Eesti Loodushoiu Keskus

Fish fauna of oxbow lakes of Emajõgi River in Alam-Pedja Nature Reserve

Tartu 2010

Summary
The current paper presents an overview of the data on fish fauna of oxbow lakes of Emajõgi River collected during the project “Saving life in meanders and oxbow lakes of Emajõgi River on Alam-Pedja NATURA 2000 area” carried out by Wildlife Estonia. The data is presented as of the end of 2010. The project is financially supported by the EU LIFE+ Programme and Environmental Investment Centre of Estonia, together with the Estonian project partners Nature Conservation Society Kotkas and Environmental Information Center. The main aim of the project is to improve the living conditions for fish in the Emajõgi River system and restore the functioning of oxbow lake ecosystems. The project activities include re-opening of migration routes for fish at the mouths of oxbow lakes by sediment removal and cleaning of spawning grounds on the flood plains at the oxbow lakes by removal of brushwood and macroflora that remarkably increases the size of spawning area required for fish reproduction. Data on fish fauna in oxbow lakes is gathered in order to assess the success of the project.
The part of the project focusing on fish fauna is executed by Meelis Tambets, Jaak Tambets, and Einar Kärgenberg. The project executors are grateful to Andres Põhjala, Meelis Sepp, Jüri Tambets, and Jüri Ketner for their helpful contribution.
1. Gillnet surveys and survey methods
Fish sampling was carried out altogether in 21 oxbow lakes of the upper course of Emajõgi River. Fish fauna data was gathered mostly by using the Nordic multi-mesh monitoring gillnets (mesh size range 5–55 mm). Both the pelagic and benthic gillnets of 1.5 meters high were used. Nets were usually set in the water on afternoon or at night before the sunset and taken out in the subsequent morning after the sunrise. The species composition as well as the total length and weight of fish were measured. Besides the distribution of fish in various sections of gillnet, the vertical distribution of individuals within the net was recorded if required. 
2. Catch results
Fish fauna and its seasonal variability. Factors influencing fish fauna
Oxbow lakes vary both in terms of species composition and catch (annex 1, table 2). Such a high variability could be expected as the variation in size and depth of oxbow lakes, the existence as well as the number and depth of connections with the main river dependent on water and other factors establish habitat diversity, thus providing conditions for a wide range of species. 
During the two years of fish sampling all in all 16 fish species were caught. This makes up 59% of all species known to live in oxbow lakes. As to the abundance, the non-predatory fish dominate in oxbow lakes with roach being especially abundant both in terms of quantity and mass (66% and 42% of total catch, respectively). In terms of mass, predatory fish form the next largest group (25%) of the caught fish (annex 1, table 2; figure 1). 
The average catch amounting to 3.4 kg per gillnet-night can be regarded as relatively high, as the figure is three times higher than the respective average of Estonian small lakes. Strong seasonal effect on the catch was recorded (figure 1). The catch figures in the spawning season reached as high as 14 kg and 671 individuals per gillnet-night (Kupu oxbow lake, April 2009). In spring and early summer roach need not necessarily be the most abundant species in the catch. The share of different species in the catch stabilizes and the catch on average drops as the high water recedes. The low catch figures are especially characteristic to oxbow lakes with low-level oxygen content. During these periods (both in winter and summer) fish may also be absent, as was in cases of Kupu oxbow lake in July 2009 and Kärkna oxbow lake in December 2009.

Figure 1. The share of species (%) and catch (CPUE, number of fish per gillnet-night) in fish sampling in 2009. 
The catch and species abundance of fish fauna is also dependent on the quality of connection between the oxbow lake and the main river. On average, respective figures are lower in the oxbow lakes with lowered mouth (figure 2). Results like this could be anticipated due to the fact that determined by the water lever, in low-mouth water bodies the migration of fish to spawning and recovery grounds and refuges can be highly limited. The water deficiency in the mouth of an oxbow lake may in some years with severe hypoxic conditions lead to the extinction of fish fauna.

