Yocum and Edsall's technique of recording individual attacks, captures, and 

 escapes was adopted. 



Both methods have intrinsic advantages and disadvantages. Bams' method 

 allows groups of predators to select between treated and untreated prey 

 simultaneously. However, the test statistic used by Bams is a biased estimate of 

 instantaneous mortality rate (2, 8). The method used by Yocum and Edsall 

 records the actual instantaneous mortality rate in terms of attacks, escapes, and 

 captures. These parameters allow a more accurate representation of changes in 

 escape capabilities. In this latter method, prey treatment groups are separated, 

 and the predator does not simultaneously compare prey groups. In a thermal 

 plume area, where predators have been observed to attack thermally-shocked 

 prey (8), it is not likely that shocked and unshocked prey would be in close 

 proximity to one another. However, in studying the effects of other pollutants, 

 simultaneous comparison of prey behavior may be an important factor in 

 differential predation, and should be considered. 



A number of biological variables which should be considered in designing a 

 laboratory predator-prey test system, including coexistence of predator and 

 prey in nature (spatial and seasonal); plausible prey-predator relationship; size 

 relation prey-predator; reproductive condition of predator; nutritive condition 

 of predator and prey; feeding periodicity of predator; and hunger state of 

 predator. Control of many of these variables has already been described in the 

 Methods section. Perhaps one of the most difficult to control is satiation 

 (hunger state) of the predator. Satiation state may affect prey risk (5). If a 

 predator is less motivated to eat, attack efficiency may not be as high, thus 

 artificially increasing escape rate of prey as measured by Yocum and Edsall's 

 method. Prey size will affect time to satiation in a predator, and must, 

 therefore, also be controlled. Optimal prey size can be estimated by calculating 

 a prey thickness to predator mouth size ratio with 0.5 as optimal (17, 26). 

 However, even with an optimal prey size and a set deprivation schedule, 

 individual variability is often substantial. Procedures to categorize motivation 

 state of the predator are recommended for laboratory predator-prey tests in 

 order to eliminate this variability. In the present study, the total mean number 

 of larvae captured per test was calculated for all tests within each prey age 

 group. A minimum percentage of this mean was chosen as an indicator of 

 adequate feeding motivation. A 75 percent Hmit described a minimum level of 

 22 larvae captured in tests with four week old M. menidia. Because six week 

 old larvae were larger, the capture minimum was narrowed to 80% of the mean 

 total larvae captured, giving a lower limit of eight larvae. All tests in wliich this 

 minimum capture level was not reached were excluded from statistical 

 treatment of data. In tests with larvae younger than four weeks, predators did 

 not reach satiation before completion of the test, and establishment of a 

 minimum capture level was unnecessary. 



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