Glass eel and elver ecology

1
Glass eel and elver ecology

• Gail Wippelhauser
• Maine Department of Marine Resources

2
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

3
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).

4
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).

5
Arrival at the coast (ASMFC 2000 data)
6
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).

7
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

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Migration into estuaries

• Patterns of migration into estuaries from SrCa
  ratios of otoliths (Jessop et al. 2002).
• entrance into freshwater as an elver
• coastal residence or estuarine residence gt1year
  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

9
Migration into estuaries

• Patterns of migration into estuaries based on
  SrCa 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.

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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).

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Anguilla anguilla glass eel mortalityDegani and
Levanon 1983
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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)

13
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).

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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/m2), and
  predation by resident eels.

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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).

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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.

17
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

18
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

19
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).