Occurrence of Eggs of Bass in Ireland

J. mar. bioi. Ass. U.K. (I968) 48, 585-592
Printed in Great Britain
OCCURRENCE OF EGGS OF BASS
DICENTRARCHUS LA BRA X ON THE
SOUTHERN COASTS OF IRELAN D
By MICHAEL KENNEDY AND PATRICK FITZMAURICE
Inland Fisheries Trust, Dublin
(Text-figs. 1-2)
During April-June, 1967, tow-netting for bass eggs was carried out at four
places on the Irish coast. Small numbers of bass eggs were obtained at two of
these places, namely the Splaugh Rock, near Greenore Point, Co. Wexford and
Youghal Harbour, Co. Cork. They were taken near the surface, in water of the
salinity range 30-34'5 %0' and of surface temperature range 10'5-13'2 °C.
The Irish material was larger than described by Bertolini (1933) from the
Mediterranean, but smaller than described by Jackman (1954), who hatched eggs
spawned in the Plymouth Laboratory Aquarium and described the larvae.
INTRODUCTION
Bass have spawned in the aquarium at Naples, and their eggs have been
found in plankton in the Mediterranean (Raffaele, 1888; Bertolini, 1933).
Bass have also spawned in the aquarium at Plymouth, and their eggs and the
resulting young stages have been described by Jackman (1954).
Hitherto, however, the eggs of bass do not appear to have been found on
the coasts of Britain or Ireland. The only young stages recorded were 14 post-
larvae from 4 to 7'5 mm long, taken in the Eddystone area in the years 1925,
1926, 1931 and 1933, with dates of capture between 17 March and 4 June
(Russell, 1935).
As part of a programme of research on the biology of bass in Irish waters,
tow-netting was carried out in 1967 at the Splaugh Rock off the Wexford
coast; at Dungarvan, Co. Waterford; at Youghal, Co. Cork; and at Baltimore,
Co. Cork (Fig. 1). The Splaugh, Dungarvan and Youghal are places where
bass 'shoal'; that is, frequently gather in shoals to attack sand-eels, herring
fry or sprats at the surface. It was felt that places where bass assemble in
numbers to prey on bait fish might also be places where they spawn. Bass do
not usually shoal at Baltimore, but they frequent the estuary of the Ilen River
flowing into Baltimore in fair numbers.
586 MICHAEL KENNEDY AND PATRICK FITZMAURICE
PROPERTIES OF WATER IN WHICH EGGS OCCURRED
Small numbers of bass eggs were taken in Youghal on several occasions
between 8 May and I June 1967; and small numbers of bass eggs were taken on
both days (1-2 June) on which tow-netting was carried out at the Splaugh
Rock. No bass larvae or post-larvae were taken.
Splaugh Rock
The Splaugh Rock is a reef! mile south-east of Greenore Point on the east
coast of Co. Wexford. It is situated in an area of shoals inside the 5-fathom
line. Tides are swift. Salinity determinations were as follows:
I June 1967
2 June 1967
Neap tides; early ebb
Late flood
32'1%0
34'5%0
o
I
MILES
__ SPLAUGH
ROCK
50
i
Fig. 1. Places on the Irish coast where bass eggs were sought in 1967,
Youghal
Youghal Harbour is the mouth of the estuary of the Cork Blackwater, which
is tidal to Cappoquin, 14 miles from the sea.
Bass occur in large numbers in Youghal Bay, in Youghal Harbour and in
the estuary above it. They often 'shoal' in the vicinity of the Bar Rocks, in
the Bay, about a mile off the Harbour mouth; they also' shoal' in the narrows
in the harbour itself.
Tides are strong in the Bay, and in the Harbour. In the Harbour, under
normal conditions of flow in the Blackwater, a surface current of brackish
water continues seawards for up to an hour after low water. Mter that, the
current reverses, and there is a strong run of tide up the Harbour from the sea
at all depths from channel bed to surface.
EGGS OF BASS OFF SOUTHERN IRELAND
Salinity determinations were not made for all tow-net hauls, but an indica-
tion of the surface salinity pattern is given by the following determinations:
Salinity
Date
Station Location Tide
(%0)
30 May 1967
2 ! mile from open sea
Neaps; H.W. 33'7
4h of ebb
24'35
L.W.
14'35
8 June 1967
4
3t miles from open sea
Springs; half ebb 31'1
L.W,
10'2
Tow-netting results
Table I gives details of tow-net hauls in which bass eggs occurred. The net
was 45 cm in diameter and had 25 meshes per centimetre. It was fished from
a small boat either slowly stemming the tide, or anchored in the tide-way,
according to conditions. Hauls series 8,9 and 13 were from the surface down
to 2 m; the remaining hauls were all at the surface. Each series of hauls, with
a few exceptions, represents a total of 1-2 h actual fishing-the net being left
out for 5 min to an hour at a time, according to the abundance or scarcity of
plankton.
