88 



J. E. SHELBOURNE 



Filter by -pass 

 closed 



.-«' 



c 

 o 

 o 



z 



o 



O 

 CO 



Figure 5. — Survival curve of marine bacteria irradiated in closed circulation. Solid line indicates 

 pour plate (1-ml. water samples in ZoBell's 2216 medium). Dashed line indicates Miles and 

 Misra method (1/50-ml. drops on ZoBell's medium). Plates incubated at 25° C. for 24 hours. 

 Mean water temperature 18° C. 



OPEN CIRCULATION AT PORT ERIN, ISLE OF MAN 



Metabolites are continually removed in 

 open circulation, and survivals of plaice 

 larvae are correspondingly higher. A 

 mean survival of 33 percent was achieved 

 in the control tanks of a salinity experi- 

 ment at Port Erin during 1961. This is 

 several times greater than the expected 

 result in closed circulation at Lo^yestoft, 

 using the same basic tank design and 

 handling technique. 



Hatchery sea water is usually of the in- 

 shore variety; certain controls are still 

 necessary. For instance bacterial control 

 is needed either in the main reservoirs or 

 locally at the point of flow into experi- 

 mental tanks. Filtration is also impor- 

 tant in open circulation since suspended 

 particles provide a "foothold" for sessile 



bacteria. The problem of temperature 

 control still persists with diurnal fluctua- 

 tions in the coastal environment; salinity 

 similarly varies according to rainfall and 

 local runoff. 



Local filtration and temperature control 



A simple apparatus to continuously fil- 

 ter incoming hatchery sea water, and to 

 lower its temperature before deliveiy into 

 experimental tanks, is shown in figure 6. 

 It consists of a 10-gallon polythene carboy 

 fitted with a high-level overflow pipe and 

 a low-level delivery tube leading into 

 an insulated 4()-gallon asbestos-cement 

 header tank. The tank contains a resin- 

 coated steel cooling coil coupled to a i^- 

 horsepower refrigerator unit outside the 

 hatchery, controlled by a sensitive thermo- 



