Correlated Data Analysis

2 down vote favorite

I am working on yield of fisheries in three different pools. The yield output was measured in terms of their length gain and weight gain. The other variables considered are length of stay, outer temperature, pool temperature, PH level of water, etc.

In this case I want to develop a model of yield in terms of other variables.

Is it possible to take both weight gain and length collectively as a dependent variable and other variables as independent? If I go for regression then weight gain and length data are correlated because the weight was being gained by fishes with each day.

Please suggest me appropriate model for analysis.