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Feeding habits of a large endangered skate
from the south-west Atlantic: the spotback skate,
Atlantoraja castelnaui
Santiago A. Barbini
A
,
B
,
D
and Luis O. Lucifora
B
,
C
A
Laboratorio de Ictiologı´a, Departamento de Ciencias Marinas, Universidad Nacional
de Mar del Plata, Funes 3350, Mar del Plata, B7602AYL, Argentina.
B
Consejo Nacional de Investigaciones Cientı´ficas y Te
´cnicas (CONICET), Argentina.
C
Instituto de Biologı´a Subtropical – Sede Iguazu´, Universidad Nacional de Misiones
and Centro de Investigaciones del Bosque Atla
´ntico (CeIBA), Casilla de Correo 9,
Puerto Iguazu´, N3370AVQ, Misiones, Argentina.
D
Corresponding author. Email: sbarbini@mdp.edu.ar
Abstract. Elasmobranch predation has important effects on marine ecosystems. Identifying the main correlates of the
feeding habits of skates is of paramount importance for determining their ecological role. We tested the hypotheses that the
diet of the spotback skate, Atlantoraja castelnaui, off Uruguay and northern Argentina, changes with increasing body size,
between seasons and regions and that prey size increased with predator’s size using a multiple-hypothesis modelling
approach. A. castelnaui preyed mainly on teleosts, followed by cephalopods, elasmobranchs and decapods. Small
individuals of A. castelnaui consumed decapods and large individuals ate elasmobranchs and cephalopods. The
consumption of teleosts was constant along the ontogeny but differed between seasons; more demersal-benthic teleosts
were consumed in the cold season, whereas more benthic teleosts were eaten in the warm season. Also, A. castelnaui
consumed more cephalopods in the warm season than in the cold season. Benthic teleosts were consumed more in the south
region, whereas decapods were eaten more in the north region. A. castelnaui is able to consume larger teleosts as it grows.
We conclude that A. castelnaui is a versatile, mainly piscivorous, consumer that shifts its diet with increasing body size and
in response to seasonal and regional changes in prey abundance or distribution.
Additional keywords: Argentina, diet variation, predation, Rajidae, Uruguay.
Received 23 July 2011, accepted 4 November 2011, published online 28 November 2011
Introduction
Large predators have significant effects on the trophic dynamics
of a variety of ecosystems, affecting community structure and
energy flow (Estes et al. 2011). In marine ecosystems,
elasmobranch predation is a major force structuring communi-
ties (Heithaus et al. 2008, 2010; Ferretti et al. 2010). Large
sharks prey on smaller sharks and batoids; removing the large
sharks results in a cascading effect that changes the structure of
the benthic community (Myers et al. 2007; Ferretti et al. 2010).
Large batoids affect the species turnover of benthic communi-
ties by disrupting the structure of the bottom and preying on
benthic invertebrates (VanBlaricom 1982; Thrush et al. 1991).
Skates, by their abundance and species diversity, may play
influential roles in marine community dynamics (Ebert and
Bizzarro 2007). Therefore, knowing and understanding the
feeding habits of skates are very important for determining their
ecological role (San Martı´n et al. 2007). The spotback skate
Atlantoraja castelnaui (Rajidae) is the largest skate of coastal
waters and one of the largest benthic batoids in the south-west
Atlantic Ocean, attaining 1400 mm in total length. It is endemic
to the south-west Atlantic from Rio de Janeiro, Brazil (228S), to
San Jorge Gulf, Argentina (468390S) (Menni and Stehmann
2000; Bovcon et al. 2011). In Argentinean waters, this species
occurs from shallow coastal waters to ,100 m depth (Cousseau
et al. 2007) and matures at 1089 mm (females) and 980 mm
(males) total length (Colonello 2009).
