Presentation on theme: "Glass eel and elver ecology Gail Wippelhauser Maine Department of Marine Resources."— Presentation transcript:
Glass eel and elver ecology Gail Wippelhauser Maine Department of Marine Resources
Metamorphosis Metamorphosis from leptocephalus to glass eel stage over the continental shelf/slope –Metamorphosis initiated when vertically migrating leptocephalus makes contact with substrate? (McCleave 1987) –Metamorphosing leptocephali (52-60mm) collected from late October to mid-March and glass eels (46-68mm) from January through July over shelf/slope (Kleckner and McCleave 1985) –European eel leptocephali held in aquarium undergo metamorphosis in one month (Grassi 1896) –Mortality unknown
Approaching the coast Glass eels may be transported toward the coast by tidal currents and directed swimming (McCleave 1987) Glass eels may orient using magnetic fields (Souza et al 1988), electrical fields (McCleave and Power 1978), olfactory cues (Sorensen 1986).
Arrival at the coast Arrival of glass eels along the coast increases with latitude (ASMFC American Eel Technical Committee) Length of arriving glass eels increases with latitude (Vladykov 1966, 1970; Haro and Krueger 1988; ASMFC American Eel Technical Committee).
Arrival at the coast Arrival at the coast (ASMFC 2000 data) StateDatesMean TL (mm)Range TL (mm) Maine3/23-5/23 Massachusetts4/3-5/450-69 Rhode Island4/4-5/2569.9 (440)50-153 Connecticut3/2-6/159.7 (457) New York3/8-4/20 New Jersey3/4-4/1358.3 (826)50-72 Pennsylvania3/7-4/11 Delaware2/24-4/75738-72 Virginia3/15-5/17 South Carolina2/3-3/1753.8 (362)47-62 Florida1/20-3/18
Anguilla anguilla trends Single dominant feature of glass eels from 9 European countries was a severe decline in abundance during 1981-1985 inclusive. A similar decline was observed in the Netherlands from 1946- 1950, but no data were available from other countries for this period (Moriarty 1986). Parallel decline in catches of 0+ elvers in 9 European countries and elvers ascending the St Lawrence supports theory that oceanic conditions are responsible for the decline (Moriarty and Tesch 1996).
Anguilla anguilla trends Negative correlations between the Den Oever glass eel recruitment index (DOI) and the North Atlantic Oscillation since 1938 (Knights 2003). –Correlations between the DOI and sea surface temperature anomalies at 100-250 m between 1952 and 1995 in the Sargasso Sea subtropical gyre spawning area
Migration into estuaries Patterns of migration into estuaries from Sr:Ca ratios of otoliths (Jessop et al. 2002). –entrance into freshwater as an elver –coastal residence or estuarine residence >1year before entering freshwater and after entering freshwater –continuous freshwater residence –freshwater residence for 1 or more years before engaging in seasonal movements between estuary and freshwater
Migration into estuaries Patterns of migration into estuaries based on Sr:Ca ratios of otoliths (Cairns et al. 2004) –54% of eels sampled in estuary migrated directly from the sea to estuarine settlement site –46% of eels sampled in estuary approached freshwater soon after arrival, then settled in the estuary –Eels sampled in freshwater above a dam settled in freshwater in the elver year and showed no subsequent change in habitat salinity –Dams may hamper normal American eel movements between rivers and estuaries, even in watercourses in which some American eels are able to colonize freshwaters.
Estuarine and riverine migration Glass eels utilize selective tidal stream transport (STST); depth selection depends on salinity (McCleave and Kleckner 1982). STST vertical migrations, timed by an endogenous clock, provide successful upstream transport under a variety of flow regimes (Wippelhauser and McCleave 1987; McCleave and Wippelhauser 1987). Glass eels at the surface are more abundant near shore than mid-channel, and at upriver sites than downriver sites (Sheldon and McCleave 1985).
Anguilla anguilla glass eel mortality Anguilla anguilla glass eel mortality Degani and Levanon 1983 DensitySurvivalCannibalismMortality 0.3 kg/m 2 55%19%26% 0.5 kg/m 2 40%32%28% 1.0 kg/m 2 37%42%21%
Anguilla anguilla glass eel mortality Daily instantaneous natural mortality (Berg and Jorgensen 1994) 0.0015 in the wild 0.0107 - 0.0233 elvers cultured, stocked Pond culture mortalities 47-88% (Klein Breteler 1992) 40-60% (Heinsbroek 1991)
Anguilla anguilla glass eel growth Average growth rates of stocked Anguilla anguilla were 40-84 mm/yr (Wickstroem 1987). Mean annual growth rates of tetracycline- marked elvers in the river Rhine was around 55 mm with increment of 100 mm for first year in freshwater (Meunier 1994).
Anguilla rostrata glass eel growth Anguilla rostrata glass eel growth (Jessop 2000) In a coastal river, finite mortality rates were 0.9945 (trap counts) and 0.9968 (mark- recapture). In a coastal river, instantaneous daily mortality rates were 0.0612 (trap counts) and 0.0675 (mark-recapture) High mortality attributed to low pH (4.7-5.0), high initial elver density (4.7 elvers/m 2 ), and predation by resident eels.
Impact of exotic parasites In 1995, exotic swimbladder parasite Anguillicola crassus reported in TX aquaculture facility and in one wild eel from SC (Fries et al. 1996). Parasite distribution limited by cold-water temperatures and high salinity (Suries and Knopf 2004).
Use of otoliths Use of otoliths (Cieri and McCleave 2000) Number of growth increments and radii of otoliths increased linearly and highly significantly with leptocephali body length. Number of growth increments and radii of leptocaphalus growth zones of glass eel otoliths were not related to body length and were lower than predicted by the relationship developed for leptocephali.
Access to habitat Access to estimated 84% of Atlantic coastal tributaries is either lost or restricted (Busch et al. 1998) Causes: –Conversion of timber-crib to concrete –Upstream anadromous passage inappropriate for eels –Eel passage beginning in 1996 –Dam construction/upgrades in 1980s (Jessop and Harvie 2003) Inflatable crests to reduce leakage
Fisheries Fisheries (ASMFC Interstate Fishery Management Plan for American Eel 2000) 1970s glass eel fisheries in FL, NC, SC, VA, MA, and ME –ceased/prohibited in NC, VA, MA in 1977 Late 1980s/early 1990s glass eel fisheries developed or reestablished in CT, RI, NY, NJ, DE, SC Glass eel fishery collapsed in 1998 –continued harvest in ME
Anguilla anguilla fishery Glass eel fishery downstream of the Arzal estuarine dam is very efficient. Compared to total catch, approximately 0.3-3.0% of the stock successfully migrates to freshwater (Briand et al. 2003).