Figure 2. The relationship between the depth of the oxbow lake mouth and abundance of fish (CPUE, number of individuals per gillnet-night), catch (CPUE, kg per gillnet-night) and species abundance (CPUE, number of species per gillnet-night).
3. Oxygen problems in oxbow lakes
Oxygen content in the water of oxbow lakes was measured during the fishing season. Clear seasonal variation in the oxygen content was detected in addition to great variation in the abundance and species composition of fish fauna. However, the data of 2010 gave no evidence for reliable cause-effect relationship of CPUE and oxygen content in the catches executed in ice-free period. 
4. Impact of deepening of oxbow lake mouth on fish fauna
Fish sampling suggested positive impact of oxbow lake deepening both on catch and species abundance (figure 3). Prior to deepening, fish fauna in the deepened oxbow lakes had been constantly poorer than average. The figures showed a manifold increase in catch after deepening took place. Species diversity increased by 1.4 times on average. All characteristics describing the abundance and species diversity of fish fauna showed rise in oxbow lakes with deepened mouth in comparison with other oxbow lakes. Similar effect of mouth opening on species diversity and catch has been known from the deepening work carried out in the 1960s. 

Figure 3. The relationship between the deepening of oxbow lake mouth and abundance of fish (CPUE, number of individuals per gillnet-night), catch (CPUE, kg per gillnet-night) and species abundance (CPUE, number of species per gillnet-night).
5. Presence of protected species in fish sampling
Four fish species listed in the Annex II of the EU Directive 92/43/EEC on the conservation of natural habitats and of wild fauna and flora are known to occur in the oxbow lakes of Emajõgi River – spined loach, European weatherfish, European bullhead, and asp. Asp and European weatherfish was caught during the netting conducted in 2009 and 2010. However, the method used can be regarded as of low-efficiency in terms of capturing other protected species most probably due to the living habits and shape of the fish. 
Asp was captured only in one oxbow lake in 2009 (table 1). In the subsequent year, it was trapped in four oxbow lakes with the overall frequency of occurrence increased by six times. In all occurrence cases juveniles (TL 9-20 cm) were caught.

Table 1. Occurrence of asp in fish sampling in 2009 and 2010.
Location	Year	
	2009	2010	
Kärkna	*	 	
Albri		*	
III Kaevand		*	
Võllinge		*	
Hobuseraua		*	
CPUE (individuals/gillnet-night)	0,011	0,069	

6. Analysis of fish sampling methods used in multi-mesh netting
Analysis of catch methods revealed a number of aspects that could be useful to consider for increasing the objectivity of fish sampling and obtaining a true picture of fish fauna status in oxbow lakes. Different species, but also assemblages of species and in some cases even assemblages within one species, demonstrated dissimilar preferences toward depth strata of a water body (figures 4, 5, and 6). These findings refer to the need of a more even coverage of all major depth strata with nets. Otherwise, the abundance of some species, assemblage of species or assemblage within a species, might be overvalued or underestimated. 
The data for the analysis was collected from July to December 2009. The ten most abundant species caught during the catches were included in the analysis. Mostly fish moving in strata near the bottom are caught with benthic gillnets, while fish from the surface layers of water are captured with pelagic gillnets. 
The analysis of catch revealed a number of differences between netting with benthic and pelagic gillnets (figure 4). When caught with benthic gillnets, the catch of predatory fish such as perch, pike, pike-perch, and ruffe turned out 2.7–5.1 higher than with pelagic nets. Interestingly, ruffe was captured only with benthic nets. In contrast, catches with pelagic nets showed higher catch only for non-predatory fish such as silver bream, common rudd, and common bleak by 1.5–6.1 times. The occurrence frequency of roach was almost the same for both net types. Similar to the figures of catch of predatory fish , also the capture of moderlieschen and golden orfe was more frequent in benthic nets than in pelagic nets, but nevertheless, not as frequent as in sampling of predatory fish (1.3–1.7 times).

Figure 4. Dependence of catch (CPUE, individuals per gillnet-night) by fish species on the net type used in sampling in 2009.

