ANOVA or Z test?

Hi.
I have a question regarding different tests. I have length-weight measurements from 3 different fish populations. Based on these measurements I calculate a value called condition factor (k) for each individual within each population.
How do I test if there is significance difference between condition factors among the three population?
Population one has 500 samples, population 2 480 and population 3 218 samples (K values).
Do I use an on way ANOVA, two way ANOVA or a Z test?
Thanks in advance
Farivar