by/par
V. Steiner, R. Pechlaner
Limnology Division, Institute of Zoology, University of Innsbruck, 6020 Innsbruck
Austria/Autriche
ABSTRACT
The ecology of the arctic char has been studied in 20 Austrian lakes. Assessment of population size has been attempted in four of these using mark-recapture methods. Recapture results indicate substantial tag loss within the lakes and the factors introducing bias are analysed. Tagging is not thought to contribute to the mortality of fish over 130 mm in length, although it may affect growth. Assessment of populations made by this method have been used for management of the commercial fishery of the lowland lakes and the sport fisheries of high-mountain lakes.
RESUME
L'écologie de l'omble arctique a été étudiée dans 20 lacs autrichiens. On a tenté de déterminer l'ordre de grandeur de leur population dans quatre d'entre eux, par des méthodes de récupération des marques. Les résultats obtenus indiquent une perte considérable de marques dans les lacs; on analyse également d'autres causes de distorsion. Il n'apparaît pas que le marquage contribue à la mortalité des poissons dépassant 130 mm de longueur, encore qu'il puisse affecter leur croissance. L'évaluation des populations par cette méthode a été utilisée dans l'aménagement des pêcheries commerciales des lacs, les basses terres, ainsi que dans les pêcheries sportives des lacs de haute montagne.
CONTENTS
1. INTRODUCTION
2. MARKING TECHNIQUES
3. DESCRIPTION AND CRITICAL EVALUATION OF SPECIFIC POPULATION ESTIMATES
3.1 Lowland Lakes
3.2 High-mountain Lakes
4. VALUE OF STOCK ASSESSMENT FOR IMPROVING FISHERY
4.1 Grundlsee and Other Lowland Lakes
4.2 High-mountain Lakes
REFERENCES
Landlocked arctic char (Salvelinus alpinus (L.)) represent a main component of the fish stocks of many cold, deep lakes in the mountain regions of Austria. Char, together with coregonids, are the only native fish having an appreciable importance for the commercial lake fisheries in the country.
It is assumed that arctic char immigrated actively into most of the glacial lakes in the lowlands both at the edges and in the centre of the Alps (Reisinger, 1953) but was planted almost exclusively from such lakes into waters above timber line by man. For standing waters in the Tyrolean mountains such planting activity is well documented for the 15th and 16th century (Diem, 1964).
There exists a rich literature on landlocked char, focusing especially on taxonomical problems at the species and ecotype level and on the ecological background of both morphological and behavioural differentiation.
Economic aspects of char as a resource for both commercial and recreational fishery are considered by some of the authors dealing with the “char problem”, but as yet the scientific basis for organizing and optimizing such fisheries is far from satisfactory. In the research programme of the Limnology Division at the University of Innsbruck char from 20 lakes are being more or less intensively studied. Assessment of stocks by mark-recapture methods has been tried in a few of these water bodies and the methods and results of such experiments are reported and critically evaluated below.
A tagging gun FD-67 and anchor tags FD-67F with “flags” (Floy Tag and Manufacturing, Seattle, Wash., USA) were used for marking. This equipment enabled quick application of the tags, introduced by an injection needle through the hypaxonic muscles beneath the dorsal fin, where each tag automatically became fixed by its elastic anchor of plastic monofilament, the “flag” being connected with the anchor by elastic monofilament, 50 mm long and 0.5 mm thick. In all cases reported here, the flags were numbered, thus enabling individual tagging. As a rule, the fish were narcotized with MS 222 (Sandoz, Switzerland) for tagging and measured for total length and weight, and scales were also taken. Narcotization was not necessary for the Vorderer Finstertaler See char (Schwarzreuter) form which are relatively apathic even in good health, and fish were simply rolled in moistened toilet paper during the tagging and measuring procedure. After tagging the fish were held under control for some time prior to release.
3.1 Lowland Lakes
Grundlsee (Styria) and Piburger See (Tyrol) are surrounded by high mountains, but are situated in the montane zone of terrestrial vegetation. Some morphometric parameters of these lakes are given in Table I.
