studies of spatial structure and dynamics of seabird ...

INSTITUTE OF ECOLOGY

Gediminas Vaitkus

STUDIES OF SPATIAL STRUCTURE AND DYNAMICS OF SEABIRD POPULATIONS IN EASTERN BALTIC

Doctoral Dissertation

Biomedical Sciences: Ecology and Environmental Research (03B)

Vilnius, 1999

The research was carried out at the Institute of Ecology during 1992-1998. The right of a doctoral degree (Biomedical Sciences, Ecology and Environmental Sciences) was granted to the Institute of Ecology and Vilnius University on 14th April, 1989 by the Resolution No.457 of the Government of Lithuanian Republic. The Committee of Doctoral Studies: Chairman and supervisor: Dr. habil., Exp. Mem. of Lithuanian Acad. of Sci. 0HþLVORYDV ä$/$.(9,ý,86 (Institute of Ecology, Biomedical Sciences, Ecology and Environmental Sciences, 03B) Members: Prof., Dr. habil., Corr. Mem. of Lithuanian Acad. of Sci. Juozas VIRBICKAS (Institute of Ecology, Biomedical Sciences, Ecology and Environmental Sciences, 03B); Dr. habil. Vytautas JONAITIS (Institute of Ecology, Biomedical Sciences, Zoology, 05B); Doc., Dr. Jurgis STAŠAITIS (Vilnius University, Biomedical Sciences, Zoology, 05B); Doc., Dr. Vytautas LOGMINAS (Vilnius Pedagogical University, Biomedical Sciences, Ecology and Environmental Sciences, 03B).

EKOLOGIJOS INSTITUTAS

Gediminas Vaitkus

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6758.7Ë526 ,5 ',1$0,.26 70); 10. Seabird flight directions at given time intervals (generated spatial data points) are calculated and stored (if density >0). All steps except 1 and 2 are completely automatic. All the original data is stored in a single database. The following inter-related databases (Fig. 2-2) are automatically updated: 1. The original data set (DS-1); 2. Technical information about separate transects (DS-2); 3. Sample of calculated data points (DS-3); 4. Rough estimates of seabird densities and species composition per transect (DS-4); 5. Non-zero point-based density estimates at given intervals of time (DS-5); 6. Rough estimates of seabird flight directions and species composition per transect (DS-6); 7. Non-zero point-based estimates of flight directions at given intervals of time (DS-7). The overall structure of the environment of inter-related databases, which is gradually developed is by the SBTC application, is based on identification and analysis of separate transects. Each transect requires a unique ID code to be assigned. The most important portion of general information about separate transects is accumulated in the DS-2 database, which is used as the main source of information to be processed in the SBDS module. All the other databases are directly linked to the DS-2, where certain pieces of summary-information are automatically accumulated during the process of updating of other databases of the SBTC environment. Detail description of the structure of SBTC data sets is given in Appendix 1.

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Manual entering of the original data

DS-3

DS-2

Figure 2-2. Functional algorithm of the SBTC application. Text boxes describe the main routines of computation sequence. Numbered circles represent interrelated databases (see text for details). Arrows represent the initial sources and flow of information while the automatic process of analysis is going on. Solid border-lines mark the most important databases and routines of the algorithm which require manual data input.

DS-1

Entering and Testing of transect parameters

Processing of transect parameters

DS-4

Rough estimates of density and species composition

DS-5

Point-based calculation of densities

Re-calculation of flight directions

DS-6

Rough estimates of flight directions and species composition

DS-7

Point-based separation of flight directions

2.1.3. Geostatistical analysis of the spatial data We used traditional gridding techniques to produce regularly spaced arrays of Z values from irregularly spaced XYZ data sets using a standard software solution (SURFER). The term "irregularly spaced" implies that the points are randomly distributed over the extent of the map area meaning that the distance between data points is not consistent over the map. When the XYZ data is randomly spaced over the map area, there are many "holes" in the distribution of data points. Gridding fills in the holes by extrapolating or interpolating Z values in those locations where no data exists. Kriging is one of the more flexible methods and is useful for gridding almost any type of data set. A specific feature of data sets obtained during transect sampling was relative concentration of data points along the original transect lines and relatively wide empty areas in spaces which were not surveyed. With most data sets, Kriging with a linear variogram was quite effective. Kriging attempted to express trends that were suggested in the original data, so that, for example, high points might be connected along a ridge, rather than isolated by bull's-eye type contours (like for example acts Inverse Distance to the Power). For more detailed information on Kriging method see Appendix 2. Further analysis of the generated grids of spatial data was performed using a combination of Geographic Information Systems (GIS) technology and statistical data analysis techniques. All the created regular spatial data grids were aggregated in a master database which was further split into different data sets corresponding to seasons characterized by different climatic conditions, etc. A special mathematical filter (a special script analyzing 12

each master database record within a reference of a geodetic coordinate system) was applied in order to obtain the average values of seabird densities (by species and season) within each cell of a geodetic grid linked to a graphical structure of GIS. Thus the final data set turned out to become an open-architecture multi-layer spatial database linked to a GIS where data records correspond to statistical averages of (calculated) data sets within each cell of a geodetic reference grid. Such a technology allowed to avoid systematic errors and occasional occurrences of high seabird densities, better describing the overall tendencies of seabird spatial distribution, because the final data set was calculated as an average of all the existing data samples which correspond to a certain season and/or climatic situation. We were using two different types of geodetic grids in the current study: the large-scale analyses were performed within 1ox1o (60’x60’) geodetic grid, whereas detailed analyses (southeastern Baltic) were performed within 1’x1’ geodetic grid. Technological process of geostatistical analysis of seabird data sets and their graphical presentation in MapInfo GIS environment is presented in Figure 2-3.

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Figure 2-3. Technological process applied for generation of an open architecture multi-factor spatial database in MapInfo GIS environment.