Due to its large body size, A. castelnaui has a high commer-
cial value and has been subjected to heavy fishing pressure; as a
result, its biomass declined by 75% between 1994 and 1999
(Hozbor et al. 2004). For this reason, A. castelnaui is categorised
as endangered by the International Union for the Conservation
of Nature (IUCN), with a decreasing trend in population
abundance (Hozbor et al. 2004). Its large size makes it ecolog-
ically important because other large skate species, such as
Zearaja chilensis or Dipturus trachyderma, do not overlap
greatly with A. castelnaui in their bathymetric range and occur
only in deeper waters (Menni and Stehmann 2000; Cousseau
et al. 2007). However, the ecology of this species, including its
CSIRO PUBLISHING
Marine and Freshwater Research, 2012, 63, 180–188
http://dx.doi.org/10.1071/MF11170
Journal compilation ÓCSIRO 2012 www.publish.csiro.au/journals/mfr
feeding habits, is poorly known. Previous studies of the feeding
habits of A. castelnaui are limited to descriptions of dietary
composition, indicating that the species feeds mainly on teleost
fishes (Laureda and Martı´nez 1981; Soares et al. 1992; Paesch
2000).
Variation in the diet of elasmobranchs can be attributed to
intrinsic and extrinsic factors (Di Gia´ como and Perier 1996;
Lucifora 2003). Intrinsic factors are traits of the predator, such
as sex, maturity stage and body size; extrinsic factors are
characteristics of the prey or the environment that affects the
availability of prey. Evaluating the interplay and relative effects
of intrinsic and extrinsic factors on the diet will help to identify
potential effects of the decline in abundance of predators
(Lucifora et al. 2009a). For example, if skate body size is an
important determinant of the consumption of a particular prey,
then fishing for the larger skates will affect the predator–prey
relationship. However, if geographic region is the main deter-
minant of the consumption of a given prey, then regional
differences in fishing effort or coastal development will have
a higher impact on the predator–prey relationship than any
intrinsic factor.
In this paper, we explored the importance of several intrinsic
and extrinsic factors in determining the diet of a large skate, the
spotback skate, A. castelnaui. Specifically, we tested the
following hypotheses: (1) the diet of A. castelnaui changes with
increasing body size; (2) the diet composition changes between
seasons; (3) there are differences in the diet between regions;
and (4) prey size increases with increasing body size of
A. castelnaui.
Materials and methods
Study site and sampling
The coastal region off Uruguay and northern Argentina
(between 348S and 418S) consists of two large ecosystems. The
first, the northern region (34–388S), is a stratified coastal zone
influenced by the very large discharge of continental waters of
the Rı´o de la Plata. The second, the southern region (38–418S), is
a homogeneous coastal zone, called El Rinco´ n, influenced by
the smaller discharges of the Negro and Colorado rivers and by
high-salinity waters of the San Matı´as Gulf (Guerrero and Piola
1997; Lucas et al. 2005).
Samples (390 individuals, 255 with stomach contents) were
obtained from scientific trawl surveys conducted by the Instituto
Nacional de Investigacio´n y Desarrollo Pesquero (INIDEP,
Argentina) during December 2005, February and June 2006
and from commercial landings of the coastal fleet of Mar del
Plata harbour (Fig. 1) during May, September, October and
November 2006 and April, May, June, July, August, October,
November and December 2007 (Fig. 1). Each specimen cap-
tured was measured (total length (TL), mm) and sexed. Also, the
maturity stage (juvenile or adult) was determined according to
the degree of calcification of the claspers and the development
of testes and reproductive ducts in males and to the presence of
eggs and observation of the uteri, oviducal glands and ovarian
follicles in females (Stehmann 2002; Colonello 2009). The
stomachs were removed and stored at 208C. In the laboratory,
prey were sorted, identified to the lowest possible taxonomic
level using published catalogues, counted and wet weight was
recorded (0.01 g).
Feeding habits
The importance of each prey was evaluated using percentage by
number (%N), mass (%M), frequency of occurrence (%F) and
index of relative importance (%IRI; Pinkas et al. 1971; Corte´s
1997).
For statistical analyses, we grouped prey into six categories:
benthic teleosts, demersal-benthic teleosts, pelagic teleosts,
elasmobranchs, cephalopods and decapods. The number of
sampled A. castelnaui with prey was tested to evaluate whether
sample size by sex, maturity stage, region and season was
sufficient for the statistical analyses. The sampling order of
stomachs was randomised 100 times and the mean cumulative
Shannon–Wiener diversity index was plotted as a function of
sample size. Sample size was considered sufficient to describe
diet if the cumulative prey curve reached an asymptote
(Magurran 2004).
