Biology and Management of European Eel (Anguilla anguilla, L) in the , ______

Biology and Management of European Eel (Anguilla anguilla, L) in the Shannon Estuary, Ireland Chapter 7 Interactions of juvenile eels, Brown trout (Salmo trutta, L) and other predators in the tidal section of the lower Shannon River

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Biology and Management of European Eel (Anguilla anguilla, L) in the Shannon Estuary, Ireland ______

Chapter 7 Interactions of juvenile eels, Brown trout (Salmo trutta, L) and other predators in the tidal section of the lower Shannon River

7.1 Introduction

The diet of the European eel in natural ecosystems and its predation on other fish has been examined in a number of European studies (Hartley, 1948; Sinha & Jones, 1967; Shafi & Maitland, 1972; Tesh, 1977; Barrack & Mason, 1992; Costa et al, 1992). Previous studies in Ireland include those carried out on yellow eels by Moriarty (1974, 1975), Proman & Reynolds (2000), and Matthews et al (2002). Walsh (1996) and Clarke et al (1996) investigated the diet and feeding of elvers and yellow eels respectively in the Shannon catchment. However, despite the various studies on the food of eels, details of the array of predators which feed on eels themselves has received relatively limited attention in the scientific literature.

Plate 40 Anglers trout fishing fropm Shannon Bridge in city in the 1990's using live baited elvers.

Although little is known about eel predation in the marine environment, eel leptocephali are not thought to be seriously affected by predation by pelagic fishes in the Sargasso Sea and

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Biology and Management of European Eel (Anguilla anguilla, L) in the Shannon Estuary, Ireland ______adjacent parts of the Atlantic (Appelbaum, 1982). Likewise, they do not appear to be important in the food of seals and cetaceans (Rae, 1973; Lunneryd, 2001). In freshwater and brackish waters, eels (particularly small ones) are known to have many predators. Anthony (1994) estimated that Atlantic salmon (Salmo salar, L.) is fed on by more than 50 predator species in coastal and riverine areas, and it is expected that many of these also feed on eels. Several studies have indicated the importance of eels in the diet of mammals such as Eurasian otter Lutra Lutra (O’ Sullivan, 1994; Fairley & Murdock, 1999; Fairley, 2001; Hayden & Harrington, 2001) and they are also eaten by the feral American Mink Mustela vison (Maitland & Campbell, 1992). Eels can be an important component of the diet of pisciverous birds such as Grey Heron Ardea cinerea (Marion et al, 2001) and Cormorant Phalacrocorcorax carbo (Feltham & Davies, 1995; Doherty & McCarthy, 1997; Engstroem, 2001), as well as other fish such as intercohort conspecifics (Jessop, 2000), and heterspecifics such as Northern Pike Esox lucius (Mann, 1982) and Brown trout Salmo trutta (Kennedy and Fitzmaurice, 1971). Predator prey interactions involving eels have been investigated by in studies such as those undertaken by Blake & Hart (1995), Feunteun & Marion (1994, Engstrom (2001) and Marion et al (2001). Studies of the interactions between eels and salmonids have generally focused on competition between the two species (Parey, 1985; Tesh, 1986; O’ Conor & Power, 1974), predation by eels on the juvenile stages of salmonids (Mann & Blackburn, 1991), and the environmental factors affecting contemporaneous riverine migrations of both groups (Hansen et al, 1995).

Brown trout (Salmon trutta, L.) occur in virtually every catchment in Ireland where suitable water quality and spawning grounds occur (Went, 1964; Kennedy & Fitzmaurice, 1971; Maitland, 1972; Fahy, 1977 Kelly-Quinn & Bracken, 1988). The anadromous variant - Sea trout - occurs mainly in ‘acidic’ coastal catchments and in some larger rivers and smoltify during their seaward migration (Went, 1964; Cross & Piggins, 1982). In many catchments throughout Ireland, trout make extensive migrations between spawning grounds in streams and feeding grounds in lakes, larger rivers or estuaries (Kennedy & Fitzmaurice, 1971). One of the best known of these migrations is that of the ‘croneen’ which migrate between Lough Derg and spawning grounds in the Little Brosna and Nenagh rivers in the Shannon catchment (Kennedy & Fitzmaurice, 1971). In the Shannon estuary ‘slob’ trout, which are believed to be recruits from the main and small catchments discharging into the Shannon estuary are present. ‘Slob trout’ is a local term used for a brown trout phenotype which has spent part of it’s life in brackish or salt water and which did not become a smolt (Cross & Piggins, 1982) before descending into tidal waters. These fish are not true sea trout and their estuarine residence is thought to be largely related to better feeding opportunities (Kennedy and Fitzmaurice, 1971). Downstream traps operated on the Lower River Shannon and Mulkear River during the period 1991-1996 have failed to indicate a significant downstream movement of trout smolts (ESB Unpublished data), confirming

