Fish, Fish, and more Fish

Hi everyone! It has been another eventful week here in Madison and working with the DNR. On Monday, the team and I headed back out into the field. Along with measuring and weighing, I also learned how to inject the small PIT tags into the fish which was very exciting. Using a small syringe that had the tag inserted in the tip of the needle, we pierced the skin of the fish and injected the tag. It reminded me of getting blood drawn in that the needle/tag is inserted directly under the skin.

After a long day in the field on Monday, I spent Tuesday in the lab working on dissecting brook trout and specifically extracting otoliths and removing the gill arches of the fish. The otoliths of the fish are made up of calcium carbonate and are located in the head of the fish. These small structures are similar to our ear bones and are important for the detection of sound and balance in fish. I was attempting to extract the otoliths so that they can be used later for fish aging. Similar to the trunk of a tree, the otoliths have rings that can be viewed under a microscope and counted to determine the age of the fish. Thick bands indicate times of fast growth while thinner and whiter bands indicate times of slow growth. The otoliths can be hard to find if you haven’t had much practice and in my first few attempts I struggled mightily to find them. I always managed to crack the fish head in slightly the wrong place which then made finding the otoliths near impossible. After asking for some advice and watching a few YouTube videos, I eventually got the hang of it. The two green circles in the picture below show the two otoliths and the next picture shows just how small these are in comparison to my hand.

In addition to extracting otoliths, I also cut the fish open to determine the gender and took out the gills of the fish to check for parasites (specifically gill lice). These are small parasitic copepods that attach to the gills and fins of the fish and decrease oxygen uptake of the fish, interfere with growth and development, and can cause an immune response. In the picture below, you can see a gill arch with the parasite indicated by the green circle.

On Thursday, I went back to the fish hatchery with the team and we spent the day tagging hatchery brook trout with visible implant elastomer (VIE) tags. These tags are a liquid when injected into the clear or translucent parts of the fish (such as the fins) and then harden. What remains is brightly colored tag that allows scientists to identify a fish when it is put in a stream. For example, fish released into a stream in April may a receive a purple tag on the left ventricle fin and those released in June may receive a green tag on a different fin. Some fish that have been recaptured can receive another tag and some fish in streams could 3 or 4 different tags on different fins depending on how often they were recaptured and tagged. It has been another fun and exciting week filled with a lot of learning and amazing opportunities. Next week, we plan driving up to Northern Wisconsin and surveying streams for a few days.