To test the hypothesis of change in the diet of A. castelnaui
with increasing body size and of differences in the diet compo-
sition between seasons and regions, we adopted a multiple-
hypothesis modelling approach (Franklin et al. 2001; Johnson
and Omland 2004; Symonds and Moussalli 2011). We assessed
whether the consumption of the prey categories varied with sex,
maturity stage (juvenile and adult), total length, season (warm ¼
October–March; cold ¼April–September) and region (north ¼
348–388S; south ¼388–418S) using generalised linear models
(GLM) (Venables and Ripley 2002). For each prey category, we
built GLMs in which the response variable was the number of
the prey consumed and the independent variables were sex,
maturity stage, TL, season and region (Lucifora et al. 2009a).
Further, models with combinations between two independent
64 62 60 58 56 54 52
42
41
40
39
38
37
36
35
34
33
Latitude S
Longitude W
Uruguay
Argentina
Fig. 1. Map of the sampling area from off Uruguay and north Argentina,
showing positions of trawls stations (black circles) and cells of the fishing
grid (black rectangles) where individuals of Atlantoraja castelnaui were
captured. The 50-m and 200-m isobaths are shown as solid and dashed lines
respectively. The rectangle in the inset shows the location of the study area in
South America.
Diet of Atlantoraja castelnaui Marine and Freshwater Research 181
variables were fitted: sex þseason, sex þregion, maturity stage þ
season, maturity stage þregion, TL þseason, TL þregion and
season þregion. Models without any of the independent variables
(i.e. null models) were fitted to test the hypothesis that none of the
variables tested had an effect on the consumption of a prey
category (Lucifora et al. 2009b). All models had a negative
binomial error distribution (i.e. a high number of zero-values
and variance much greater than the mean) and a log link (Crawley
2005).
For each model fitted within a prey category, we calculated
the Akaike information criterion (AIC) and the model with the
lowest AIC was selected as the best model. AIC measures the
amount of information lost when fitting a model, so the model
with the lowest AIC is the best one explaining the observed data
(Crawley 2005). To obtain the likelihood of each model fitted,
Akaike’s weight (w) was calculated (Franklin et al. 2001;
Johnson and Omland 2004). If wdid not provide strong support
for any model fitted, we used model averaging to measure the
effects of the variable explaining most of the variation (Johnson
and Omland 2004; Symonds and Moussalli 2011).
The hypothesis that prey size increased with increasing body
size of the predator was assessed using the TL of A. castelnaui
and the TL of prey teleosts. Regressions on the 5, 50 and 95%
quantiles were fitted to test an increase in minimum, medium
and maximum prey size consumed with increasing TL of
A. castelnaui respectively (Scharf et al. 1998).
Results
Of the individuals with food in the stomachs (n¼255), 121
were female (243–1365 mm TL) and 134 were male (332–
1400 mm TL). The cumulative diversity curves reached an
asymptote, indicating that sample size was sufficient for all
groups considered (see Fig. S1, available as Supplementary
Material to this paper).
Forty-five prey were identified to the lowest taxonomic
level: 27 teleosts, eight decapods, six elasmobranchs, three
molluscs and one cephalochordate (Table 1). Teleosts were
the dominant prey consumed by A. castelnaui. Decapods,
cephalopods and elasmobranchs were less important compo-
nents of the diet. The most important teleost by number was
Dules auriga, followed by Raneya brasiliensis,Porichthys
porosissimus and Trachurus lathami. In terms of weight, Cynos-
cion guatucupa was the most important prey, followed by
P. porosissimus and Prionotus nudigula.D. auriga and
R. brasiliensis were the most important prey by frequency of
occurrence. The decapods, cephalopods and elasmobranchs
consumed were predominantly shrimps, octopi and skates
respectively.
Relationships between number of prey consumed with TL,
season and region were found (Table 2). The effect of each of
these variables was dependent on the prey group; therefore,
below we present the results for each prey group. In all models,
the residual deviance was less than the residual degrees
of freedom, indicating that the models had a good fit to the
data (see Table S1, available as Supplementary Material to
this paper).
