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MARINE MAMMAL SCIENCE, 18(3):734-745 (July 2002) 0 2002 by the Society for Marine Mammalogy
DIET OF SOUTH AMERICAN FUR SEALS (A RCTOCEPHA LUS A USTRA LIS ) IN
ISLA DE LOBOS, URUGUAY DANIEL E. NAYA
Seccicin Zoologia Vertebrados, Facultad de Ciencias, Universidad de la Republica, Uruguay
E-mail: [email protected]
MAT~AS AKIM Seccicin Ecologia, Facultad de Ciencias, Universidad de la Rephblica, Uruguay
RODRIGO VARGAS Seccicin Zoologia Vertebrados, Facultad de Ciencias,
Universidad de la Rephblica, Uruguay
ABSTRACT
The diet of Arctocephalus uustrulis was studied through fecal analysis, from September of 1995 to May of 1998, in Isla de Lobos (3S001'50"S- 54"53'00"W), Uruguay. A total of 539 scats analyzed indicated that the weak- fish (Cynoscion guatucupu), the cutlasfish (Trichiuruj lepturus), the anchoveta (Engradis unchoitu), the anchovy (Anchoa marznii), and cephalopods were the main prey consumed by these fur seals. Prey richness was higher during January. The diet of A. uustrulis varied between years, apparently related to changes in prey availability. C. gzutucupu and E. unchaitu decreased while 7: lef,turu~ and cephalopods increased from 1996 to 1998. South American fur seals fed on fish ranging in length from 5.4 to 104.8 cm and from 0.7 to 629.9 g in wet mass. However, the mean mass of the main items was never more than 200 g . Observations suggests that A. australis is a trophic gen- eralist with the potential to prey on a wide range of species, although most of the diet is comprised of relatively few species. Fur seals and artisan fisheries both took C. guuti~cupu, with fur seals taking younger individuals (1-2 yr) than artisan fisheries ( > 3 yr).
Key words: Arctocephulus austrulis, South American fur seal, dietary analysis, feeding ecology, mammalifisheries interactions, pinniped diet.
The South American fur seal, Arctocepbufzls azlstrufis, is present along both coasts of the South American continent, being one of the most widely dis- tributed otariid seals in the Sourhern Hemisphere (Bonner 1981). In Uruguay the main breeding colonies are on Isla de Lobos, and on Torres and Castillo
7 34
NAYA E T A L . : SOUTH AMERICAN FUR SEAL DIET 735
59O 55O 51°
lsla de ARGENTINA Lobos
3 3 O
35O
370
Figure 1. Map of study area showing location of Uruguay and Isla de Lobos.
Grande Islands, just north of the Rio de la Plata Estuary (King 1983). With about 300,000 individuals and an annual growth rate of 1.4% (Lima and Piez 1997), the Uruguayan population is the largest of the species.
Pinnipeds are thought to be important top predators in marine ecosystems (Riedman 1990) and, thus, an understanding of their diets is critical to this assessment. However, to date no large studies about the A. australis diet for Uruguayan waters have been published.
Foraging behavior and diet composition of many seal species is related to the abundance and distribution of prey (Lowry et al. 1990, Bowen and Har- rison 1994, Boyd et al. 1994, Lawson et al. 1995, Merrick and Loughlin 1997). For otariid seals in temperate and tropical climates, interannual diet Auctua- tion appears to be more important than seasonal variation (Ochoa and Francis 1995, and reference therein). Previous work on the diet of A. australis in the Peruvian coast suggested that this species responded to annual changes in prey availability by broadening its diet when the main prey, the anchoveta (En- graulis ringens) was scarce (Majluf 1989). The differences between the ecosys- tems of Peru (a coastal upwelling system) and Uruguay (a shallow shelf zone with an important mixing of salt and fresh waters) provide an opportunity to contrast the diets of one predator in two contrasting habitats. The objective of this work was to determine the diet of A. australis along the Uruguayan coast over two years.
MET H o D s
Samples were collected in different areas of Isla de Lobos (35"01'50"S- 54'5 3'00"W), Departamento de Maldonado, Uruguay (Fig. 1). A. australis cohabits this island with the southern sea lion, Otarzajavescens. Thus, all the
716 MARINE MAMMAL SCIENCE, VOL. 18, N O . 3, 2002
samples were taken from sites where species mixing was less than 5% (follow- ing Thompson et al. 1996) in a minimum of 100 individuals counted. The scats were collected in six periods: September 1995 ( n = loo), December 1995 (n = 51), January 1996 (n = 76), March 1996 ( n = 51), January 1998 ( n = 137), and May 1998 ( n = 122). Interannual variation was evaluated from January samples only.
