Background Information | Data Activity
Mike Arendt, Fisheries Scientist
Mike Arendt completed his Master's thesis in 1999 under the direction of Jon Lucy at the Virginia Institute of Marine Science. Mike's research on "Seasonal Residence, Movement, and Activity Patterns of Adult Tautog, Tautoga onitis, in Lower Chesapeake Bay" involved tagging and monitoring tautog and produced some interesting results.
The first objective of Mike's study was to determine at what times during the year tautog inhabit certain natural and man-made habitats within the lower Chesapeake Bay. Specifically, Mike wanted to determine if tautog in Chesapeake Bay moved to areas of warmer water in the winter and cooler water in the summer, as tautog do in New England states, or if tautog remained within the Bay year-round. Mike also wanted to determine if tautog remained active year-round or if they reduced their activity during the summer and winter, periods of very high and very low temperatures. Tautog populations that remain active year-round are more likely to be adversely affected by fishing pressure than tautog that are only active seasonally, because tautog are almost never caught unless they are active.
Mike's Research Methods
Tautog spend much of their time around reefs. The reefs at the study area were all about 1 meter tall. When a fish is resting very close to the reef, the reef structure can block the transmitter signal from reaching the receiver. To be sure that all areas were monitored, Mike deployed two acoustic receivers at each site, one on the eastern side and the other on the western side of the site.
Mike has created five graphs as samples of his data for you to examine. The first four graphs (fall, winter, spring, and summer) show data for an individual tautog for a single week in each season, the fifth graph shows data for an individual tautog during Hurricane Dennis. For fall, winter, and spring the graphs are from the same tautog; for the summer and storm graphs the data are from a second tautog. In this exercise, you will examine each tautog's diel activity pattern (detections per hour versus time of day). Print out and compare each of the graphs. If you would like to see the data behind the graphs, click here to download Mike's Excel worksheets.
Compare your answers to the above questions with our answer key.
The data in the graphs and worksheets are greatly simplified for ease of viewing. The behavior of the fish chosen for each seasonal graph is representative of the overall trend that Mike observed. To confirm this, take a look at two mean detection graphs for the entire data set for all fish combined. The first figure shows black lines for sunrise and sunset times. The second figure shows the same data compared with water temperature (white line). You will observe that the tautog monitored were more active in fall and spring, when the water temperatures were more to their liking, and less active in winter and summer when temperatures were more extreme. (Arendt, M.D., J.A. Lucy & D.A. Evans. Diel and seasonal activity patterns of adult tautog, Tautoga onitis, in lower Chesapeake Bay, inferred from ultrasonic telemetry. Environmental Biology of Fishes, In Press).
If you are familiar with Microsoft Excel, continue on with the advanced data exercise. If not, skip to the results section.
Advanced Data Activity
There certainly was individual variation among fish, and the following is a more advanced Microsoft Excel exercise to demonstrate this. Mike has selected a single sampling day and included data for three fish. Open the "Detections" spreadsheet and look at the "interactive" worksheet. You will see that the data from fish #18 has already been graphed. Using the instructions below, have your students tally the number of detections per hour for the other two fish and paste their answers into the worksheet. After they have done that, the worksheet will automatically graph the data for these fish.
HINT: The column called "Counts" is there to assist you with your counting. If the number of detections in a given hour is very high, a simple way to count them is to click and drag with your mouse in the "Counts" column, creating a box around all the counts next to that hour. If you then look down towards the bottom right corner of the worksheet, you will see "Sum = ", with a number representing those counts. You can then enter that number into the appropriate box in the "No. of Detections" column.
You should now be able to see that there was individual variation among fish in Mike's study. Compare your detections tally and graph to Mike's.
Mike found that 70% of the fish he tagged remained at their respective release sites for up to 6 months (transmitter battery life) and were never detected or recaptured away from their respective release sites. Although there certainly was individual variation among fish, the overall trend indicated that tautog in the lower Chesapeake Bay remain resident year-round, but decrease their activity levels in winter and summer. Understanding these patterns is necessary for proper management of the resource, especially since the increased activity in spring and fall corresponds to our tautog fishing seasons here in the Chesapeake Bay. Extended periods of residence in the winter at inshore sites may increase the potential for over-exploitation due to prolonged activity and, thus, catchability. Mike's results have been presented at numerous regional, national, and international scientific meetings and have been published in the scientific literature.
Mike's results were also presented to the Virginia Marine Resources Commission with the hopes that his findings will be taken into consideration in future discussions of tautog stock management. Current regulations for tautog harvest include a 14" minimum size and 7 fish per person per day bag limit for recreational fishers, which account for approximately 95% of total tautog harvest throughout this species' distribution. Although only a small percentage of tautog are harvested commercially, no commercial fishing is allowed between 1 May and 31 August, corresponding to the extended spawning season for tautog. Regulations such as these are necessary to protect and rebuild tautog stocks.
After finishing his master's degree, Mike Arendt became a biologist with the Marine Resources Division of the South Carolina Department of Natural Resources. We are extremely grateful to Mike for giving us firsthand insight into his research. Be sure to check back for our next Spotlight On A Scientist.
For more resources, be sure to visit the Bridge's Commercial & Recreational Fisheries page. If you have questions or comments about the Spotlight, contact Laura Rose, Bridge Data Project Manager.
Previous Spotlight on a Scientist: Kate Mansfield, Sea Turtle Biologist
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