J. Mar. Biol. Ass. U.K. ꢀ2001), 81, 1015^1018
Printed in the United Kingdom
Feeding habits and dietary overlap in spiny dog¢sh
Squalusacanthias ꢀSqualidae) and narrowmouth catshark
Schroederichthysbivius ꢀScyliorhinidae)
V .V . Laptikhovsky, A.I. Arkhipkin* and A.C. Henderson
Falkland Islands Government Fisheries Department, PO Box 598, Falkland Islands.
*
Corresponding author, e-mail: aarkhipkin@¢sheries.gov.fk
Spiny dog¢sh Squalusacanthias and narrowmouth catshark Schroederichthysbivius are the only two shark
species commonly inhabiting the Falkland Islands shelf. The present study was undertaken to investigate
their feeding habits and possible dietary overlap between the species. The diet of the spiny dog¢sh was
dominated by Falkland herring Sprattus fuegensis and squid Loligo gahi, though it also preyed on a wide
range of other ¢sh and invertebrates. Catsharks showed a preference for invertebrates. The diet of both
species showed marked variations with season, and ontogenic shifts were also evident. Dietary overlap
was minimal in winter ꢀ15.4%) and maximal in spring ꢀ61.1%), the latter being due to both species
preying upon spawning aggregations of S. fuegensis.
INTRODUCTION
level, and the percentage frequency of occurrence ꢀ%O)
and percentage by number ꢀ%N) of each item calculated
Spiny dog¢sh Squalusacanthias L., 1758 ꢀSqualidae) and
narrowmouth catshark Schroederichthys bivius ꢀMuller &
«
Henle, 1841) ꢀScyliorhinidae) are the only two near-
bottom sharks inhabiting the Falkland Islands shelf. Both
species attain similar sizes and occur at depths from 50 to
ꢀ
Cortes, 1997). Levin's measure of niche breadth ꢀB) was
¨
calculated following Krebs ꢀ1989) and niche overlap was
estimated by Pianka's symmetrical measure ꢀKrebs, 1989).
2
50 m. The biology of neither species has been studied in
RESULTS
Falkland Islands waters, although the spiny dog¢sh is
known to be a piscivorous and opportunist feeder ꢀe.g.
Ellis et al., 1996; McMillan & Morse, 1999). Schroederichthys
bivius, however, is known to feed predominantly on inverte-
brate species ꢀmainly crustaceans), although ¢sh are also
Schroederichthysbivius
Catsharks 540 cm L fed mostly on small crustaceans
T
and polychaetes ꢀTable 1). Planktonic crustaceans were less
important in the diets of specimens 440 cm L , although
T
preyed upon ꢀCorte
¨
s, 1999).
euphausiids were sometimes found in sharks as large as
55 cm L , and amphipods in those up to 65 cm L . Thus,
there was an ontogenic transition from planktonic crusta-
ceans to squid Loligo gahi and small-sized ¢sh, especially
Falkland herring Sprattus fuegensis. Large decapod crusta-
ceans represented a total of 10.5% of prey in sharks of
Despite the overlap in their geographical and bathy-
metric distributions, little is known about possible feeding
competition between these two shark species. The present
study was undertaken to investigate seasonal and onto-
genic variations in the diets of both sharks, to compare
their feeding niches, and to estimate the dietary overlap
in Falkland Islands shelf waters.
T
T
40^59 cm L and 18.2% in those larger than 60 cm.
T
Although squid predominated the diet of the largest
sharks, diet breadth was greatest in this size-class. Apart
from squid, ¢sh, and large crustaceans, catsharks
consumed a wide range of benthic invertebrates ꢀTable 1).
The diet of the catshark showed marked variations with
season. In winter, the most common prey was L. gahi and
polychaetes. In spring S. fuegensis and lobster-krill Munida
MATERIALS AND METHODS
Specimens of both species were collected opportunist-
ically onboard commercial and research vessels operating
in the Falkland Islands Conservation Zone, between May
1
999 and February 2001. Dog¢sh were found at depths gregaria became very important ꢀTable 1), whilst poly-
ranging from 95 to 231m ꢀmean 148 m), while catsharks
chaetes remained of the same importance. In summer,
occurred from 35 to 231m ꢀmean 141m). During biological Munidaspp., crabs and L.gahidominated the diet.
analysis on board ship, the total length ꢀLT) of each shark
was measured and its stomach contents were examined.
Atotal of 223 spiny dog¢sh ꢀL T44^98 cm, mean
Squalusacanthias
6
4.7 cm) and 280 catsharks ꢀL 20^81cm, mean 59.3 cm)
Dog¢sh were found to feed mostly on the Falkland
herring, and to a lesser extent on L. gahi, though the
feeding spectrum included a wide range of di¡erent ¢sh
and invertebrates ꢀTable 2). Smaller ꢀ44^65 cm) and
larger ꢀ66^98 cm) sharks had similar diets and niche
T
were found with stomachs containing food at di¡erent
stages of digestion. Undigested food ꢀi.e. prey that had
been eaten during hauling) was not quanti¢ed. Prey
items were identi¢ed to the lowest possible taxonomic
Journalofthe MarineBiological Associationofthe United Kingdom 2001)