Biology and Management of European Eel (Anguilla anguilla, L) in the Shannon Estuary, Ireland __________________________________________________________________________________________ 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 __________________________________________________________________________________________ www.EuropeanEel.com| www.ecofact.ie |www.OldRiverShannon.com Page | 222 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 Limerick 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 __________________________________________________________________________________________ www.EuropeanEel.com| www.ecofact.ie |www.OldRiverShannon.com Page | 223 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 river Shannon 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 __________________________________________________________________________________________ www.EuropeanEel.com| www.ecofact.ie |www.OldRiverShannon.com Page | 224 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 River Maigue 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 __________________________________________________________________________________________ www.EuropeanEel.com| www.ecofact.ie |www.OldRiverShannon.com Page | 225 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