Scats were immersed in water mixed with detergent for 12 h and strained through a sieve with 0.5-mm mesh. The otoliths and cephalopod beaks were separated, briefly washed in 98% isopropyl alcohol, air-dried, and stored. Fish species were identified by comparing recovered sagittal otoliths with reference material collected from local fishes or with published otolith identification guides (Torno 1976, Baldas et a/ . 1997, Volpedo and Echeverria 2000). Only beaks of cephalopods were counted.
Diet diversity was estimated with a rarefaction approach (Heck et al. 1975, Magurran 1988, Krebs 1989). This involves the use of the hypergeometric distribution to calculate the expected richness for a series of samples from the species abundance distribution in the total samples (Heck et al. 1975, Krebs 1989). To evaluate the importance of each species or taxa consumed (i), the following indexes were calculated: percent relative frequency (PRF, = 100* observed occurrence of i/x observed occurrence of all items) and percent wet mass (PWM, = 100*x of individual weight of i in each scadestimated weight of all items). This last index was calculated only for the main teleost items. Interannual changes in PRF were evaluated by a Pearson xL (Steel and Torrie 1988).
To estimate prey size, otoliths with low or no erosion were measured (max- imum length) to the nearest 0.05 mm, using dial calipers. For those otoliths that showed signs of little erosion, a correction factor (15% of maximum length) was applied (Dellinger and Trillmich 1988, North 1996). Length and wet mass of fish prey were estimated from regressions (see Table 1). Because fur seals usually feed upon schooling fish species (Bowen and Harrison 1996, and references therein), a within-scat correlation in prey size is expected (Rec- chia and Read 1989). Thus, the length and wet mass prey frequency distri- butions were constructed using the mean value of each species for a given scat. Between-year differences in length and wet mass were compared, for each prey separately, by one-way ANOVAs.
RESULTS
Fish Species and Diet Richness
Of 539 scats examined, 58.3% contained otoliths or cephalopod beaks. However, great variation between years was observed: during January 1996, 85.5% contained otoliths and/or cephalopod beaks, whereas in January 1998 only 35.0% contained at least one of these groups. Fourteen teleost taxa were identified from saggital otoliths, of which 11 were identified to species (Table 2). Of the reported species, the Brazilian coding (U. brasiliensis) and the sand
Tabl
e 1.
L
engt
h an
d w
et m
ass
regr
essi
ons
used
to
estim
ate
prey
siz
e. O
L =
oto
lith
leng
th (
mm
). T
L =
tot
al l
engt
h (m
m).
WM
= w
et
mas
s (g
).
Len
gth
Wet
mas
s
Eau
atio
n So
urce
a E
auat
ion
Sour
ces
Anc
hovy
(An
cboa
mar
inii)
TL
= -
2.19
9 +
28.0
25"O
L
1 W
M =
8.4
06*1
0-7*
TL
3457
1
Anc
hove
ta (
Eng
radi
s anc
boita
) T
L =
-7.
401
+ 38
.880
*0L
1
WM
= 1
.46*
10-6
*TL3
.294
1
Wea
kfis
h (C
ynos
czon
guat
ucup
a)
TL =
e[L
n(O
L/11
.211
55)1
0.7
1x1
2 W
M =
0.0
293*
TL
L-'"
"'
3 C
utla
sfis
h (T
ricb
iuru
s lep
turu
s)
TL
= -
171.
424
+ 17
6.71
8*0L
1
WM
= 5
.07*
10-8
*TL
3 342
1 1
a So
urce
: 1.
Per
sona
l co
mm
unic
atio
n fr
om M
. Hai
mov
ici,
Uni
vers
idad
e do
Rio
Gra
nde,
Cx
P. 4
74, R
io G
rand
e, 9
6201
-900
Bra
zil;
2. B
alda
s et
al.
1997
; 3. A
nthn
ez,
P. 1
983.
