|WI - Cormorant Research Group||Meetings||last updated on 13/09/01|
Abstracts of poster and oral
presentations on cormorants
International Symposium on
Interaction between fish and birds:
implications for management
University of Hull, England
3 - 6 April 2001
Relations between fisheries and cormorants (Phalacrocorax carbo) for the exploitation of the fish community in a temperate lake during ten years: are they competing? (POSTER)
CARPENTJER, A., PAILLISSON, J.M. & MARION, L. Université de Rennes 1, Laboratoire dévolution des systèmes naturels modifìés, UMR Ecobio 6553, 35042 Rennes Cedex. France
The establishment of a cormorant colony in the Lake Grand-Lieu twenty years ago has caused conflict with fishery interests due to a rapid increase of the number of breeding pairs (from 2 in 1 98 1 to 600 in 2000). The fish community studied in 199 1-92 and 1999-2000 is characterized by an increase of cyprinids (32% to 64 %) mainly due to bream (20 % to 40% in biomass). Analysis of the database of fisheries revealed that catches are focused on eel (79% in biomass), whereas this species is few consumed by cormorants. Conversely, diet of cormorants is dominated by cyprinids (77%) which are represented a tiny proportion of fishery catches. Competing could occur on pike with 14% in the cormorants diet and 5.4% in the fishery catches (2nd most caught fish). We discuss on these data and show how these two components of this ecosystem could be in interaction with fish community.
The relationship between cormorant and fish populations at a fishery system in England: an overview
DAVJES J.M. School for Professional and Continuing Education, University of Birmingham, Birmingham B1522T; WJLSON B.R., HOLDEN T., FELTHAM M.J. School of Biological and Earth Sciences, Liverpool JMU, Byrom Street, Liverpool, L3 3ÄF; BRFPTON J.R., HARVEY J.P. &. COWX J.G. University of Hull International Fisheries Institute, Hull
This paper provides a synthesis of work presented at this conference from a three-year study of cormorants (Phalacrocorax carbo) at a cluster of fisheries in Nottingham, England (see Britton et al., Holden et al.). Taking an overview, the paper will examine the extent to which cormorants have been responsible for changes in fish populations, relative to other factors. The findings of this study, like many others that have estimated the impact of fish-eating birds, are equivocal. The reasons for this are (i) the complex nature of these systems (ii) difficulties in relating cormorant depredation to changes in fishery performance and (iii) separating the effects of piscivorous birds from other factors affecting fish populations. This paper will conclude by considering ways in which future research should investigate the relationship between fish-eating birds and fisheries, and the importance of adopting a holistic approach to such studies.
A theoretical assessment of cormorant impact on the fish stocks of England and Wales
DIAMOND M., APRAHAMJAN M.W. & NORTH R. Environment Agency, North West Region, Richard Fatrclough House, Knutsford Road, Warrington, WA4 IHG, UK
A simple empirical model was developed to estimate the impact of cormorant predation on the stock of fish within England and Wales. The study used GJS data to estimate the surface areas of waters, published estimates of standing stock, fish production and cormorant consumption rates. These data were used to determine the weight of fish eaten by cormorants on a national basis in relation to the total annual production of fish. Impact was assessed in terms of the amount of fish flesh produced annually and consumed as a proportion of the standing stock of fish present. Impacts were also assessed on a local scale m order to provide guidelines as to whether the number of days cormorants were feeding on a particular still water were having an impact.
Do cormorants and fishermen compete for fish resources in Estonian coastal areas?
ESCHBAUM R. & VEBER T. Institute of Zoology and Hydrobiology, University of Tartu, Tartu and Estonian Marine Institute, Tallinn, Estonia
The first colony of Phalacrocorax carbo in Estonia was established in the Väinameri (West-Estonian Archipelago) in 1984. The number of breeding pairs in area has reached 3500 and is still increasing. Simultaneously, the intensity of coastal fishery increased in early 1990s, which has lead to overfishing of the most important stocks and conflicts between fishery and fish-eating animals (cormorants and seals). The calculated consumption of fish in Väinameri by cormorants was 51 t in 1998 and 669 t in 1999; commercial catch in the Väinameri amounted to l 242 t and 676 t, respectively. To analyze proportion of fish species in cormorant diet samples were taken from April to august. The most important fish in cormorant diet was viviparous blenny with 203,7 tons in 1998. The ratio cormorants/fishermen in removal of the most valuable species (by weight) is 1,06 for perch, 0,53 for pike, 6,99 for pikeperch, 40,6 for burbot, 3,63 for smelt, 0,79 for eel, etc. However, cormorants prefer smaller specimens and the number of fish eaten by cormorants is even more important as compared to the fishery.