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Here n(Xi>0) – number of non-zero intervals in the sample of densities; xi>0 – non-zero density in a sample. Coefficient of aggregation can also provide some indirect information on intra-specific tolerance of seabirds in different seasonal and climatic situations. Seabird density samples were analysed separately within three seasons of the year: summer, autumn and winter. 10 selected species of seabirds were either those forming large concentrations in the Baltic Proper during the non-breeding period, or those living in offshore areas all around the year. The selected species were: Razorbill (Alca torda), divers (Gavia arctica, G. stellata), Velvet Scoter (Melanitta fusca), Long-tailed Duck (Clangula hyemalis), also Herring (Larus argentatus), Common (L. canus), Great Black-backed (L. marinus), Lesser Black-backed (L. fuscus), Little (L. minutus) gulls and Kittiwake (Rissa tridactyla). We used mean density (n/km2), standard error of estimate, maximum density (n/km2), Cr and Ca values to describe seasonal seabird density samples. Results are summarised in Tables 3-2, 3-3 and 3-4. Results presented in the following tables lead to a speculation what species or ecological groups of seabirds inhabit large offshore areas of the Baltic Proper in different seasons of the year. Here it would be reasonable to look at the importance of factors like average density, Cr and Ca values in order to find out which of them are of primary importance in the description of seabird population structure. 23

Table 3-2. Results of statistical analysis of density samples of 10 selected seabird species taken in the Baltic Proper in summer seasons 1993 and 1995; n=228. Species Alca torda Gavia stellata, G. arctica Melanitta fusca Clangula hyemalis Larus argentatus Larus canus Larus marinus Larus fuscus Larus minutus Rissa tridactyla * – one observation only.

Mean (birds/km2) 0.09 3.58 1.94 0.43 0.12 2.55 -

Sd 0.51 6.44 5.46 1.24 0.54 13.27 -

Maximum (birds/km2) 5.3 1.2* 0.0 0.0 39.1 43.2 8.3 5.9 180.9 1.3*

Cr

Ca

0.04 0.57 0.32 0.16 0.07 0.19 -

2.21 6.27 5.98 2.65 1.67 13.53 -

Table 3-3. Results of statistical analysis of density samples of 10 selected seabird species taken in the Baltic Proper in autumn seasons 1992-1994 and 1997; n=635. Species Alca torda Gavia stellata, G. arctica Melanitta fusca Clangula hyemalis Larus argentatus Larus canus Larus marinus Larus fuscus Larus minutus Rissa tridactyla

Mean (birds/km2) 3.37 0.83 32.93 100.39 8.11 6.04 0.04 0.01 1.29 0.03

Sd 8.72 4.31 156.26 795.57 268.3 10.66 0.32 0.14 7.44 0.24

Maximum (birds/km2) 122.4 93.5 1848.5 >10000 470.0 99.3 5.0 2.6 157.1 2.5

Cr

Ca

0.45 0.20 0.25 0.45 0.69 0.64 0.02 0.01 0.17 0.02

7.48 4.07 133.57 220.70 11.77 9.56 1.88 1.58 7.52 1.57

Table 3-4. Results of statistical analysis of density samples of 10 selected seabird species taken in the Baltic Proper in winter seasons 1993-1996 and 1998; n=489. Species Alca torda Gavia stellata, G. arctica Melanitta fusca Clangula hyemalis Larus argentatus Larus canus Larus marinus Larus fuscus Larus minutus Rissa tridactyla

Mean birds/km2 1.33 1.55 256.98 292.87 30.02 8.45 0.28 0.20 1.07 0.06

Sd 4.17 6.37 1364.51 2270.09 117.08 35.45 0.99 1.62 11.22 0.45

Maximum birds/km2 38.2 71.0 >10000 >10000 2170.8 545.7 10.9 29.7 270.8 5.7

Cr

Ca

0.22 0.17 0.28 0.52 0.66 0.52 0.11 0.05 0.06 0.03

5.94 9.06 932.76 567.22 45.68 16.30 2.48 3.88 18.05 2.39

Sea ducks were not recorded in the Baltic Proper during the summer movements and post-breeding moult, H[FHSW %ODFN 6FRWHU ZKLFK SHUIRUPV PDVVLYH PLJUDWRU\ PRYHPHQWV LQ VXPPHU -DNREL äDODNHYLþLXV  

Even though average and maximum densities of this group of seabirds may reach extremely high values in specific shallow areas during non-breeding season (and are definitely the highest among all other species during all seasons of the year), their different Cr indices suggest, that different species of sea ducks have different ecological adaptations determining different tendencies to expand into offshore marine aquatories. Long-tailed Duck (in corresponding seasons) indicated almost the same level of Cr coefficient as Common and Herring Gulls, which are actually the most common birds in the Baltic Proper in general (see further text). Another specific feature of sea ducks is surprising stability of Cr indices during the whole period of their stay in the Baltic Sea area. Roughly, Velvet Scoters were present in ~25% of the offshore areas studied, while Long-tailed Ducks – in ~45%. 24