At the Splaugh Rock, 18 pelagic fish eggs were taken in three hauls. Of
these, six were bass eggs occurring in two hauls. At Youghal, 124 eggs were
taken in 16 hauls, including 25 bass eggs in six hauls. At Youghal, the bass
eggs were obtained only at stations 2 and 3 (! mile and It miles respectively
from the open sea). None occurred in the Bay (Station I) nor at the north end
of the Harbour, 3t miles from the sea (Station 4). No bass eggs occurred in
hauls made during the latter part of the ebb tide.
EGGS AND LARVAE OF BASS
The bass egg taken in haul 17 at Youghal on 18 May contained an advanced
embryo estimated to be about 3 days old. All the other bass eggs taken at
Youghal and at the Splaugh Rock were recently fertilized. They were in an
early stage of cell division and contained mostly two, sometimes three, oil-
globules which had not yet fused together.
Some of the eggs were preserved on capture; others were brought back to
the laboratory and hatched. The surface temperature was recorded when the
eggs were obtained, and the water temperature in the laboratory was also
recorded. However, between capture and arrival in the laboratory, the eggs
were in uninsulated containers in boats, hotels and cars, so no incubation
temperature can be given,
The bass eggs taken at Youghal ranged in diameter from 1'20 to 1'386 mm
in diameter. When a single oil-globule was present or was formed during
development, its diameter ranged from 0'36 to 0'42 mm. An egg of 1'20 mm
had two globules 0'32 and 0'20 mm across; an egg of 1'275 mm had three
globules measuring 0'30, 0'27 and 0'20 mm; and an egg of 1'22 mm had two
Vl
00 00
TABLE 1. DETAILS OF TOW-NET HAULS IN WHICH BASS EGGS OCCURRED
Fish eggs
:s:
Hauls Temp.
~------~-------~Larvae or post-larvae - series
Date Station Tide (0C) Bass Others (nos. in parentheses) (nos. in parentheses)
(')
::r::
Youghal
:»
tI1
8
8 May 2 Springs, flood 10'5
6
Phrynorhombus regius (I) Semi-metamorphosed
r-<
Platichthys flesus (I)
~
9
8 May
3
Late flood - 2
Nil
'Gadus' spp. (3); Liparis tI1
montagui (I)
Z
13
9 May 2 Flood II'O 7 Microstomus kitt (I)
Semi-metamorphosed
Z
tI1
Platichthys flesus (3);
tj
semi-metamorphosed Solea
><::
solea (I); Clupeoids (2);
:»
'Gobius' sp (I)
Z
17
18 May 2 Neaps, H.W. to·S
I
Scomber scombrus (I); Sprattus
'Gadus' spp, (7); Rocklings (2) tj
sprattus (I); Rocklings (10);
"d
Phrynorhombus regius (5);
:»
Ctenolabrus rupestris (I I)
>-l
19
20 May 2 Neaps, H.W. 13'2
I
Rocklings (21); Phrynorhombus Nil
:-0
- regius (3); Ctenolabrus
(')
rupestris (3);
? (I)
~
23
I June 2
Late flood II·8 8
Rocklings (3); Phrynorhombus Clupeoids (2) 'TI
regius (2); Ctenolabrus
-
>-l
rupestris (2)
N
Splaugh Rock
:s:
26
I June Neaps, early ebb
- 3
Rocklings (2); Trachinus vipera
Callionymus sp (5)
:»
c (3); Phrynorhombus regius (I);
:-0
Ctenolabrus rupestris (I)
-
(') 28
2 June
-- Late flood]
12 3 Rocklings (5)
'Gadus' spp. (2); 'Gobius' spp. tI1
(3); symmetrical Solea solea (I);
symmetrical Buglossidium luteum (I)
EGGS OF BASS OFF SOUTHERN IRELAND
globules measuring 0'40 and 0'175 mm. The eggs taken at the Splaugh Rock
ranged from 1'20 to 1'30 mm in diameter.
The Irish eggs were thus bigger than those from the Mediterranean (1'15
to 1'16 mm diam., Bertolini, 1933); but were smaller than those from the
Plymouth Aquarium, (1'26-1'51 mm diam., Jackman, 1954). They were of
similar size to the unfertilized eggs of bass from the Plymouth Aquarium
described by Holt & Byrne (1898) (1'25-1'34 mm diam.). In appearance the
eggs were as described by Bertolini and Jackman except for their size dif-
ference. They had a small perivitelline space, an undivided yolk, and a
capsule without any special features. The oil-globules fuse together to form
a single globule during development. The oil-globule is somewhat yellowish.
