Biogeography of epibenthic crustaceans on the shelf and upper slope off the Iberian Peninsula Mediterranean coasts : implications for the establishment of natural management areas *

Epibenthic crustaceans are one of the most valuable resources of the Mediterranean demersal fishery. Some of them are widely known and heavily exploited throughout most of their distribution range, such as the Norway lobster (Nephrops norvegicus), the shrimps Aristeus antennatus, Aristaeomorpha foliacea and Parapenaeus longirostris or the stomatopod Squilla mantis (e.g. Ardizzone et al., 1990; Abelló and Martín, 1993; Demestre and Lleonart, 1993; Sardà, 1998; Carbonell et al., 1999). The species richness of epibenthic crustaceans in the Mediterranean is high, especially that of decapod crustaceans (Sardà and Palomera, 1981; Tunesi, 1986; Abelló et al., 1988; Relini et al., 1986; Mura and Cau, 1994; Falciai, 1997). They constitute an important part of the total biomass throughout the continental shelf and SCI. MAR., 66 (Suppl. 2): 183-198 SCIENTIA MARINA 2002

SUMMARY: The patterns of occurrence and bathymetric distribution of epibenthic crustaceans on the continental shelf and upper slope down to a depth of 800 m are analysed based on data gathered during six demersal trawl surveys performed annually in spring along the Mediterranean coasts of the Iberian Peninsula (from the Straits of Gibraltar to Cape Creus) between 1994 and 1999.A total of 598 valid hauls has been studied providing a total of 108 species of decapods, two stomatopods, one euphausiid, one mysid and one isopod.The study area has been subdivided into seven sectors according to their geomorphological characteristics, and the patterns of occurrence and abundance by depth have been analysed separately for each of the sectors.Detailed data on bathymetric distribution are presented for each species.Two main biogeographical areas can be discerned along the study area, which can approximately be separated at Palos Cape: the Alborán Sea to the southwest, and the northwestern Mediterranean (Levantine and Catalan Seas) northwest of Palos Cape.The continental shelf in the Alborán Sea (the most western area of the Mediterranean) is extremely narrow whereas it is much wider in the northwestern Mediterranean.The influence of Atlantic waters entering the Mediterranean is particularly strong in the Alborán Sea which shows a particularly high species richness of Atlantic affinity.Within the context of the western Mediterranean Sea, the Alborán Sea region shows important faunistic characteristics such that it might be considered as a possible separate natural management area for demersal fisheries.
slope, especially in the middle and lower slope (Cartes, 1993;Cartes andSardà, 1992, 1993;Cartes et al., 1993Cartes et al., , 1994)).Well over a hundred decapod crustacean species have been reported on the trawlable bottoms of the Mediterranean (Relini et al., 1986;Tunesi, 1986;Abelló et al., 1988;Mura and Cau, 1994;etc.).Faunistic studies constitute essential tools to understand the dynamics of exploited communities and accordingly, food webs can be much better understood, as well as other interspecific relationships, competence mechanisms, changes in relative abundance of the species, interannual changes in community structure and dynamics, etc.The samplings performed within the framework of the MEDITS international trawl survey (Bertrand et al., 2000) have provided an unique opportunity to study and delimit, with a high degree of accuracy, the geographical and bathymetric distribution of many species, as well as their interrelationships, given the large geographical area surveyed, the bathymetrical intensity of sampling, which encompassed the trawlable bottoms between 25 and 800 m depth, and the time duration of the surveys over several (1994-1999) years.
The identification of major geographical areas with communities sharing similar specific composition may provide an insight into their characteriza-tion as ecological assemblages sharing similar problems.Thus, from the management point of view, their identification should be taken into account when designing specific policies for the sustainable management of regional fisheries.The marked geomorphological differences between different Mediterranean basins and regions, may provide or facilitate the differentitaion of a reasonably large number of relatively isolated subpopulations within species, especially so in those with a short planktonic larval life.Boundaries such as sills, capes and associated stable or semi-permanent hydrographical circulation features (frontal zones, anticyclonic and cyclonic gyres, eddies, etc.) may constitute effective semi-permeable boundaries able to differentiate communities and population units.
The main objectives of the present work were to provide detailed information on the patterns of geographical and bathymetric distribution of decapod crustaceans on the trawlable bottoms of the continental shelf, upper and middle slope off the Mediterranean coasts of the Iberian Peninsula, and to geographically analyse and delimit the main biogeographical faunistic assemblages, especially aimed at the identification of natural management areas, recognized as one of the present time key issues in Mediterranean management (Caddy, 1998).

