The Summer Bass is a Spring/Summer fish, just like the Winter Squid is a Fall/Winter fish. Players sought clarification on fish versus pet spawning seasons from Lead Content Designer Cory "Mumper" Stockton, and received it:
Seasonal pets will follow the pre-existing seasons starting in patch 5.3. Note that the existing fish are available for spring/summer (1/2)- Cory Stockton (@mumper) March 22, 2013
and fall/winter. The pets are available only in the Summer and Winter seasons so the dates are not the exact same. (2/2)- Cory Stockton (@mumper) March 22, 2013
So, the seasonal fish span multiple seasons, and the two seasonal pets span single seasons. Single seasons that loosely mirror our real-world seasons. Be on the lookout for the Guardling around the time Summer hits the Northern hemisphere -- in-game we're guessing it'll be May or June. Are you ready for your Zookeeper title?