As anyone who has ever spent time near our lake sturgeon touch tank knows, sturgeon aren’t covered in scales like most fish. Instead, they have five rows of bony scutes on their back and sides which are made of a substance called ganoin. These rigid plates act as a suit of armor for the fish, especially for young sturgeon where the scutes are still sharp- the scutes wear down as the fish ages and become smaller in relation to the body size of the sturgeon.
Scientists and fisheries managers use these scutes to keep track of the age of sturgeon that we raise and reintroduce into the wild. Every fish that leaves our TNACI facility has a specific scute removed to identify the age of the fish. On odd-numbered years (i.e. 2007, 2009) we remove a lateral scute from the right side of the fish; on even years we remove lateral scutes from the left side of the fish. That way, when a scientist captures a fish from the river, he or she can run their fingers down the lateral scutes of the fish, and identify the soft spot where a scute has been removed. By counting how many scutes back from the head of the fish to where the missing scute is, the scientist can identify which year the fish was born and use this data in conjunction with length and weight measurements to judge how fast the fish has grown.
The scute removal process is inexpensive and quick, but it does not allow for identification of individual fish and over many years, it becomes increasingly difficult to tell which scute was removed because the remaining scutes will migrate together and partially fill in the gap. Next week, we will talk about another tagging method we use that allows for identification of individual fish called “PIT” tagging.