This study presents the periodical but irregular appearance of pike (Esox lucius L.) in the Danube River and its inundated ponds. During 14 years, 1076 pike were captured at nine sites. The fish appeared in large numbers in years when the average water level was high in spring (R = 0.738, P < 0.01, n = 14). During the study, there were two periods when there were no pike, either in the river or in the inundated ponds. Furthermore, observation showed that prey fish might also affect the presence or absence of pike in late spring in the study area.Rad govori o periodičkom, no nepravilnom pojavljivanju štuka (Esox lucius L.) u rijeci Dunavu i njenim naplavljenim barama. Tijekom 14 godina na 9 lokaliteta ulovljeno je 1076 štuka. Ribe su se u velikom broju javljale u godinama kad je prosječna razina vode bila visoka u proljeće (R = 0.738, P < 0.01, n = 14). Tijekom istraživanja postojala su dva razdoblja kada štuka nije bilo ni u rijeci niti u naplavljenim barama. Osim toga primijećeno je da riblji plijen također može utjecati na prisutnost ili odsutnost štuka na istraživanom području tijekom kasnog proljeća

Thomas Oliver Mérő

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original scientific paper / izvorni znanstveni radPERIODICAL APPEARANCE OF PIKE(ESOX LUCIUS LINNAEUS, 1758) IN THEDANUBE RIVER IN NORTH-WESTERNVOJVODINA (N SERBIA)THOMAS OLIVER MÉRÕMilana Raki}a 20, 25000 Sombor, SerbiaMérõ, Th. O.: Periodical appearance of pike (Esox lucius Linnaeus, 1758) in the Danube Riverin north-western Vojvodina (N Serbia). Nat. Croat., Vol. 20, No. 1., 201–207, 2011, Zagreb.This study presents the periodical but irregular appearance of pike (Esox lucius L.) in the DanubeRiver and its inundated ponds. During 14 years, 1076 pike were captured at nine sites. The fish ap-peared in large numbers in years when the average water level was high in spring (R = 0.738, P < 0.01,n = 14). During the study, there were two periods when there were no pike, either in the river orin the inundated ponds. Furthermore, observation showed that prey fish might also affect the pres-ence or absence of pike in late spring in the study area.Key words: water level, Spearman correlation, river, inundated ponds, Pike, Esox lucius, »GornjePodunavlje«Mérõ, Th. O.: Periodi~ko pojavljivanje {tuka (Esox lucius Linneaus, 1758) u Dunavu u sje-verozapadnoj Vojvodini (sjeverna Srbija). Nat. Croat., Vol. 20, No. 1., 201–207, 2011, Zagreb.Rad govori o periodi~kom, no nepravilnom pojavljivanju {tuka (Esox lucius L.) u rijeci Dunavu i nje-nim naplavljenim barama. Tijekom 14 godina na 9 lokaliteta ulovljeno je 1076 {tuka. Ribe su se u ve-likom broju javljale u godinama kad je prosje~na razina vode bila visoka u prolje}e (R = 0.738, P < 0.01,n = 14). Tijekom istra`ivanja postojala su dva razdoblja kada {tuka nije bilo ni u rijeci niti u naplav-ljenim barama. Osim toga primije}eno je da riblji plijen tako|er mo`e utjecati na prisutnost ili od-sutnost {tuka na istra`ivanom podru~ju tijekom kasnog prolje}a.Klju~ne rije~i: razina vode, Spearmanova korelacija, rijeka, naplavljene bare, {tuka, Esox lucius,»Gornje Podunavlje«INTRODUCTIONThe pike (Esox lucius) is a top freshwater predator that feeds mainly on fish. It iscommon in lakes, slow flowing rivers and inundated ponds in the arctic and tem-perate zones of the northern hemisphere (DIANA et al., 1977; RAAT, 1988). Pike aredescribed as keystone predators of fresh waters and through this function they playan important role in the aquatic ecosystem. Through their predation they are ableto control the prey fish community composition (PREJS et al., 1994; BERG et al., 1997).NAT. CROAT. VOL. 20 No 1 201¿207 ZAGREB June 30, 2011Croatian Natural History Museum, Demetrova 1, Zagreb, CroatiaFurthermore, the pike is an important target species for recreational anglers (PIERCEet al., 1995; ARLINGHAUS & MEHNER, 2004).The appearance of fish in the Danube River is strongly connected to their migra-tion and to the floodplain areas (CIOLAC, 2004). They come to certain parts of theDanube because of spawning or food supply (CIOLAC, 2004). We do not knowmuch about the appearance of pike in parts of Danube River, and about their spa-tial movements. Such studies have been much more common in lakes and pondsthan in rivers (CHAPMAN & MACKAY, 1984; MASTERS, 2005; ARLINGHAUS, 2008; KOBLERet al., 2008). Their appearance and movements differin still water and in rivers(MANN 1980; BURKHOLDER & BERNARD, 1994; GERLIER & LUQUET, 1999; KOBLER et al.,2008). The distance of pike movement varies. Investigations done on the River Ill inFrance mention a movement range between 400–12,260 m (GERLIER & LUQUET, 1999),while English studies done in the River Frome give a maximum movement of 3 kmin