Opposite tendencies of Cr and Ca values in both species lead to an assumption, that generally Velvet Scoters tended to aggregate into dense flocks over their specific feeding habitats, whereas Long-tailed Ducks preferred to disperse over large offshore areas in considerably lower densities. This assumption seems to be valid mostly for the period of autumn and normal winters, mainly because Long-tailed Ducks used to form large and dense concentrations in offshore temporary stop-over areas during autumn migration, as well as during cold winter periods which could cause massive escape movements of this species (see next chapters for more details). Basing on the comparison of statistical coefficient and Ca values, it seams to be reasonable to assume, that aggregation of both Long-tailed Duck and Velvet Scoter may drastically increase during the seasonal transition from autumn to winter, and particularly in severe winter conditions, when the maximal densities of both species were recorded. This assumption especially fits Velvet Scoter, which Ca value has increased by nearly 7 times in winter. On the other hand, the autumn period maximal densities of e.g. Long-tailed Duck were found to be of roughly the same level (>10000 birds/km2) which practically means, that extremely dense staging aggregations of sea ducks are possible both during the period of migratory movements and formation of wintering concentrations (autumn) and in constant wintering sites, especially if those are exposed to rapid formation of ice-fields in specific meteorological conditions. The only difference here is physiological state (i.e. seasonal pattern) of birds, that form dense offshore staging concentrations during their migratory movements in autumn or escape movements in cold winter. Another group of seabirds with similar values of statistical coefficients and seasonal trends is the group of “dominating gulls” with only two species – Herring and Common Gull. Average densities of both species are low during all seasons in comparison to those of sea ducks. Both species were present in the Baltic Proper all around a year and indicated a clear tendency of increase of their densities on the transition from breeding season (when only non-breeding birds were constantly present in offshore areas and breeding adults were foraging in coastal waters) to post-breeding moult (late summer and autumn; local breeders with their fledglings flow into the sea from inland breeding sites) and winter (populations from wide geographic range aggregate in ice-free offshore aquatories and harbours of the eastern and southern Baltic). Both species of gulls in this group indicated low or medium Cr values during all seasons of the year (with corresponding lowest values in autumn). In practice it would mean, that Herring and Common gulls inhabited 50-70% of the whole aquatory of the Baltic Proper all around the year. Herring Gulls indicated slightly higher tendency of expansion into offshore areas (Cr values were lower in summer and winter). Surprisingly similar Cr indices were indicated by both dominating gull species in autumn, when migratory movements take place along the main axis of the Baltic Proper (NE to SW direction) and frequent sea-storms scatter offshore populations of gulls. Aggregation of both gull species also tended to increase along the pass from summer to winter. We assume, that the ecological background of this process lays in constant aggregation of gulls within their wintering areas in offshore waters of the southern part of the Baltic Proper during the period of autumn and winter. The non-breeding aggregations of both species indicate similar low aggregation patterns. Wintering concentrations of Herring Gull were ~3-4 times more dense than those of the Common Gull. It is worth mentioning in this aspect, that in most cases large aggregations of dominating gull species were recorded in offshore areas of intensive fishery, so we may assume, that this particular group of seabirds survive at a significant extent due to their successful scavenging strategy. This was proved by intensive studies in the North Sea as well (Camphuysen et al., 1995). The third ecological group of seabirds is divers (Red-throated and Black-throated) and Razorbill. The main ecological feature distinguishing those seabird species among the others is their foraging strategy based on utilisation of small-size fish, that is usually taken from upper layers of the water column (up to 30, maximum – 40 m depth). Both species were characterised by very low average densities and Ca values during all seasons of the year, which is an indirect reflection of their species-specific tactics of foraging solitary or in small groups. The only exception here (Cr=0.45; Ca=7.48) was Razorbill “invasion” in autumn 1997, when rather high densities of this species were found in central and south-eastern sectors of the Baltic Proper. However, the calculated Cr values in winter were different than those one could expect - the preliminary assumption was, that solitary-foraging seabirds should be dispersed in low densities over wide offshore areas (consequently – with high Cr values). What we actually found out was rather low Cr values of both species, which means, that diving seabirds were patched in some specific, but far not all the offshore areas. The only exception of this rule was Razorbill in autumn with its rather high Cr value, meaning, that this species was present over ~45% of the offshore aquatory surveyed. The last group of seabirds was represented by several species of gulls which all seem to have rather different ecological features, nevertheless fall into one category due to their low (very low, in fact) mean densities and Cr values. Those species were Great and Lesser Black-backed Gulls, Little Gull and Kittiwake. The first two species are known to use solitary offshore foraging strategy. In the eastern Baltic Lesser Black-backed Gull is very rarely observed both in offshore areas and in coastal roosting sites of gulls during the non-breeding season. Great Black-backed Gull is more common in coastal sites (e.g. city dumps or fishing farms), where groups up to 10

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birds can be frequently observed in winter. Kittiwakes were also observed quite rarely (solitary birds were recorded in most cases). Little Gull seems to have some specific ecological adaptations to live in pelagic zone of the Baltic Sea. In winter and autumn it was often recorded in flocks up to several hundred birds at the edge of surface fronts where vertical water currents meet together causing vertical transport of deposits and suspended matter. Distribution of the Little Gull is extremely patchy, especially during the post-breeding molt in shallow coastal waters of the southeastern Baltic (estuarine areas were given especially high preference). Table 3-2 gives comparatively high density indices and Ca value for this species in summer, in autumn Cr and Ca values indicate, that Little Gulls still are widely dispersed in offshore Baltic Proper, but are significantly less aggregated, whereas in winter they again aggregate into dense patches (the highest maximal density level; Tab. 3-4), but their observations become much more random in comparison to other seasons of the year (less than 10% of the offshore area covered). So, the last group of seabirds, artificially constructed basing on interpretation of statistical parameters of their density samples, seems to be formed of species employing rather different survival strategies, like scavenging in areas of intense fishery or flocking in certain turbulent marine environments. To finalise the discussion which ecological groups of seabirds actually prevail in the marine environment of the Baltic Proper, we must consider first of all the spatial and seasonal continuity of the species’ presence in the marine aquatory under consideration. The most “common” ecological group of seabirds in the Baltic Proper with no doubt is gulls, dominated by Herring and Common Gulls. They are present here all year round and inhabit the major part of the marine aquatory. On the other hand, the most “abundant” (an important in both ecological and conservation aspect) group is sea ducks, which form constant dense aggregations in the most vulnerable range of the marine environment – its coastal zone – during autumn and winter seasons, i.e. the most dangerous periods in case of any ecological disasters. 3.3. Mapping of seabird distribution We performed the analysis of spatial distribution of the calculated average densities per sampling unit (10min. period as a standard) within the whole range of our study area in different seasons of the year. Depending on frequency of occurrence of certain species along the transects covered (i.e. proportion of non-zero density points in a sample), we used different techniques to present our data: 1. Plotting of average densities (birds/km2) of certain species per standard sampling interval (in most cases – 10 min.). This measure was used for further calculation of average densities of certain species within 1ox1o sample areas covering offshore aquatories of the Baltic Proper. The calculated within-rectangle averages were used for the comparison of certain aquatories of the study area on a very rough scale. 2. When species were observed rather occasionally, so that it was impossible to outline their concentration areas (basing on a series of successful non-zero average density points along the transect), we applied a method of simple plotting of the recorded densities at corresponding geographic locations, as well as numbers of birds observed outside the transect (calculation of densities was impossible in the latter case). The threshold indicator for the selection of the method of data presentation was Cr value of a certain density sample, which at its values >0.8 indicates occasional observations of a species, whereas calculation of average density values in a sample which consists of almost exclusively zero-densities, is not reasonable and incorrect. The whole data sample of our study was represented by 29 species of waterfowl and seabirds. We will further discuss the species in detail following their systematic order. In rich enough data samples we performed the analysis of density frequencies. 3.3.1. Black-throated and Red-throated Divers Red-throated and Black-throated Divers were recorded in the Baltic Proper during autumn and winter seasons exclusively. In south-eastern Baltic solitary birds were observed along the coastline in summer as well. In autumn maximum average densities of divers (up to 2.5 birds/km2; Fig. 3-1 ) were recorded south of the Gotland Island in autumn 1992 when strong migratory passage of divers and sea ducks was observed following a sudden cold spell in late October. Most of the birds present in the area were flying in a continuous stream of migrants, so it is practically impossible to treat high densities of divers recorded there as a stable concentration. The other areas where divers were found in densities of ~2 birds/km2 were eastern part of the Gulf of Riga and Latvian coastal area between Pape and Liepaja. It is evident, that divers tended to avoid deep far offshore areas of the Baltic Proper (Fig. 3-1). In winter maximal densities of divers were recorded in Lithuanian sector of the Baltic Sea during the cold winter period in February 1994 (Fig. 3-2; see Chapter 4.2.1 for details). In normal winters rather high densities of divers (up to 3 birds/km2 on the average) were constantly recorded in the Gulf of Riga near Mersrags and in Irbe Strait, also along the south-eastern part of the coastline. Diver concentrations were found to be dynamic depending on dynamics of small fish stocks. This was clearly evident in the eastern part of the Gulf of Riga with large stocks of sprat (Sprattus sprattus DQG QHDU .ODLS¡GD LQ VRXWKHDVWHUQ %DOWLF ZLWK ODUJH PLJUDWRU\ FRQFHntrations of smelt (Osmerus eperlanus) in February. 26



























'HQVLW\ QNP

Number of observations (vertical axis in log-decimal scale) in corresponding density classes (horizontal axis) in the autumn period sample of divers densities.

