Telemetry-Based Mortality Estimates of Juvenile Spot in Two North Carolina Estuarine Creeks

Abstract

We estimated natural mortality rates (M) of age-1 Spot Leiostomus xanthurus by using a sonic telemetry approach. Sonic transmitters were surgically implanted into a total of 123 age-1 Spot in two North Carolina estuarine creeks during spring 2009 and 2010, and the fish were monitored by using a stationary acoustic receiver array and manual tracking. Fates of telemetered Spot were inferred based on telemetry information from estimated locations and swimming speeds. Potential competitors of age-1 Spot were assessed through simultaneous otter trawl sampling, while potential predators of Spot were collected using gill nets and trammel nets. The number of inferred natural mortalities was zero in 2009 (based on 29 telemetered Spot at risk) and four in 2010 (based on 52 fish at risk), with fish being at risk for up to about 70 d each year. Catches of potential competitors or predators did not differ between years, and age-1 Spot were not found in analyzed stomach contents of potential predators. Our estimated 30-d M of 0.03 (95% credible interval = 0.01–0.07) was lower than that predicted from weight-based (M = 0.07) and life-history-based (M = 0.06–0.36) estimates. Our field-based estimate of M for age-1 Spot in this estuarine system can assist in the assessment and management of Spot by allowing a direct comparison with M-values predicted from fish size or life history characteristics. The field telemetry and statistical analysis techniques developed here provide guidance for future telemetry studies of relatively small fish in open, dynamic habitat systems, as they highlight strengths and weaknesses of using a telemetry approach to estimate M.

Received November 23, 2011; accepted September 11, 2012

ACKNOWLEDGMENTS

We thank all of the field personnel who assisted with the data collection and analysis, especially Tyler Averett, Cheston Peterson, Paul Rudershausen, Joe Facendola, Jon Vanderfleet, Ben Kornegay, and Melissa Brewer. We are grateful to Craig Harms, Eric Anderson, and Betsy Stringer for Spot surgical guidance and advice and to Craig Sullivan, Andrew McGinty, and Mike Hopper (Pamlico Aquaculture Field Laboratory) for their assistance with and facility use for the Striped Bass experiment. We also thank Nate Bacheler and two anonymous reviewers for their thoughtful and critical reviews of this manuscript. Funding was provided by North Carolina Sea Grant (R/MRD-57 and R/10-HCE-1). Any use of trade, product, or firm names is for descriptive purposes only and does not imply endorsement by the U.S. Government. The North Carolina Cooperative Fish and Wildlife Research Unit is jointly supported by North Carolina State University, North Carolina Wildlife Resources Commission, U.S. Geological Survey, U.S. Fish and Wildlife Service, and Wildlife Management Institute.