Growth response and carcass composition of rabbitfish , Siganus canaliculatus ( Park ) fed diets supplemented with dehydrated seaweed , Enteromorpha sp .

A trial to incorporate dehydrated Enteromorpha in the diet of rabbitfish fry averaging 0.25 ± 0.04 g was undertaken for a period of 12 weeks. Four isonitrogenous (39% crude protein) and isoenergetic (19 kJ/g) diets were fed to fish. The dehydrated Enteromorpha was incorporated in the diets at a rate of 0 (control), 10, 20 and 30%. A fifth group of fish was fed control diet and supplemented with known weight of fresh Enteromorpha placed in plastic baskets at the bottom of the rearing tanks. The survival rate, growth performance and feed utilization efficiency were observed to decrease with increasing inclusion levels of dehydrated algae (p < 0.05). The best results of all parameters were achieved by fish fed control diet and fresh Enteromorpha (P< 0.05). Carcass protein was not affected by the different treatments while lipid content was observed to increase in the group of fish supplemented with fresh Enteromorpha (P< 0.05). It is concluded that further studies are needed on the dehydration techniques of Enteromorpha as well as on the digestibility and assimilation of diets containing dehydrated algae.


Introduction
The potential of siganids for mariculture has been recognized for many years.This potential was attributed to their palatability, hardiness under culture conditions, ability to reproduce easily in captivity and to the fact that the fish are primarily first trophic level feeders and thus can be grown on cheap feeds (Ben-Tuvia et al., 1973;Lam, 1974;Von Westernhagen and Rosenthal, 1976;Tseng and Chan, 1982).In the coastal waters of the United Arab Emirates (UAE) two species are found, Siganus canaliculatus, which is the commoner species and one of the most favored food fish; and S. javus, which is less abundant and less valuable (MAF, 1984;Al-Ghais, 1993).The fish is herbivorous, feeding predominantly on filamentous green algae.Early studies of stomach contents and food preference revealed that among the many different algal species and vascular plants eaten, the presence of Enteromorpha was high and was the preferred species.This preference for Enteromorpha by siganids is not directly related to the calorific value of the algae but is related to the texture of its thalli which are crispy and thin (Von Westernhagen, 1973;1974).Enteromorpha is one of the most widespread genera of the green algae with a long history of food use, both for humans and animals (Jeong et al., 1993).The alga is rich in minerals, carbohydrates and vitamins (Nisizawa et al., 1987;Naidu et al., 1993;Reviers and Leproux, 1993).The alga grows along the coasts of UAE throughout year in the littoral zones exposed to shallow water.The only use of this alga in UAE is that by fishermen as bait to catch siganids (Al-Ghais, 1993).
Efforts to reduce the cost of fish feed have concentrated on the use of low-cost by-products and plant materials (Yousif et al., 1994).The present study was conducted to evaluate the possibility of utilizing the dehydrated Enteromorpha as a dietary ingredient in the formulated feed for S. canaliculatus fry.

Materials and Methods
Experimental procedure and diets Rabbitfish fry, S. canaliculatus, were collected by seine netting from Umm Al Qaiwain lagoon and stocked in a 40-t holding concrete tank at the Marine Resources Research Center.The fish were fed a 40% crude protein formulated diet ad.libitum twice daily.After two weeks of adaptation the fish fry averaging 0.25 ± 0.04 g (mean ± s.d.) were randomly distributed in 15 outdoor 1000-L circular fiberglass tanks.The tanks received a continuous flow of filtered sea water (40 ± 0.5 ‰) at a rate of 30 L/min.The fish were stocked at a rate of 200 fry per tank.
The fresh algae were collected from the seagrass area of Umm Al Qaiwain lagoon and immediately washed with running filtered seawater to remove the epiphytes and other foreign matter such as sand and shells.The algae were then airdried in the sun for two weeks.After complete drying the algae were finely milled using a laboratory mill.
Proximate analysis of major feed ingredients was performed prior to formulation of the experimental diets (Table 1).A lyophilized extract of fresh algae was analyzed for amino acids composition (Table 2).Four isonitrogenous (39% crude protein) and isocaloric (19 KJ/g) test diets were formulated (Table 3).The ingredients of each experimental diet were thoroughly mixed with water and oil in a laboratory feed mixer (Nakayasu Co., Japan) until a uniformly moist mixture was obtained.Pellets were made by passing the diet mixture through a laboratory feed pelletizer (Fuji Mzuho, Model 52-k, Japan) equipped with a 1-mm die.The pellets were then air-dried for 48 hours.The dry pellets of each experimental diet were packed in sealed plastic bags and stored in a freezer at -20°C until used.A fifth group of fish was fed a control diet with addition of a known weight of fresh Enteromorpha placed in plastic baskets at the bottom of the rearing tanks (treatment 4).
Each experimental diet was fed to triplicate tanks of fish fry for a period of 12 weeks.The fish were fed three times at a rate of 7% body weight per day.Fish from each tank were bulk weighed biweekly and the daily feed allowance was adjusted accordingly.At the start and end of the experiment, fish were retained from each treatment for subsequent proximate analysis.12,000,000 IU, B 1250 mg, B 2 12,000 mg, B 6 1250 mg, B 12 4250 mg, E 12,500 mg, K 5000mg, C 50,000 mg, Nicotinmide 30,000 mg, CaO panthotecat 15,000 mg, Folic acid 1500 mg, Niacinamide 20,000 mg, Insitol 112,500 mg, Lecithin 25,000 mg and Cholin 37,500 mg) 2 Gross energy calculated based on the conversion factors: protein 23.51 kJ/g, fat 39.75 kJ/g and carbohydrates (as NFE) 17.20 kJ/g (Yousif et al.,1996).