MATERIAL AND METHODS
The study area comprised the entire Mediterranean continental shelf, upper and middle slope off the Iberian Peninsula from Gibraltar Straits to Cape Creus, excluding most of the Balearic islands (Fig. 1).An overall area of 45331 km 2 was surveyed.The Alborán Sea and the Vera Gulf are characterised by a narrow continental shelf, as well as by the direct influence of surface Atlantic waters, which lead to local areas of high productivity.The continental shelf broadens in the Alacant-Eivissa area, where surface inflowing (modified) Atlantic and outflowing Mediterranean waters meet around the Eivissa sill.Around the Ebro delta and the Columbretes islands the continental shelf is widest (up to 70 km), and the area is highly productive due to the combined effect of the Liguro-Provençal-Catalan front at the continental slope and the runoff of the Ebro river.In the central-north Catalonia area the continental shelf becomes narrow again, and is indented by submarine canyons, with the Liguro-Provençal-Catalan front flowing southwestwards along the continental slope.
The information presented herein was obtained from a total of six trawl surveys performed annually in spring (May-June) from 1994 to 1999 using the research vessel "Cornide de Saavedra".A depth stratified random sampling procedure, taking into account the surface area of five depth intervals (0-50 m, 50-100 m, 100-200 m, 200-500 m and 500-800 m) and three broad geographical sectors, (Alborán Sea, Alacant region, and Catalan Sea) was used (MEDITS protocol) (Bertrand et al., 2000(Bertrand et al., , 2002)).The samples were obtained with a bottom trawl (model GOC-73) with a 4-m vertical opening and 20-mm codend mesh size, trawled at a speed of 3 knots.Hauls performed at depths of less than 200 m had a duration of 30 minutes; those performed deeper had a duration of 60 minutes.A total of 598 valid samples was obtained (Table 1).The depths sampled ranged between 25 and 798 m.
All crustaceans in the samples were identified, counted and weighed.It is however worth remarking here that for crustacean phyla typically with small-sized species, such as Euphausiacea, Isopoda and Mysidacea, the sampling was not adequate, given the net selectivity characteristics.Species belonging to these phyla were included in the present analysis given the focus centered on presence and absence distribution characteristics.Sampling can be considered adequate for Decapoda and Stomatopoda.Except for portunid crabs and some other exceptions, nomenclature of the decapod crustacean species has mainly followed d'Udekem d' Acoz (1999).The patterns of distribution are analysed as a function of depth stratum and geographical sector.Intervals of 50 m depth down to 200 m and of 100 m on the slope were used to analyse the distribution patterns of occurrence (Table 2).The narrower depth strata of the shelf zone were selected to account for the greater steepness of the environmental gradients associated with depth variations within this zone.The study area was additionally divided into eight geographical sectors, according to their different   In order to ascertain biogeographical similarities among geographical sectors, cluster analysis based on the faunistic composition resemblance between geographical sectors has been performed using Yule's coefficient as a similarity index and UPGMA (Unweighted Pair Group Method using Arithmetic Averages) aggregation algorithm.Yule's index has been chosen since it adequately summarizes presence/absence data in biogeographical analyses (Macpherson, 1991).Jackson et al. (1989) indicated that this index reduced the size-influence associated with frequency of occurrence.Yule's index was used because it adequately summarized the major groupings of samples (sectors).Identical groupings were obtained using Jaccard's similarity index.Species with less than three occurrences have been excluded from the analysis due to the little information afforded.Samples from the Eivissa Island sector have also been excluded due to the extreme scarceness of samples taken on the continental shelf in that sector (Table 2) which would preclude a proper analysis of the data.A resulting matrix of 92 species and seven geographical sectors encompassing the whole Iberian Peninsula Mediterranean coasts has been used in the analysis.

