Sculpin Predation on Lake Trout Eggs in Interstices: Skull Compression as a Novel Foraging Mechanism

Lake Trout (Salvelinus namaycush) spawn in fall and hatch in spring; to protect their eggs over the winter they spawn on rocky reefs, where the eggs settle deeply into interstices. Uptake of water swells the eggs, further protecting them by lodging in crevices. Freshwater sculpin (Cottidae), the primary predator of salmonid eggs, are small, benthic fishes with malleable bodies and large heads. To acquire the lipid-rich resource contained in eggs, sculpin must penetrate deeply into interstices, and have a method to extract lodged eggs. We placed Slimy Sculpin (Cottus cognatus) into a tunnel containing a sequence of funnels with progressively smaller openings to determine the smallest hole they could enter. Forty-three of 138 Slimy Sculpin (31%) moved through openings up to 19.7% smaller than the maximum width of their head, indicating that they compressed their skull to move through the opening. We examined egg acquisition behavior by placing Lake Trout eggs into holes drilled in a Plexiglass plate. Holes varied in diameter from 5.6 to 8.7 mm, and were either shallow (5 mm) or deep (9 mm); after water hardening, eggs were lodged in the smallest holes and loose in the larger holes. Sculpin consumed eggs in the large, shallow holes first, then used suction to extract eggs from small, shallow holes. To extract eggs from small, deep holes, they used a sequence of suction attempts to draw the eggs upward. Sculpin are highly adapted to forage on Lake Trout eggs, including the novel capacity to compress their skulls to access eggs.