Biological Observations on Shark Species Taken in Commercial Fisheries to the West of Ireland

Abstract

During an investigation into the biology of three shark species commonly taken in Irish fisheries, biological information was also collected from an additional six less-common species: Galeorhinus galeus, Lamna nasus, Mustelus asterias, Galeus melastomus, Hexanchus griseus and Dalatias licha. Data on age, reproduction, feeding and parasites were collected from a total of 137 sharks. Age data from G. galeus and M. asterias suggest possible differences in growth-rate relative to other areas. The state-of-maturity of all specimens agreed with earlier studies, although data regarding the reproductive cycle of M. asterias and G. melastomus were inconsistent with data commonly reported for these species. Stomach contents were unremarkable, with the exception of 1.3kg of cetacean material recovered from a specimen of H. griseus. Results from the parasite component of the investigation extend the known range of two monogenean species, while a further three parasites were recorded from new hosts.

Full text

BIOLOGICAL OBSERVATIONS ON SHARK
SPECIES TAKEN IN COMMERCIAL
FISHERIES TO THE WEST OF IRELAND
A.C. Henderson, K. Flannery and J. Dunne
ABSTRACT
During an investigation into the biology of three shark species commonly taken in Irish fisheries,
biological information was also collected from an additional six less-common species: Galeorhinus
galeus,Lamna nasus,Mustelus asterias,Galeus melastomus,Hexanchus griseus and Dalatias licha. Data on
age, reproduction, feeding and parasites were collected from a total of 137 sharks. Age data from
G.galeus and M.asterias suggest possible differences in growth-rate relative to other areas. The
state-of-maturity of all specimens agreed with earlier studies, although data regarding the
reproductive cycle of M.asterias and G.melastomus were inconsistent with data commonly reported
for these species. Stomach contents were unremarkable, with the exception of 1.3kg of cetacean
material recovered from a specimen of H.griseus. Results from the parasite component of the
investigation extend the known range of two monogenean species, while a further three parasites
were recorded from new hosts.
A.C. Henderson
(corresponding author;
e-mail:
aaronh@squ.edu.om),
Department of Marine
Science and Fisheries,
Sultan Qaboos
University, P.O. Box 34,
Al-Khod 123, Oman; K.
Flannery, Department of
the Marine and Natural
Resources, Dingle, Co.
Kerry; J. Dunne,
Department of Zoology,
National University of
Ireland, Galway.
Received 3 November
2000. Read 16 March
2002. Published 31
July 2003.
INTRODUCTION
The Irish fishing fleet is diverse, ranging from
inshore and deep-water trawlers to pelagic and
bottom-set gill-netters. With most marine habitats
targeted, it is not surprising that a wide range of
shark species is regularly taken. However, since the
collapse of the basking shark fishery on the west
coast, directed fisheries are few, although certain
species remain valuable by-catch.
Despite increasing fishing pressure, relatively
little biological work has been conducted on sharks
in these waters, and our understanding of their
biology and stock divisions is limited. With this in
mind, the Department of Zoology at National
University of Ireland, Galway, developed a
research programme in 1996. The aim of this
programme was to collect biological information
from commercially caught/landed sharks, paying
particular attention to those species that are taken
in greatest numbers in pelagic and inshore fisheries,
i.e. blue shark, Prionace glauca L., spiny dogfish
Squalus acanthias, and lesser-spotted dogfish,
Scyliorhinus canicula L. While information collected
from these species has been reported previously
(Henderson et al. 2001; Henderson et al., 2002a;
2002b; 2002c; Henderson and Casey, 2001), the
aim of this work is to report biological information
collected from other species. The data collected
were typically concerned with age, reproductive
biology, feeding and parasitology. The species
examined were tope, Galeorhinus galeus (L.);
porbeagle, Lamna nasus (Bonnaterre); starry smooth
hound, Mustelus asterias Cloquet; blackmouth
dogfish, Galeus melastomus Rafinesque; six-gill
shark, Hexanchus griseus (Bonnaterre); and kitefin
shark, Dalatias licha (Bonnaterre).
