The Little nets are commonly used to sample fish; however, little is known about post-capture effects of this gear type on the fish captured. We conducted experiments to evaluate the effects of trammel net sampling on survival and cortisol levels of razorback sucker (Xyrauchen texanus), bonytail (Gila elegans), and roundtail chub (Gila robusta), at 15 °C, 20 °C, and 25 °C. Fish between 139 mm to 388 mm in total length (TL) were obtained from both captive hatchery stock and wild populations, quarantined for two weeks, and acclimated in two 18,000 L tanks for 13 days. Treatment fish were entangled in a trammel net for two hours and control fish were captured by a seine. After capture, all fish were weighed, measured, and individually tagged. We extracted approximately 0.37 mL (between 0.1 mL and 0.5 mL) of blood from the caudal vasculature of 30-80% of fish in both the treatment and control groups. All fish were placed in a 36,000 L holding tank, where they were monitored for delayed mortality for 14 days.
The captured by trammel net experienced up to 94% mortality within the first two weeks after capture, whereas seined fish experienced less than 24% mortality. There was little immediate net mortality for both control and treatment fish, which is consistent with field observations. Mortality of trammel-netted fish, however, was observed up to two weeks after capture. In warmer water, both bonytail and razorback sucker experienced significantly increased mortality rates. Roundtail chub did not show this trend. Even higher levels of mortality may occur in nature, as any increased stress and/or physical injury that fish incur during capture and handling may lead to a competitive disadvantage and impair foraging ability. Post-capture mortality of wild fish may have gone undocumented thus far because of the time delay between capture and death. Our results suggest management agencies should re-examine the appropriateness of trammel net sampling for imperiled fish populations, especially when water temperatures are above 20 °C.
Cortisol as an Indicator of Stress:
The the magnitude of the cortisol response varied among species, cortisol levels were higher for fish captured by trammel net than for fish captured by seine. Fish captured at the highest temperature (25 °C) were more likely to have elevated cortisol levels than fish captured in the same way at lower temperatures. Higher fish mortality was observed after capture by trammel net than seine, and more mortality occurred at the warmest experimental temperatures. For bonytail and razorback sucker, elevated cortisol levels were an effective predictor of mortality. However, roundtail chub did not demonstrate a significant association between cortisol levels and subsequent mortality. These results suggest that cortisol could be used as a potential index of stress and post-capture mortality, at least for some species.
The results from this project suggest that post-capture mortality should be evaluated where trammel nets are used in systems with water temperatures warmer than 20°C unless high mortality is irrelevant. We have found that cortisol may be used as an indicator of mortality for razorback suckers and bonytail, but we did not find this correlation for roundtail chub.
This project was funded by the USGS Grand Canyon Monitoring and Research Center as part of ongoing research activities.