The size spectrum (SS) is an allometric relationship between body mass and population density that recognizes specimens by their individual size, not their taxonomic identity. In aquatic ecosystems, where many taxa experience ontogenetic shifts in feeding behavior, the ataxic SS approach may be more useful than traditional, taxic models that use a single, average body mass for each taxon. NEON fish samples (3-pass depletion) were used to test for SS relationships in a diverse selection of temperate streams. Negative mass vs. density relationships were detected for all streams, with linear model slopes ranging from -1.51 to -0.89 (mean slope = -1.22). Notably, the SS slopes were significantly flatter than the theoretically predicted slope (-1.75), indicating that densities of the largest individuals are much higher than expected and/or densities of the smallest individuals are lower than expected. Subsequent tests for secondary SS structure revealed distinct ‘domes’ within each of the fish assemblages. These domes are hypothesized to reflect intraguild predation (i.e., large predators consuming smaller intraguild prey). This work suggests two fundamental discoveries: (1) densities of the largest stream fishes are consistently and exceptionally high; and (2) intraguild predation may have a pervasive and largely underestimated influence on stream ecosystems.