METHODS
Specimens examined were landed by the Dingle
fishing fleet throughout 1998. Sampling was
necessarily opportunistic; in addition to examining
commercially landed specimens, fishermen were
asked to retain (when possible) those shark species
which would normally be discarded. Such
specimens were held on ice while at sea, and
examined immediately upon landing. With the
exception of H.griseus,D.licha, and G.melastomus
(which were taken in deep-water trawls in the
region of the Porcupine Bank), all specimens were
captured in bottom-set gill-nets in the shelf-waters
west of Ireland. Commercially important species
such as porbeagle and tope were commonly landed
gutted, and data collection in such cases was
therefore limited to size and external parasites.
Pre-caudal length (PL), fork length (FL), and
total length (TL) of each specimen were measured
to the nearest centimetre. In the case of porbeagle,
very little time was available for examination
and, in many cases, only PL was measured.
Nonetheless, all length measurements reported
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should be read as TL unless otherwise indicated.
Information collected on reproductive biology
consisted of gonad weight/fecundity and
oviducal-gland width /clasper length. The gills
and body surface were examined for parasites, and
the alimentary tract was examined for food remains
and parasites (see Henderson et al. 2002b for
procedures).
Vertebral centra were used for age estimation
(Cailliet et al. 1986), except in six-gill and kitefin
sharks, where vertebrae were too poorly calcified
to use. Vertebrae were cleaned in a trypsin
solution, which was kept at 37°C for up to 48
hours. Porbeagle vertebrae were stained in a
solution of alizarin red S (LaMarca 1966) for
approximately five minutes, and examined under a
binocular microscope. Smooth hound vertebrae
were stained in the same manner although, due to
their relatively deep-coned nature, half of the
vertebra was first ground away (along the
anterior-posterior axis). Tope vertebrae required
more processing, as follows:
1. Using the coarse wheel on a bench grinder, the
vertebra was ground down to within 2.0mm of
the focus, after which the fine wheel was used
to continue grinding until the focus was
reached.
2. The ground face was polished on fine grain
(P600) wet sandpaper, and the vertebra was
glued to a glass slide (ground face down) using
araldite resin.
3. The resin was allowed to set for 48 hours, and
the remaining half of the vertebra was ground
(as above) until only a thin section of the
vertebra remained on the slide.
4. This was again polished on fine grain wet
sandpaper, rinsed in water, and immersed in an
alizarin red staining solution (see above) for
about five minutes.
5. The slide was examined under a low power
(×20) binocular microscope.
The solitary specimen of blackmouth dogfish
was aged using the acid decalcification technique
of Correia and Figueiredo (1997).
RESULTS
A range of biological data recorded from individual
sharks of five of the species is presented in Table 1.
TOPE (GALEORHINUS GALEUS)
Nineteen male and three female tope were
examined. They ranged in length from 70–162cm.
Vertebrae were collected from four individuals,
and the estimated age ranged from 10–12 years
(see Table 1). Of the three female tope examined,
one still had its reproductive tract in situ (despite
being gutted), and this 132cm individual was
completely immature. Testis weights were
available from four individuals and clasper length
was measured in all nineteen males (Table 1).
Seven alimentary tracts were examined, all of
which contained teleost remains. However, no
identification of prey was possible due to the
advanced state of digestion of the prey. Beaks from
two specimens of octopus, Eledone cirrhosa
(Lamarck), were also recorded from one indi-
vidual.
A total of six parasite species were recorded,
the most common being the nematode Anisakis
simplex L
3
Rudolphi, which was recorded in four
of the seven sharks examined. In most instances
this parasite was recorded from the stomach lumen,
but in one shark it was also found in the spiral
valve. The intensity of infection ranged from
1–53. One of the sharks harbouring this nematode
was also infected with two specimens of
Hysterothylacium aduncum L
3
(Rudolphi), which
were also found in the stomach. The only other
intestinal parasite recorded was the cestode
Anthobothrium cornucopia van Beneden, which was
found in the spiral valve of three sharks. Two
copepod species were recorded. A single specimen
of the more common of these, Pandaras bicolor
Leach, was attached to the respective flanks of two
sharks, while a third shark had two specimens
attached to the ventral surface of its left pelvic fin.
Lernaeopoda galei Kroyer, was found attached to the
cloacal region of one shark. The monogenean
Erpocotyle canis (Cerfontaine) was the only gill
parasite recorded. A total of four of these parasites
occurred on one of the sharks examined.
PORBEAGLE (LAMNA NASUS)
Data were collected from a total of 91 porbeagle
sharks, although the viscera and vertebrae of only
one of these sharks were available for examination.
