Not to be quoted without prior reference to the authors
Fisheries Research Services Internal Report No 17/03
NORTH SEA COD SPAWNING GROUNDS
P J Wright, F M Gibb, I M Gibb, M R Heath and H A McLay
September 2003
Fisheries Research Services
Marine Laboratory
375 Victoria Road
Aberdeen AB11 9DB
NORTH SEA COD SPAWNING GROUNDS
P J Wright, F M Gibb, I M Gibb, M R Heath and H A McLay
Fisheries Research Services, Marine Laboratory,
PO Box 101, 375 Victoria Road, Aberdeen, AB11 9DB
INTRODUCTION
The current discussions on recovery plans for cod stocks have highlighted the need for
updated information on spawning grounds. Some information can be derived from the
catches of mature adults in fisheries surveys, but more accurate information is provided by
egg surveys or surveys designed to determine the densities of spawning adults. However,
there has never been a comprehensive survey of eggs or spawning cod throughout the
North Sea and most survey data pre-dates the recent wide-scale decline of the stock.
Available information indicates that spawning takes place from December through to April,
offshore in waters of salinity 34-35 ‰ (Riley and Parnell, 1984). Around the British Isles
there is a tendency towards later timing with increasing latitude (Hislop, 1984; Brander,
1994a – Fig. 1). Only a few studies have sampled eggs or spawning fish throughout this
protracted season. There are some old reports of spawning in the autumn, but these are
probably not significant (Brander, 1994b).
This review summarises information on cod spawning grounds in the North Sea currently
available from i) compilations of historic data (Anon, 1971; Daan, 1978; ICES, 1994). ii)
various icthyoplankton and trawl surveys carried out between 1919 and 2003 and iii)
interviews with fishermen conducted in 2002.
DATA SOURCES
Spawning Adults
Information on spawning areas based on the occurrence of spawning adults during the early
part of the last century was presented by Graham (1934), West (1970) and Anon (1971).
More recent information is available from the ICES International Bottom Trawl Survey.
However, as the surveys were not designed with the intention of mapping spawning fish
distribution there are several limitations to the use of the data for this purpose. The timing of
the surveys in January-February covers only part of the spawning season of cod and thus
the importance of certain spawning areas may not be reflected by these surveys. The
survey takes a length based sub-sample of fish from hauls to produce sex, maturity, age
and length keys (SMALK). Only SMALK data collected since 1991 include reference to
spawning fish. Due to the nature of sub-sampling, it is not possible to derive annual indices
of spawning density distribution. The STEREO project (EC FAIR-CT98-4122) attempted to
generate a spatial distribution of the spawning areas based on the prevalence of spawning
fish in the decade 1991–2000. Spawning prevalence was estimated from the proportion of
years in which an ICES rectangle contained one or more spawning fish. Prevalence
therefore varied from zero (no years in which a spawning fish was taken) to unity (at least
one spawning fish captured in every year sampled).
1
North Sea Cod Spawning Grounds
Data on all pre-spawning and spawning cod caught by Scottish bottom trawl surveys in the
north west North Sea (IBTS) and sub-Area VIA have been collected as part of two EU
funded projects, STEREO in 1999 and METACOD (EC-Q5RS-2001-00953) in 2002 and
2003. It was necessary to combine pre- and spawning fish since the Scottish IBTS surveys
were found to precede the onset spawning north of 59°N. The recent low catch rates of cod
in surveys limit the usefulness of this information but these surveys do provide some
indication of the extent of spawning areas in recent years.
Information about cod spawning areas was also gathered as part of an investigation of
fishermen’s knowledge carried out by FRS and the University of Aberdeen in 2002. Twenty
five fishermen, skippers of demersal fishing vessels fishing in the northern North Sea and to
the west of Scotland were interviewed on a range of topics and asked about the location and
timing of cod spawning. Most commented generally and eight volunteered information on
cod spawning grounds.
