DIET OF LOGGERHEAD SEA TURTLES (Caretta caretta) ON THE CONTINENTAL SHELF OF URUGUAY
FURG
Gustavo Martinez
12 Souza ;
Paul G.
3 Kinas ;
Andrés
25 Estrades ;
Fabrizio
45 Scarabino
1 Programa de
Pós-Graduação em Oceanografia Biológica, Universidade Federal do Rio Grande, Rio Grande, RS, Brazil – souza_oceano@yahoo.com.br 2 Karumbé, Av. Giannastasio km 30.5, El Pinar, Canelones CP 15008, Uruguay 3 Instituto de Matemática, Estatística e Física, Universidade Federal do Rio Grande, Rio Grande, RS, Brazil. 4 Dirección Nacional de Recursos Acuáticos, Montevideo, Uruguay 5 Museo Nacional de Historia Natural y Antropología, Montevideo, Uruguay
Introduction
Material & Methods
o Later juvenile and adult specimens of Loggerhead turtles are known to occur in the temperate coast of
o Analysis of gut contents of loggerhead stranded in the coast of Uruguay (see map);
the Southwestern Atlantic Ocean (SWAO);
o Data analysis by:
o The following have not been evaluated: the diet of adults or late juveniles; when they are in the same
o Index of Relative Importance (IRI) conventional (Pinkas et al 1971);
feeding area on the continental shelf; whether they use different food resources, or if there are differences
o Index of Relative Importance (IRI) by bayesian framework (Kinas & Martinez-Souza in press);
in the diet depending on the season;
o General Overlap Index (GO) and the Specific Overlap Index (SO) (Petraitis 1979).
o If there are subgroups, are any differences in diet ignored, or can the importance of food items in the diet be biased by differential subgroup sampling.
Study Area
Objectives o What is the diet of Loggerhead turtles on the continental shelf of Uruguay? o Do differences exist in loggerhead diet given the stage of maturity and season?
Results: Table 1 Prey items found in gut contents of sea turtles Caretta caretta (n =
Table 2 The main prey items found in the gut contents of two subgroups of sea turtles Caretta caretta: %IRI conventional = Relative
52). %O = Percent frequency of occurrence; %N = Percentage by number; %W = percentage by weight (proxy); %IRI bayes (mean) = Posterior mean of %IRI estimate; Cr I 95% = 95% posterior credibility intervals for %IRI
Prey items
%O
%N
%IRI %W conventional
CNIDARIA ANTHOZOA Actiniaria sp. 5.77 1.60 0.20 CNIDARIA HYDROZOA Plumularia setacea 1.92 0.04 0.00 Tubularia crocea 1.92 0.04 0.00 MOLLUSCA POLYPLACOPHORA Chaetopleura 1.92 0.04 0.01 angulata MOLLUSCA GASTROPODA Tonna galea 7.69 0.19 2.08 Buccinanops 34.62 18.76 14.36 cochlidium Zidona dufresnei 3.85 0.27 5.87 Pachycymbiola 19.23 0.69 27.91 brasiliana Natica Isabelleana 1.92 0.19 0.01 Olivanilaria urseus 1.92 1.07 1.84 MOLLUSCA BIVALVA Corbula patagônica 11.54 0.42 0.06 Piter rostratus 3.85 0.11 0.13 Mytilus edulis 1.92 0.04 0.01 Brachidontes 1.92 0.04 0.01 rodriguezi Ostrea puelchana 1.92 0.04 0.03 MOLLUSCA CEPHALOPODA Argonauta nodosa 1.92 0.04 0.42 Octopus tehuelchus 1.92 0.15 0.92 CRUSTACEA MALACOSTRACA Dardanus insignis 36.5 2.90 2.88 Platyxanthus 3.85 0.08 0.00 crenulatus Platyxanthus 1.92 0.04 0.00 patagonicus Libinia spinosa 59.6 8.14 23.4 Hepatus pudibundus 9.62 0.46 0.28 Loxopagurus 28.85 63.20 19.44 loxochelys Ovalipes punctatus 11.54 0.34 0.10 CRUSTACEA CIRRIPEDIA Lepas sp. 3.85 0.11 0.02 Balanus sp. 5.77 0.84 0.01 FISHES 23.08 Trichiurus lepturus 17.3 Scianidea 11.54
%IRI bayes (mean)
Importance Index estimate according to Pinkas et al.(1971); %IRI bayes (mean) = Posterior mean of %IRI estimate; Cr I 95% = 95% posterior credibility intervals for %IRI. Only dietary items for which conventional %IRI estimate exceeds 2%. %IRI conventional
0.603
[0.069 - 0.685]
0 0
0.11 0.09
[0 - 0.037] [0 - 0.04]
0
0.132
[0 - 0.049]
0.54
1.441
[0.034 - 5.114 ]
20.11
16.394
[8.694 - 22.562]
0.38
0.796
[0.019 - 5.08]
4.85
6.417
[0.315 - 37.607]
0.01 0.12
0.108 0.19
[0.002 - 0.078] [0.011 - 0.591]
0.2 0.01 0
0.369 1.86 0.123