Figure 3-1. Distribution of the average densities (n/km2) of Red-throated and Black-throated divers in the Baltic Proper in autumn. 





































'HQVLW\ QNP

Number of observations (vertical axis in log-decimal scale) in corresponding density classes (horizontal axis) in the winter period sample of divers densities.

Figure 3-2. Distribution of the average densities (n/km2) of Red-throated and Black-throated divers in the Baltic Proper in winter. The same as in autumn, wintering divers tended to concentrate in coastal zone of the Eastern Baltic with lower average densities in far offshore aquatories of the Baltic Proper. It is still considered to be a problem to distinguish between the species of divers in winter plumage while sampling in offshore waters (Webb, Durinck 1992). Those birds were usually taking off rather far in front of the ship (1-1.5 km), and it was possible to identify them only in ideal sampling conditions and with high-quality optical devices. Therefore, in most studies it was agreed, that divers should not be identified to species treating them as one group. In the Baltic Sea this usually meant, that Black-throated and Red-throated Divers will fall into one category. Nevertheless, it is interesting to find out which of the two species occurs more frequently, or what locations are preferred by one or another diver species. To make a comparison of Red-throated and Black-throated Diver distribution in the study area, we plotted known cases of calculated densities and observations of the two species. Red throated Divers occurred more frequently (16 cases; Fig. 3-4) in our data sample in densities up to 5 birds/km2. Most of the records took place in south-eastern Baltic, whereas some birds were observed in the Gulf of Riga and even in the Gotland Fall area (220-250 m depth). 69% of observations took place in autumn (easier to identify birds in their summer plumage), 25% - in winter and 6% - in summer. Black-throated Divers (single birds, up to 2-3 ind/km2 densities) were observed inside the transect just several times, mainly in Lithuanian sector of the Baltic (Fig. 3-3) where the overall coverage was the best. All the records took place in autumn seasons. On the other hand, Black-throated Divers seemed to be less numerous in the study region. Earlier studies revealed, that offshore aquatories along the eastern coast of the Baltic Sea are clearly predominated by Red-throated Divers; half of the regional diver population winter in the Gulf of Riga and Irbe Strait (Durinck et al., 1994). 27

Comparison of density distribution of data samples in autumn and winter (histograms in Fig. 3-1, 3-2) clearly indicates, that generally diver densities tended to increase during the winter season (the main interval of densities reached up to 6-7 birds/km2 in autumn and up to 10-12 – in winter). The flocking tendency of divers was stronger under severe winter climatic conditions (up to 30 birds/km2), especially in temporary stop-over area along the Curonian Spit during the cold winter period in February 1994 (see Chapter 4.2.1 for details). Rather high densities of divers were also observed in offshore aquatories characterised by intensive fisheries or migratory concentrations of small fish species (smelt and sprat in particular).

Figure 3-3. The recorded densities (n/km2) of Blackthroated Diver in the Baltic Proper.

Figure 3-4. The recorded densities (n/km2) and observations of Red-throated Diver in the Baltic Proper.

3.3.2. Great Crested and Slavonian Grebes South-eastern Baltic was earlier pointed out as one of the major wintering sites of this species in the Baltic Sea region (Durinck et al., 1994). We found out, that Great Crested Grebe was also wintering in the southern part of the Gulf of Riga (Fig. 3-5), whereas the absolute majority of records took place in Lithuanian coastal waters during the severe winter 1996 (see Chapter 4.2.2 for details).

Figure 3-5. The recorded densities (n/km2) and observations of Great Crested Grebe in the Baltic Proper.

Figure 3-6. The recorded densities (n/km2) of Slavonian Grebe in the Gulf of Riga.

Slavonian Grebe was also recorded in small densities along the Curonian Spit (south-eastern Baltic) in normal winters (Durinck et al., 1994). We didn’t find Slavonian Grebes there, but one record of this species took place in southern part of the Gulf of Riga (Fig. 3-6) where density of this species reached ~5 birds/km2. The most reasonable explanation of the occurrence of Great Crested and Slavonian Grebes in the southern part of the Gulf of Riga is close neighborhood of large river mouth, harbor and wetland areas in the coastal zone ¡GD

QHDU -XUPDOD )HHGLQJ IORFNV RI *UHEHV ZHUH FRQVWDQWO\ REVHUYHG QHDU WKH JDWHV RI HJ .ODLS

28

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along the coasts of brackish Curonian Lagoon in winter time. In the Baltic Sea context, both Great Crested and Slavonian Grebes should be considered as exceptionally coastal species common both in marine and brackish water habitats in coastal lagoons and wetlands. 3.3.3. Great Cormorant The species was observed in offshore waters of the south-eastern Baltic in summer (foraging birds from coastal colonies) and autumn (Fig. 3-7; see Chapter 4.2.3 for details). Constant movements of Cormorants to their foraging areas in coastal and offshore waters were observed at the watch-points on Lithuanian coastline during all the summer (Vaitkus et al., 1994). In coastal and inland waters cormorants use to spend normal and mild winters âYDåDV 9DLtkus 1995). Figure 3-7. The recorded densities (n/km2) of Great Cormorant in the south-eastern Baltic.

Figure 3-8. Observations of Mute Swan in the Baltic Proper.

Figure 3-9. Observations of Whooper Swan in the Baltic Proper.

