TAGGING EXPERIMENTS FOR THE ASSESSMENT OF THE SIZE OF FISH POPULATIONS IN THE RIVERS ELBE AND TRAVE
EXPERIMENTATIONS DE MARQUAGE POUR L'EVALUATION DE L'IMPORTANCE DES POPULATIONS PISCICOLES DE L'ELBE ET DE LA TRAVE

by/par

H. Koops

Institut für Küsten-und Binnenfischerei
der Bundesforschungsanstalt für Fischerei, Hamburg
Federal Republic of Germany/République fédérale d'Allemagne

ABSTRACT

Three different tagging experiments are described (a) a tagging experiment carried out with burbot in the River Elbe (example of a commercial fishery). The catch in the lower section of the river is known. The catches in two sections further upstream, as well as the total stock size, could be estimated by means of the recapture rates of tagged fish; (b) a second tagging experiment on pike in the River Trave is an example from angling catches. The size and density of population could be estimated by the catch statistics of the anglers' club and by the recapture rate of tagged fish; and (c) in another tagging experiment on mirror carp the stock size was known, all carp being stocked. The percentage of tagged fish recaptured was comparable with the percentage of the total number of carp caught by angling. By this method it is possible to check the reliability of the tagging experiments by means of the catch statistics.

RESUME

Cette communication décrit trois expérimentations de marquage: (a) avec la lote de rivière dans l'Elbe (exemple de pêcherie commerciale). Les prises de la section inférieure du fleuve sont connues. Les prises de deux autres sections en amont, ainsi que la totalité du stock ont pu être estimées d'après le taux de recapture du poisson marqué; (b) avec le brochet dans la Trave (example de prises à la ligne), on a pu évaluer l'importance et la densité de la population d'après les statistiques de prises du Club des pêcheurs à la ligne et le taux de recapture de poisson marqué; et (c) avec la carpe miroir, l'importance du stock était connue toutes les carpes étant stockées. Le pourcentage de poisson marqué recapturé est comparable au pourcentage du nombre total des prises à la ligne. Cette dernière méthode permet de contrôler la fiabilité des expérimentations de marquage au moyen des statistiques de prises.

1. INTRODUCTION

This paper describes three different tagging experiments which have been carried out to assess the size of fish populations in the Rivers Elbe and Trave.

From 1957 to 1959 investigations on the biology and fishery of burbot in the River Elbe included large-scale tagging experiments to obtain information on the migration behaviour and to estimate the population size of mature fish (Koops, 1959 and 1960). It was anticipated that the brief 3-month catching period for burbot (October-December) would simplify inter- pretation of the data.

One of several angling waters of a local angling club, is the River Trave. Apart from those conclusions drawn from catch statistics and stocking data (which cover the last 24 years), it was hoped to obtain more detailed and useful information for the management of the waters by carrying out tagging experiments. One of the fish species considered was the pike and the results of these studies are of special interest here.

The size of a fish population can be estimated by means of tagging experiments. It is supposed that the chance of normal capture is equal for tagged and non-tagged fish. Frequently the rate of recapture has to be adjusted in view of such factors as tagging mortality, loss of tags varying with time in freedom, etc. In most of the European sea fisheries the total catch is usually reliably reflected by the landings at the fish markets; in freshwater fisheries, however, reliability of catch statistics is questionable to a greater or lesser degree. Summarized below are the results of two tagging experiments with mirror carps carried out to check the catch statistics of an angling club.

2. DESCRIPTION OF THE TAGGING SITES

2.1 Elbe

The Elbe is one of the three principal rivers of the Federal Republic of Germany and empties into the North Sea. The river originates in Czechoslovakia, passes through south Poland and the German Democratic Republic and enters the Federal Republic of Germany at Schnakenburg, 125 km upstream of Hamburg; for 95 km it forms the border between the German Democratic Republic and the Federal Republic of Germany and 155 km later flows into the North Sea. At Schnakenburg the river is about 150–200 m wide and 3 m deep. From Hamburg downstream it enlarges to a width of a few kilometres and to a depth of 12–16 m. The bottom is sandy to muddy. The water carries much detritus; and pollution and lack of oxygen are common in dry summers and long, cold winters. The tidal area reaches as far as the Geesthacht Dam. The majority of fish are cyprinids. Fishing is by stownets, various types of fyke nets and traps being used mainly and especially for the catch of eels. Other species of economic value are pike, pike-perch, burbot and river lamprey.