Impact assessment of Great Cormorant (Phalacrocorax carbo sinensis Blumenbach) predation on the population of roach (Rutilus rutilus L.) in the river Meuse, Belgium (Poster)
EVRARD G. Facultés Universitaires Notre-Ðame de la Paix, Laboratory of Freshwater Ecology; URBQ, 61 rue de Bruxelles, B-5000 Namw; Belgium
Since few years, the wintering population of Great Cormorant reach 3000 individuals who are distributed on 5 dormitories along the river Meuse. Therefore, it appears a situation of contact between piscivorous birds and sport fishing interests.
Our fundamental study will assess impact on Roach population between 2 comparative areas, with and without (or less) predation. Roach is common species in the river and also well appreciate by anglers. Moreover, estimations of density and biomass are practicable. Density of 4300 ind./ha and biomass of 230 kg/ha have been recently evaluated. Our aims are to determine fishing pressure of Cormorants (number of individual/day on study area) by evaluation of range limits round dormitory site and importance of dispersal axis. And in the same time, an analysis of regurgitation pellets and an estimation of population dynamics of Roach give us characteristics and changes of daily food ration.
Managing a Balance Between Double-crested Cormorant Numbers and Warmwater Fish Abundance in the Eastern Basin of Lake Ontario, New York; Preliminary lnsights from a Management Program.
FARQUIIAR J.F., 111, MCCU1LOUGH R.D. & SCHIAVONE A.S. New York State Department of Environmental Conservation, Watertown, NY 1360
Since Double-crested Cormorants (Phalacrocorax auritus) began nesting on the U.S side of the eastern basin of Lake Ontario in 1974, populations have increased from 22 pairs to over 5,000 pairs. By the early 1990s, complaints from local recreational fishing interests related to cormorant impacts to sportfish began to escalate. In response, cormorant diet studies were initiated in 1992, and continue presently. A series of studies conducted by the New York State Department of Environmental Conservation(NYSDEC) and the United States Geological Survey(USGS) in 1998, established a link between cormorant predation and reduced abundance of smallmouth bass, an important sportfish and keystone predator in the lake ecosystem. Based on these studies, the NYSDEC determined to manage cormorant abundance for the social benefit of a viable fishery, and for the ecological integrity of the fish community.
In 1999, a cormorant population reduction program was implemented in the eastern basin of Lake Ontario. On Little Galloo Island, the largest cormorant colony in the basin, reproductive suppression via egg oiling in 1999 and 2000 has resulted in an estimated 98% reduction in reproductive success. Estimated smallmouth bass consumption has fallen more than 28% relative to a no management condition, largely as a result of fewer overall cormorant feeding days.
Cormorant management efforts on Lake Ontario suggest that cost effective techniques are available to substantially reduce problems on a local level. While several years of effort will be required to achieve the desired balance between cormorant numbers, recreational fishery benefit and fish community objectives, initial results from two years of management are encouraging.
Possible effects of double-crested cormorants and largemouth bass predation on crappie in an Arkansas oxbow lake
FENECH A.S., LOCHMANN S., WOOTEN D., HOY M. & RADOMSKI A. Aquaculture and Fisheries Center, University of Arkansas at Pine Bluff 1200 N. University Drive, MaiJ Slot 4912, Pine Btuff Arkansas 71601, USÄ
Mortality rates of age-0 to age- 1 crappie (Pomoxis sp.) in Lake Chicot, an oxbow lake of the Mississippi River have been estimated to be approximately 90% in recent years. Increasing numbers of Double-crested cormorants (DCCOs)(Phalacrocorax auritus) feeding on Lake Chicot are suspected by many anglers to be the cause of high mortality rates in sportfish populations. However, Arkansas Game and I5sh Commission report an increase in the density of largemouth bass (Micropterus salmoides) which may be having an affect on crappie survival. Collections of largemouth bass and DCCOs will be taken seasonally with an additional collection of largemouth bass taken in the summer months. The diet and potential impact of largemouth bass and wintering DCCOs will be investigated over a one-year period. We will attempt to use bioenergetic models to determine which of these factors is having a greater impact on the mortality of young sportfish in Lake Chicot.
