I wanted to share some tips on how to count lateral line scales and the scales around the caudal peduncle as well as caudal peduncle proportions.
I am just wrapping up a Black redhorse illustration and figured this would be a good example since these measurements are ket to properly id'ing this species.
The caudal peduncle is the tail section of the fish. It starts and the posterior end of the base of the anal fin and continues to the posterior edge of the hypural plate.
The Hypural plate is the last bone in a fishes vertabre. It is not typically visible under the skin on most species. Although some species have a small noticable dent or divet at this point. It can however be felt under the skin. Its where the muscle tissue stops on the tail and the tail fin begins. But be aware that the tail actaully starts beneath the skin and there are normally scales that continue onty the fin. If you hold a fish and pinch the base of its tail lightly and move the tail from side to side you can feel the "knuckle" as som call it. This is the edge of the hypural plate.
The Cleithrum is the collar bone found behind the gills on the body of the fish.
Lateral line scale count
When counting scales on a lateral line you start at the first fully formed poored scale behind the cleithrum.
The scale count continues to the last scale on the hypural plate. Its important to not count the scales past this point. Often they can continue past this and well onto the fin. This will give you a false scale count in the lateral line leading to issues with identification.
The fish below has a lateral scale count of 47. Black redhorse typically have a scale count of 45 or more and golden redhorse a scale count of 44 or less.
Caudal peduncle scale count
When counting the scales around the caudal peduncle this is the method I was taught to use in the field. Often it can be tricky to get this count if the fish is still alive and moving. So rather than count all the way around I count one side while its laying flat. Find a good center point on the peduncle. I count the scales on the side of the peduncle folloing the arc of scale. I disregard the ventral scale and the dorsal scale.
For example on the fish pictures there are 5 scales on the side, 1 dorsal, and 1 ventral. There are 5 scales on the oppisite side. 5+5+1+1 = 12 scales around the caudal peduncle.
Caudal peduncle proportions
Measuring the proportions of the caudal peduncle can be tricky in the feild. a calipers is helpful but often I find doing this at home with a good photo of the fish laying flat is far easier.
First the caudal peduncle length. From the posterior edge of the base of the anal fin to the posterior edge of the hypural plate. Then the depth or height of the caudal peduncle at its center.
These measurements are taken and then compared.
In the case of the fish below it is 1:0.59 The caudle peduncle depth is 59% of its length.
Black redhorse typically have depth that is 59.9% or less and Golden redhorse have a depth of 62% or more.