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Biology and Management of European Eel (Anguilla anguilla, L) in the Shannon Estuary, Ireland ______that a significant Sea trout population is absent from the Shannon. The ‘slob trout’ term describes a continuum between trout found in slightly brackish water to fish almost impossible to distinguish from genuine sea trout. Unlike sea trout, slob trout (with the exception of spawning migrations) appear to move up and down an estuary only in the search of food (Kennedy and Fitzmaurice, 1971). Anglers in Limerick City have said that these trout follow upstream migrating glass eels from the estuary to the tidal head at Limerick city. Brown trout are important species in the context of the Irish angling tourism industry, however little attention has been paid to trout angling in Ireland in the scientific literature.

Although the Lower River Shannon is noted mainly as a salmon fishery, extensive and important stocks of Brown trout also exist (Southern, 1937, Kennedy & Fitzmaurice, 1971). Indeed the Irish record rod caught brown trout (9.07 kg) was captured in this area in 1957 (Anon, 2000). Recent upgrading of fish counting facilities at Ardnacrusha generating station has allowed the census of all fish species ascending into the upper Shannon via the Borland fish-lift, and significant numbers of trout were recorded each year during the period 1994- 1997 (see chapter 6). Trout fishing is more important than salmon fishing in the tidal section of the Shannon River near the centre of Limerick city. Trout are fished early in the season using artificial minnows, with fly-casting becoming more successful from April onwards. In spring, when the peak migration of elvers through this area occurs, large numbers of anglers concentrate on the city bridges and fish with live-baited elvers. Some bank and boat angling also occurs. A standard size 8-12 full shank hook generally is used to mount glass eels and a sliding float rig is used to present the bait near the bed of the river. This type of fishing normally takes place during the spring tides of April and May.

7.2 Materials and methods

Angling catches were monitored by direct observations and through the examination of trout sold into a Limerick fishmongers (‘Ray’s Plaice’, Parnell Street, Limerick) by local rod and line fishermen. During 1995 samples were collected from May and June only, while in 1996 monthly samples from March to June were analysed. Although every effort was made to ensure that all trout examined at the fishmongers came from the lower Shannon, there is a small possibility that fish came from other local rivers. However, interviews with anglers and local observations of fishing activities suggested that it was very unlikely that any trout from areas other than the Lower River Shannon and perhaps the were sold to the fishmongers at that time.

During 3rd and 4th of May 1995 all trout captured by anglers using lines baited with live elvers and glass eels at Sarsfield and Shannon bridge in Limerick city were examined and the

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Biology and Management of European Eel (Anguilla anguilla, L) in the Shannon Estuary, Ireland ______time of capture of each fish was noted. Trout recovered at the fishmongers, were examined for length and weight, and the head and viscera from each fish was retained and stored in a domestic freezer prior to further examination in the laboratory. Condition factor (k) was calculated for males and females from both years using the equation K = W/L3 given in Tesh (1971). Examinations of stomach contents were carried out on each sample and percentage occurrence and abundance indices (Liao et al, 2001) of various food items in the trout stomachs were derived. The sex of each fish was determined by visual examination of their gonads. Lengths of undigested glass eels recovered from trout stomachs were recorded. Additional observations of eel predators were undertaken during the servicing of elver traps and nets (see chapters 3 and 4).

Plate 41 Shannon bridge (foreground) and Sarsfield bridges on the River Shannon in Limerick City.

7.3 Results

The greatest angling activity occurred, and the largest catches were observed during the last three hours of a spring tide. Although on some occasions elvers were seen to be actively migrating on the surface of the river, angler’s baits were presented near the bottom of the river (1-4m deep). Likewise, most of the surface eel activity was seen to be on the sides of ______www.EuropeanEel.com| www.ecofact.ie |www.OldRiverShannon.com Page | 226

Biology and Management of European Eel (Anguilla anguilla, L) in the Shannon Estuary, Ireland ______the river, while anglers fished at regular intervals across the river. Anglers fishing the middle of the river did not report smaller catches that those fishing the side of the rivers.