3.3.4. Mute and Whooper Swans Migratory flocks of both swan species were occasionally observed in offshore areas of the Baltic Proper (Fig. 3-8, 3-9) either flying close to the water surface or in compact resting flocks. Mute Swan flocks were much larger (up to ~60 birds). All offshore observations of swans took place in autumn seasons. Locations of flocks observed laid along the Eastern Baltic migratory flyway (Matsalu and Kolka in the Gulf of Riga – offshore waters in the Baltic Proper near Liepaja – Lithuanian sector of the Baltic). Earlier studies (Durinck et al., 1994) revealed, that Mute Swans winter in coastal waters all around the Baltic except wide open beaches along its eastern (Lithuania and Latvia) and southern (Poland) coasts. We didn’t find swans wintering in offshore (> 10 m depth) areas of the Baltic Proper.

29

3.3.5. Mallard and Teal Mallards were recorded in offshore Baltic waters just several times in 1994 and 1997 autumn seasons and in early summer 1995 (Fig. 3-10). All records took place in Lithuanian sector of the Baltic (coastal zone and far offshore). A flock of teals were observed inside the transect in offshore areas near Liepaja only once, namely in early June 1995 (Fig. 3-11). Like swans, Mallards and Teals most likely were temporary visitors in offshore Baltic Proper recorded on their local or migratory movements. Mallards were found wintering in low densities around all the coast of the ice-free Baltic (Durinck et al., 1994). Teals were not recorded in offshore Baltic before.

Figure 3-10. The recorded densities (n/km2) and observations of Mallard in the Baltic Proper.

Figure 3-11. The recorded densities (n/km2) of Teal in the Baltic Proper.

3.3.6. Common Eider Common Eiders are known to have several small wintering sites along the coast of the south-eastern Baltic, whereas their major wintering areas are located in the north-west Kattegat, also in shallow Danish waters and Kiel Bay of Germany (Durinck et al., 1994). Sampling in offshore waters of central and eastern part of the Baltic Proper, we didn’t find any Common Eiders in winter, except a flying flock observed in Lithuanian sector of the Baltic Sea in winter 1995. This generally supports the earlier statement, that this species is not common in the eastern Baltic in winter quarters in comparison to the large population constantly wintering in the western sector of the sea. Rather few offshore observations of Common Eiders took place in south-eastern Baltic in autumn seasons (Fig. 3-12). Those were groups of birds on the water or flying flocks in offshore shallows near the border of Lithuania (see Chapter 4.2.4 for details) and Latvia. Unfortunately, technical characteristics of the ship didn’t allow sampling in shallow waters closer to the coast where Common Eiders were usually observed during coastal counts both in autumn and winter seasons. Figure 3-12. The recorded densities (n/km2) and observations of Common Eider in the Baltic Proper.

30

In autumn seasons 1992 and 1993 we had quite a lot observations of migrating or staging flocks of Common Eiders in the Gulf of Riga, north-western part of the Baltic Proper and especially south of the Gotland Island where a continuous stream of migrating seabirds – mainly Common Eiders, Long-tailed Ducks and divers – was recorded immediately after a cold high-pressure atmospheric front in autumn 1992 (Fig. 3-12). Most of the birds were flying at 15-25 m height heading south-westwards (roughly – towards Bornholm Island). In snapshots Common Eider densities reached 100-300, and even up to 2000 birds/km2, whereas very few flocks were recorded on the water surface over the area. Basing on the locations of the migratory flocks observed and the analysis of prevailing flight directions, we assume, that the one branch of the main migratory route of Common Eiders in autumn cross the Estonian archipelago and offshore waters of the northern part of the Gulf of Riga and the other lays down along the coast of Bothnian Bay. Those two branches collect into one flow in the area of Gotland Island and than flow down over shallow waters south of the Gotland Island touching the southern end on Oland Island and Bornholm Island, crossing the southern end of Scandinavian Peninsular towards their constant wintering areas in Kattegat and south-western Baltic. 3.3.7. Long-tailed Duck Long-tailed Duck is definitely the most numerous sea duck species wintering in the Baltic Sea region with the total estimated population of more than 4 million birds (Durinck et al., 1994). The major wintering areas of this species were found in the Pomeranian Bay, the Gulf of Riga and south of the Gotland Island. The species was one of the most common in offshore waters of the Baltic Sea during our surveys as well. The autumn-period sampling in the Gulf of Riga and south of the Gotland Island revealed very important offshore staging areas of sea ducks (mainly Long-tailed Ducks) in shallow banks in the northern part of the Gulf of Riga (densities >10,000 birds/km2) and massive south-west heading migratory movements over the offshore waters south of the Gotland Island (Fig. 3-13; the case was described in detail in Chapter 3.4.7). Rather high densities of Long-tailed Ducks were recorded in deep far offshore waters (~200-250 m depth) in the Gotland Fall area, which seem to be a clear indication of the existence of sea duck migratory route laying from the northern part of the Gulf of Riga across the Baltic Proper towards the southern end of Gotland Island where it is influxed by another branch coming down from the Bothnian Sea and collecting birds from northern part of Scandinavian Peninsular. Long-tailed Ducks were also observed quite frequently along the eastern coast of the Baltic Sea, as well as in far offshore aquatories in central Baltic Proper. Here we may suspect the existence of the other sea duck migratory branch coming down along the eastern coast of the Baltic towards its south-eastern and southern regions. Those findings demonstrate, that Long-tailed Duck is important in the Baltic Sea region not only as numerous and constantly wintering species, but also as massive migrant over the entire region, forming large staging concentrations in offshore waters already in autumn.





























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Number of observations (vertical axis in log-decimal scale) in corresponding density classes (horizontal axis limited to 500 birds/km2) in the autumn period sample of Long-tailed Duck densities. Figure 3-13. Distribution of the average densities (n/km2) of Long-tailed Duck in the Baltic Proper in autumn. In winter quarters the largest concentrations of Long-tailed Ducks were found in the southern part of the Gulf of Riga and Irbe Strait in particular (Fig. 3-14), where their densities could reach >10000 birds/km2 level during the period of rapid development of ice fields. Practically the same extremely high density level of the Longtailed Ducks was recorded in ice-free offshore waters of the south-eastern Baltic 5-7 days after the Gulf of Riga was completely covered with ice (for detailed information see Chapter 6). Thus it was demonstrated, that large wintering sea duck populations may be highly dynamic in extreme winter climatic situations, and a framework of marine habitats exists within the Baltic Sea Region to support those large dynamic wintering populations. An31

