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Using environmental cues and individual traits to predict sea trout (Salmo trutta) smolt migration behaviour and phenology

Forsberg, Herman (2025) BION03 20241
Degree Projects in Biology
Abstract
The decision of when to exchange habitats represents a significant moment for organisms that utilize migration as a strategy to access growth and feeding opportunities surpassing those available in their natal habitat. For juvenile anadromous salmonids, the initial transition from freshwater to the marine environment marks a critical point in the life cycle characterized by high mortality with migration timing being a major determinant of subsequent survival and success. In general, migration is expected to occur during conditions that enhances the individual likelihood of survival. Whereas the population timing of migration is mostly attributed to environmental conditions, individual timing is generally attributed to condition-dependent... (More)
The decision of when to exchange habitats represents a significant moment for organisms that utilize migration as a strategy to access growth and feeding opportunities surpassing those available in their natal habitat. For juvenile anadromous salmonids, the initial transition from freshwater to the marine environment marks a critical point in the life cycle characterized by high mortality with migration timing being a major determinant of subsequent survival and success. In general, migration is expected to occur during conditions that enhances the individual likelihood of survival. Whereas the population timing of migration is mostly attributed to environmental conditions, individual timing is generally attributed to condition-dependent differences in risk and reward. Consequently, studies on juvenile salmonid migration often focus solely on population or individual where potential interactions between environmental conditions and individual traits are overlooked. This case study investigated the migration behavior and timing of juvenile anadromous brown trout (sea trout; Salmo trutta) during their seaward migration in a low-land river in southern Sweden through a combination of population- and individual-level data. Manually and automatically collected data on the number of downstream migrating smolts in spring were used to construct a phenological model to identify key environmental cues affecting population migration patterns during 2021-2024. Using passive telemetry from juvenile trout parr tagged in autumn 2023, individual variation in migration propensity and timing was further explored as a subsample of the 2024 migration season. The results from the phenological model indicated that the combined effects of accumulated photo-thermal units and discharge peaks in spring could be used as adequate proxies to capture the distribution of migrating smolts within a season. The synergistic relationship between temperature and photoperiod had a strong effect across the entire migration period. In contrast, sharp increases in river flow had a promoting but not necessary effect on smolt migration, triggering small groups of smolts to migrate early in the season. Using individual data, I further demonstrate that individuals that were larger at the time of tagging migrated earlier and at lower photo-thermal accumulation than smaller individuals. Moreover, larger individuals were more likely to migrate during an early discharge peak than smaller ones who deferred migration to a later instance suggesting that environmental influences on migration timing are condition-dependent at the time it occurs. Lastly, I found that the probability of spring out-migration varied with body-size and increased with migration distance. The effect of body-size on migration probability reached a maximum at ~100mm autumn length above which migration occurred in similar proportions. Interestingly, the highest observed proportion of migrants came from the upper parts of the river, contradicting the common notion that migration distance is negatively correlated with smolt production. Overall, this study demonstrates that apparent migration patterns on a population level is to some extent underpinned by seasonal gradients in individual traits and that early life conditions are important predictors of out-migration probability. (Less)
Popular Abstract
Havsöringens smoltutvandring: Miljösignaler och individens roll

Överlevnad och framgång hos vandrande arter, som havsöringen (Salmo trutta), beror ofta på förmågan att välja rätt tid för att migrera. För unga havsöringar innebär övergången från sötvattensmiljö till havet en avgörande livsfas och tidpunkten för migration behöver synkroniseras med gynnsamma förhållanden för att tas sig från ån och ut i havet. Miljösignaler som temperatur och vattenflöde samt individuella egenskaper är avgörande i detta avseende. I min studie undersökte jag hur dessa faktorer samspelar för att forma utvandringsmönster hos havsöringssmolt i Nybroån, i södra Sverige.

Miljösignaler och populationsmönster

Med hjälp av automatiskt insamlade data från en... (More)
Havsöringens smoltutvandring: Miljösignaler och individens roll

Överlevnad och framgång hos vandrande arter, som havsöringen (Salmo trutta), beror ofta på förmågan att välja rätt tid för att migrera. För unga havsöringar innebär övergången från sötvattensmiljö till havet en avgörande livsfas och tidpunkten för migration behöver synkroniseras med gynnsamma förhållanden för att tas sig från ån och ut i havet. Miljösignaler som temperatur och vattenflöde samt individuella egenskaper är avgörande i detta avseende. I min studie undersökte jag hur dessa faktorer samspelar för att forma utvandringsmönster hos havsöringssmolt i Nybroån, i södra Sverige.

