# Presentation: 468177

##                          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:
-       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

## 2005 Workshop Participants (18)

- Experts representing:
-   3 countries
-   9 universities
-   5 federal agencies
-   3 state/province agencies
-   All life stages of American eel

## 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

##       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

##    habitat

- 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

## 2005 Questionnaire Results

## 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

## 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

## 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

## 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

##    habitat

- 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

## Plausible relations among growth, dispersal, mortality, and sex determination

- D

- M

- D

- G

- M

- G

- D

- M

- S

- D

- G

- S

- D

- G

##                                     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

## 2005 Breakout Results

##             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

##                  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

##                 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

##                   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

## Review of 2006 Workshop Literature

##    habitat

- 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

## 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:   ?

## 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

## 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

## 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

## L

- 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

- M

- M

-           sex
- determination

**L**

- M

- D

-    periodic
-     habitat
-      shifts

-     habitat
- availability

- population
-   density

- G

- G

## Knowledge of  Eel Population Dynamics

- 4. Yellow    Silver
-           Threats:
- Dams
- Disease
- Exotic species
- Fishing
- Habitat degradation / loss

## 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

## 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

##              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?