The analysis of catch of fish sampling with different net types by three different height zones (upper, middle, and lower third) allows specifying the findings of analysis of catch with different net types. Ruffe clearly prefers benthic habitats (figure 5). Accordingly, 80% of this species was captured in the lower third of benthic gillnets. Similar results were recorded for perch, the share of which in the lower third of benthic gillnets made up about 50% of its total catch. Of the cyprinids that were relatively more frequent in the pelagic gillnets, common bleak and common rudd were those clearly preferring the upper 0.5-meter-high layer of the pelagic zone. Silver bream is abundant also in somewhat deeper layers. Despite the fact that moderlieschen and golden orfe are more abundant in benthic gillnets, they are still more frequently captured in the middle parts of the nets. Moderlieschen, however, is common rather in the upper part of the nets. In a similar way, the middle water layer is also favored by roach.

Figure 5. Distribution frequency (%) of fish in vertical zones by fish species and net type. 

Species caught in larger quantities enabled to analyze habitat preferences of fish of different size. In case of roach, the size classes were not always evenly distributed between different depth layers. In the early season (from July to the first half of August) the abundance of roach of smaller size (TL up to 13 cm) in the upper water strata was several times higher than in lower layers (figure 6), while the roach of bigger size (TL 14 cm or more) had unvarying distribution between water layers. In the late season (from the second half of August to October) no apparent depth preferences specific to length of fish were detected. 

Figure 6. Dependence of catch (CPUE, individuals per gillnet-night) of roach of different size on the net type used in sampling in 2009. 

Habitat preference determined by fish length was also recorded in sampling of perch. The effect was more apparent in the late season when smaller individuals (TL up to 18 cm) mainly chose to inhabit deeper water layers (figure 7). Similar to roach, perch of bigger size (TL 19 cm or more) showed no depth preference specific to length of fish.

Figure 7. Dependence of catch (CPUE, individuals per gillnet-night) of perch of different size on the net type used in sampling in 2009.

Literature
Ristkok, J. 1969. Emajõe vanajõed. Tartu Riikliku Ülikooli Toimetised. Zooloogia-alaseid töid V. Tartu, lk. 3-85.
Veeroja, R., Luig, J., Tambets, J., Tambets, M., Järvekülg, R., Rosentau, A., Liimand, K. 2003. Emajõe ülemjooksu vanajõgede piirkonna kalastik ja suurselgrootud. Liikide ja elupaikade kaitse. Eesti Loodushoiu Keskus, Tartu, 2003, 95 lk. (Käsikiri EV Keskkonnaministeeriumis

Appendix 1

Table 2. The catch (CPUE, number of individuals (n) per gillnet-night) and abundance (CPUE, grams (m) per gillnet-night) in samplings in oxbow lakes.
Species	Rõhu	II Kaevand	Albri	Nasja	Kupu	IV Kaevand	Sibula	I Kaevand	Rõngaskoold	Lustivere	Kärkna	
	n	m	n	m	n	m	n	m	n	m	n	m	n	m	n	m	n	m	n	m	n	m	
Roach 	97,7	3352	95,6	2792	111,5	2893	78	1951	84,8	1559	69	2143	68,3	1916	69,3	1797	48	1179	52,8	630	34,8	1090	
S. bream	17	186	26,2	337	4	25	12,3	89	6,2	136	24,8	318	2,3	13	1,3	13	2	28	1,5	3	15,5	648	
C. bleak	31	350	14,4	153	6,5	76	11,7	140	4,8	46	2,8	35	10,8	160	2,5	29	17	151	0,3	4	13,1	140	
Perch	8,7	878	4,2	698	6,3	665	1,7	82	4,1	203	2,5	431	8,8	601	8,5	510	3,5	317	12	328	5,7	367	
C. rudd	9,3	596	5,8	474	3,5	355	4	522	4,8	145	1,3	236	4,8	339	5,8	332	5,5	261	8,3	236	4,2	429	
Moderlieschen	1,7	3	 	 	6,8	12	1,3	2	3,5	4	1,3	1	2,5	5	9,8	13	 	 	2,8	4	0,8	1	
Ruffe	6	72	6	59	0,8	11	4,3	61	0,8	7	4,5	48	1,5	22	1,3	13	4	28			0,7	5	
Golden orfe	1	500	0,8	246	0,3	92	 	 	0,5	238	 	 	1,3	605	0,5	316	1,5	569	1	612	0,9	396	
Tench			0,4	387	0,8	456	 	 	0,1	21	0,8	645	 	 	0,3	138	 	 			0,1	29	
Pike	0,3	98	 	 		 	 	 	0,4	440	0,3	201	0,8	922	0,5	777	 	 	0,5	613	0,3	297	
C. bream	0,3	112	0,8	168	0,5	51	 	 	0,2	82	0,3	39	0,3	59			 	 			0,2	28	
Pike-perch	0,3	3	0,2	1		 	 	 			0,5	3	1,8	4			 	 			<0,1	<1	
Asp			 	 	0,3	6	 	 			 	 	 	 			 	 			<0,1	2	
Chub			 	 		 	 	 			0,3	132	 	 			 	 			 	 	
Crucian carp			 	 		 	 	 			 	 	 	 			 	 			<0,1	13	
Gibel carp			 	 		 	 	 			 	 	 	 			 	 			 	 	
Total	173	6150	154	5315	141	4642	113	2847	110	2881	108	4234	103	4646	100	3938	82	2532	79	2431	76	3444	