Est
udio
del
cre
cim
ient
o en
la
pesc
adill
a (C
ynos
czon
strz
atus
) de
la z
ona
com
lin d
e pe
sca
Arg
entin
-Uru
guay
a du
rant
e 10
s ai
ios
1976
-197
8. T
esis
de
Lic
enci
atur
a, F
HC
, UR
. 11
3 pp
. (n
rn
F
Tabl
e 2.
Pe
rcen
t re
lativ
e fr
eque
ncy
(PR
F),
and
perc
ent
wet
mas
s (P
WM
) of
the
pre
y co
nsum
ed b
y fu
r se
als
in e
ach
colle
ctio
ns p
erio
d.
PW
M w
as e
stim
ated
onl
y fo
r th
e m
ain
tele
ost
fishe
s. n =
num
ber
of s
cats
.
All
Sep
1795
D
ec 1
995
Jan
1996
M
ar 1
796
Jan
1998
M
ay 1
798
colle
ctio
ns
PRF
PW
M
PRF
PW
M
PRF
PW
M
PRF
PW
M
PRF
PW
M
PRF
PW
M
PRF
PW
M
Scia
nida
e Cy
nosc
ion g
uatu
cupa
M
icrnp
ngon
ias f
umie
ri
Men
ticiw
bus a
mer
ican
us
Um
brin
a ca
nosa
i E
ngra
ulid
ae
Engr
aulis
anc
boita
An
choa
mar
ini
Tri
chiu
rida
e Tr
icbi
urus
lept
urus
Mer
lucc
ius
bubb
si M
erlu
cida
e
Car
angi
dae
Phyc
idae
Serr
anid
ae
Tvac
buru
s lat
bam
i
Uro
phyc
is br
asi li
ensis
Acan
tbis
tius
bras
ilian
us
Oth
er f
ishe
s C
epha
lopo
ds
n Num
ber
empt
y sc
ats
16.8
0.9 -
-
68.1
1.
8
1.8
-
0.9
-
-
0.9
8.8
34.2
-
-
-
42.1
-
10.5
2.6
-
-
-
-
10.5
26.3
40
.2
--
1.
0 -
24.2
14
.7
20.2
19
.6
8.1
25.5
5.1 -
3.0 -
1.0 -
1.0 -
1.0 -
9.1 -
76
11
20.7
9.
3 6.
9 5.
0
10.3
34
.8
3.4 -
--
12.1
-
19.0
-
51 7
15.0
1.
7 -
-
10.0
18
.3
25.0
1.7
1.7
-
-
5.0
21.7
26.6
1.
6 -
-
12.5
-
42.2
4.7
-
-
-
-
12.5
23.2
0.
5 0.
2 0.
2
33.1
8.
6
14.4
2.3
1.6
0.2
0.2
2.8
12.7
37.1
-
-
-
29.4
3.
6
29.7
-
-
-
-
-
-
5 37
225
NAYA ET AL.: SOUTH AMERICAN FUR SEAL DIET 739
Sep 95
// Mar96
I
0 50 100 I50 200 250 300 -350 400 480
individuals Clasified
Figure 2. Rarefaction curves for each collection period.
whiting ( M . amerzcanus), constituted the first records for the A. australis diet in Uruguayan waters. The rarefaction curves were highest in January, but there was no change in prey richness among years (the difference between Januarys was less than one species; Fig. 2).
Main Taxa and Interannual Changes
According to the percent relative frequency, the taxa that comprised the diet of A. australis can be separated in two groups: main taxa (PRF 2 5% overall) and secondary taxa (PRF < 5% overall). The first group consisted of the anchoveta (E . ancboita), the weakfish (C. guatucupa), cutlasfish (T. lepturus), the cephalopods, and the anchovy (A. marinii), and represented 92.0% of the total. Significant between-year variation in A. australir diet was detected (x24 = 17.78, P < 0.01). PRF of E. ancboita and C. guatimdpa declined, while PRF of T. lepturus and cephalopods increased from 1996 to 1998 (Table 2).
A great variation in the biomass contribution to species to the diet among collections was recorded (Table 2). Three of the four main fish species con- tributed >70% in at least one period and <15% in others. Except for the first collection, where E. ancboita was the most important item, the percentage wet mass was dominated by C. guatucupa and T. lepturus. These species showed an inverse relationship, with a progressive increase in ?: lepturus and decrease in C. guatmupa.
