Fishing, lazy style

So we stopped at this boat dock for the hell of it and I got some really good pictures like this one. This is some type of contraption they use to either catch fish or crabs. Can someone fill me in on this? I do know that they traditionally make these out of palm tree parts, but this particular one is metal. I also understand the general principal behind them--there is only one way in so the animal in question gets stuck inside and then somebody comes and collects whatever is inside. I just want to know what they are catching.