User blog comment:D2r/Unanswered Questions of Redwall/@comment-1630956-20100125215446

ok, i'll answer this as best as i can.

My answer comes from ''The Long Patrol, Chapter 40

8) In Mossflower, Martin the Warrior diverted the River Moss and flooded the depression in which Kotir stood. When building Redwall, all the dirt and debris that was dug up for the foundations was poured into the lake that covered Kotir causing it to dry up.  It became little more than a swamp, the only trace of it being a spring that bubbled up some distance from the original lake site.  This became the abbey pond.

Background info finished, time to attempt to answer the question.

Since the abbey pond is fed, presumably, by the river moss which forces it's way through the dirt and gravel into the pond. I think it is safe to say that loose fish eggs, small fish, and watershimp could be sucked along with the current and end up in the pond. This, coupled with the fact that the Redwallers seem to not eat a lot of fish, would keep the population of fish from going extinct.

I hope I threw a little light on the answer.