Question

how have seals, penguins, and small baleen whales of antarctic waters benefited from the decline of large baleen whales?

Answer 1

The seals, penguins, and small baleen whales have benefited a lot from the decline in the large baleen whale populations.

The reason for the the benefit is that there's much more food available with the smaller number of large baleen whales feeding in the waters. The large baleen whales eat up to 3.6 tons of food per day in half of the year. The food they eat is mostly zooplankton and small fishes. The small fishes are also the pray for the penguins, seals, and the small baleen whales. So with the declining number of the large baleen whales, the other three species mentioned have much more food available, and also one competitor less.