It’s easy to understand why so many boats are showing up in the Atchafalaya Spillway now.

The fish are biting: Sac-a-lait started the procession, and bass are helping spur angler’s passion for getting on the water with hopes of finding what has been a terrific run on big slabsides.

Launches are packed by 9 a.m., and there’s a constant procession of fishermen putting in throughout the day. Some are hitting the launch ramps after 2 p.m. to get in on the warmth of the afternoon sun and some of the best late-winter sac-a-lait and bass action in years.

While the word is out on these spots, it’s worth taking time to understand why launching ramps are filled from Bayou Sorrel on the north all the way to Russo’s on the south end near Morgan City.

It’s simple: Water temperatures are on the rise, up from the dead-of-winter waters that dipped into the 40s, a level that sends freshwater species across south Louisiana into a lethargic state, where survival is more important than feeding, a time when sac-a-lait, bass, bluegill and other panfish, catfish and small baitfish rely on stores of fat they accumulated during fall feeding sprees.

And there is more sunshine during the days leading up to the vernal equinox, the day when the sun crosses from south to north over the equator. Even 30 additional minutes of sunshine helps water temperatures rise, or, at least, be able to hold a constant temperature under decreased hours of darkness, even nights when the thermometer dips into the lower 30s.

Simple, too, is understanding the Atchafalaya River is unusually low for this time of year. In a “normal” year, this deepest of Louisiana rivers is taking water from the Red River and 30 percent of the flow of the Mississippi River, and both of those rivers that drain more than two-thirds of our country are overflowing with late-winter runoff. In the country’s northern climes, a cold winter has reduced snow melt, and ice continues to cover miles of smaller rivers.

That means the Atchafalala River has stayed in its banks and has not sent cold water rushing into the Spillway’s backwater areas.

While this delays the run on late-winter crawfish, it also means the two species of sac-a-lait — you can find them listed as black crappie and white crappie in fish identification books — have found water warm enough to push them to spawning banks.

Take Sunday’s trip into a canal off Big Bayou Pigeon, a location east of a cross-over canal from Little Bayou Pigeon. The take was 24 sac-a-lait, 22 of which weighed a pound or more, and was about evenly divided between these two subspecies.

Of the 11 black sac-a-lait in the catch, there were four males and seven females, and all seven of the females had spawned.

We were in water that hit 57 degrees at noon, and relying on biological information that indicates this species is the first of the sunfishes to spawn — when water temperatures are between 56-59 — and knowing that sunfishes like to spawn on a full moon — and that was Feb. 3 (and Sunday was Feb. 8), it was easy to understand why the fish were in water less than two feet deep.

Noted, too, at the filleting table, was that of the 13 white sac-a-lait, seven were females (six males), and all females had their eggs, almost fully developed eggs. Again, relying on books that indicate that this subspecies spawns when water temperatures run between 59-62, it was easy to know why the females held their eggs. The presence of the males indicated their spawn isn’t far away, and could come on next week’s new moon phase.

OK, so that does little for bass fishermen. True, there are some ardent pursuers of the largemouth bass who’ll never admit the same level of desire to catch panfish —but most will put down those high-dollar rods and reels for jigging poles and lighter tackle — the first reports of sac-a-lait catches should push them to a couple of nights spent readying their bass tackle.

That’s because largemouths and what spotted bass live in Louisiana waters begin their prespawn rituals when the sac-a-lait spawn hits high gear. Bass that have found the deepest water their home areas offer begin moving to traditional spawning territories.

In the Spillway and nearby waters in the Verret and Des Allemands basins, Bayou Black and the marshes and backwaters areas in Delacroix and Lafitte, water depth generally is as little as two feet. It’s now when bass will move to clear banks, areas void of overhangs that assures the eggs deposited on spawning beds receive as much warming sunshine as possible and eggs hatch as quickly as possible.

Water temperature between 62-65 is the range listed for the bass to spawn. Reports that bass were inhaling offerings at or near the bank in the past week is an indication they’re ready to make little bass, hopefully lots and lots of little bass.