The size distribution is presented in Fig. 1. Of the
nineteen females examined, only one individual
had its reproductive tract still in place, and this
170cm female was completely immature. The
distribution of clasper lengths for 72 males is
presented in Fig. 2.
The stomach contents of the single individual
examined (one year old, 100cm TL) yielded the
remains of many euphausiids (16g) and four
polychaete worms (7.2g). Both prey types were
highly digested, and no further identification was
possible. This individual and a further thirteen
porbeagle were examined for external (skin)
parasites. Of these, two were also examined for
gill parasites. Two specimens of the copepod
Dinemoura producta (Mu¨ller) were recorded from
the pelvic fin of a 122cm female, and a further six
specimens of this copepod were aggregated on the
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Table 1—Biological data recorded from sharks taken in commercial fisheries west of Ireland.
Species Sex Length
(
cm
)
Estimated age Clasper/nidamental
(
cm
)
Gonad weight
(
g
)
G.galeus M 139 16
F 119
M 140 16
F98
M 151 14
F 132 0.5
M70 4
M 152 15 185
M 161 14.5
M 147 15
M 156 15
M 151 14
M 152 16
M 141 15
M 162 15
M 103 7 103
M 133 15
M 141 10 14 123
M 148 12 15
M 149 10 14.5 80
M 152 11 15 185
M 162 15
H.griseus M 93 3.4 0
M 122 6 0
F 165 1 0
M79 4 0
F66 0 0
F92 0 0
F 86 0.5 0
M 101 5.2 0
M84 0 0
F 98 0.8 1
M 86 3.5 0
F85 0 0
M.asterias F 87 6 2.7 27
F 89 6 3.2 59
F 109 8 2.8 13
D.licha M 45 1.6 0
M 45 1.8 0.2
M 42 1.75 0
F39 0 0
F51 0 0
F40 0 0
F45 0 0
F42 0 0
F44 0 0
G.melastomus F 67 6 2.0 30
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dorsal surface of the left pelvic fin of a 250cm
female. One specimen of the isopod Natatolana
borealis (Lilljeborg) was found attached to the roof
of the mouth of a 233cm female, although it is most
likely that this was a post-mortem occurrence.
STARRY SMOOTH HOUND (MUSTELUS
ASTERIAS)
A total of three female smooth hounds was
examined (Table 1). All three individuals had
encapsulated eggs in their uteri. The fact that
embryos were not yet visible to the naked eye seems
to imply that encapsulation had occurred relatively
recently. Table 2 presents the number of eggs and
oocytes recorded from the three specimens. The
87cm specimen had remains of crustacean material
in its stomach, but they were too fragmented for
identification. The 89cm specimen contained one
squat lobster, Galathia squamifera Leach (6.1g), and
the remains of a swimming crab, Liocarcinus depurator
(L.) (3.0g).
Four parasite species were recorded. Two
smooth hounds were infected with an
indeterminate species of the nematode Proleptus:
two were found in the stomach lumen of the 89cm
specimen and three in that of the 109cm specimen.
An indeterminate species of the cestode
Anthobothrium was the only other intestinal parasite
recorded. Four of these parasites were recovered
from the spiral valve of the 87cm specimen and two
from the spiral valve of the 109cm specimen. Two
species of gill parasite were recorded: the copepod
Kroyeria lineata van Beneden, and the monogenean
Erpocotyle laevis Beneden and Hesse. A single
specimen of the latter was recorded from the 109cm
specimen, while nine E.laevis specimens and one of
K.lineata were recorded from the 89cm and 109cm
specimens respectively. The specimens of K.lineata
recovered are of interest in that their form is much
smaller and highly transparent compared with
specimens recovered from blue sharks (Henderson
et al. 2002c).
Fig. 1—Size distribution of porbeagle sharks (Lamna nasus) taken in bottom-set
gill-nets in shelf-waters to the west of Ireland. The 90cm length-class indicates a
range of 90–99cm, the 100cm length-class indicates a range of 100–109cm, etc.
Table 2—Numbers of encapsulated eggs and
oocytes recorded from Mustelus
asterias. Multiple values indicate
distinct size classes.
Oocytes
(
meanTotal Encapsulated
eggslength diameter
)
(
cm
)
10109 9 (1.6cm)+4 (1.2cm)
87 10 1 (1.3cm)+2 (1.0cm)
10 (1.5cm)289
Fig. 2—Clasper lengths recorded from porbeagle sharks, Lamna nasus.