Eggs
The pelagic eggs of cod typically hatch over a period of 2-3 weeks, depending on water
temperature (i.e. 12 days at an average temperature of 5.5ºC; Meek, 1924). The
unambiguous identification of fish eggs of several species remains problematic; cod,
haddock and whiting spawn eggs that are all of similar appearance and over-lap in size
range. The egg diameter range is given by Russell (1976) as 1.16-1.89 mm for cod eggs,
1.2-1.7 mm for haddock and 0.97-1.32 mm for whiting. Only when the eggs have reached
developmental stage V can they be confidently identified based upon developing patterns of
pigmentation in the embryos. Previous egg surveys have been hampered by this limitation.
A genetically based method for distinguishing gadoid eggs has now been developed
(Taylor et al., 2001). However, data are available from several egg surveys, particularly in
the southern and central North Sea, including those conducted by Harding et al. (1974),
Daan (1978; 1981), Heessen and Rijnsdorp (1989) and Van der Land et al. (1990).
Brander (1994a) presents the results of egg and larval surveys carried out in the westcentral areas, the Southern Bight and the German Bight and around the British Isles.
Around the coast of Scotland, icthyoplankton surveys have been undertaken by
Saville (1959) in the 1950s and Heath et al. (1994) in 1992. The Saville surveys were
primarily concerned with haddock but Raitt (1967) re-analysed the data and presented maps
for cod.
SPAWNING DISTRIBUTIONS
Evidence from plankton surveys and ripe females suggest that spawning cod occur
throughout much of the North Sea. However, there appear to be several spatially
segregated concentrations of spawning fish and eggs and the importance of these have
changed over time.
Based on the occurrence of spawning fish, Graham (1934) identified spawning areas in the
central North Sea to the west and north of Dogger Bank, and to the south east and south
west of the Fladden Ground. The ICES North Sea Roundfish Working Group on North Sea
Cod (Anon., 1971) noted other sites at Silver Pit, north east Dogger and the German and
Southern Bight, while West (1970) reported several mostly coastal sites around Scotland
(see Fig. 2).
2
North Sea Cod Spawning Grounds
The prevalence of spawning cod in the 1990s indicated consistent use of three areas: the
German Bight, to the north-west of Dogger Bank, and in a band from the Moray Firth to the
north-east of Scotland (see Fig. 3). High spawning prevalence was generally consistent
with the spawning areas previously reported (Anon., 1971; Brander, 1994a). However, a
relatively low spawning prevalence was found around Shetland and Orkney, which is not
consistent with earlier reports and might be a sampling artefact arising from the late
spawning times in these areas.
Data from Scottish trawl surveys in 1999, 2002 and 2003 are presented in Figure 4. Data
on pre-spawning cod are included because the North Sea survey takes place in the early
part of the spawning season. No pre- or spawning cod were found in several of the ICES
rectangles previously regarded as spawning areas by the ICES demersal group
(Anon, 1971). In particular, very few spawning cod were found in the Forties region and off
the east coast of Scotland.
Fishermen reported that cod in spawning condition can be found dispersed over relatively
large areas of the northern North Sea, and they were keen to dispel the notion that cod
always congregate in large shoals to spawn. Some, however, identified particular areas as
important spawning grounds. These are shown in Figure 5 which indicates two main areas,
one in the Norwegian sector in the vicinity of the Viking and Bergen banks and second west
of Orkney and Shetland which includes Papa Bank. The north east coast of Scotland
between Fraserburgh and Banff was identified by several fishermen as a traditional cod
spawning area, but one which is no longer used by the fish. They commented that stock in
this area had been fished out in the early 1990s by the seine netters and not recovered
since. One skipper identified the Barnacle, a ground north of Flamborough, as an area
previously known to fishermen as important for spawning cod.
In relation to the spawning period, fishermen said that cod spawn between January and
April, at present mainly in February and March. Several noted that there seems to be a
trend for cod and other gadoid species to spawn later in the year than 10 years ago.
Fishermen said that the fish tended to spawn earlier in more southerly waters and that the
last spawning cod were located further north. One Shetland fisherman said that the cod
spawn earlier on the west side of Shetland than on the east side.