Fish and shrimp biomass consumption by Olivaceus cormorant population in Los Olivitos estuary,Venezuela
GIL DE WEIR K. & WEIR F.H. La Universidad del Zulia, Venezuela
Study of abundance and diet of the Olivaceus cormorant (Phalacrocorax olivaceus) was undertaken from August 1998 to July 1999, in the Wildlife Refuge and Fishing Reserve, Los Olivitos Estuary, Zulia State, Venezuela. Total cormorant abundance was estimated by direct counts at their resting colony and diet composition was determined via stomach content analysis (ten birds/month) and through fish otoliths identification in sixty regurgitated pellets/month. Monthly changes in abundance showed that the cormorant population is migratory and maximum abundance reached 1 7,000 individuals in July 1999. Diet analysis showed variation in the composition and proportion. The biomass consumed by cormorant increases more than double before migration takes place. Main items are species of catfish and herring. We have discussed the proportion of the impact on the fishery production in the Los Olivitos estuary, but the results indicated that they eat species which are in high proportion in aquatic habitat.
Piscivorous bird licenses in England
GILL E., BUTT P. & BAKER S. Farming & Rural Conservation Agency (Ministry of Agriculture, Fisheries & Food) Block C, 98, Epsom Road, Guildford, Surrey, GU] 2LD. UK
Licenses to take a limited number of piscivorous birds on stillwaters and rivers to reinforce scaring are issued annually by MAFF. The poster will summarize the licensing procedure and present information gathered during monitoring and from carcass analysis. 88 licenses were issued in 1999-2000, 71 of which were for cormorants; 236 birds were shot. Licenses are monitored throughout the license period (normally September to March) and carcasses of shot birds are retrieved. Birds are usually shot early in the morning. Most of the shot cormorants are juvenile males, and gut contents generally reflect the species composition of the area where a bird was shot; marine species are rarely found. Amphibian and mammalian remains have been identified in herons. Most licensees regard combining licensed shooting with non-lethal measures as the most successful approach to protecting fisheries.
Do cormorants influence the parasitic fauna of fish in Vistula Lagoon?
GOMULKA P., WLASOW T., MARTYNIAK A., KOZLOWSKJ J. Faculty of Environment Protection and Fisheries, Olsztyn University, Poland
Cormorant (Phalacrocorax carbo sinensis) has been taken under species protection in Poland in 1952. Since that, cormorant populations show continuous increase of growth. The cormorant colony in Katy Rybackie exists since XIX century. Between 1 959 and 1 997 number of specimens fluctuated from 1 1 7 to 5 1 92 couples. In 1995-97 research on the influence of cormorant on Vistula Lagoon (VL) ichthyofauna was carried out. The research included health inspection of the fish vomited by cormorants and fish netted in VL waters. No significant statistical differences in the structure and intensity of parasitic infestation between both examined groups were noticed. However, conducted research showed that the ruffe is the main paratenic host of the nematode Anguillicola crassus. This nematode is a dangerous parasite of the eel Anguilla anguilla and because of its common occurrence is a serious problem for fisheries in Poland. The prevalence of A. crassus infestation in ruffe was 50,0% and the mean intensity 2,6 larvae per fish (SD 2,71). The ruffe turned out to be the absolutely dominant species in cormorant diet and his weight rate was 58,0%, 70,5% and 75,5% respectively in1995, 1996 and 1997, 1997). Cormorants consumed on average 538,3 t of fish during nesting period. The results suggest that cormorant doesnt play a sanitary role in VL ecosystem. However, through the mass-elimination of ruffe, cormorant contributes to decreasing of the pool of invasive Ä. crassus larvae.
