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 Catch Frequency, All Gears, All months
30 N = 6,911 28 26 24 22 20 18
16
14 12 10
Frequency % Frequency 8 6 4 0.25 2 0 Length and Mass Gain
1000 7
(t(17) = -13.48, p < 0.001) 6 800
5
600 4
400 3 Mass (kg) Mass
Length (mm) Length 2 200 1 (t(17) = -10.39, p < 0.001)
0 0
Length
Mass
4/1/2012
9/14/2011 11/3/2011 2/11/2012 5/21/2012 7/10/2012 8/29/2012 12/7/2012
12/23/2011 10/18/2012 Growth Rates and Water Temperature
40 0.2
r(4) = 0.84, p < 0.05)
35 SGR
0.15
30
C °
25 0.1
Length/Mass Length/Mass 20
0.05 Water Temp. Temp. Water 15 (r(4) = 0.80, p = 0.07)
10 0
Mean Water Temp. (°C)
6/5/20120:00 8/4/20120:00
4/26/2012 0:00 4/26/2012 0:00 5/16/2012 0:00 6/25/2012 0:00 7/15/2012 0:00 8/24/2012 0:00 9/13/2012 0:00 10/3/2012 Length SGR
Mass SGR Growth Rate: Illinois and Louisiana
0.08
0.07 0.06
0.05 (% /Day) (%
IL 0.04 0.03 LA 0.02 (t(42) = -5.54, p = 0.13 Length SGR SGR Length 0.01 0 0 200 400 600 800 1000 1200 TL (mm)
0.3
0.25
/Day) 0.2
0.15 IL 0.1
Mass SGR (% (% SGR Mass LA 0.05 (t(42) = -1.49, p = 0.14 0 0 1 2 3 4 5 6 7 Mass (kg) Length Gain: Illinois and Louisiana
1100
1000
900
800 IL 700
TL (mm) TL LA 600
500
400
300 350 400 450 500 550 600 Age in days Body Condition: Illinois and Louisiana
4
3.5
3 IL Merwin Preserve (n = 18)
LA Port Sulphur (n = 54) Mass (g) Mass
10 2.5 LA Bayou Dularge (n = 48) Log
Log Mass = 3.7654 (Log Total Length) - 7.4777 2 10 10 n = 120 R² = 0.98 p < 0.01
1.5 2.5 2.6 2.7 2.8 2.9 3 3.1
Log10 TL (mm)
Diets
Diet Content Frequency 100%
90%
80%
70%
60% Unknown Fish 50% Gizzard Shad 40% Empty
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 Gizzard Shad R² = 0.9523 35 Mean 20.3 ± 1.1 n = 339 y = 0.0324x + 4.1315 30 R² = 0.7872 Mean 12.0 ± 1.0 25
20 Eye Diameter Caudal Peduncle 15 Linear (Eye Diameter)
10 Linear (Caudal Peduncle)
5 Caudal Peduncle from Alligator Gar
Diet Eye Diameter/Caudal Peduncle (mm) Peduncle Diameter/Caudal Eye 0 Caudal Peduncle from Shortnose 0 50 100 150 200 250 300 350 400 Gar Diet
Total length mm
41% of Predators Length Diet Contents Over Time
Alligator Gar Diets Over Time 8
7
6
5 Gizzard Shad 4 Unknown Fish
3 Empty Samples Collected Samples 2
1
0 May (2) June (2) July (1) August (1) September (4) October (7)
Shortnose Gar Diets Over Time 8 7
6 5 Gizzard Shad 4 Unknown Fish 3 Empty
Samples Collected Samples 2 1 0 May (0) June (1) July (1) August (5) September (7) October (7) Food Selection Index 1 0.8 0.6 0.4
0.2
0 Score
-0.2 L -0.4 -0.6 -0.8 -1 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 = Selection 0 = No selection (opportunistic) -1 = Avoidance/inaccessibility
Prey Abundance Over Time
July & August Electrofishing CPUE
20 15 Common Carp 10 Gizzard Shad 5 Lepomis
Catch/Hour 0
Largemouth Bass
40 90
190 240 290 340 390 440 490 550 140 Pomoxis Total Length (mm)
September & October Electrofishing CPUE
20
15 Common Carp 10 Gizzard Shad
5 Lepomis Catch/Hour
0 Largemouth Bass
40 Pomoxis
100 150 200 250 300 350 400 450 500 550 630 Total Length (mm) June October Recapture Success and Mortality
Alligator Gar: Gear Mortality 7
6
5
4 Lived
catch 3 Mortality
2
1
0 Modified Gill (6) Experimental Gill (2) Trap (8) Electrofishing Modified (1) • May – August: Mortality = 50%
• September – October Mortality = 38% 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
300
250 Bigmouth Buffalo
200 Common Carp Largemouth Bass 150
Gizzard Shad Total catch Total 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 Trent Thomas Illinois Natural History Survey 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. 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