Upstream struggle for predatory fish
Predatory fish such as brown trout and eel face a greater struggle for survival in smaller rivers than larger ones, Canterbury scientists have discovered.
A study of 29 sites across New Zealand has found that shrinking rivers are less able to support larger predatory fish, triggering new questions about how much water can be taken from rivers.
The research, published in the journal Science Advances, showed that reducing the size of a habitat undermines its capacity to support larger predators.
Professor Angus McIntosh, a freshwater ecologist at the University of Canterbury who led the study, said: ‘‘We need to incorporate that understanding in the ways that we set minimum flows and the way that we allocate environmental flows in New Zealand and around the world.
‘‘At the moment we assume that a unit of water in a big river can support the same amount of fish as the same unit of water in a small river, and that’s just not true.’’
McIntosh said the study potentially explained why reductions in habitat size are such a powerful driver of predator loss globally.
‘‘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,’’ he said.
‘‘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.
‘‘We show that smaller rivers support fewer 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.’’
Fish were examined in rivers all around the country, including the Rakaia and Waimakariri in Canterbury, the latter of which has been the focus of recent concern over water flow as irrigators push to delay the introduction of long-promised regulations.
The researchers hope their work will influence decisions on resource use, particularly regarding flow rates.
Canterbury University 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.
‘‘These effects haven’t generally been considered in how we make decisions about natural resource management.’’