2.2 Trave

The Trave is a lowland river flowing into the Baltic. About 22 km of its central area is fished by the members of the Oldesloe angling club. This part is divided into two sections. The upper part of about 6 km lies between two watermills without fishways and only at times of heavy rainfall are salmonids, such as sea trout, able to pass these barriers upstream through the floodgates. The way downstream is usually open, especially at weekends, when the water does not run the turbines but flows through the gates. The current in this section is moderate (about 10–30 cm/sec).

The average depth is between 0.8 and 1.2 m and the river is 20–25 m wide. The bottom is sandy and weeds are dense in summer. River regulations, formulated some 35 years ago, left 5 arms with still water open to the river only at their lower reaches, some of which are fairly deep (2 m and more) and are the places preferred by carp and bream for over- wintering. The lower part of the river below the watermills is open to the Baltic.

Within the first 2 km the current is stronger (about 0.8–1 m/sec) and then the river gradually reverts to its former character.

The dominant species caught are various cyprinids, pike-perch and eel (Table I). In this part of the river there is no commercial fishery only sport-fishing.

2.3 Angling Pond

A tagging experiment in a small angling pond of only 60 m in diameter will be discussed because of the very clear results. This pond is nearly circular, 2 m deep in average and surrounded by trees. The bottom is muddy. There is little renewal of water by wells at the bottom of the pond (only about 5 1/min in dry periods).

This pond was first fished in 1959 and had a natural stock of Crucian carp, trench and eel. Both the Trave and this pond have been firstly stocked with 2-summer-old mirror carp in 1959. There is no natural propagation of carp in both waters. Carp smaller than the size of fish used for stocking have never been caught or observed, not even after a 15-year stocking period. By club regulation the minimum size of carp is set to 35 cm for the river and 40 cm for the pond, the maximum catch to 3 carp/angler/day. Both waters are fished exclusively by anglers.

3. TECHNIQUE OF TAGGING

The tagging was done as described by Mann (1962) by means of small coloured plastic plates (0.5 × 1.5 cm) which were anchored in the back of the fish under the beginning of the dorsal fin with perlon yarn. In the first tagging experiment with burbot the fish were narcotized by means of urethane and after tagging they were kept in fresh water until swimming in normal position. However, this method gave very poor results. Probably the fish had not fully recovered and die in the mud of the river bottom. The return was only 1 percent (4 out of 400) compared with 14.2 percent (172 out of 1 202) in the following series, when the tagging was done without applying narcotics. The pike and carp also were tagged without using narcotics.

The burbot used for tagging in the River Elbe were taken from the commercial fyke-net fishery. The pike and carp were undersized fish caught by anglers. All fish were released at the place they were caught.

4. TAGGING EXPERIMENT ON BURBOT IN THE RIVER ELBE

4.1 Migration

Burbot are caught in the lower parts of the River Elbe from October to January. Ninety-six percent of the burbot are sold at the Hamburg fishmarket during these months (October 12.3 percent, November 45.3 percent, December 34.3 percent, January 4.1 percent). This is the period of an upstream-directed spawning migration from the tidal area (between Hamburg and the brackishwater zone) to the upper regions of the river. In November-December 1957/58, 583 burbot were tagged in the Hamburg area at the time of upstream migration and 619 more burbot were tagged after the spawning period (January-February) near Schnakenburg in February and April 1958/59. The return from the first group was 92 (15.8 percent) and from the second group 80 (12.9 percent). Most of the returns from the spring tagging were from the following winter. The difference in the recaptures between autumn and spring taggings may indicate a few losses of tags from spring to next spawning season. Heavy losses of tags, however, must occur after about one year; only 6 of the 172 returns were from the second year after releasing and only one from the third year. For the time of the upstream-directed spawning migration, the average distance covered per day was 2.4 km (maximum 10 km). The spawning is in late January. By extrapolation beyond day and place of recapture we can fix a calculated spawning place and we find a spawning area from 90–190 km upstream of Hamburg which is upstream the Geesthacht Dam. Twenty-one tagged burbot were actually caught in the Havel, a secondary river to the Elbe in the German Democratic Republic. These recaptures were taken 240 km upstream of Hamburg.

4.2 Estimate of Stock Size

The only statistical data available are the landings at the Hamburg fishmarket. We may take these figures as representative of catches in the tidal zone from the brackish water up to Hamburg. Some fish may be sold locally but most of the fish are caught by boat fishing and at least these are being landed at the fishmarket. If the recaptures from this area correspond to the landings we may estimate the catches from upper regions from the percentage of recapture.

