Is it time to junk Adaptive Harvest as a waterfowl management tool?

The question demands to be asked after Adaptive Harvest unveiled its latest and most shocking conclusion -- the maximum sustainable kill over the long term can be achieved only by reducing the average mallard breeding population to 5.3 million, a near-record low. (See Too Many Mallards? and Reuniting Waterfowl Management , posted Mar. 10, 2004.)

Not only is this idea contrary to common sense, it discards past waterfowl management practices. It calls for an examination of the biological data so that we can make a judgment for ourselves.

We will look at the years 1974-2001. We will confine our study to the traditional North American breeding survey area, excluding Minnesota, Wisconsin and Michigan. This reduces Adaptive Harvest’s mallard breeding population goal from 5.9 million to 5.3 million.

We begin by looking at the mallard breeding population.

Figure 1. North America’s mallard breeding population during the years 1974-2001 ranged from a low of 5 million in 1985 to a high of 10.8 million in1999. The long-term average is 7.1 million. Source: USFWS.

As you can see, Adaptive Harvest’s 5.3 million population goal is 51 percent below peak and 25 percent below the long-term average. More importantly, it is only six percent larger than the lowest breeding population ever recorded. (The record low occurred in 1985, a year in which hunters saw few ducks and experienced dismal shooting.)

It is important to remember that we do not hunt the breeding population. It should be viewed as the standard measure by which we judge waterfowl management’s success or failure to preserve sufficient breeding stock to provide for a decent fall flight and the continuation of hunting. We judge the hunting season by the fall flight – the number of ducks we see and the number of ducks we kill. The fall flight includes both post-breeding adults and young-of-the-year.

Adaptive Harvest concludes that as the breeding population increases, the number of juveniles produced per adult decreases – a biological phenomenon known as density dependence. Thus, for maximum reproductive efficiency, it attempts to determine the breeding population that will produce the greatest number of young per adult that over the years will provide the maximum sustainable kill. It concludes the most efficient production level is when the spring breeding population is 5.3 million.

What does historical biological data say about this?

We can examine the average hen mallard age-ratios (the number of juveniles per adult in hunters’ bags) for the Mississippi and Central Flyways by the average North American breeding population. This data covers the so-called mid-continent mallard population and the area where most mallards from the traditional survey area are shot.

For the years 1974-2001 we find:

As you can see, the historical data more or less confirms Adaptive Harvest’s assertion that the age-ratio increases as the number of breeding mallards decreases, although the highest age-ratio occurs when the average population size is in the 6-million range, rather than the 5-million range cited by Adaptive Harvest. (Populations marked with an asterisk denote a one-year sample size.)

Does this mean Adaptive Harvest is correct?

The answer to this question depends on whether the juvenile-production rate increase is sufficient to offset the decline in numbers of breeders.

We can construct a simplified fall-flight equation to judge Adaptive Harvest’s assertion.

We assume adult summer-survival of 80 percent. We also adjust for juvenile gunning vulnerability, assumed to be 1.25. The equation is:

Fall Flight = (Breeding Population x .8) x (1 + [Age-Ratio/1.25]).

The calculated fall flights, broken down by average breeding populations for the years 1974-2001, work out like this:

As you can see, not only is the increase in juvenile productivity insufficient to offset the decline in numbers of breeders, the fall flight under Adaptive Harvest’s 5.3 million breeding population proposal would produce the smallest fall flight of all possible options. The largest fall flights and the best gunning occur in years when the breeding population is highest – a fact borne by our simplified population model and the experiences of veteran waterfowlers.

The same equation, using the data from years in which the mallard breeding population was within 10 percent of Adaptive Harvest’s 5.3 million goal, produced an average fall flight estimate of only 9.6 million.

This suggests that reducing the mallard breeding population goal from the current 8.1 million to 5.3 million would be a horrible mistake. It would result in abysmal fall flights and bleak hunting seasons.

More importantly, our simplified analysis argues we need to increase the breeding population. Adaptive Harvest estimates there is sufficient habitat under average wetland habitat conditions to support a mallard breeding population of 11.5 million. This suggests that to improve hunting waterfowl management should increase our average breeding population from 8.1 million to 10.6 million to take advantage of unoccupied habitat. This works out to one hen per pothole on the northern prairies. It would insure bountiful fall flights and quality gunning for decades to come.

In the meantime, management should dismiss Adaptive Harvest and toss it on the scrap heap. As the data shows, and as duck hunters have discovered in recent years, it is the road to ruin.