Grundlsee: Char forms the main fish stock of Grundlsee. Other fish present comprise Salmo trutta f. fario L. and S. trutta f. lacustris L., Coregonus sp., Esox lucius L., Lota lota (L.), Louciscus cephalus (L.), Alburnus alburnus (L.) and Phoxinus phoxinus (L.)
The char population is one of the most homogenous concerning form, size, pigmentation and behaviour known from Austrian lakes “normal” form, average total length 29 to 30 cm, average weight 200 to 250 g. The yield of the commercial fishery has been recorded since 1919 (Table II) and other activities connected with the preservation of the stock are also carefully documented since that time.
Grundlsee char are caught by trawl net in large quantities during the peak of their spawning activity in November. Having marked 23 mature char as a methodological test in autumn 1969, we randomly marked 977 char out of a catch of 4 000 on 8 November 1970. After tagging all marked fish were kept under control in large holding tanks for two days. No mortality and no loss of tags were observed in this period.
Recapture was done one year later, on 22 November 1971, at the same place and under
the same fishing conditions. 73 marked individuals were picked out of a total sample of
approximately 4 000 char. 16 of these marked fish lost their tags before we could identify
them. Further 66 “empty” tags had been found throughout the year before, mainly hanging
in fishing nets. (See Table III). The population estimate derived from the data in
Table III gives: 
= 49.918 ± 5.603 (Robson and Regier, 1971). This figure is valid only
for the size classes represented in the catch (total length between 15.5 and 32 cm); the
length frequencies are given in Figure 1.
The relationship between length classes and age groups has not yet been attempted. In a critical evaluation of this experiment the following conditions for a valid use of this method were met:
There was a homogenous population of the species studied.
The locality, equipment and effort for catch and recapture were comparable.
There was a random distribution of marked and recaptured fish in the sample.
The following sources of error were present:
The small number of marked and recaptured fish.
No control marking to detect tag losses in the lake.
Only a selection of size classes is covered by the estimate.
However, the main purpose of the tagging in Grundlsee was to study the ecology and especially the growth rate of this char.
Piburger See:
The Grundlsee is a typical char-lake whereas the Piburger See is only an experimental lake for char. The main fish stock consists of Cyprinids (Rutilus rutilus (L.), Scardinius erythrophthalmus (L.) and Leuciscus cephalus (L.), perch (Perca fluviatilis L.), brown trout (Salmo trutta f. fario and f. lacustris L.) and stocked rainbow trout (Salmo gairdneri Rich.). The lake is meromictic (monimolimnion approximately below 19 m), with a sharp thermocline in summer in about 5 m of depth (Pechlaner, 1968). Eutrophication almost caused a fish kill under ice in the winter 1969/70, but meanwhile the lake has been restored by an ‘Olszewski-tube’ (Pechlaner, 1971). At present the Piburger See is intensively studied within the Lake Eutrophication Programme of OECD, and as one of Austria's contributions to ‘Man and Biosphere’. Marking experiments with char were done to learn about the development of this species in a rather eutrophic, well documented milieu. Some observations nevertheless might be useful for discussing the methodology of mark-recapture experiments. In January 1972, 757 artificially reared two-year - old chars (mainly from the Grundlsee population) were marked with FD-67 flag-tags. After tagging, the fish were kept under observation in a circular rearing tank containing approximately 1 000 litres of water. Within 16 days 135 chars had lost their tags, most of them shortly after tagging. This tag loss (18%) must be judged in association with the artificial rearing conditions which influence both the histological properties of muscle and skin and the behaviour of the crowded fish, and might intensify tag loss in several ways. Specimens which had lost their tag were stocked without repeating the tagging, but were discernible as belonging to this batch. The char were released on 21 January 1972. Recapture were from all seasons between November 1972 and January 1974.
The results given in Table IV were surprising: we would have expected to find an increase in the proportion of marked fish in our recaptures, since catching was done mainly by gill nets, which should bring a positive bias toward individuals with flag tags. The actual decrease seems to indicate that tag loss continued in the lake at a substantial rate.
