Ann. Zool. Fennici 58: 243–253 ISSN 0003-455X (print), ISSN 1797-2450 (online) Helsinki 30 August 2021 © Finnish Zoological and Botanical Publishing Board Vendace (Coregonus albula) in Lake Inari — what has changed in 50 years? Erno Salonen Natural Resources Institute Finland (Luke), Saarikoskentie 8, FI-99870 Inari, Finland (e-mail: erno. [email protected]) Received 5 Oct. 2020, final version received 17 June 2021, accepted 18 Mar. 2021 Salonen, E. 2021: Vendace (Coregonus albula) in Lake Inari — what has changed in 50 years? — Ann. Zool. Fennici 58: 243–253. Lake Inari is a subarctic, oligotrophic, regulated lake in northern Finland, connected via the Paatsjoki (Pasvik) River to the Arctic Ocean. A new coregonid species, the ven- dace (Coregonus albula), was introduced into the watershed in the 1960s and gradually established a population in the lake becoming part of its ecosystem. The annual vendace catch in 1989 reached 300 tonnes, while during the following 25 years the vendace catches declined stabilizing at a very low level. Vendace introduction radically changed the fishing practices within a short time, and its population became firmly established in the lake. It benefited both fisheries and predatory fishes, especially salmonids. The vendace year-class strength varied greatly between 1983 and 2019. According to winter seine CPUE data, the strongest year-classes were up to 100 times greater than the weak- est ones. Winter seine CPUE of one-year-old fish correlated positively with catches in the coming years. The growth data for the period 2015–2021 revealed a decrease in the youngest vendace age-groups. Introduction Such fluctuations were observed in the invasive vendace population in Lake Inari (Finland) and The vendace (Coregonus albula) is not native to further downstream in the Paatsjoki watershed Lake Inari or the Paatsjoki (Pasvik) watershed. (Norway and Russia) (Salonen et al. 2007). A native coregonid is the dwarf densely-rakered In 1956, some vendace larvae escaped acci- European whitefish (reeska in Finnish) (Corego- dentally from an Inari fish farm into the Juutua nus lavaretus), which is similar to the vendace River, which flows to nearby Lake Inari. In both biologically and ecologically. Its gillraker 1964–1966, vendace was introduced into nearby count, however, is between 30 and 40, while Alajärvi (Fig. 1). According to local accounts the vendace gillraker count is over 40 (Salonen (J. Kyrö pers. comm.), newly hatched vendace 1992, and author’s own unpubl. data). In general, larvae were also stocked directly into the Ivalo- an introduced or invasive species that became joki. The distance from Alajärvi along the Ivalo- established in a new environment may, even joki to Lake Inari is about 30 km. In some way, long after establishment, undergo substantial and perhaps by active swimming or drifting with unpredictable population fluctuations, including the current down the river, sufficient amount of also boom-and-bust cycles (Williamson 1996). larvae managed to enter the southernmost part of Editor-in-charge: Timo Marjomäki 244 Salonen • ANN. ZOOL. FENNICI Vol. 58 24°E 27°E 30°E 33°E 70°N Arctic Ocean Skrukkebukta Vaggatem NORWAY Lake Inari i 69°N Inari fish atsjok farm KaitakosPa Juutua ki power Ivalo i plan Ivalojok Alajärvi* t RUSSIA Porttipahta 68°N Lokka Kitine n Luiro Kelujärvi* 67°N Iso-Venejärvi White Se Raudanjoki Ounasjok Kemijärvi a Sinettäjärvi* i Kemijoki Kemijärvi Rovaniemi Fig. 1. Map of the study area with Lake Inari, watersheds, and possible 66°N founder lakes (Kelujärvi, Legend FINLAND Sinettäjärvi, Alajärvi) into Lakes and reservoirs which vendace were intro- Kemijoki watershed duced. Data on water- Baltic Sea Paatsjoki watershed sheds from the Finnish Rivers Flow direction Environment Institute, and 0525 0 100 km State border on lakes and reservoirs 65°N * possible founder lakes from The National Land 24°E 27°E 30°E 33°E Survey of Finland. Lake Inari, where vendace were observed for the resource for predatory fishes in the lake, espe- first time in 1973. There, the species gradually cially for valuable salmonids (e.g., Salmo trutta), established a new fish population in Lake Inari which before vendace introduction preyed upon (Sergejeff 1985, Salonen et al. 2007), and about C. lavaretus (dwarf densely rakered whitefish; two decades later (between 1989 and 1995), reeska in Finnish) as well as three-spined (Gas- vendace was found downstream in the Norwe- terosteus aculateus) and nine-spined (Pungitius gian part of the Paatsjoki (Pasvik) watershed pungitius) sticklebacks (Toivonen 1966). On the (Amundsen et al. 1999, 2012). In 2013, a genetic other hand, the alien vendace became quite a study confirmed that the vendace in Lake Inari strong competitor for resources with native C. and in Vaggatem and Skrukkebukta in the Pasvik lavaretus (reeska) (Salonen et al. 2007). watershed (Norway) were both of Sinettäjärvi Here, I describe history of the vendace in origins from introduction/stocking in 1964–1966 Lake Inari. The material for this work comprised (Praebel et al. 2013). yearly vendace catches and catch sample data Between 1973 and 2021, vendace changed since the mid-1980s up to the present day. fisheries in Lake Inari, and became a new prey ANN. ZOOL. FENNICI Vol. 58 • Vendace in Lake Inari — what has changed in 50 years? 