86 



J. E. SHELBOURNE 



A small electric pump operated by a 

 mercury float switch transfers sea water 

 from the reservoir to a header tank in the 

 hatchery. From there it is g-ravity-fed to 

 nine 5- by 2- by 2-foot black polythene 

 rearing tanks on metal stands. Although 

 much bigger than the glass incubators 

 shown in figure 1, their design is essen- 

 tially the same. Water returns to the 

 reservoir along a connnon exhaust tube. 

 The inside of the hatchery is painted 

 white ; glass "blisters" in the hatchery roof 

 provide overhead illumination during the 

 day, augmented by four 80- watt fluores- 

 cent tubes. Hatchery air temperature is 

 stabilized by heaters in cold weather and 

 two small air coolers in summer. 



Metal surfaces in contact w^ith sea water 

 must be kept to a minimum; among the 

 cheaper metals only stainless steel is safe. 

 Metallic fittings are best coated with epoxy 

 resin to prevent corrosion and the release 

 of harmful ions; mild-steel coils treated in 

 this manner can be used for cooling sea 

 water by direct immersion. Filters are 

 necessary to check undesirable phyto- 

 plankton blooms in the reservoir and to 

 reduce the surfaces available for bacterial 

 attachment. There is some doubt about 

 the efficiency of our bactericidal arrange- 

 ments in 1961 ; the matter is discussed in 

 the next section. 



In retrospect, it was a mistake to sep- 

 arate hatchery and reservoir. Both com- 

 ponents should be housed in the same 

 insulated building equipped with air-tem- 

 perature control. Sunken concrete reser- 

 voirs can be replaced by fiberglass tanks 

 above floor level ; direct water-temperature 

 control systems then become superfluous. 

 Natural lighting may be an advantage for 

 efficient algal photosynthesis, and could be 

 provided by double-glazed panels in the 

 roof. Fluorescent lighting at night is in 

 this case preferable to tungsten, on 

 grounds of heat production, which can be 



further minimized by siting lamp-control 

 gear outside the l)uilding. 



Bacterial control in closed circulation 



Oppenheimer (1955) demonstrated the 

 possible value of bacterial control by anti- 

 biotics in marine fish hatcheries. Walne 

 (1958) also used antibiotics to increase the 

 survival rates of oyster larvae in experi- 

 mental tanks. Bacterial populations are 

 usually higher inshore than ofi^shore (Zo- 

 Bell, 1946). The plaice and many other 

 sea fishes spawn in virtually oceanic waters 

 with a low bacterial count. In the rich 

 organic environment of a closed circula- 

 tion the bacteriological problem must be 

 greatly magnified. Even at low incuba- 

 tion temperatures less than 6° C, plaice 

 egg shells become covered with epiphytic 

 bacteria, a condition seldom occurring in 

 the sea. The effect of shell contaminants 

 on final survival to metamorphosis has not 

 yet been systematically assessed; a start 

 is to be made during the spring of 1962. 



Wood (1961) has studied the use of ul- 

 traviolet light for bacterial control in sea- 

 water circulations, and our system is based 

 on his design. Eeservoir sea water flows 

 at a total rate of 300 gallons an hour 

 through three plywood boxes lined with 

 epoxy resin (fig. -t). Each lid contains 

 two 15-watt low^-pressure ultraviolet tubes 

 backed by an aluminium reflector. Water 

 enters the box at a low level and passes 

 over a longitudinal plywood weir before 

 overflowing into the filters. We were un- 

 able to test the bactericidal efficiency of the 

 system before installation; an opportunity 

 to do so occurred later on, when the plaice- 

 rearing experiments were finished. 



A supply of 450 gallons of turbid estu- 

 ary Waaler in a concrete tank was continu- 

 ously pumped through two of our 

 ultraviolet boxes at a fast rate of 520 gal- 

 lons per hour. Water samples were with- 

 drawn at frequent intervals, and bacterial 



