Below the surface

Counting fish

ONE of the fundamental aspects of managing any animal population is being able to record and monitor the number of individuals present in a given environment. This could equally be songbirds in a hedgerow, elephants on the African savanna, or fish in a stretch of river. Without this information it is impossible to know how a population is performing and how it is changing over time.

Of all the places that we want to record the number of animals present, under water is among the most difficult. After all, most of the time we cannot see the animals, they don’t make any sound that we can record and catching them can be very difficult.

Challenges faced

Even if we look at a really simple environment, such as a small pond that contains no snags, is relatively shallow and is not too weedy, catching all of the fish present is virtually impossible. In fact, the only way to get close to an exact number would be to drain the pond completely. Even then, it is likely that some fish will be missed as they become buried in the mud at the bottom.

The problem of gaining an accurate estimate of the number of fish in a pond, or a stretch of river, or canal has vexed fishery scientists for more than a hundred years. But over this time they have developed many innovative ways of estimating the numbers of fish present.

Measuring techniques

One of the most commonly used techniques is catch depletion. In this method the fish are removed by making several passes through the pond, either with a net or by electric fishing. Not all of the fish will be caught in the first pass. In fact, often less than half will be caught. Fish that have been caught are not returned ( they are normally held safely in vats of water), so in the subsequent passes more new fish will be caught. Hopefully, with each run through the number of fish will decrease and this enables scientists to estimate the total number of fish present using a mathematical formula.

The removal of fish using successive

passes is referred to as a catch depletion approach. In fact, other methods can be used that employ a similar tactic. Trapping fish can also give catch depletion data that can be extrapolated in a similar way to give an estimate of the number present.

Undertaking any form of catch depletion can be labour intensive and expensive. Even in a small enclosed pond it can take a day of work by a team of trained staff to collect enough data to create an estimate of the fish population. Now imagine trying to do the same thing in a large gravel pit or a river. In these environments typically only a small part of the fishery will be netted off and samples taken from this area that can then be multiplied up to give an estimate for the whole venue.

Running water

Rivers represent an additional level of difficulty as very often the fish populations are not static and can move long distances. A shoal of bream may, by chance, happen to be sampled one year, but a repeat sampling of the same stretch the next year may find the fish gone. This can make investigating changes in fish populations over time and space very difficult and prone to error.

Most techniques for estimating fish populations only give a rough estimate of the fish present, which can mask subtle changes in the number and size of fish that may be required to manage the fishery properly. The larger the sample size the more accurate the results are likely to be, but there is always a trade- off with the time and effort involved in collecting this data. Most of the time it is safest to view these data as showing trends rather than exact figures. They can tell us whether a fish population is increasing or decreasing, perhaps getting older, or being bolstered by young fish, but rarely can we view them as more than estimates.

Effect on anglers

Although a huge quantity of fish number and population data has been collected in the UK over many years it is often very difficult, if

“Angler catches can bolster our understanding of fish populations”

not impossible, to correlate this with angler catches not least because it is actually quite rare for the two types of data to be collected simultaneously. One important question that remains unanswered is ‘ how does the number of fish in a pool or river influence angler catches?’ To manage fisheries for anglers this is perhaps the key question that we need to answer, but there has been relatively little work in comparing the two sets of catch data. This is potentially a major omission as not only can angler data provide a great deal of additional information, particularly in difficult to sample fisheries, but the population estimates and forecasts can also predict how angler catches may change in the future.

From match results to records of specimen fish, angler catches can bolster our understanding of fish populations and should form an important part of our monitoring of the health of fisheries.