I am by no means an expert at anything, but I have caught my share of snook, tarpon, sharks, snapper etc around bridges with fast moving water. I feel like a lot of people don't understand the concept of the "stack" or don't know how useful it can be. I hope to help others learn the things it has taken me years to figure out for myself, because I sure as hell would have loved if someone had taught me when I first started.
We've all heard of fish "stacked up," but what exactly does this mean? It means there are multiple fish sitting in one place, waiting for food to come by. Fish stack up in areas with good water movement, because, obviously, this means more food will be passing by. Stacking up in fast current is the most efficient way for fish to eat - they expend virtually no energy chasing down their prey - it is literally swept into their mouths.
So how do we use this to our advantage? Well first off the fast current means the fish have a limited window of opportunity to eat so they won't be asking too many questions lest they miss their opportunity. Secondly, when fish are stacked up we can reliably predict where they will be sitting and how they will be oriented. The classic example is bridge fishing for snook. The fish will always be facing into the current and will be positioned in front of the structure. If there is a well defined shadow line, snook love to sit just shy of the lighted area, and down low. Additionally, any kind of bottleneck, like the freshwater culverts you see going under roads, are excellent places for fish to stack up around. In the case of a culvert, the fish can be both in front of and behind the culvert (so they will be facing into the current on both sides of the road. After a heavy rain when the spillways are open and the water is moving quickly through these culverts, you can catch many trophy largemouth bass, snook and tarpon by fishing these culverts.
Understanding the position of the fish is only part of the equation. The second part is figuring out the presentation. Any bait presented to stacked up fish will have to be the right size/shape as what the fish are expecting, will have to be fished at the right depth, and will have to be moving in a convincing manner. A lot of times when fishing with live mullet or live shrimp, the difference between a bite and a pass is if your live bait makes the right kind of movement (usually a desperate attempt to flee the predator) at the right time. A shrimp that is too lazy or a mullet that is too dead to bother trying to get away from the predator is useless. They will ignore it. Sometimes twitching the line a little just as you think the bait is approaching a stacked up fish will trigger the strike.
When dealing with bridges, like in the keys for example, how the fish will stack up dictates your preferred tidal movements. For example, channel two is absolutely useless to fish on an incoming tide because the tide is moving south to north - and the fishing bridge is to the north of the road bridge. You simply won't be able to present a bait to the fish that are facing into the current and in front of the road bridge. Long Key bridge, however, is the opposite. the fishing bridge is to the south of the road bridge, so you'll be able to fish long key bridge on an incoming tide. In my experience fish will eat on both an incoming and outgoing tide. They may be more finnicky on an incoming tide because of the clearer water but by downsizing your leader and being persistent you can coax them into a bite.
The last thing I want to mention is that a lot of bridges have sight fishing opportunities for stacked up fish. Seven Mile Bridge in marathon is a great example. If you go to seven mile bridge just before dark on a good outgoing tide (as in an hour or two after the high tide - that's when the water really starts to move) you can literally see the tarpon and sharks sitting there. This is the case for many bridges, especially in the keys. Throw a bait in front of them and let the current bring it back to you. With the right bait and the right presentation you will hook up - I guarantee it.