Glass eel and elver ecology

							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)     Range TL (mm)
Maine            3/23-5/23
Massachusetts    4/3-5/4                        50-69
Rhode Island     4/4-5/25      69.9 (440)       50-153
Connecticut      3/2-6/1       59.7 (457)
New York         3/8-4/20
New Jersey       3/4-4/13      58.3 (826)       50-72
Pennsylvania     3/7-4/11
Delaware         2/24-4/7      57               38-72
Virginia         3/15-5/17
South Carolina   2/3-3/17      53.8 (362)       47-62
Florida          1/20-3/18
          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
               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 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
                  (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.
           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).