The Reintroduction of Alligator Gar (Atractosteus spatula) at Merwin

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
History In Illinois
• Last vouchered
record from 1966
• Delisted in 1994
(extirpated)
• Reintroduction efforts
began in 2009 by
IDNR
Kaskaskia River
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
Frequency %
Catch Frequency, All Gears, All months
30
28
26
24
22
20
18
16
14
12
10
8
6
4
2
0
N = 6,911
0.25
12/7/2012
10/18/2012
8/29/2012
7/10/2012
5/21/2012
4/1/2012
2/11/2012
1000
(t(17) = -13.48, p < 0.001)
600
4
400
3
2
200
(t(17) = -10.39, p < 0.001)
0
0
Mass (kg)
800
12/23/2011
11/3/2011
9/14/2011
Length (mm)
Length and Mass Gain
7
6
5
1
Length
Mass
10/3/2012 0:00
9/13/2012 0:00
8/24/2012 0:00
8/4/2012 0:00
7/15/2012 0:00
6/25/2012 0:00
6/5/2012 0:00
5/16/2012 0:00
4/26/2012 0:00
Water Temp. °C
40
35
r(4) = 0.84, p < 0.05)
0.15
30
25
0.1
10
Length/Mass SGR
Growth Rates and Water Temperature
0.2
20
0.05
15
(r(4) = 0.80, p = 0.07)
0
Mean Water Temp. (°C)
Length SGR
Mass SGR
Growth Rate: Illinois and Louisiana
Length SGR (% /Day)
0.08
0.07
0.06
0.05
IL
0.04
LA
0.03
0.02
(t(42) = -5.54, p = 0.13
0.01
0
0
200
400
600
800
1000
1200
TL (mm)
Mass SGR (% /Day)
0.3
0.25
0.2
0.15
IL
0.1
0.05
LA
(t(42) = -1.49, p = 0.14
0
0
1
2
3
4
Mass (kg)
5
6
7
Length Gain: Illinois and Louisiana
1100
1000
900
TL (mm)
800
IL
700
LA
600
500
400
300
350
400
450
500
Age in days
550
600
Body Condition: Illinois and Louisiana
4
Log10Mass (g)
3.5
3
IL Merwin Preserve (n = 18)
LA Port Sulphur (n = 54)
2.5
LA Bayou Dularge (n = 48)
Log10 Mass = 3.7654 (Log10 Total Length) - 7.4777
n = 120
R² = 0.98
p < 0.01
2
1.5
2.5
2.6
2.7
2.8
Log10 TL (mm)
2.9
3
3.1
Diets
Diet Content Frequency
100%
90%
80%
70%
60%
Unknown Fish
50%
Gizzard Shad
Empty
40%
30%
20%
10%
0%
Shortnose
Alligator Gar
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).
Prey Size selection
y = 0.0831x + 0.2483
R² = 0.9523
Mean 20.3 ± 1.1
Gizzard Shad
Eye Diameter/Caudal Peduncle (mm)
35
n = 339
y = 0.0324x + 4.1315
R² = 0.7872
Mean 12.0 ± 1.0
30
25
Eye Diameter
20
Caudal Peduncle
15
Linear (Eye Diameter)
10
Linear (Caudal Peduncle)
5
Caudal Peduncle from Alligator Gar
Diet
0
0
50
100
150
200
250
Total length mm
41% of Predators Length
300
350
400
Caudal Peduncle from Shortnose
Gar Diet
Diet Contents Over Time
Alligator Gar Diets Over Time
8
7
Samples Collected
6
5
Gizzard Shad
4
Unknown Fish
3
Empty
2
1
0
May (2)
June (2)
July (1)
August (1)
September (4)
October (7)
Shortnose Gar Diets Over Time
8
Samples Collected
7
6
5
Gizzard Shad
4
Unknown Fish
3
Empty
2
1
0
May (0)
June (1)
July (1)
August (5)
September (7)
October (7)
Food Selection Index
L Score
1
0.8
0.6
0.4
0.2
0
-0.2
-0.4
-0.6
-0.8
-1
May/June
-0.0649
July/August
-0.078
September/October
-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
Ameiurus
+1 = Selection
0 = No selection (opportunistic)
-1 = Avoidance/inaccessibility
Prey Abundance Over Time
20
15
Common Carp
10
Gizzard Shad
5
Lepomis
Largemouth Bass
550
490
440
390
340
290
240
190
140
90
0
40
Catch/Hour
July & August Electrofishing CPUE
Pomoxis
Total Length (mm)
September & October Electrofishing CPUE
15
Common Carp
10
Gizzard Shad
630
550
500
450
400
350
300
250
Largemouth Bass
200
0
150
Lepomis
100
5
40
Catch/Hour
20
Total Length (mm)
June
October
Pomoxis
Recapture Success and Mortality
Alligator Gar: Gear Mortality
7
6
catch
5
4
Lived
3
Mortality
2
1
0
Modified Gill (6)
Experimental Gill (2)
• May – August:
Mortality = 50%
• September – October
Mortality = 38%
Trap (8)
Electrofishing Modified (1)
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”
Some Diet Predictions
Total catch
300
250
Bigmouth Buffalo
200
Common Carp
Largemouth Bass
150
Gizzard Shad
100
Ameirus
50
Pomoxis
0
200 210 220 230 240 250 260 270 280 290 300 310 320
TL (mm)
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).
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
Illinois Natural History Survey
Trent Thomas
Dr. Leon Hinz
Doug Carney
Nerissa Michaels
Matt O’Hara
Levi Solomon
Rich Lewis
John Wisher
Karen Miller
The Nature Conservancy
Todd Rettig
Tharran Hobson
Nate Goetten
Dr. Michael Lemke - Thesis Advisor
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. BandaValdes. 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