This study is based on field data that were collected
by hatcheries. These data were not collected for scientific
reasons and therefore can be less accurate. However,
because the data are (partly) collected for financial
systems, the accuracy should be high. Furthermore,
there is the advantage of a large data set, with which
this study deals and which tends to counteract the effect
of errors. Analyzing field data results in finding relations
without being able to test the causality. Therefore,
the found relations will be discussed in the light
of experimental literature to suggest causality and to
indicate where knowledge is lacking.