The banding, which is a feature of the larva, is obvious in the embryo before
hatching. Eggs, which were in an early stage of development when collected,
hatched in 3-4 days.
Fig. 2A shows a larva less than a day old, drawn when alive and anaesthe-
tized at 4 p.m. on 12 May 1967. It was then 4'45 mm long. It had hatched
during the previous night. When examined at 10 a.m. that morning, it had
measured 4'25 mm. It came from an egg in the size-range 1'20-1'30 mm.
From Bertolini's figures and descriptions, it differs chiefly in that the coloured
pigment, by reflected light, is more a brownish yellow than the clear yellow
of Bertolini's specimens. By transmitted light, the coloured pigment is brown,
not yellow. At lower magnifications, the combination of black and yellow-
brown pigments gives a grey-green effect to the oil-globule and to the banded
areas of the body. This is a feature also of the advanced embryo.
Fig. 2B shows a more advanced larva, 4 days old, drawn alive on 16 May.
It probably came from a smaller egg than specimen 2A, as it measured only
4'35 mm. The eyes were as yet unpigmented. The anterior vertical band on
the body, over the yolk sac, had faded out on the sides, but was represented
by dark touches dorsally and ventrally. Again, at high magnifications, the
coloured pigment was brownish yellow rather than clear yellow by reflected
light. Some of the melanophores-on the snout; anteriorly on the yolk sac;
along the top of the gut and the ventral edge of the tail; at the vent; and in
the band over the vent-were still black, but the melanophores elsewhere
were becoming brownish black. Some, but not all, of the melanophores were
conspicuously stellate. There was some diffused brown pigment, the colour
of pale milk chocolate, on the head and about the eye. At lower magnifications,
by reflected light, the pigment now appeared amber-brown where only
coloured pigment occurred, and brown-grey (rather than grey-green) where
dark and coloured pigments occurred together.
This example was less advanced than a 3-day-old specimen figured by
Bertolini; but this may be attributable to the water temperature fluctuating
in the laboratory during the period 12-16 May between 12 and 15'5 DC, the
mean temperature being 13'5 dc.
590 MICHAEL KENNEDY AND PATRICK FITZMAURICE
Fig, 2C shows a larva 3 days old, drawn alive on 6 June 1967, This hatched
on 3 June, from an egg collected on Youghal on I June (hauls series 23).
Structurally it was very similar to the 3-day-old larva, 4'5 mm long, figured
by Bertolini, and the arrangement of the pigment was very similar-though,
as in other Irish examples, the coloured pigment was not clear yellow. In this
~
~'~': ••~.. ' -ct~~'~~"'J~<~
c
Fig, 2, Larval bass; A, larva hatched from egg, less than a day old, 4'45 mm; B, 4 days old,
reared at a mean temperature of 13'5 °C, length 4'35 mm; c, larva 3 days old, reared at a
mean temperature of 17'5 °C, length 5'0 mm,
specimen, the pigment was greenish brown. It was bigger than the Mediter-
ranean larva, being 5'0 mm long. The eye was pigmented, with a bluish
metallic sheen, and the larva was swimming actively and right way up. Its
more rapid development, as compared with example 2B, is probably due to
higher temperatures. The water temperature in the laboratory during the
3 days 3-6 June fluctuated between 16'5 and 19°C, the mean temperature
being 17·5 °C.
DISCUSSION
Examination of the gonads of Irish bass indicates a spawning period ex-
tending from A~ril to about mid-June; and some spawning may well take
EGGS OF BASS OFF SOUTHERN IRELAND
591
place as early as March in west Cork and Kerry. This would be consistent
with the capture of post-larval bass off Plymouth in mid-March (Russell,
1935). During the present Irish investigations, eggs were obtained during the
period 8 May to 2 June 1967; but no doubt a more systematic sampling
would disclose their presence over a longer period of the year. The surface
temperatures when Irish bass eggs were obtained varied between 10'5 and
13'2 0c.
Spawning off Ireland, and presumably in English waters also, is later than
in the Mediterranean, where it occurs from January to March. This is pre-
sumably attributable to differences in sea temperatures. No data are available
for seasonal surface temperatures in Youghal Harbour, but sea temperatures
along the Irish coast can be deduced from the monthly charts published by
LC.E.S. Sea temperatures in the Bay of Naples from January 1957 to January
1958 have been given by Hapgood (1960). Details are given in Table 2.
