Welcome to Pond Boss.

I've never fertilized, so let's see what others say.

My gut response to question 1 is that various factors affect catch rates in the spring. Spawning would be one, and maybe reduced visibility due to natural or induced blooms.

You'll probably get asked more questions about fish details for question 3.

Excerpt from Robert Crais' "The Monkey's Raincoat:"
"She took another microscopic bite of her sandwich, then pushed it away. Maybe she absorbed nutrients from her surroundings."