Oddly enough, there’s decent evidence that preserves that ban fishing are effective for preserving the biodiversity in the larger region that includes zones where fishing is allowed.

Thinking about how many sea animals produce large quantities of offspring, I wonder if the answer is that while normally competition for food & predators limit how many survive in those situations a fair number can expand out of the protected area, with the predator population artificially low after having crashed along with whatever was overfished.