Biogeographical analysis
The geographical sector with the highest species richness was the Ebro delta sector, with 82 species, followed by Eastern Alborán and Vera Gulf sectors, with 79 species each (Table 4).Eivissa Island was the sector with the fewest number of species, with 49 species recorded, but very few samples were obtained at depths shallower than 200 m (Table 2) and data were therefore not directly comparable.Alacant with 58 species was therefore the sector with the fewest recorded species.
The percentage occurrence of each species within each geographical sector (Table 4), allows the characterization of the geographical sector(s) in which a species was more frequently recorded.One main pattern was found, in which some  ).-Crustacean species captured on the shelf, upper and middle slope along the Spanish Mediterranean during the MEDITS_ES cruises (1994)(1995)(1996)(1997)(1998)(1999).The peculiar faunistic characteristics of the Alborán Sea are further emphasized by the cluster analysis performed among geographical sectors based on the resemblance obtained with the qualitative Yule's index, which is based on presence/absence analysis of the species by geographical sector (Fig. 2).By using this approach, two main groups of geographical sectors were clearly obtained: one, in which the two Alborán Sea sectors, plus Vera Gulf, are grouped together (Alborán Sea cluster), and the other, which encompassed the rest of geographical sectors north of Palos Cape: Alacant, Valencia, Delta and northern Catalonia (Northwestern Mediterranean cluster).Identical groupings were obtained using Jaccard's similarity index.

Bathymetrical patterns of occurrence
The overall depth range, mean depth of occurrence and the percentage occurrence of each species within each depth stratum are presented in Table 5.This table provides detailed information on the depth strata in which each species was most frequently found throughout the study area.Thus, for example, the portunid crab Macropipus tuberculatus was present in over 70% of the samples taken between 200 and 400 m; the species showed decreasing occurrence values at depths shallower and deeper than these strata, reaching zero values at the shallowest and deepest.As another example, the red shrimp Aristeus antennatus clearly showed an increasing occurrence with depth, being absent at depths shallower than 400 m and reaching occurrences of 95.2% within the deepest sampled stratum (700-800 m).
From these data, information on the commonest species within each depth stratum can be obtained (Table 6).Thus, very few species were commonly found on the continental shelf bottoms, which were mainly dominated by the portunid crab Liocarcinus depurator and the hermit crab Dardanus arrosor.Caridean and penaeid shrimps, such as Plesionika heterocarpus and Parapenaeus longirostris, together with the portunid crabs Liocarcinus depurator and Macropipus tuberculatus, were the commonest species on the upper slope bottoms between 150 and 300 m.A large number of species with high occurrence values was present in the 300-400 m depth interval, in which nektobenthic and benthopelagic shrimps such as Solenocera membranacea and Pasiphaea sivado, together with the burrowing benthic lobster Nephrops norvegicus, were the most characteristic species.At depths of over 400 m, the nektobenthic shrimp Plesionika martia was the species most frequently occurring in the hauls; other deep-sea species, such as the benthic polychelid lobster Polycheles typhlops, the bathypelagic shrimp Sergia robusta, or the nektobenthic shrimp Plesionika acanthonotus, were also present with high occurrence values.Species such as the red shrimp Aristeus antennatus, the crab Geryon longipes, the benthopelagic shrimp Pasiphaea multidentata and the hermit crab Pagurus alatus, clearly increased their percentage occurrence towards the deepest strata.
Differences between the depth occurrence patterns among the different large groups of decapod crustaceans are clearly apparent when analysing the depth distribution range and the mean depth of occurrence of each species within each taxonomic group.
