You can find astrocetus in the space biome of gen 2. From my experience they seem to spawn more during ambergris space (although I have no actual evidence to back this up). If you manage to find a good one it's pretty easy to trap them in the narrow area of the space bridges with a few behemoth gates.
Theres an issue for me in Singleplayer where they teleport away never to be found again if their close to being knocked out using the Bloodstalker and Rocket Launcher method. The easiest way for me is to lure it near the land and fire a Cannon Shell from a mek. People say to use the Skiff and Cannon method but it might teleport away aswell.