The total catch must be in the order of 11.7 tons (Table II). In the previous section the percentage of recaptures from burbot caught at the spawning migration was given as 15.8 percent which corresponds to a catch of about 11.7 tons so that the total population size of mature fish must have been about 70 tons (approximately 150 000 mature fish) in the tidal zone.

5. TAGGING EXPERIMENT ON PIKE IN THE RIVER TRAVE

5.1 Migration and Growth Estimate

Most of the pike were caught at the same place or very near to where they were tagged (Table III).

There is no significant difference in growth for the two regions of the river judging by data of recapture. From mid-October until mid-April the daily growth was calculated (n = 29) at 0.0056 cm/day (about 1 cm in total). The important months for growth are from spring to autumn with a daily growth rate of 0.0595 cm/day (n = 55) or 1 cm in about 17 days. The total growth in one year is 12 cm on average for pike between 35 and 55 cm.

5.2 Estimate of Stock Size

From 1957 to 1962/73 pike between 23 and 55 cm caught by anglers were tagged; 16.7 percent (2 out of 12) of the fish from 23–30 cm were recaptured and also 16.7 percent (6 out of 36) of the fish from 31–40 cm, but 26.4 percent (33 out of 125) of the bigger ones. This difference is explained mainly by the size of bait. For a calculation of the catch rate, an estimation of losses of tags is necessary (Table IV).

The sharp decrease in the beginning is not explained by tag losses only, but probably more by seasonal catch distributions (Table V).

The main catching months are followed by periods of low catch (or no catch in January to April). The low number of recaptures after more than 12 months is caused by tag losses. In the first month no losses of tags occurred. By means of the seasonal catch distribution the number of recaptures can be calculated (as can be expected) and compared with the number actually caught. We thus arrive at a factor of 2.1, by which the total return rate of tags has to be multiplied and we obtain 25.4 × 2.1 = 55 percent of pike annually. This rate of loss will include angling, cannibalism and migration out of the club's river of which angling will be the most important factor for fish near minimum size and bigger. An annual catch of 300 pike in the lower part of the river and 200 in the upper region would correspond to a stock of 500 and 370 pike bigger than 50 cm respectively. This would mean 1 pike/730 m² for the lower region of the river of 1/320 m² for the upper region. The high fishing rate results in a relative low average weight of 1 450 g and 1 650 g respectively.

6. TAGGING EXPERIMENTS ON CARP

6.1 Migration and Growth of Carp in the River Trave

It is obvious that carp need the still water arms of the River Trave for overwintering. From June to September about 60 percent of the carp are caught in the river and 40 percent in the still water arms. This relation changes to 0:100 in October/November and to 20:80 in March-May. Some of the fish, those not entering the still water arms, are drifted down- stream of the floodgates to the lower section of the river in winter. This results in a low but steady increase in the number of carp caught annually in the lower part.

No difference in growth between tagged and untagged carp could be detected for the river fish and also not in a pond experiment to be described later. The annual average growth for the river is given in Table VI.

6.2 Checking of Catch Statistics by Tagging Results

Pond experiment: In 1959 57 carp between 24 and 30 cm were tagged. Most of the tagged fish were recaught several times (Table VII).

The constant rate of recaptures shows that even a frequent catch does not cause mortality in the fish returned to the water. The total number of 198 fish released and 167 recaptures leads to a fishing efficiency of 84 percent. This figure fits well with the catch statistics over a 6-year period, when 82 percent (213 of 260 fish stocked) were caught (fish above 40 cm are taken for consumption).

River experiment: The upper section of the Trave was first stocked with mirror carp of 24–32 cm length in 1959;62 undersized carp caught in 1959–62 were tagged and 38 (61 percent) were caught again in the following years. Again the catch rate of fish frequently caught was constant. This means that no tagging mortality or losses of tags occur. Thirty-eight (61 percent) of 62 tagged carp were recaptured, 10 of them put back in the river (because of small size); 6 (60 percent) were caught a second time and 1 out of 2 (50 percent) a third time.

The catch rate of the 1959–62 tagged carp has been calculated as 61 percent. The cumulated catch rate by the club's statistics for a 10-year period amounts to 67 percent (minimum 58 percent after 3 years; maximum 73 percent after 7 years), which is in good agreement with the tagging experiment.

As already mentioned, some of the carp drift downstream to the lower section of the river. There the cumulated percentage of catch gradually increased from 2.4–5.1 percent of the cumulated number of fish stocked.