Intestinal parasites of cormorants
KIRK R.S. School of Life
Sciences, Kingston University, Penrhyn Road, Kingston upon
Thames, Surrey KTI 2EE, UK
LEWIS J.W. School of Biological Sciences, Royal Holloway, University of London, Egham, Surrey UK
In the UK, cormorants are protected under the EU Birds Directive and the Wildlife & Countryside Act (1 98 1). They are widely perceived, however, to significantly reduce and damage exploitable stocks of fish. Concern has increased due to the rising number of birds overwintering at inland sites. Parasites are an important part of the ecological interaction between predators and prey, but this relationship has not been fully researched in relation to piscivorous birds. This paper presents the results from a preliminary parasite survey of two races of cormorants, Phalacrocorax carbo carbo and P. carbo sinensis, and forms part of a multi-disciplinary analysis of cormorant carcasses shot under MAFF license. Most birds (74%) were infected. Parasites recovered from the intestine included the nematodes Contracaecum rudolphii, Camallanus lacustris, Syncuaria sp. and the tapeworm Schistocephalus solidus. The importance of parasite surveys in impact assessment studies and knowledge of avian biology is discussed.
The impact of black cormorant on the eel fishery in the catchment area of the river Havel/Germany
KNOSCHF R. Institute of lnland Fisheries, Jaegerhof Gross Glienicke, D-4476, Germany
The percentage of the eel in the feed ration of the black cormorant is controversially discussed between ornithologists and fishermen. The river Havel drains off a total lake area of nearly 70,000 ha. Having available long-term data about eel stocking intensity, eel yield and CPUF from a 32-m stow net in the river Elbe below the river Havel mouth we analyzed the influence of different factors (e.g. water-drain, stocking rate and the number of cormorants) by multiple regression. Roughly 28 % of the variability of the eel yield can be explained by the number of cormorants. It means that the portion of eel in the cormorants food amounts to about 19 %. We analyzed the stomach content of 58 cormorants, and found 42 % eel in midsummer and 4,5 % in winter. From it follows a weighed mean of 16 % eel in the food of the cormorant.
Pilot trials to assess the efficacy of fish refuges in reducing the impact of cormorants on inland fisheries.
McKAY H.C., Central Science
Laboratory, Sand Hutton, York Y04 JLZ, UK
RUSSELL I.C. & REHFISH M.M. Lowestofi Laboratory, Pakefield Road, Lowestofi, Suffolk NR33 OHT, UK
Fish refuges are artificial structures introduced into a water body with the aim of providing refuge to fish from avian predators. In a series of pilot trials in southern England, cylinders of plastic mesh were placed into small drainable ponds stocked with three size classes of carp (Cyprinus carpio). Observations were made of the number and behaviour of cormorants visiting the site and ponds were drained at the end of the trial periods to assess numbers of fish remaining and levels of wounding. Comparisons between trials with and without refuges, and between ponds with and without cormorants (netted over) were made. A 60% difference in mortality between netted and un-netted ponds was assumed to be the result of predation by cormorants. Cormorant dive durations were significantly longer, and the percentage of large fish with wounds was significantly lower, when refuges were present. These results suggest refuges may reduce the availability of carp (particularly of larger size classes, >28cm) to cormorants.
Experiences of managing the impact of cormorants on large recreational trout fisheries
MOORE D. Recreation Development manager, Anglian Water, UK
Rutland Water and Grafham Water are two of the best known still-water trout fisheries in Europe. Cormorant populations of 1200 and 600 individuals respectively have been recorded in the vicinity of these reservoirs and recovery by anglers of stocked trout felt to unacceptable proportions. Management action, including increasing the size of trout stocked, varying the timing of stocking and other actions without direct action on the cormorants was believed to be effective in increasing the recovery of stocked fish by anglers and reducing the proportion of damaged fish. Other factors may have been the depletion of easily caught coarse fish prey by the birds and this may have been a factor in reducing the local cormorant population. Anglers were involved in collecting extensive data and the strategy brought benefits to them which compensated for increased costs of stocking larger fish.
The potential use of fish refuges to reduce damage to inland fisheries by cormorants - a review and preliminary results.
RUSSELL I.C., DARE P.J. CEFÄS,
Lowestoft Laboratory, Pakefield Road, Lowestoft, Suffolk NR33
McKAY H.V. Central Science Laboratory, Sand Hutton, York Y04 JLZ, UK
IVES S.J. CEFÄS, Lowestoft Laboratory, Pakefield Road, Lowestoft, Suffolk UK
Recent research in the UK has concluded that cormorants can have a major impact on some freshwater fisheries. There is thus a need for effective management measures that will reduce the interaction between these predators and their prey and reduce the level of impact at such sites. A recent comprehensive review of possible management measures indicated that underwater refuges could affect cormorant foraging behaviour and reduce levels of damage to fish. It also recommended further studies to test the effects of refuge design on the behaviour of different freshwater fish and the number and behaviour of cormorants visiting a site. This paper reports on an ongoing investigation to assess the efficacy, practicality and
Cormorant predation, commercial fishery and fish stocks in the Vistula Lagoon, N Poland.
