SEA-WATER SYSTEMS FOR REARING FISH LARVAE 



91 



top. Hinged plywood flaps over large 

 rectangular holes in the cover give access 

 to the inner tanks. These flaps are pro- 

 vided with central slits covered by trans- 

 lucent polythene film, for admitting light. 

 A prefabricated steel framework carries all 

 components other than the reservoir, which 

 stands on the floor, partly hidden under 

 the water bath. 



The volume of salinity-adjusted sea 

 water made up each day is always more 

 than that required for 24 hours' irriga- 

 tion. Residual water is measured the 

 following morning with a dipstick, and 

 the amount of distilled water in the mix- 

 ture calculated. Measured volumes of 

 hatchery sea water and distilled water are 

 then added to provide the next 24 hours' 

 supply. 



By this method it is possible to maintain 

 a continuous flow of sea water adjusted to 

 a salinity below that of hatchery supply, 

 for long periods of time. Adjustments 



the other way can be made by adding suit- 

 able mixtures of salts. The experimental 

 tanks are buft'ered against air tempera- 

 ture variation only. Stricter temperature 

 standards would require a hatchery air- 

 conditioning plant ; alternatively, the wa- 

 ter bath could be made part of an auxili- 

 ary closed fresh- water circulation with im- 

 mersion heating or cooling. These triple 

 tank units are virtually self-contained and 

 can be used in replicate to test any long- 

 term chemical eft'ect on larval survival, 

 without running the risks inherent in a 

 static system. 



All the closed and open circulations de- 

 scribed in this paper have been successfully 

 used to rear plaice larvae. Technical de- 

 tails such as optimum temperatures, light, 

 salinity, pH, flow rate, and diet will vary 

 from species to species, but these basic de- 

 signs are recommended as a starting point 

 in the experimental study of hatchery re- 

 quirements for all marine fish with pelagic 

 eggs. 



WATERBATH 

 COVER 



INCUBATOR 

 SCREEN 



EXHAUST 

 TUBE 



AIR PUMP 



WATERBATH 

 INLET 



AIR-LIFT 

 DEVICE 



Figure 8.— Open-circulation apparatus designed to test long-term effect of salinity variation on the 



survival of plaice larvae : side view. 

 712-029 O — 64 7 



