Scales of Stress

How Fish Populations Handle Stress Differently

Stress is a ubiquitous part of life and one we are all familiar with. As humans we can easily recognise the signs of stress and are aware of the wide range of long-term effects stress can have. We develop various coping mechanisms or tools to help us deal with everyday stressors, but everyone experiences and deals with stress in their own way. For some people giving a presentation at work may elicit a high level of stress, for others they may experience little to no stress but are instead highly stressed by moving house. People experience the same stressor in different ways and to different levels. Well, it turns out the same is true of fish.

In our recently published paper, we explore how where a fish is born influences how it copes with stress. Like us, fish also experience a wide range of stressors in their daily lives, from encountering a predator to changes in the water around them. When a fish encounters a stressor, the brain perceives it and sends an array of signals through the body. These signals are communicated through hormones such as cortisol and adrenaline. As different parts of the body receive changes in hormone levels a variety of physiological and behavioural changes take place. Heart rate increases, the spleen contracts to increase circulating red blood cells, they may flee to escape the stressor.

Within one stream individuals of the same species occupy different sections. In the case of juvenile brown trout, Salmo trutta, where an individual is found is related to several factors including where their parents chose to spawn, habitat quality and the number of other individuals in the area. All these factors can contribute to the type of individual produced. In our study we focused on how response to a stressor, in this case being chased, differed between fish produced in the furthest upstream section of a stream and a downstream section close to the river mouth. We found that fish from downstream had a stronger response to stress, indicated by parameters such as higher levels of cortisol and higher levels of circulating red blood cells, compared to fish upstream.

But why is it important to understand how individuals differ in different stream sections? Freshwater systems are increasingly under human-induced pressures that are shaping the environment these fish find themselves in. Habitat fragmentation, for example through restructuring of streams or construction of barriers, cuts off individuals from previously accessible areas. If fish from these different sections differ this could have knock on effects for the types of traits we find in a stream.

Now we have established there is local variation in stress response within a stream we are keen to see what other differences there may be between individuals in different streams sections. We have ongoing work based on this and are hoping to uncover differences in behaviours like aggression and boldness as well as morphological characteristics such as fin size and body shape.

Written by Madeleine Berry, PhD student with the SEG.