DECREASING ANNUAL NEST COUNTS IN A

GLOBALLY IMPORTANT LOGGERHEAD SEA

TURTLE POPULATION

Blair Witherington

Paul Kubilis

Beth Brost

Anne Meylan

2009

Ecological Applications

19:30–54.

Life Stage Durations for Caretta

Benthic Juvenile

13-20 yr

Pelagic Juvenile

7-11 yr

Egg to Post-hatchling

0.5 yr

Adult

>25 yr

Age at Maturity

~30 yr

Morning-after Nest Counts Based on Crawl Evidence

Estimated annual number of loggerhead nests in the southeastern U.S.,

The Bahamas, and Mexico, 2001-2005.

Northern

Recovery Unit

5,215 nests/yr

1989-2008

Peninsular Florida

Recovery Unit

64,513 nests/yr

1989-2007

Northern Gulf

Recovery Unit

906 nests/yr

1995-2007

Dry Tortugas

Recovery Unit

246 nests/yr

1995-2001, 2003, 2004

US Atlantic Loggerhead Recovery Units (Metapopulations)

Sea turtle nesting beaches in

Florida where seasonal nest

counts have been made. Gray

shoreline-shading represents all

Statewide beaches surveyed

during the period 2001–2006.

Dark shading represents Index

beaches used in this study; these

were surveyed daily from 1989 to

2006 for the Index sampling

season (15 May – 31 August).

The average percentage of total

recorded Statewide nesting by

loggerheads from 2001 to 2006 is

presented for each region.

FWC Partners: Florida State Parks, National Parks,

National Seashores, National Monuments, National

Wildlife Refuges, National Marine Fisheries Service, U.S.

Air Force bases, U.S. Naval stations, DEP, counties,

municipalities, conservation organizations, oceanaria,

marine laboratories,consulting companies, university

professors & students, private individuals, and others.

Who Does the Work?

Statewide and Index Nesting Beach Survey Programs

Loggerhead Index Nesting Data

Time Series: 1989—2008

Data Resolution: 109-day season x 368 zones (mean =

69% of Florida loggerhead nesting, SD = 5%)

Probability that a nest counted by an Index surveyor was

actually a nest: 0.96 (95% CI: 0.92–0.99)

Missing data: 0.7% of zone-level biweekly observation

intervals were not surveyed

19

8919

9019

9119

9219

9319

9419

9519

9619

9719

9819

9920

0020

0120

0220

0320

0420

0520

0620

0720

08

25000

35000

45000

55000

65000

Surv ey y ear

To

tal n

um

ber

of

ne

sts

Annual total nest counts for loggerhead turtles on Florida Index Beaches, 1989-

2008. The trend line was estimated by fitting a 5-knot restricted cubic spline curve

to the total counts via negative binomial regression.

19

8919

9019

9119

9219

9319

9419

9519

9619

9719

9819

9920

0020

0120

0220

0320

0420

0520

0620

0720

08

25000

35000

45000

55000

65000

Surv ey y ear

To

tal n

um

ber

of

ne

sts

Annual total nest counts for loggerhead turtles on Florida Index Beaches, 1989-

2008. The trend line was estimated by fitting a 5-knot restricted cubic spline curve

to the total counts via negative binomial regression. Change % (95% CI) shown

relative to 1998 point in time series. All p<0.025 (% change=0%).

+26% (2 to 55%)

-41% (-52 to -28%)

-26% (-42 to -5%)

19

8919

9019

9119

9219

9319

9419

9519

9619

9719

9819

9920

0020

0120

0220

0320

0420

0520

0620

0720

08

25000

35000

45000

55000

65000

Surv ey y ear

To

tal n

um

ber

of

ne

sts

Annual total nest counts for loggerhead turtles on Florida Index Beaches, 1989-

2008. The trend line was estimated by fitting a 5-knot restricted cubic spline curve

to the total counts via negative binomial regression. Nest-count estimate for 2009

is from linear regression of May-June nest counts against total season nest

counts 1989-2008.

2009(estimate from

May-June data,

95% CI from linear,

R2=0.91)

Do Index nest counts represent total nests?

Have there been spatial or temporal shifts relative

to the Index zones and season?

0 50 100 150 200 250 300 350 400 450 500 550 900 950

05

00

10

00

15

00

10

00

Num

ber

of nests

Shoreline position along the Florida coast (km)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 1718

19 20 22 21 23 24 28 27 26IndexBeach

2008

NE CNE CE CSE SE SW

Shoreline distribution of annual nest counts for loggerhead turtles from 368

Florida Index Beach zones surveyed during the 2008 nesting season. Shoreline

extents of Index Beaches and sub-regions are indicated above and below the

upper edge of the plot. The horizontal axis represents approximate shoreline

distance from the Florida/Georgia state border (30.1 N, 81.4 W). Abbreviations for

Florida sub-regions are NE (Northeast), CNE (Central Northeast), CE (Central

East), CSE (Central Southeast), SE (Southeast), and SW (Southwest).

