Negli ultimi decenni l'impattante intervento antropico ha modificato pesantemente gli ambienti fluviali. Anche il Sessera, torrente biellese affluente del Sesia, ha subìto profonde modificazioni che limitano lo sviluppo delle comunità ittiche. Lo scopo di questo lavoro consiste nel delineare un quadro strutturale del popolamento ittico, valutando le criticità in rapporto alle influenze antropiche. Sono state scelte nove stazioni rappresentative, sette sul Sessera e due su gli affluenti Dolca e Rio Caneglio, campionate tramite elettropesca nel giugno 2007 e nel 2010 (solo due stazioni); sul Sessera due siti di prelievo sono prima della diga di Mischie (927 m s.l.m.), due nel tratto sotteso e tre dopo la restituzione. In ogni stazione i pesci sono stati classificati per specie e ordine di lunghezza. Sono stati inoltre rilevati i parametri fisico-chimici più importanti (T°, pH, O2 disciolto) e i dati di Arpa e dell'Amministrazione Provinciale inerenti a precipitazioni atmosferiche, portate e parametri chimico-fisici del torrente. Nelle prime due stazioni il popolamento è risultato monospecifico a trota fario di ceppo mediterraneo e atlantico, a volte ibridati, ben strutturato e in grado di riprodursi. Gli individui di ceppo atlantico provengono dalla riserva Zegna sita in questo tratto. Nella terza stazione gli individui di trota fario, assenti sotto i 10 cm, condividevano il tratto con un'esigua comunità di scazzoni. Nella quarta stazione la fario era accompagnata dai ciprinidi reofili, barbo e vairone, con popolamenti non molto strutturati. Nelle stazioni più a valle i popolamenti di vairone, sanguinerola e alborella, sono risultati predominanti e ben strutturati anche se carenti di individui molto giovani, seppure con differenze tra un tratto e l'altro; viceversa quelli di barbo e cavedano sono risultati poco strutturati. Si è rilevata la presenza di alcuni ibridi fario-marmorata a Pray e del raro cobite a Crevacuore. La trota fario era sempre presente grazie ai continui ripopolamenti,dai due incubatoi di valle. In nessuna stazione sono stati rilevati esemplari di trota marmorata e temolo. La gestione di Salmo trutta è condizionata dalle riserve emerse sull'autoctonicità della fario mediterranea, mentre è riconosciuta quella della trota marmorata. Sono state individuate anche le influenze antropiche più invasive per l'ecosistema del torrente. E' emersa una situazione molto differenziata tra la zona a monte della diga di Mischie, ancora naturale e ben gestita, e quella a valle, con una situazione più critica. Il torrente ha perso gran parte della sua naturalità a causa delle opere di captazione e dell'utilizzo di acqua per fini idroelettrici che incide sulle portate, spesso inferiori al deflusso minimo vitale stabilito per legge. Questa situazione ha favorito l'insabbiamento e l'infangamento dei possibili letti di frega delle specie ittiche. Il torrente inoltre è sede di continui lavori in alveo e sulle sponde che lo rendono simile ad un canale artificiale. La conseguente mancanza di vegetazione spondale e di rifugi per la fauna ittica dà la possibilità alla fauna avicola ittiofaga essenzialmente aironi e cormorani) di potersi alimentare senza difficoltà. La situazione del Sessera è seria, ma per fortuna non ancora completamente compromessa; un'attenta politica di conservazione ambientale e il recupero delle situazioni critiche restituirebbero al torrente Sessera una certa naturalità e un popolamento ittico vario e strutturato.
