Does hunting mortality have significant long-term influence on waterfowl populations? This debate continues within both the hunting and biological research communities, but the answer isn’t as simple as some hunters might like.

Some biologists believe hunting to be additive. That is, birds killed by shotgun are additional to other types of mortality, including predation, disease and accidents. According to this model, as harvest increases, populations decrease. Others believe hunting to be compensatory. Under this model, certain percentage of duck populations will die each year no matter the cause, and those killed by hunters are simply part of that total.

Biologists with the USFWS have, for the most part, considered previous hunting mortality when setting limits and lengths for upcoming seasons. Based on the number of ducks flying south each fall for the past 20 years or so, those scientists in the “additive” camp appear to have been right. Duck numbers have been booming for nearly two decades. As a result, hunting seasons have been governed by a liberal framework, the most generous of the Service’s three frameworks.

But not all species of ducks are thriving. Those same biologists have been tinkering with daily bag limits on some species  for years. Pintails, for instance, have been ping-ponging between one, two and three birds per day since the late 1980s. In the 1970s, hunters were allowed as many as seven pintails in a day.

Why? Despite generally banner duck recruitments, Pintails continue to struggle. According to the USFWS, the breeding population was about 2.4 million in 2018, down from around 7 million in 1970. As their numbers started falling, biologists with the USFWS reduced bag limits, and in some years, season lengths, on pintails, assuming hunting to be additive. Their goal was to boost populations by reducing hunting mortality.

Recent research, however, seems to suggest that hunting mortality has little impact on pintails and at least two other species of ducks: redheads and scaup. Reductions in bag limits haven’t correlated with increased populations for any of those species. University of Nevada-Reno research scientist Dr. Ben Sedinger examined data in three published papers and concluded that hunting mortality has little, if any, impact on populations of those three species.

“Despite daily bag limits that range from 1 to 10 pintail per day over the last 40 years, peer-reviewed studies have shown no corresponding change in pintail annual survival rates. That is, drastic differences in daily bag limit for pintail have had zero effect on annual survival,” Sedinger wrote in a 2016 California Waterfowl article.

As it turned out, nesting conditions in the Prairie Pothole Region appear to have a far more significant impact on how many birds make their way south each fall than the number killed by hunters the previous winter. Nesting conditions have been generally favorable for nearly two decades and, in correlation, hunters have enjoyed excellent waterfowl hunting. So why are pintails struggling?

According to Sedinger, “The current decline in pintail populations corresponds with a shift in agricultural practices: Farmers started leaving harvested fields in stubble to avoid erosion. Those stubble fields look similar to the shortgrass prairies that pintails evolved to nest in and farmers typically start disking these stubble fields about two weeks after pintails initiate their nests. This has resulted in an ecological trap for pintail that caused the population to decline.”

Another study Sedinger cited found that restrictions have not benefited redheads, either. A third study that examined lesser scaup also didn’t find a correlation between population trends and daily bag limits. Although they don’t follow the same nesting habits as pintails, redheads and bluebills have also faced significant habitat-related challenges.

In the same article, Sedinger addresses the additive-compensatory debate, suggesting that while hunting tends to be additive, duck numbers self-regulate based on environmental conditions.

“Every summer duck populations double or more in size as newly hatched ducklings get their first view of the world. This often results in more ducks than the environment can support, especially in late winter when food starts to become more scarce. If ducks are culled by hunters, then there are more resources for the ducks that were not shot and the remaining ducks may be able to be even more productive.”

Even if hunting does impact population, harvest tends to follow populations. That is, when lots of ducks fly south each fall, more are killed by hunters. That may seem like an obvious correlation, but points to another potential factor. When duck numbers are high, more hunters take to the marshes. When duck numbers are low, passive waterfowl hunters tend to stay home and never toss out a decoy spread. In some ways, hunting-related mortality may self-regulate, at least to some extent.

That’s true with other game species, too, particularly upland game birds. Far fewer hunters visit such traditional pheasant states as Nebraska, Kansas and South Dakota when bird numbers are down. Those states typically don’t alter daily bag limits, even during years of drought, which can result in poor chick recruitment, or after significant winter mortality. Such events occur regularly in the Plains, and pheasant numbers ebb and flow with environmental conditions. When those conditions are prime and habitat is abundant, populations boom.

That said, we also know that harvest rates can influence populations of some game animals.

“Canada geese are a prime example,” Delta Waterfowl senior vice-president John Devney said.

That became clear in the 1990s when the Atlantic Flyway population of Canada geese crashed. The USFWS actually closed the season for six years after a string of poor nesting seasons. At the same time, hunters were shooting an estimated 30 to 40% of the flyway’s wintering population. Heavy harvest and poor recruitment was a one-two punch that led to significant population declines.

Geese have different nesting and breeding biology than most duck species, though. Canadas don’t reach sexual maturity until they are at least 2-years-old, their broods tend to be smaller than most puddle duck species and they won’t renest if their first nest fails. The upcoming Canada goose season is again being restricted for the same reasons it was closed in the 90s: The past few nesting seasons have been poor.

“Limits and seasons need to be adjusted for some species,” adds Devney. “Sea ducks also seem to respond to hunting mortality more so than dabbling ducks.”

All this seems to suggest that  our initial question—Does hunting mortality have significant long-term impact on populations—can’t be answered with a simple yes or no. The answer to that question is likely seasonal, situational and species dependent. And even in good years when certain populations are booming, we should abandon daily bag limits entirely. History has shown us what can happen when we completely disregard the impacts of hunting on our game animals.

Feature image via Phil Kahnke.