STEMPMEWICZ L., Department of
Vertebrate Ecology & Zoology, University of Gdansk, Legionów
9, 80-441 Gdansk, Poland
MARTYNIAK A. Olsztyn University of Agriculture and Technology, Oczapowskiego 5, 10-957 Olsztyn, Poland
BOROWSKI W. Sea Fishery Institute, Kollqtaja 1, 81-322 Gdynia, Poland
GOC D.M. Department of Vertebrate Ecology & Zoology, University of Gdansk, Gdansk, Poland
Fish community, and Cormorant Phalacrocorax carbo breeding and feeding ecology were studied in the Vistula Lagoon in 1995-1997. Most common fish species constituted the bulk of preys. Ruffe Gymnochephalus cernuus was preferred (22-29% in sample catches, and 58.0-75.5% in diet, by numbers), followed by Roach Rutilus rutilus (8.2-25.5% and 5.3-12.1%), Perch Perca fluviatilis (12.6-13.8% and 3.5-6.9%) and Herring Clupea harengus (0.9-25.5% and 1.2-7.7%, respectively). During one season 2200-3100 t. of small fish was removed from the lagoon (by catch: ca 1484-2267 t and cormorants: ca 712-816 t). During three years proportions of main fish species in sample catches did not change significantly. Proportions of Ruffe in the Cormorant diet increased from 58.0% in 1995 to 75.5% in 1997. Cormorants selected smaller fish than available, i.e. caught as a by catch. Commercial catches decreased considerably during last 25 years, except of Herring. Pike Esox lucius disappeared from catches in late 1 970s. Due to its small size and species composition fish taken by Cormorants has negligible economic value.
The impact of cormorants 011 extensive fish culture and guidelines for a possible management of cormorant populations ¡n Flanders (Belgium)
VERREYCKÆN H. Provincial
Fisheries Committee of Flemish Brabant, Belgium
DEVOS K. Institute for Nature Conservation, Belgium
BELPAIRE C. Institute for Forestry and Game Management, Belgium
The cormorant Phalacrocorax carbo sinensis population in Flanders (Belgium) has increased exponentially during the last decade. The cormorant population now consists of 900 nesting birds and of a hibernating group between 2000 and 2500 birds. The total (ecological and economical) impact of this large number of fish eating birds on a small area like Flanders (with only 257 km2 water surface) is yet unclear and needs urgent investigation. For the extensive fish culture in Flanders (mostly cyprinids) losses in yield are estimated to be almost 50 % and economical losses are even higher as investments have to be made to prevent further damage. In the framework of an international "Action Plan for the Management of the Great Cormorant in the African-Eurasian region" (1 997), an advice with guidelines for a possible management of the cormorant population in Flanders was handed over to the proper environmental administration. Emphasis was laid on financial support and/or compensation for fish culturists rather than the reduction of the cormorant population.
Management of the cormorant (Phalacrocorax carbo) and endangered whitefish (Coregonus lavaretus) populations of Haweswater, UK
WINFJELD I.J., CEH Windermere,
The Ferry House, Far Sawrey, Ambleside, Cumbria LA22 OLP, UK
CRAWSHAW D.H. North West Water Limited, Dawson House, Great Sankev, Warrington WAS 3L W UK
CAMERON DURIE N. Environment Agency, Gkvll Mount, GiIlan Wav; Penrith 40 Business Park, Penrith, Cumbria CA]] 9BP, UK
Within the U.K., the whitefish (Coregonus lavaretus) is protected under nature conservation legislation. The status of one population in Haweswater, a reservoir in the English Lake District, has declined since the 1 960s due to increased variations in lake level during the spawning period. A substantial reduction in leakage from the water distribution system has reduced overall demand for water, and this development, aided by recent rainfall patterns, has resulted in a series of years with good spawning conditions. Nevertheless, recruitment has continued to be poor and studies have indicated that foraging by a local cormorant (Phalacrocorax carbo) colony, which has grown rapidly from its establishment in 1 992, is likely to be responsible. In the present paper, management measures undertaken on the cormorant colony in 1 999 and 2000, together with those proposed for 200 1, are described. The context of this conflict of conservation issues is also explored with respect to future potential developments within the wider Lake District.
