Summary
Stress can have sublethal effects that are manifested either immediately or at spatial or temporal scales that are removed from the stress event (i.e., carryover effects). We tested whether a short-term elevation of plasma cortisol would result in seasonal carryover effects in wild largemouth bass Micropterus salmoides. Using exogenous hormone implants, we raised circulating cortisol concentrations in a group of wild fish for approximately 5 d in October 2007. We then compared activity (velocity, distance traveled) of cortisol-treated animals with that of sham-treated and control animals throughout the winter using an automated acoustic telemetry array. Immediately following treatment, the cortisol-treated fish showed increased activity relative to controls. However, this difference disappeared following the cessation of the elevation of circulating cortisol. During the winter of 2007 to 2008, the lake experienced a nearly complete winterkill event, providing insight into how a transient stress response can influence the response of wild animals to subsequent challenges. Most fish carrying acoustic transmitters succumbed during this winterkill event, but cortisol-treated fish died earlier than fish in other groups and showed a decrease in activity relative to controls and sham-treated fish before mortality. This study provides preliminary evidence of seasonal carryover effects in wild fish and yields insight into the ecological consequences of stress across broad temporal scales.
Methodology
Study Site
Warner Lake is a small (8.2-ha) private research lake in eastern Ontario (44°31' N, 76°22' W). In 2003, Warner Lake was equipped with a whole-lake acoustic telemetry array that allows near-real-time positioning of fish in the wild (fully described in Cooke et al. 2005). Briefly, 13 hydrophones were positioned strategically throughout the lake so that the locations of fish equipped with transmitters could be tracked with submeter precision in near real time. This system is unique in the world in terms of coverage of an entire lake to assess the behavior and fate of wild freshwater fish. Warner Lake is a closed system for fish, with no potential for immigration or emigration, and the lake is closed to public fishing.
Study Animals and Treatments
From October 10 to 13, 2007, largemouth bass were captured by rod-and-reel angling from Warner Lake and transported to shore in coolers of lake water. On shore, fish were weighed (to the nearest g), and fish greater than 450 g were deemed suitable to carry the intraperitoneal acoustic transmitters (Model MD11-18, mass p 8.4 g in the air; Lotek Wireless, Newmarket, Ontario) according to the accepted 2% rule of biotelemetry (where the transmitter must not be greater than 2% of the animal’s total mass; Winter 1983). Suitable fish (n = 25) were assigned to one of three treatment groups: control (n = 8; 638.5 ± 125.6 g), sham treatment (n = 8; 637.6 ± 72.3 g), and cortisol treatment (n = 9; 693.6 ± 113.0 g). All groups contained fish with the same size distributions. All fish were anesthetized in an induction bath containing 50 MG L-1 clove oil (10% clove oil emulsified in ethanol; Prince and Powell 2000) in fresh lake water. After loss of equilibrium, fish were placed on a surgery table that allowed the gills to be irrigated with an aerated solution of fresh lake water containing 10 mg L-1 clove oil. The fish were implanted with transmitters following procedures described in Cooke et al. (2003). Cortisoltreated fish were injected intraperitoneally with 10 mg mL-1 of cortisol (hydrocortisone; Sigma H4001, Sigma-Aldrich, St. Louis, MO) emulsified in coconut oil (Cocos nucifera; Sigma C1758, Sigma-Aldrich, St. Louis, MO), at 0.005 mL g-1 body weight. This is an established method of elevating circulating cortisol in fish in a controlled manner (Gamperl et al. 1994). A laboratory pilot study confirmed that in this species, this dose raises plasma cortisol for approximately 5 d to ∼2,000 ng mL-1 (O’Connor et al. 2009), a supraphysiological level that is higher than that elicited by exposure to a typical stressor such as exhaustive exercise (O’Connor et al. 2009). Sham-treatment fish were given intraperitoneal injections of pure coconut oil at 0.005 mL g-1 body weight. Control fish were not injected. All fish were immediately released from the same location on regaining equilibrium.