In the Portuguese fraction of the hydrographic basin of the Guadiana River, there are three dozen species, 13 of which (about 43%) correspond to non-native species.
The majority of these introductions took place during the twentieth century, with some of these taxa showing a remarkable acclimatization to water courses, representing a serious threat to several Iberian endemisms, such as saramugo (Anaecypris hispanica). The species of exotic origin have undergone a marked increase over the last decades, and it should also be noted that the respective areas of distribution have been significantly expanded.
The expansion of many of the exotic species has been favored by the transformations induced in the river systems, namely the construction in the last decades of medium/large dams, a particularly relevant fact in the Alentejo. The construction of these hydraulic infrastructures, in addition to breaking the lotic continuum and increasing depth, almost always offer significant fluctuations in the water level, with the consequent presence of unguarded margins of vegetation. This change in ecological conditions rarely benefits native species, which are not ecologically prepared to adapt to these environments, which means that artificial lentic systems are dominated, both in number of species and in biomass, for non-native taxa.
Specifically on the Guadiana river basin, the introduction of non-native fish species has several origins:
i.) existence of several common water courses with Spain, inducing that some of the introductions in that country have expanded, through natural dispersion or by anthropic intervention, to national territory - e.g., channel catfish (Ictalurus puntactus).
ii.) introduction of a taxa of consumers of malaria vector mosquitoes - gambusia (Gambusia holbrooki) -, which in periods of the 20th century was a very widespread disease in Portugal;
iii.) Provide greater income of inland waters, taking advantage primarily of the value of some species for sport fishing - e.g., largemouth bass (Micropterus salmoides) and lucioperca (Sander lucioperca) -, extensible to forage species (e.g., bleak in relation to largemouth bass); this item can also consider the availability of protein for human consumption - e.g., common carp (Cyprinus carpio) and;
iv.) release of specimens used in aquarium - e.g., chanchito (Australoheros facetus);.
Next, the freshwater fish species referenced for the Portuguese fraction of the Guadiana basin are listed, indicating the likely reason for their introduction as well as a synthetic summary of their ecology.
From the following list these species were excluded:
i.) the tinca (Tinca tinca): despite being classified by some authors as exotic, doubts remain as to the nature, native to the allotone, of the taxon, in view of its dispersed and very localized distribution pattern;
ii.) the mummichog (Fundulus heteroclitus): for being a species that occurs mainly in brackish waters, more specifically in the estuary of the Guadiana River;
iii.) the goby (Gobio lozanoi): in the nineties some specimens were captured in the Xévora river but it was not identified again; and,
iv.) the tinfoil barb (Barbonymus schwanenfeldii): which is a species with wide use in aquarium, having only two specimens been captured in 2005 in the Lucefécit dam.
CYPRINIDAE
Common carp (Cyprinus carpio, Linnaeus 1758)
The common carp originated in Eastern Europe and Western Asia, having been introduced by the Romans in Italy and since then spread to other European countries. It is one of the most widespread species on the globe and the oldest introduction to the Iberian Peninsula, probably since the 16th-17th centuries. It is a very resistant species that occupies mostly reservoirs, also subsisting in lotic areas with a weak current, with deep vases and that preferably offer some depth. It presents a benthic eating behavior and omnivorous regime, consuming insect larvae, algae, macrophytes, small molluscs and fish fry.
Goldfish (Carassius auratus, Linnaeus 1758)
The goldfish is a taxon that was presumably imported from China in the early 17th century, for ornamental reasons. It is a species that is mainly associated with lentic systems, and may also use some of a lotic character - in the Guadiana basin, in particular, the river sectors of greater number immediately upstream of the reservoirs - particularly the sections with weak current, abundant vegetation and fine texture beds. It presents a high resistance, withstand relatively unfavorable conditions from an environmental point of view. It exhibits a benthic eating behavior and omnivore regime, essentially consuming aquatic macroinvertebrates, detritus and material of plant origin - e.g., algae, macrophytes.
