Chondrostoma nasus - Biology Centre of the Academy of Sciences of

More documents

Recommendations

Info

(Rodriguez-Ruiz & Granado-Lorencio 1992; Penaz 1996; Winter & Fredrich 2003) have been demonstrated. Several native cyprinids of the Danube undertake spawning migrations into tributaries (Schiemer & Waidbacher 1992), among which nase Chondrostoma nasus (L.) is an indicator species of good river ecosystem health over the breadth of its distribution in Europe (Lelek 1987; Winfield & Townsend 1991; Schiemer et al. 2004). In many large rivers its abundance and distribution has dramatically declined (Penaz 1996) because of habitat fragmentation (Welcomme 1994; Ward & Tockner 2001). Studies about spawning behaviour (Zbinden & Maier 1996; Huber & Kirchhofer 1998), spawning sites composition (Lusk et al. 1995; Keckeis 2001) and large-scaled migrations (Steinmann et al. 1937; Povz 1988) have been carried out, but very little is known about the finescaled patterns and environmental triggers and cues for short-term variability of its spawning migration. In the present study, we used horizontal echo sounding for online monitoring of the spawning migration of nase into an Austrian tributary of the Danube River and related the observed pattern to a range of abiotic parameters (i.e., water temperature, water level and turbidity) on a fine temporal scale. Beyond a general seasonal component, these factors fluctuate considerably in natural systems. We provide evidence that the fluctuations of these environmental cues – short-term increases or decreases within the spawning season – are relevant stimuli for describing the magnitude and timing of the spawning migration in Danube nase. Fig. 1. The Danube River in Austria and its tributary, the Fischa River, showing spawning areas of nase, locations of the echo sounder and traps and where environmental variables were recorded. N Study area The Fischa River is a tributary of the Danube River east of Vienna, originating at an altitude of 240 m in southern Lower Austria (Haschendorf ⁄ Ebenfurth) and flowing northeast for 35 km before joining the Danube (Fig. 1). Together with its main tributary, the Piesting River, which originates at an altitude of 1200 m, the total catchment area is 549.4 km 2 , with a pluvio-nival drain-regime (Mader et al. 1996) and a mean annual discharge of 7.5 m 3 Æ s )1 . Spring (March ⁄ April) and winter (November ⁄ December) floods are common (Kresser 1989). The water temperature does not exceed 16.0 °C during summer. Its hyporithral character changes into epi- and metapotamal within a transition area approximately 4.0 km upstream of the confluence with the Danube, where discharge is mainly influenced by the Danube water level. In spring, the river is characterised by a sequence of single-species spawning events starting with pike Esox lucius Linnaeus and dace Leuciscus leuciscus (Linnaeus), followed by nase, barbel and ide Leuciscus idus (Linnaeus) (Keckeis & Rakowitz 2005). Two nase spawning sites are located 4.5 km upstream the mouth, where the Fischa splits into several smaller branches, creating a local floodplain area (Fig. 1). Two of these branches contain riffles with high current velocity and are regularly frequented by the same nase individuals from year to year (Keckeis 1999; Keckeis & Rakowitz 2006). Adults are present in this river system only during spawning season, from March to early June (Keckeis 1999). Spawning area Weir Echo sounder Traps 2004 Traps 2005 Water temperature Water gauge Turbidity Environmental cues in nase spawning migration Fischa river 0 50 100 km Danube 0 500 m 503
Rakowitz et al. Materials and methods Hydroacoustic recording The hydroacoustic investigation of the spawning migration of nase into the Fischa River was conducted in the transition area at a site 4.0 km upstream of the river mouth (Fig. 1). The cross-sectional riverbed topography here is well suited for fixed location horizontal beaming: a triangular cross-section with a gently sloping right shore and steeply sloping left shore. At average discharge the river cross-section at the sampling site is 15.0 m wide with a maximum depth of 2.15 m near the left shore. The bottom consists of gravel and has an even gradient with small obtrusions. The site is located about 500 m downstream of the spawning areas and about 4 km from the confluence, a sufficient distance to minimise hydroacoustic recording of other Danubian fish species migrating in the lower part of the tributary. Also, water level fluctuations during spring floods are less extensive than in the area of the confluence without needing to dismantle the hydroacoustic setup because of high water levels. From 30 March to 27 May 2004 and from 12 March to 3 May 2005, hydroacoustic data