TABLE 2, TEMPERATURES OFF SOUTHERN IRELAND AND NAPLES
Range of observed
temperatures
Bay of Naples
(Jan, 1957-Jan, 1958)
(0C)
15'°1-15'°7 (8 Jan, 1958)
13'0-13'78 (1957)
13'48-15'38 (1957)
15'2-16'17 (1957)
17'09-18'53 (1957)
20'74-25'79 (1957)
\
Splaugh Rock
(July 1955-
Dec, 1958)
(0C)
9-9'5
7-8'5
7'5-9'5
8-9'5
9'7-II
II'5-12'5
Youghal
(July 1955-
Dec, 1958)
(0C)
9-9'5
7'5-8'5
8-9'5
8'5-9'5
ro'5-II
12-13
Range of mean temeratures
A
Jan,
Feb,
Mar.
Apr,
May
June
The bass eggs taken at the Splaugh Rock were in the open sea, though close
to shore and in shallow water. At Youghal, bass eggs, as already noted, were
obtained only on the flood, at high water and on the early ebb. This suggests
that they were spawned in the Bay and were being swept into and out of the
mouth of the estuary by the strong tides. It is possible that some eggs might
drift into big eddies in the estuary where they would remain long enough to
hatch. The possibility of spawning in the estuary itself cannot be excluded on
the basis of the limited data at present available. All that can be said with
certainty at this stage is that the bass eggs which were obtained occurred in
water of fairly high salinity.
The eggs found associated with those of bass are of interest. While eggs of
rockling, which are ubiquitous, predominated, the other species most fre-
quently represented were Phrynorhombus regius, Ctenolabrus rupestris and
Trachinus vipera-species which live in inshore waters and whose eggs are
usually shed close to the shore. Eggs of species which normally spawn off-
shore were represented in the collections made at Youghal and the Splaugh
Rock only by two mackerel eggs, and one lemon sole egg.
592 MICHAEL KENNEDY AND PATRICK FITZMAURICE
The invertebrate plankton associated with the bass eggs was essentially
inshore in character.
The difference in size between the eggs of Irish bass and Mediterranean
bass is consistent with observations on many other species-that eggs tend to
be larger in waters of lower salinity and temperature. In the Bay of Naples,
where the diameter of bass eggs does not exceed 1'16 mm (Bertolini, 1933),
or at most 1'2 mm (Raffaele, 1888), the surface salinity in the early months
of the year varies between 36'82 and 38'01 parts per thousand (Hapgood,
1960). The Irish bass eggs, measuring from 1'20 to 1'386 mm, were taken in
waters in the salinity range 30'0-34'5 parts per thousand.
When adult bass 'shoal', with the gulls working over them, they are
feeding, not spawning. However, the present investigations have shown that
bass eggs are likely to be taken in places where 'shoaling' occurs. Spawning
probably takes place in a variety of scattered situations, some of which, at
least, are in inshore waters where there are swift tides. The tides probably
create the traffic patterns which result in bass becoming concentrated for a
time in such places and may also create conditions favourable for spawning.
It seems likely that some spawning takes place whenever temporary assemblies
of bass occur throughout a rather long spawning season.
The authors' thanks are due to their colleagues Desmond Brennan, Kevin Linnane
and Patrick Sheehy who participated in the tow-netting; to Dr Julie Fives of the
Zoology Department, University College, Galway, who identified the invertebrate
plankton; and to Jack O'Brien of Rosslare, John Casey of Dungarvan and Cecil Pratt
of Youghal, for their cooperation in the project.
Thanks are also due to Dr Alwyne Wheeler of the British Museum, Dr J. E. Smith
of the Marine Biological Association of the United Kingdom, and Dr P. Dohrn of
the Stazione Zoologica, Naples, for their help in tracing references to the young stages
of marine fishes and to hydrographical conditions.
REFERENCES
BERTOLINI, F., 1933. Serranidae; in Uove, Larve e stadigiovanili di Teleostei: Fauna u.
Flora Golf. Neapel, Monogr. 38, pp. 3IO-I3.
HAPGOOD,W., 1960. Hydrographic observations in the Bay of Naples (Golfo di
Napoli) January I957-January 1958 (Station Lists). Pubbl. Staz. zooI. Napoli;
Vol. 31, pp. 337-7I.
HOLT, E. W. L. & BYRNE, L. W., 1898. Notes on the reproduction ofteleostean fishes
in the south-western district. J. mar. bioi. Ass. U.K., Vol. 5, pp. 333-5.
JACKMAN, L. A. J., 1954. The early development stages of the bass, Morone labrax (L.).
Proc. zool. Soc. Lond., Vol. 124, NO.3, pp. 531-4.
RAFFAELE, F., 1888. Le uove galleggianti e Ie larve dei Teleostei nel Golfo di Napoli.
Mitt. zool. Stn Neap., Bd. 8, pp. 1-84.
RUSSELL,F. S., 1935. On the occurrence of post-larval stages of the bass Morone
labrax (L.) in the Plymouth area. J. mar. bioi. Ass. U.K., Vol. 20, pp. 71-2.