0 50 100 150 200 250 300 350 400 450 500 550 900 950

0500

1500

1000

Num

ber

of

nests

Shoreline position along the Florida coast (km)

1989

NE CNE CE CSE SE SW

0 50 100 150 200 250 300 350 400 450 500 550 900 950

0500

1500

1000

1990

0 50 100 150 200 250 300 350 400 450 500 550 900 950

0500

1500

1000

1991

0 50 100 150 200 250 300 350 400 450 500 550 900 950

0500

1500

1000

1992

0 50 100 150 200 250 300 350 400 450 500 550 900 950

0500

1500

1000

1993

NE CNE CE CSE SE SW

0 50 100 150 200 250 300 350 400 450 500 550 900 950

0500

1500

1000

1994

0 50 100 150 200 250 300 350 400 450 500 550 900 950

0500

1500

1000

1995

0 50 100 150 200 250 300 350 400 450 500 550 900 950

0500

1500

1000

1996

0 50 100 150 200 250 300 350 400 450 500 550 900 950

0500

1500

1000

1997

NE CNE CE CSE SE SW

0 50 100 150 200 250 300 350 400 450 500 550 900 950

0500

1500

1000

Num

ber

of

nests

Shoreline position along the Florida coast (km)

1998

NE CNE CE CSE SE SW

0 50 100 150 200 250 300 350 400 450 500 550 900 950

0500

1500

1000

1999

0 50 100 150 200 250 300 350 400 450 500 550 900 950

0500

1500

1000

2000

0 50 100 150 200 250 300 350 400 450 500 550 900 950

0500

1500

1000

2001

0 50 100 150 200 250 300 350 400 450 500 550 900 950

0500

1500

1000

2002

NE CNE CE CSE SE SW

0 50 100 150 200 250 300 350 400 450 500 550 900 950

0500

1500

1000

2003

0 50 100 150 200 250 300 350 400 450 500 550 900 950

0500

1500

1000

2004

0 50 100 150 200 250 300 350 400 450 500 550 900 950

0500

1500

1000

2005

0 50 100 150 200 250 300 350 400 450 500 550 900 950

0500

1500

1000

2006

NE CNE CE CSE SE SW

0 50 100 150 200 250 300 350 400 450 500 550 900 950

0500

1500

1000

Num

ber

of

nests

Shoreline position along the Florida coast (km)

2007

NE CNE CE CSE SE SW

0 50 100 150 200 250 300 350 400 450 500 550 900 950

0500

1500

1000

2008

Our zone-level model showed strong, positive, within-zone autocorrelation (R > 0.93) between annual counts, indicating remarkable

year-to-year consistency in the longshore spatial distribution of nests over the survey region.

0

10

20

30

40

50

60

70

80

1988

1989

1990

1991

1992

1993

1994

1995

1996

1997

1998

1999

2000

2001

2002

2003

2004

2005

2006

2007

2008

Year

Season Day (of 109) on which Florida Index

Loggerhead Nesting Reached 25%, 50%, and 75% of

Seasonal Totals

75%

R=0.22

P=0.35

50%

R=0.30

P=0.20

25%

R=0.15

P=0.40

July 18

June 10

July 1

Season

Day

Do Index nest counts represent total nests? Yes

Have there been spatial or temporal shifts relative

to the Index zones and season?

None that would explain nest count changes

Do nest counts represent adult females?

2—3 year re-migration interval

Sea Turtle

Reproductive

Cycles

No temporal trends known in

loggerhead clutch frequency

(varied approximately 20%

around a median rate over 10

years; Hughes 1974, Frazer

and Richardson 1985)

No temporal trends known in

loggerhead remigration

intervals (vary approximately

7% around a central value;

Richardson et al. 1978,

Bjorndal et al. 1983, Hughes

1982, Limpus 1985)

H0: A decline in loggerhead adult females is

similar to the decline measured in nesting

H1: A decline in loggerhead adult females is less

than the decline measured in nesting

H2: A decline in loggerhead adult females is

greater than the decline measured in nesting

Do nest counts represent adult females? We are

not able to demonstrate otherwise

What can the nesting data tell us about potential

causes of the decline?

What is the relative importance of terrestrial and

marine threats?

% T

ota

l N

ests

19

89

-20

08

Co

rre

cte

d fo

r B

ea

ch

Le

ng

th

Loggerhead Nests Green Turtle Nests

0

10

10

20

20

30

30

Ft.

Clin

ch S

P

Am

elia

Isla

nd

Litt

le T

albo

t Isl

and

SP

Atla

ntic

-Jax

Bea

ch

Gua

na R

iver

SP

Ft.