Seasonal and spatial variation of cormorants predation in a lowland floodplain river
WOLTER C.& PAWLIZKI R. Institute of Freshwater Ecology and Inland Fisheries, Müggelseedamm 310, 12561 Berlin
In 1998 altogether 523 cormorants pellets have been collected, before and after hatch, after fledge, and before wintering. From pellets a total of 9278 fish was identified belonging to 15 species. Roach (Rutilus rutilus), perch (Perca fluviatilis), and ruffe (Gymnochephalus cernuus) formed 84 % of the diet. The mean number of specimens per pellet was 17.7 _ 13.8, with a range of 1 - 128. Cormorants preferred small fish between 6 - 16 cm total length (85.7 % of all prey items). The back-calculated fish mass per pellet varied between 9.3 g and 23 17.8 g with a mean of 490.9 260.7 g, corresponding to the mean daily food consumption of a cormorant. Significant highest consumption was detected in June during rearing period, lowest in September before wintering. The investigated cormorants colony of 730 breeding pairs had a daily fish consumption of716.9 kg corresponding to 179.2 t annual catch and a mean yield> 100 kg/year respectively. These results are discussed with special reference to fisheries.
Wintering cormorants on Loch Leven, Scotland, 1968 to 1999: impacts on the trout fishery and the effect of cormorant shooting
WRIIGHT G. 19 Cairnfore Avenue, Troon, Ayrshire, and KÄ1O 7JL, Scotland
Perceived conflicts between piscivorous birds and commercial fisheries are common in the United Kingdom and abroad. Such a perception exists at Loch Leven, a wetland of international importance for nature conservation and a famous commercial brown trout (Salmo trutta) fishery, where cormorants (Phalacrocorax carbo carbo) have been shot in large numbers. This paper summarizes changes in cormorant numbers wintering on Loch Leven recorded over 30 winters, and compares them with fish stocking rates, angling catches and fishing effort in order to explore the impact of cormorants on the fishery. The extent of cormorant wounding of brown trout is analyzed from samples caught in gill nets. Numbers of cormorants shot on Loch Leven for the purposes of fishery protection are compared with wintering numbers during the same and subsequent winters, and with fish catches, in order to explore the effect of shooting. Although the commencement of trout stocking coincided with an increase in wintering cormorants, there is no evidence of a causal relationship between one and the other. There is no evidence of any detrimental effect of wintering cormorants on angling catches. The principal determinant of the number of fish caught is angling effort and there has been no decrease in catch per unit effort with increasing cormorant numbers. The proportion of brown trout found to be wounded by cormorants is low, rising from 0.4% in June/August to 4.7% in February/March. Wounded fish are significantly larger than unwounded fish. There is no evidence of a reduction in wintering cormorant numbers, or of an increase in angling catches, as a consequence of shooting large numbers of cormorants.
