To Manage a Fishery, You Must Know How The Fish Die

Recreational anglers and commercial fishermen understand you need good fishery management to make sure there will be healthy populations of fish for generations to come. And making good management decisions rests in large part on understanding the mortality of fish species – how many fish die each year as a result of natural causes and recreational and commercial fishing. Now researchers at North Carolina State University have utilized a new research method that can give fishery managers a better idea of how fish are dying, so they can make informed decisions on how to ensure a healthy fish population.

Fisheries scientists from NC State have, for the first time, implemented a research strategy that uses both “conventional” tags and ultrasonic telemetry tags (transmitters) to estimate mortality rates. The approach was used in a study on mortality rates of “sub-adult” red drum, which are red drum that are close to adult in size but have not yet begun to reproduce. However, the research methods pioneered in this study could be applied to many other species, including popular fish such as striped bass. Red drum are popular among recreational anglers in many parts of the country, and are also important to commercial fishermen in North Carolina.

The conventional tags offer rewards to recreational and commercial fishermen who catch the tagged fish, creating an incentive for them to contact researchers. This approach lets researchers know how many of the tagged fish have been caught and how many of the fish were subsequently released or harvested, explains Dr. Jeff Buckel, an associate professor of biology at NC State and co-author of the study. This approach provides particularly good data on mortality resulting from commercial and recreational fishing, Buckel says.

The telemetry tags transmit uniquely coded sounds to receivers, allowing researchers to track fish movement in a given area. In this instance, the researchers were using both stationary receivers and mobile hydrophones to track tagged fish in the Neuse River estuary in eastern North Carolina, Buckel says. Telemetry tags provide excellent data on natural mortality, because the tags stop moving once the fish has died. These tags can also detect that a fish has been caught by commercial or recreational fishermen, because the tag will disappear from the study area without swimming past any of the receivers.

“The methodology we used in this study combined good natural mortality data from the telemetry tags with good recreational and commercial fishing mortality data from the conventional tags to give us a more precise estimate of overall mortality for sub-adult red drum,” Buckel says. “This is important because, if you have a good understanding of mortality rates, you can make informed decisions about how to manage a fishery in order to ensure its long-term health.” For example, the information generated by this study contributed to state and regional assessments of the red drum population.

“This is the first time this approach, using both kinds of tags, has been used in the field,” Buckel says, “and it could have significant applications for other species, such as striped bass.” One limitation is that the telemetry tags are only useful in relatively confined areas, such as lakes, estuaries or reservoirs – where researchers can place listening devices near exits to determine if a fish has left the waterbody on its own, rather than being caught by a fisherman.

The researchers, led by then-NC State Ph.D. student Nathan Bacheler, focused on sub-adult red drum because North Carolina only allows recreational and commercial fishermen to keep drum that are between 18 inches and 27 inches long. These fish are generally not old enough to reproduce. So researchers wanted to determine whether natural mortality and fishing mortality were limiting the long-term viability of the fishery. Good news fish fans: “Natural mortality was much lower than we previously assumed,” Bacheler says, “and the fishing mortality was similar to previous estimates.”

The research, “A combined telemetry – tag return approach to estimate fishing and natural mortality rates of an estuarine fish,” was funded by North Carolina Sea Grant and is published in the August issue of the Canadian Journal of Fisheries and Aquatic Sciences. The study was co-authored by Bacheler, Buckel, NC State biology professors Dr. Joseph Hightower and Dr. Kenneth Pollock, and Lee Paramore of the North Carolina Division of Marine Fisheries.

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Note to editors: The study abstract follows.

“A combined telemetry – tag return approach to estimate fishing and natural mortality rates of an estuarine fish”

Authors: Nathan M. Bacheler, Jeffrey A. Buckel, Joseph E. Hightower, Kenneth H. Pollock, North Carolina State University; Lee M. Paramore, North Carolina Division of Marine Fisheries

Published: Online August 2009, Canadian Journal of Fisheries and Aquatic Sciences, 66:1230-1244

Abstract: A joint analysis of tag return and telemetry data should improve estimates of mortality rates for exploited fishes; however, the combined approach has thus far only been tested in terrestrial systems. We tagged subadult red drum (Sciaenops ocellatus) with conventional tags and ultrasonic transmitters over 3 years in coastal North Carolina, USA, to test the efficacy of the combined telemetry – tag return approach. There was a strong seasonal pattern to monthly fishing mortality rate (F) estimates from both conventional and telemetry tags; highest F values occurred in fall months and lowest levels occurred during winter. Although monthly F values were similar in pattern and magnitude between conventional tagging and telemetry, information on F in the combined model came primarily from conventional tags. The estimated natural mortality rate (M) in the combined model was low (estimated annual rate ± standard error: 0.04 ± 0.04) and was based primarily upon the telemetry approach. Using high-reward tagging, we estimated significantly different tag reporting rates for state agency and university tagging programs. The combined telemetry – tag return approach can be an effective approach for estimating F and M as long as several key assumptions of the model are met.