Interannual variation of fish assemblage structure in a Mediterranean river: implications of streamflow on the dominance of native or exotic species

Abstract

Streams in mediterranean-type climate regions are shaped by predictable seasonal events of flooding and drying over an annual cycle, but also present a strong interannual flow variation.The Guadiana River is one of the most important rivers in the Iberian Peninsula. The fish fauna presents 11 native freshwater species, including eight with high conservation status. Several exotic species are present, the most important being the American centrarchids pumpkinseed fish and largemouth bass. As a typical mediterranean-type river, the Guadiana has an irregular hydrological regime with severe drought periods and floods; the interannual variation of discharge presents a ratio of c. 100 to 1.From 1980 to 1995 several dry years were observed, culminating in the drought of 1991/92-1994/95. Analysing the variation of the fish assemblage structure during this period, exotic species (mostly pumpkinseed) progressively increased, strongly dominating in 1995. Indigenous populations dramatically decreased and a previously common endemic cyprinid (Anaecypris hispanica Steindachner) became endangered and one of the most threatened fishes of Europe. However, the following years presented above-average flows with several flood events and an inverse process occurred, with native species increasing their contribution in a short period.The importance of floods as a disturbance factor in the control of lentic or slow flowing water exotics is discussed. Native species apparently possess adaptive responses to high flows which exotics lack. This hypothesis is consistent with probabilityof-use curves and preflood-postflood surveys. Results emphasize the importance of floods in the environmental flows of mediterranean-type rivers. In the absence of flooding disturbance, exotic fish populations predictably grow, increasing the pressure on native species; abiotic disturbance may supersede deterministic outcomes of predation or competition and influence community structure by reducing populations of the exotic species.

A comparison of artificial neural networks and random forests to predict native fish species richness in Mediterranean rivers

“…Another important aspect in ecological modelling involves the evaluation and interpretation of the results. The presented models were in accordance with the literature due to the fact that the selected input variables, such as water quality, flow regime and the status of riparian forest, are of great importance for the Mediterranean fish populations (Granado-Lorencio, 1996, 2000Bernardo et al, 2003;Ferreira et al, 2007). Both models had three variables in common to predict fish richness: drainage area (DRA), quality index of the riparian forest (QBR) and the biological index for water quality (IBMWP).…”

“…Similarly, the RF model built with 20 variables also shows a notable difference in training and validation performances. Also for the RF case, the most important predictors in this model have been identified by previous studies as relevant to freshwater fish species (Bernardo et al, 2003;Costa et al, 2012). The high differences between training and validation performances confirm the hypothesis that a very complex model with many degrees of freedom is not robust (Maier and Dandy, 2000) Looking at the models built after variable selection, ANN and RF showed no significant differences of performance in the cross-validation procedure (R 2 = 68% for RF and R 2 = 66% for ANN, Table IV).…”

“…Although the two statistical techniques considered different HMU types, one can observe how the spatial distribution and dynamics of mesohabitats can be of great importance for fish conservation (Fausch et al, 2002). According to Bernardo et al (2003) riffles and runs can influence the composition of Mediterranean fish communities; particularly for those dominated by the family Cyprinidae (Granado-Lorencio, 2000;Ferreira et al, 2007), because these mesohabitats can offer good conditions in terms of food availability and shelters. Mean annual flow rate (FMA), the coefficient of variation of mean monthly flow (FIM) along with the channel length without artificial barriers (CWB) were only selected by ANN.…”

“…The number of individuals of these native fish species have decreased consistently in the last decades as a consequence of habitat modifications (including barriers) and pollution in the lower river reaches (Doadrio, 2001;Costa et al, 2012). 24 environmental variables (Table II) were used to construct the ANN and RF models, which were selected considering their ecological importance for fish life cycle (Granado-Lorencio, 1996;Jackson et al, 2001;Bernardo et al, 2003;Costa et al, 2012). These variables belong to three categories: physicochemical water quality, hydro-morphology, and biologicallybased indicators of water and riparian habitat quality.…”

Fish-based groups for ecological assessment in rivers: the importance of environmental drivers on taxonomic and functional traits of fish assemblages

“…Native species richness per site is generally low (Griffiths, 2006;Reyjold et al, 2007) and had undergone a steep decline during the last decades (Doadrio, 2002;Cabral et al, 2005;Smith and Darwall, 2006;Ribeiro et al, 2009). Moreover fish assemblages from Iberain Peninsula may present a high variability both at seasonal and inter-annual scale determined by the influence of Mediterranean climate (Gasith and Resh, 1999;Bernardo et al, 2003;Ilhéu, 2004). All these aspects cause considerable difficulties in the development of tools to assess the ecological status of Mediterranean climate streams, namely regarding fish fauna.…”