Collect 40 gems (fish cannery) is a mission in Daxter. After the "Collect 30 gems from the tanker" mission, Taryn gave Daxter a flamethrower attachment for his spray gun. In return, she requested that he handle bug infestations by collecting skull gems in the fish cannery, as well as the same request at the distillery.
Upon arrival to the fish cannery you will face several slashers, after which you arrive at a larger open room. Take the right-most door to face a new gameplay mechanic involving giant ice blocks. These can be moved and melted with the flamethrower attachment. Melt this one just enough to use it as a platform, then move or melt the second one to move on. In the next room you must hover from cage to cage while dodging ice blocks that move from rails on the ceiling. After fighting several bugs you will run into Brutter, followed by a fight with a new enemy called a spawner. These spawn a limited number of metal backs and are vulnerable to the spray gun.
Shortly after you will encounter small lurker sharks. They have varying paces at which they swim around but each of them attack noticeably fast. The moment you fly above a shark in the direction it is heading it will jump up, killing you instantly. Either give them a wide berth or time flying across their path the moment they swim past you. You will gain the required 40 skull gems quickly enough as you fight more bugs and spawners throughout the cannery, occasionally solving simple puzzles with ice blocks.
Once outside, but still inside the cannery complex itself, you must dodge several more sharks as you hover from ice floe to ice floe. Use an elevator at the end to return inside and prepare for a tricky ice block puzzle. After using one block to climb to the higher platform you must hit two levers. One new ice cube must be moved one space away and the other two spaces, after which it fills back up to full size. Fly from one to the other and from there to a nearby cage hanging from the ceiling, you will just about reach the edge of the cage if you time your double jump and flame hover correctly.
Afterwards you return outside once more to finally deal with a live version of the large blue shark seen earlier. Heat up a frozen fish cube that come from the right so the rail takes them over the water and lures the shark into trying to eat it, which incapacitates it for a short while. Use this time to hover across the water. Several remaining bugs now stand between you and the exit.