Mat

anza

s N

M

Can

aver

al N

S

Mer

ritt I

slan

d N

WR

Can

aver

al A

F S

tatio

n

Pat

rick

AF

Bas

e

Sou

th B

reva

rd C

ount

y

Seb

astia

n In

let S

RA

Wab

asso

Bea

ch

Ft.

Pie

rce

Inle

t SR

A

Hut

chin

son

Isla

nd

St.

Luc

ie In

let S

P

Hob

e S

ound

NW

R

Jupi

ter I

slan

d

Juno

Bea

ch

Boc

a R

aton

J.D

. Mac

Art

hur

SP

John

U. L

loyd

SR

A

Mia

mi B

each

es

San

ibel

Isla

nd

Wig

gins

Pas

s S

RA

Kee

way

din

Isla

nd

Relative Loggerhead and Green Turtle Nesting Density by Beach

Loggerhead Nests

Green Turtle Nests

Loggerhead and Green Turtle Nests

on Florida Core Index Beaches, 1989-2008

-26%

1989-2008

+537%

1989-2008

Life Stage Durations for Caretta

Benthic Juvenile

13-20 yr

Pelagic Juvenile

7-11 yr

Egg to Post-hatchling

0.5 yr

Adult

>25 yr

Age at Maturity

~30 yr

Table 2. Concordance of loggerhead threat factors with attributes of the

loggerhead nesting decline.

Factor Consistent

with

timeframe of

nesting

decline?

Consistent

with

magnitude

of nesting

decline?

Consistent

with

temporal

pattern of

nesting

decline?

Similar

effects on

nesting

between

subregions?

Different

expected

effects on

green turtle

and

leatherback

nesting?

Score*

Decreased hatchling production

(from predation, hurricane effects,

habitat deterioration)

No No No No No 0

Direct take No No No Yes No 1

Fisheries bycatch Yes Yes Yes Yes Yes** 5

Disease (including harmful algal

blooms)

Possibly Possibly Possibly Yes No 2.5

Boat-related mortality Yes No No No No 1

Pollution (oil, plastics, discarded

fishing nets)

Yes Possibly Possibly Yes No 3

Global warming Possibly No Unknown Yes No 1.5

Decline of food resources Yes Possibly Unknown Yes Yes 3.5

*Score is the sum of categories in which the effect of the factor is concordant with the patterns observed in

the nesting data. Possible concordance was scored as 0.5.

**Longlines principally affect loggerheads and leatherbacks. Trawling

affects principally loggerheads.

Where are loggerheads

in the eastern Gulf of Mexico?

Questions ?

Northern

Recovery Unit

Peninsular Florida

Recovery Unit

Northern Gulf

Recovery Unit

Dry Tortugas

Recovery Unit

Mean Total Nests

Nesting Trend

Annual Change

5,215 nests

1989-2008 1.3% 1983-2008

64,513 nests

1989-20071.6% 1989-2008

906 nests

1995-20074.7% 1995-2007

246 nests

1995-2001, 2003, 2004No detectable

Trend

0

500

1,000

1,500

2,000

2,500

3,000

Loggerh

ead N

ests

Year

Summed annual loggerhead nest counts from 11 Northern Recovery

Unit beaches, 1983-2008.

Northern

Recovery Unit

20,000

25,000

30,000

35,000

40,000

45,000

50,000

55,000

60,000

65,000

Loggerh

ead N

ests

Year

Summed annual loggerhead nest counts from 26 Peninsular Florida

Recovery Unit beaches, 1989-2008.

Peninsular Florida

Recovery Unit

0

50

100

150

200

250

Loggerh

ead N

ests

Year

Summed annual loggerhead nest counts from three Northern Gulf of

Mexico Recovery Unit beaches, 1997-2008.

Northern Gulf

Recovery Unit

0

50

100

150

200

250

300

350

400

1994 1995 1996 1997 1998 1999 2000 2001 2002 2003 2004 2005

Loggerh

ead N

ests

Year

Summed annual loggerhead nest counts from three Dry Tortugas

Recovery Unit beaches, 1995-2004, excluding 2002.

Dry Tortugas

Recovery Unit

Year

Total #of

Nests

Statewide

Total #of

Nests from

Sample

Beaches

(# actually

inventoried)

Weighted

Mean

Emergence

Success%

# of

Hatchlings

Produced

2001 69,681 31,949 (2,276) 44 3,436,611

2002 62,905 27,964 (1,574) 53 3,733,262

2003 63,446 27,085 (1,879) 61 4,326,154

2004 47,173 19,912 (1,482) 40 2,105,260

2005 52,469 23,167 (1,709) 51 2,988,159

2006 49,786 20,841 (1,626) 67 3,730,146

2007 45,051 18,158 (1,867) 62 3,116,054

2008 61,440 25,288 (2,598) 49 3,381,124

Loggerhead hatchling production

from Florida beaches, 2001-2008