Size mediates relative risk and anti-predation response in juvenile sturgeon

ABSTRACT Objective The strength of anti-predation behavior should be proportional to predation risk. Predation risk can vary with predator density, prey size, and other ecological attributes. Threat-sensitive anti-predation behavior has been documented in a variety of organisms. Here, we describe predation risk and anti-predation behavior in juvenile White Sturgeon Acipenser transmontanus across body sizes. We hypothesized that sturgeon would show threat-sensitive anti-predation behavior by reducing predator avoidance when at a lower risk of predation. Methods We began by quantifying size-dependent predation risk within 24-h trials in which juvenile White Sturgeon (9.5–25.9 cm total length [TL]) were exposed to Largemouth Bass Micropterus nigricans predators (mean TL = 37.9 cm, SD = 4.6). We then documented the strength of an anti-predation behavior (reduction in activity) by exposing the juveniles to a predator when the sturgeon were at high predation risk (mean TL = 7.5 cm, SD = 1.1) and low predation risk (mean TL = 23.6 cm, SD = 1.3). Results Predation risk trials were conducted over a 3-month period starting when sturgeon were 61 days posthatch. As sturgeon increased in size, predation rates decreased sharply from a maximum of 70%, when sturgeon were 12.0 cm TL (SD = 0.6), to less than 5%, when sturgeon were 23.5 cm TL (SD = 1.3). When anti-predation behaviors were tested, sturgeon in the smaller size-class (higher risk) showed a stronger anti-predation response (activity reduced to 3.5–18.2% of baseline) than sturgeon in the larger size-class (lower risk; activity reduced to 30.6–67.2% of baseline), thus supporting our hypothesis. Conclusions White Sturgeon experienced an ontogenetic shift in predation risk and displayed a corresponding change in anti-predation behavior, supporting our hypothesis. Further research on the distribution, movement, and behavior of juvenile life stages could be valuable to identify whether this shift is correlated with migration or habitat shifts.