Dendrobranchiate shrimps (Fig. 3A) are species clearly living on the middle slope bottoms (400-800 m), with only three, out of twelve, species present on the continental shelf; two of those, however, Parapenaeus longirostris and Solenocera membranacea, presented their mean depth of occurrence in the upper slope (200-400 m).Some species, such as P. longirostris, S. membranacea or Sergestes arcticus presented large bathymetrical occurrence depth ranges.
Most caridean shrimps (Fig. 3A) showed large occurrence depth ranges and were clearly continental slope dwelling species, both in the upper (approx.200-500 m) and in the middle slope (>500 m).Only three species (two of them occasional) presented their mean depth of occurrence on the continental shelf, of which the commonest was the crangonid Aegaeon cataphractus.It is worth noting the large number of pandalid and crangonid species occurring throughout the study area which often show a marked interspecific bathymetrical segregation that can be more clearly observed when analysing their percentage occurrence by depth strata (Table 5).
Anomuran crabs (hermit crabs and squat lobsters) presented a shallow continental shelf distribution (Fig. 3B).Only one species of hermit crab (Pagurus alatus) and occasional specimens of Dardanus arrosor and Pagurus prideaux were present on the middle continental slope, at depths of over 400 m.Three species of the genus Munida were present on the continental slope, with a rather marked bathymetrical segregation between them: M. rutllanti, M. intermedia and M. tenuimana.
Brachyuran crabs (Fig. 3C) are also a typical group of species on the continental shelf.Most of the species recorded in the current surveys presented their mean depth of occurrence within the continental shelf, some of the commonest being, for example, Atelecyclus rotundatus, Pilumnus spinifer or Inachus dorsettensis.Some species had their mean depth of occurrence on the continental shelf but were also rather commonly found on the upper slope and sometimes in the middle, such as Medorippe lanata, Macropodia longipes, Liocarcinus depurator, Homola barbata or Calappa granulata.A few species presented their mean depth of occurrence on the middle slope but were rather commonly found on the shelf and sometimes on the upper slope, such as Goneplax rhomboides and  Macropipus tuberculatus or the benthopelagic crab Polybius henslowi whose distribution was restricted to the Alborán Sea.Monodaeus couchi was rather common throughout the middle and upper slope, but presented also some occurrences on the continental shelf.A few species appeared as characteristic of the middle slope, at depths of over 500 m, of which the commonest were Geryon longipes, Dorhynchus thomsoni and Paromola cuvieri.Some of these deep-sea species presented their distribution area exclusively (or almost exclusively) restricted to the Alborán Sea geographical sector, such as Ergasticus clouei, Bathynectes maravigna, Rochinia carpenteri, Cymonomus granulatus and Euchirograpsus liguricus.
Other crustacean taxonomic groups (Fig. 3D) presented a much smaller number of species.Thus, eryoneid polychelid lobsters were only represented by Polycheles typhlops, present in the middle and, sometimes, upper slope.Palinurid lobsters appeared scarcely, the commonest species being Palinurus elephas on the continental shelf and Palinurus mauritanicus on the slope.Stenopodid shrimps (Richardina fredericii) were only recorded once in the middle slope.The commonest thalassinid shrimp was Calocaris macandreae, frequently recorded on the slope.Nephropid lobsters were mainly represented by Nephrops norvegicus, recorded on the upper continental slope, with some occurrences on the shelf.Stomatopods were represented by Squilla mantis on the shelf, and by Rissoides pallidus on the upper slope.Other crustacean groups appeared more scarcely in the samples.

