90 



J. E. SHELBOURNE 



of slight differences in salinity. An open 

 circulation is essential, since accumulating 

 metabolites may mask a salinity effect. 

 Figures 7 and 8 illustrate an experimental 

 unit successfully used in 1961. 



A measured amount of hatchery sea 

 water is filtered through toweling into a 

 leached asbestos-cement reservoir (100- 

 gallon capacity) each morning, and a cal- 

 culated volume of distilled water is added 

 to give a desired lower salinity. After 

 thorough mixing, this water is continuous- 

 ly transferred by a small airlift pump into 

 a polythene header tank above the reser- 

 voir, at a rate slightly in excess of incu- 

 bator requirements. A delivery manifold 

 emerging from the front lower face of the 

 header, conducts water at an equal and 

 steady rate of 4 liters per hour into each of 

 three 2- by 1- by 1-foot glass incubators. 

 The header overflow is channelled back 

 into the reservoir. This arrangement pro- 



vides a constant head to facilitate delivery 

 control. 



The three glass incubators have the same 

 design as that illustrated in figure 1. 

 Their sides are shrouded in close-fitting 

 jackets of black polythene film. The inlets 

 deliver their flow below the water surface ; 

 each perforated outlet pipe has a vertical 

 extension, also below the surface, screened 

 with a fine nylon mesh bag held at a dis- 

 tance from the pipe by three polythene 

 disks. The incubators sit side by side in 

 a black fiberglass tank through which sea 

 water slowly circulates from the main 

 hatchery supply. This water bath is cal- 

 culated to limit the effect, if any, of diurnal 

 variations in hatchery air temperature. 

 The outlet (strictly overflow) pipes of the 

 inner glass tanks are extended through the 

 side of the fiberglass water bath, to dis- 

 charge into a common drain gutter. The 

 bath has a plywood cover, treated under- 

 neath with epoxy resin, and painted on 



HEADER 

 OVERFLOW 



INLET 

 MANIFOLD 



WATERBATH 

 INLET (BACK) 



EXHAUST 

 GUTTER 



FIBREGLASS 

 WATERBATH 



ASBESTOS 

 CONCRETE 

 RESERVOIR 



POLYTHENE 

 HEADER TANK 



AIR PUMP 



WATERBATH 

 OUTLET(FRONT) 



INCUBATOR 

 OUTLET 



GLASS 

 INCUBATOR 



AIR-LIFT 

 DEVICE 



Figure 7. — Open-circulation apparatus designed to test long-term effect of salinity variation on the 



survival of plaice larvae: front view. 



