I started doing some secretive work in the spawn chunks (where my command blocks live) earlier today in preparations for 1.9, when I noticed some squids floating about in the ocean several chunks out, I went out to investigate saw more.. and more.
As it turns out, squids had been spawning and seemingly getting stuck on the outer edges of the spawn chunks and never being unloaded. Hundreds of squids were out there, forming a wall around the outer edge of the always loaded chunks around spawn.
To fix this, I have replaced all water with beautiful blue glass in the chunks in the always loaded region, plus a one-chunk border. This gives squids no more room to spawn in the spawn chunks and should allow them to spawn elsewhere.
Early reports seem to suggest an abundance of Squids, it is unclear whether this affects other mob spawning but I would assume there'll be an increase generally for mob spawns too.