Species	Pudru	Võllinge	III Kaevand	Vanaviht	Hobuseraua	Samblasaare	Väike Kullasaare	Puhja	Kõverik	Lempsi	Total	Average share (%) 	
	n	m	n	m	n	m	n	m	n	m	n	m	n	m	n	m	n	m	n	m	n	m	n	m	
Roach 	49,8	1441	53,5	889	45	1459	43,8	718	29,3	1089	20,3	515	27,5	990	24,3	788	23	708	7,6	263	1133	30163	66,3	42,4	
S. bream	3,8	31	1,5	9	1,8	21	2	3	0,3	5	12,8	369	 	 	1	8	0,3	6	0,2	1	136,6	2250	8	3,2	
C. bleak	2,3	29	4,5	50	3,5	33	0,8	3	2	16	3,6	48	 	 	0,5	8		 	1,2	7	133	1479	7,8	2,1	
Perch	5,3	253	2,5	228	9,3	983	5,3	391	4,3	365	1,9	135	9,3	572	4,5	527	4,3	304	1,8	248	114,1	9087	6,7	12,8	
C. rudd	3,5	590	2,8	67	3,3	260	4,3	446	4,8	530	2,1	148	1,8	153	0,5	79	1,5	153	0,4	55	81,9	6406	4,8	9	
Moderlieschen	1,3	2	 	 	0,3	<1	0,5	1	2,3	6	1,5	2	0,3	1	1	1	0,5	1			37,8	60	2,2	0,1	
Ruffe	0,5	4	0,3	4	1,8	27	1	9		 	1,3	38	 	 		 	0,3	7	0,4	6	35,3	421	2,1	0,6	
Golden orfe	0,8	408	0,5	232	0,5	335	0,8	496		 	0,5	245	0,5	352	1	646		 	0,2	47	12,3	6333	0,7	8,9	
Tench	0,8	576	 	 	0,5	380	0,5	255	1	676	0,1	66	0,8	420	0,5	233	1	620	0,2	74	7,6	4975	0,4	7	
Pike	1	1292	0,8	1416			0,8	1474	0,3	72	0,3	185	 	 		 	0,5	552	0,6	188	7,2	8529	0,4	12	
C. bream	0,8	168	 	 			 	 	0,3	16	0,3	92	0,3	72		 	0,8	201			4,8	1088	0,3	1,5	
Pike-perch			 	 			 	 		 			 	 		 		 			2,8	12	0,2	<0,1	
Asp			0,3	2	0,3	3	 	 	0,3	4			 	 		 		 			1,1	17	0,1	<0,1	
Chub			 	 			0,3	252		 			 	 		 		 			0,5	385	<0,1	0,5	
Crucian carp			 	 			 	 		 			 	 		 		 			<0,1	13	<0,1	<0,1	
Gibel carp			 	 			 	 		 	<0,1	2	 	 		 		 			<0,1	2	<0,1	<0,1	
Total	70	4796	67	2896	66	3500	60	4050	45	2780	45	1846	40	2559	33	2290	32	2553	13	889	81	3391