Dealing with dynamics

Fluctuating crappie populations in fluctuating reservoirs Part II

Mississippi’s flood-control reservoirs — Arkabutla, Sardis, Enid and Grenada — provide outstanding crappie fishing. But like crappie fisheries everywhere, the quality of the fishing in any one of these lakes varies widely from year to year.

Length-limit regulations can improve the size of fish available to anglers, but only under some conditions do regulations affect abundance.

The abundance of crappie is largely determined by successful spawning and good survival of the young. Recent research at Mississippi State University sheds some light on the annual production of young crappie.

It’s well established in natural lakes and impoundments that abundant young crappie — what biologists refer to as strong year classes — are produced in high-water years. Crappie spawn in vegetation and brush, and the cover helps protect the young crappie. High water expands the amount of spawning habitat and cover.

The importance of flooded vegetation creates a problem in Mississippi’s flood-control reservoirs that fluctuate 12 to 15 feet throughout the year.

This not a sinister plot by the Corps of Engineers to make fishing and fishery management tough; the reservoirs were built to prevent downstream flooding in the Yazoo River floodplain in the spring when the rains come and the Mississippi River typically rises and impedes discharge from the Yazoo.

To accomplish their flood-control purpose, each reservoir has an operation mandate called the rule curve that specifies target water levels throughout the year. Water is released in the fall to lower the water levels to create flood storage capacity. The rule curve for Mississippi’s flood control reservoirs prescribes that water is lowered to a minimum stage by December.

In Sardis, Enid, and Grenada, the water is held at winter pool until mid-January and then steadily raised to summer pool by May. In Arkabutla, the rule curve dictates holding the water at winter pool until April.

So say the rule curves, but the reservoirs only fill if it rains after January. On the other hand, heavy winter or early spring rains can result in high water in the reservoirs if the rains cause flooding downstream. In this soggy scenario, reservoir managers hold water in the reservoirs to help minimize additional downstream flooding.

The rule curves might reduce downstream flooding in most years, but they are not conducive to successful crappie spawning.

Due to the wide annual fluctuations, these reservoirs are devoid of cover in the fluctuation zone. Aquatic and wetland plants can’t grow if they are out of water most of the year. Terrestrial vegetation can’t survive when underwater during the growing season, so it only grows at elevations above normal summer pool.

A wet winter and spring can create early high water that makes the flooded terrestrial vegetation useful for spawning and provides cover for the young crappie. Such was the case in 2010, but these high-water years are infrequent and unpredictable.

Benefits of backwaters

The flood-control reservoirs were rivers before they were impoundments. The upper ends of these reservoirs, where the river or rivers flow in, retains the rivers’ pre-impoundment floodplain. When the river rises, the floodplain is flooded and creates extensive backwaters — maybe even connecting to some floodplain ponds and lakes.

Do crappie use these backwaters? Let me give you a hint: Crappie existed in rivers throughout the Southeast for millions of years before man built the first dam.

Dr. Steve Miranda, U.S. Geological Survey fisheries scientist and fisheries professor at Mississippi State University, has been doing crappie research to guide Mississippi Department of Wildlife, Fisheries and Parks crappie management for more than two decades. Recently, Miranda and his graduate students took a close look at the value of upper-reservoir backwaters for crappie spawning and recruitment.

The MSU researchers found crappie spawned and the young were reared in the backwater areas. Indeed, abundance of young crappie in the backwater areas measured as fish per net in the fall was more than 10 times greater than in main reservoir coves.

Expansive backwaters are crappie factories, but they need water, too. The backwaters become flooded at water levels well below summer pool levels that inundate the terrestrial vegetation that provide the bulk of the spawning and rearing habitat in the coves. Considering reservoir water levels in the flood-control reservoirs from 1989 through 2010, sufficient water was available to flood at least part of the backwaters in more than nine out of 10 years. Water level sufficient to flood vegetation in the coves occurred in a little more than seven out of 10 years.

The water only benefits crappie if it occurs when they are spawning. Miranda and graduate student Jonah Dagel determined the backwaters of the reservoirs were flooded when the water reached 60 degrees, a temperature at which crappie usually begin spawning, in five out of 10 years. The coves, on the other hand, had sufficient water to flood vegetation before the crappie spawned in only three out of 10 years.

Not only do the backwaters produce more young crappie than main reservoir coves, but they do so more often.

There is one more piece to the puzzle. Obviously the benefits of high water depend on whether it arrives in time for the crappie spawn, but too much water too early had a negative effect on crappie abundance.

This suggests that the amount or maybe the quality of the vegetation in the backwaters was reduced by high water in the winter. This makes sense; these plants’ life cycle is adapted to several million years of relatively low water during winter and rising water in late winter and early spring.

Knowing more about what affects crappie year class abundance will help advance management. Fishery managers can’t control the amount of rain or when it comes, but now they better know which years are likely to produce strong year classes and which years will produce a weak year class.

That knowledge, in turn, can help managers determine when to use length limits to help take some of the fluctuations out of crappie fisheries.

Click here to read Part I of Dealing with Dynamics.