In the last decades, impacting human intervention has heavily modified the fluvial environment. The Sessera, Biella tributary of the river Sesia, has undergone profound changes that limit the development of the fish communities. The purpose of this work is to outline a structural framework of the fish population, assessing the criticality in relation to anthropogenic influences. We have chosen nine representative stations, seven on Sessera and two in tributaries (Rio Dolca and Caneglio), sampled by electrofisher in June 2007 and June 2010 (only two stations). Two Sessera sampling sites were first Mischie dam, (927 m above sea level), two in the intercepted segment and three where water returns back to the river downstream. In every stations fishes have been classified by species and length. We also assessed the most important physico-chemical parameters: temperature, pH, O2 dissolved), statistics received by ARPA and Provincial Adminstration related to related to rainfall, flows and physico-chemical parameters of the stream. In the first two sampling stations the population was monospecific in brown trout of mediterranean and atlantic strain, sometimes hybridized, well structured and able to reproduce. Atlantic strain comes from the Zegna reserve located in this section. In the 3rd station, individuals of brown trout, absent below 10 cm, shared the stretch with a small community of bullhead. In the 4th station, the trout was accompanied by rheophilic species, barbel and roach, with populations not very structured. In further downstream stations, populations of suffia, bleak and minnow, were predominant and well structured although deficient in very young individuals, albeit with difference among the segments, and conversely barbel and chub populations were less structured. We assessed the presence of few hybrid brown-marble trouts in Pray and rare spined loach in Crevacuore .The brown trout was always present, thanks to continue repopulations by two valley hatcheries. In no station were found speciments of marble trout and grayling. The management of Salmo trutta is conditioned by the uncertainty about Mediterranean brown trout nativeness, while it is recognized that the marble trout. Anthropogenic influences more invasive to the ecosystem of the river have also been identified. There is a great difference between the upstream area over the Mischie dam, which is still free from devastating man intrusion and is well supervised, and the area under the dam, where the situation is critical due to anthropogenic influences. The river has lost much of its naturalness because of the works of uptake and use of water for hydroelectric purposes that affect flow rates, often below the minimum vital flow required by law. This situation has encouraged the sanding-up and the mudslide of possible spawning beds of fish species. The river is also home to continue work in the river bed and banks that make it similar to an artificial canal. The resulting lack of vegetation, river banks and shelters for fish makes it possible to ichthyophagous birdlife (essentially herons and cormorants) to feed themselves without difficulty. The situation of Sessera is serious, but not yet fully compromised: careful policy of environmental conservation and recovery of critical situations would return to the river Sessera a certain naturalness and a diverse and structured fish population.
INDAGINE SULL'ITTIOFAUNA DEL TORRENTE SESSERA IN RELAZIONE A FATTORI DI ORIGINE ANTROPICA E NATURALE
CERREIA VARALE, ANDREA
2011/2012
Abstract
In the last decades, impacting human intervention has heavily modified the fluvial environment. The Sessera, Biella tributary of the river Sesia, has undergone profound changes that limit the development of the fish communities. The purpose of this work is to outline a structural framework of the fish population, assessing the criticality in relation to anthropogenic influences. We have chosen nine representative stations, seven on Sessera and two in tributaries (Rio Dolca and Caneglio), sampled by electrofisher in June 2007 and June 2010 (only two stations). Two Sessera sampling sites were first Mischie dam, (927 m above sea level), two in the intercepted segment and three where water returns back to the river downstream. In every stations fishes have been classified by species and length. We also assessed the most important physico-chemical parameters: temperature, pH, O2 dissolved), statistics received by ARPA and Provincial Adminstration related to related to rainfall, flows and physico-chemical parameters of the stream. In the first two sampling stations the population was monospecific in brown trout of mediterranean and atlantic strain, sometimes hybridized, well structured and able to reproduce. Atlantic strain comes from the Zegna reserve located in this section. In the 3rd station, individuals of brown trout, absent below 10 cm, shared the stretch with a small community of bullhead. In the 4th station, the trout was accompanied by rheophilic species, barbel and roach, with populations not very structured. In further downstream stations, populations of suffia, bleak and minnow, were predominant and well structured although deficient in very young individuals, albeit with difference among the segments, and conversely barbel and chub populations were less structured. We assessed the presence of few hybrid brown-marble trouts in Pray and rare spined loach in Crevacuore .The brown trout was always present, thanks to continue repopulations by two valley hatcheries. In no station were found speciments of marble trout and grayling. The management of Salmo trutta is conditioned by the uncertainty about Mediterranean brown trout nativeness, while it is recognized that the marble trout. Anthropogenic influences more invasive to the ecosystem of the river have also been identified. There is a great difference between the upstream area over the Mischie dam, which is still free from devastating man intrusion and is well supervised, and the area under the dam, where the situation is critical due to anthropogenic influences. The river has lost much of its naturalness because of the works of uptake and use of water for hydroelectric purposes that affect flow rates, often below the minimum vital flow required by law. This situation has encouraged the sanding-up and the mudslide of possible spawning beds of fish species. The river is also home to continue work in the river bed and banks that make it similar to an artificial canal. The resulting lack of vegetation, river banks and shelters for fish makes it possible to ichthyophagous birdlife (essentially herons and cormorants) to feed themselves without difficulty. The situation of Sessera is serious, but not yet fully compromised: careful policy of environmental conservation and recovery of critical situations would return to the river Sessera a certain naturalness and a diverse and structured fish population.| File | Dimensione | Formato | |
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https://hdl.handle.net/20.500.14240/57232