Environmental Biology of Fishes 54: 371–387, 1999. © 1999 Kluwer Academic Publishers. Printed in the Netherlands. Distribution patterns of indigenous freshwater fishes in the Tagus River basin, Spain Jose´ A. Carmonaa, Ignacio Doadrioa, Ana L. Marquez´ b, Raimundo Realb, Bernard Huguenyc & Juan M. Vargasb a Museo Nacional de Ciencias Naturales, Jos´e Guti´errez Abascal, 2, 28006, Madrid, Spain (e-mail: [email protected]) b Departamento de Biolog´ıa Animal. Facacultad de Ciencias, Universidad de Malaga,´ 29071, Malaga,´ Spain c CNRS, URA 1974, Laboratoir du Ecologie des Eaux Douces, Baˆt. 403, 43 Bd du 11 Novembre 1918, 69622 Villeurbanne Cedex, France Received 19 December 1997 Accepted September 1998 Key words: biogeography, chorotypes, environmental boundaries Synopsis Classification and ordination methods used to examine the internal complexity of the Mediterranean Tagus River catchment based on fish distribution revealed that it is not a homogeneous biogeographical unit. The indigenous fishes analyzed in this study are distributed through the basin forming geographical communities (chorotypes), some of which are associated with environmental factors like river morphology, water quality or geographical location. Nevertheless, 40% of the variation in species occurrence remains unexplained by either environmental or geographical variables, suggesting that historical factors may influence the freshwater fish distribution patterns. Three main biogeographical areas, delimited by significant boundaries, were identified. Two of them are identified as the upper and the middle-lower basins of the Tagus River catchment; the third corresponds to the Alagon´ River and seems to be linked to historical factors of the catchment. Introduction the Iberian Peninsula; 19 species are introduced and 35 are indigenous of which 23 are endemic (Collares- Two main areas have been established in Europe on the Pereira 1980, Coelho 1985, Doadrio et al. 1991, Elvira basis of freshwater fish fauna (Ban˘ arescu˘ 1989): Cen- 1995). tral Europe, with a fish fauna consisting of widespread Historical factors, such us the Miocenic origin species, and Southern Europe, characterized by many of the primitive endorheic basins that developed endemic species. The Iberian Peninsula belongs to the into the current hydrographic basins in the Pliocene southern subdivision and has the highest proportion of (Lopez-Mart´ ´ınez 1989) have exclusively been used endemic fish species in Europe. Nevertheless, it is also to explain freshwater fish distribution in Iberian characterized by low diversity compared with other Peninsula (Almac¸a 1978, Doadrio 1988, Hernando & areas in the southern European region and with central Soriguer 1992). However, ecological factors may Europe. Most of the Iberian species belong to genera also play a basic role in the fish distribution that also inhabit central Europe such as Barbus Cuvier patterns and communities at smaller time scale and Cloquet, 1816, Chondrostoma Agassiz, 1835, (Grossman et al. 1982, 1985, Pusey et al. 1993, Leuciscus Cuvier, 1817, Rutilus Rafinesque, 1820 Oberdoff et al. 1993) and can contribute valuable and Cobitis Linnaeus, 1758, but few species are information, since the historical explanations do not shared by both biogeographic areas. Currently, 54 imply a homogeneous distribution of freshwater fishes freshwater fish species with stable populations inhabit within a hydrographic basin. 372 Hydrographic basins are geographical units for In this paper we examine biogeographical com- studying freshwater fish distribution since they have plexity variaton in the Spanish Tagus basin and the well-defined boundaries that primary fishes (Myers nature of species replacement patterns. We tested for 1951) rarely cross under natural circumstances. As a the existence of (a) distribution patterns shared by result, various authors have considered river basins several indigenous freshwater fishes, which we call as operational units for the biogeographical analysis ‘chorotypes’ after Baroni-Urbani et al. (1978), (b) bio- of fishes or other freshwater taxa (see Doadrio 1988, geographical boundaries within the basin, which would Matthews & Robison 1988, Real et al. 1992, 1993). explain fish distribution patterns, and (c) environmen- Nevertheless, the distribution of freshwater fishes tal factors that could explain such shared distributions within a hydrographic basin may be heterogeneous. and biogeographical boundaries. Changes in fish composition through the basin can occur either continuously or discontinuously. The River Continuum Concept (Vannote et al. 1980) refers to Material and methods gradual changes in community structure and a gradual increase in species richness from upstream to down- Study area and environmental variables stream