The “high-mountain” area is synonymous with the “alpine” horizon of plant sociologists, i.e., the region above the timber line of mountains all over the world. The two lakes, which are considered here, lie in Tyrol, in the Central Alps. Morphological data are given in Table V.
Vorderer Finstertaler See: The Vorderer Finstertaler See is the lower and larger of two neighbouring lakes, drained by the same river. It was - and still is - studied as an IBP site. Since 1968 limnological research included as much as possible all important aspects of both the physical environment and the food web of this relatively simple ecosystem (Pechlaner et al., 1972 a, b).
Salvelinus alpinus (L.) is the dominant fish in Vorderer Finstertaler See, both by numbers and biomass. It lives there sympatrically with brown trout, Salmo trutta f. fario L.; a third species, the minnow Phoxinus phoxinus L., is very rare.
Population estimates were made by the mark-recapture method. From July 1968 until July 1971 approximately 500 char had been tagged individually. At the beginning of July 1971 the number of marked char in Vorderer Finstertaler See was 419 (released marked fish minus earlier recaptured ones). In this month 29 tagged fish were recaptured, giving a population estimate of 6 315 ± 1 111 individuals.
The actual confidence limits are certainly larger than the standard error given above. We cannot quantify the various sources of error but can give a general idea of potential bias:
Positive bias due to artificial mortality:
Catching: Gill nets largely had to be used for catching fish, but they were checked rather frequently to minimize the harm to the fish. Char from more than 15 m of depth were caged for several hours in gradually reduced depth of water to enable adaptation to surface pressure. Only fish with all the signs of good health were tagged, nevertheless some marked fish died in the days prior to release. Artificial mortality from catching injuries therefore cannot be ruled out entirely.
Tagging: From all our experience the tagging procedure does not cause mortality in the size classes above approximately 130 mm. Some effect of carrying a tag on the growth of the carrier is to be expected. Our material will permit us to trace and to quantify such growth reductions, but the data are not processed as yet.
A more serious effect towards the positive bias mentioned above may result from intensified predation of brown trout on char carrying our dark blue tags (and even more on fish with the red tags used in Hinterer Finstertaler See).
We have found several tags in trout stomachs, but these few observations do not tell us more than that trout of a certain size do prey on char.
Positive bias due to tag loss:
Rather improbable; never observed in holding tanks with char of this lake, and little chance of getting caught at the tag in a lake with stony substrate only.
Negative bias from increased catchability of marked fish in gill nets:
This effect certainly exists in those fish marked with FD-67 tags with normal flag, since this flag (5 × 10 mm) is fixed at the distal end of the tag like the bar on a “T”. Only the first third of the char marked got such “double- flags”, then half of the T-bar was cut off, thus minimizing the probability of it getting caught in gill nets due to the tag.
Finally there exists selectivity of fishing with gill nets and hook and line on both marked and unmarked fish, thus making the population estimate valid for the preferably caught fraction of the stock only. In our catches there is a clear peak in the frequency curve at 160 mm of total length (5 and 6 years of age). Pechlaner (Pechlaner et al., 1972b) has tried to fill this gap by back- calculating a probable age pyramid under the assumption of 10 percent mortality per year in char 2, 3 and 4 years old. On this basis a stock of 10 000 (106 kg) of char of age 2 years or older was calculated for Vorderer Finstertaler See. This would be 640 individuals (or 6.8 kg) of these age-groups per hectare. Though these figures are only approximate, we found satisfactory agreement between the food demand of salmonids calculated on this basis, and the availability of both autochthonous and allochthonous food in Vorderer Finstertaler See (Pechlaner et al., in preparation).
Mölser See: Mölser See is one of the warmer high-mountain lakes, sometimes even showing the thermal features of a pond. In our period of observation char of the “Schwarzreuter” form were living allopatrically in this lake.
