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

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 1.Overall abundance is limited by number of yellows and spawners, and perhaps by number of freshwater silvers, leptocephali, and elvers 2.Growth limits yellows 3.Dispersal rates may limit number of yellows and spawners 4.Much uncertainty surrounding mechanisms of eel decline

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

11. Summary of gaps in knowledge of population dynamics 1.Distribution and abundance of leptocepahali and elvers are poorly understood 2.Dispersal (all stages) and sex determination are poorly understood 3.Top-priority gaps to fill include various aspects of dispersal 4.Consensus on top-priority gaps is weak

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

13. GDM D GM G DM S DG S DG Plausible relations among growth, dispersal, mortality, and sex determination

14. Data Availablity Dist/Abun Duration Growth Mortality Dispersal 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. fertilized eggs oceanic glass eels leptocephali freshwater elvers estuarine elvers {freshwater yellows} {estuarine yellows} {freshwater silvers} {estuarine silvers} {oceanic migrants} spawners M G G M D G M G M D G M G M M M M fecundity sex ratio L L L L D M D habitat accessibility spawning habitat availability size geographic origin habitat accessibility D D D D D habitat accessibility

22. Knowledge of Eel Population Dynamics 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 Population Dynamics 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 Population Dynamics 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 Population Dynamics 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. salt yellows silvers staging to migrate brackish yellows freshwater yellows continental elvers freshwater yellows brackish yellows freshwater silvers brackish silvers G M D G M G M D G M G MM sex determination L M D periodic habitat shifts habitat availability population density G G

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

28. Knowledge of Eel Population Dynamics 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 Population Dynamics 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?