Try the following
1. Do you have Time Series Data of the following Variables for a Period of 20 Years ?
(I) Population (A) (II) Population (B) (III) Water Temperture
2. If you feel changes in the sizes of population are due to natural causes, Examine their trends by fitting appropriate trend curves also supplement with a suitable statistical Test for testing the equality of population sizes after adjusting the data for the trends.
3. If you feel water Temparature is a contributing factor, run appropriate Linear or Non Linear Regression of each population on Temperature. If both the regressions are significant, test the equality of population sizes after adjusting the data for Water Temperature. In the event of non significant relations, carry out ususal statistical tests of Significance ( such as t - test for testing the equality of means
I hope this may serve as a good starting point
Dr N S Gandhi Prasad
Professor of Statistics ( Retd. )