BLACKMOUTH DOGFISH (GALEUS
MELASTOMUS)
The estimated age and measurements recorded from
this single individual are presented in Table 1.
Unfortunately, the ovary did not preserve well, and
no measurements could be taken. The uteri
contained five fully-formed egg-cases. The only
parasite recorded was the copepod Eudactylina similis
(T. Scott), four specimens being recovered from the
gills. No food items were recorded.
SIX-GILL SHARK (HEXANCHUS GRISEUS) AND
KITEFIN SHARK (DALATIAS LICHA)
The only food recovered from the six-gill sharks
was 1.3kg of cetacean material found in the stomach
of the 165cm specimen. Four of the eight kitefin
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sharks contained digested teleost material, while
three contained crustacean remains. However, the
material was digested beyond identification.
Three species of parasite were recorded from
the twelve six-gill sharks examined: the nematode
A.simplex L
3
and the cestodes Phyllobothrium dohrni
(Oerley) and P.sinuosiceps Williams. Anisakis
simplex L
3
was recorded from three sharks,
occurring in the stomach and spiral valve and
encysted in the stomach wall. Individual cestodes
were recovered from the anterior of the spiral
valve in each of three sharks.
Two kitefin sharks contained nematodes in
the stomach lumen but all three parasites were in
poor condition. While it appears that there may
have been three separate species, only one could
be identified, i.e. A.simplex L
3
. One of the
remaining nematodes also appeared to be a larval
anisakid (a larval tooth was evident): based on the
morphology of the head it may have possibly been
a species of Raphidascaris.
DISCUSSION
As discussed by Francis and Mulligan (1998),
growth rates in tope (G.galeus) from various parts
of the world seem to vary, a problem compounded
by the use of different ageing techniques in studies.
Sectioned centra were employed during the
present study, which greatly increased the clarity of
distal growth bands when compared with whole
stained centra (A.C. Henderson, pers. obs.), so it
seems unlikely that the present specimens were
under-aged to the degree that they were by
Moulton et al. (1992), as reported by Francis and
Mulligan (1998). The present study showed larger
sizes at age than studies of tope from southern
Brazil (Ferreira and Vooren 1991) and from New
Zealand waters (Francis and Mulligan 1998): if,
indeed, age determination was accurate during the
present study, the larger sizes at age indicate that
tope may display a faster growth rate in the
north-eastern Atlantic than in other areas.
Capape´and Mellinger (1998) report that the
reproductive organs of female tope display
increased development from 100cm onwards. It is
therefore surprising that the reproductive tract
of the 132cm individual examined appeared
immature. Tope are known to commonly prey on
fish (as found here) and cephalopods (Whitehead et
al. 1989), and the octopus E.cirrhosa found in this
study has been previously recorded from this shark
in the north-eastern Atlantic (Ellis et al. 1996).
According to Compagno (1984), the size at
birth of the porbeagle, Lamna nasus,is60–75cm,
with specimens in the north-eastern Atlantic
attaining sizes up to 300cm (Gauld 1989). The
determination that the 100cm specimen was one
year old is in close agreement with Aasen (1963)
for porbeagles in the north-western Atlantic.
Unfortunately, no comparable data exist for the
north-eastern Atlantic. Females do not mature
until at least 200cm (Aasen 1961), as was also
found here. Relative clasper lengths are similar to
those recorded by Aasen (1961), which indicates
that size at maturity is similar to elsewhere in the
North Atlantic. Few published data exist on the
biology of the porbeagle, but it is known to
commonly prey on a wide variety of small
schooling fishes, as well as squid (Compagno 1984;
Vas 1991). The specimen in the present study is
unusual in that it contained only crustacean and
annelid material, although it is possible that small
invertebrates are more important in the diet of
younger porbeagle. The occurrence of annelid
worms indicates bottom feeding, evidence of
which was also found by Gauld (1989).
From the age determination for the smooth
hounds examined during the present study, the
growth rate of this species may be slower than
previously thought. Francis (1981) calculated a
growth coefficient, K, for this species using
existing data on the length of the gestation period,
the size at birth, and the maximum observed
size—a technique first proposed by Holden
(1974). However the growth bands observed here
indicate this approach may have resulted in an
overestimate of K, and the age at size is almost
twice that predicted in the Von Bertalanffy
equation proposed by Francis (1981).