[0.042 - 0.337] [0.003 - 14.601] [0.001 - 0.072]
0
0.135
[0 - 0.069]
0
0.129
[0.001 - 0.143]
0.12 0.46
1.109 1.158
[0.001 - 0.918] [0.003 - 0.918]
5.18
5.419
[1.882 - 6.355]
0.01
0.414
[0.002 - 0.153]
0
0.095
[0 - 0.07]
16.36 0.21
19.49 0.421
[11.464 - 31.175] [0.049 - 0.627]
50.68
41.71
[23.457 - 58.203]
0.19
0.461
[0.042 - 0.417]
0.02 0.2
0.407 0.42
[0.002 - 0.057] [0.031 - 0.32]
Juveniles
Adults
Juveniles
Adults
Juveniles
Adults
Loxopagurus loxochelys
24.47
20.49
27.352
18.777
[16.414 – 44.662]
[8.143- 33.16]
Buccinanops cochlidium
22.65
28.30
20.946
20.874
[13.132 – 32.109]
[8.991 – 36.113]
Libinia spinosa
38.45
30.43
20.564
19.743
[8.752 – 34.08]
[8.136 – 37.811]
Pachycymbiola brasiliana
8.07
13.69
17.982
28.708
[0.415 – 36.527]
[0.655 – 54.652]
Dardanus insignis
4.04
4.5
4.971
4.582
[2.339 - 17.57]
[1.355 – 10.127]
Table 3 The main prey items found in the gut contents of two subgroups of sea turtles Caretta caretta: adults and later juveniles. %IRI conventional = Relative Importance Index estimate according to Pinkas et al.(1971); %IRI bayes (mean) = Posterior mean of %IRI estimate; Cr I 95% = 95% posterior credibility intervals for %IRI. Only dietary items for which conventional %IRI estimate exceeds 2%.
%IRI conventional
%IRI bayes (mean)
Cr I 95%
Main Species
Figure 2 Box-plot summaries for marginal posterior distributions of %IRI estimates for the 5 main prey items for loggerheads classified as adults and later juveniles. Boxes denote 50% credibility intervals, bold line within boxes the posterior median
Summer
Autumn
Summer
Autumn
Summer
Autumn
Loxopagurus loxochelys
22.6
70.86
16.504
43.934
[9.205 – 26.859]
[21.521 – 71.208]
Buccinanops cochlidium
32.6
6.44
21.928
4.4
[13.317 – 33.338]
[1.319 – 9.056]
Libinia spinosa
33.3
1.38
39.224
3.679
[24.46 – 54.564]
[0.547 – 11.57]
Pachycymbiola brasiliana
1.31
12.22
10.899
33.076
[0.131 – 27.333]
[0.614 – 62.146]
Dardanus insignis
6.38
2.08
4.63
1.563
[2.156 – 8.687]
[0.385 – 3.986]
Conclusions
Figure 3 Box-plot summaries for marginal posterior distributions of %IRI estimates for the 5 main prey items for loggerheads classified by season. Boxes denote 50% credibility intervals, bold line within boxes the posterior median
oThe studied area constitutes a feeding zone for C. caretta, the southernmost reported for the southwestern Atlantic; o The loggerhead mainly eats there on megaepibenthic and shallow megaendobenthic gastropods and decapods that inhabits muddy or muddy sand bottoms, between circa 10 to 50 meters; o Regardless of state of maturity., is a single pattern in the use of the coastal environment by loggerhead turtles, o According to the Bayesian posterior inference of the %IRI, the main species of prey rank differently in summer and autumn. o We interpret the fish contents as the result of feeding on by-catch that reached the bottom
Table 4 Diet overlap analyses between the main prey items. GO = general overlap index; V = the statistic to test the null hypothesis that GO = 1; df = degrees of freedom; P = p-value of test statistic; SOik = specific overlap of group i onto group k; U = statistic to test the null hypothesis that SOik = 1. The number of prey occurrences in each category are indicated in parentheses. Specific Overlap Index SOik U
i
k
Juvenile (40)
Adult (9)
0.989
Adult (9)
Juvenile (40)
Autumn (10)
Summer (40)
Summer (40)
Autumn (10)
Figure 1 Box-plot summaries of marginal posterior distribituions for %IRI estimates for the dominating dietary items. Boxes denote 50% credibility intervals, bold line within boxes the posterior median
Cr I 95%
Main Species
Cr I 95%
0.33
%IRI bayes (mean)
df
P
1.57312531
4
0.81361403
0.988
0.50260358
4
0.97324703
0.208
75.358
5
7.8331x10-15
0.797 30.030 General Overlap Index GO V
5
1.4552x10-15
df
P
State of maturity
0.99800129
0.38013412
4
0.98407083
Season
0.94189204
10.7756319
5
0.05601456
Acknowledgements