FISHERY BULLETIN; VOL. 74, NO. 2 



effect of both prolonged and short exposure to in- 

 creased temperatures on survival and reproduc- 

 tion of D. pulex. 



METHODS AND MATERIALS 



Two stocks of D. pulex were cultured at two 

 acclimation temperatures and subjected to three 

 tjT^es of tests to determine their thermal toler- 

 ance. One stock was obtained from the Columbia 

 River and the other from a small pond north of 

 Seattle, Wash. They were cultured separately and 

 will be referred to as the Columbia group and the 

 Seattle group. Stock cultures were maintained in 

 5-liter battery jars of Lake Washington water 

 filtered through No. 25 Swiss silk bolting cloth to 

 remove zooplankton and phytoplankton, but not 

 bacteria. Taub and Dollar (1968) felt that bac- 

 teria were important to the nutrition ofDaphnia, 

 especially in relation to reproduction. Stock cul- 

 tures were reared and acclimated at either 15° or 

 20°C in a controlled temperature incubator Con- 

 tinuous fluorescent lighting (45-50 foot candles, 

 cool white) provided similar lighting in the in- 

 cubator and in the laboratory and was consistent 

 for all animals, test and control. Algae, Chlorella 

 and Chlamydomonas, were cultured using me- 

 dium No. 63 developed by Taub and Dollar (1968) 

 and fed to D. pulex. Water in the test vessels was 

 changed weekly, and the animals were fed three 

 times a week. 



The test temperatures were maintained by 

 using primary and secondary water baths and 



immersion heaters activated by temperature con- 

 trollers (Figure 1). The primary bath was a 

 Plexiglas^ tank 150 x 30 x 23 cm supphed with 

 flowing water at 10° to 15°C. The secondary baths 

 consisted of six or seven 5- liter battery jars, 23 x 

 14 X 17 cm, placed in the primary bath. The 

 temperature in each of these secondary baths was 

 raised progressively from the water inlet end to 

 the outlet end of the primary tank. Temperatures 

 in the secondary baths could be maintained from 

 10° to 36°C ± 0.5°. Air continually bubbling into 

 each secondary bath eliminated stratification. 

 Experimental subjects were held in 50-ml jars of 

 filtered lake water suspended in the secondary 

 baths and equilibrated to the test temperature in 

 those baths. 



Parthenogenetically produced animals of the 

 same age, either young females (less than 24 h 

 old) or mature females (approximately 1 wk old), 

 were selected from the stock cultures and held in 

 10-ml vials for a day before the start of the exper- 

 iment to check for handling mortality. At the 

 start of an experiment, the bulk of the water in 

 the vials was canted off, and the appropriate 

 number of test animals was poured directly into 

 the 50-ml test chamber at the test temperature. 

 The control groups were treated identically with 

 the others, except that they were held at acclima- 

 tion temperatures. A large bore pipette was used 

 when individual animals were handled. 



^Reference to trade names does not imply endorsement by the 

 National Marine Fisheries Service, NOAA. 



Figure l. — Experimental equip- 

 ment used to study temperature ef- 

 fects on zooplankton showing primary 

 and secondary water baths, test ves- 

 sels, and temperature controllers. 



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