Dover (1992-93) or Ramsgate (1994-98). Four trained ornithologists, eventually assisted by other seabird watchers, carried out all ship-based surveys. The.
Seabird distribution
CHAPTER TWO DISTRIBUTION PATTERNS OF SEABIRDS IN BELGIAN MARINE WATERS
Jan Seys1*, Henk Offringa2, Jeroen Van Waeyenberge1, Patrick Meire3, Magda Vincx4 & Eckhart Kuijken1 1 Institute of Nature Conservation, Kliniekstraat 25, B-1070 Brussel, Belgium Ministry of Transport, Public Works and Water Management, Directorate General of Public Works and Water Management, North Sea Directorate, P.O.Box 5807, 2280 HV Rijswijk, The Netherlands 3 University Antwerpen, Department Biology, Universiteitsplein 1, B-2610 Wilrijk, Belgium 4 University Gent, Biology Department, Marine Biology Section, K.L.Ledeganckstraat 35, B-9000 Gent, Belgium * present adress: Flanders Marine Institute (VLIZ), Victorialaan 3, B-8400 Oostende, Belgium 2
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Distribution patterns of seabirds in Belgian marine waters J. Seys, H. Offringa, J. Van Waeyenberge, P. Meire, M. Vincx & E. Kuijken
ABSTRACT Intensive seabird surveying during seven years (1992-98) in the Belgian part of the southern North Sea revealed the existence of a land-sea and a longitudinal gradient from the Schelde estuary in the east to the deeper, less turbid waters in the west. Piscivorous species preferring clear water and mid- to offshore conditions (auks, Kittiwake and Northern Gannet) are more abundant in the west. Divers, grebes and Larus-gulls are commoner in the more turbid waters near the mouth of the Schelde estuary. Depth and topography are less dominant as explanatory variables for the distribution of most of the 17 dominant species/taxa. Multivariate and correlative analysis of the abundance of these species could not reveal strong temporal or spatial coherence of seabirds in communities. Highest correlations were found among Larus-gulls scavenging at trawlers, and in the group of auks, Kittiwake and Little Gull. The auks (Razorbill, Common Guillemot) and both gull species were often seen in short-lived multi-species feeding associations over presumed fish shoals. Razorbill is the species that associated most frequently (in 28% of all observations) and it appeared to be a more ‘attractive’ target for Kittiwake (34%) and Little Gull (23%) than the Common Guillemot. Kleptoparasitic behaviour was rarely observed (2.9-6.3% of the observations in skuas). The impact of fishery activities on the distribution of scavenging seabirds (8 of the 17 dominant species) is large. Some 65-70% of all large gulls in the study area were observed in association with trawlers. The general patterns of distribution described in this paper provide the basis for new future research. Major emphasis should go to the interactions between hydrography, prey-availability (pelagic fish) and the specific geomorphologic characteristics of this study-area. Priority species for more detailed research are proposed.
INTRODUCTION Since the start of systematic seabird counting in the North Sea in the late 1970s, eight atlases have been published of the area (BAPTIST & WOLF 1993, CARTER et al. 1993, CAMPHUYSEN & LEOPOLD 1994, ANONYMOUS 1995, CAMPHUYSEN et al. 1995, SKOV et al. 1995, STONE et al. 1995, OFFRINGA et al. 1996). The maps produced in these publications are very often based on smoothing small-scale data over large areas and time-intervals, obscuring the detailed spatial and temporal patterns. As a result the generalised and rather rough pictures have only a limited value for those interested in the community structure and interactions between seabirds and their environment (CAMPHUYSEN 1996). Seabirds may use very fine-scale cues in exploiting the environment, often much finer or more ephemeral than those that can be detected with large-scale surveys (BEGG & REID 1997, RIBIC et al. 1997, TASKER & REID 1997, WANLESS et al. 1997, CAMPHUYSEN & WEBB 1999).
1998, we put in a great effort (16,000 km) in a relatively short period and clear patterns in occurrence of seabirds could be observed. In this paper we want to analyse the spatial and temporal distribution patterns and try to find out whether communities of seabirds can be distinguished in relation to environmental conditions. The analysis will focus on how distance to the coast, longitude (distance to the Schelde estuary and the Straits of Dover), depth, topography (swale, slope or crest of a sandbank) and period of surveying may structure seabird populations. By studying coexistence patterns of seabird species, we will try to gain more insight in the relationships among species and in the coherence of communities. Do communities of seabirds at sea exist or are they basically the artificial result of lumping and smoothing seabird data over large time-spans and areas, creating pictures of coexistence in species that are rarely found together? An analysis of associations and correlations between species can also help to answer whether certain seabird species may behave as ‘umbrella species’, i.e.: species with such demanding habitat requirements that saving it will automatically save many other species (SIMBERLOFF 1998). In general certain species might tell more about their environment or about associated species than others, and deserve more attention in the future in terms of research investment.
The unique geomorphologic characteristics of the Belgian marine waters - with four major formations of linear sand ridges (‘sandbank systems’) each with its own strike, distance to the coast and topography -, and its location in between the Straits of Dover and the mouth of the Schelde estuary, hold prospects to investigate distribution patterns of seabirds in more detail. During extensive seabird surveys in Belgian marine waters from September 1992 till December
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Seabird distribution
MATERIAL & METHODS STUDY AREA 1979), with various sand ridges resulting from sediment and melt water displacements during several glacial periods. Six subregions – including four major sandbank systems each with a characteristic strike, profile and distance to the coast – can be distinguished (Fig. 1, Table 1, CATTRIJSSE & VINCX 2001). These subregions were further divided in 36 ‘sandbanks’, each split up into three strata. The strata ‘crest’, ‘slope’ and ‘swale’ are defined as 20 m, except for the inshore Oostkust- and Westkustbanken where < 5m, 5-10 m and >10m respectively.
The study-area is situated in the southern North Sea, characterised by its shallowness and well-mixed waters. It forms the main and central part of what is internationally referred to as the ‘Flemish Banks’ (OSPAR COMMISSION 2000). Belgian marine waters occupy a small area (3500 km2), are intensively used (MAES et al. 2000) and near to important sources of water input (situated at c. 55-75 km east of the Straits of Dover and immediately west of the mouth of the Schelde estuary). The structural variation in topography is unique for the North Sea (HOUBOLT 1968, EISMA et al.
Fig. 1. Belgian marine waters with: a) setting within the Southern Bight of the North Sea; b) subdivision in six subregions and 35 sandbanks; c) most important sandbanks
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Table 1. General characteristics of the Belgian marine waters, divided into six subregions on the basis of the occurrence of sandbank systems: OK=Oostkustbanken, WK=Westkustbanken, VB=Vlaamse Banken, ZB=Zeelandbanken, HB=Hinderbanken, DW=Deepwater subregion. The figures indicated with Seys et al this study, have been derived from the official maritime charts and from a GIS application tool. VARIABLE Area (km2) Mean depth (m MLLWS) Minimal depth (m MLLWS) Mean elevation sandbanks to seafloor (m) Length sandbanks (km) Distance to coast (km) Strike of the sandbanks (°) Mean surface temperature in February (°C) Mean surface salinity in February (‰) Max. tidal current velocity at surface (m.s-1) Depth 1% daylight penetration in Dec. (m) Non-living suspended matter (ppm)
OK 310 0-10 0 2-5 10-20 0-13 70 4.0 30.5 1.8
