Spatiotemporal trends in Schistocephalus solidus infection among populations of Alaskan threespine stickleback (Gasterosteus aculeatus).

Understanding the dynamics of host-parasite interactions is essential for uncovering the ecological and evolutionary processes shaping natural populations. In this study, we investigated the spatial and temporal patterns of prevalence of the cestode Schistocephalus solidus in 4 Alaskan populations of threespine stickleback (Gasterosteus aculeatus) over 11 years (1996 to 2006). Additionally, we assessed host tolerance through key indicators such as the Parasite Index and the presence of fibrotic scar tissue, a specialized form of resistance in this system. We observed fibrosis formation in both infected and uninfected fish across all populations, with notable year-to-year variability in Cornelius Lake. Here, fibrosis prevalence ranged from 9% in 1997 to 63% in 1998, coinciding with the highest cestode prevalence recorded in this population. Willow Lake stickleback displayed persistently high Parasite Indices, with parasite weights occasionally exceeding host eviscerated weights, and experienced a potential epizootic from 2000 to 2005. Cestode prevalence remained stable in Rocky and Cornelius Lakes and was consistently low in Lazy Lake. Our findings demonstrate significant variation in cestode prevalence, parasite burden, and resistance mechanisms among populations and between years. Importantly, the decoupling of fibrosis formation from infection status highlights the multifaceted nature of host-parasite interactions. Our study emphasizes the value of long-term datasets in revealing spatial and temporal patterns in host-parasite dynamics. Future research integrating ecological and genetic frameworks will be critical for elucidating the drivers of resistance and tolerance and their evolutionary implications.