You cannot target species that are not open to retention.
I dont think that's true at all and it's doesnt imply that anywhere in the regs I am aware of.
People target trout, dollies, bulls, cutties, salmon all the time in systems with no retention and we seem ok with that (ie Skagit, Squamish, Upper Pitt).
When there is conservation concern like there currently is for sockeye, the DFO is asking to limit impact on that species, which is fair enough. Targetting an otherwise closed species seems legal unless specified in the regs (ie no fishing for salmon, no fishing for sturgeon etc). Whether we should be doing so is another debate.