Fisheries Management and Ecology Fisheries Management and Ecology, 2015, 22, 286–294
Evaluation of downstream migration mitigation actions for eel at an Archimedes screw pump pumping station D. BUYSSE, A. M. MOUTON, R. BAEYENS & J. COECK Research Institute for Nature and Forest (INBO), Brussels, Belgium
Abstract Eel, Anguilla anguilla (L.), mortality was studied in a Belgian lowland canal after downstream passage through a large and small de Wit-adapted Archimedes screw pump over a 12-month period. The hypothesis tested was the minimisation of fish injuries with the de Wit adaptation. Simultaneously, downstream migration through a Dutch pool and orifice fishway alongside the pumping station (PS) was monitored. Nets were mounted on the outflow of the pumps, and a cage was placed in the fishway. Based on the condition of the fish and injuries sustained, the assessed maximum mortality rates ranged from 19 4% for the large de Wit Archimedes screw pump to 14 8% for the small de Wit Archimedes screw pump. The screw adaptations did not substantially minimise grinding injuries and overall mortality, and the fishway did not mitigate downstream eel migration. To achieve escapement targets set in the eel management plans, fish-friendly pump designs and effective PS bypass solutions are needed. KEYWORDS:
Anguilla anguilla, barrier, delayed mortality, downstream migration dynamics, fish friendly, fish
passage.
Introduction Although protection of downstream migrating European eel, Anguilla anguilla (L.), is gaining importance (Buysse et al. 2014), further insight into eel migration behaviour is key for effective preservation. During migration from their respective freshwater habitats to the Sargasso Sea, eels are particularly vulnerable to intake screens, turbines and pumping stations (PSs) due to their elongated morphology and poor burst swimming capabilities (Piper et al. 2013 and references herein). Yet, studies on fish injury and mortality after pump passage are scarce, and more information is needed on the extent of eel losses after migration through PSs. Although the worldwide distribution of PSs is poorly quantified, in Belgium and the Netherlands alone more than 150 and 3000 PSs, respectively, are needed to evacuate water towards the sea (Moria 2008; Stevens et al. 2011). Previous research showed that these PSs may kill between 17 7% and 97 5% of the downstream migrating eel (Buysse et al. 2014). Safe passage at PSs could be achieved by installing fish-friendly pumps and fish bypass facilities. To imple-
ment these actions effectively, additional insight is needed into the seasonal eel migration dynamics as well as downstream passage at PSs. For instance, innovative pumps such as the de Wit Archimedes screw pump were recently designed to convey fish without damage, but very little is known about their efficacy. Fishways have been designed to mitigate fish migration at barriers other than PSs and are at times suggested to facilitate downstream migration at these PSs. Although strong evidence suggests that eels make their route selection choices based on localities with highest flow (Jansen et al. 2007; Piper et al. 2013), fish migration at PSs remains poorly understood, leading to the current lack of effective bypass designs. The aim of this study was threefold, namely (1) to assess the efficacy of downstream eel migration facilitated through an innovative de Wit Archimedes screw pump pumping station (DWASP PS); (2) to gain insight into the timing, magnitude and seasonal downstream migration dynamics of the European eel at this particular PS; and (3) to assess whether the downstream migrating eel use the Dutch pool and orifice fishway (hereafter fishway) without any guidance at the same PS. During a
Correspondence: David Buysse, Research Institute for Nature and Forest (INBO), Kliniekstraat 25, B-1070 Brussels, Belgium (e-mail:
[email protected])
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doi: 10.1111/fme.12124
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12-month survey of the PS and fishway, the number, size and (delayed) mortality rates of eel migrating through a small (S-DWASP) and a large de Wit Archimedes screw pump (L-DWASP) were quantified. Eel condition and the different injuries sustained after pump passage were assessed. Simultaneously, the timing of eel migration was assessed, and the number and size of eel migrating through the fishway was quantified. Materials and methods Study site and data collection
The Isabella PS evacuates water from the Leopold Canal in Flanders (Belgium) (1.40 m asl) to the Isabella Canal (1.97 m asl in summer, 1.42 m asl in winter) and drains a drainage and wetted area of 17156 and 193 ha, respectively (Figs 1 and 2). Since 2012, the PS has had two SDWASP and three L-DWASP installed, resulting in a
total discharge capacity of 14 m³ s 1 (see Buysse et al. 2014 for detailed description of the PS). The DWASP PS has adapted screws according to the de Wit principle. The first windings of the screws have curved edges. As a result, the three blades move more smoothly into the water and are thus less likely to damage fish (Moria 2008; Kemper et al. 2011). This design should minimise eel mortality caused by blade strikes of the leading edges. The PS and the fishway were sampled from 19 March 2012 and 23 April 2012, respectively. Between 19 March and 23 April 2012, only one eel was caught in the PS. Both sampling campaigns ended on 22 March 2013. The fish were routinely sampled every Monday, Wednesday and Friday, with additional sampling if the pumps were operating during periods of high rainfall. One L-DWASP and one S-DWASP were sampled at the same location as in a previous study on the original screw pump (Buysse et al. 2014). Forty-metre-long and
Figure 1. Map of study area showing the Leopold and Isabella canal and the location of the de Wit Archimedes screw pump pumping station (DWASP PS).
