Guest post by Chloé Nater

Size‐ and stage‐dependence in cause‐specific mortality of migratory brown trout (2020) Nater at al., Journal of Animal Ecology, https://doi.org/10.1111/1365-2656.13269

The Crux

When it comes to dying, not everyone is equal.

The corona-virus pandemic has reminded us of that over the last months: the same disease that passes often without any symptoms in young children is life-threatening for the elderly. Age, in this case, seems to influence how likely someone is to die from the disease. For other risks of death – take, for example, car accidents – age is not that important, but location may be: the chance of dying in a car accident is higher for someone who spends two hours per day commuting by car on a busy highway, than for someone who only needs to walk across one car-free road to get to work.

For animals, this is very much the same. They can die from a variety of causes (starving, predation, disease, hunting, etc.), and an individual animal’s risks of dying from any of these depend on characteristics like age, size, or colour, and on location. How many and which animals die from different causes then has consequences for the size of populations, and sometimes also for other species in the area, including humans.

The latter is particularly true when the animals of interest are being harvested. If we want to manage harvested populations sustainably, we need to know how much of the total mortality risk of different individuals is due to harvest, and how much is due to natural (and possibly other human-related) causes.

Freshwater fish are among the species most heavily impacted by humans through harvest, but also other factors such as habitat destruction, pollution, and hydropower production. In this paper, we estimated the mortality of Norway’s largest trout due to harvest and due all other causes, and how it varies depending on individual body size and spawning location.

What We Did

The trout studied in our paper embark on a journey up the river Lågen every second year to spawn (lay eggs). On their way, they often make use of a fish ladder built by humans which allows them to traverse a hydropower dam. Some cross the dam and spawn upstream of it, while others are content to spawn downstream of the dam. Almost 15,000 unique trout have been captured and marked in this fish ladder between 1966 and 2016.

Recaptures of already marked trout in the fish ladder in a later year provide information on survival, while reports of harvest of marked trout by fishers provided information on deaths due to harvest. We then analysed all these records using statistical models that account for differences in individual body size and spawning location (above versus below the dam).

Did You Know: Barriers and Fish Ladders

Hydropower production is a major stressor for a wide variety of species in freshwater ecosystems. For migratory fish, like the trout in this study, dams can be particularly detrimental: they act as barriers between habitats, may cut off historical spawning grounds, and increase mortality of both spawning adult fish and their offspring. Fish ladders are structures meant to facilitate upriver (and sometimes also downriver) migration. There are many different types (including some “innovative” recent developments involving cannons!), but they are often selective and not sufficient to compensate for the habitat loss and mortality risks induced by the dams.

What We Found

For the majority of trout – irrespective of their body size and spawning location – harvest mortality was substantially higher than mortality from other causes. Intermediate-sized trout had higher harvest mortality than small or large trout, while mortality from other causes decreased with size for above-dam spawners and increased with size for below-dam spawners.

Problems

Since recaptures only happen in the fish ladder, recapturing marked trout in years in which they spawned below the dam was not possible. At the same time, not all trout were equally likely to use the fish ladder. Large trout in particular were much more likely to stay below the dam.

This made them “unobservable”, which not only made it difficult to estimate parameters for them, but also left us with a new big question as to why these large trout have such high mortality from non-harvest causes when they spawn below the dam.

So What?

Our study highlights that harvest, even when mostly recreational, can be the dominant source of mortality for adult trout. How high the total harvest pressure is, and how susceptible different-sized individuals are to it, are thus important considerations for sustainable management.

Somewhat surprisingly, we did not find high mortality of adult trout associated with passing the hydropower dam. Instead, we found evidence that spawning below the dam came with a large mortality risk for large trout. We can only speculate on reasons (e.g. crowded spawning grounds leading to more aggression or disease transmission), but this shows that some human impacts, i.e. hydropower production, can affect fish populations in complex ways.

Taken together: different human activities in freshwater ecosystems can feed into a variety of mortality causes for fish, but individual characteristics like body size and spawning location determine susceptible each individual is to be dying from any of the causes.

Chloé Nater is a quantitative ecologist currently employed as a Postdoctoral researcher at the Norwegian University of Science and Technology. She’s supposed to be working on birds, but cannot quite let fish and (some select!) mammals go yet. Fortunately, she can use similar models to study all of them. You can follow her on Twitter @chloe_nater.

Title Image Credit: Atle Rustadbakken, CC BY 2.0