Rough assessment of the stock by fishing with hook and line and by direct observation in 1952 had led to the assumption that not more than 100 to 150 adult char would live in this lake (Gutmann, 1962). In 1969 we caught 300 individuals for experiments on temperature tolerance and other research. Twenty individuals of these were marked by yellow Floy tags FD-67F in order to get a more realistic figure of the number of catchable fish. Recapture was done one year later, again by the same gill nets as in 1969 (mesh sizes from 15 to 20 mm), and was performed in the same area. Four marked fish were in the second sample of 300 char. The estimate of the population, derived from these figures, gives 1 500 ± 666 individuals in the marked size classes (180 to 200 mm of total length).
The small number of marked fish certainly does not fulfil the conditions for a valid assessment, but the aim of this experiment was only a rough estimate. On the basis of area, the figure for catchable fish of Mölser See is more than four times higher than the estimate for Vorderer Finstertaler See. Nevertheless we do not believe to have essentially over- estimated the stock of Mölser See, since the catch per effort was not reduced when the second group of 300 char was caught. The higher population density in Mölser See might be explained by the fact that this lake is considerably warmer than the Vorderer Finstertaler See, and the char in the smaller lake live allopatrically whereas in Vorderer Finstertaler See they have to compete with brown trout.
The present fishery situation in most of our lowland lakes demands fundamental improvement. Only few lakes seem to have stable fish stocks and satisfying yields, due to a well organized and careful fishery. We believe that even in these lakes a better knowledge of the fish present, their population dynamics, diseases etc. would give a better basis for regulations and could raise the yields.
The char stock of Grundlsee was known as the best of Austrian lowland lakes until a few years ago. Since 1967 a sudden decrease in the length and weight of char has been observed in the regular catches. In autumn 1973 the size of Grundlsee char reached the lower limit for successful marketing (Figure 2).
From 1909 until 1965 there was little change in the size and weight of Grundlsee char. The average weight in this period fluctuated between 270 and 330 grammes. Length measurements are not documented, but the average length must have been between 330 and 350 millimeters. At the present point of decline the Grundlsee char have an average length of 270 to 290 mm, at a weight of about 180 grammes.
In many Austrian lakes the fisheries are declining due to eutrophication, but there is no indication that in Grundlsee consequences of the enrichment of the lake with plant nutrients are involved. The native human population of the catchment area has decreased in the last 13 years rather than increased, and tourism has expanded only very moderately. The few data available on Secchi disk transparency and oxygen stratification do not show an increase of the trophic state of this lake and algal blooms have not been observed.
A decrease in the standing crop of zooplankton is, however, evident. Daily catches of zooplankton for fish feeding in a hatchery showed a substantial decline in the availability of this food in the last years. In the same years unusual low water temperatures have been observed, and this might be one reason of a reduced production rate of both phytoplankton and zooplankton. Road construction work on the north shore of Grundlsee has brought considerable quantities of sedimenting material into the lake over the last three years. In Mondsee a similar situation has directly and indirectly reduced the food available to char and other fish (Einsele, 1963); the decrease in zooplankton of Grundlsee might be explained by a similar process. Oil pollution by ships is certainly negligible, but the detrimental effects of a gypsum factory existing on this lake for 10 years cannot be ruled out.
Furthermore changes in the sympatric fish population might play a role; pike appeared in those years when the decline of the char began. Competition for prey fish suitable for bigger char could be a factor reducing the average size attained by this species. Man also competes for forage fish; approximately 20 000 specimens of the minnow Phoxinus phoxinus (L.) are removed annually from this lake to be sold as bait.
A third competitor, in the form of several thousand hatchery-raised coregonids, 10 to 20 cm long, have been planted into Grundlsee two and a half years ago. Prior to this, large numbers of eggs and fry of Coregonus had been planted without success, but since 1973 the planted fingerlings show up as a relatively high proportion of the catches. Coregonids certainly do compete with char for invertebrate food. In the nearby Hallstätter See (508 m a.s.l., 8.58 km2, 152 m deep) the opposite population changes have occurred. Char were negligible in this lake until 1958, but then became important, largely parallel to a decline of Coregonus catches (Table II, Figure 3).