other sea duck species with the same tendency to perform massive escape movements along the eastern coast of the Baltic Sea in extreme winter climatic situations was Velvet Scoter (see Chapter 3.3.9 for details). The same as in autumn seasons, rather high densities of Long-tailed Ducks were found in far offshore aquatories of central Baltic Proper where water depths exceed 50 and even 100 m level. The earlier studies didn’t show any sea duck presence in far offshore waters of the Baltic Sea. So far it is impossible to describe the status of those flocks of sea ducks. Nevertheless, two alternatives seem to be realistic: flocks of sea ducks may be chasing some source of pelagic food (like small fish stocks) or they may be staging during some regional movements origin of which is so far not clear. The only concrete statement at a moment could be this: not all wintering sea ducks in the Baltic Sea are aggregated in shallow coastal and offshore waters. A substantial proportion of the regional wintering population is either constantly or temporarily living in far offshore areas where large water depths cut them off the supplies of traditional benthic food. This regards first of all the most plastic and expansive species like Long-tailed Duck. Comparison of Long-tailed Duck density samples in autumn and winter indicated a tendency of higher aggregation of the species during the winter quarter (the main density ranges were reaching 300 birds/km2 level in autumn and 500 – in winter); maximal calculated densities of the species were considerably higher as well. Nevertheless, both autumn and winter samples were characterised by similar slopes within the interval up to 300 birds/km2, and this seems to be a reflection of some “normal” flocking pattern (i.e. dispersion in groups and small flocks) of Long-tailed Ducks. It seems to be reasonable to assume, that higher (up to 500 birds/km2) densities and extremely high (> 10,000 birds/km2) density values are caused either by extreme climatic situations, or specific highly favourable ecological conditions (large temporary stocks of food available in certain locations).





























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Number of observations (vertical axis in log-decimal scale) in corresponding density classes (horizontal axis limited to 500 birds/km2) in the winter period sample of Long-tailed Duck densities. Figure 3-14. Distribution of the average densities (n/km2) of Long-tailed Duck in the Baltic Proper in winter. Figure 3-15. The recorded densities (n/km2) and observations of Black Scoter in the Baltic Proper.

3.3.8. Black Scoter In offshore waters of the Baltic Proper Black Scoters were observed very few times (Fig. 3-15). Several large flocks of this species were observed in central part of the Baltic Proper during early morning hours in June 1995 32

(densities up to 60 birds/km2). Those birds were most probably staging on their summer migratory movements. The same regards observations of Black Scoters in coastal waters of the Lithuanian sector of the Baltic in August 1993. High altitude offshore migratory movements of Black Scoters in summer were studied with radar along WKHLU (DVWHUQ %DOWLF IO\ZD\ äDODNHYLþLXV   -DNREL äDODNHYLþLXV   7KH PDMRU ZLQWHULQJ DUHDV RI

this species were found in Kattegat, Pomeranian Bay and shallow Danish waters (Durinck et al., 1994). In autumn Black Scoters (up to 30 birds/km2) were observed in the Irbe Strait area of the Gulf of Riga. No winter-period records were made during our studies in offshore Baltic, whereas other studies (Durinck et al., 1994) demonstrated the existence of low-density Black Scoter concentrations in shallow waters along the eastern coast of the Baltic Sea. 3.3.9. Velvet Scoter The most important wintering sites of this sea duck species were found in the western part of the Gulf of Riga and Irbe Strait, Pomeranian Bay and south-eastern Baltic (Durinck et al., 1994). This species was the second most numerous wintering sea duck species in the Baltic Sea with total estimated population of more than 900 thousand birds. The autumn period distribution of Velvet Scoters in the Baltic Sea was very much similar to that of other sea ducks, and that of the Long-tailed Duck in particular (see Chapters 3.3.6 and 3.3.7). The most important offshore staging areas were found in shallow banks of the northern part of the Gulf of Riga (densities up to 2000 birds/km2). An intense south-west heading migratory passage of the species was recorded together with other sea ducks over shallow offshore aquatories south of the Gotland Island (Fig. 3-16). Rather low densities of Velvet Scoters were found in offshore areas along the eastern coast of the Baltic Sea during the period of formation of wintering concentrations in autumn. 



























'HQVLWLHV QNP

Number of observations (vertical axis in log-decimal scale) in corresponding density classes (horizontal axis limited to 500 birds/km2) in the autumn period sample of Velvet Scoter densities. Figure 3-16. Distribution of the average densities (n/km2) of Velvet Scoter in the Baltic Proper in autumn.





























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Number of observations (vertical axis in log-decimal scale) in corresponding density classes (horizontal axis limited to 500 birds/km2) in the winter period sample of Velvet Scoter densities. Figure 3-17. Distribution of the average densities (n/km2) of Velvet Scoter in the Baltic Proper in winter.

33

All the above mentioned facts suggest, that all sea duck species, but especially Long-tailed Duck and Velvet Scoter, perform rather similar migratory movements with practically the same stop-over areas and the main flyway routes in the Baltic Sea region. This species seems to avoid far offshore areas during the autumn migration and formation of wintering concentrations. Wintering Velvet Scoters were found in the largest concentrations in the western part of the Gulf of Riga (which supports the earlier finding of Danish colleagues; Durinck et al., 1994) and in offshore aquatory along the Curonian Spit (see Chapter 4.2.6 for details). Both situations when large Velvet Scoter concentrations were exceeding 10000 birds/km2 density level were resulted by rapid formation of ice fields in the Gulf of Riga – the main wintering area of sea ducks in the Eastern Baltic region (see Chapter 6 for details). Maximal density ranges of Velvet Scoter data samples in autumn and winter were of the same level (up to 400-500 birds/km2). The overall distribution of density ranges in both samples was also very similar: most of cases fell into the range of densities up to 200 birds/km2. There seemed to be no clear tendency to form dense aggregations in normal winter conditions (except the cases when massive escape movements were taking place). Thus we may expect, that Velvet Scoters develop their species-specific spatial structure of foraging flocks already in autumn, and it seems to remain constant throughout the winter. 3.3.10. Smew All records of Smew took place in the Gulf of Riga in winter 1993 (Fig. 3-18) when ice field formation was in progress over shallow coastal areas of this semi-closed water basin. Birds were observed in small flocks (up to 50) or groups (outside the transect along the edge of ice fields. During the earlier studies (Durinck et al., 1994) wintering Smews were found only along the coasts of Estonian Islands (Saaremaa and Hiumaa) in depths less than 10 m. It could be possible, that ice formations forced wintering birds to leave their usual wintering sites and stay in offshore aquatories.

Figure 3-18. The recorded densities (n/km2) and observations of Smew in the Baltic Proper.

Figure 3-19. The recorded densities (n/km2) and observations of Red-Breasted Merganser in the Baltic Proper.