with a maximum located in the medium course. However, typical features of fluctuating Mediterranean The Tagus River (Figure 1) is located in the mid- rivers, especially the variety of available microhabi- dle of the Iberian Peninsula, between the Duero tats (Grossman et al. 1987), could produce high het- and Guadiana basins. It runs mostly from east to erogeneity in ecological factors throughout the basin west and flows into the Atlantic Ocean. However it and generate more complexity in distribution patterns presents the typical southern European river typol- of freshwater fish species than expected in a homo- ogy because most of its basin is regulated by the geneous or gradually changing biogeographical unit. Mediterranean climate (Arenillas & Saenz´ 1987). It Southern European rivers show a singular typology contains 24 freshwater fish species, 14 of them native. characterized by short streams with water level fluc- The number of recorded endemic species is espe- tuations and an intermittent flow that depends on alter- cially high and, except for Salmo trutta Linnaeus, nating dry and wet seasons (Arenillas & Saenz´ 1987). 1758 and Atherina boyeri Risso, 1810, all native In most cases the rivers consist of extensive pools con- species are also endemic. They are Barbus bocagei nected by shallower runs. These pools become isolated Steindachner, 1865, Barbus comizo Steindachner, during the dry season, giving way to a high number 1865, Barbus microcephalus Almac¸a, 1967, Chondros- of fragmented microhabitats isolated by physical and toma polylepis Steindachner, 1865, Leuciscus pyre- ecological gradients (Balon 1981). This phenomena naicus Gunther,¨ 1868, Leuciscus carolitertii Doadrio, may preclude traditional river zonation (Huet 1959) 1987, Rutilus arcasii (Steindachner, 1866), Rutilus into upper, medium and lower streams as a function of lemmingii (Steindachner, 1866), Tropidophoxinellus gradient, current velocity and temperature of the water alburnoides (Steindachner, 1866), Cobitis paludica (see Balon & Stewart 1983). (De Buen, 1930), Cobitis vettonica Doadrio & The Tagus basin is characterized by a rich endemic Perdices, 1997 and Cobitis calderoni Bacescu, 1961. fish fauna due to its ancient and endorheic origin. It was We excluded A. boyeri and B. microcephalus from our formed during the Miocene through the connection of analysis because they are extremely localized. We con- the endorheic basins of Loranca, Madrid and Lisbon sider the samples of S. trutta collected for this work as (De la Pena˜ 1995) as a result of tectonic movements natural populations. Fish reintroducted in the basin are that raised, and later tilted the Iberian Peninsula toward very localized and the genetic introgression detected the Atlantic Ocean (Lopez-Mart´ ´ınez 1989). Such abun- was moderately low in natural populations (Garc´ıa- dance of endemic fishes and the large size of the basin Mar´ın & Pla 1996, Garc´ıa-Mar´ın et al. 1991). (54 769 km2 and 731 km of principal course length) Two main biogeographical areas were defined make the Spanish Tagus River an excellent model for (Doadrio 1988) within the Iberian Peninsula on the studying the influence of microhabitats on the biogeo- basis of freshwater fish distribution: the northern graphical complexity of a basin, and allow examination region containing some central european species and of heterogeneity in fish distribution patterns. the southern Iberian region inhabited by the bulk 373 Figure 1. Location of the sampling points in the study area: ◦ = sampling points with only physical variables available, •=sampling points with chemical and physical variables available. of the endemic species. Within the latter region the capture of rare species at each station. We performed Tagus River basin is transitional between the Galician- at least three succesive catches at each sampling point, Portuguese and southern ichthyofaunal sectors. It with a similar sampling effort. Three people conducted forms the southern limit for species such as B. bocagei, the sampling over 150–200 m of river length. Fish were R. arcasii, L. carolitertii and C. calderoni, and the present at only 77 sampling points (Figure 1) because northern limit for others such as B. microcephalus, many stretches were dry and others were too polluted. B. comizo and R. lemmingii. The subspecies of Ch. Physical variables indicative of river morphology polylepis also have their distribution limits in the were measured at each of the 77 sampling points, but Tagus basin (Doadrio et al. 1991). chemical variables indicative of water quality were We performed the sampling between 1991 and 1993, available only from 51 (Hydrographic Confederation always in late summer-early autumn, using electrofish- of River Tagus unpublished data). Both sets of data,