Freshwater
was obtained from the city supply and was aerated for at least 5 days before use to avoid
any problems with chlorine. Low salinities (20 and 30%0) were obtained by diluting sea
water with fresh water. The high salinities (40 and 50%~) were obtained by mixing
filtered sea water with sea salt. This concentrated stock was then diluted with sea water
to produce the required salinities. Salinity was adjusted daily, as necessary, to keep the
variation within f 1.5%0 and changes were effected by replacing portions of water in the
test containers with equal volumes of either fresh water or concentrated sea water.
Temperature was controlled to within +0.5”C using thermostatted heaters. Each tank
was continuously aerated by an air stone and photoperiod was standardised at 12 L: 12 D
using timer-controlled lights. Dissolved oxygen, temperature and salinity were measured
each morning and afternoon using an oxygen meter (YSI Model 57, Yellow Springs
Instrument, OH, USA), a portable refractometer (ATAGO), and a mercury thermometer.
Every morning before feeding, faeces and other detritus in each container were
siphoned out, and a sufficient quantity of water at the same salinity and temperature was
added to maintain levels.
Freshwaterwas obtained from the city supply and was aerated for at least 5 days before use to avoidany problems with chlorine. Low salinities (20 and 30%0) were obtained by diluting seawater with fresh water. The high salinities (40 and 50%~) were obtained by mixingfiltered sea water with sea salt. This concentrated stock was then diluted with sea waterto produce the required salinities. Salinity was adjusted daily, as necessary, to keep thevariation within f 1.5%0 and changes were effected by replacing portions of water in thetest containers with equal volumes of either fresh water or concentrated sea water.Temperature was controlled to within +0.5”C using thermostatted heaters. Each tankwas continuously aerated by an air stone and photoperiod was standardised at 12 L: 12 Dusing timer-controlled lights. Dissolved oxygen, temperature and salinity were measuredeach morning and afternoon using an oxygen meter (YSI Model 57, Yellow SpringsInstrument, OH, USA), a portable refractometer (ATAGO), and a mercury thermometer.Every morning before feeding, faeces and other detritus in each container weresiphoned out, and a sufficient quantity of water at the same salinity and temperature wasadded to maintain levels.
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