were collected nearcontinuously with a portable 120 kHz digital echo sounder (Simrad, EK 60, Horten, Norway) and an elliptical (4.4° ·8.3°) splitbeam transducer. The transducer was mounted on a tetrapod-scaffold and aimed horizontally across the river from the right shore towards the left shore using a dual-axis remote controlled rotator (Sub-Atlantic, 1128-MAS, Aberdeen, Scotland). A panel fence was placed downstream of the transducer to guide upstream migrating fish out of the transducer’s nearfield. The panel fence was manufactured from solid plastic tubes with a lattice spacing of 1.5 cm, fixed on the ground and equipped with floating bodies on the top. Prior to continuous echo-sounding, beam mapping was conducted to adjust the horizontal beam as close to the bottom as possible in order to detect bottom-oriented upstream migrating fish. The beam was tilted downwards by 7.6° degrees and panned downstream with 13.0° degrees. Once properly oriented, the echo sounder system was calibrated in situ using the tungsten-carbide standard calibration sphere (Ø = 23.00 mm) with a nominal target strength (TS) value of TS = )40.40 dB and kept stationary for the whole sampling period. Basic information about the sound propagation in the horizontal bottom-aligned beam is given by Rakowitz & Kubecka (2006). During monitoring, sound pulses were transmitted in average with the duration of 0.10 ms, the power of 63 W, the pulse width of 0.064 ms, the bandwidth of 11.80 kHz and the pulse repetition rate of 10 pingsÆs )1 . The sensitivity threshold 504 value of the detected echoes was set to )50.00 dB because of the level of the background noise. To increase the detectability in the horizontal bottomaligned beam, the echo length detector was set to 0.8– 1.5 times the transmitted pulse, the maximum phase deviation was set to 1.2° and the maximum gain compensation was enhanced to 6 dB (one-way). Signals were automatically compensated for off-axis distance. The echo sounder was in operation for an average 23.5 h a day for 59 days in 2004 and 53 days in 2005. Changing environmental conditions (water level, turbidity) did not alter the efficiency of the hydroacoustic system. Analog data from the transducer were digitalised in the echo sounder’s GPT unit (General Purpose Transceiver), recorded on a laptop and stored on an external hard disk. Hydroacoustic analysis For this study, a total of 66 days – 33 days each year – were analysed with Sonar5-Pro version 5.9.1 postprocessing software (Balk & Lindem 2000, 2002; Lindem Data Acquisition, Oslo, Norway), which was updated regularly (http://www.fys.uio.no/~hbalk). Data recorded in 2005 was additionally corrected for biased TS-values because of inhomogeneous sound propagation in the horizontal bottom-aligned beam (Rakowitz & Kubecka 2006). To obtain the total number of upstream migrating nase (Fig. 2e,f), the remaining 26 and 23 days, respectively, were interpolated using the mean of upstream migrants of the day before and the day after. Linear regression analysis between daily counts and interpolated data from a continuous series of 11 days in 2005 revealed a highly significant relationship (R 2 = 0.91, P < 0.001). After conversion of raw data, the acoustic analysis was carried out by single echo detection (SED). For SED, we did not apply a traditional detector based on echo length (Soule et al. 1997), but the alternative Crossfilter detector (Balk 2001) found in Sonar5. This detector has been developed to improve track quality and to reduce erroneous detections in data with low signal-to-noise ratio (SNR). The detector contains a suggestor obtaining trace candidates, an evaluator sorting out unwanted candidates, a SED former which defines the individual echo detections within a trace and an echo quality estimator. The suggestor works with filters identifying the targets frequency response while the evaluator operates with a set of min and max trace feature tests. In 2004 and 2005, the cross-filter settings which provided best results for tracking fish in the Fischa were for the suggestor: foreground filter: height = 1 and 1, width = 1 and 1; background filter: height = 23 and 11, width = 7 and 13; offset = +6 and +6 dB. For the evaluator, trace length and trace area were used. Minimum and maximum values for the
Page 1: Ecology of Freshwater Fish 2008: 17
Page 5 and 6: Rakowitz et al. all migrants in the
Page 7 and 8: Rakowitz et al. Lag period (days) -
Page 9 and 10: Rakowitz et al. y -1 N o. of migran
Page 11 and 12: Rakowitz et al. Table 2. Summary of
Page 13: Rakowitz et al. Northcote, T.G. 198

Chondrostoma nasus - Biology Centre of the Academy of Sciences of

You also want an ePaper? Increase the reach of your titles

Delete template?

Save as template?