the study area, while only a few pike dispersed more widely (MANN, 1980). Inboth studies there are static and mobile specimens in the pike population, a num-ber of individuals staying within a range of movement, but some of them makingmuch longer journeys (MANN, 1980; GERLIER & LUQUET, 1999). BURKHOLDER & BER-NARD (1994) showed that pike are disposed to make movements longer than 100 kmduring their migration between summer and winter locations. Studies showed thatpike from rivers have extensive movements with mean upstream spawning migra-tions in the Ourthe and Ambléve rivers (OVIDIO & PHILIPPART, 2002).The aim of this study was to i) present the number of pike captured per year ofobservation and ii) exhibit the appearance of pike in correlation with water levelsin the Danube River and its floodplain area in the Gornje Podunavlje Special Natu-re Reserve, Serbia.STUDY AREA AND METHODSStudy areaThe study was performed in and along the Danube River (in the text: the river)and its ponds in the floodplain area on the territory of the Gornje Podunavlje Spe-cial Nature Reserve (SNR) (central coordinate: E18°58’; N45°37’) in northwest Voy-vodina (N Serbia) – between 1,366 and 1,433 river km. Pike were captured at ninesites: five in the river and four in ponds (Fig. 1). In this region the average width ofthe river is about 750 m with a water level of 450 cm measured in Apatin (NWVoyvodina), with average depth of 15 m (range: 2.2–32 m). The river meanders andcreates canals and many ponds. Ponds were formed from former arms of the riverand some of them were established through excavation when soil and clay wereneeded for the building of dams. The ponds are often surrounded by trees, mainlywith willow (Salix). The banks are mainly overgrown with grass, sedge (Poaceae)and blackberry (Rubus).On the territory of the Gornje Podunavlje SNR the descent of the river is be-tween 4–6 cm/km which means that it has a slow current velocity. Algal vegetationis very poor but it is rich in zoo- and phytoplankton, and primitive algae. In theponds there are many emerged and submersed vascular plant species (Tab. 1.).202 Mérõ, Th. O.: Periodical appearance of pike in the Danube River in NW Vojvodina (N Serbia)MethodsPike were captured from the beginning of June until mid of November over 14years (1995–2008). The total number of visits was 297, average per year 21 (range:15–30). The study sites were visited in the morning, and the time of catching perday varied from 3 to 6 hours. Angling rods and natural baits or lures were used tocollect pike. The pike were measured with a precise scale and ruler. Only forklength of each individual was taken into consideration. The length of the fishes wasmeasured to a precision of 1 cm, and the weight to a precision of 50 g. Mean lengthand weight was calculated as the arithmetic mean of all fish lengths and weights.Nat. Croat. Vol. 20(1), 2011 203Fig. 1. Locations where pike (Esox lucius) were captured in Gornje Podunavlje SNR,northwest Serbia.Tab. 1. Main characteristics of the locations on the Danube River and in flood pondswhere pike (Esox lucius) were captured.WaterbodyDepth of sites wherepikes appearedGround Aquatic vegetation(dominant species)Hiding placesfor pikeDanubeRiver1.5–4.5 m(mean: 3 m) at water levelof 100 cm, Apatinsand, clay,mud, rock– dead trees,dimples, rocksInundatedponds0.8–2.5 m(mean: 1.65 m)mud, sand seaweed species,Trapa natans,Nymphoides peltatadead trees, dimples,living and deadseaweed vegetationIn this study the mean water levels of four spring months (March, April, Mayand June) were used as a key factor. I used the data measured in Apatin at thehydrologic station (value given in cm). In the mentioned four spring months thewater level mostly varies between 250 and 750 cm. During summer and early autumnthe water level is mostly low or very low with a range between 50 and 200 cm.The main question of this paper is: does the appearance of pikes in the investigat-ed area correlate with the average water level during these four months? Our ques-tion was tested with a non-parametric test – Spearman rank correlation (PODANI,1997)RESULTSDuring the 14 years a total of 1076 pike were captured with a mean length of42.9±7.48 cm (SD range 30–96 cm) and a mean weight of 847.8±725.64 g, (SD range250–6900 g).The appearance of large numbers of pike depends on high water level in springmonths. There was a significant positive correlation between the number of cap-tured pike and the average water level in the four spring months (Tab. 2).204 Mérõ, Th. O.: Periodical appearance of pike in the Danube River in NW Vojvodina (N Serbia)Tab. 2. Spearman correlation between number of pike and mean water level in the fourspring months.Number ofspecimensR