Shrinking rivers affect fish populations

New research from the University of Canterbury published today has found that a shrinking river is less able to support larger predatory fish, such as the highly-valued sports fish like brown trout or at-risk native fish like galaxiids and eels.

Reducing a river's size is likely to reduce its capacity to support predatory fish. Underwater view of the Waimakariri River, Canterbury. Photo credit: Angus McIntosh/University of Canterbury
Reducing a river's size is likely to reduce its capacity to support predatory fish. Underwater view of the Waimakariri River, Canterbury. Photo credit: Angus McIntosh/University of Canterbury

New research from the University of Canterbury distributed today has discovered that a shrinking river is less able to help larger predatory fish, for example, esteemed valuated sports fish like brown trout or in danger local fish like galaxiids and eels.

For this study, Scientists collected the data from 29 New Zealand rivers. Their findings potentially explain why decreases in natural surroundings estimate are such a great driver of predator misfortune around the world.

Lead author Freshwater Ecology Professor Angus McIntosh of the School of Biological Sciences said, “If you make a habitat smaller, for example by taking water out of a river, you shrink the physical dimensions of the space which can reduce the size of predators, such as fish, that live there. When predators are smaller, they are not as efficient in their energy use so the food, such as stream insects, available in the habitat will support fewer and smaller fish.”

Capacity to support predators like this brown trout scales with river size. Photo credit: Angus McIntosh/University of Canterbury
Capacity to support predators like this brown trout scales with river size. Photo credit: Angus McIntosh/University of Canterbury

We show that smaller rivers support fish per unit of prey resource compared to larger rivers and we derive a theory explaining why this happens. The theory is based on how the dimensions of a habitat constrain the body size of individual predators, the fish in our case, and how that affects the efficiency of prey use.”

“One of the neat things about this study is that the mathematical theory actually helped us to understand the underlying causes of population declines. This means we can be better informed when making decisions that affect habitats for key species in New Zealand.”

How much water is taken out of rivers is a hot topic and the subject of great public debate and lengthy court battles. The researchers expect this work will have an important influence on decisions about resource use, particularly regarding the flow in rivers.

UC biologist Dr. Helen Warburton said, “We make many decisions about how to manage natural resources which affect the size of habitats, for example when we take water from rivers. Our work shows that those changes in habitat size affect how food webs work and that they could have a detrimental effect on the capacity of those habitats to support fish in rivers. Moreover, these effects haven’t generally been considered in how we make decisions about natural resource management.”

Using data from 29 New Zealand rivers, a new research paper by lead author Freshwater Ecology Professor Angus McIntosh of the School of Biological Sciences, UC Science, and co-authored by UC mathematician Associate Professor Michael Plank ,UC biologist Dr Helen Warburton, and collaborators from NIWA and overseas, has been published in the online journal Science Advances.