。
So that means there are at least 20 rainbow trout in this section.
Literal
This-[with-で] this one-section [topic-は] at least 20 [counter-匹] rainbow-trout [subject-が] exist [it follows that-わけだ].
~わけだ is a logical-conclusion marker: 'so it follows that / that means that.' Used when the speaker has just worked out a consequence from previously-stated facts. 匹 is the counter for small/medium animals including fish. 一画 'one section, one block' divides a larger area into subsections. The sentence sounds like a fishing-survey observation, working out a population estimate from direct counting.