Feeding habits on Enteromorpha prolifra and growth characteristics of juvenile Siganus guttatus

The development of fisheries economy has caused increasing environmental problems,such as water degradation and the overgrowth of Enteromorpha prolifra. Chemicals are often used to control the growth of E. prolifra,but they always bring side effects to the culture water. Siganus guttatus ingests E. prolifra and it may become a new method to control the growth of E. prolifra. To provide reference for the use of S.guttatus to control the overgrowth of E. prolifra,living habits,feeding rhythm,daily food intake and growth characteristics of juvenile S. guttatus were studied.Living habits and feeding rhythm of S. guttatus were studied by underwater photography and direct observation. 200 fish were cultivated in 5. 0 m × 5. 0 m × 5. 0 m cage with mesh size of 4. 0 mm. S.guttatus was fed with enough fresh E. prolifra and the residual feed was duly removed everyday. Results showed that the fish inhabited in the middle and low layer and moved around in groups. They distributed dispersedly in water and liked to gnaw the attachment on cages,such as microalgae and detritus. S. guttatus had strong stress reaction. They were easily scared by surrounding sounds and vibration caused by breeders.The fish escaped quickly when they sensed danger. S. guttatus was the daytime feeding type. There were three apparent feeding peaks in daytime at 5∶ 00 a. m.,9∶ 00 a. m. and 14∶ 00 p. m. Unlike the peaks around9∶ 00 a. m. and 14 ∶ 00 p. m.,the formation and decline of the peak around 5 ∶ 00 a. m. took shorter time.They seldom fed from 19∶ 00 to 20∶ 30 p. m. and had no feeding behavior from 20 ∶ 30 p. m. to 3 ∶ 00 a. m.,indicating that light intensity could affect their feeding activity.To investigate the daily food intake and growth characteristics of juvenile S. guttatus,90 fish were cultivated from June 24 th to September 19 th in three 2. 5 m × 1. 5 m × 1. 5 m cages( 30 fish per cage) with mesh size of 0. 5 mm. The average body length( BL) was( 39. 9 ± 2. 3) mm and average body weight( W)was( 2. 1 ± 0. 2) g. The residual feed was collected and weighted every day at 5∶ 30 a. m.( summer),and the fish was fed with enough fresh E. prolifra( 500. 0 g) concurrently. The difference between end value and start value was calculated as daily food intake for three weeks. Fish size was measured every 30 days. During the culture period,temperatures ranged from 23. 0- 26. 5 ℃,p H between 7. 8 and 8. 2,dissolved oxygen 5. 0 mg·mL- 1. Results showed that the average body length and weight increased to( 66. 0 ± 5. 8) mm and( 9. 8 ± 2. 2) g after 90 days. Daily body length growth was 0. 29 mm and daily body weight gain was 0. 09 g. A linear relationship between total length( TL) and body length( BL) could be indicated as TL = 1. 1439 BL + 2. 1082( R2= 0. 944 9),and a power function relationship could be indicated as W = 0. 000 024TL2. 938 7( R2= 0. 954 3) between body weight( W) and total length. The power index was nearly 3,so S.guttatus grew at a uniform speed. The average daily food intake of 30 fish [average body length( 64. 5 ± 5. 6)mm and average body weight( 7. 6 ± 1. 3) g]was 260. 3 g of E. prolifra( wet weight),8. 7 g feed per fish,1. 1 times of body weight. The daily food consumption changed in wave motion,and the maximum daily food consumption was 490. 9 g of E. prolifra( wet weight) during the experiment.According to the experiment,feed consumption of fish with larger sizes would be larger. So S. guttatusprobably could effectively inhibit overgrowth of E. prolifra and it has no side effects. The way that uses S.guttatus to control E. prolifra deserves deeper study,probably S. guttatus could play an important role in the matter.