The consumption of benthic teleosts was affected by season
and region. Benthic teleosts were consumed more in the warm
season than in the cold season (Fig. 2). In the warm season, the
most important benthic teleosts in the diet were P. porosissimus,
Etropus longimanus and Percophis brasiliensis (Fig. 3). The
number of the benthic teleosts consumed was higher in the south
region than in the north region (Fig. 2).
Season was the only factor affecting the consumption of
demersal-benthic teleosts. More demersal-benthic teleosts were
consumed in the cold season than in the warm season (estimated
number of demersal-benthic teleosts by GLM: warm ¼0.388;
cold ¼0.747). D. auriga was the most consumed demersal-
benthic teleost by A. castelnaui in the cold season (Fig. 3).
The model of consumption of demersal-benthic teleosts had a
low w, so we computed model averaging. The averaged coeffi-
cient was 0.642 (s.e. ¼0.195) for the cold season, with a
combined wof 0.981. The consumption of pelagic teleosts
was independent of sex, maturity stage, TL, season or region.
Body size was the only variable with a significant effect on
the consumption of elasmobranchs by A. castelnaui; consump-
tion of elasmobranchs increased with increasing TL of
A. castelnaui (Fig. 4). Body size and season significantly
affected the consumption of cephalopods by A. castelnaui.
The number of cephalopods consumed increased with the size
of A. castelnaui and was higher in the warm season than in the
cold season (Fig. 4). The main cephalopod consumed in the
warm season was the octopus Octopus tehuelchus.
A combination of body size and region was the most
plausible explanation for the pattern of consumption of deca-
pods. Contrary to the pattern found for elasmobranchs and
cephalopods, the consumption of decapods decreased with
increasing TL of A. castelnaui (Fig. 4). Decapods were
consumed more in the northern region than in the southern
region (Fig. 4).
For elasmobranchs, cephalopods and decapods, model
averaging was computed. For elasmobranchs, the model aver-
aged slope for TL was 0.002 (s.e. ¼0.0007) with a combined w
of 0.636. The estimated averaged coefficients for cephalopods
were 0.004 (s.e. ¼0.002) for TL and 0.740 (s.e. ¼0.630) for
the cold season with a combined wof 0.90. The averaged
coefficients for decapods were 0.004 (s.e. ¼0.001) for TL
and 0.259 (s.e. ¼0.252) for the south region with a combined
wof 0.778.
Significant relationships between predator and prey body
size were found. As TL of A. castelnaui increased, minimum,
medium and maximum TL of teleosts increased (slope and
intercepts of 5, 50 and 95% quantile regressions ¼0.081 and
15.673; 0.131 and 32.817; 0.366 and 39.170, respectively,
P,0.05) (Fig. 5).
Discussion
Dietary composition
Corroborating our results, a previous study conducted off Mar
del Plata (388S) found that teleosts were the main prey con-
sumed by A. castelnaui, followed by molluscs, crustaceans and
other invertebrates (Laureda and Martı´nez 1981). However, the
main species of teleosts consumed were different between
studies. The most important species of teleosts observed by
Laureda and Martı´nez (1981) were flatfishes, Symphurus spp.
and R. brasiliensis. In our study, D. auriga,R. brasiliensis,
182 Marine and Freshwater Research S. A. Barbini and L. O. Lucifora
P. porosissimus and T. lathami were the most consumed teleosts.
This difference in consumption of teleosts may be associated
with local differences in prey availability, because the results
from Laureda and Martı´nez (1981) reflected the diet of samples
taken from off Mar del Plata whereas the samples in our study
covered a much larger area.
