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A Conceptual Model for Population Dynamics of American Eel Paul L. Angermeier

A Conceptual Model for Population Dynamics of American Eel Paul L. Angermeier U.S. Geological Survey – BRD Virginia Cooperative Fish & Wildlife Research Unit. 2005 Workshop on American Eel Population Dynamics Primary Objectives:

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A Conceptual Model for Population Dynamics of American Eel Paul L. Angermeier

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  1. A Conceptual Model for Population Dynamics of American Eel Paul L. Angermeier U.S. Geological Survey – BRD Virginia Cooperative Fish & Wildlife Research Unit

  2. 2005 Workshop on American Eel Population Dynamics Primary Objectives: 1. Refine a conceptual model of American eel population dynamics 2. Identify key data sources, gaps relevant to the model 3. Prioritize studies needed to advance understanding of eel population dynamics Instruments/Activities: 1. Questionnaires 2. Overview presentations 3. Breakout topics 4. Group discussions

  3. 2005 Workshop Participants (18) Experts representing: 3 countries 9 universities 5 federal agencies 3 state/province agencies All life stages of American eel

  4. My Objectives Today: • Review 2005 workshop findings • relevant to American eel threats • 2. Review literature for 2006 USFWS workshop relevant to how threats • affect eel population dynamics • 3. Explore utility of conceptual model as tool for organizing scientific knowledge

  5. Utility of conceptual models as organizational tools - summarize data - identify data gaps - entire life cycle, specific life stage, specific region - make assumptions, hypothetical relations explicit - suggest new hypotheses - set stage for future simulations, field studies

  6. geographic origin fertilized eggs size fecundity M sex ratio spawners spawning habitat availability leptocephali M G D M oceanic glass eels {oceanic migrants} L G M D M M {freshwater silvers} {estuarine silvers} estuarine elvers freshwater elvers L L G G G M G M M D {estuarine yellows} {freshwater yellows} D habitat accessibility M L

  7. 2005 Questionnaire Results

  8. Summary of historical vs current population dynamics • a. Observable declines began 1970 – 1985 • b. Limiting factors have changed substantially for yellows and adult (oceanic) migrants, and perhaps for leptocephali, and freshwater elvers • c. Mortality, including fishing mortality, may have changed substantially for nearly all stages • d. Dispersal may have changed substantially for nearly all stages • e. Much uncertainty in identifying historical vs current differences in population dynamics

  9. Summary of causes of eel decline • Overall abundance is limited by number of yellows and • spawners, and perhaps by number of freshwater silvers, • leptocephali, and elvers • Growth limits yellows • Dispersal rates may limit number of yellows and spawners • Much uncertainty surrounding mechanisms of eel decline

  10. 14. Top-priority assumption or uncertainty (model-wide) • aspects of dispersal – 3 • dispersal of yellows and silvers; effects of D on M and G • causes of mortality • mortality, sex determination and distribution • genetic influences on leptocephalus metamorphosis (is larval • recruitment a random consequence of panmixia?) • contribution of large females to overall recruitment • interactions among D, G, and sex determination; geographic variation in reproductive contribution

  11. Summary of gaps in knowledge of population dynamics • Distribution and abundance of leptocepahali and elvers • are poorly understood • Dispersal (all stages) and sex determination • are poorly understood • Top-priority gaps to fill include various aspects of dispersal • Consensus on top-priority gaps is weak

  12. geographic origin fertilized eggs size fecundity M sex ratio spawners spawning habitat availability leptocephali M D G D M oceanic glass eels {oceanic migrants} L G M D D D M M habitat accessibility {freshwater silvers} {estuarine silvers} estuarine elvers freshwater elvers L L G D D G D G M G M M M {estuarine yellows} {freshwater yellows} D habitat accessibility habitat accessibility L

  13. D G G D M G M D M Plausible relations among growth, dispersal, mortality, and sex determination S G D S G D

  14. Data Availablity Dist/AbunDurationGrowthMortalityDispersal Eggs Lo Lo -- Lo Lo Lepto’s Med Med Hi Lo Hi Elvers Lo Lo Lo Lo Lo Yellows Hi Med Hi Lo Hi Silvers Hi Lo -- Hi Lo Spawners Lo Med -- Lo Lo

  15. 2005 Breakout Results

  16. Factors Regulating Leptocephalus Abundance Importance Uncertainty Geogr. Var. Uncertainty No., location Hi Med N-S, E-W Med of eggs Transport Hi Med N-S, E-W Hi Food avail’y Hi Hi N-S, E-W Hi Predation Hi Hi N-S, E-W Hi Contaminants Med Hi None Lo