The northern North Sea icthyoplankton surveys from the 1950s found high egg
concentrations in the important spawning areas reported by Graham (1934), as well as the
areas reported by West (1970) in March off the Butt of Lewis, west of Orkney and Shetland
and off the Moray and Tay Firths (Saville, 1959; Raitt, 1967; Fig. 6a). Cod eggs were more
abundant by April and cod larvae were common across the survey area. By May the
occurrence of cod eggs and larvae was much reduced. The survey by Heath et al. (1994) in
1992 indicated that the distribution of cod eggs was far less extensive with the notable
absence of concentrations south of the Moray Firth (Fig. 6b). Positively identified (late
stage) cod eggs were only found around Bergen Bank and Orkney. However, this survey
was conducted in March-April so some early spawning could have been missed.
Nevertheless, a Norwegian plankton survey conducted in March 2003 suggests a similar
picture with cod eggs being extremely scarce, and restricted to scattered areas of the
central and northern North Sea and off the Danish coast (ICES, 2003).
Brander (1994b) reported a peak in egg concentrations between Flamborough on the north
east English coast and the southwestern flank of Dogger Bank, and another in the Southern
Bight off the Dutch coast. Other investigations show further cod spawning at the
southeastern edge of the Dogger Bank towards the Horns Reef area (Daan, 1978; Heessen
and Rijnsdorp, 1989; see Fig. 7). These parts of the southern North Sea were apparently
3
North Sea Cod Spawning Grounds
the most important spawning sites in the 1970s and 1980s. Based on surveys conducted by
Harding et al. (1987), cod spawning also occurs off the NE coast of England. Prior to the
1970s, significant spawning was found in the vicinity of Great Fisher Bank and Aberdeen
Bank (Graham and Carruthers, 1925; Raitt, 1967). Daan (1978) found no evidence of a
contraction in the central and southern North Sea spawning areas from his analysis of data
up to the 1970s. In egg surveys of the southern and eastern North Sea in 1987 and 1988,
Heessen and Rijnsdorp (1989) found a similar spatial distribution of eggs as seen in the
early 1970s (Daan, 1978), with the exception that in 1987 they did not observe the centre of
high egg production in the eastern English channel. No information is available on the
recent extent of spawning grounds in the southern, central and north east North Sea.
POPULATION STRUCTURING
A recent genetic survey of cod in European continental shelf waters using micro-satellite
DNA detected significant fine scale differentiation suggesting the existence of at least 3-4
genetically divergent cod populations, resident in the northern North Sea off Bergen Bank,
within the Moray Firth, off Flamborough Head and within the Southern Bight
(Hutchinson et al., 2001). As is typical of marine fishes, the level of detectable genetic
differentiation among these populations was low, which is to be expected from the large
population sizes and high dispersal potentials. The biological significance of such low
differentiation is often questioned in part because the temporal stability of the observed
patterns is generally unknown and where different studies exist these have sometimes
provided conflicting results. However, long-term stability in population structure has been
demonstrated in north west Atlantic cod (Ruzzante et al., 2001) and there are now other
examples of fine scale population structuring of cod in other regions (Jonsdottir et al. 2001;
Beacham et al., 2002). If the findings of Hutchinson et al., 2001 reflect distinct populations
they indicate significantly more genetic sub-structuring of cod within European continental
shelf waters than had previously been detected. Interestingly, this new genetic evidence is
largely consistent with the limited movements suggested by historical tagging studies
(Anon, 1971). Clearly, this has implications for the way we view historic changes in cod
spawning areas and indeed population dynamics. Further investigations into population
structuring and dynamics, including the temporal stability of genetic differentiation are being
made by Hutchinson and as part of the EU-funded METACOD project.
SUMMARY
Cod spawn throughout much of the North Sea but spawning adult and egg survey data and
fishermen’s observations indicate a number of spawning aggregations.
It is not possible to quantify long-term changes in the use of spawning grounds because of a
lack of comprehensive survey data on eggs or spawning adults, and the lack of suitable
sampling within ICES bottom trawl surveys.
However, the limited data available do suggest a contraction in significant spawning areas,
beginning with the loss of sites at Great Fisher Bank and Aberdeen Bank by the 1980s, and
more recently other coastal spawning sites around Scotland and in the Forties area.
