For nearly three-quarters of a century waterfowl management has keyed on northern-prairie potholes to set season lengths and bag limits. As a general rule authorities have called for long seasons and large bag limits in years when potholes are plentiful, and short-seasons and low bag limits during dry years when pothole numbers drop alarmingly. This reflects the highly variable precipitation on the Great Plains and its effects on waterfowl behavior.

Biologists long ago observed that breeding ducks descend in great numbers on the northern prairies in years when the region’s shallow basins are filled with water. Nest success increases and hens fledge larger broods. This produces an abundance of juveniles that swell the fall flight, allowing for longer seasons, larger bag limits and a higher total kill. In dry years, many ducks over-fly the parched prairies to spend the spring and summer in the boreal forest or tundra. Many hens do not nest. Others fledge fewer ducklings. This results in a smaller fall flight and restrictive hunting seasons.

The long-season/high-bag limit in wet years and short-season/low-bag limit in dry years has been the general waterfowl management policy since the 1930s when drought devastated the Great Plains. It represents an attempt to fine-tune the regulations and prevent hunters from over-shooting our breeding flocks during times of drought.

But the prairie landscape has changed considerably in the past three-quarters of a century. Myriads of potholes have been drained, upland nesting vegetation has been plowed under and new egg- and hen-eating predators have invaded the grasslands to reduce nest success. These changes raise a question:

Is the high water-high kill, low water-reduced kill rationale still justified?

To answer this question, we will focus on mallards, our most abundant duck. We begin with the year 1974, the first year biologists counted potholes in both Canada and the United States. (Prior to this date biologists counted only Canadian potholes.) The number of potholes found each May from 1974-2002 is shown in the following graph.

Figure 1. Potholes ranged from 2.1 million to 8.2 million during 1974-1999. The long-term average is 5 million with nearly two-thirds found in Canada’s prairie provinces. The fluctuations in abundance reflect the highly variable climate on the northern plains. Source: USFWS

As you can see, the number of potholes has varied considerably over the years, but the nearly horizontal trendline tells us that from 1974-2002 the prairies did not suffer a significant long-term decline in numbers of potholes, only the normal fluctuations that reflect the region’s highly variable precipitation.

A second issue involves the magnetism of the prairies. Are potholes still attracting ducks as they did in the past? To find out, we can compare the years 1974-89 and 1990-2002 by water conditions. (Drought conditions are defined as less than four million potholes, average water from four to six million potholes, and high water as more than six million potholes. The long-term average is five million potholes.)

Prairie Mallards

Figure 2. The graph at left shows the average prairie mallard breeding population under varying water conditions for the years 1974-89 and 1990-99. The graph at right shows the average percentage of North American mallards found on the prairies for the same time periods and environmental conditions. Source: USFWS

As you can see, the number of mallards on the prairies increased significantly in the 1990s, but the percentage of the overall North American population remained nearly the same. On average, 57 percent of North America’s mallards from 1990-02 were found on the prairies in low-water years, 65 percent during medium-water years and 68 percent in high-water years. This tells us breeding hens are still attracted to prairie potholes.

A related but seldom mentioned factor involves the temporary nature of many May potholes. A significant number become dry as summer progresses. A lack of replenishing spring rains can accelerate these losses and doom many duckling broods. In the past biologists have found that the higher the percentage of potholes retaining water in July, the greater the survival of broods and the higher percentage of young in the fall-flight.

We can examine pothole losses by looking at the following graphs that show the percentage of May potholes retaining water in July. The graphs are arranged by water

conditions.

Potholes Remaining Wet in July

Figure 3. The percentage of prairie potholes retaining water in July increased significantly under all water conditions from 1990-2002. The solid blue line shows the long-term trend. Source: USFWS.

The rising trendlines tell a remarkable story. The average percentage of potholes retaining water in July increased during the years 1990-2001 under all water conditions. In dry years from 1974-89 only 54 percent of the potholes on average remained wet in

July. This increased to 83 percent from 1990-2001. In average years the percentage increased from 51 percent to 70 percent. In wet years it increased from 42 percent to 58 percent.

According to standard biological theory, the high percentage of mid-summer potholes under all potholes conditions should have produced a signficant increase in the number of juveniles winging southward in the autumn. We can determine whether this occurred by looking at mallard age-ratios. These tell us the number of juveniles per adult in the hinters’ bag. In years of good production, age-ratios are high. In poor production years, age-ratios are low.

The following graphs show the long-term age-ratios by water conditions. The 2002 mallard age-ratio is provisional.

Juveniles Per Adult

Figure 4. Age-ratio graphs show the ratio of juveniles per adult declined significantly under all water conditions in the late 1990s. The declines were especially severe when prairie water conditions were average or excellent. This tells us the prairie pothole region is losing its former productivity. Source: USFWS

As you can see, juvenile productivity has declined under all water conditions, especially in high-water years. This is probably due to the decrease in the amount of upland nesting vegetation and the increase of such nest-destroying predators as red foxes and raccoons.

More importantly, we can compare average production rates by water conditions for the years 1974-89 and 1990-02.

Figure 5. Average juvenile productivity declined under all water conditions from 1974-89 to 1990-02. The declines ranged from .99 to .90 juveniles per adult in dry years, 1.25 to 1.05 in average-water years and 1.51 to 1.02 in high-water years. Source: USFWS.

As you can see, the difference today between dry and wet years is minimal -- .90 in low water years to 1.02 in high water years. This compares to prior years when the difference ranged from .99 to 1.51 -- a 53 percent increase in juvenile productivity between low-water and high-water years. This compares to a paltry 13 per cent difference today.

The age-ratio differences under all water conditions today are so small as to be nearly identical. This tells us the explosive productivity increases formerly associated with greater numbers of potholes is a thing of the past. – a phenomenon that has gone the way of the passenger pigeon.

Yet today’s regulations under Adaptive Harvest allow for season lengths and bag limits in the Mississippi Flyway ranging from 20-days and two mallards during low water to 60 days and four mallards in high water – a 100 percent bag-limit and 200 percent season-length increase at a time when the juvenile production increase is only 13 percent. Similar differences are found in other flyways.

How can waterfowl management cite pothole abundance to justify this enormous disparity in season lengths and bag limits during these times of depressed juvenile production?

The answer is that it cannot.

This strongly suggests waterfowl management is more interested in maintaining a high kill than rebuilding our beleaguered and declining breeding flocks. It further suggests that in the future we abandon as no longer valid the wet year-long season, dry year-short season practice of the past.

One might “excuse” our waterfowl managers’ failure to recognize the changing realities on the breeding grounds on grounds they are slow-learners. But this would be a mistake. They have taken a bad situation and made it worse. Not only do they ignore today’s new biological truths, they refuse to heed the old truths of the past.

How else can one explain that for the past three years the number of potholes has averaged only 3.7 million – a level that describes prairie drought conditions. Yet for the past three years waterfowl management and its malignant Adaptive Harvest protocol has refused to restrict hunting and reduce the kill as we did in times past. It has allowed the most liberal regulations in nearly half a century.

Is it any wonder our mallard breeding flocks have declined for the past three years – and are expected to further decline this spring?