SEA-WATER SYSTEMS FOR REARING FISH LARVAE 



87 



ALUMINIUM 



REFLECTOR 



lid: u-v control 

 gear outside 



15 WATT U-V TUBE 



POLYTHENE 

 INLET PIPE 



WEIR 



PLYWOOD BOX 

 LINED WITH 

 EPOXY RESIN 



OVERFLOW 

 PIPE 



Figure 4. — Design of ultraviolet bactericidal unit. 



populations were assessed by mixing 1-ml. 

 volumes with ZoBell's 2216 medium using 

 the standard pour-plate method. Dupli- 

 cate bacterial counts were also derived 

 from the dropping technique of Miles and 

 Misra (1938) . Even at a flow of 260 gal- 

 lons per hour per box, water leaving the 

 outlets was virtually sterile after treat- 

 ment. Within 6 hours, the originally 

 high bacterial counts characteristic of in- 

 shore water had dropped to a level similar 

 to that in the open sea ( fig. 5) . The treat- 

 ment was continued overnight, and sam- 

 pling resumed the following morning. 

 By this time the water in the reservoir was 

 perfectly clear and the filters were caked 

 with estuarine debris. The filter bypass 

 was then closed and the whole flow was 

 directed through tlie ultraviolet boxes 



alone. Figure 5 demonstrates (a) the 

 bacterial efficiency of these units at a high 

 rate of flow and (h) the recontaminating 

 influence of dirty filters in a bypass 

 circuit. 



The hourly flow in our 1961 plaice- 

 rearing circuit was only one-tenth of the 

 reservoir volume. Thus the bactericidal 

 potential of the system was not fully ex- 

 ploited. In addition, the filters were in- 

 correctly sited — they should be placed 

 before and not behind the source of ultra- 

 violet radiation. Given rapid water turn- 

 over, reservoir bacteria can undoubtedly 

 be controlled by direct irradiation. How- 

 ever, the method is not easily applied to 

 bacterial control in actual rearing tanks, 

 where periodic dosing with antibiotics may 

 be necessary, at least during the egg phase. 



