Pick Our Brain - September 2013

We hear that some fish populations surge while others crash. How are fish populations counted?

Because it is impossible to actually count all of the individuals in any wildlife population over a territory as large as the Delta, scientists and managers generally use sampling techniques and statistical models to obtain estimates of a population size.

Direct estimates of population sizes are easiest to understand and interpret, but can be very costly to obtain, so they are typically only used for very local, intensive studies. The main direct estimate methods are mark-recapture and removal sampling. Both methods involve collecting at least two samples of the population by a method such as electrofishing (see figure) or catching fish in nets or traps. In a mark-recapture study, fish caught in the first sampling event are counted and marked, then released. When the next sample is collected, scientists count how many of the captured fish were previously marked and how many were not. They assume that the fraction of marked individuals caught in the second sample is equal to the fraction of the total population caught in the first sample, so a simple calculation provides an estimate of the total population in that location.

To obtain a population estimate by removal sampling, scientists assume that a fixed amount of sampling effort will capture a constant fraction of the population. For example, maybe they’ll get 20 percent of the population each time they sample a section of a small stream. If this assumption is correct, then the scientists will catch fewer fish each time out-if they did this enough times, they’d eventually catch all of the fish and would have a complete count. That usually isn’t practical, but graphical and statistical methods can be used to estimate the population from a relatively small number of samples.

Researchers sample largemouth bass with a boat-mounted electrofisher in the San Francisco Bay-Delta
Researchers sample largemouth bass with a boat-mounted electrofisher in the San Francisco Bay-Delta
(Image courtesy of Fred Feyrer, USBR)

At any scale much larger than a pond or a short stretch of a stream, it is unlikely that scientists will be able to use the methods described above. If there is a commercial or recreational fishery for the population they’re interested in, they may be able to use an “index” method to obtain a number that is proportional to the population size and can be compared from year to year to assess trends. In fisheries, the most important index method is called “catch per unit effort.” Similar to removal sampling, scientists assume that each unit of fishing effort (e.g., hooks trolled per hour, volume of water swept with a net) will catch a consistent proportion of the population. Thus, if the catch per unit effort starts declining, scientists know that the population is probably declining, and can recommend corrective management actions.

In the absence of a fishery for a particular species or life stage (e.g., delta smelt or protected salmon species on their spawning grounds), managers have to conduct monitoring surveys to obtain a catch per unit effort index, but the principle is the same. Monitoring surveys may also use visual methods such as counts of fish passing by a camera or underwater observation by snorkelers.

More involved methods of population estimation exist, but all require that researchers make assumptions about the sampling process and whether fish move in and out of the area being sampled. It is always important to assess how well these assumptions are satisfied, and to report the statistical uncertainty associated with a given estimate.