Summary
Urbanization and agricultural practices can lead to alterations in stream habitat quality. However, there have been few attempts to understand if these alterations influence the health and condition of fish, or their ability to respond to multiple stressors. This study examines a sentinel fish in streams adjacent to different types of land-use practices and how they respond to an experimental stressor. Creek chub (Semotilus atromaculatus) from replicate agricultural, urban, and reference streams were subjected to an experimental manipulation of cortisol, the primary stress hormone in fish. A single intraperitoneal injection raised circulating plasma cortisol for ~3 days, mimicking the physiological effects of a prolonged stressor. We compared the survival, health, and condition of cortisol-treated, sham-treated, and control fish across the different land-use types. While marginally non-significant (P = 0.06), cortisol-treated fish displayed an ~50% increase in mortality in streams adjacent to agricultural areas. We did not observe differences in blood glucose, condition factor, splenic index, and gonadosomatic index, or parasite burden among the treatment groups or relative to land-use type. However, within the agricultural watersheds, the hepatosomatic index value of fish receiving a sham treatment was ~20% greater than fish in the control treatment, a significant result that appears to be spurious given that a similar effects was not observed in other land-use types. Overall, these results suggest that in the wild, there are apparently compensatory mechanisms that enable creek chub to persist despite being exposed to a significant challenge with little evidence that the outcome is modulated by variation in habitat quality, at least among the three types of sites (i.e., urban, agricultural, and reference) studied here. Nonetheless, we encourage additional field-based studies with larger sample sizes or better recapture rates to improve statistical power and provide more clarity on how variation in habitat quality influences how fish respond to other challenges.
Methodology
All fish were sampled under an Ontario Ministry of Natural Resources Scientific Collection Permit (Licence No.: 1061994; Granted to S.J.C) and were processed in adherence to the guidelines set out by the Canadian Council on Animal Care, as issued by Carleton University (B10-9). As creek chub generally spawn in the spring beginning at temperatures of 12.8°C (Scott & Crossman, 1973), all sampling occurred outside this range to avoid confounding effects of the reproductive period. Captured fish were individually anaesthetized in an induction bath of 70 ppm clove oil emulsified in ethanol (1 part clove oil to 10 parts ethanol; Anderson et al., 1997). Once unresponsive, fish were randomly distributed across the following three groups: cortisol-, sham-treated and control. Cortisol-treated fish received a single intra-peritoneal injection of 10 mg ml−1 of cortisol (hydrocortisone; Sigma H2882, Sigma-Aldrich) suspended in cocoa butter (Cocos nucifera; Sigma C1758, Sigma-Aldrich, St. Louis, MO) at 0.005 ml g−1 body weight (reviewed in Gamperl et al., 1994; but see O’Connor et al., 2010). Sham-treated fish received a single intra-peritoneal injection of pure cocoa butter at 0.005 ml g−1 body weight (i.e., without cortisol to evaluate the effects of the carrier and the injection procedure), while control fish were handled in a manner identical to treatment fish, but received no injections. This method of experimentally raising circulating plasma cortisol concentrations to physiologically and ecologically relevant level is an established technique used to study the effects of stressors on fish (Gamperl et al., 1994). The dosage administered was confirmed by Nagrodski et al. (unpublished data) to experimentally raise cortisol levels in creek chub to 753 ± 256 ng/ml, for ~3 days relative to control values of 203 ± 60 ng/ml. This cortisol dosage was intended to emulate the stress that might be encountered during ecologically relevant events such a hypoxic event (e.g., Herbert & Steffensen, 2005), a short-term starvation (e.g., McConnachie, 2010), heat shock (e.g., McConnachie et al., 2012), exposure to supercooling and frazil ice (e.g., Brown et al., 1999), droughts and floods (e.g., Flodmark et al., 2002). For context, wild creek chub exposed to heat shock in a laboratory experienced an 11× increase in cortisol titers relative to baseline values, reaching over 1,200 ng/ml (Blevins et al., 2013), validating that the experimental manipulations achieved in this study were realistic and ecologically relevant. After treatment, fish were placed in separate, aerated recovery bins (~52 l). All fish were also identified individually using fluorescent orange visual implant (VI) Alpha Tags (Northwest Marine Technology, Inc., Tumwater, WA) and collectively, according to treatment, by small partial caudal fin clips (Rounsefell & Kask, 1945). While cortisol- and sham-treated fish received upper and lower fin clips, respectively, control fish received both upper and lower fin clips. Prior to field application, a laboratory pilot study found that VI Alpha Tag had an ~88% retention rate after 14 days when implanted in the cheek epidermis of creek chub in an outdoor holding tank (~4290 l) at Queen’s University Biological Field Station (QUBS; 44°31′N, 76°20′W).