SEA-WATER SYSTEMS FOR REARING FISH LARVAE 



89 



HATCHERY 



SEAWATER 



SUPPLY 



TOWELLING 

 FILTER 



CARBOY 

 OVERFLOW 



POLYTHENE 

 CARBOY 



THERMOGRAPH 



THERMOSTAT 



INSULATED 

 HEADER TANK 



STEEL COIL 

 COATED WITH 

 EPOXY RESIN 



TO 



EXPERIMENTAL 

 TANKS 



Figure 6. — Open-circulation apparatus providing local filtration and temperature control. 



Stat in the header. Hatcheiy sea water 

 passes into the carboy through two layers 

 of fine-weave toweling, at a rate slightly 

 in excess of experimental requirements. 

 The filtered overflow runs to waste. 

 Temperature- adjusted water leaving the 

 header is continually replaced by filtered 

 water from the carboy. Some mechani- 

 cal stirring may be necessary in the con- 

 trol tank; no float switches or ball cocks 

 are required. If the temperature of the 

 incoming seawater is lower than required, 

 an aquarium heater replaces the cooling 

 coil. Both heater and cooling coil can be 

 used to counter one another when the 

 temperature of the incoming supply is 

 very variable. In practice, the unit deliv- 

 ered 30 gallons per hour of filtered sea 

 water at 7° C, with the main hatchery 

 supply standing at 10° C. 



The horizontal stippled strip below the 

 supporting platform of the header tank is 

 a sheet of expanded polystyrene insula- 

 tion board, stuck on the underside. It 

 represents a not-too-necessary refinement 



if the platform is of thick timber, and 

 could be eliminated if one so chooses. The 

 vertical and topside stippling similarly 

 represents the insulating sheath of the 

 tank. 



Port Erin sea water is relatively clear, 

 but in turbid conditions a finer filter sys- 

 tem will be needed, with an increased filtra- 

 tion surface. A small-wattage bactericid- 

 al unit can easily be interposed between the 

 filter and the constant-level carboy. The 

 pipeline to rearing tanks, and the tanks 

 themselves, must be insulated if hatchery 

 air temperature is uncontrolled. An ap- 

 paratus of this sort ran for 3 months dur- 

 ing 1961 with no attention other than a 

 daily change of filter. 



A variable salinity apparatus in open cir- 

 culation 



Most salinity experiments on marine fish 

 eggs and larvae have been confined to 

 testing the effect of extremes on short-term 

 survival in static conditions. We are 

 more interested in the long-term effect 