245 8500 gillnet winter seine trapnet trawl 8000 7500 7000 6500 6000 5500 5000 4500 4000 3500 3000 2500 2000 Fig. 2. Sampled and aged Sampled and aged vendace (indiv.) 1500 vendace caught with dif- 1000 500 ferent fishing gear in Lake 0 Inari. Fish caught in 1986 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 were not analysed. 198 198 198 198 198 199 199 199 199 199 199 199 199 199 199 200 200 200 200 200 200 200 200 200 200 201 201 201 201 201 201 201 201 201 201 Material and methods sebago) until 2001 and lake trout (Salvelinus namaycush) until 2012. There are many white- Study area fish morphs in the lake, including two dwarf forms: densely-rakered (reeska in Finnish) and Lake Inari in northern Finland (68°57´N, sparsely-rakered (rääpys in Finnish) whitefish. 27°40´E) is a large (around 1100 km2) central They can be identified by their morphology lake in the Paatsjoki (Pasvik) watershed, drain- (jaws, number of gill rakers), and they also differ ing northeast into the Arctic Ocean, first through from each other in their ecology (food selec- Russia and then as a borderline watercourse tion and growth) (Toivonen 1966, Marttunen et between Russia and Norway (Fig. 1). The water- al.1997). shed has been regulated since the 1940s, and the uppermost power plant, Kaitakoski, regulates the water level of Lake Inari (annual level change Data ~1.5 m). The ice-free season in Lake Inari lasts from May–June to October–November (often Information on the catch statistics was collected December in large open-water areas). The lake from commercial, local subsistence- and recrea- is quite deep (max. depth 95 m, mean depth tional fisheries with regular yearly mail inquires. 14.4 m), clear and oligotrophic. Its Secchi-depth These were complemented by interviews and ranges from 3–4 m to 7–10 m in the southern and book-keeping (daily catch and effort records) in northern parts, respectively. The fish community connection with the obligatory monitoring pro- includes 13 species, with whitefish (Coregonus cedures related to Lake Inari water-level regula- lavaretus s.l.), brown trout (Salmo trutta), Arctic tion (see above). char (Salvelinus alpinus), grayling (Thymallus Before the 1990s, during the vendace boom thymallus), perch (Perca fluviatilis), pike (Esox years, vendace samples originated from the lucius) and burbot (Lota lota) being the most entire Lake Inari, and afterwards mostly from abundant native species. Minnows (Phoxinus the southern part of the lake. The sampling phoxinus) are the only cyprinid species in the was performed mainly by commercial fishermen lake (Salonen 2004). Large obligatory stocking using gill nets, winter seines, trawls and trap nets programmes including whitefish and piscivorous (e.g. Salonen 1998). The data on the vendace for species such as brown trout and Arctic char have the 1985–2019 period include of over 40 000 been carried out since the 1970s. In addition specimens. Especially high numbers of vendace to vendace, two other non-native species were were caught (sampled) in 1989 particularly by stocked: land-locked salmon (Salmo salar m. trawling (Fig. 2). In recent years, only around 246 Salonen • ANN. ZOOL. FENNICI Vol. 58 150 samples per year were taken from catches by Alajärvi (Fig. 1). The vendace larvae which had winter seine and gillnets, as only these two types escaped in 1956 were from the Kelujärvi stock, of fishing still targeted vendace. and the vendace introduced in 1964–1966 were The catch per unit of effort (CPUE) for from the Sinettäjärvi stock. Both lakes are in the vendace were calculated from the records of Kemijoki watershed (waters flow to the Baltic the commercial winter seining (1996–2020, 1–2 Sea) 150–250 km south of Lake Inari (Serge- seine teams), and gillnet fishing (data provided jeff 1985, Mutenia & Salonen 1992) (Fig. 1). by local subsistence fishermen in yearly mail According to local accounts (J. Kyrö pers. inquires). Commercial fishermen caught vendace comm.), newly hatched vendace larvae were also using gillnets only very occasionally, and com- stocked directly into the Ivalojoki in the vicin- mercial trawling had practically ceased already ity of the bridge in the village of Ivalo. Those in the1990s after the vendace boom years. After- larvae had the same origin (Sinettäjärvi stock) wards, one or two commercial trawlers operated and, as the ones in Alajärvi, were also introduced until 2017, but only very occasionally (no reli- in 1964–1966.