514 LOZANO AND KITCHELL 



artificial substrate samplers, modified from Hester and Dendy 

 (1962), were suspended 3 m offshore and 0.5 m below the surface at 

 each sampling site. The modified Dendy sampler consisted of six 

 squares (64 cm' ) of fiberboard alternating with five layers of 

 conservation webbing. Samplers were presoaked before use and were 

 suspended in the cooling lake for a 2-week sampling period each 

 month. Preliminary test runs showed that numbers of H. azteca in 

 samplers reach a constant level within 2 days. 



Upon collection, samplers were placed in separate plastic bags 

 filled with 90% ethyl alcohol (EtOH). Organisms were removed 

 from the sampler, washed into a 0.28-mm Nitex mesh concentrator 

 (Tetko, Inc.), and preserved in 70% EtOH. Amphipods were 

 separated from other invertebrates, cleared in xylene, and mounted 

 in Hoyer's solution on glass slides (Cooper, 1965). 



Amphipod abundance was recorded as number of Hyalella per 

 sampler. Head length was measured with a microprojector and 

 digitizer (Hewlett—Packard Co., 9197 A) along the dorsal margin 

 from the posterior edge to the base of the first antennal segment. 

 Antennal segment number was determined as the total number of 

 segments from the first and second antennae (Cooper, 1965). 



Laboratory Techniques 



We conducted laboratory experiments in November 1976 to 

 show the relative importance of density and temperature on the 

 colonization of samplers by H. azteca. Amphipods collected from the 

 field were acclimated for 1 week at 13, 21, and 23 ± 0.5°C in 60-hter 

 aquariums. Myriophyllum sp. and Cladophora sp. were added for 

 substrate and food. In one experiment 25, 50, and 100 amphipods 

 were placed in separate 4-liter vessels maintained at their acclimation 

 temperature. Small samplers (15 cm" ) were suspended in the vessels 

 for 12 hr. In a second experiment, groups of 35 amphipods were 

 placed in 4-liter vessels at their acclimation temperature, with six 

 replicates for each temperature. The small samplers suspended in the 

 vessels were removed after 3 hr. The number of amphipods per 

 sampler was recorded for both experiments. We measured the size of 

 the amphipods from the second experiment to test for differential 

 colonization of substrate samplers by large amphipods. Amphipod 

 head lengths from the second experiment were measured according 

 to the methods described. Nonparametric statistical tests, the 

 Mann— Whitney U test and Kruskal— Wallis H test, were used to 

 determine whether differences existed between median scores (Gib- 

 bons, 1976). 