Off the Rı´o de la Plata, between 50 and 100 m depth, the diet
composition of A. castelnaui is also dominated by teleosts
Table 1. Diet composition of Atlantoraja castelnaui off Uruguay and northern Argentina
%N, percentage by number; %M, percentage by mass; %F, percentage frequency of occurrence; %IRI, percentage index of relative importance
Group Prey %N %M %F %IRI
Teleosts
A
74.40 92.52 89.41 97.81
Unidentified teleosts 19.40 14.54 30.20
Congridae Conger orbignyanus 0.22 1.70 0.39
Engraulidae Engraulis anchoita 0.22 0.03 0.39
Ophidiidae Raneya brasiliensis 6.13 4.77 10.59
Batrachoididae Porichthys porosissimus 5.47 7.60 9.02
Triathalassothia argentina 2.19 1.54 3.14
Triglidae Prionotus nudigula 3.50 7.32 5.88
Serranidae Dules auriga 11.82 3.84 12.55
Carangidae Trachurus lathami 5.47 4.37 5.88
Parona signata 0.22 0.62 0.39
Sparidae Pagrus pagrus 0.44 2.07 0.78
Sciaenidae Cynoscion guatucupa 2.62 16.10 4.31
Umbrina canosai 0.87 2.04 1.57
Paralonchurus brasiliensis 0.22 0.14 0.39
Mullidae Mullus argentinae 2.62 2.72 2.74
Cheilodactylidae Nemadactylus bergi 0.44 2.10 0.78
Percophidae Percophis brasiliensis 2.84 6.17 4.31
Pinguipedidae Pinguipes brasiliensis 1.31 5.41 1.96
Gobiidae Gobiosoma parri 0.87 ,0.01 0.39
Stromateidae Stromateus brasiliensis 0.22 0.46 0.39
Paralichthyidae Paralichthys orbignyanus 0.22 5.25 0.39
Paralichthys patagonicus 0.66 1.53 1.17
Xystreurys rasile 0.22 0.29 0.39
Achiropsetta tricolepis 0.66 0.07 0.78
Etropus longimanus 3.72 0.55 4.70
Unidentified Paralichthyidae 0.66 1.15 1.18
Cynoglossidae Symphurus spp. 0.87 0.10 1.59
Elasmobranchs
A
4.16 2.63 5.49 0.24
Triakidae Mustelus schmitti 0.22 0.55 0.39
Rajidae Atlantoraja castelnaui 0.22 0.05 0.39
Psammobatis extenta 0.22 0.43 0.39
Psammobatis spp. 0.22 0.52 0.39
Sympterygia bonapartii 0.22 0.07 0.39
Unidentified Rajidae 3.06 1.00 3.53
Cephalochordates
A
Branchiostoma platae 0.22 ,0.01 0.39 ,0.01
Molluscs
A
Cephalopods
A
3.50 4.09 5.10 0.25
Unidentified squid 0.44 0.18 0.78
Octopodidae Octopus tehuelchus 3.06 3.90 4.31
Gastropods Unidentified Fissurellidae 0.22 0.03 0.39
Crustaceans
A
Decapods
A
17.50 0.73 14.12 1.69
Penaeidae Artemesia longinaris 1.31 0.07 1.96
Solenoceridae Pleoticus muelleri 3.28 0.02 0.78
Unidentified shrimps 8.31 0.04 5.10
Majidae Collodes rostratus 0.22 ,0.01 0.39
Libinia spinosa 0.44 ,0.01 0.39
Libidoclaea granaria 0.44 0.01 0.39
Portunidae Ovalipes trimaculatus 0.66 0.47 1.18
Unidentified crabs 2.84 0.12 4.31
Total number of prey 457
Total mass (g) 17 813
A
Major taxonomic group.
Diet of Atlantoraja castelnaui Marine and Freshwater Research 183
(Paesch 2000). In contrast, in Ubatuba Bay (248S, Brazil), the
diet composition consisted of two main prey items: teleosts and
decapods (Soares et al. 1992). The high consumption of dec-
apods may be due to a bias in the frequency distribution of
samples towards small skates (range of TL: 217–865 mm), since
our results indicate a negative relationship between skate size
and decapod consumption. The pattern found in Ubatuba Bay
could also be a result of lower sample size (n¼24).
Relationships between diet and body size,
season and region
The diet of A. castelnaui was affected by ontogeny, season and
geographic area. Body size (i.e. total length) has an important
effect on the diet composition of A. castelnaui; small individuals
consume decapods and large individuals consume elasmo-
branchs and cephalopods. An increase in the consumption of
elasmobranchs with body size has been reported for sharks
(Smale 1991; Lowe et al. 1996; Lucifora et al. 2005, 2009a).
