Dealing with dynamics

Part I: Fluctuating crappie populations in fluctuating reservoirs

Mississippi’s flood-control reservoirs — Arkabutla, Sardis, Enid and Grenada — have developed a national reputation for outstanding crappie fishing. But crappie populations in these lakes, like crappie populations everywhere, are very cyclic, with one or two strong year classes followed by several years of weaker year classes.

Strong year classes — years when abundant young crappie are produced — occur during high-water years when flooded vegetation provides good spawning areas and cover for the young crappie. During low water years, spawning habitat and refuge for young crappie are limited.

Differences in the amount of flooded vegetation between high- and low-water years is extreme in the flood-control reservoirs, where water level fluctuates 15 to 20 feet during a year — even more among years.

Although some shoreline might grow during extended droughts, the large annual fluctuations generally prevent the establishment of vegetation — both terrestrial and aquatic — along the shorelines throughout most of the lake.

A simple rule of thumb in fisheries is that abundant fish tend to have slow growth when they compete for limited food resources, and this rule especially applies to crappie.

Strong year classes might produce a lot of crappie if young-of-the-year survival is high, but these fish will grow slowly. Anglers will catch a lot of crappie for a couple years after a strong year class, but they will be small.

Back-to-back strong year classes can result in extremely slow-growing, paper-thin crappie.

The converse is equally true: Weak year classes tend to have fast growth rates because there are fewer crappie sharing the food resources.

But Mississippi’s flood-control reservoirs don’t follow the rules. The same high-water conditions that favor a good spawn and early survival also offer the young crappie a greatly expanded reservoir with more aquatic area and volume and several thousand acres of recently flooded shoreline and vegetation. The expanded habitat triggers food production that feeds the abundant crappie, and growth is good.

High minimum length limits, like 10 or 12 inch, have proven effective in improving the size of crappie available for anglers, but they are not a silver bullet.

High crappie length limits only work when crappie have fast growth and low mortality. Because of the extraordinary fast growth, high length limits generally work in the flood-control reservoirs.

High minimum length limits can have a fringe benefit that can be exploited by astute biologists if they can annually adjust length limits. The high length limits keep decent-sized crappie available to anglers longer than no length limit or a low length limit, like 8 or 9 inches.

Let me give you an example for Mississippi waters. A strong year class is produced in 2010. Growth is fairly fast, and the crappie will reach 9 inches in their second year (2011). Although far from the quality of crappie Mississippi reservoirs can produce, most anglers will harvest a 9-inch crappie.

Many crappie will survive the first year that they are vulnerable to harvest (2011), and these fish will be 12 inches by 2012. 2012 will be a good year for crappie fishing, with or without a high minimum length limit.

If another strong year class is produced in 2011 and the growth is good, anglers will enjoy great crappie fishing in 2012 — they will have 12-plus-inch crappie from the 2010 year class and 9-plus-inch crappie from the 2011 year class.

Any length limit will be OK, but a length limit really isn’t even needed in 2012. If it ain’t broke, don’ fix it.

But let’s say 2011 is a very weak year class. These fish, too, might grow to 9 inches during their second summer (2012), but there won’t be many.

A good length limit in 2012 would be a high length limit, like 12 inches. Anglers will have to release some keeper crappie, but they will have big fish to clean. Most importantly, they will have good numbers of giant crappie in 2013. Their catch will include fish that are 11 to 13 inches from the 2011 year class and the survivors of the 2010 year class that are now 12 to 15 inches.

Had the high length limit regulation not conserved some of the relatively abundant fish from the 2010 year class, anglers in 2013 would only have the few fish from the very weak 2011 year class plus fish from the 2012 year class.

Unfortunately, biologists can’t forecast the strength of a year class (in this case, the 2012 year class) until it is too late to implement a regulation. So it is better to implement a high minimum length limit as soon as a weak year is detected.

What biologists need is a way to better forecast crappie year class strength.

Crappie in these flood-control reservoirs grow fast and enter the fishery in their second year. They are pretty well depleted by the end of their third year. A high size limit can stretch the time a year class is in the fishery from two years to three.

While regulations can be an effective way to change the size of fish available to anglers, successful spawning and good survival of the young are essential to quality crappie fishing.

Next month I’ll share some recent research at Mississippi State that may help improve and stabilize crappie spawning and recruitment.