The North Sea cod stock may comprise a number of reproductively isolated populations,
although further corroboration is needed.
4
North Sea Cod Spawning Grounds
REFERENCES
Anon, 1971. Report by the North Sea Roundfish Working Group on North Sea Cod.
ICES/Demersal Fish Comm F:5:1-35. International Council for the Exploration of the
Sea, Copenhagen.
Beacham, T.D., Brattey, J., Miller, K.M., Le, K.D. and Withler, R.E. 2002. Multiple stock
structure of Atlantic cod (Gadus morhua) off Newfoundland and Labrador determined
from genetic variation. ICES Journal of Marine Science, 59, 650-665.
Brander, K.M. 1994a. The location and timing of cod spawning around the British Isles.
ICES Journal of Marine Science, 51, 71-89.
Brander, K.M. 1994b. Spawning and life history information for North Atlantic cod stocks.
International Council for the Exploration of the Sea. Rapport des recherches
Collectives, 205.
Daan, N. 1978. Changes in cod stocks and cod fisheries in the North Sea. Rapp. P.-v.
Reun. Cons. Int. Explor. Mer., 172, 39-57.
Daan, N. 1981. Comparison of estimates of egg production of the Southern Bight cod stock
from plankton surveys and market statistics. Rapp. P.-v. Reun. Cons. Int. Explor.
Mer., 178, 242-243.
Graham, M. 1934. Report on the North Sea cod. Fishery Invest. Lond. Ser., 2, 13, 1-160.
Graham, M. and Carruthers, J.N. 1925. The distribution of pelagic stages of the cod in the
North Sea in 1924 in relation to the currents. Fishery Invest. Lond. Ser. 2, 8(6), 1-31.
Harding, D., Nichols, J.H. and Riley, J.D. 1974. Preliminary estimates of egg production
and mortality in the 1968 North Sea cod spawning. ICES CM 1974/F: 21 (mimeo).
Harding, D. and Nichols, J.H. 1987. Plankton surveys off the north-east coast of England in
1976: an introductory report and summary of results. Directorate of Fisheries
Research, Fisheries Research Technical Report, 86, 55 pp.
Heath, M., Rankine, P. and Cargill, L. 1994. Distribution of cod and haddock eggs in the
North Sea in 1992 in relation to oceanographic features and compared with
distributions in 1952-1957. ICES Marine Science Symposium 198: 244-253.
Heessen, H.J.L. and Rijinsdorp, A.D. 1989. Investigations on egg production and mortality
of cod (Gadus morhua L.) and plaice (Pleuronectes platessa L.) in the southern and
eastern North Sea in 1987 and 1988. ICES Symp. on the Early Life History of Fish,
Bergen (Norway), 3-5 Oct 1988.
Hislop, J.R.G. 1984. A comparison of reproductive tactics and strategies of cod, haddock,
whiting and Norway pout in the North Sea. In: Potts, G.W. and Wootton, R.J. (eds),
Fish reproduction: strategies and tactics, Academic Press, London.
5
North Sea Cod Spawning Grounds
Hutchinson, W.F., Carvalho, G.R. and Rogers, S.I. 2001. Marked genetic structuring in
localised spawning populations of cod Gadus morhua in the North Sea and adjoining
waters, as revealed by microsatellites. Marine Ecology Progress Series, 223, 251260.
ICES, 1994. Spawning and life history information for North Atlantic cod stocks.
International Counc. for the Exploration of the Sea, Copenhagen (Denmark).
Recruitment Processes Working Group. Cod and climate Change Study. ICES cooperative research report. Copenhagen, 1994, 150 pp.
ICES, 2003. PGEGGS (Planning Group on North Sea Cod and Plaice Egg Surveys) report,
IJmuiden, 24-26 June 2003. C Fox (chair).
Jonsdottir, D., Danielsdottir, A.K., Naevdal, G. 2001. Genetic differentiation among Atlantic
cod (Gadus morhua L.) in Icelandic waters: temporal stability. ICES Journal of
Marine Science, 58, 114-122.
Meek, A. 1924. The development of the cod. Fishery Invest., Lond., Ser.2, 7(1). 26pp.
