Monitoring of Alligator Gar (Atractosteus spatula) Reintroduced into Merwin Preserve

Nathan Grider - University of Illinois, Springfield

Rob Hilsabeck - Illinois Department of Natural Resources

Status

• Populations have declined

• Vulnerable to extinction

• Reintroduction in AL, AR, FL, KY, LA, MS, MO, OK, TN, and TX

M.D. W. F. P

Field & Stream

River Monsters

Jeremy Wade and Mark Spitzer with a 7 ft, 111 lb alligator gar http://colourofautumn1216.blogspot.com/2010/06/river-monsters.html

Kaskaskia River

History In Illinois

• Last vouchered record from 1966

• Delisted in 1994 (extirpated)

• Reintroduction efforts began in 2009 by IDNR

Why Reintroduce Them?

• Increase biodiversity – resist invasion

• Apex predators provide top-down control

• May control “rough fish” and invasive species

• May help prevent stunting of sportfish

• Popular food fish

• Angling and bowfishing

Big Fish Bowfishing Texas™

Merwin Preserve (Spunky Bottoms)

• Approximately 590 ha

• 100 alligator gar were tagged with passive integrated transponders (PIT), released 9/29/2011

• Average length was 538 mm and weight 886 g

Objectives 1) Measure growth rate and compare to data from the

southern range

2) Determine condition (fitness) and compare to data from the southern range

3) Investigate prey selection and potential use as a management tool

4) Compare sampling methods used to capture alligator gar

Methods

Sampling •Sampled May - October = six events

• Sample event = two days and one night of extensive gear effort

Gears • Modified multifilament gill nets – 3” bar mesh, dyed black

• Experimental monofilament gill nets

• Trap nets, 1.5” mesh

• Mini fyke nets

• DC Electrofishing

Diet Analysis

• Gastric lavage

• Strauss (1979) index used

to determine prey selection

• Compares abundance of prey items

in diet to abundance in environment

-1 = avoidance/inaccessibility,

0 = no selection (opportunistic)

+1 = selection

Results

American Fisheries Society

0.25

0

2

4

6

8

10

12

14

16

18

20

22

24

26

28

30

Freq

uen

cy %

Catch Frequency, All Gears, All months

N = 6,911

Length and Mass Gain

0

1

2

3

4

5

6

7

0

200

400

600

800

1000

9/1

4/2

01

1

11

/3/2

01

1

12

/23

/20

11

2/1

1/2

01

2

4/1

/20

12

5/2

1/2

01

2

7/1

0/2

01

2

8/2

9/2

01

2

10

/18

/20

12

12

/7/2

01

2

Mas

s (k

g)

Len

gth

(m

m)

Length

Mass

(t(17) = -13.48, p < 0.001)

(t(17) = -10.39, p < 0.001)

Growth Rates and Water Temperature

0

0.05

0.1

0.15

0.2

10

15

20

25

30

35

40

4/2

6/2

01

2 0

:00

5/1

6/2

01

2 0

:00

6/5

/20

12

0:0

0

6/2

5/2

01

2 0

:00

7/1

5/2

01

2 0

:00

8/4

/20

12

0:0

0

8/2

4/2

01

2 0

:00

9/1

3/2

01

2 0

:00

10

/3/2

01

2 0

:00

Len

gth

/Mas

s S

GR

W

ater

Tem

p. °

C

Mean Water Temp. (°C)

Length SGR

Mass SGR

r(4) = 0.84, p < 0.05)

(r(4) = 0.80, p = 0.07)

Growth Rate: Illinois and Louisiana

0

0.01

0.02

0.03

0.04

0.05

0.06

0.07

0.08

0 200 400 600 800 1000 1200

Len

gth

SG

R (

% /

Day

)

TL (mm)

IL

LA

0

0.05

0.1

0.15

0.2

0.25

0.3

0 1 2 3 4 5 6 7

Mas

s SG

R (

% /

Day

)

Mass (kg)

IL

LA

(t(42) = -5.54, p = 0.13

(t(42) = -1.49, p = 0.14

Length Gain: Illinois and Louisiana

300

400

500

600

700

800

900

1000

1100

350 400 450 500 550 600

TL (

mm

)

Age in days

IL

LA

Body Condition: Illinois and Louisiana

1.5

2

2.5

3

3.5

4

2.5 2.6 2.7 2.8 2.9 3 3.1

Log 1

0M

ass

(g)

Log10 TL (mm)

IL Merwin Preserve (n = 18)

LA Port Sulphur (n = 54)

LA Bayou Dularge (n = 48)

Log10 Mass = 3.7654 (Log10 Total Length) - 7.4777 n = 120

R² = 0.98 p < 0.01

0%

10%

20%

30%

40%

50%

60%

70%

80%

90%

100%

Shortnose Alligator Gar

Diet Content Frequency

Unknown Fish

Gizzard Shad

Empty

Diets

Estimating Prey Length from Remains

• Knowing prey size allows us to estimate predator impact

• How do we estimate prey length from diet remains?

