FISH NESTS IN MIXED-SEDIMENT STREAMS, SOUTHEAST PENNSYLVANIA: INSIGHTS FROM TEXTURAL ANALYSIS OF POST-FLOOD BOTTOM SEDIMENT

Nesting structures of freshwater fish along shallow (< 1m) stream bottoms represent a relatively understudied aspect of fluvial zoogeomorphology. In forested mixed-sediment creeks of southeastern Pennsylvania, numerous nests, many produced by bluegill sunfish (Lepomis macrochirus), were photographed and measured during late spring and summer months. These circular structures (diameter: 15-60 cm) are composed of inward-coarsening sediment, with fragments of freshwater bivalves often incorporated into the center. In gravel-dominated reaches adjacent to point bars, such nests are less well expressed. In a 0.3 m reach of Pennypack Creek (Montgomery County), stream bottom sediments at three cross-channel sites with observed fish activity were sampled following a record flood (Ida; September 2021). At each site, two subsamples (A – suspected nest; B – adjacent patch) were collected to assess granulometric trends. Mean particle size, sorting, and skewness were determined for three size fractions (coarser than -2.7 φ (6.6 mm sieve), -2.7 – 0.0 φ (1 mm), and finer than 0.0 φ (Camsizer analysis). In the majority of intra-sample trends, the suspected nests (sites A) had a slight coarsening (2-3 mm for coarse gravel fraction) and better sorting (sand fraction range: 0.7-08 φ; fine gravel: 1.1-1.5 φ). Mostly positive skewness is consistent with typical fluvial sediments, with insufficient fine fraction winnowing likely due to relatively short timing since nest formation. It is likely that fin action and hydro-jetting allow the nesting fish to remove particles as coarse as fine gravel, increasing with body size. Therefore, more mature nests are expected to have increasingly negatively skewed fraction toward the center. Although seemingly ephemeral by nature, some nests may escape erosion if covered by a thick layer of fine sediment (e.g., legacy mud) or peat in abandoned stream sections. Our study provides the basis for identifying similar structures as an integral part of ancient fluvial and lacustrine ichnocoenoses, as well as paleo-water-level indicators.