3.3.11. Red-breasted Merganser In the Eastern Baltic region wintering Red-breasted Mergansers were found in rather low densities along the FRDVW OHVV WKDQ  P GHSWK RI WKH *XOI RI 5LJD LQ .ODLS¡GD3DSH DUHD DQG LQ WKH *GDQVN %D\ 'XULQFN HW 2

all., 1994). Except the latter area, we did find approximately the same densities (up to 6-7 birds/km ) of this species (Fig. 3-19). All the observations took place in autumn seasons or winter (1993) with intense formation of ice-fields in coastal shallows of the Gulf of Riga. Thus we may assume, that wintering Red-breasted Mergansers are normally present in shallow coastal waters already in autumn, but they may be driven into offshore waters of the Gulf of Riga by the process of ice formation. Similar tendency was identified for Smews wintering in the Gulf of Riga (see Chapter 3.3.10). 3.3.12. Goosander

Goosanders are wintering in rather high densities along almost entire eastern and western coast of the Baltic Sea. The highest densities were found in the Pomeranian Bay, other areas holding large numbers of wintering Goosanders are the Gulf of Riga, Curonian Lagoon, Gdansk Bay (Durinck et al., 1994). All earlier findings of Goosanders took place in the coastal zone, i.e. in depths less than 10 m.

34

We had more than 60 measurements of densities (usually 1-5, but up to 20-25 and even 50 birds/km2) and observations of those birds in offshore areas of the Eastern Baltic. Goosanders have been observed in small groups and flocks of up to 20 birds. One specific characteristic of situations when Goosanders were present in offshore waters was progress of ice formation along the eastern coast of the Baltic. Practically all the observations of Goosander in the Gulf of Riga (Fig. 3-20) took place in ice-free waters in southern and eastern part of the basin and especially in the area of Irbe Strait where huge concentrations of sea ducks were present in early March 1993 (see Chapter 6 for details). Figure 3-20. The recorded densities (n/km2) and observations of Goosander in the Baltic Proper.

Another situation when rather high densities of Goosanders were recorded in offshore waters of the southeastern Baltic was cold winter period in February 1994. It is impossible to say where did those birds come from, but it is worth mentioning, that rather large local wintering population of Goosanders normally stay in the neighERXULQJ &XURQLDQ /DJRRQ DQG XVXDOO\ OHDYHV WKLV FORVHG ZDWHU EDVLQ ZKLOH LW EHFRPHV FRYHUHG ZLWK LFH âYDåDV

Meissner, Nehls 1994). Therefore we may conclude, that all three species of this group are not common in offshore waters of the Eastern Baltic in autumn and normal winter conditions. The main factor forcing those birds move into deeper waters is development of ice formations in coastal lagoons and wetlands, as well as along the coastline. The ecological adaptation mechanism which ensures survival of their wintering populations of this fish-eating seabirds group in severe winter climatic conditions so far is not clear. 3.3.13. Little Gull Earlier studies have revealed, that Little Gulls are wintering in small numbers in the Gulf of Riga and far offshore areas of central and western part of the Baltic Proper (Durinck et al., 1994). We have paid a special attention to that pelagic species of gulls during our offshore studies in the Baltic Sea in order to clarify its status in the region during the non-breeding period. We found out, that Little Gulls are present in the coastal zone of the south-eastern Baltic and the Gulf of Riga, as well as in the far offshore areas during all seasons of the year (except the breeding season), while average densities of that species in certain situations and areas may reach rather high levels. The most important finding in this aspect was, that, in contrary to the results of the earlier studies, the species constantly inhabits both coastal and far offshore pelagic areas of eastern and central part of the Baltic Proper. Nevertheless it should be stated, that distribution of Little Gulls was very patchy, and the only ecological background for this fact could be offshore fishing activities and turbulence on the water surface with concentrations of suspended material. Up to 60-70% of Little Gull records in offshore areas were directly associated with the occurrence of such surface water turbulence zones. One of important findings during our studies was identification of an internationally important Little Gull moulting area in offshore waters of the south-eastern Baltic. In addition to certain indirect indications of an existence of such a moulting area referred in other reports (Meltofte 1993), we have obtained a rather detailed sample of density measurements of the species over the continental shelf close to the border to Lithuanian and Latvian territorial waters in the late summer 1993 (Fig. 3-21; see Chapter 4.2.8 for details). In autumn, when the coverage was the most extensive, we found out, that the main concentration areas of Little Gulls were in the Gulf of Riga and Irbe Strait, as well as in central and southern part of the Baltic Proper (Fig. 3-22). Maximal recorded density of the species during that period reached the level of 160 birds/km2, which was observed in the intensive fishing area in the southern part of the Baltic Proper. Generally, the distribution of the Little Gull in the Baltic Sea in autumn best of all resembles the one found by other researchers 35

(Durinck et al., 1994) during mild winters. This suggests the assumption, that the species has an advanced development of its wintering aggregations already early in autumn, and the patterns developed may stay constant throughout the normal winter season unless unfavourable climatic conditions cause certain perturbations in spatial structure of wintering population of Little Gull in the Baltic Proper.





























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Number of observations (vertical axis in log-decimal scale) in corresponding density classes (horizontal axis limited to 20 birds/km2) in the summer period sample of Little Gull densities. Figure 3-21. Distribution of the average densities (n/km2) of Little Gull in the Baltic Proper in summer. 



























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Number of observations (vertical axis in log-decimal scale) in corresponding density classes (horizontal axis limited to 20 birds/km2) in the autumn period sample of Little Gull densities. Figure 3-22. Distribution of the average densities (n/km2) of Little Gull in the Baltic Proper in autumn. 

























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Number of observations (vertical axis in log-decimal scale) in corresponding density classes (horizontal axis) in the winter period sample of Little Gull densities. Figure 3-23. Distribution of the average densities (n/km2) of Little Gull in the Baltic Proper in winter.