It is not probable that fishing practices had a negative effect on the average size of the char of Grundlsee. Only gill nets with mesh size of 36 to 38 mm have been used for many years and catches with these nets have become very small recently. The equipment for seining in the spawning areas remained the same and most of the small fish caught by seine were released immediately into the lake. From spawners eggs and roe were collected for hatchery purposes; most of the spent fish being sold (representing the bulk of the annual catch given in Table I) and the rest being released alive, although perhaps not in good health.
There is no evidence of disease in freshly caught char, but high mortalities occur in artificially-bred eggs and fry, the reason of these losses being still unknown.
The tagging programme for improving the assessment of both stock and growth of char under field conditions is a major component of our research, both as a main tool to judge the response of this salmonid to environmental conditions and to plan successful management of the char fishery.
There are only a few lakes above the timber line in the Austrian Alps, which provide good recreational fishing. In many of the elevated lakes there are no fish, but in most of those that have been stocked sport fishermen catch char and brown trout less than 200 mm long which are below the preferred.
From our population estimates, from production studies at all levels of the food chain of alpine lakes, and from transplantations of char into richer environments, we know that most salmonid stocks in high-mountain lakes are overcrowded and could yield good sport fishing if the carrying capacity would be used by a smaller number of fish. This knowledge is the prerequisite of rational management toward improved utilization of the recreational potential of such lakes.
| Grundlsee | Piburger See | |
| Elevation a.s.l.(m) | 709 | 915 |
| Area (km2) | 4.14 | 0.134 |
| Length (m) | 5 800 | 800 |
| Breadth (m) | 900 | 250 |
| Maximum depth (m) | 63.8 | 24.7 |
| Grundlsee | Hallstätter See | ||||
| S. alpinus | Total | S. alpinus | Coregonus | Total | |
| 1910 | 3.4 | 4.95 | . | 2.6 | 3.3 |
| 1915 | 5.5 | 8.62 | . | 2.9 | 3.5 |
| 1920 | 1.3 | 3.47 | . | N | N |
| 1925 | 3.1 | 4.65 | . | N | N |
| 1930 | 3.3 | 4.11 | . | 7.9 | 8.7 |
| 1935 | 2.9 | 3.8 | . | 4.6 | 5.3 |
| 1940 | 4.2 | 7.7 | . | N | N |
| 1945 | 4.9 | 5.5 | . | 6.3 | 7.3 |
| 1950 | 4.3 | 5.5 | . | 9.3 | 10.1 |
| 1955 | 2.9 | 4.2 | . | 0.6 | 1.6 |
| 1960 | 6.8 | 8.8 | 1.6 | 0.7 | 2.9 |
| 1965 | 3.7 | 4.8 | 1.0 | 2.6 | 5.2 |
| 1970 | 4.1 | 5.5 | 1.1 | 4.3 | 6.7 |
| year | size of catch | marked | recaptured | loss of tags (known) |
| 1969 | ca 4 000 | 23 | - | - |
| 1970 | ca 4 000 | 977 | - | - |
| 1971 | ca 4 000 | - | 73 | 66.16 |
| 1972 | ca 4 000 | - | 2 | - |
| 1973 | ca 4 000 | - | 0 | - |
| marked | without tags | proportion of marked (%) | |
| Released January 1972 | 622 | 135 | 82.2 |
| Recaptured up to January 1974 | 47 | 26 | 64.4 |
| Vorderer Finstertaler See | Mölser See | |
| Elevation a.s.l. (m) | 2 237 | 2 338 |
| Area (m2) | 157 000 | 8 430 |
| Length (m) | 597 | 144 |
| Breadth (m) | 380 | 65 |
| Maximum depth (m) | 28.5 | 6.8 |
Figure 1 Size-distribution of the marked char
Figure 2 Average weight of Grundlsee char caught from 1910 to 1973
Figure 3 Average weight of Hallstätter See char from 1910 to 1972
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