Oecologia (2003) 136:155–160 DOI 10.1007/s00442-003-1236-1 BEHAVIOURAL ECOLOGY Christa M. Woodley · Mark S. Peterson Measuring responses to simulated predation threat using behavioral and physiological metrics: the role of aquatic vegetation Received: 28 August 2002 / Accepted: 28 February 2003 / Published online: 9 April 2003  Springer-Verlag 2003 Abstract An organism’s daily activities are affected by predation and predation risk that have behavioral and physiological costs, which translate into long-term pop- ulation and community consequences. We tested the hypothesis that the perception of predation risk from sand seatrout, Cynoscion arenarius, affects the behavior, and immediate and intermediate physiological responses of longnose killifish, Fundulus majalis. We further hypoth- esized that prey responses change if prey are buffered by artificial submerged aquatic vegetation (SAV), a potential refuge from predators. Experiments were conducted to quantitatively estimate the behavior, plasma cortisol (PC) concentration, mass-specific oxygen consumption, and short-term growth rate changes relative to full, partial, and no visual exposure to the predator. The partial visual exposure treatment involved the use of artificial SAV. Our results indicate that there are significant behavior and physiological responses of longnose killifish to predation threat. Longnose killifish in the full visual and partial exposure treatments displayed different behaviors than the control treatments by shifting towards the rear of the aquaria. In addition, longnose killifish in the full visual exposure compared to the partial exposure and the control treatments responded by exhibiting an elevation of PC and mass-specific oxygen consumption rate, and through decreased short-term growth. These responses were less intense in the partial exposure, when artificial SAV was present. The significance of this study is that it examines a suite of responses from cellular to the whole-organism level as they are affected by predation threat and modified by the presence or absence of artificial SAV. Keywords Predator-induced stress · Plasma cortisol · Respiration · Growth · Tradeoffs Introduction Predation or predation threat affects the behavior of prey by forcing them to assess the cost of predator avoidance during activities such as foraging, habitat use, mate choice, and metamorphosis (Jordan et al. 1996; Lima 1998; McPeek et al. 2001). Non-lethal effects of predation on prey can be more prominent than the direct effects (i.e., mortality) through the diversion of energy from growth, reproduction and overall fitness (He and Kitchell 1990; Houston et al. 1993; Von Buskirk and Arioli 2002). These non-lethal effects can influence the lifetime fitness of prey exposed to predation threat, thereby affecting population dynamics and community structure (Werner and Anholt 1993). In fact, recent studies indicate that predator-prey interactions are an important causative agent in the evolution of life history traits and population dynamics (Boonstra et al. 1998; Peckarsky et al. 2002; Relyea 2002). A number of studies have quantified the influence of predator-derived stress on aquatic metazoans using met- rics correlated to lifetime fitness such as delay in age-at- maturity (Belk 1998), reduction in growth (Anholt et al. 2000), and fecundity (Fraser and Gilliam 1992). We consider stress to be a diversion of metabolic energy from an organism’s normal metabolic budget (Barton and Schreck 1987) that can result from natural and anthro- pogenic factors. For example, McCormick (1998) found that competitors and predators elevated the maternal cortisol levels in a damselfish, Pomacentrus amboinensis. The stress-related response resulted in the reduced size of the nutritive yolk sac, and thus the size of the larvae at hatching. C. M. Woodley ( ) ) Department of Wildlife, Fish and Conservation Biology, University of California, One Shields Avenue, Davis, CA, 95616–8571, USA e-mail: cmwoodley@ucdavis.edu Tel.: +1-530-7528659 Fax: +1-530-7524154 M. S. Peterson Department of Coastal Sciences, University of Southern Mississippi, 703 East Beach Drive, Ocean Springs, MS, 39564, USA