Reproductive pattern and offspring performance in brown trout (Salmo trutta)

Abstract: This thesis deals with how reproductive traits and reproductive behaviour in femalebrown trout (Salmo trutta) are linked to offspring performance.In the small stream Tullviksbäcken, the choice of spawning sites was stronglyinfluenced by short-term fluctuations in stream flow and by substrate quality. Coarsegravel gives high egg and alevin survival and was preferred, but substrate selectivityvaried among years. Stream accessibility in the autumn, as determined by fluctuations instream flow and sea level, affected both the number of spawners and the timing ofspawning.In Tullviksbäcken, the distribution of stocked trout two months after emergence wasbest explained by a mathematical model that considered fry to consist of one componentwith resident behaviour and one component with dispersive behaviour. This is opposedto the general view that fry dispersal behaviour is uniform within populations. Dispersionfrom the redds was spatially limited and this may be an adaptation to poor survival farfrom the redds. Initially small sibling trout appeared to be more dispersive than wereinitially large siblings. Egg size had no general effect on fry performance in localaggregations, and trout survival was not affected by differences in egg size. Thus, theprevailing 'bigger-is-better' paradigm concerning larval fish performance wascontradicted. However, initially large trout appeared to be favoured in stream sectionswith high biomass of underyearling trout and good growth opportunities, and it issuggested that selection for initial size may occur locally.Resident trout in the stream Jörlandaån had relatively larger eggs than both sympatric migratory trout and trout from five other Swedish streams. The fitness in residentJörlandaån females is probably enhanced by the production of extremely large offspring, possibly through increased fry survival relative the more numerous fry descending from larger migratory parents.In conclusion, redd-site selection is important for egg and alevin survival but also forjuvenile performance, because fry will not disperse far from their original redd andselection pressures may vary locally. Female egg production strategies may also influenceoffspring survival. Thus, the presented results demonstrate a strong coupling betweenfemale reproductive pattern and offspring performance in brown trout.