ACTA ICHTHYOLOGICA ET PISCATORIA (2012) 42 (4): 321–327 DOI: 10.3750/AIP2012.42.4.05 POPULATION STRUCTURE, CONDITION, AND REPRODUCTION CHARACTERISTICS OF NATIVE MONKEY GOBY, NEOGOBIUS FLUVIATILIS (ACTINOPTERYGII: PERCIFORMES: GOBIIDAE ), IN THE BULGARIAN DANUBE Markéta Konečná * and Pavel Jurajda Institute of Vertebrate Biology, Academy of Sciences of the Czech Republic, Brno, Czech Republic Konečná M., Jurajda P. 2012. Population structure, condition, and reproduction characteristics of native monkey goby, Neogobius fluviatilis (Actinopterygii: Perciformes Gobiidae ), in the Bulgarian Danube. Acta Ichthyol. Piscat. 42 (4): 321–327. Background. Monkey goby is one of many Neogobius species that have invaded European waters in recent decades. Little interest has been paid to this species up till now, probably due to its lower rate of invasiveness compared to some other goby species, e.g., round goby. As with any non-native species, however, it represents a potential danger for native fauna. The aim of this study is to fill in the missing data concerning monkey goby biology and population structure in its native area of distribution, thereby providing a baseline for future compar - ative studies in non-native distribution areas . Materials and methods. Samples of monkey gobies, Neogobius fluviatilis (Pallas, 1814), were obtained season - ally from the lower, Bulgarian stretch of the Danube River during 2006. The fish collected were sexed, meas - ured, and weighed. After that the gonads were removed and weighed. These data were used to calculate sex ratio, length-frequency distribution, condition coefficient, and gonadosomatic index. Dissected ovaries were used for histological analysis of oocyte distribution . Results. The monkey goby population displayed a relatively balanced sex ratio, while length-frequency distribu - tion demonstrated two- (April), and later three (October), clearly visible peaks, representing three age classes. The condition factor of both males and females was influenced by season. We were able to estimate the starting time and the duration of the spawning season using both gonadosomatic index and ovarian histological analysis . Conclusion. Population of monkey goby in the Bulgarian Danube is an example of a stable native population defined by balanced sex ratio, three-year life span, and recruitment represented by strong cohort of 0+ fish, some of them maturing in their second year of life . Keywords: monkey goby, invasion, Bulgaria, Danube, condition, GSI, ovary structure INTRODUCTION of monkey goby was confirmed on the German Rhine at The monkey goby Neogobius fluviatilis (Pallas, 1814) the port of Duisburg (Stemmer 2008), and in 2009 in the is a moderately large goby (up to 195 mm total length) Dutch Rhine (van Kessel et al. 2009), probably having with a short life span of 5–6 years, reaching sexual matu - migrated downstream. As there has been no published rity in its second- or third year of life (Plachá et al. 2010). record of monkey goby between Austria and the German Males exhibit territorial nesting behaviour and parental upper Rhine (joined by the Rhine–Main–Danube canal), care (Grabowska 2005, Grabowska et al. 2009). The the occurrence at Duisburg almost certainly represents an length of spawning season varies from just two months introduction via shipping, either in ballast water or (March–April) in Turkey (Sasi and Berber 2010) to very sand/gravel as cargo. The species has also extended its protracted spawning (April to August) in the Volga and range through the second invasion pathway i.e., the Danube deltas (Pinchuk et al. 2003). Dnieper–Pripyat–Bug–Vistula river system (Neilson and The monkey goby is natively distributed throughout the Stepien 2011), causing the expansion of this species into Ponto-Caspian region (Neilson and Stepien 2011), inhabit - Poland (Kostrzewa et al. 2004, Grabowska et al. 2009). ing both inland freshwater habitats and saline waters. Their Comparison of native and non-native invasive popula - range expansion from a formerly static natural distribution tions can provide better insights into the biological or was first observed during the 1960s (Bănărescu 1970), and environmental factors that contribute to range extension quite recently this fish reached the Slovakian section of the in such species (Neilson and Stepien 2011). Due to its Danube River (Polacik et al. 2008). In 2008, the presence apparent low rate of invasiveness and low predation rate on * Correspondence: Dr Markéta Konečná, Ústav biologie obratlovců, Akademie věd České republiky, v.v.i., Květná 8, 603 65 Brno, Czech Republic, phone: +420 543 422 522, e-mail: [email protected] . 