Summary
Male largemouth bass (Micropterus salmoides) provide sole parental care over a 4–6‐wk period to a single brood, fanning the eggs to keep them oxygenated and free of silt and defending the brood until the offspring develop antipredator tactics. During this period, fish are highly active and have few opportunities for feeding, so this activity is energetically costly. To understand some of the consequences of stress during this challenging period, we injected fish with cortisol suspended in coconut oil to experimentally raise circulating cortisol in parental males for the first week of the parental care period. We compared parental care behavior between cortisol‐treated, sham‐treated (injected only with coconut oil), and control parental males. We further compared physiological parameters associated with metabolism and reproductive function between cortisol‐treated and control males. The cortisol injections resulted in supraphysiological levels of circulating plasma cortisol, giving us insight into potential maximal effects of stress during parental care. At these supraphysiological levels, the cortisol‐treated fish displayed higher concentrations of circulating glucose and cholesterol and lower concentrations of circulating triglycerides when compared with control fish, with no change in plasma concentrations of total protein. Plasma concentrations of androgen were similarly unaffected by cortisol treatment. In the short term (initial 1–2 wk), parental care of eggs and egg‐sac fry was maintained by all groups, with no differences observed in behavior (e.g., tending, vigilance, defense) among the groups. However, the cortisol‐treated fish abandoned their offspring at a higher rate than in the control or sham groups. The fish treated with cortisol also tended to develop external Saprolegnian infections, indicative of compromised immune function. These data demonstrate that exogenous cortisol elevation during parental care results in changes in energy use and a decrease in immune function. Interestingly, the data also suggest resistance to stress during parental care in largemouth bass, with no changes in parental care behavior before abandonment.
Methodology
All fish were sampled under an Ontario Ministry of Natural Resources Scientific Collection Permit (to S.J.C.) and handled in accordance with the guidelines of the Canadian Council on Animal Care as administered by Carleton University (B07-02) and Queen’s University. From May 10 to 28, 2007, 52 largemouth bass guarding nests with fresh eggs (0–1 d old) were identified by snorkeling in Long Lake, a private research lake managed by the Queen’s University Biological Station (QUBS) in eastern Ontario (44°31'N, 76°20'W). No angling or other disturbance was permitted during the study period, so there was no possibility that fish were captured or harvested during this period. Mean daily water temperatures (∼1 m below the surface) increased from 15° to 21°C across parental care. The nests were individually marked with a numbered tile, and the fish were randomly assigned to one of four treatment groups: (1) control (n = 15), (2) sham treatment (n = 8), (3) low-dose cortisol treatment (n = 14), and (4) high-dose cortisol treatment (n = 15). The control animals were neither captured nor handled in any way. Treatment fish were captured by rodand-reel angling, landed with a rubber-mesh net within 10 s, and placed in a foam-lined trough filled with fresh lake water. Fish were then uniquely identified by intraperitoneal implantation of a passive integrated transponder (PIT) tag (2 mm × 9 mm). The fish were briefly air-exposed (∼10 s) while mass was determined using a portable electronic balance and returned to the water-filled trough for administration of an intraperitoneal injection of coconut oil (Cocos nucifera; Sigma C1758, Sigma-Aldrich, St. Louis, MO) containing cortisol (hydrocortisone; Sigma H4001, Sigma-Aldrich). This method is an established means of raising plasma cortisol in fish and creates a slow-release cortisol implant that will elevate plasma cortisol for approximately 5–6 d (see Gamperl et al. 1994). All fish were injected with 0.005 mL of coconut oil per gram of fish body weight. Sham treatment fish (total length = 358 ±12 mm [mean ± SE]) were injected with pure coconut oil. Low-dose cortisol treatment fish (total length = 365 ± 7 ) were injected with 10 mg mL-1 mm cortisol in coconut oil, while high-dose cortisol treatment fish (total length = 377 ± 10 mm) were injected with 40 mg mL-1 cortisol in coconut oil. These doses were chosen on the basis of literature values used to induce exogenous stress responses in teleost fish (see Gamperl et al. 1994). During treatment, a snorkeler was deployed to protect the eggs from nest predators and remained with the nest until the parental male returned following release. All techniques were performed without anesthesia (see Cooke et al. 2005 for rationale), and total treatment time was typically <90 s.