Anadromous blueback herring (Alosa aestivalis) interact with numerous species during their spawning migrations between marine and freshwater systems. As such, the drastic declines in blueback herring populations and altered population dynamics over the last centuries may be affecting not only the stability of blueback herring, but also the species they interact with, including freshwater mussels. Effectively evaluating the impacts of blueback herring population changes on freshwater mussels requires unbiased demographic assessments of blueback herring data, such as age and number of spawning events, and a robust understanding of blueback herring as hosts for mussels. To assess the quality of spawning mark data, I examined the precision and bias between paired spawning estimates of 8,698 blueback herring collected by the U.S. Fish and Wildlife Service (USFWS) over the last decade. Systematic bias was found in 70% of years, and 30% showed coefficient of variation (CV) values >10% indicating imprecision. Bias was absent and precision was highest in most recent years where new training requirements and reference collections were used. These findings support future spawning estimation improvements through standardized precision thresholds and uniform training. To explore freshwater mussel use of blueback herring as hosts within the Connecticut River, the frontmost gills of 1,011 fish from the 2023 USFWS survey were examined for mussel glochidia (larvae). I observed 18,738 glochidia from four genera, including a species not known to inhabit the watershed (Utterbackia imbecillis), and a 64% infection prevalence (i.e. # of infected fish / # of total fish examined). Water temperature significantly affected the prevalence on blueback herring, with prevalence peaking at 14.2 ºC for alewife floater (Utterbackiana implicata) and 18.8 ºC for eastern elliptio (Elliptio complanata). For U. implicata, higher infection prevalence was related to higher densities of blueback herring, highlighting the linkage between these two species. Maximum infection intensities were predicted for smaller fish that were female and virgin spawners, which may be primarily explained by temporal patterns in migrating blueback herring and correspondence to glochidia release by mussels. These results highlight the value of understanding and monitoring organisms with unique, coupled life cycles toward developing effective, comprehensive conservation strategies.