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Figure 2. Schematic drawing of the de Wit Archimedes screw pump pumping station (DWASP PS) and the fishway with indications of water level differences, flow directions and research set-up.
twenty-metre-long fyke nets with reducing funnel were permanently mounted, respectively, on the outflows of these large and small pumps. The net mesh size decreased from front to back, from 2 cm, over 1.5 to 0.5 cm. In this study, the sampling method was slightly adapted. The fyke net was extended with a floating frame net, which was made of knotless material (length: 4 m, width: 2 m, depth: 2 m, mesh size: 0.5 cm) to minimise the damage to fish epidermis. It was assumed that the difference in sampling method will not have had any significant influence on the condition and injuries of eels. Pumping activity changed the flow conditions in the canal from non-flowing to slow flowing. Following the automated PS operation, the pumps only evacuated water when the water level upstream exceeded a fixed threshold (1.50 m asl). In this study, the monitored pumps automatically switched on first. If the water level kept rising once the pumps were operating, the remaining pumps also started. Consequently, downstream migrating fish could be missed during sampling. Pumps stopped when the level upstream had reached a fixed base level of 1.30 m asl.
After emptying the nets, all eel were immediately transferred into large aerated holding 90-L tanks. The condition (dead or alive) of each individual eel and its physical status were examined based on visible external or internal injuries. Fish injuries were divided into four categories: injury-free (1), presence of minor superficial scratches (2), (internal) bruising, swelling or bleeding (3) and presence of cuts/slashes (4). Eel characteristics were measured for eels longer than 300 mm: body mass (to the nearest g), total length (to the nearest mm), pectoral fin length and horizontal and vertical eye diameters (to the nearest 0.1 mm) (Durif et al. 2005). The six-stage classification of Durif et al. (2009) was used to divide each eel into growing eel (stages I to FII), pre-migrant females (stage FIII) or migrant females and males (stages FIV, FV and MII). Eels smaller than 300 mm were automatically classified as stage I. Silver eel sex ratios were also based on size, with all eels >450 mm assumed to be female. To study delayed mortality, all pumped eels were kept in two nets mounted on floating frames in the canal. Their condition (dead or alive) and physical status were examined after 48 to 72 h. The nets were placed in the bypass channel in the canal, a calm environment, and were covered with a lid to minimise fish disturbance. To investigate whether the eel can bypass the PS via a fishway, a Dutch pool and orifice fishway adjacent to the PS was monitored. Due to the unnatural water level difference at the PS, the water flows through the fishway in the opposite direction to the main channel. Therefore, eel have to switch their ‘downstream migrating modus’ into an ‘upstream swimming movement’ (i.e. against the water current in the fishway). The fishway consists of a 21-mlong channel with 17 regularly spaced weirs with incorporated underwater orifices (width 0.20 m and height 0.5 m) and a series of 16 steplike pools (width 1.80 m and length 1.00 m) (Figs 1 and 2). Eels were caught by placing a fyke cage in a pool with the cage opening-oriented upstream. The cage mesh size was 0.6 cm, and a funnel in the cage opening prevented eels from returning. Precipitation and pump discharge data were obtained from the Flemish Environment Agency (www.hydronet.be). Due to late replacement of pump flow meters during the revision of the PS, discharge data were not available at the beginning of the study, from 19 March to 5 June 2012. Data analysis
Catch-per-unit-effort (CPUE) was calculated as the number of eel pumped by the S-DWASP or L-DWASP divided by the total volume of water pumped by the respective pumps during the sampling event. A sampling © 2015 John Wiley & Sons Ltd
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event is the period between placement and emptying of the net. The number of eels was rounded to the nearest number. Based on the condition and injuries, eel mortality rates could be assessed according to different scenarios: for example, a minimum or ‘direct mortality’ scenario and a maximum or ‘delayed mortality’ scenario (Buysse et al. 2014). The maximum mortality rate was applied, calculated as the ratio between the number of dead eel plus the number of living eel with lethal visible external or internal injuries (injury category 3 and 4) and the total number pumped. Results Sex ratio and migratory status
In total, 375 eel were caught at the PS: 301 eel (length range: 205–955 mm) passed the L-DWASP, while 74 eel (length range: 212–968 mm) passed the S-DWASP. Characteristics of 336 pumped eel were measured (>300 mm) (Durif et al. 2005). All eels