Miljösignaler och populationsmönster

Med hjälp av automatiskt insamlade data från en fiskräknare och manuellt räknade smolt från en fälla utvecklade jag en modell för att identifiera nyckelfaktorer som styr populationsmässiga migrationsmönster under 2021–2024. Resultaten visade att kombinationen av ackumulerad fototermisk energi (ökad vattentemperatur och dagslängd) samt ökat vattenflöde (flödestoppar) under våren spelar en central roll. För populationen stimulerade dessa faktorer inte bara migrationens start utan också periodens längd där den tidiga migrationen utlöstes av flödestoppar medan den huvudsakliga migrationen påverkades av ökad temperatur och dagslängd. Temperatur hade således en reglerande effekt på smoltutvandringen medan flödestoppar hade en främjande effekt där grupper av smolt nyttjade ökande flöden när tillräckligt med temperatur ackumulerats.

Individuella variationer

För att förstå olika individers beteende under migrationsperioden använde jag data från individer som fångades och märktes på sina uppväxtområden under hösten 2023. Genom att placera ut detektionsutrustning på olika platser i ån kunde de märkta individernas vandringsbeteende övervakas under smoltutvandringen 2024. Analysen visade att sannolikheten att vandra till havet varierade med individens storlek föregående höst och att den ökade med avståndet som en individ behövde vandra för att nå havet. Större individer tenderade också att migrera tidigare än mindre individer och var mer benägna att nyttja en tidig flödestopp för utvandring medan mindre smolt avvaktade till senare delar av säsongen när vattnet blivit varmare och dagarna längre.

Praktiska konsekvenser

Studien visar att faktorer som styr populationsmönster delvis är beroende av variationer i individuella egenskaper och att olika individer behöver miljösignaler av olika intensitet för att avgöra när det är optimalt att byta miljö. Dessa insikter är viktiga för att förstå hur klimatförändringar, som ändrade temperatur- och flödesmönster, kan påverka havsöringens migration och därmed återvandring till sina ursprungsområden. Genom att kombinera data på populations- och individnivå belyser denna studie också vikten av att förstå samspelet mellan miljö och individ om effektiva bevarandeåtgärder ska utformas och genomföras.


Masterexamensprojekt i akvatisk ekologi 60 hp 2025
Biologiska institutionen, Lunds universitet

Handledare: Anders Persson & Anders Nilsson
Avdelningen för Funktionell ekologi, Biologiska institutionen, Lunds universitet (Less)
Please use this url to cite or link to this publication:
author
Forsberg, Herman
supervisor
organization
course
BION03 20241
year
type
H2 - Master's Degree (Two Years)
subject
language
English
id
9185004
date added to LUP
2025-02-12 15:53:43
date last changed
2025-02-12 15:53:43
@misc{9185004,
  abstract     = {{The decision of when to exchange habitats represents a significant moment for organisms that utilize migration as a strategy to access growth and feeding opportunities surpassing those available in their natal habitat. For juvenile anadromous salmonids, the initial transition from freshwater to the marine environment marks a critical point in the life cycle characterized by high mortality with migration timing being a major determinant of subsequent survival and success. In general, migration is expected to occur during conditions that enhances the individual likelihood of survival. Whereas the population timing of migration is mostly attributed to environmental conditions, individual timing is generally attributed to condition-dependent differences in risk and reward. Consequently, studies on juvenile salmonid migration often focus solely on population or individual where potential interactions between environmental conditions and individual traits are overlooked. This case study investigated the migration behavior and timing of juvenile anadromous brown trout (sea trout; Salmo trutta) during their seaward migration in a low-land river in southern Sweden through a combination of population- and individual-level data. Manually and automatically collected data on the number of downstream migrating smolts in spring were used to construct a phenological model to identify key environmental cues affecting population migration patterns during 2021-2024. Using passive telemetry from juvenile trout parr tagged in autumn 2023, individual variation in migration propensity and timing was further explored as a subsample of the 2024 migration season. The results from the phenological model indicated that the combined effects of accumulated photo-thermal units and discharge peaks in spring could be used as adequate proxies to capture the distribution of migrating smolts within a season. The synergistic relationship between temperature and photoperiod had a strong effect across the entire migration period. In contrast, sharp increases in river flow had a promoting but not necessary effect on smolt migration, triggering small groups of smolts to migrate early in the season. Using individual data, I further demonstrate that individuals that were larger at the time of tagging migrated earlier and at lower photo-thermal accumulation than smaller individuals. Moreover, larger individuals were more likely to migrate during an early discharge peak than smaller ones who deferred migration to a later instance suggesting that environmental influences on migration timing are condition-dependent at the time it occurs. Lastly, I found that the probability of spring out-migration varied with body-size and increased with migration distance. The effect of body-size on migration probability reached a maximum at ~100mm autumn length above which migration occurred in similar proportions. Interestingly, the highest observed proportion of migrants came from the upper parts of the river, contradicting the common notion that migration distance is negatively correlated with smolt production. Overall, this study demonstrates that apparent migration patterns on a population level is to some extent underpinned by seasonal gradients in individual traits and that early life conditions are important predictors of out-migration probability.}},
  author       = {{Forsberg, Herman}},
  language     = {{eng}},
  note         = {{Student Paper}},
  title        = {{Using environmental cues and individual traits to predict sea trout (Salmo trutta) smolt migration behaviour and phenology}},
  year         = {{2025}},
}