Summary
In this study, we explored the growth, survival, and potential population-level effects of short-term experimentally induced stress in largemouth bass (Micropterus salmoides). Cortisol implants [50 mg·(kg body mass)–1] were used to increase circulating stress hormones in a group of wild fish in a research lake for ∼6 d in June 2007. Through mark-and-recapture, we compared survival, growth, and plasma biochemistry of cortisol-treated, sham-treated, and control fish at liberty until October 2007. Cortisol-treated fish displayed persistent growth rate depression compared with other groups. However, neither plasma biochemistry nor mortality rates differed among treatments. In a complementary study, we found that the standard metabolic rates (SMR) of cortisol-treated fish were higher than control fish ∼56 h following treatment. Bioenergetics modelling revealed that a transient elevation in SMR alone was insufficient to explain the observed growth depression. Finally, we constructed a simple population model to explore the potential consequences of growth depression. We found that a 10% reduction in population growth rate is conceivable when 39% of the population experiences a stress causing the growth rate depression documented in this study. Our study is novel in highlighting that individual and potentially population-level growth depression can result from a single stress event of short duration.
Methodology
To study the long-term (5 month) costs of a short-term (6 d) cortisol elevation, we captured 207 mature (>250 mm) largemouth bass by rod-and-reel angling from Warner Lake, a small (8.2 ha surface area) private research lake in eastern Ontario (44°31'N, 76°22'W). To avoid any confounding effects of reproduction, initial treatment occurred between 24 and 28 June 2007, after the cessation of all spawning and parental care activities in this lake. Warner Lake is closed to immigration and emigration for fish, and fishing (including recreational angling) is prohibited aside from research purposes. All captured fish were landed within 20 s, and placed in a foam-lined trough that exposed the ventral side while keeping the gills submerged in fresh lake water. To establish baseline plasma biochemical parameters indicative of feeding and fasting (Congleton and Wagner 2006), approximately 1.5 mL of blood was withdrawn by caudal puncture into lithium-heparinized 3 mL vacutainer-style syringes (B.D., Franklin Lakes, New Jersey) from a random subset of animals. All fish were then scanned for a passive integrated transponder (PIT) tag (12.5 mm X 2.0 mm) using a PIT tag reader (Biomark, Boise, Idaho), and given a unique intracoelomic PIT tag if necessary. Warner Lake is a research lake, and fish have been PIT-tagged since 1993 for routine population monitoring. Of the 207 fish captured, 55 fish already carried PIT tags. Fish were measured (total length, TL), and size-matched by TL into three treatment groups: cortisol, sham, or control (Table 1). Fish requiring new PIT tags were also distributed evenly among treatment groups. Cortisol-treated fish were injected with cortisol (hydrocortisone; Sigma H4001, Sigma-Aldrich Inc., St. Louis, Missouri) emulsified in coconut oil (Cocos nucifera; Sigma C1758, Sigma-Aldrich Inc.). Sham-treated fish were injected with pure coconut oil. Control fish were not injected. Cortisol was mixed with coconut oil at a constant ratio of 10 mg·mL-1, and cortisol-treated fish were injected with the cortisol--coconut oil mixture based on body size at a dose of 0.005 mL·g1, leading to a final dose of 0.05 mg·g-1 based on fish mass. Sham-treated fish were injected with pure coconut oil at the same dose of 0.005 mL·g1]. For all fish, body mass was estimated from a mass-TL relationship, (mass in g) = (1.359 x 10-4) x TL in mm2.996. This relationship (R2 = 0.949) was previously developed from 68 mature largemouth bass captured between 2003 and 2006 in Warner Lake during the summer post-spawning period (i.e., June and July). All fish were released following treatment. Sexing largemouth bass externally is unreliable out of the breeding season, and so fish were not sexed, and were likely a mixture of males and females.
To monitor the fate and growth of the three treatment groups, between 20 and 25 August (i.e., 54 to 63 d following initial treatment), 206 mature largemouth bass were captured by rod-and-reel angling from Warner Lake, sampled for blood, measured (TL), and scanned with a PIT tag reader as described above. The process was repeated between 10 and 15 October (i.e., 105 to 114 d following initial treatment), when 151 mature largemouth bass were captured, sampled for blood, measured (TL), and scanned with a PIT tag reader.
All blood samples were held in water-ice slurries for no more than 1 h, until being centrifuged at 10 000g for 5 min (Compact II Centrifuge, Clay Adams, New Jersey). Plasma samples were flash-frozen in liquid nitrogen and stored at -80°C until analysis.
Water temperatures were stable between 24 June and 25 August, ranging from 23°C to 25°C, with the highest temperatures occurring between 20 July and 10 August. After 25 August, water temperatures declined steadily through the fall, reaching 11°C by 15 October.