Prussian- carp (Carassius auratus, Bloch 1782)
The prussian carp has been referred to Portugal since 2012, being originally from the Asian continent. It is a species with a wide distribution worldwide, and the main reason for introduction corresponds to its use as an ornamental. In the Guadiana basin, its populations are not very significant, occurring in reservoirs and in the final section of its larger tributaries. The ecological characteristics are identical to those of the goldfish.
Bleak (Alburnus alburnus, Linnaeus 1758)
Bleak is a species originating in Europe that was introduced in Spain in the nineties of the 20th century, as a forage species for ictiophages. It has patented a considerable expansion in recent years, having reached Portugal through international water courses, associated with translocations by sport fishermen, as it is a species that is very much appreciated by largemouth bass as food. The first records for the Guadiana basin date back to the Caia and Odeleite reservoirs at the beginning of the 21st century, having subsequently colonized most river systems, particularly in the upper and middle sector of this hydrographic basin. This cyprinid has gregarious habits and exhibits great plasticity, occurring in lotic and lentic systems, despite preferentially selecting the shallowest areas. It has a carnivorous diet, consuming mainly aquatic insects, crustaceans and zooplankton.
Rainbow trout (Oncorhynchus mykiss, Walbaum 1792)
Rainbow trout is native to North America and was introduced in the Iberian Peninsula in the late 19th century, being one of the most produced salmonids in aquaculture. The occurrence of this taxon in national watercourses will have resulted from escapes from fish farms, together with restocking carried out by official bodies, as it is very interesting from a fishing and food point of view. To date, reproduction in Portugal in natural conditions has not been proven, which suggests that its preservation derives from successive restocking. In the course of the A3 actions of the LIFE Saramugo project, a specimen was captured in the Xévora River, which most likely resulted from the regular restocking actions that are carried out in fishing harbors on the Spanish stretch of this river. Its preferred habitat corresponds to lentic systems, and can also occur in clean water rivers with strong or moderate currents. It is a carnivore that feeds mainly on aquatic macroinvertebrates and small fish.
Eastern mosquitofish (Gambusia holbrooki, Girard 1859)
Eastern mosquitofish is a small-sized species originating in North America, which is spread over several dozen countries on five continents. Its introduction in Spain dates back to the second decade of the 20th century, where it was intended to control the vectors of malaria propagation; later, and with the same objective, it was also released in Portugal, where it quickly expanded. Nowadays it will probably be the exotic species with the widest distribution at the level of the hydrographic basin of the Guadiana River, sometimes showing very expressive densities. It preferably chooses areas of weak or even no current, with a high density of aquatic macrophytes, being an extraordinarily resistant species, which can withstand high temperatures and considerable levels of pollution. It is carnivorous, feeding mainly on mosquito larvae, but also on other aquatic invertebrates and even some terrestrial ones that appear on the aquatic surface.
Northern pike (Esox lucius, Linnaeus 1758)
Northern pike was a taxon introduced in Spain at the beginning of the second half of the 20th century, to promote sport fishing, which later reached Portugal via the international rivers (Tagus and Guadiana). It is currently referenced to the main section of the Guadiana corresponding to the Alqueva reservoir, as well as in the terminal part of some of its larger tributaries, namely the Xévora River. It preferably chooses lotic sections of weak current and vast vegetation, where it is hidden to attack prey. It is a territorial and very voracious carnivore, which as an adult is a piscivore, feeding occasionally on large invertebrates - e.g., crayfish - and even amphibians or water birds.
Largemouth bass (Micropterus salmoides, Lacépède 1802)
The largemouth bass is a species originating in the USA and Mexico, which was introduced in our country at the beginning of the second half of the last century, currently constituting a very important species from a sports and gastronomic point of view. In Portugal it had excellent acclimatization, currently occupying almost all hydrographic basins. The present taxon is mostly restricted to lentic areas - reservoirs and ponds, which are very abundant in the Guadiana watershed - and to lotic sectors of abundant vegetation and with not very strong current; corresponding mainly to the middle and lower sections of water courses. It is a carnivorous species that feeds mainly on fish, and can also eat crustaceans, amphibians, reptiles, birds and micro-mammals.