The gestation period of this species is reported
as twelve months by Compagno (1984), with
parturition occurring in summer (Kennedy 1969;
Wheeler 1969; Compagno 1984). This in-
formation implies that ovulation must also take
place in summer, so it is therefore surprising that
the three specimens examined during the present
study all contained recently fertilised ova. Without
knowing the spermatozoa-storing capabilities of
the oviducal-gland of the species, little can be
inferred as to when mating occurs. However, the
present results clearly indicate that the breeding
season begins as early as spring. The number of
fertilised ova in the uteri of the 87cm and 109cm
specimens falls within the fecundity range reported
for the species (Compagno 1984; Vas 1991);
however, the number of fertilised ova in the uteri
of the 89cm specimen is very low. In all three
specimens, the size of the oocytes indicates that
ovulation for the present season was complete,
which would suggest that fertilised ova were
possibly aborted during capture. Smooth hounds
feed primarily on crustaceans and invertebrates
(Compagno 1984), and both Galathia and
Liocarcinus have been previously recorded from
smooth hounds in the north-eastern Atlantic (Ellis
et al. 1996).
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The estimated age of the blackmouth dogfish
agrees with results obtained by Correia and
Figueiredo (1997) for specimens in Portuguese
waters. Little information exists on the
reproductive biology of this shark, but Compagno
(1984) noted that females mature between 39cm
and 45cm and may have up to thirteen eggs
present in the oviducts. The current specimen may
therefore not yet have formed a full complement
of egg-cases. Wheeler (1969) notes that egg-cases
ready for extrusion are abundant off the
south-western coast of Ireland in early July.
However, the current specimen was captured in
spring, indicating that either egg-laying may begin
earlier in the year than previously thought or the
process of forming a full complement of egg-cases
may take a considerable amount of time.
The virtual absence of gonad tissue from the
six-gill and kitefin sharks is indicative of wholly
immature animals: this agrees with Berrow’s
statement that the majority of six-gill sharks taken
in Irish waters are immature (Berrow 1994a). The
six-gill shark is commonly reported as being
mainly piscivorous (Compagno 1984; Vas 1991).
However, Ebert (1994) also recorded mammalian
remains from six-gill sharks off southern Africa,
indicating that mammals may also be important
prey. The scientific literature reports that the
kitefin shark is mostly piscivorous (Wheeler 1969;
Whitehead et al. 1989), but Compagno (1984) and
Vas (1991) have also reported crustacean material
from this species.
Many of the parasites recorded here are
commonly recorded from their respective hosts
(see Yamaguti 1959, 1961; Williams 1960; Scott
1963; O’Riordan 1966). However, certain species
are of interest. The occurrence of the nematode A.
simplex L
3
in H.griseus appears to be a new host
record, at least for the north-eastern Atlantic.
Similarly, this parasite has not been previously
reported from any of the other species for which it
was recorded here. Additional host records are K.
lineata on M.asterias, and E.similis on G.
melastomus. The latter record is also interesting
because it is the first occurrence of this parasite on
a non-rajid host.
Dawes (1947) reported the occurrence of E.
laevis on M.asterias from Plymouth, but apart from
occurring on this host elsewhere on the south coast
of England (A.C. Henderson, pers. obs.), there are
no further records around Britain and Ireland. The
current record therefore extends the known range
of this parasite. This is also the case for its tope
parasitising congener E.canis, which has hitherto
not been reported anywhere in British or Irish
waters, the most northerly record being that for
the French and Belgian coasts (Dawes 1947). The
occurrence of the isopod N.borealis is also
interesting. This species is generally regarded as a
scavenger, feeding on both dead and dying fish,
particularly those trapped in nets (Keable and
Bruce 1997). Berrow (1994b) recorded N.borealis
feeding on a variety of fish caught in bottom-set
gill-nets off the Irish coast, including some shark
species, but the present record appears to be the
first instance involving L.nasus.
ACKNOWLEDGEMENTS
The authors would like to thank the Dingle fishing
fleet for providing specimens, and also the
management and staff of O’Cathain Iasc Teo for
allowing examination of their landings. We would
also like to thank Dingle Oceanworld for
providing dissection facilities. The expertise of
Mark Holmes (Natural History Museum, Dublin)
in confirming the identity of the Kroyeria lineata
specimens is also gratefully acknowledged.
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The subvention granted
by the National
University of Ireland,
Galway, towards the
cost of publication of
papers by members of
its staff is gratefully
acknowledged by the
Royal Irish Academy.
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