Prey Size Estimatej
Because few otoliths were recovered from the secondary taxa, the prey size estimates were determined for the main teleost prey. Fur seals fed upon prey ranging from 5.4 cm to 104.8 cm in estimated length and 0.7 g to 629.9 g in wet mass. Length and wet mass frequency distributions of the main fish
740 M A R I N E MAMMAL SCIENCE. VOL. l X , NO. 3. 2002
species are shown in Figure 3. The size overlap among species was low; the two species that overlapped in length ( E . ancboita and C. guatucuapa), had different wet mass, while the two species that overlapped in mass (C. guatucupa and 1: lepturus) did not in length. Size variation between years was significant for E. anchoita (F,,96 = 7.00, P < 0.01 for length; Fl,g6 = 5.11, P < 0.05 for weight) and 7: leptwzs (F1 ,52 = 4.26, P < 0.05 for length; Fl ,52 = 5.52, P < 0.05 for weight). For the former species the individuals consumed in 1996 were larger than those ingested in 1998, while for the second species they were smaller.
Methodological Limitations
Fecal analysis is favored for studies of pinniped diet because scats are rela- tively abundant, easy to collect, and their collection is non-invasive (Carey 1992). However, important methodological biases should be taken into ac- count when interpreting the results (see Croxall 1993, Bowen and Harrison 1994). Diet of animals making short foraging trips is more likely to be rep- resented in the scats than that of animals making longer trips (Tollit et a f . 1997). These differences in trip duration, combined with the variation in gut transit time among species, will directly affect the composition of the samples (Goodman-Lowe et af . 1997). Moreover, the fact that erosion of hard parts varies markedly among prey, implies that the detection of each item is de- pendent upon its size and morphological structure. Bowen (2000) showed that prey numeric proportion is affected by several factors (e.g., predator species and level of activity, prey size), and recommended the use of prey-number correction factors (NCFs). Because NCFs are not available for A. australis, percent relative frequency and percent wet mass were used as diet descriptors in our work. In addition, only those otoliths without erosion or with little erosion (using corrections factors published for otariid seals) were considered in reconstructing the prey-size distribution.
Dietary Composition
The rarefaction curve is the only diversity measure that is sensitive to rare species and is unbiased by sample size (Magurran 1988). In addition, species richness is easy to interpret and the shape of the curve reflects evenness (GoteIIi and Graves 1996). Other approaches commonly used, such as Shannon or Simpson indexes cannot be interpreted in a biologically meaningful way (Gas- ton 1996). Furthermore, these indexes lack a probability basis, so i t is im- possible to evaluate difference between estimators (see Gotelli and Graves 1996).
The number of species consumed by A. australis was similar to those re- corded for other species of the same genus based on scat samples of similar sizes (e.g., A. gazeffa, Green et al. 1989, Reid 1995, North 1996; A. forsteri,
NAYA E T A L . : SOUTH AMERICAN FUR SEAL DIET 741 ~
m c 0
0 > f a, m
0
L 0
.- i
n
k a, z)
E 3 z
Anchoa marinii Anchoa marinii Anchoa marinii
Engruulis anchoita Engruulis anchoitu Engraulis anchoiia
40 40 30 30 20 20 10 10
12.0 16.0 20.0 0.0 70.0 40 0 60.0
Cpnoscion guutucupa Cvnoscion guatucupa Cynoscion gun iucupa
25 25 20 20 15 15 10 10 5 5
0 10.0 20 0 30.0 0.0 100.0 200 0 300.0
Trichiurus Iepiurus Trichiurus lepiurus Trichiurus lepiurus
16 20 1 :o ;6::(14.1) I 16 WM = 174.8 (142.4) 12
8 8 4 4
13
%CO = 18% 0 NF = 45
40.0 60.0 80.0 100.0 0.0 200.0 400.0 600.0
Length (cm) Wet mass (g)
Figwe 3. Length and wet mass distribution of main fish species. L = mean length in cm (+ 1 SD). WM = mean wet mass in g (& 1 SD). NO = total number of otoliths measured. %CO = percentage of otoliths that were corrected; NF = number of scats from which otoliths were taken.
Carey 1992; A. philipii, Ochoa and Francis 1995), but lower than those re- ported for the same species in Peru (Majluf 1989). In this last environment, A. australis diet is dominated by the anchoveta (E. ringens), and a markedly increase in prey richness occurs when this preferred prey item is dispersed or less abundant (Majluf 1989).
