First record of the brown bullhead Ameiurus nebulosus (Lesueur, 1819) in the Neman River basin in Belarus

The brown bullhead Ameiurus nebulosus (Lesueur, 1819) is recorded for the first time in the Neman river basin in Belarus. In August 2021, 41 specimens were caught in closed reservoir of the Novodvorka River (Novy Dvor village, Shchuchyn district, Grodno region), and 2 specimens were caught from closed reservoir of the Livanka River (Zasulye village, Stolbtsy district, Minsk region). Both rivers belong to the Neman river basin. In our opinion, the main reason for the origin of brown bullhead in the studied reservoirs is associated with intentional release in nature by anglers.


Introduction
The brown bullhead Ameiurus nebulosus (Lesueur, 1819) is a small catfish, of the family Ictaluridae (Ictaluridae: Siluriformes) (Scott and Crossman 1973).The native range of the brown bullhead is the eastern part of North America.It is successfully established in France (Vivier 1951), Germany (Schindler 1957), Poland (Nowak et al. 2008), the Czech Republic (Lhotský 1995), Hungary (Harka and Pintér 1990), Slovakia (Balon 1966;Koščo et al. 2010;Rutkayová et al. 2013), Ukraine (Ivlev and Protasov 1948).This species is found in lakes, ponds, oxbows, reservoir meliorative canals characterized by dense aquatic vegetation and a muddy bottom.The brown bullhead is a benthic fish and can tolerate low oxygen levels and high water temperatures.This species has been found in Belarus since the end of the 19 th century, when it was intentionally introduced into the lakes and ponds of the Malorita region of western Belarus (Makushok 1951).Our previous studies have revealed the widespread distribution of brown bullhead in most areas of the Brest region (Okhremenko and Gajduchenko 2021).
The spread of this species further north from its initial invasion point was noted by Rizevsky and Ermolaeva (2012).Until now, no brown bullhead have been found within the Grodno region.Therefore, any information regarding the research object location, distribution, biology, and ecology in the aquatic ecosystems of Belarus would make a significant contribution to the understanding of this species outside its natural range.
This article provides information on new records of brown bullhead beyond of the initial intentional introduction.

Materials and methods
In August 2021, water bodies of the Neman River basin were monitored.The brown bullhead was recorded only in two reservoirs of 20 studied.We caught 41 specimens of brown bullhead in the closed reservoir of the Novy Dvor village (53.815266; 24.566339), Grodno region, and 2 specimens in the closed reservoir of the village of Zasulye (53.584707;26.837159),Minsk region (Figure 1).Fish were collected using an umbrella-type traps (for crayfish) with a mesh size of 0.5 mm, placed near the coast, at depth of 1.5 m.The brown bullhead specimens were caught in traps mainly at night.We analyzed a total of 29 morphometric and 5 meristic characters.Table 1 shows the main characters with the highest and lowest standard deviation.Morphometric data were expressed as a percentage of the length of the entire fish or the length of the head.The age of the individuals was determined by the growth rings on the vertebrae (Chugunova 1959).

Results
Our morphometric data of brown bullhead is significantly lower than that of other authors for the non-native range (Table 1).
All specimens identified as brown bullheads based on the sharply serrated posterior edges of the pectoral fins (Figure 2).Specimens caught in the reservoir  Novy Dvor village has a wider range of the number of rays in the dorsal, pectoral, ventral and anal fins compared to specimens from their native range and in the number of rays in the dorsal and ventral fin compared to the non-native range.The meristic characters of specimens from the reservoir Zasulye village fall within the range of variations in the number of rays in the fins for both the native and non-native ranges and have a wider range only in the number of rays in the caudal fin (Table 2).Furthermore, individuals from their native range has a broader range of the number rays in the anal fin.

Discussion
Information on the invasion history of brown bullhead was presented by Rutkayová et al. (2013).Until this time, no brown bullhead has been found in the Neman river basin (Fish Base 2023).It is also known, that the spread of brown bullhead can go through canals between lakes and other river systems.Hovewer we caught the brown bullhead specimens in closed reservoirs that have no connection with the Pripyat River basin and the Bug River basin, where the species was previously confirmed to exist.Therefore, we assume that the appearance of brown bullhead in these water bodies is assoсiated with human introduction (intentional release, use of live bait, etc.).Probably a risk of spread of this species in the Belarusian part of the Neman River basin and outside Belarus is exists.
The brown bullhead is a well-adapted species for widespread dispersal and rapid colonization of new areas.Taking into account the high rate of spread of brown bullhead in Belarus (Okhremenko and Gajduchenko 2021) we can predict its further expansion into the Neman River basin in the next few years.
The continuation of the spread of this species is extremely undesirable, due to the rapid increase of this species abundance when it established in a new water body, and the difficultly to predict the results of invasion.It is known that the brown bullhead is characterized by rapid adaptation to new environment, increasing of population size for short time and competition with native fish species (Mieczan et al. 2022).At the same time, this species has no natural enemies among native fish species due to its sharp spines in the dorsal and pectoral fins.The brown bullhead is a host of parasites that can cause new diseases in native fish species (Grabowska et al. 2010).Also the brown bullhead quickly becomes the dominant component of the fish community in some water bodies due to predation on the eggs of native fish species (Kornijów 2001;Kornijów et al. 2003;Rechulicz and Plaska 2018).However, it does not reach large sizes in the reservoirs of its nonnative range.In Florida waters it reaches a length more than 50 cm and weight of up to 3 kg (Scott and Crossman 1973), while in Central Europe the average size of this species is 20-25 cm and a weight of 250 g (Kapusta et al. 2010).

Figure 1 .
Figure 1.Sampling sites of Ameiurus nebulosus: the initial introduction (shaded green), the Neman River basin shaded blue, red dots -specimens found, white dots -not found.

Table 1 .
Morphometric characters of brown bullheads from the closed reservoir of Novy Dvor village and Zasulye village.

Table 2 .
Meristic characters of brown bullheads from the closed reservoir of Novy Dvor village and Zasulye village.