Yocum and Edsall’s technique of recording individual attacks, captures, and 
escapes was adopted. 
Both methods have intrinsic advantages and disadvantages. Barns’ 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 limit 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 which 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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