Diet composition of the South American fur seals was dominated by four teleost fishes and the cephalopods. The fact that the bulk of the diet was derived from only a few species is in agreement with previous studies on other pinniped species (see Bowen and Siniff 1999 for review). The two most rele- vant species in terms of biomass are demersal-pelagic. C. guatucupa is a shelf species (relatively abundant from 0 to 165 m of depth) that occurs from Rio de Janeiro (Brazil) to Argentina, while T lepturus is mainly an outer shelf species, worldwide distributed in tropical and warm-temperate waters (Hai- movici et al. 1994). The two engraulid species are typically marine; E. anchoita
742 MARINE MAMMAL SCIENCE, VOL. 18, NO. 3 , 2002
is pelagic and occupies practically all the shelf, while A. marinii is more limited to coastal waters (Weiss and Feij6 de Souza 1977).
Although A. australis feed on a wide range of prey size, the mean mass of the main fish species was never more than 200 g. This value is in agreement with size data reported for A. australis in Peruvian waters, where the most important prey items were all less than 300 g in wet mass (Majluf 1989). The most remarkable difference in prey size between the two locations was the predation on a large species (T lepturus) in Uruguayan waters.
Temporal Vuriations und Fisheries Interactions
The high diversity during January was probably related to the reproductive cycle of A. australis. Births of A. austrulis in the Uruguayan islands occur in November and December, reaching a maximum during the last week of No- vember and the first and second weeks of December (Vaz-Ferreira and Ponce de Le6n 1987). Lactating females are restricted in their foraging range by the necessity of returning to pups at regular intervals (Boyd and Croxall 1992). The need to supply energy requirements in a short time might lead to a reduction in prey selectivity and, consequently, an increase in female diet diversity after the birth of her pup. However, higher diversity also could result from two different biases. First, short foraging trips may increase the propor- tion of scats per forage trip that are defecated on land, affecting prey detection probability in a positive way. Second, the concentration of adult and subadult males that takes place at this time on the island (Vaz-Ferreira and Ponce de Le6n 1984), implies a greater heterogeneity in the sex-age classes sampled.
The diet of A. australis varied strongly between years. This change was probably caused by variation in prey availability as observed in many otariid seals (Green e t al. 1989, Majluf 1989, Lowry et al. 1990, Boyd et al. 1994, Ochoa and Francis 1995, Koen et al. 1999). Although data on prey distri- bution and abundance for Uruguayan waters are not available, changes in the fishery catches paralleled those in the diet. The decrease in the capture of C. gmtucupa by the artisan fisheries (from 69 tons in 1997 to 33 tons in 1998), and the increase in the capture of 7: lepturus by the industrial fisheries (from 12.5 tons in 1997 to 30.8 tons in 1998), provide indirect evidence that fur seals responded to variation in prey abundance (1.NA.PE. official report 1997,' 1998*).
Three fish species important to the artisan fisheries were present in the A. australis diet, but only C. guatucupa appears to be an important resource for both. In addition, the size of this prey consumed by fur seals (1 and 2 yr) was lower than those caught by the artisan fisheries (3 yr and older).% An under-
' Anonymous. 1997. Informe Sectorial Pesquero, Instituto Nacional de Pesca. Available from
Anonymous. 1998. Informe Sectorial Pesquero, Instituto Nacional de Pesca. Available from
Personal communication from D. Szteren, Facultad de Ciencias, Universidad de la Repliblica,
I.NA.PE., Constituyente 1497, C.P. 11200, Montevideo, Uruguay. 54 pp.
I.NA.PE., Constituyente 1497, C.P. 11200, Montevideo, Uruguay. 47 pp.
Montevideo 11400, Uruguay, December, 1999.
NAYA ETAL.: SOUTH AMERICAN FUR SEAL DIET 743
standing of the diets of pinnipeds is one of the first steps toward understanding interactions with fisheries.
ACKNOWLEDGMENTS
The authors wish to thank F. Brum, A. Canavero, N. Gutierrez, A. Suanez, A. Turell, and the 1.NA.PE. island staff, for the help during the field work; and C. Altuna, M. Clara, A. Le Bas, M. Loureiro, E. Pat%, and A. Ponce de Ledn for their suggestions and comments. Special thanks to D. Szteren and M. Haimovici for providing unpub- lished data; and to F. Trillmich and two anonymous reviewers for the constructive criticism of a draft of the manuscript.
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Received: 26 February 2001 Accepted: 18 October 2001