Analytical Methods
Analysis for chemical composition of feed ingredients, experimental diets and fish carcass were made according to the standard methods of AOAC (1984).

Statistical Analysis
All values are reported as means ± SEM.
Performance and carcass composition means were compared using the MSTAT 4 package (Nissen, 1987).Duncan's multiple range test of the same program was used to test for differences among treatment means at P < 0.05.

Results and Discussion
The comparison of growth response and feed performance of S. canaliculatus fed the experimental diets is shown in Table 4. Survival, weight gain, specific growth rate, feed conversion ratios, protein efficiency ratio, protein productive value and energy utilization of fish fed the control diet supplemented with fresh algae were significantly better (P<0.05)than those of fish fed diets including various levels of dehydrated algae.All these parameters were observed to decrease with increasing inclusion levels of the dehydrated algae (P<0.05).The poorest performance (P<0.05) was observed in the group of fish receiving diet supplemented with 30% dehydrated algae.The carcass composition (Table 5) showed no significant differences (P<0.05) in the body moisture and protein content among all treatments.The lipid and energy contents were higher (P<0.05) in the fish fed the control diet and fresh Enteromorpha whilst the values of other treatments decreased with increasing the level of inclusion of dehydrated algae.The lowest (P<0.05)ash content was produced by fish fed the control diet supplemented with fresh Enteromorpha sp.
When comparing the chemical composition of local Enteromorpha sp.used in this study with those from other countries (Nisizawa et al., 1987;Naidu et al., 1993), it is found that local algae have a lower protein content and high carbohydrate level.The lipid and fibre contents do not differ from those reported elsewhere.The amino acid composition indicated that the green algae used in this study are poor in histidine, tyrosine, methionine and cystine and rich in aspartic acid, glutamic acid and alanine.This tendency was reported by other workers for Enteromorpha.(Amano and Noda, 1992).Nevertheless, these values fall within the recommended ranges for finfishes (NRC, 1983;De Silva and Anderson, 1995).Ash content was observed to increase with increasing levels of dehydrated algae in the diets.This was probably due to the presence of sand grains and shells or sea salts resulting from the inadequate washing of the algae.Jauncey and Ross (1982) recommended a level of less than 10% in the diets for tilapia.Ash per se is not known to be harmful to cultured organisms.However, ash content, such as silica, affects the digestibility of the diets (De Silva and Anderson, 1995).Since siganids usually naturally thrive on algae, the ingestion by S. canaliculatus of high ash levels contained in the experimental diets might not be the direct cause of growth retardation.A possible cause could be the drying process of the algae followed in this study.The sun-drying technique might have affected the nutritional properties of the algae as well as its palatability.The presence of high antinutritional factors in the dehydrated algae might have also affected the growth of the fish.Naidu et al. (1993) attributed the growth retardation of rats fed seaweed meal, among which was Enteromorpha sp., to the presence of lectins (haemagglutinins).Lectins are proteins which bind to specific receptor sites in the intestinal epithelial cells resulting in impairment of nutrient absorption (Naidu et al., 1993;De Silva and Anderson, 1995) In the present study, it has been observed that, the fish body color became dark before death and the fish were observed to swim near the water surface, then completely cease feeding.These symptoms are characteristic of streptococcal infection described by Foo et al. (1985) in diseased S. canaliculatus.This best survival in the treatment receiving supplement of fresh Enteromorpha was probably due to the antibacterial activity found in the algae.This activity has been widely reported for Enteromorpha spp.and other marine algae (Ballantine et al., 1987;Kumar and Rengasamy, 2000).On the other hand this improved survival rate could be attributed to the qualitative lipid composition of the fresh algae.Enteromorpha, like the other marine green algae, usually contain highly polyunsaturated C 16 and C 18 fatty acids (Akinin et al., 1992;Jones and Harwood, 1993;Khotimchenko, 1993).The supplementation of dietary unsaturated fatty acid in the diet of marine fish larvae have been reported to improve the survival rate (NRC, 1983;Dhert et al., 1991;Izquierdo, 1996).
In conclusion, the present study shows that the dehydrated Enteromorpha incorporated in the diets of S. canaliculatus produced poor growth response and feed utilization efficiency.However, further studies on the digestibility and assimilation of diets containing dehydrated algae by S. canaliculatus are recommended.Also, further studies are needed to determine the proper techniques of processing the algae.Cooking and vacuum drying of seaweeds has been reported to increase digestibility of the plant (Naidu et al., 1993).Optimum protein-to-energy ratio for two size groups of rabbitfish, Siganus canaliculatus (Park).Aquaculture Nutrition 2: 229-233.

Table 3 . Formulation and chemical composition of the experimental diets Ingredients (% as fed)
vitamin-mineral mix: shrimp tonic, JV Marine East, Co. Ltd., Taiwan (A 30,000,000 IU, D 3 1

Table 5 . Carcass composition of S. canaliculatus fry fed the experimental diets
Means in this row are not significantly different (p< 0.05) *