DISCUSSION
The detailed analysis of the patterns of occurrence of epibenthic and benthic crustaceans on the Mediterranean coasts of the Iberian Peninsula has allowed proper delineation of the bathymetric distribution range of the commonest species, as well as provided valuable information on scarcer species.Also, the wide geographical range of the samples studied, of well over a thousand km (from the Straits of Gibraltar in the south-west to Cape Creus in the north-east), has objectively delimited broad biogeographical boundaries along the regional sectors studied.The actual faunistical composition of the Mediterranean Sea is a product of its geological oceanographical changes, and, given the characteristics of it being a semi-enclosed sea with rather dif-ferent oceanographic characteristics from those of the neighbouring Atlantic Ocean, has favoured speciation in many decapod families (Zariquiey-Álvarez, 1968;Almaça, 1985aAlmaça, ,b, 1988;;Pérès, 1985).
Thus, one of the most relevant findings of the present study has been the identification of the Alborán Sea as a biogeographically distinct unit which can be separated from the rest of the analysed western Mediterranean sectors by a boundary, or biotone, around Palos Cape.The western and eastern Alborán Sea, together with Vera Gulf, share a number of geomorphological and oceanographic characteristics, namely the virtual absence of a continental shelf and the strong influence of the Atlantic currents entering the Mediterranean through the Straits of Gibraltar on the upper levels of the water column, and of the Mediterranean waters exiting the basin into the Atlantic at the deeper levels (Hopkins, 1985;Millot, 1987;García-Ladona et al., 1996).The well-documented stable presence of the Almería-Orán hydrographical front (Millot, 1987(Millot, , 1999;;Font et al., 1998) undoubtedly contributes to the biogeographical differentiation between the two identified areas, one southwest of Palos Cape, characterized by the occurrence of species with strong Atlantic affinities (i.e.Rochinia carpenteri, Polybius henslowi, Ergasticus clouei, Bathynectes maravigna, etc.), and the other northeast of that area.The present results provide, therefore, strong evidence for the Alborán Sea to be considered a distinct separate management unit when dealing with demersal fisheries from an ecosystem point of view (Caddy, 1998).
The particular geomorphological and associated oceanographical characteristics of the Alborán Sea make this region a distinct transition zone between Mediterranean and Atlantic faunas.The overlap area between the three classical marine biogeographical regions of the northeastern Atlantic (Mauritanian, Lusitanian and Mediterranean) is found in the Alborán Sea.According to Pérès and Picard (1964), this region is characterised by the rarity or absence of certain Mediterranean endemisms and by the presence of Atlantic species that are not found in other Mediterranean regions.The Alborán Sea basin has been previously identified as a region of great biogeographical importance in several faunistic groups, such as sponges (Templado et al., 1986;Maldonado, 1992;Boury-Esnault et al., 1994;Maldonado and Uriz, 1995;Pansini, 1996), bryozoans (Harmelin and D'Hondt, 1993), pycnogonids (Stock, 1987) or hydrozoans (Medel and López-González, 1996).Bouchet and Taviani (1992), working on marine gas-tropods, established the hypothesis of Atlantic species establishing pseudopopulations, non-able to succesfully reproduce, in the Mediterranean basins.Harmelin and D'Hondt (1993) also stated that several endemic Atlantic bryozoan species or morphotypes can live in conditions of typical Mediterranean water (high temperature and salinity) but do not spread beyond the eastern limits of either the Gibraltar Straits or the Alborán Sea.Concerning decapod crustaceans, García-Raso (1984, 1996) reported that the Alborán Sea fauna showed a higher similarity with that of the northern Ibero-Moroccan Bay, suggesting an influence of the Mediterranean outflow on the benthic fauna.Additionally, several deep-sea decapod species with epipelagic larvae able to overcome the Gibraltar sill with the inflowing surface currents have only been reported to date in the westernmost basin of the Mediterranean Sea (i.e. the Alborán Sea region), such as Penaeopsis serrata or Hymenopenaeus debilis (Abelló and Torres, 1998;Cartes et al., 2000).
In addition to bathymetrical boundaries, which constitute the most evident results arising from crustacean faunistic assemblages studied at small geographical scales (e.g.Abelló et al., 1988;Cartes, 1993;Cartes and Sardà, 1993;Cartes et al., 1994;Haedrich et al., 1975Haedrich et al., , 1980;;Ungaro et al., 1999), large-scale geographical and latitudinal boundaries have been identified in a much fewer number of studies on distribution data (e.g.Mas-Riera et al., 1990;Macpherson, 1991;Williams et al., 2001).Indeed, important faunistic differences, dealing mainly with different relative abundance composition of both faunistic groups and feeding guilds, have been identified in the western Mediterranean at scales of a few hundreds of km (Moranta et al., 1998;Maynou and Cartes, 2000).On the other hand, seasonality may also affect the faunistic composition and structure of demersal assemblages (e.g.Wenner and Wenner, 1989;Demestre et al., 2000), although given the limited temporal characteristics of the analysed trawl surveys this aspect cannot be analysed with the present data.