• Use linear relationship of eye diameter or caudal peduncle to total length (Scharf et al. 1997).

y = 0.0324x + 4.1315 R² = 0.7872

Mean 12.0 ± 1.0

y = 0.0831x + 0.2483 R² = 0.9523

Mean 20.3 ± 1.1

0

5

10

15

20

25

30

35

0 50 100 150 200 250 300 350 400

Eye

Dia

met

er/C

aud

al P

edu

ncl

e (m

m)

Total length mm

Gizzard Shad

Eye Diameter

Caudal Peduncle

Linear (Eye Diameter)

Linear (Caudal Peduncle)

Caudal Peduncle from Alligator Gar Diet

Caudal Peduncle from Shortnose Gar Diet

n = 339

Prey Size selection

41% of Predators Length

Diet Contents Over Time

0

1

2

3

4

5

6

7

8

May (2) June (2) July (1) August (1) September (4) October (7)

Sam

ple

s C

olle

cte

d

Alligator Gar Diets Over Time

Gizzard Shad

Unknown Fish

Empty

0

1

2

3

4

5

6

7

8

May (0) June (1) July (1) August (5) September (7) October (7)

Sam

ple

s C

olle

cte

d

Shortnose Gar Diets Over Time

Gizzard Shad

Unknown Fish

Empty

+1 = Selection

0 = No selection (opportunistic)

-1 = Avoidance/inaccessibility

May/June July/August September/October

Ameiurus -0.0649 -0.078 -0.1688

Bigmouth Buffalo -0.0119 -0.0395 -0.063

Common Carp -0.2727 -0.3192 -0.529

Gizzard Shad 0.68515 0.74139 -0.0504

Largemouth Bass -0.1862 -0.0968 -0.0554

Lepomis -0.341 -0.1774 -0.063

Pomoxis -0.0586 -0.0305 -0.0705

-1

-0.8

-0.6

-0.4

-0.2

0

0.2

0.4

0.6

0.8

1

L Sc

ore

Food Selection Index

Prey Abundance Over Time

June October

0

5

10

15

20

40

90

14

0

19

0

24

0

29

0

34

0

39

0

44

0

49

0

55

0

Cat

ch/H

ou

r

Total Length (mm)

July & August Electrofishing CPUE

Common Carp

Gizzard Shad

Lepomis

Largemouth Bass

Pomoxis

0

5

10

15

20

40

10

0

15

0

20

0

25

0

30

0

35

0

40

0

45

0

50

0

55

0

63

0

Cat

ch/H

ou

r

Total Length (mm)

September & October Electrofishing CPUE

Common Carp

Gizzard Shad

Lepomis

Largemouth Bass

Pomoxis

Recapture Success and Mortality

• May – August: Mortality = 50% • September – October Mortality = 38%

0

1

2

3

4

5

6

7

Modified Gill (6) Experimental Gill (2) Trap (8) Electrofishing Modified (1)

catc

h

Alligator Gar: Gear Mortality

Lived

Mortality

Discussion Objective 1 & 2

• No significant difference in growth rate or condition compared to Louisiana

• Factors that effect growth rate: salinity, temp., prey, and habitat

Objective 3

• No sportfish found in diet.

• Did they eat a few? Probably.

• Selection or opportunistic feeding on gizzard shad?

“Optimal Foraging Theory”

Abundance of potential prey items at Merwin Preserve within the preferred prey size range (200 – 320 mm) of 137 – 183 cm alligator gar

described by Goodyear (1967).

Some Diet Predictions

0

50

100

150

200

250

300

200 210 220 230 240 250 260 270 280 290 300 310 320

Tota

l cat

ch

TL (mm)

Bigmouth Buffalo

Common Carp

Largemouth Bass

Gizzard Shad

Ameirus

Pomoxis

Discussion

Objective 4 • Trap nets and modified gill nets worked best

• Modified gill nets produced less bycatch, but higher mortality

• Sampling in September & October is recommended DC Electrofishing

• 3,500 watt generator (small boat) = no Alligator Gar

• 5,000 watt generator (big boat) = 8.5 hours produced 1 Alligator Gar @ 30 cycles/sec & 7 amps

What’s Next?

• Continue reintroduction and monitoring

• Consider further harvest restrictions

• Public education and outreach

• Maybe develop catch and releasing fishing opportunities in dedicated waters

Could help fund continued conservation work!

Bass Pro Shop - Springfield, MO

Acknowledgments

Illinois Department of Natural Resources

Rob Hilsabeck

Trent Thomas

Doug Carney

Matt O’Hara

Rich Lewis

Karen Miller

Todd Rettig

Nate Goetten

Dr. Michael Lemke - Thesis Advisor

Illinois Natural History Survey Dr. Leon Hinz

Nerissa Michaels Levi Solomon John Wisher

The Nature Conservancy Tharran Hobson

Literature Cited Brinkman, E. L. 2008. Contributions to the life history of alligator gar (Atractosteus spatula,

lacepede), in Oklahoma. Master’s Thesis, Oklahoma State University. Oklahoma City, Oklahoma

Garcia De Leon, F. J., L. Gonzalez-Garcia, L. M. Herrera-Castillo, K. O. Winemiller, & A. Banda-Valdes. 2001. Ecology of the alligator gar, Atractosteus spatula, in the Vicente Guerrero Reservoir, Tamaulipas, Mexico . The Southwestern Naturalist 46(2):151-157

Ianni, R. C. 2011. Monitoring diets and growth rates of native predatory fish stocked to suppress non-native tilapia. Master’s Thesis, Nicholls State University. Thibodaux, Louisiana

Scharf, F. S., J. A. Buckel, F. Juanes, & D. O. Conover. 1997. Estimating piscine prey size from partial remains: Testing for shifts in foraging mode by juvenile bluefish. Environmental Biology of Fishes 49: 377-388

Strauss, R. E. 1979. Reliability estimates for Ivlev’s Electivity Index, the forage ratio, and a proposed linear index of food selection. Transactions of the American Fisheries Society 108: 344-352