(A) would be relevant to a conclusion about the global population of this shark species, but this argument is only about the shark population in South Australia.

Even if this species were entirely extinct everywhere else, that wouldn’t affect the shark population in South Australia.

(B) says fishers (sharksters?) prefer to catch stationary sharks, which doesn’t threaten the CPUE as a measure of shark population.

Like maybe (B) suggests our particular species of shark is a stationary shark, so sharksters love hunting them and their population has declined significantly. The researchers’ take is that if this were true, it would show up in the CPUE – it would be harder for dudebro to catch as many sharks as he used to. (B) gives us no tools to challenge that narrative.

(C) introduces another potential threat to the shark population aside from intentional hunting, but this doesn’t undermine the CPUE as a measure of the shark population.

Let’s say tons of sharks are getting caught up in tuna nets and the population has declined significantly. The researchers’ take is that if this were true, it would show up in the CPUE – it would be harder for dudebro to catch as many sharks as he used to. (C) gives us no tools to challenge that narrative.

(D) draws a distinction between number of sharks and pounds of sharks. It would be relevant if the CPUE measured one thing and the conclusion was about the other. But both the CPUE and the conclusion are about the number of sharks.

If the CPUE were about tonnage and not number, then (D) would weaken the argument by suggesting commercial boats have been catching lots of tiny sharks, leaving behind a smaller population of big sharks. That’s probably the lens through which (D) is most tempting. But again, the CPUE is about number.

Whereas the stimulus hypothesizes that the reason CPUE has remained constant is because the shark population has remained constant, (E) suggests an alternative hypothesis: perhaps the shark population has been in decline but we've simultaneously gotten better at catching sharks.

It could be that back in 1973 commercial boats were out there dragging their nets around randomly. They caught a few sharks through sheer luck because the sharks were so plentiful. Now there are way fewer sharks, but the commercial boats plop themselves right on top of them before casting their nets.

Same CPUE; smaller shark population.