322 Konečná and Jurajda native fish species in invaded areas (Ahnelt et al. 1998, Although KF is known to be somewhat misleading due to Grabowska et al. 2009), the biology of monkey goby in seasonal differences in male and female WG (Gonçlalves both its native- and non-native distribution areas has been and Almada 1997), it is still the most frequently used index poorly studied. Until recently, the research in its non-native in scientific publications concerning length–weight rela - distribution area has focused mainly on growth (Plachá et tion in fish. In order to allow wide comparability with al. 2010) and food composition (Grabowska el al. 2009). other monkey goby publications, therefore, we have used A number of research projects targeted the Black Sea both the routine index and a less often used index ( KC) that (native distribution area) in the 1960s (Bil’ko 1965, 1968). will provide more accurate data on seasonal fish condition. The apparent lack of comparable methods to describe Gonad maturity and delimitation of the spawning sea - population structure and reproductive characteristics calls son were determined using the gonadosomatic index for up-to-date standardised studies. Recently, longitudinal (GSI, %), calculated as –1 distribution, density and habitat preference were GSI = 100 WG · WT described by Polacik et al. (2008) and Polačik et al. where: WG = gonad weight, WT = total weight [g] (2008) along the lower Danube (native distribution; In multiple-spawning fishes such as monkey goby, Bulgarian stretch); however, no other characteristics (e.g., GSI is a not a very reliable indicator of ovarian maturity population structure, reproduction, feeding habits) impor - as eggs at varying stages of maturity can be present at the tant for successful colonisation were investigated. same time in the ovary. For this reason, histological In this study, we focused on the population structure analysis was also undertaken in order to assess the distri - and reproductive biology of monkey gobies to provide bution of ovarian developmental stages. Ovaries of differ - missing data from their native range along the Bulgarian ent sizes (i.e., representing different maturation stages) stretch of the Danube River. were recovered from 20 females (5 from each sampling session) in order to determine the size of particular oocyte MATERIALS AND METHODS stages. The ovaries were first transferred to 70% ethanol, Monkey goby populations were sampled in 2006 near than embedded in a paraffin matrix and thin (7–10 µm) the town of Vidin (river km 790; Bulgaria) using elec - sections of each ovary cut from the middle part of the trofishing gear (Lena, Bednář Olomouc) and/or a 7-m ovary. The sections were stained with Harris’ hema - beach seine (4 mm mesh), depending on habitat condi - toxylin-eosin using the Papanicolau and Mallory methods tions. Sampling took place in April, June, July, and (Bancroft and Gamble 2008). A digital image of each his - October, i.e., before-, during-, and after the presumed tological section was taken using a digital camera spawning season (see Pinchuk et al. 2003). Sampled fish (Olympus DP 70) connected to a binocular-microscope were administered a lethal dose of anaesthetic (clove oil) (20×, 25×, and 32× zoom; Olympus SZX7). Individual and stored in 4% formaldehyde for further analysis. oocyte developmental stages (primary All fish were sexed, based on the shape of the urogen - growth/previtellogenic, cortical alveoli, vitellogenic and ital papilla (Miller 1984), measured to the nearest mm mature oocytes; Table 1) were characterised according to using digital callipers (TL = total length, SL = standard Cinquetti and Rinaldi (1987), Halačka et al. (2000), and length), and weighed to the nearest 0.01 g. The data were Murua and Saborido-Rey (2003), and measured using further used to calculate the sex ratio and the population LUCIA 5 image analysis software (Laboratory Imaging age structure based on length-frequency distribution. Ltd.). In addition, a digital image was taken of a sample Because age determination in fish using anatomical meth - from the remaining untreated ovary, using a digital cam - ods (e.g., counting annuli on scales, otoliths, or vertebrae; era (uEye-1540C) coupled with a binocular microscope, Campana 2001) is substantially influenced by the skills of for precise size adjustment and a count of each oocyte the reader, and the reading procedures require a great deal stage, again using LUCIA 5 image analysis software. of time (İlkyaz et al. 2011), we concentrated on length- As data in this study were not normally distributed frequency distribution as an indicator of age groups (Kolmogorov–Smirnov test of normality; P > 0.05), the non- (Kumar and Adams 1977, Anderson and Neumann 1996) . parametric Man–Whitney U test was used for examination of On each sampling date, 50 adult fish (25 males and 25 differences between male and female length and condition females) were randomly chosen from the sample for fur - and Kruskal–Wallis test with post hoc multiple comparisons ther analysis (condition, gonadosomatic index, ovary for dynamics in male and female GSI. The Spearman corre - structure). The fish were dissected and the gonads and lation was used to determine the influence of female SL on gastrointestinal tract removed. The eviscerated fish and number of mature oocytes and GSI, and the chi-square test to the gonads were then weighed. explore sex ratio. The influence of SL on GSI and number Changes in body condition were expressed using of oocytes was counted using linear regression.