Fisheries Biology: Understanding Length Frequency
Most readers are aware of the new black bass rules that are now in effect throughout Florida beginning July 1, 2016 (see Issue 5 for details). These rules were developed with a tremendous amount of public involvement and decades of research on the effectiveness of various regulations.
Data from hundreds of fish population samples were also considered in creating these new black bass regulations. One of the most important components of this data used to establish statewide size limits is length frequency, which is the size distribution of fish within a population. This information is vital to determining what size fish can be harvested with the least impact on the population.
How do FWC biologists obtain length frequency data from a fish population? Well, it starts with the fish! The illustrations below show a simplified version of how a length frequency graph is created and how that data contributes to establishing sound fishing regulations.
Lake full of bass
1. Above is a lake full of largemouth bass. As you can see, there are many different sizes. Of course, there are also sunfish, crappie, catfish and other species in the lake, but to manage bass it helps to look specifically at the length frequency of the bass.
Population of bass
2. Fisheries biologists sample the bass in the lake. There are various sampling methods that can be used, but electrofishing is one of the most efficient and also allows the fish to be caught, measured and then released unharmed (see "Shocking Truths of Electrofishing" at the Florida Sportsman FWC Freshwater Blog). Biologists can essentially "capture" the fish from the lake, but in the form of data -- the numbers go back to the lab, but the fish go back in the water!
Bass population sorted by size
3. Back at the lab, biologists sort the fish (actually, the fish data) by size category. The example above illustrates this process as if the actual fish, rather than the data, have been sorted. Commonly used size categories are increments of 1 cm (0.4 inches) and 2 cm (0.8 inches), but in our simplified example we're going to use large 2-inch size groups. As you can see, this fish population has more 10-inch category bass than any other size group. You can also see that the fewest bass in the population are in the 16- and 18-inch size groups.
Length Frequency graph
4. Of course, biologists don't use fish symbols. They use the bass length data to create a length frequency graph. This is how biologists are trained to look at fish population size distributions, and gives a better visual picture of the different sizes of fish in a bass population.
Length Frequency graph
Those are the steps in creating a length frequency graph for a bass population. Above is an authentic graph generated from actual sampling data. These bass size distributions will vary from waterbody to waterbody, and also within the same water over time (especially following events such as a very good spawning year or a devastating drought). However, this graph is very typical of most Florida waters in that the vast majority of largemouth bass are less than 16 inches long. This is a good example of why the new statewide regulation for largemouth bass is "5 Black bass, only one of which may be 16 inches or longer in total length." With an abundance of smaller bass, you can also see why there is often no need for a minimum length limit.
The goals of the new statewide bass rules are to streamline regulations, allow anglers to keep smaller, more abundant bass, and protect larger bass desired by most anglers. Length frequency information helps FWC biologists determine which regulations are appropriate for the most enjoyment and best conservation of Florida's bass.