However, elasmobranchs are not important prey in the diet of
skates and this pattern has not been documented before in any
skate. In other studies on the diet of A. castelnaui, elasmo-
branchs such as angel sharks, Squatina spp. and skates (Laureda
and Martı´nez 1981; Paesch 2000) were also found. Skates were
the main elasmobranchs consumed by A. castelnaui and there
was even one case of cannibalism by an adult male (TL ¼
1063 mm) that consumed a juvenile individual. As the mor-
phology of skates as prey (i.e. dorsoventrally flattened)
complicates the handling and suction by small individuals of
A. castelnaui, large individuals may be more able to capture this
prey. The importance of body size in determining the
consumption of elasmobranchs indicates that shifting size dis-
tributions towards smaller individuals, a common result of
overfishing (Bianchi et al. 2000), would relax the predation
pressure on these prey by A. castelnaui. Body size has been
identified as the main determinant of elasmobranch predation by
the copper shark Carcharhinus brachyurus (Lucifora et al.
2009a) and the sand tiger shark Carcharias taurus (Lucifora
et al. 2009b), which indicates that body size may be a general
determinant of elasmobranch consumption regardless of the
taxonomic identity of the predator. A decrease in the con-
sumption of decapods with increasing body size has been
described for other skates (Koen Alonso et al. 2001; Treloar
et al. 2007). In A. castelnaui, this pattern may be associated with
an increase in the quality of the diet, where decapods are
replaced by more energetically profitable prey such aselasmo-
branchs and cephalopods. The most important cephalopod in the
diet of A. castelnaui is the octopus O. tehuelchus and its higher
consumption in the warm season may be related to the behaviour
of this prey. The warm season is a period of intense reproductive
(mating) and feeding activity by O. tehuelchus, potentially
increasing its exposure and vulnerability to predation (Iribarne
1991; Re´ and Go´ mez Simes 1992). Season was also the main
factor affecting the consumption of teleosts by A. castelnaui.
The higher consumption of demersal-benthic teleosts in the cold
season and the higher consumption of benthic teleosts in the
warm season may be related to seasonal shifts in the distribution
and abundance of teleosts. In the same study area, other coastal
skates, such as Psammobatis extenta (Braccini and Perez 2005),
Psammobatis bergi (San Martı´n et al. 2007) and Rioraja agas-
sizi (Barbini and Lucifora 2011), also have seasonal shifts in diet
composition. These skates have evolved strategies to cope with
temporal variability in prey abundance (Caddy and Sharp 1986;
Braccini and Perez 2005).
The dominant benthic teleosts in the diet during the warm
season were P. porosissimus,E. longimanus and P. brasiliensis,
while D. auriga was the most important in the cold season. In the
Table 2. Best models explaining the consumption in number of the main prey categories of Atlantoraja castelnaui
TL, total length; AIC, Akaike information criterion; w, Akaike’s weigths; standard errors are in parentheses. South and cold are levels of factors region and
season respectively
Prey categories Intercept Coefficient AIC w
Benthic teleosts 1.357 (0.262) 0.706 (0.293) south 0.346 (0.250) cold 372.4 0.559
Demersal-benthic teleosts 0.944 (0.148) 0.653 (0.197) cold 500.9 0.325
Elasmobranchs 5.981 (1.136) 0.003 (0.001) TL 128.4 0.244
Cephalopods 6.831 (1.463) 1.443 (0.680) cold þ0.004 (0.001) TL 109.5 0.421
Decapods 1.700 (0.674) 0.881 (0.449) south 0.004 (0.001) TL 286.6 0.279
Warm Cold
Estimated mean number of benthic teleosts per stomach
0.0
0.2
0.4
0.6
0.8
1.0
Fig. 2. Changes in consumption of benthic teleosts (in number) with
season and region of Atlantoraja castelnaui estimated by generalised linear
models. The models had a log link and a negative binomial error distribution.
Black, north region; white, south region.
184 Marine and Freshwater Research S. A. Barbini and L. O. Lucifora
warm season, male P. porosissimus migrate to rocky habitats
where they establish and maintain a territory, emitting low
frequency sounds (Brantley and Bass 1994) and producing a
bioluminescent display to attract females (Crane 1965). This
reproductive behaviour may increase the vulnerability and
availability of this prey to elasmobranch predators in the warm
season, due to the increased visual exposure or audible detection
related to the breeding activity (Lucifora et al. 2006).