Observations of anglers using lines baited with live elvers and glass eels at Sarsfield and Shannon bridges in Limerick city during the 3rd and 4th of May 1995, provided information on the temporal distribution of trout catch around HWT (high water time). These results are presented in figure 7.1. On these nights, anglers begun to gather on the bridges approximately 2.5 hours before high tide, and the numbers present peaked at 35 and 20 respectively, approximately 1 hour before HWT. After high tide, many anglers left with less than ten anglers fishing Sarsfield Bridge one hour after the tide had turned on both nights. Many anglers fishing on Sarsfield Bridge moved down to Shannon Bridge, where the deeper water is thought to allow an extended fishing period. The greatest angling activity recorded coincided with the main movements of elvers through the area (see chapter 3-4). The largest catches were made during evening spring flood tides, and this is also the time when the greatest glass eel movements were recorded (see chapter 3). Anglers were generally catching trout at a depth of 4m, and trout were not generally seen feeding on the surface. This may suggest that elver movements also take place along the bottom of the river.

During 1995 and 1996, examination of all trout from the fishery that were sold into a local fishmonger provided a total of 282 specimens from which length, weight, sex and stomach content determinations were made. Figure 7.2 compares the length frequency distributions of the entire catch (both retained and returned) of 115 trout captured by anglers, and of 155 trout sampled at the at the fish mongers during 1995. This figure indicates that (in general) smaller trout are not sold but probably released. The survival rate of these fish after release is unknown. The percentage length frequency distributions of male and female trout from both years are given in figure 7.3. The sex ratio of males: females was identical for the two years under consideration. The size range of trout recorded at the fishmongers was greater in 1996 than in 1995. This may be the result of the longer sampling period during 1996. The regression equations derived for length and weight during 1995 are presented in table 7.1.

The condition factor (k) percentage frequency distribution of male and female trout from the Limerick fishery examined during 1995 and 1996 is presented in figure 7.4. Percentage occurrence and abundance indices of various food items in the trout stomachs were derived and are presented in tables 7.2 and 7.3 respectively. It is apparent that young eels were the most important component of trout diet in this area during March and April 1995, and during April 1996. Eels were less important in the diet of trout examined during March 1996. Amphipods and caseless Caddis fly larvae were very important in the diet early in the season ______www.EuropeanEel.com| www.ecofact.ie |www.OldRiverShannon.com Page | 227

Biology and Management of European Eel (Anguilla anguilla, L) in the Shannon Estuary, Ireland ______of both years, with trout apparently switching to eels as soon as they become available. Only limited numbers of trout were available for examination during May and June, but results indicate that eel is less important in the trout diet at this time. The % length frequency distribution of juvenile eels examined from March/April trout stomach samples, March shore sample from Limerick Docks and May sample from Ardnacrusha ramp trap (chapter 4) are presented in figure 7.5. Non-parametric tests (p=0.05) indicated that the samples from the shore at Limerick docks and from trout stomachs were not significantly different (Mann Whitney, p= >0.5), while the sample from the Ardnacrusha elver trap was significantly different (Mann Whitney, p= <0.01) from both the shore and stomach samples.

Table 7.1 Linear regression equations for length and weight measurements of Brown trout from the River Shannon at Limerick City.

Sex Year Regression equation R-sq.

Females 1995 Log Length = 0.682 + 0.311 Log Weight 83.0 %

Males 1995 Log Length = 0.704 + 0.299 Log Weight 78.3 %

Females 1996 Log Length = 0.788 + 0.269 Log Weight 78.3 % Males 1996 Log Length = 0.716 + 0.297 Log Weight 88.4 %

A small number (n=7) salmon smolt (Salar salar) were recorded in the fishermen’s catch during the period 3-4 May, 1995 (8.2% of total catch). Although none were recorded during this study, fishermen accounted that occasionally larger eels were captured using elvers as bait.

During the study period, significant quantities of elvers were regularly observed attempting to ascend the spillway channel located to the east of the Ardnacrusha powerhouse during both day and night. Here large concentrations of trout gathered and could easily be seen to feed on the young eels. During the study, trout were also observed to be feeding on elvers downstream of the traps on the River Maigue. At both Ardnacrusha and Adare, otters were seen to eat elvers at the base of the traps and predator protection nets were regularly damaged by the activities of these mammals. Otters at Ardnacrusha regularly climbed up the elver ramps under the tensarmat layer in order to avoid the anti predator barrier. Grey Herons regularly interfered with riverine traps and fed in the areas immediately downstream of the traps. At Ardnacrusha, resident Herons (n=10-12) learned to land in deep water and swim towards the base of the traps to predate on the migrating elvers.

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Biology and Management of European Eel (Anguilla anguilla, L) in the Shannon Estuary, Ireland ______

During 1997, Mallard (Anas platyrhynchos) (n=4) – a species not noted for piscivority - were regularly observed feeding opportunistically on elvers at the base of the traps at Ardnacrusha. During 1997, significant numbers of salmon smolt (Salmo salar), which appear have passed downstream through the turbines, were seen to be actively feeding on elvers in the tailrace over the period 9-14 April.

