The General Rules of Big Water Fishing
1) Fish will seek comfort zones when not really hungry or not amorous (i.e. spawning).
Know the preferred temperature preferences for the species you want to pursue. SInce a large body of water can have a variety of temperatures (temperature layering or stratification) at various depths, a fish will seek out the temperature it is most comfortable at and the depth associated with that temperature. Also, if the species is structure seeking (black bass, crappie, brown trout are a few examples in freshwater) it will seek structure at these corresponding depths. This rule applies to saltwater as well as freshwater, as any party boat captain can tell you. In order to find these fish use a thermometer (see tools below) on a metered line to obtain the temperatures at various depths, then fish at the appropriate depth for your fish. In the spring and fall a phenomena called 'turnover' will make it very difficult to predict fish depth, and just sampling water temps at various depths will not work. In these cases, however, the fish will be very hungry or amorous anyhow (See #5 below)!
2) Fish seek gradients (areas of sharp changes in depth, temperature, pH, light, weed lines, or clarity).
Remember that contour chart we discussed above, take another look at it. Areas where the depth contour lines of different colors (depths) come close together indicate a drop-off. Predatory fish often sit on the deep side of the drop-off waiting for shallow water swimming baitfish to swim over the edge. If the drop off has structure on the shallow side (see the submerged tree areas), predators will also sit on the deep edge and shallow edges of the cover so that they can jump out and snatch up a hapless baitfish. Areas with sharp changes in temperature will also attract fish. Where a river enters a lake, there is often a zone where the river water merges with the lake water (sometimes easily visible by the change in water clarity or whirlpools). In summer and winter the lake water will form layers. In the summer the water near the surface will be warmer than the water near the bottom. The depth at which the water temperate sharply changes from warm to cold is called the 'thermocline'. This thermocline is sometimes visible on sonar as a line. Fish sit near the thermocline to be both comfortable, and able to pop into the less comfortable water to get prey. The thermocline also hides some vibration from above or below it, so it will appear as a line on a sonar screen, and like an attack submarine, predatory fish will use this characteristic to hit prey.
3) Hungry fish in big water are either schoolers, cruisers, or ambushers.
Every predatory fish (this includes nearly all sportfishes) are 'built' to catch their prey in one of three ways: ambush (hide until prey comes by and then jump out to catch it), cruise (swim alone or in groups of 2 ot 3 in a likely prey holding area until it finds something to eat), and school (swim in large groups to encircle prey or use other predators to help find prey-'two eyes are better than one, four are better yet', and to help evade even bigger predators). Know which of these three types your fish is. Some species will assume all three roles, depending on age and the type of prey pursued. As an example, smaller striped bass will school together to ambush schools of small minnows, large stripers will cruise near schools of larger prey when prey is plentiful, and then will ambush when seeking structure prone prey. Largemouth bass are largely ambush predators (big mouth, sprint muscles, upturned eyes& mouth), but will cruise in the spring and fall, and small-midsized largemouth will school in the fall to encircle shad schools in larger lakes. Bottom line-know what your fish is for the conditions and lake and customize your lures/baits accordingly. For ambush predators-find structure in there comfort zone, or gradients the fish can use (see rule #2) and you will find the fish. For cruisers, use noisy/smelly/colorful baits/lures to attract their attention over large distances, and move lots over likely water. ALSO-note that one of the best ways to seek out schooling or cruising predators is to troll (at the right depth, see rules #1&2) until you find them, or if they are schooling near the surface look for diving seagulls and water birds (see rule#4).
4) Watch the birds, look for anything out of the ordinary, watch the weather.
Birds are an excellent source of reconnaissance for large water. They have a good viewpoint, and better eyes, and can spot schools of bait (which is where the predators will be) better than most anglers. Look for fish-eating birds diving into the water, and look for flocks of birds circling a section of water surface. Also, look for 'nervous' water (water that looks like something is moving underneath it). Look for shadows in the water and 'boils' on the water. Also, look for groups of other fishers (remember courtesy and don't crowd them).
A weather change can cause fish to feed or stop feeding. Dropping pressure will usually trigger bites, while rising pressure will turn the fish away from feeding. Lots of rain will make some fish harder to catch (sight feeders like bass) and some easier to catch (scent, vibration dependent fish like catfish). Watch the weather forecast (see rule #6).
5) Remember to be aware of turnover in Fall and Spring.
In spring and fall, the surface water goes to the bottom and the bottom water comes to the surface, which will excite the fish and make them hungry. Unfortunately, it may also make the fish harder to find, since their is no correlation of depth to temperature during turnover. Use rules #2,3,4 to find the fish. Also, if your species is spawning seek its spawning areas (before and after, preferably not during-let the fish do their thing and pick another species).
6) Be Safe!
Always respect a large body of water, and never underestimate it.