  17. Factors Regulating Elver Abundance Importance Uncertainty Geogr. Var. Uncertainty Habitat loss Hi Med-Lo Region-spec. Lo Fishing Hi-Med Med E-W, U-D Lo Parasitism Med Hi N-S, F-E Med Predation Hi Med F-E, U-D Med Contaminants Med Hi Locale-spec. Lo

  18. Factors Regulating Yellow Abundance Importance Uncertainty Geogr. Var. Uncertainty Fishing Hi Med Locale-spec. Med Predation Med Hi N-S, F-E, U-D Hi

  19. Factors Regulating Silver Abundance (All factors operate sex-specifically) Importance Uncertainty Geogr. Var. Uncertainty Barriers Hi Lo N-S, F-E, U-D Lo Fishing Med Lo Locale-spec. Lo Predation Med Hi N-S, F-E, U-D Hi Contaminants Med Hi Locale-spec. Hi

  20. Review of 2006 Workshop Literature

  21. geographic origin fertilized eggs size fecundity M sex ratio spawners spawning habitat availability leptocephali M D G D M oceanic glass eels {oceanic migrants} L G M D D D M M habitat accessibility {freshwater silvers} {estuarine silvers} estuarine elvers freshwater elvers L L G D D G D G M G M M M {estuarine yellows} {freshwater yellows} D habitat accessibility habitat accessibility L

  22. Knowledge of Eel PopulationDynamics 1. Egg  Leptocephalus little known Threats: ? 2. Leptocephalus  Elver - changes in ocean currents, temp., nutrients may affect survival - transported by surface (<300m) currents - changes in Gulf Stream may impair transport, esp. to hi latitudes - may be general population bottleneck for Anguilla - >99% mortality - growth affected by marine “snow” (?) - detraining is size-dependent (?) Threats: ?

  23. Knowledge of Eel PopulationDynamics 3. Elver  Yellow - salinity-regime residency est. w/in 1 yr of arrival - great variation in timing, extent, duration of inland movement - major declines in abundance observed - low pH induces high mortality - instantaneous daily mortality = 6-7% Threats: Dams Fishing Habitat degradation / loss Poor water quality

  24. Knowledge of Eel PopulationDynamics 4. Yellow  Silver (general) - growth: brackish > fresh - age: brackish < fresh; south < north - sex determination is density-dependent - inter-habitat shifts increase with age - trends in catch data mostly negative - most fishing occurs in estuary - reproductive potential esp. sensitive to freshwater fishing

  25. Knowledge of Eel PopulationDynamics 4. Yellow  Silver (freshwater) - growth: females > males - size, age, % female increase w/ distance inland - density inversely related to distance inland - survival is density-dependent - sex ratio: female-biased, esp.at hi latitudes - wide variation in duration of stay in freshwater - short-term movement is limited - pH < 5.5 may be lethal - condition of individuals not affected by Anguillicola infection - turbine mortality is size/sex - selective

  26. silvers staging to migrate G M salt yellows sex determination periodic habitat shifts habitat availability G G G continental elvers brackish yellows brackish silvers brackish yellows D D population density M M M D freshwater yellows freshwater silvers freshwater yellows M G M G M G L

  27. Knowledge of Eel PopulationDynamics 4. Yellow  Silver Threats: Dams Disease Exotic species Fishing Habitat degradation / loss

  28. Knowledge of Eel PopulationDynamics 5. Silver  Available Spawner General - fecundity, size increase with latitude - viral infection lethal in simulated migration - Anguillicola infection reduces swimming speed, endurance, cap’y - catch trends mostly negative Freshwater - movement rate: ~380 km downstream / yr (A. anguilla) - mortality in St. Lawrence system during migration: 53% in lower 500 km; turbines > fishing Threats: Dams Disease Fishing

  29. Knowledge of Eel PopulationDynamics 6. Available Spawner  Egg - probability of mating density-dependent (?) - spawning timing, location affected by variable ocean currents (?) - contaminants concentrate in gonads - lipophilic contaminants impair gametogenesis, fecundity (?) - fecundity is a function of size (assumed) Threats: Contaminants

  30. 2006 American Eel Status Workshop Key objective: Evaluate, rank importance of selected threats in regulating American eel distribution, abundance Can the framework presented above, or another framework, help us organize the vast eel literature in a way that helps us meet this objective?

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