The last time my wife and I were there the lake contained all the usual flotsam expected from an urban jungle "pit" but I didn't see any signs of fish life though.
One of the horse & carriage drivers told us that there are some small bass but not much else.
Off topic, if you want a really good laugh take the carriage ride around the park and ask the driver to take you to all the places where the bodies are found on the TV programmes like Law & Order.
Typically, you will be treated to a 5 minute long treatise from the driver on how safe the Park really is, and that the TV producers are doing NYC no favours at all.