P nDanube River and inundated ponds (SUM) 1,076 0.738 <0.01 14Danube River 722 0.642 <0.05 14Inundated ponds 354 0.639 <0.05 14R: correlation value; P: value of significance; n: number of years studied;Fig. 2. Changes in the mean water level in the four spring months and the number ofpike over 14 years on the Danube River and in flood ponds.However, the average spring water level was higher than 350 cm in 2000, 2001and 2005 and yet pike did not appear in large number. In 1996, they appeared inquite large number although the average water level in spring was only 336 cm(Fig. 2). There were two periods (1997–1998; 2007–2008) during these 14 years inwhich there no pike were caught, either in the river or in the ponds (Fig. 2). Be-tween 2001 and 2004, pike appeared only in ponds. In these years the average wa-ter level in spring varied from 220 (2003) to 378 cm (2001) (Fig. 2).Average spring water level between 200 and 400 cm did mainly result in the ap-pearance of pike, but in a very low number, while average water level between 400and 600 cm mostly resulted in the appearance of a large number of pike (Fig. 3).DISCUSSIONThis study showed in general that pike do not appear every year in large num-bers in the studied part of the Danube River and ponds. Their appearance per yearis irregular because it depends upon the height of the average spring water level. Itis very important that the flow reaches and holds an adequately high water level inthe floodplain area of the river for a longer period of time (pers. obs.). Constant andprolonged (min. 1–2 months) duration of the high water level in spring is impor-tant because only then do pike and other fish species use the inundated areas forspawning. In some years the water level was not high enough at the beginning ofMarch when pike spawn. The high water usually came later, nevertheless pikes ap-peared. There were also exceptions in their appearance, for example in 1996, whenthere was no high water in spring but they appeared in large numbers. Results alsoshowed that high water level did not result in a high number of pike. I presumethat average water height in spring is not the only factor responsible for the ap-pearance of pikes. The prey fish resource is also an important parameter. A long--term study done in the United Kingdom reported that change in prey fish abun-dance affects the population of pikes (WINFIELD et al., 2008). Low food availabilityNat. Croat. Vol. 20(1), 2011 205Fig. 3.Appearance of pike in relation to mean water level.in a habitat could lead to cannibalism which is common behaviour among pikes(RUTZ, 1996), and over the long term could destabilise the population. In our re-gions the prey fish are mostly represented by the carp family (Cyprinidae). Theirspawning starts in the beginning of April and lasts until the end of June. This mo-ment is important for pikes, because after the exhausting reproduction period, a lotof energy must be compensated for and spawning cyprinids are an easy prey inlate spring.According to my observations, most of the pike left the studied parts of the Dan-ube River at the end of October or by beginning of November (temperature of thewater is then about 10 °C), probably to their wintering habitats. Other studies alsoshowed that pikes have different winter and summer habitats. Studies about habi-tat choice in mid-summer and mid-winter were done on lakes where pikes selecteddifferent habitats for summer and winter (KOBLER et al., 2008). In the winter seasonpikes choose habitats closer to shore, but in summer they select positively sub-merged macrophyte beds (KOBLER et al., 2008). During the spawning period in earlyspring pike mainly inhabit shallow water habitats with rich aquatic vegetation(NILSSON, 2006). The ponds in the floodplain area of the Danube River are also wa-ter habitats with rich submerged vegetation and I presume that they make excellentspawning sites.According to the results of this study the high water level in the spring plays animportant role in the appearance of pike in the studied part of the Danube Riverand its floodplain area. Furthermore, I presume that prey fish abundance in latespring can also be an important factor that keeps pike in the study site.AknowledgementsThanks to Dejan ^ergi} for important data and Jelena Rakovi} for checking themanuscript.Received July 9, 2009REFERENCESARLINGHAUS, R. & MEHNER, T., 2004: A management-orientated comparative analysis of urban andrural anglers living in a metropolis (Berlin, Germany). 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Periodičko pojavljivanje štuka (Esox lucius Linneaus, 1758) u Dunavu u sjeverozapadnoj Vojvodini (sjeverna Srbija)

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