Pumpkinseed (Lepomis gibbosus, Linnaeus 1758)
Coming from the Northeast of the USA and Southern Canada, it was first imported into Europe in the late 19th century, most likely for use as ornamental fish. Pumpkinseed fish has been referred to Spain since the first decade of the 20th century, having been introduced in Portugal in the late seventies. Pumpkinseed is abundant in lentic areas and in some river sections with low current speed and high density of macrophytes, being one of the species with the greatest spread in the Guadiana watershed. It tolerates adverse environmental conditions, such as low oxygen levels, high temperatures and poor water quality. It has a carnivorous diet, consuming mainly insects, but also crustaceans, eggs and small fish.
Chameleon cyclid (Australoheros facetus, Jenyns 1842)
The chameleon cyclid is originally from South America, having been referenced for the first time in Portugal at the end of the first half of the 20th century, most likely the result of the release of specimens used in aquariums, as the Cichlidae family is one of the most used in the referred hobby. In Mediterranean conditions, the main reason for the survival of this taxon compared to other cichlids was due to its greater tolerance to lower temperatures. It revealed a good acclimation to national waters - particularly at the level of the hydrographic basins of Guadiana and Sado -, in rivers and reservoirs, where it is generally not very abundant, although occasionally it may have high population densities. In its region of origin it is referred to as a predator of larvae and small crustaceans, while the existing food data for Portugal indicate the primordial consumption of material of plant origin.
Black Bullhead (Amieurus melas, Rafinesque 1820)
The black bullhead is a species originating in North America, which was introduced in Spain at the beginning of the 20th century, from where it probably expanded to the Portuguese fraction of the Tagus basin; it will also have been transferred by fishermen, which explains its occurrence in lentic and lotic water bodies in the Guadiana and Sado river basins. Currently this species predominates in reservoirs as well as in some adjacent lotic sectors. The black bullhead occupies river sections from weak to null current, with abundant vegetation and sandy or muddy beds. It is extremely tolerant of pollution, subsisting in waters with low levels of dissolved oxygen and high temperatures. It is an omnivorous taxon that exhibits benthic eating behavior, including plant material, macroinvertebrates and small fish in its diet.
Channel catfish (Ictalurus punctatus, Rafinesque, 1818)
Species originating in the eastern USA and northeastern Mexico, having been widely disseminated worldwide for its interest in aquaculture, and also its sporting interest. The first registrations in Spain correspond to the eighties of the 20th century, with the Portuguese fraction of the Guadiana basin being initially detected in 2011, in the Alqueva reservoir, in its upstream sector (Ponte da Nossa Senhora da Ajuda). It is currently referred to the Alqueva and Pedrógão reservoirs, in addition to the section of the Guadiana river between this last dam and the Mértola area. It occurs preferably in reservoirs and deep water courses, tolerating a wide range of housing conditions. From a trophic point of view, it is omnivorous, with larger specimens feeding almost exclusively on other fish. It is also important to highlight some reproductive characteristics, namely the high fertility and parental care since the males protect the fry during the first weeks.
Zander (Sander luciperca, Linnaeus 1758)
Zander is a species of European origin that was introduced in Spain - during the seventies of the 20th century - to promote sport fishing. Currently, it is booming in our country, occupying many of the medium to large reservoirs existing in the hydrographic basin of the Guadiana River; it also occurs in the terminal sections of some larger tributaries (e.g., river Xévora and ribeira do Vascão), as well as in the section of the river Guadiana between the dam of Pedrógão and Mértola. Zander appears mostly associated with lotic sections with weak current and some depth, and can also occur in lentic systems. At the trophic level, it is said that fry feed on zooplankton, juveniles of invertebrates while adults have a mostly piscivorous diet, with Louisiana red crayfish (Procambarus clarkii) becoming abundant.
Article written by: Paulo Pinheiro
[Translated by: Natasha Silva]
