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
(ASMFC 2000 data)
State
Dates
Mean TL (mm)
Maine
3/23-5/23
Massachusetts
4/3-5/4
Rhode Island
4/4-5/25
69.9 (440)
Connecticut
3/2-6/1
59.7 (457)
New York
3/8-4/20
New Jersey
3/4-4/13
Pennsylvania
3/7-4/11
Delaware
2/24-4/7
Virginia
3/15-5/17
South Carolina
2/3-3/17
Florida
1/20-3/18
Range TL (mm)
50-69
50-153
58.3 (826)
50-72
57
38-72
53.8 (362)
47-62
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 19461950, 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
Degani and Levanon 1983
Density
Survival
Cannibalism Mortality
0.3 kg/m2
55%
19%
26%
0.5 kg/m2
40%
32%
28%
1.0 kg/m2
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 tetracyclinemarked 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
(Jessop 2000)
• In a coastal river, finite mortality rates were
0.9945 (trap counts) and 0.9968 (markrecapture).
• 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/m2), 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
(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
(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).