaria lemaneiformis, Porphyra haitanensis, Corallina sessilis, Sargssum fusiforme), which are common available around Na Ao sea area, and examined its affecting factors from several aspects including the nutrient contents and physical morphology of seaweeds, apparent digestibility of fish to the tested macroalgae and effects of different types of foods on the activities and distribution of digestive enzymes. Main results are as following:1. Preference of S. canaliculatus to seven macrophytes and its influencing factors1.1 When seven algae were synchronously supplied to S. canaliculatus, the order of consumed mass of fresh macroalgae by fish during 1.5 h was: E. intestinalis>G. sjoestedtii>C. linum>U. lactuca>P. haitanensis>S. fusiforme>C. sessilis, which indicated that E. intestinalis, G. sjoestedtii and C. linum are the most preferred algae. When one to four relatively preferred algae were removed, the order of food preference was not changed.1.2 Rabbitfish's food preference showed relationship with nutrient contents of algae. The preferred seaweeds E. intestinalis, G. sjoestedtii and C. linum generally contain higher contents of protein, soluble sugar and energy, and thus rabbitfish could obtain more dry matter, protein, carbohydrate and energy during unit time. In the un-preferred seaweeds such as S. fusiforme and C. sessilis, the contents of protein and energy were relatively low, while that of ash was high. In addition, S. canaliculatus showed a preference to filamentaous and then flat algae, rather than to calcified algae, indicating that algal morphology also affects the feeding preferences. However, the food preference didn't show obvious relationship with apparent digestibility for maocrophytes by the rabbitfish.2. Effects of different diets on digestive enzymes of S. canaliculatusAfter five kinds of diets (a, G. sjoestedtii; b, C. linum; c, U. lactuca; d, mixed diet of algae constituting of above three algae; e, mixed diet of algae plus fresh fish constituting of above three algae and fresh fish meat) were fed to different groups of S. canaliculatus for 3 weeks, the activities and distribution of protease, amylase and lipase in stomach, intestine and liver were compared among groups. The results showed that: 1) Protease activity was in the order of intestine>liver>stomach, while those of amylase and lipase were in the order of intestine>stomach>liver. 2) Effects of foods on the activity of digestive enzymes differed with dietary species. Pepsin activity was high in fish group fed with U. lactuca, but showed not significant difference among five groups (P<0.05). Highest trypsin activity in foregut was in mixed diet of algae plus fresh fish group, which was significantly higher than that in the plant-mixture diets and C. linum fed groups (P<0.05). However, trypsin activity in midgut, hindgut and liver showed no significant difference among five groups (P<0.05). Gastric lipase activity was highest in groups fed mixed diet of algae plus fresh fish or C. linum, and that in group fed mixed diet of algae plus fresh fish was significantly higher than that of other three groups (P<0.05). However, lipase activity in foregut, midgut, hindgut and liver showed no significant difference among five groups (P<0.05). Amylase activity was generally higher in fish group fed mixed diet of algae plus fresh fish than that in other groups.The results will be of important theoretical significance for enriching nutritional ecology of fish, and provide evidence and guidance for the development of algae-based compound feed and making full use of seaweed resources.