Well yeah, but saving one better than saving none
Is it? if your mission is to save lobsters buying a lobster which then encourages the fishmonger to get more lobsters from the market, and indirectly makes the price of lobster marginally higher and encourages fishermen to catch more it might be counter productive. What would save the lobsters is if the fishmonger can’t sell it, ends up taking it home for his tea and makes a loss on it, so is less likely to buy lobster next time, the market sees less demand, the fishermen get lower prices and so focus on other species or look to other ways to make a livelihood ….
Of course if your local market won’t support the lobster supply then the other alternative is the fisherman still goes out, still catches lobster but now finds that the only market for his produce is hundreds of miles away and so food miles go up and the planet gets a little more destroyed.