7.4 Discussion

The chances that a fish will be attacked will depend on the density of the prey, the density of the predators and the size of the predators (Wooton, 1990). The number of prey taken by predators over a defined time is determined by the functional, numerical and developmental responses of the predator-prey relationship (Taylor, 1984). The risk of predation can vary either directly (compensatory risk) or inversely (despensatory risk) with prey density. Despensatory mortality can occur during fish migrations when migrating fish pass through an array of predators over a short time period (Ricker, 1962), and upstream migrating elvers are likely to be impacted on by this phenomenon. Many studies have suggested that predation can be responsible for a high proportion of the mortality in a natural fish population (Piggins, 1959; Anthony, 1994; Alexander, 1979), and predation on glass eels and elvers in estuaries and the lower reaches of rivers is though to contribute significantly to mortality rates (Jessop, 2000; Hansen et al, 1995).

To mitigate against predation, eels have developed a number of behavioral defenses. For example, the upstream migration of glass eels generally takes place at nighttime to perhaps reduce the possibility of being detected by predators. Elvers also shoal in large groups and migrate upstream over a relatively short period. However, elvers are very vulnerable to predation as they enter rivers and osmotic stress and difficulties in clearing instream obstacles may contribute to a high predation rate. Moreover, anthropogenic perturbations on the River Shannon have probably exacerbated the impacts of some predators, and may have contributed to increases in some predator populations. The impact of dam construction on the predation of salmonids on the Columbia River was discussed by Collins et al (2002). Doherty & McCarthy (1997) described the foraging activity of cormorants in the tailrace of Ardnacrusha Generating Station.

Many studies have been published on the feeding of trout in Irish waters (Fitzmaurice, 1979; O'Grady, 1983; Lobon-Cervia & Fitzmaurice, 1988; Kelly-Quinn & Bracken, 1989; Bolger, Bracken & Dauod, 1990; Bridcut & Giller, 1993; Bridcut & Giller, 1995). Trout are opportunistic feeders, and will consume a wide range of available allochtonous and autochotonous food ranging from zooplankton to other trout. This opportunistic feeding has been confirmed during the current study, with lower Shannon trout feeding on elvers at

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Biology and Management of European Eel (Anguilla anguilla, L) in the Shannon Estuary, Ireland ______times when they are available. Kennedy and Fitzmaurice (1971) reviewed the age and growth of brown trout in Irish waters. In alkaline waters trout typically achieve lengths of 10cm at the end of their first year of life, 20cm at the end of their second and 30cm at the end of their third. In the current study the trout were in the 10-45cm range.

Accurately quantifying the importance of prey taxa and understanding the contributions of various prey to predator well-being is essential for effective management of fisheries resources (Bowen, 1996). However, Laoi (2001) postulated that rating and comparing prey taxa in the diets of fish on an importance scale may lead to misleading or erroneous conclusions. It is clear that numerical abundance (%N) data can underestimate the importance of small prey due to the influence of prey size, shape and energy content on digestion rates (Anderson, 1999; Chapman et al, 1989), while percentage occurrence (%O) data can overestimate the importance of small prey. However, compound diet indices have also been critisised due to nonadditivity and loss of information (McDonald & Green, 1983). In the current study both %N and %O indices were used, and despite the limitations of these methods, it is concluded that the results clearly indicate a seasonal presence and relative importance of juvenile eels in the diet of trout in the lower Shannon. Assessing the importance of juvenile eels in the diet of trout is important for fisheries managers in Ireland, due to recent proposals for the expansion of glass eel fishing and elver trapping activities (McCarthy et al, 1994b) and concerns that such fisheries may impact on the food of trout. The current study confirms opportunistic feeding behavior by trout; however the overall importance of elvers in supporting trout populations in the lower Shannon is unknown. Significant interannual variations in predation rates on migratory fish have been reported (SOAFD, 1993; Anthony, 1994), and the role of trout as predators of eels on the lower Shannon may also vary. It is clear from this study that juvenile salmon smolts also feed on elvers. However the coincidental immigration of elvers and emigration of smolts may be due to Reponses to similar environmental stimuli rather than the existence of a predator-prey relationship (Hansen et al, 1995).

The relationship between trout abundance and angler’s catch is not fully understood at this time. Likewise the impact of trout predation on eel recruitment in the Shannon is unknown. The current fishing activity may be illegal due to the use of live baits, and fishermen casting lines from busy inner city bridges can be hazard. However, this traditional activity remains popular and has received little interest from the authorities. Nonetheless, the fact that significant quantities of juvenile trout are captured may be of significance for local trout recruitment. It is clear that trout are important predators of juvenile eels in the lower Shannon, and the displacements of trout upstream at Ardnacrusha fish-lift may be a result of ‘slob’ trout attempting to follow their prey upstream.

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