36

In winter the recorded densities of Little Gulls reached their absolute maximum in the Baltic Sea area (230 birds/km2), which again took place in intensive fishery area in southern part of the Baltic Proper (Slupsk Bank; Fig. 3-23). The periods of our sampling in the Gulf of Riga were associated with intensive development of ice formations, therefore most o gulls tended to abandon the offshore aquatories of that part of the Baltic. Higher densities of Little Gulls were recorded in the Gotland Fall area and along the eastern coast of the Baltic, which may be an indication of flux of wintering Little Gull populations from the region of the Gulf of Riga to southern part of the Baltic Proper. It is rather surprising, that shallow coastal waters over the continental shelf of the south-eastern Baltic harbour rather large numbers of moulting Little Gulls only in late summer. During other periods of the non-breeding season those birds seem avoid shallow coastal waters in the south-eastern Baltic. The basic range of densities of the Little Gull in most cases was bordered by the level of 20 birds/km2, whereas maximal densities in dense patches could reach the level of 140-160, and even up to 220-240 birds/km2. Independently to the period of the non-breeding season, both recorded maximal densities and the derived regional average densities of the species were of similar level (150-230 and 7-15 birds/km2, respectively). Comparison of spatial distribution of the Little Gull densities in the Baltic Proper during different periods of the nonbreeding season indicate, that the largest total regional population of the species in the Baltic Proper can be found in autumn, whereas the lowest – in winter. Its considerable decline and aggregation during the winter season indirectly supports the assumption, that large numbers of Little Gulls leave the Baltic Sea on their way to constant wintering areas in the Mediterranean Sea and northern Africa. 3.3.14. Common Gull Common Gull is one of the most usual both in the coastal zone and offshore areas of the Baltic Proper (see Chapter 3.2). Even though earlier studies reported presence of the species in central part of the Baltic in winter (Durinck et al., 1994), insufficient data in offshore areas of the eastern Baltic Proper didn’t indicate presence of the Common Gull in this aquatory. The Gulf of Riga was pointed out as the area with highest density of this gull species in all the Baltic Sea region. Our studies in offshore waters of the eastern Baltic Proper revealed, that Common Gulls live here in rather high densities allaround the year, being considerably less numerous during the breeding period in summer (Fig. 3-24). On the other hand, so far it is impossible to make assumptions about non-breeding populations of any gull species living in offshore waters of north-eastern Baltic, because those aquatories were not surveyed during this time of the year, whereas local breeding populations of e.g. Common or Herring Gulls are known to be quite numerous. Concerning the south-east Baltic region, Common Gulls in summer were observed mostly in coastal waters, which may be due to foraging adults of local breeding population (see Chapter 4.2.9 for details).



























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Number of observations (vertical axis in log-decimal scale) in corresponding density classes (horizontal axis) in the summer period sample of Common Gull densities. Figure 3-24. Distribution of the average densities (n/km2) of Common Gull in the Baltic Proper in summer. In autumn Common Gulls were recorded in comparatively high densities (up to 100 birds/km2) over large offshore areas of the Baltic Proper (Fig. 3-25), and it would be complicated to point out certain areas where those birds were more abundant. The only clear tendency of Common Gull distribution in autumn was concentration of this species along the eastern coast of the Baltic Sea and gradual declining in densities further westwards, i.e. into the central part of Baltic Proper. Offshore waters of the Gulf of Riga were also inhabited by this species in autumn with higher densities recorded in its northern part (intensive fishery). Offshore fishing areas in the open Baltic were also attractive to those birds in autumn (the highest densities were recorded in offshore fishing area in the Gulf of Gdansk). 37





























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Number of observations (vertical axis in log-decimal scale) in corresponding density classes (horizontal axis) in the autumn period sample of Common Gull densities. Figure 3-25. Distribution of the average densities (n/km2) of Common Gull in the Baltic Proper in autumn. In winter Common Gulls were indicating even higher average densities than those recorded in autumn. The maximal recorded densities of this species were reaching the level of 400-450 birds/km2 in the areas of intensive fishery. The major concentration areas during this season were more clearly distinguished (Fig. 3-26). The largest offshore wintering populations of Common Gull in eastern Baltic Proper were found in the intensive fishing area in southern Baltic (Slupsk Bank) and in deep transitional area between the Gulf of Riga and Gotland Island. Unfortunately, the existing data from the Gulf of Riga doesn’t represent a normal winter situation, which, basing on the earlier studies performed in normal winters (Durinck et al., 1994) and our own data collected in autumn, should harbour rather large local wintering population of Common Gulls.







































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Number of observations (vertical axis in log-decimal scale) in corresponding density classes (horizontal axis limited to 75 birds/km2) in the winter period sample of Common Gull densities. Figure 3-26. Distribution of the average densities (n/km2) of Common Gull in the Baltic Proper in winter. Comparison of the location of areas with lowest densities (up to 3 birds/km2) of Common Gulls in autumn and winter (Fig. 3-25, 3-26) suggests that, on a rough scale, the main wintering areas of this species tend to shift southwards by approximately 1o on the transition from autumn to winter season. Offshore waters at the latitude of Liepaja seem to be abandoned in winter, whereas offshore waters of the Gulf of Gdansk and Slupsk Bank indicate a considerable increase of Common Gull densities in winter. The basic range of densities of the Common Gull indicated a continuous increase on the transition from the breading and moulting season (up to 30 birds/km2) to autumn migration and formation of wintering concentrations (both up to 50-60 birds/km2), whereas maximal densities could reach up to 100 birds/km2 in autumn and even up to 500-600 birds/km2 in winter. A very specific feature of Common Gull offshore populations in Eastern Baltic was a clear tendency of doubling its maximal average densities (in the most preferred areas, respectively) within the seasonal gradient: ~7 birds/km2 in summer, ~15 birds/km2 – in autumn and ~30 birds/km2 – in winter. Comparison of spatial distribution of the Common Gull densities in the Baltic Proper during different periods of

38

the non-breeding season indicate, that the largest total regional population of the species in the Baltic Proper could be found in winter, whereas the lowest – in summer. 3.3.15. Herring Gull Herring Gull seems to be one of the most common, yet definitely not the most numerous species in the Baltic Sea region (see Chapter 3.2) with major breeding colonies in northern Baltic (Kilpi 1988). Earlier offshore studies of wintering seabirds in the Baltic Proper demonstrated presence of this species even in the most offshore areas of western Baltic Proper and the coastal areas of the eastern Baltic (Durinck et al., 1994). Due to insufficient coverage of the offshore areas, distribution pattern of wintering Herring Gulls was not complete. The most important wintering areas of this species along the eastern coast of the Baltic Sea in normal winters were found LQ WKH ZHVWHUQ SDUW RI WKH *XOI RI 5LJD LQ WKH 6OXSVN %DQN 3ROLVK VHFWRU DQG QRUWK RI .ODLS¡GD /LWKXDQLDQ

sector). Our studies in eastern part of the Baltic Proper clearly demonstrated, that Herring Gulls are present in rather high densities in far offshore aquatories all around the year. The lowest densities of this species were found in summer (Gulf of Riga not surveyed), when the highest proportion of non-breeding Herring Gulls were observed in the Slupsk Bank of the southern Baltic (Fig. 3-27). Maximal densities of this species in summer reaching the level of 35-40 birds/km2. Rough proportion of young (