Control of invasive fish species in the "Krka" National Park with an emphasis on pike (Esox lucius)

Rijeka Krka spada u vodotoke Jadranskog slijeva s najvećim brojem zabilježenih slatkovodnih vrsta, zbog čega spada u najzanimljivija ihtiološka područja Hrvatske. Od ukupnog broja zabilježenih vrsta šest je stranih, a osam vrsta jesu zavičajne endemske vrste. Tema ovog rada su invazivne vrste unesene u rijeku Krku iz drugih europskih i svjetskih staništa te translociranih vrsta iz Dunavskog slijeva. To je rezultiralo promjenama u ribljem fondu i brojnosti populacija autohtonih i endemičnih vrsta riba poput visovačkog glavočića, oštrulje, mrena, drlje te ilirskog i turskog klena. Osnovni cilj ovog rada je utvrđivanje kvantitativnog stanja populacija stranih vrsta naročito štuke te razvoj programa uklanjanja invazivnih vrsta kako bi se pokušale određenim mjerama osigurati adekvatni uvjeti koji će omogućiti opstanak autohtonih riba u Nacionalnom parku „Krka“. Od svih zabilježenih vrsta, tijekom istraživanja najbrojnija vrsta je bila gambuzija (3749 jedinke, 35,5%), bezribica (2728 jedinke, 25,8% ) i sunčanica (2023 jedinke, 19,1%), a slijedi ih drlja (1223 jedinke, 11,6%). Navedene četiri vrste čine 92,1% ukupne brojnosti uzorka, dok su ostale vrste zastupljene s manje od 2%. Od ostalih alohtonih i unesenih vrsta, crni somić je zastupljen s 1,8% (199 jedinki), grgeč s 1,4% (154 jedinki), linjak s 0,8% (93 jedinke), štuka s 0,1% (19 jedinki), babuška s 0,05% (pet jedinki), šaran s 0,02% (dvije jedinke) te bijeli amur s 0,01% (jedna jedinka) Utjecaj invazivnih vrsta na zavičajnu ihtiofaunu je višestruk, a najznačajnije je da se odrasle jedinke, posebice štuka, hrane prvenstveno mlađi zavičajnih vrsta smanjujući njihov reproduktivni uspjeh. Također, uzrokuju kompeticiju za hranu i pogodna staništa, a mijenjaju i stanišne uvjete. U tom je pogledu štuka posebno problematična, s obzirom na veliku gustoću populacije u rijeci Krki i njenu sposobnost da se vrlo brzo širi u svim vodotocima u koje je ubačena. Unesene predatorske vrste (posebice štuka) uzrokovale su promjene čije posljedice tek treba sagledati. Kroz trenutna, ali i ranija istraživanja, utvrđeno je kako se strane i invazivne vrste riba, posebno štuka, kao vršni predator, tijekom visokih voda iz Šarenih jezera šire u rijeku Kosovčicu, a zatim i u rijeku Krku i na taj način narušavaju ekološko i biološko stanje iste unosom stranih vrsta riba koje imaju negativan utjecaj na cijelu ihtiofaunu rijeke te je potrebno mehaničkim rješenjima poput pregrada, nasipa i rešetki te električnim mjerama odvraćati i sprječavati rasprostranjenost tih invazivnih vrsta. Usporedno s tim tehničkim rješenjima, potrebno je izvoditi selektivni izlov stranih (invazivnih) vrsta kako na navedenim područjima tako i u samom Nacionalnom parku „Krka“ te zabraniti i spriječiti daljnje poribljavanje neželjenim vrstama pogotovo u njezinim ujezerenim područjima rijeke idealnim za mrijest, rast i širenje tih stranih vrsta, kao što su Carigradska draga i Jezero Brljan.The Krka River belongs to the watercourses of the Adriatic Basin with the largest number of recorded freshwater species, which is why it belongs to the most interesting ichthyological areas in Croatia. Of the total number of recorded species, six are foreign, and eight are native endemic species. The topic of this work is invasive species introduced into the Krka River from other European and world habitats and translocated species from the Danube Basin. This resulted in changes in the fish stock and the number of populations of autochthonous and endemic fish species, such as the Flounder from Visovac,Dalmatian barbel gudgeon, barbel, Dalmatian rudd, and Illyrian and Turkish chub. The main goal of this work is to determine the quantitative state of populations of foreign species, especially pike, and to develop a program to remove invasive species in order to try to ensure adequate conditions that will enable the survival of indigenous fish in the "Krka" National Park. Of all the recorded species, during the research the most numerous species were the gambusia (3749 individuals, 35.5%), topmouth gudgeon (2728 individuals, 25.8%) and Pumpkinseed (2023 individuals, 19.1%), Dalmatian rudd (1223 individuals, 11.6%). The mentioned four species make up 92.1% of the total abundance of the sample, while the other species are represented by less than 2%. Of the other non-native and introduced species, Black bullhead is represented by 1.8% (199 individuals), bass by 1.4% (154 individuals), tench by 0.8% (93 individuals), pike by 0.1% ( 19 individuals), prussian carp with 0.05% (five individuals), carp with 0.02% (two individuals) and white grass carp with 0.01% (one individual) The impact of invasive species on native ichthyofauna is multiple, and the most significant is that adults, especially pike, feed primarily on juveniles of native species, reducing their reproductive success. They also cause competition for food and suitable habitats, and change habitat conditions. Introduced predatory species cause changes whose consequences have yet to be seen. In this regard, the pike is particularly problematic, given the high population density in the Krka River and its ability to spread very quickly in all watercourses into which it is introduced. Introduced predatory species (especially pike) caused changes whose consequences have yet to be seen. Through current and earlier researches, it was established that foreign and invasive species of fish, especially pike, as the top predator, during high waters from Šareni Jezera spread into the Kosovčica River and then into the Krka River, thus disrupting the ecological and biological state they pollute it by bringing in foreign species of fish that have a negative impact on the entire ichthyofauna of the river, and it is necessary to use mechanical solutions such as barriers, embankments and gratings and electrical measures that deter and prevent the spread of these invasive species. In parallel with these technical solutions, it is necessary to carry out selective fishing of foreign (invasive) species both in the mentioned areas and in the National Park "Krka" itself, and to prohibit and prevent further stocking with unwanted species, especially in its lake areas of the river ideal for spawning, growth and spread of these foreign species, such as Carigradska draga and Jezero Brljan