A new crappie regulation went into effect July 22 on all four flood-control reservoirs — Arkabutla, Enid, Sardis and Grenada.

If you haven’t already seen it, the minimum length limit is 12 inches, the daily creel limit is 15 fish and the number of poles is limited to four per angler (two hooks or lures per pole).

In addition, boats with three or more anglers may only keep up to 40 crappies per boat.

These regulations standardized crappie limits at these reservoirs to simplify things for anglers who often travel from lake to lake.

And pole limits are not new. Concern about pole limits was triggered by increased harvest and potential overfishing by trollers fishing unlimited numbers of poles.

However, long before recommending any pole-limit regulations, Mississippi Department of Wildlife, Fisheries and Parks’ fisheries biologists Keith Meals and Arthur Dunn, and Mississippi State University fishery researcher Steve Miranda carefully designed a creel survey and analyzed the data so they could assess the effect of multi-pole trolling.

Here’s the science behind pole limits.

Trolling has become increasingly common on Mississippi reservoirs, especially Arkabutla, Enid, Grenada and Sardis — each of which has vast areas of open water where summertime crappie can hide from anglers.

If you haven’t tried trolling, you should because it’s so effective. Among the three flood-control reservoirs studied, catch and harvest rates of trollers were two to three times higher than polers.

Trolling used to be primarily a late-spring through summer strategy, but Meals’ and Dunn’s recent creel surveys show it has become a year-round technique.

Creel data also reveal that trollers have higher catch rates than polers throughout the year.

And trolling is becoming more prevalent. In 2004-06, fishing effort was about equal between polers and trollers — each method comprised about 50 percent of the total crappie fishing effort.

Meals shared that trollers now make up about 80 percent of the fishing effort.

Not only does trolling result in a higher catch rate (crappie per angler hour), the catch rate of trollers increases exponentially with the number of poles.

For example, the average catch rate with two poles per party is about .5 crappie per angler hour.

A simple linear relationship would predict one crappie per angler hour with four poles and 1.5 crappie per angler hour with six poles.

Rather, the researchers found catch rate with four poles was 1.6 per angler hour — more than triple the catch rate with two poles.

And the catch rate was 2.2 crappie per angler hour with six poles — far more than four times the catch rate with two poles.

It should be apparent that catch and harvest will increase if more anglers are trolling, and this increased harvest might jeopardize the quality of the fishing.

Fishery models revealed that if the fishery was a 50/50 mix of polers and trollers the annual harvest rate would be 1.3 times greater than if all anglers were polers under a 30-crappie-per-day/no-pole-limit regulation. If all the fishing was by trollers, the annual harvest would be 1.7 times that of a poler-only fishery under the same crappie/pole regulations.

Now, hook this together: higher catch and harvest as trollers increase, and catch and harvest rates go up even more when trollers use more poles.

More trollers with an unregulated number of poles is a path that could easily lead to overfishing and deplete the numbers of quality-size crappie. Everybody loses.

The MDWFP-MSU team used their population models to look at the effect of harvest limits and pole limits. In a fishery that was a 50/50 mix of polers and trollers, a 30-crappie-per-day/three-pole limit would reduce the catch approximately the same as a 10-crappie-per-day limit with no pole limit.

In the case of 100 percent of the anglers trolling, it would require a daily creel limit of less than 10 fish to lower the harvest as much as a 30-fish-per-day/three--pole limit.

Plain and simple: Managers can protect these high-class crappie fisheries better with a pole limit than a highly restrictive creel limit.

But there are some unknowns that prevent precise estimates from the models.

It is well known among fisheries managers that not all anglers catch a limit. The proportion of crappie anglers who catch a limit of crappie is not known for the flood-control reservoirs because creel surveys obtain data while anglers are fishing, and access-point creels that obtain completed trip data are needed to determine the proportion of anglers that catch a limit.

Nevertheless, it is easy to accept that not all crappie anglers catch a limit. Thus, lowering the creel limit, say from 30 to 15 fish per angler per day, does not reduce harvest by 50 percent but rather by a lesser amount.

Size limits have been used effectively to reduce harvest. But a problem with size limits is that short fish must be released. Studies at Mississippi State found 97 percent of angler-caught crappie survived after release when water temperature was 60 degrees, but an Illinois study found only 71-percent survival at 73 degrees.

Judging from studies of largemouth bass, mortality is expected to increase at higher temperatures.

So a high size limit might reduce the number of crappie in the cooler (although harvest in pounds mught be the same or greater, as with a lower size limit), but with poor survival of the released fish, the regulation might not reduce the number of crappie killed — especially in Mississippi’s warm summertimes when trollers are most active.

Managers are concerned about retaining a large number of anglers. How much can management reduce the creel limit before the number of anglers starts to fall off?

Low creel limits are a double-edged sword. First, anglers are more likely to be selective in the fish they retain with a low creel limit, so more fish would be released.

The other edge of the sword is how many anglers keep fishing but release fish after they catch a limit? This number would probably increase under a highly restrictive creel limit, and the problem would be magnified for trollers, who catch limits faster than polers and, thus, would be more likely to catch and release.

Both scenarios take us back to the question of survival of caught and released crappie.

There is some interesting crappie biology and angler tips buried in all this management science.

Trollers have higher catch rates than polers. One obvious explanation is efficiency: Trollers are effectively fishing the expansive open-water zone, and they are quickly covering a lot of water, both horizontally and vertically.

And for anglers into the effectiveness of modern sidescan and 360-degree depth finders, they are fishing while looking.

But there might also be something to the effect of multiple baits “swimming” in the water that, by sight or by sound, might send crappie the message of a school of bait. Imagine the effect of trolling multiple umbrella rigs, which is illegal in the FCRs.

The biologists recommended a three-pole limit. MDWFP always solicits public comments on fishing regulations.

The Fisheries Bureau listened to anglers and recommended a compromise of a four-pole limit with a reduction in creel limit.

The MDWFP Fish and Wildlife Commission approved the revised regulation. A good regulation is based on sound science. An effective regulation is based on evaluation.

Now we wait to see the effectiveness of this regulation.