P. brasiliensis has a constant spatial distribution in this area
(Barreto 2007), but higher abundances were observed during
spring, possibly associated with reproductive movements of
adult individuals from deep areas towards shallower spawning
areas (Perrotta and Ferna´ndez Gime´nez 1996; Barreto 2007).
These seasonal patterns indicate that A. castelnaui is a versatile
consumer of teleosts and can shift its diet in response to seasonal
changes in the abundance or distribution of these prey.
The consumption of pelagic teleosts was not related to any of
the variables tested in our study. In Carcharhinus brachyurus,
the consumption of pelagic teleosts is a bell-shaped function of
predator age (Lucifora et al. 2009a). We did not test age as a
potential variable explaining consumption of any prey in
A. castelnaui. It remains a question for future studies to test
whether the consumption of pelagic teleosts in skates is also
affected mostly by age or some other variable.
Relationships between prey size and predator size
The consumption of teleosts is homogeneous throughout the
ontogeny of A. castelnaui, but this species is able to consume
larger teleosts as it grows. The same pattern has been observed in
other piscivorous skates, such as Zearaja chilensis in the south-
west Atlantic (Lucifora et al. 2000) and Dipturus gudgeri and
Porichthys porosissimus
Triathalassothia argentina
Percophis brasiliensis
Gobiosoma parri
Paralichthys orbignyanus
Xystreurys rasile
Etropus longimanus
Symphurus spp.
Mean number of
benthic teleosts
0.0
0.1
0.2
0.3
0.4
0.5
Raneya brasiliensis
Prionotus nudigula
Dules auriga
Pagrus pagrus
Cynoscion guatucupa
Umbrina canosai
Mullus argentinae
Nemadactylus bergi
Pinguipes brasiliensis
Mean number of
demersal−benthic teleosts
0.0
0.1
0.2
0.3
0.4
0.5
(a)
(b)
Fig. 3. Mean number and standard error of (a) benthic and (b) demersal-benthic teleosts consumed for Atlantoraja
castelnaui with season. Black, warm season; white, cold season.
Diet of Atlantoraja castelnaui Marine and Freshwater Research 185
Dipturus whitleyi in south-eastern Australian waters (Treloar
et al. 2007). Skates are suction feeders that ingest their prey
whole by creating a hydrodynamic flow (Dean et al. 2005).
Then, gape size imposes a limit on the size and type of prey
consumed (Scharf et al. 2000). Some elasmobranchs, such as
sharks, evade gape limitation by cutting prey with their teeth
(Frazzetta 1988; Lucifora et al. 2006; Braccini 2008), but skate
teeth are not able to cut. Therefore, the only way to increase prey
size in skates is by increasing body size.
Our results identified a heterogeneous array of factors
affecting the consumption of different prey groups in a large
endangered skate, suggesting a complex situation for managers
attempting to maintain the ecological function of this predator.
This array includes both extrinsic factors (e.g. season and
region) and intrinsic factors such as body size. Piscivorous fish
generally achieve the largest body size within their community
and have potentially large impacts on their communities through
predation (Juanes et al. 2002). Overfishing alters the size
structure of the populations because larger fishes are selectively
removed from the marine community (Bianchi et al. 2000). The
removal of large predators has indirect effects that involve
trophic interactions at the community level (e.g. trophic cas-
cades and non-lethal risk effects, indirect effects such as appar-
ent competition, Heithaus et al. 2008, 2010). Thus, the decline
and removal of large elasmobranchs, such as A. castelnaui, may
have marked ecological consequences in marine ecosystems.
Acknowledgements
We thank the Instituto Nacional de Investigacio´n y Desarrollo Pesquero
(INIDEP) for specimens collected from different research cruises and two
referees and the editor A. J. Boulton for their helpful comments.
S. A. Barbini was supported by a scholarship from the Comisio´n de Inves-
tigaciones Cientı´ficas de la Provincia de Buenos Aires (Argentina).
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