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.

To define suitable holding conditions for fish used in experimental studies on food consumption and growth, the effect of cover on growth at two ration levels was quantified for immature individuals of two common, freshwater species: the threespine stickleback (Gasterosteus aculeatus: Gasterosteidae) and the European minnow (Phoxinus phoxinus: Cyprinidae). In the European minnow, the presence of cover significantly increased growth rates and also the rate of food consumption at ad libitum rations; however, cover had no significant effect on the growth of the threespine stickleback. In both species, fish fed ad libitum rations had higher growth rates than fish fed 4% of body weight per day. This study illustrates the need for species specific holding conditions and illustrates an approach to welfare using measurements of traits closely related to fitness.

The cestode Schistocephalus solidus is a common parasite in freshwater threespine stickleback populations, imposing strong fitness costs on their hosts. Given this, it is surprising how little is known about the timing and development of infections in natural stickleback populations. Previous work showed that young-of-year stickleback can get infected shortly after hatching. We extended this observation by comparing infection prevalence of young-of-year stickleback from 3 Alaskan populations (Walby, Cornelius and Wolf lakes) over 2 successive cohorts (2018/19 and 2019/20). We observed strong variation between sampling years (2018 vs 2019 vs 2020), stickleback age groups (young-of-year vs 1-year-old) and sampling populations.

We performed a long-term natural experiment investigating the impact of the diphyllobotriidean cestode Schistocephalus solidus on the body condition and clutch size (CS) of threespine stickleback Gasterosteus aculeatus, its second intermediate host, and the growth of larval parasites in host fish. We tested the hypothesis that single S. solidus infections were more virulent than multiple infections. We also asked whether the metrics of mean and total parasite mass (proxies for individual and total volume, respectively) were consistent with predictions of the resource constraints or the life history strategy (LHS) hypothesis for the growth of, hence exploitation by, larval helminths in intermediate hosts. The samples were drawn from Walby Lake, Alaska in eight of 11 years. Host body condition and CS (egg number per spawning bout) decreased significantly with intensity after adjustments for host size and parasite index. Thus, infections have an increasingly negative impact on measures of host fitness with greater intensity, in contrast to the hypothesis that single infections are more harmful than multiple infections. We also found that mean parasite mass decreased with intensity while total parasite mass increased with intensity as predicted by the LHS hypothesis.

An analysis of the metrics of Schistocephalus solidus infection of the threespine stickleback, Gasterosteus aculeatus, in Walby Lake, Alaska, showed that an epizootic ended between 1996 and 1998 and another occurred between 1998 and 2003. The end of the first epizootic was associated with a crash in population size of the stickleback, which serves as the second intermediate host. The likely cause of the end of that epizootic is mass mortality of host fish over winter in 1996–1997. The deleterious impact of the parasite on host reproduction and increased host predation associated with parasitic manipulation of host behaviour and morphology to facilitate transmission might also have played a role, along with unknown environmental factors acting on heavily infected fish or fish in poor condition. The second epizootic was linked to relatively high levels of prevalence and mean intensity of infection, but parasite:host mass ratios were quite low at the peak and there were no apparent mass deaths of the host. A number of abiotic and biotic factors are likely to interact to contribute to the occurrence of epizootics in S. solidus, which appear to be unstable and variable. Epizootics appear to depend on particular and, at times, rare sets of circumstances.

The objective of this study was to investigate the means by which Schistocephalus solidus might reduce annual fecundity in female threespine stickleback fish (Gasterosteus aculeatus) through processes of oocyte development. Histological examinations of specimens from one lake in Alaska in 2000 and 2001 were used to analyse the effects of S. solidus on recruitment of primary growth oocytes into vitellogenesis, atresia of vitellogenic oocytes, and the interspawning interval. The ratio of primary growth to late secondary growth (late vitellogenic) oocytes was significantly greater (P<0·01) among infected fish than uninfected ones in early-season samples from 2000 and 2001, revealing a decrease in recruitment of oocytes from primary growth into vitellogenic oocytes among infected females. The difference was marginally non-significant (P=0·087) in a mid-season sample from 2001 due to reductions in the entire pool of vitellogenic (early and late secondary growth) oocytes recruited prior to the spawning season in this determinate-fecundity species. Atresia among all vitellogenic oocytes was low and did not differ between infected and uninfected females. Histological estimations of the interspawning interval using post-ovulatory follicles showed no significant differences between infected and uninfected fish, suggesting that the number of spawnings in stickleback females each spawning season is unaffected by S. solidus infection. Thus, annual fecundity appears to be reduced only through recruitment of oocytes into vitellogenesis.