Salmon Need Stress
The hormones that fuel the annual salmon run
I’m truly captivated by this topic, not just for the fascinating insights it provides into salmon behaviour, but also for what it teaches us about ourselves. There’s something deeply relatable about how stress affects us all, leading to similar responses like social anxiety or disrupted eating patterns. Understanding these shared experiences allows us to connect not only with each other but also with our aquatic counterparts. It’s a reminder that even the most overlooked creatures, like salmon, deserve our attention and advocacy for their conservation.
The Salmonidae family encompasses 220 species of Teleosts, a group of bony fish that have evolved to thrive in both marine and freshwater environments. Among them are peripheral freshwater fish like lampreys, sturgeons, gobies, and the iconic salmon. These fish have intricate systems to manage stress responses, which are crucial for their survival. Two key stress response systems in these fish are the hypothalamus-sympathetic-chromaffin cell (HSC) axis and the hypothalamic-pituitary-interrenal (HPI) axis. These systems kick into action when the fish perceive threats, releasing neurotransmitters and hormones like serotonin, dopamine, norepinephrine, and epinephrine to cope with the stressors they encounter.
Human activities, such as boat noise, chemical pollution, increased predation, and overfishing, pose significant threats to salmon populations worldwide. These stressors can disrupt their reproductive cycles and ultimately threaten their survival. Understanding how stress impacts salmon’s metabolism, behaviour, and social interactions is essential for effective conservation efforts, particularly considering the vital role of hormonal regulation during their annual breeding migrations.
Behavioural Responses
When faced with stressors, salmon undergo behavioural changes as part of their survival strategy. For instance, they may adjust their food intake and appetite to balance the risk of foraging with the threat of predation. Corticotropin-releasing factor (CRF) and Urotensin I (UI) are two key players in regulating these responses. They influence the salmon’s feeding behaviour and gastric function, helping them prioritize immediate survival needs over long-term processes like reproduction.
Food intake and appetite are highly conserved evolutionary responses to stress, marking physiological and behavioural tradeoffs between foraging and predation risk. Food intake is regulated by CRF and UI in Rainbow Trout (Oncorhynchus mykiss), acting through negative feedback to impact gastric function and appetite signals directly, integrated by CNS peripheral CRF signals in the forebrain peptide neurons (Craig et al. 2005). In the case of a sustained decrease in feeding induced by hypoxia, chasing, or isolation, UI and pre-optic area CRF mRNA increases may continue for up to six hours after exposure (Bernier et al. 2005).
Anorexic behaviour, defined by a significant decrease in food intake, has been documented across vertebrates as a conserved response deviating from an organism’s homeostasis to prioritize current survival over long-term system processes such as reproduction, foraging, digestion, growth, and differentiation (Siegfried 2003). For migrating Salmonidae, the evolutionary trade-off between appetite and reproduction is a primary driving force of their life history traits, including their semelparous annual breeding cycle and consequent senescence onset once and then die. In the case of salmon, this is the result of a dramatic increase in glucocorticoids from stress released during migration that results in death by malnutrition (Mills 2009). However, this affects survival rates per spawning mass yearly, especially if the stressors are maintained long-term within the environment (Bernier & Craig 2005).
Such is the case with anthropogenic disturbances. Hvas et al. (2021) found Atlantic salmon post-smolt (transfer to seawater) responded to exercise stressors with significantly higher plasma cortisol levels deviating from the homeostatic norm but a starvation stressor of 4-week fasting treatment had decreased plasma cortisol. Other studies such as Conde-Sieira et al. (2018) support that there is heavy integration of anorexigenic and orexigenic properties which affect physiological changes necessary for adapting to heightened stress during migration within their life cycle. Hvas et al. (2021) argue that osmoregulatory acute stress is a more significant concern for population adaptation and survival than other environmental stressors that may be caused by increased climate change on community composition and anthropogenic influence, but many other studies highlight other environmental stressors that negatively impact species normal HPI system response, life history traits, and fecundity (Lai et al. 2021; Wade et al. 2019; Mills 2009; Paul & Craig 2005)
During their migratory journeys, salmon face intense stressors that can impact their ability to feed and reproduce. Elevated levels of stress hormones, such as glucocorticoids, can lead to malnutrition and even death among migrating salmon populations. These challenges are exacerbated by human-induced disturbances in their habitats, further jeopardizing their survival. Recent research has highlighted the intricate balance between appetite regulation and stress responses in salmonids, shedding light on how they adapt to challenging environmental conditions during their life cycle.
