Task Three: The method, the data and a map |
To Think About
For this portion of the process, you will read the following sections about the study as well as the study area. Keep in mind that you will print out the image below and draw in some points of detail. You can either right-click on the image to save it or copy it to another application, or you can CLICK HERE to download a PDF version of it.
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Where the study was done and why
Schooner Creek is a salty tidal creek that runs along side the JCNERR field station in the estuary. It was chosen as the site for this study because in a past tagging study at this site, large gaps of time left several unanswered questions regarding when the fish left and the extent of their creek use. The creek was divided into five sections based on depth and width, but we will be looking at three:
- Mouth: the first 50 meters from the creek mouth (0 meters)
- Flat: a shallow flat area just inside the creek mouth (less than 0.2 meters deep at low tide, 50 to 150 meters up from the mouth
- Upper creek: the creek area above the stationary hydrophone which is located at 150 meters on a point of land
Now you can begin filling in points on your map, starting with the mouth, flat, upper creek, and the stationary hydrophone.
Tagging
Five summer flounder were captured using fyke (funnel type) nets approximately 150 meters up the from the creek mouth. Four were captured using hook and line near the tide station, which was approximately 350 meters up from the creek mouth. All nine of the fish were determined to be less than one year old based on information about size and age. (Here is some more information for your map.)
They were tagged and released at different times. Each fish was temporarily put to sleep with an anesthesia and an ultrasonictransmitter was attached with a strong thin thread by sewing through the muscle just below the dorsal fin. The transmitters were manufactured by Sonotronics, Tucson, Arizona, weighed 4 grams in the air and had a 40-day life expectancy. So, there were 40 days, or about a month and 10 days to collect the data. It was the beginning of August.
Since it wasn't known how the fish would recover from the tagging procedure, they were kept for observation in a flow-through seawater tank for about 1-48 hours. The release site was approximately 530 meters up the creek from the mouth. The tagging-release procedure began August 9, with the last fish tagged and released on August 27th. Fish movements were tracked until the last signal was lost on Sept. 27, 1990.
Tracking
A 150-meter coaxial cable connected the hydrophone to a continuously scanning receiver. After a signal is detected from a tagged fish, the tag number, time, and date were stored in a computer. After some testing, it was determined that the stationary hydrophone was good for approximately 130 meters directly in front of it. Sometimes a fish would swim back and forth in front of it making multiple detections, so multiple detections within 5 minutes were counted as one. The other way fish were tracked was by portable directional hydrophones from a small boat or kayak. Initial detection was recorded on a detailed map of the creek. Then the creek was searched every six hours during slack water. Slack water is the time just before a high or low tide when the current is minimal.
If a fish had not been detected for a while, a search was on dividing the mouth and a short distance beyond in 1 KM transects or grids from the last known location. Transect searches were repeated three time before the signal was considered lost.
A "missing" category accounted for the time it was known the fish was in the creek but could not specifically locate it. For example, if a particular fish was located at the mouth at 6:00am and in the upper creek at 12:00 pm, but did not detect it moving up the creek, 6 hours were placed in the "missing" category.
Each time a fish was detected by the portable hydrophone, the time and stage of the tide was recorded as well as some physical and chemical data. The tide station was located 335 meters from the creek mouth. Surface salinity was measured with a refractometer, bottom temperatures and dissolved oxygen (DO) concentrations with a YSI meter.
Important Note: you are going to have to explain what these things are when explaining your data
The Questions
Now that you have a better understanding of the study and it's whereabouts, answer the following questions:
- Why do you think they only tagged nine fish?
- Do you think the creek was a large enough study area?
- What are some challenges in choosing this tagging and tracking method?
- When did they track using the portable method? Why?
- Where was the stationary hydrophone and in what direction was it pointing?
- What is "missing time?"
- What is a refractometer?
