Fish Surveys

Introduction

These recommendations are based on the techniques discussed by Perrow, Côté & Evans in Sutherland’s ‘Ecological Census Techniques: A Handbook’.

Personal Safety

Please read this safety information before carrying out a survey.

Seasonality

Certain species can only be located at certain times of the year. If you are undertaking a one-off survey of a site you will normally need to make more than one visit to ensure you maximize the proportion of species present at the site that are found during surveying. If you are surveying specific species you should ensure all visits are carried out at the appropriate time of year. If in any doubt consult literature about the species or an acknowledged expert before deciding when to visit.

Fish will generally be more active during the summer and will often be less mobile in winter and seek deeper water. Therefore, surveying for fish will normally take place during the summer months.

Monitoring Frequency

In order to monitor population trends, survey work should be carried out on a regular basis (e.g. once a year), with the visit(s) at or near the same period each year. It is important that exactly the same method is followed to allow comparisons to be made.

Equipment

Method 1 – Bankside counts – dull coloured clothes, polarised sunglasses, census forms.

Method 2 – Appropriate nets or traps, appropriate protective clothing, census forms

Methodology

Two methods are described. Conspicuous species in pools and slow-moving, shallow streams and small rivers can be measured using bankside counts. Other species can be monitored by netting or trapping. A third method that enables all fish to be counted is electrofishing. This method works by stunning the fish but requires a high level of training and should only be undertaken by experienced surveyors. It is not described here.

Method 1 – Bankside Counts

(from Perrow, Côté & Evans, pp180-181)

Method

Bankside counts are a good technique in shallow, slow-moving, and clear waters with minimal vegetation, such as streams or even lake shores. The stretch of water to be surveyed is usually divided into contiguous but non-overlapping sections. The sections should be small enough that all fish can be counted from a single vantage point. The use of landmarks on the shore helps in delimiting the sections.

Observers, wearing polarised sunglasses, to reduce glare, and dull coloured clothes, should move slowly to the vantage point and conceal themselves behind riparian vegetation. Artificial structures, such as docks or bridges, may be used when available.

Once in position, wait motionless for at least five minutes before counting to minimise the effects of disturbance. Counts are best made on sunny days or at least bright overcast days. Intensive sunshine can create problems with glare, and shadows can betray the presence of the observer, whilst rain, wind, and surface ripples make observations nearly impossible. After the count, move away from the bank before proceeding to the next observation site.

Fish density can be calculated by measuring the area surveyed.

Advantages and disadvantages

Shore based visual counts are cheap, fast, and easy. They are particularly appropriate when the water is too shallow to be sampled easily by other techniques and are especially useful for censusing particular age classes of fish (e.g. young-of-the-year) which seek shallow water habitats. Direct observation allows parameters such as fish size and sex and many features of the environment (e.g. water depth, current speed, substrate type, vegetation cover) to be recorded. It also allows detailed behavioural observations to be made. Stress to the fish is minimal.

One potential problem is that fish are very aware of human presence, and any disturbance will reduce the accuracy of visual estimates. Fish can take a long time to leave cover and resume their activities following disturbance. Visual counts from banks typically have high inter-observer variability (Hankin & Reeves 1988), although having a few, well-trained people counting fish can reduce this problem.

Biases

Big and brightly coloured fish are easier to see than smaller, duller-coloured fish. This may introduce sex-bias since males are often larger and/or more brightly coloured than females. Age-related behaviour made also introduce a bias if some age-classes seek cover more than others. Fish in deeper water, turbulent areas, turbid waters, areas of high surface glare, or dense habitat may be overlooked.

Method 2 – Netting and/or trapping

Netting and/or trapping is suitable in most habitats for fish with the exception of fast-flowing water, and enables monitoring to be undertaken in habitats where conditions are not suitable for bankside counts. It also enables some of the more secretive species to be caught that could not be monitored by counting, including species that are active at night.

There are a large number of different netting and trapping techniques, and the most appropriate technique will vary depending on the habitat, the species and the aims of the survey. Details of various techniques are given in Perrow, Côté & Evans, but another good source of information about appropriate methods will be local experts and fisheries or angling organisations.

When choosing a technique, it is important to consider the aims of the survey and the amount of surveyor time available. If traps are to be put in place for extended periods, they will require regular monitoring and maintenance. For ongoing monitoring, a further consideration is how easily the survey can be repeated in future years. In this case, markers can be used to ensure that traps are set in the same location each year.

Conditions may vary considerably from year to year, so it is also important that habitat recording is undertaken so it can be considered in the analysis (e.g. depth of water, speed of flow, extent of vegetation)

References

Perrow, Martin R., Côté, Isabelle M. & Evans, Michael ‘Fish’ in Sutherland, William J. (Ed) ‘Ecological Census Techniques : A handbook’, Cambridge University Press, 1996.