In agreement with previous faunistic studies restricted to the Catalano-Balearic Sea, the northernmost part of the Iberian Peninsula Mediterranean (Abelló and Valladares, 1988;Abelló et al., 1988;Cartes, 1993;Cartes and Sardà, 1993;Cartes et al., 1994), the present results have shown that brachyuran crabs constitute the Infraorder of decapod crustaceans with the highest species richness along the coasts of the Iberian western Mediterranean, fol-lowed by caridean shrimps and anomuran crabs.Notwithstanding, neither of those species constitute an important fraction of commercial catches in the western Mediterranean and many of them are discarded.Only a few crab and pandalid shrimp species are commercialized and hardly ever constitute the target species of any fishery (with the exception of Plesionika edwardsi, for which a targeted pot fishery takes place (García-Rodríguez et al., 2000)).The important target crustaceans in the Mediterranean belong to the Dendrobranchiata suborder, a decapod crustacean group that is not represented by many species in the Mediterranean (the coastal penaeid Melicertus kerathurus and the deep water shrimps Aristeus antennatus, Aristeomorpha foliacea and Parapenaeus longirostris), together with Nephrops norvegicus which belongs to the infraorder Nephropidea, with only two species occurring in the Mediterranean.
Concerning the depth distribution of the species, the present results support and expand the findings reported by several authors from the Catalano-Balearic Sea (e.g.Abelló et al.,1988;Cartes, 1993;Cartes andSardà, 1992, 1993;Cartes et al., 1993;Maynou and Cartes, 2000).Previously published data based on trawl surveys were practically missing from the Alborán Sea and the southern part of the Catalano-Balearic basin (Gil de Sola, 1993;García-Raso, 1996).
Some interesting bathymetrical distribution features can be identified from present data (Table 5), such as the occurrence in several benthic species, mainly with burrowing or burying habits, of clear bimodal peaks of occurrence, one clearly on the continental shelf at depths shallower than 100-150 m, and another on the continental slope at depths of 300-500 m, clearly avoiding the shelf-slope break.Species showing this bimodal depth distribution patterns are some crabs such as Goneplax rhomboides, Medorippe lanata, Monodaeus couchii, or the Norway lobster Nephrops norvegicus.To a lesser extent, this pattern is also shown by the crabs Liocarcinus depurator and Calappa granulata, and by the hermit crabs Dardanus arrosor and Pagurus prideaux.
Phylogenetical constraints on distribution characteristics can also be debated based on the differential patterns presented by the several large groups of decapods present in the study area (Fig. 3).Thus, dendrobranchiate shrimps thrive in deep water in the middle continental slope, deeper than 400 m, and are very scarce on the continental shelf.Caridean shrimps, mainly species with nektobenthic habits, are also clearly inhabitants of the continental slope, usually shallower than dendrobranchiate shrimps.Several species are found on the upper slope, between 200-400 m, but very few still occur on the continental shelf.Hermit crabs are clearly a shallow continental shelf group of species, with only two to three species being present on the upper and middle slope.Squat lobsters present two groups of species, with Galathea species present on the shallow continental shelf and Munida species on the upper and middle slope.It is worth remarking the common occurrence of Munida rutllanti in the studied samples, taken in the decade of the 1990's, in contrast with the scarceness reported by other authors in previous decades (Sardà and Palomera, 1981;Abelló et al., 1988).Brachyuran crabs are also clearly a shallow continental shelf group of species, with some of them occupying the upper continental slope; the middle slope is characterized by a distinct grouping of crab species, most of them very scarce.
As a concluding remark, it is worth noting that trawl surveys, as exemplified by present results, can be very useful, not only to study the distribution and recruitment patterns and population characteristics of the target species of a fishery, but also to establish, from an ecological and biodiversity point of view, the geographical areas and depth strata sharing common characteristics which could therefore be managed under common policy rules (Caddy, 1998).Detailed studies on biodiversity characteristics, faunal assemblages and autoecology of non-commercial, but important species in the food webs, can also be performed and be useful for appropriate and sustainable management.
FIG. 3. -Bathymetric distribution of the crustaceans collected during the MEDITS trawl surveys performed along the Iberian Peninsula Mediterranean coasts, ranked according to their mean depth of occurrence.(A) Dendrobranchiata and Caridea; (B) Anomura; (C) Brachyura; (D) Other Crustacea groups.

TABLE 1 .
-Dates and number of samples taken in the six studied trawl surveys.

TABLE 2 .
-Distribution of the number of samples taken as a function of the geographical sector and depth stratum (MEDITS_ES94-99).
ragona to Cape Creus (Fig.1, Table1).Sectors WALB, EALB, VERA, ALAC and EIVI are located in the Algerian Basin of the Western Mediterranean and receive a stronger Atlantic influence than the rest of sectors (VALE, DELT and NCAT), located in the Catalano-Balearic Sea and strongly influenced by the Liguro-Provençal-Catalan slope current.

TABLE 5 .
-Depth range, mean depth of occurrence, and percentage occurrence of each species within each depth interval.Species are presented in alphabetical order.

TABLE 5 (
Cont.).-Depth range, mean depth of occurrence, and percentage occurrence of each species within each depth interval.Species are presented in alphabetical order.

TABLE 6 .
-Species whose percentage occurrence within each depth stratum is larger than 50%.