SAMPLING OF FISH IN A LAKE WHERE THE POPULATION IS CONCENTRATED IN AN AREA OF LIMITED ACCESSIBILITY
ECHANTILLONNAGE DE POISSON DANS UN LAC OU LA POPULATION EST CONCENTREE DANS UNE ZONE D'ACCES LIMITE

by/par

J.F. Craig
Freshwater Biological Association, Ambleside
Westmorland, England/Angleterre

ABSTRACT

Peroh in Slapton Ley were found to concentrate in the Higher Ley, an area of reed swamp. The only sampling gear that could be used were traps. The limitations of this gear are discussed and it is concluded that if a choice is available, traps should not be used for sampling a population. If there is no choice, then catches from traps can be used to determine various population parameters. This can include population density if the experiment is carried out over many years but mark-recapture experiments can be of little value.

RESUME

Dans le Slapton Ley, les perches ont été trouvées principalement dans le Haut-Ley, une zone marécageuse à roseaux. Le seul engin de pêche pouvant y être utilisé est la nasse. Les possibilités d'utilisation limitée de cet engin sont discutées et on conclut que si d'autres méthodes sont praticables les nasses ne doivent pas être employées pour l'échantillon-nage d'une population. Si l'on n'a pas le choix, les prises des nasses peuvent servir à déterminer quelques paramètres de population. Ceux-ci peuvent inclure la densité de population si la phase expérimentale se déroule sur plusieurs années, mais les expérimentations marque-recapture ne sont guère valables.

CONTENTS

1. INTRODUCTION

2. SLAPTON LEY

3. USE AND RESULTS OF FISHING GEARS

3.1 Seine Netting
3.2 Traps

4. DISCUSSION

REFERENCES

1. INTRODUCTION

The employment of traps for catching fish and using the fish for estimating population parameters has been criticized (Stott, 1970; Bagenal, 1972) since the catch depends on the behaviour of the fish.

Traps are simple to manufacture. They are also easy to operate. In some waters they may be the only type of gear that can be used and their limitations must be fully understood. This paper is based on studies of the perch population in Slapton Ley, South Devon, where the fish had to be caught in traps since they were concentrated in an area where no other gear could be used.

2. SLAPTON LEY

Slapton Ley is a small freshwater coastal lagoon divided into two main regions, the Higher Ley with an area of 36.4 ha, and the Lower Ley with an area of 76.1 ha. The average depth of the lake is 2.13 m.

The Lower Ley is mainly open water with a discontinuous reed fringe. Seine netting is possible from a number of points around the lake. The Higher Ley is mostly reed swamp, open water only occurring in a narrow discontinuous channel along its length and in small pools. A number of islands occur made up mainly of Phragmites communis peat and all bearing a willow/alder carr. Stretches of open water between these islands alter with changes in the water level. Stretches of open water may also be filled in with floating pieces of vegetation which break away from the islands. The water in this area is obstructed by vegetation and too deep for electro-fishing and, since the areas of open water are continually changing, gill netting is difficult. Only about a third of the area is accessible to set any fishing gear.

The main fish species in the Ley are perch, Perca fluviatilis L.; pike, Esox lucius L.; rudd, Scardinius erythrophthalmus (L.); roach, Rutilus rutilus L.; and eel, Anguilla anguilla L. Rare species found are brown trout, Salmo trutta L.; brook lamprey, Lampetra planeri Bloch; three-spined stickleback, Gasterosteus aculeatus L.; and bullhead, Cottus gobio L.

3. USE AND RESULTS OF FISHING GEARS

3.1 Seine Netting

From May to October during 1970 and 1971, seine netting was carried out at various points in the Lower Ley. It was also operated once in the Higher Ley in October 1970. The net had wings of 1 inch square mesh narrowing to half inch square mesh with a codend of a quarter inch square mesh.

The seine net caught perch, pike, roach, rudd and eels. Of the perch population, the catch in the Lower Ley was predominantly small fish. It caught a representative sample of I+ fish and toward the end of June it caught a representative sample of O+ fish (having an average length of 3.4 cm). The monthly average number of perch ≥ 9.5 cm caught per haul are shown in Table 1. I+ perch reached an average length of 9.5 cm during August which accounted for the increase from July but it was obvious from the September and October figures that the majority of perch had moved from the Lower Ley after August. The October value was based on four hauls at three different sites. One haul in the Higher Ley at this time caught 157 perch (≥ 9.5 cm).

3.2 Traps

Initially thirty traps were set in the Ley, twenty-five in the Lower Ley and five in the Higher Ley. They were made of half inch square wire mesh. A full description of their construction and use has already been given (Craig, a and c in press).

The monthly average number of perch (≥ 9.5 cm) caught per trap in the Higher Ley and the Lower Ley are shown in Table I for the same months as the seine net was in operation. It was quite obvious from the catches that the population of perch concentrated in the region of the Higher Ley. This meant that most of the data on the population had to be collected from fish caught in traps. It also meant that only a small portion of the population could be sampled due to the physical difficulties of the area.

The following points were noted when using traps:

1. Traps with half inch square mesh caught fish from 9.5 cm upward. The recruitment of I+ perch to the traps occurred in August (Table I) when the I+ group reached this threshold value. When traps with a quarter inch square mesh were fished alongside half inch square mesh traps, they caught fish from 5.0 cm upward. These included the O+ ripe males during the spawning season. The upper size limit must be dependent on the size of the trap opening. The largest fish caught was 27.75 cm. Ford Walford plots gave L_∞ = 25.0 for females and L_∞= 22.0 cm for males indicating that the traps recruited all sizes above the lower limits. As Le Cren (1958) found, there appeared to be no selection for size between the lengths mentioned.

2. During the spawning season, the males made up 95 percent or the catch. There was little selection for sex at other times of the year.

3. There was wide variability between the catches of individual traps. This was probably due to changes in the habitat (Craig, c in press). Bagenal (1972) suggests that many traps should be used (> 10) for meaningful comparisons between catches to be made. However, catches from groups of five traps in Slapton Ley were found to give valuable information. They indicated changes in density of perch in various parts of the Ley and all showed seasonal trends. Especially noted was an increase in numbers during the spawning season shown by all groups of five traps.

   Groups of five traps have been used for many years in Windermere and have given meaningful results when the catches have been compared year to year over many years.

4. Perch caught in traps were marked for a multi-mark recapture experiment to estimate population numbers. Recaptures of marked fish had to be made using traps. Since the total catch depended on the behaviour of the fish, the assumption that the catch of marked fish should be unselective could not apply. An attempt to estimate numbers by this method (taking into account the problems of handling and marking perch which were very susceptible to stress) gave the wide variations.

5. An examination of the stomachs of trapped fish only gave a qualitative account of food and feeding habits. Since traps were laid for quite long periods (one week) no time could be given when the fish had entered the trap. Therefore no value could be applied to the quantity of food present in the stomach.

4. DISCUSSION

If possible, traps should not be used as a method of sampling fish due to the limitations already mentioned. However, if no other sampling gear is appropriate the following information can be obtained about a population (see Craig, a and b in press).

1. Over many years, changes in density with time can be monitored, as in Windermere. Traps are of little value in short term mark-recapture experiments except as perhaps a method for catching fish for initial marking.

2. Catches from traps can indicate changes in density with space. They can also be used to follow migrations.

3. Fish caught can be used to determine growth, age composition, age of sexual maturity, fecundity, the onset and duration of spawning, length-weight relationships, condition and the incidence of disease. If the spawning season is known, then fish caught outside this period can indicate the sex composition of the population. Trapped fish have little value in food or feeding studies.