Foraging behaviour of wintering cormorants on Loch Leven, Scotland
WRIGHT G. 19 Cairnfore Avenue, Troon, Ayrshire, and KÄlO 7JL, Scotland
Foraging behaviour of wintering cormorants has been observed to change over recent years, in particular with the development of socia1 or flock fishing. This has been attributed to feeding on shoals of coarse fish, and a1so proposed as a response to eutrophication causing reduced underwater visibility and loss of macrophytes. This paper describes the foraging behaviour of cormorants at Loch Leven, a wetland of international importance for nature conservation and a famous commercial brown trout (Salmo trutta) fishery, where cormorants (Phalacrocorax carbo carbo) have been shot in large numbers. Loch Leven was surveyed on 520 occasions to determine the numbers and distribution of wintering cormorants. Fifty-nine flock feeding events were studied in detail and the timing, position and movements of the flock were recorded. Fish abundance and distribution on Loch Leven was investigated through gill net sampling of 24 randomly located sites. Cormorants spent most of the time roosting, and the peak of feeding activity occurred early in the morning. Birds were more active during mid-winter. There were three different feeding behaviours, the most prominent of which was flock feeding, followed by solo feeding and group feeding. The commencement of flock feeding was concentrated to the west of St. Serfs Island. Seven adjacent 250m x 250m grid squares out of a total of269, accounted for 25% of flock feeding initiations, and 1 3 adjacent squares accounted for 32% of initiations. The intensity of flock feeding was not evenly distributed. Of the 269 grid squares, 97 (36%) were not used, and 1 10 (41%) were rarely used. 78% of flock-feeding activity took place in 23% of the grid squares and 59% took place in 13% oft he grid squares. The most intensively used squares were concentrated to the west of St. Serfs Island, adjacent to the site of peak flock-feeding initiation. The intensity of use of squares for flock feeding was compared with minimum, maximum and mean depth, sediment type, distance from loch shore and distance from loch shore and islands. Flock feeding intensity of use increased very significantly with water depth. Flock feeding intensity of use increased very significantly with sediment type, though sediment type also correlated with depth. Flock feeding intensity of use decreased significantly with distance from the loch shore. Flock feeding intensity of use increased very significantly with brown trout abundance in gill net samples. The mean distance covered by feeding flocks was 3,757m, with a range from 600m to 8,490m. The mean duration of a11 flock-feeding events was 68min, with a range from 13 to l3Smin. The mean flock foraging speed was 0.9mis and the December speed was significantly higher than in January. Flock speed in the most frequently used squares was over 1 .2mis, very significantly higher than in other squares. At the time of formation, flocks usually comprised around 75% of birds present. Flock size gradually declined until the flock dwindled away, or all remaining birds flew en-masse to roost. Flocks with a size of the order of 250 to 450, tended to lose about 1 .5% per minute for the first 40 minutes, after which the rate of loss doubled to 3%. When more than one flock-feeding event was recorded in a day, size declined from a mean of 232 for the earlier event to 179 for the later event. Solo feeding was distributed over a wider area with a principal concentration west and south of St. Serfs, and other concentrations in the north and south-east.
Turn-over within a wintering cormorant population and its effect on the impact of cormorant shooting - results from radio tracking on Loch Leven, Scotland
WRJGHT G. 19 Cairnfore Avenue, Troon, Ayrshire, andK4IO 7JL, Scotland
Loch Leven is a wetland of international importance for nature conservation and a famous commercial brown trout (Salmo trutta) fishery. A conflict is perceived to exist between piscivorous birds and the fishery, and cormorants (Phalacrocorax carbo carbo) have been shot in large numbers. Work on Loch Leven has shown that shooting had no effect in reducing the wintering cormorant population. It has been suggested that this is attributable to a high rate of turn-over within the population, with a constant influx of new birds replacing departing and shot birds. This paper reports the results of radio tracking of cormorants, using hand tracking and satellite tracking techniques in order to investigate the movements of individual cormorants wintering on loch Leven. Eight cormorants were fitted with short-range radio transmitters and successfully tracked by hand. One bird left the loch almost immediately and did not return. Two others left within six days but returned briefly before departing permanently shortly afterwards. Five birds remained for up to three months but in the intervening period departed for up to a month at a time. Individual percentages of time present on the loch were 23%, 30%, 35%, 50% and 80%, and absences increased later in the winter. One bird was tracked flighting out to feed 011 another loch in the early morning, before returning to Loch Leven. It was also tracked to a nearby marine site for an extended period. Nine cormorants were fitted with long-range transmitters and successfully tracked by satellite. 572 fixes were obtained, ranging from the Scottish west coast and Clyde estuary to offshore areas of the North Sea. The 64 most accurate fixes were within 43 km of Loch Leven. Due to the fixing technique the most likely times when the cormorant will be located is either at roost or in extended flight, rather than whilst feeding. There is therefore the likelihood of a bias towards recording roost sites rather than feeding sites, possibly underestimating time spent away from Loch Leven by birds which roost there but feed elsewhere. Diurnal variation in the satellite position may also induce bias, and fixes are more likely to be obtained at particular times. For example, whilst 49% of fixes were obtained during the six-hour period from 1400 to 2000, only 4.5% were obtained between 2 100 and 0300. The radio tracking data confirms a high rate of turn-over within the wintering cormorant population, and indicates that it may be more pronounced in late winter. It also indicates that cormorants present on Loch Leven do not necessarily feed there exclusively. Satellite tracking reinforces the concept of turn-over within the Loch Leven wintering population with individual birds ranging widely across Scotland.
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