Social Hierarchy
Social interactions and dominance hierarchies play a crucial role in salmonid populations, influencing their stress responses and overall survival. Corticotropin-releasing factor (CRF) levels in the brain mediate behaviours like aggression, which help establish social hierarchies among salmon. However, the effects of CRF can vary depending on its concentration. Low levels of CRF may prompt subordinate individuals to challenge higher-ranking ones, while high levels can lead to increased cortisol levels, resulting in anxiety-like behaviour and reduced social status.
The Stress Response System evolved physiological and behavioural responses to acute stress, namely the Fight or Flight response and general adaptations. Within the fight or flight response, behavioural aggression has evolved to retort acute stress and threats to an individual’s survival (Maximino et al. 2010). Several studies since 2000 have evaluated the significance of sociality on salmonid survival (Gilmour et al. 2005). Backstrom et al. (2011) demonstrate the significance of dose-dependent effects of CRF on behaviour and consequently, how CRF levels mediate life history traits such as reproduction and sociality. Social dominance in juvenile O. mykiss is determined by aggression, where attacking an individual establishes a dominance-subordinate hierarchy and is a product of pervasive fitness benefits during migratory mating season. Backstrom et al. (2011) found intracerebroventricular (ICV) injection of CRF at low levels induced a subordinate individual to increase aggression and challenge their position in the social hierarchy (via downstream increased Da and Ep), however, injection of CRF at high levels resulted in a significant increase in plasma cortisol levels. High cortisol levels produced anxiety-like behaviour in juvenile O. mykiss, treatment individuals were more likely to social isolate and decrease their social position. Cortisol at high, acute doses, therefore, has negative impacts on fitness and survival but is a necessary proponent of survival instincts (Backstrom et al. 2011).
Understanding the interplay between stress hormones and social dynamics is essential for comprehending how salmon populations cope with environmental challenges. By studying these interactions, researchers can gain valuable insights into the factors shaping salmon behaviour and survival strategies in the face of human-induced disturbances and environmental changes.
AI-generated illustration of salmon in water — via substack
Salmon Need Stress (we do too)
While significant progress has been made in understanding the stress responses of salmonid species, there is still much to learn about how they will adapt to rapidly changing environmental conditions. Ongoing research efforts are crucial for informing conservation initiatives and sustainable fisheries management practices. Because these species have specialized endocrine and reproductive systems that rely on a Goldilocks level of stress hormones, the threshold for failure is fairly low. Boat noise, pollution, and other environmental stressors threaten to alter this balancing act by unravelling the complex relationships between stress hormones, behaviour, and social interactions, scientists can develop strategies to protect salmon populations and preserve their vital role in aquatic ecosystems.
I particularly enjoy this topic for the insights it unveils about the human condition as well. Before entertaining this tangent I must note this disclaimer — model species and the evolution of the endocrine system provide insight into shared ancestral states but there is danger in haphazardly drawing 1:1 comparisons, especially when addressing behaviour. Pop science has taken biological comparisons across species too far from time to time — I’m looking at you alpha wolf bros. Nonetheless, not unlike salmon, we too need epinephrine to work in conjunction with our motivation and reproductive systems to get on with life — following the proverbial river upstream. Comparative physiology allows us to identify that an overload of stress causes similar patterns of behavioural shutdown in a vast array of species. As we saw in fish, we are not the only ones to exhibit social anxiety, isolation, and disturbed eating patterns as a result of acute stress changes. My hope is that this knowledge can allow us to more freely recognize the effects of stress in our daily lives — and better empathize with our aquatic friends. Marine life has notoriously been disregarded and placed at the bottom of our collective idea of the tree of life, historically treated as some of the least complex and least important organisms. My intention is to illustrate the intricacies of the Salmonidae family so that we can better advocate for and make decisions about their conservation.
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Literature Cited
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