How to Take Measurements of Fish
Fisheries biologists use Relative Weight (Wr) to determine the growth of a given species of fish. To do this, one needs to take two key metrics: one, length, and two, weight. For this example, we will collect this information from a Large Mouth Bass (Micropterus salmoides).
Preparation:
There are two items needed to collect the measurements from the fish.
1. A measuring board. If one is not available, you can make one with any flat surface. Just make marks at each inch or centimeter for the length of the surface; and,
2. A Scale. You will want a low weight scale for most pond systems, preferably one that will measure under a pound (lbs.) or measures in grams (g).
Measurements:
Note: the order you take these measurements does not matter; however, it is essential to stay consistent in your measurements. Also, be consistent with the measurement system you use. In most cases, the metric system will be easiest to calculate Standard Weight (Ws) and Relative Weight (Wr).
Total Length.
Total length is the entire length of the fish from the head to the tail fin’s end. Hold the fish on the measuring board with the head at 0 and pinch the tail fin to a point. Record the number at the end of the tail fin.
Weight:
Weight is the simplest to measure but the more challenging to collect. For total weight, you can just hold the fish down on the measuring board. However, most fish scales have a hook, and when you let go of a fish, they tend to jump. Holding on to the fish will result in a false weight reading, and letting completely go of the fish could result in the fish damaging its gills. To prevent hurting the fish and quickly record its weight, hold on to the fish until it is done thrashing. Then remove your hands from the fish but keep one close to the fish to catch it if it begins to thrash again.
Now that you have collected the needed information from each fish in the sample, you can calculate the Standard Weight (Ws) then find relative weight.
The calculations for Standard Weight and Relative Weight will be explained in a later post.