Raitt, D.F.S. 1967. Cod spawning in Scottish waters. Preliminary investigations. ICES CM
1967/F:29 (mimeo).
Riley, J.D. and Parnell, W.D. 1984. The distribution of young cod. pp. 563-580. In: E.
Dahl, D.S. Danielssen, E. Moksness & P. Solemdal (ed.) The Propagation of Cod
Gadus morhua L. Flodevigen rapportster, 1, Institute of Marine Research, Arendal.
Russell, F.S. 1976. The Eggs and Planktonic Stages of British Marine Fishes. Academic
Press, London. 524 pp.
Ruzzante, D.E., Taggart, C.T., Doyle, R.W., and Cook, D. 2001. Stability in the historical
pattern of genetic structure of Newfoundland cod (Gadus morhua) despite the
catastrophic decline in population size from 1964 to 1994. Conservation Genetics 2:
257-269.
Saville, A. 1959. The planktonic stages of the haddock in Scottish waters. Mar. Res., 1959
(3), 1-23.
Taylor, M.I., Fox, C.J., Rico, I. and Rico, C. 2001. Species-specific TaqMan probes for
simultaneous identification of cod (Gadus morhua L.), haddock (Melanogrammus
aeglefinus L.) and whiting (Merlangius merlangus L.). Molecular Ecology Notes 2:
599-601.
Van der Land, M.A., Heesen, H. and Rijnsdorp, A. 1990. The result of the 1989 egg
surveys for cod and plaice. ICES CM 1990/G:27.
West, W.Q.-B. 1970. The spawning biology and fecundity of cod in Scottish waters. Ph.D.
Thesis, Aberdeen University.
6
Figure 1.
Abundance of stage I cod eggs and fitted normal distributions with mean dates
of spawning and standard deviations. For the English Channel cod eggs of all
stages are amalgamated because abundance of stage I eggs was too low to
give an estimate (Brander, 1994a).
Figure 2.
Cod spawning areas in the North Sea, assimilated data by ICES
(Anon, 1971).
62
61
60
59
LING
BANK
58
FORTIES
57
FISHER
BANK
56
N.E.
DOGGER
55
FLAMBOROUGH
CLAY
DEEP
SILVER
PIT
54
53
52
from Graham 1934
from West 1970
51
BUSSURELLE
BANK
ICES WG 1971
50
-10
-8
-6
-4
-2
0
2
4
6
Figure 3.
Spawning prevalence (proportion of hauls which have a stage 3 maturity fish)
of cod in the North Sea and West Coast of Scotland.
Figure 5.
Past and present cod spawning areas as identified by fishermen.
Figure 4.
Distribution of pre-spawning and spawning cod in Scottish bottom trawl
surveys in a) 1999, b) 2002 and c) 2003. Shading reflects average catch
range of 0-13 h 1.
a)
62
61
13
12
60
11
59
10
9
58
8
7
57
6
56
5
4
55
3
2
54
1
53
0
52
-10
-8
-6
-4
-2
0
2
4
62
b)
61
10
9
60
8
59
7
58
6
57
5
4
56
3
55
2
54
1
53
0
52
c)
-10
-8
-6
-4
-2
0
2
4
62
61
7
60
6
59
5
58
4
57
3
56
2
55
54
1
53
0
52
-10
-8
-6
-4
-2
0
2
4
6
Figure 6.
Cod stage 3-6 eggs in the northern North Sea found in the a) 1950s (scale
bar represents no’s eggs per 15 minute haul; 0 = 0, 1 = 1-10, 2 = 11-50, 3 =
>50) and b) 1992 (scale bar represents no’s eggs per m-1; 0 = 0, 1 = 1-10, 2 =
11-20, 3 = >21)
a) Raitt, 1967
3
62
61
60
2
59
58
57
1
56
55
-10
-8
-6
-4
-2
0
2
4
6
0
b) Heath et al., 1994
3
62
61
60
2
59
58
57
1
56
55
-10
-8
-6
-4
-2
0
2
4
6
0
Figure 7.
Distribution of stage 1 cod eggs in the Southern Bight, 9-18 February 1968
(from Brander, 1994a).