Fact 1: Harming an animal species causes that animal 's population to decrease.

Fact 2: Fish are wild animals.

Given the two facts above, answer the question "Doing what to fish species causes their population to decrease?" with the following options: 
- deforestation
 - repopulating them
 - Pesticides
 - hypothyroidism
 - kill them
 - harming them
 - light
 - helping them


 

      harming them   