Compendiums

Selectively Harvesting: does hunting and fishing change populations?

Around the world, people harvest fish and wildlife to make a profit, provide sustenance, and honor a cultural heritage. Harvest of wild animals occurs across a gradient of scale, from a single person hunting a white-tailed deer to fill their freezer with meat to the commercial process of harvesting thousands of fish at a time in the ocean. Yet, the way we harvest animals, such as the method or the intensity of harvest, has the potential to cause dramatic change in populations.

In recent decades, there has been growing concern among researchers, managers, and the public surrounding the way that we harvest wildlife and the potential consequences that harvest practices may have on populations. Harvest that removes animals based on specific traits or characteristics can affect the presence of those traits in the population. For example, people often harvest animals based on the body size, horn and antler size, or behaviors that make certain individuals more vulnerable. Over time, if harvest of animals with specific traits is consistent and high, it can result in an evolutionary change in a population. Evolutionary effects of harvest have important implications for how we manage wild populations, especially if through that selective process of harvest we change populations in ways that result in lower quality individuals or those that possess undesirable traits.

Selective harvest graphic showing spawning fish and harvested fish

Left: Selective harvest can change the body size of fish populations over time. As large fish are removed from the population via harvest, slower growing and smaller bodied fish are left to reproduce. Over time, continual harvest that removes large and fast-growing fish can result in smaller and smaller individuals in a population.

What is evolutionary change?

For evolutionary change to occur through harvest, two things must be true. First, the traits that are targeted by hunters and anglers must be heritable; in other words, those traits are passed from parents to offspring. Second, harvest of animals with those traits by hunters and anglers must be strong enough to remove those animals before they have the opportunity to reproduce and pass those traits on to another generation.

Evolutionary change, because of selective harvest, has been shown across populations and species. Some of the best-known examples occur in commercial fisheries. Average body size of fish in some heavily fished populations have declined through time. Body size of fish is a heritable trait, and depending on the net size used by anglers, fish that are smaller and grow slower are able to escape capture. Those fish that grow slower and have smaller bodies then are able to survive and continue to reproduce in the population, whereas the fish that grow quickly and have larger bodies are removed through harvest. Over time, small and slow-growing fish become more common, and have increased opportunity to reproduce. Over many generations, individuals in that population may become smaller and slower growing than they were before

How would this happen in ungulates?

In ungulate species, horns and antlers are heritable. Selection for males with
large horns and antlers has the potential to change the size of horns and antlers
in populations in the same way that commercial harvest can change the body
size of fish.

Harvest that selects for males that grow large horns or antlers quickly can remove those individuals before they have the chance to reproduce. Age, body size, and weaponry all play an important role in breeding success; if males that grow horns and antlers quickly are selectively removed at a relatively young age, males that grow slowly, or have smaller horns or antlers, may benefit with a reproductive advantage.

Does it occur?

While the potential for evolutionary change exists for ungulates, there are many factors that can quickly dilute a potential effect of hunter selection on the size of male horns or antlers. First, the way we harvest ungulates is vastly different than how we commercially fish. With fish, nets are incredibly efficient at removing all animals that are larger than the net size. With ungulates, however, even despite the best efforts of hunters, large males still occasionally escape harvest. Second, females contribute 50% of the genetic material to their offspring, but female ungulates aren’t often harvested. While both male and female fish are equally selected for with a fishing net, in many ungulate populations
the females are not hunted, and when they are, it’s almost never based on their horn size. Finally, nutrition and disease can play an important role on the condition of an animal. Allotment of resources to horns and antlers occurs after investment in body growth, and animals in compromised conditions may not grow horns and antlers to their full potential. Combined with the role of females, nutrition, and disease, evolutionary changes in ungulates species are often more nuanced and complicated than commercial fisheries. Given these nuances, the pressure simply does not exist at a level to result in an evolutionary change in many populations of ungulates.

What does this mean for harvest?

Harvest of wildlife is a fundamental part of wildlife conservation and management, and understanding the consequences of harvest practices on populations and specific traits is critical to the continued sustainable use of wildlife. Identifying where harvest is resulting in unwanted change is important for managers to understand where harvest practices may need altered. For big game species in North America, in certain populations where harvest is highly selective and intensive, genetic changes can occur; however, most of our current harvest practices do not appear to have evolutionary consequences for populations.

Definitions:

selective harvest - the hunting or fishing of animals based on a specific characteristic, behavior, or trait

evolutionary change - genetic changes to a population or species that results in changes in the characteristics of certain traits over time

Case Study: Bighorn Sheep

Changes to horn size are occurring in some populations of bighorn sheep that
experience both high rates of harvest, with that harvest being very selective (i.e., based on a size requirement). Some agencies only require a minimum size requirement for an animal to be legally harvested, that is, all males that possess horns of a certain size are legally able to be harvested. This size requirement for harvest can result in a similar effect as nets in fishing; only the largest animals are harvested, and males with smaller or slower growing horns are left to reproduce. Because this size requirement often removes animals before they reach their peak in reproductive success, males do not have the opportunity to reproduce before they are harvested. In some populations where size requirements are implemented as a management tool and there is unlimited harvest, there is evidence for genetic changes to horn size. Throughout most of North America, however, harvest of bighorn sheep is less intense and less selective, and there are not evident declines in horn size in most populations across North America.

bighorn sheep