Nursery grounds of dab (Limanda limanda L.) in the southern North Sea

299

Netherlands Journal of Sea Research 32 (3/4): 299-307 (1994)

NURSERY GROUNDS OF DAB (LIMANDA LIMANDA L.) IN THE SOUTHERN NORTH SEA LOES J. BOLLE, ROB DAPPER, JOHANNES IJ. WlTTE and HENK W. VAN DER VEER Netherlands Institute for Sea Research, P.O. Box 59, 1790ABDen Burg, Texel, The Netherlands

ABSTRACT The distribution of 0-group dab, Limanda limanda L., in the open North Sea, the Dutch coastal zone and the estuarine Wadden Sea was investigated and patterns of seasonal abundance and growth were compared. The purpose was to locate the nursery grounds of dab and to assess their relative importance. Settlement was observed in the North Sea, both in coastal and offshore waters. Highest densities of 0-group dab were found in coastal areas. Since the coastal zone covers only a relatively small part of the North Sea it is not necessarily the most important nursery area for dab. No settlement of dab occurred in the Wadden ,Sea. Large numbers of juveniles were caught in the tidal channels and subtidal areas in autumn, but their size range indicated that migration rather than settlement was responsible for the observed increase in numbers. A similar process of inshore migration occurred in shallow coastal waters in October. Growth of 0-group dab was more or less similar in all areas. Mean length increased to 6-7 cm by the end of the first year of life.

1. INTRODUCTION The dab Limanda limanda L. is the most abundant flatfish species in the North Sea (Wheeler, 1978; Daan et aL, 1990), and adults are caught throughout the North Sea to a depth of 150 m. Highest densities are observed in the southeastern North Sea at depths of 20 to 40 m (Bohl, 1957; Daan et aL, 1990; Rijnsdorp et aL, 1992). Females reach sexual maturity at age 2 or 3, while males are all mature by the age of 2 (Bohl, 1957). Dab are high-fecundity serial spawners with a prolonged spawning period (Daan et aL, 1990). Egg production takes place from January to September, with major spawning activity in February to April (Harding & Nichols, 1987; Van der Land, 1991). Spawning occurs throughout the southeastern North Sea with offshore concentrations of eggs in the German Bight, north of the Frisian Islands, along the southern edge of the Dogger Bank (Van der Land, 1991 ; Rijnsdorp et aL, 1992) and northeast of Flamborough Head (Harding & Nichols, 1987). Postlarvae settle at a size of 13 to 20 mm, shortly before metamorphosis is completed (Russell, 1976). Settlement of dab is known to occur in shallow open bays along the coasts of Scotland and Wales (Macer, 1967; Edwards & Steele, 1968; Gibson, 1973; Poxton et aL, 1982). In all areas except the Clyde Sea (Poxton et aL, 1982) the 0-group showed no tidal migration and they did not enter the intertidal region. Maximum population densities are reported at depths of 3 to 5 m. However, in these cases sampling was

restricted to shallow waters (less than 10 m). Surveys along the coasts of England and Wales, which covered a greater depth range (0 - 30 m), indicate that the distribution of 0-group dab is not restricted to waters less than 10 m in depth (Riley et aL, 1981). Although juvenile dab seem to be mainly concentrated in coastal areas, small specimens (<9 cm) have been caught at offshore stations in the Dogger Bank area and in the eastern central North Sea in February (Rijnsdorp et aL, 1992). It is unclear whether these juveniles have migrated from coastal nursery grounds or originate from open sea nursery grounds. Another common flatfish species in the southern North Sea, the plaice, Pleuronectes platessa L. settles in open coastal waters as well (Macer, 1967; Edwards & Steele, 1968; Gibson, 1973; Lockwood, 1974; Riley et aL, 1981; Poxton et aL, 1982). However, the most important nursery area for North Sea plaice is the estuarine Wadden Sea, which accounts for 50 to 60% of the recruitment to the parent stock (Zijlstra, 1972; Anonymous, 1985; Van Beek et aL, 1989). Growth and survival conditions are favourable in the Wadden Sea, due to the large food resources and the relative scarcity of predators. Optimal growth rates and relatively low mortality rates have been observed for 0-group plaice in the Wadden Sea (Zijlstra et aL, 1982; Van der Veer, 1986; Bergman et aL, 1988; Van der Veer et aL, 1990; lies & Beverton, 1991; Beverton & lies, 1992; Van der Veer & Witte, 1993). Although juvenile dab have been caught in appreciable numbers in the tidal channels of the

300

L.J. BOLLE, R. DAPPER, J.IJ. WlTTE & H.W. VAN DER VEER

56°

~

L

I

4°

I

.. ~°~

I /

i

. ~

5°

6q 56°

&i

55~_

55°

,o.;;.a. ., • t .. "";

" .•

" "t

• o " ~,o 54~.

54°

,,o • .o : .

5

~

~

.

•

:

N. / , ~ _ j - ~ o / ' - - - - ~

"

11%

5s °

.. °° °°

o

°

,

," °

Sampling took place from 1972 to 1980 with RV 'Aurelia'. A total 461 hauls were made (Fig. 1). Sampling was spread throughout the year, but only 2 or 3 months were sampled within one given year. Samples were collected with a 5.5-m beam trawl with two tickler chains and 5 x 5 mm mesh size in the cod end of the net. Haul duration was 10 min and the distance fished was estimated using Decca Track plots. Hauls were carried out during the daytime only and without regard to the tidal phase. Parts of the southeastern North Sea were not sampled due to the presence of large rocks• Coastal survey

...

•°~

North Sea survey

,' ;

Four stations along the Dutch North Sea coast were sampled regularly between 1978 and 1982 (Fig. 2). A total of 250 hauls were made. Fishing was done using a 2-m beam trawl with one tickler chain and a 5 x 5 mm mesh size net, towed from a rubber dinghy. Haul distance was recorded by an odometer fitted to the frame• At each station one sample was collected at depths of 0.5, 1,2, 3, 5 and 7 m, irrespective of the tidal phase.

* ,° •!

,, ,, , ;.,

Terschey 52'

~ ,o 5

4o

;o

Amelan~

,- 52 °

Vlielany

6°

/

Fig. 1. Position of sampling stations of the North Sea survey 1972-1980. Dutch estuaries (Fonds, 1979; Wolff et aL, 1981; Rijnsdorp et aL, 1992), they are hardly ever found on the tidal flats of the Wadden Sea (De Vlas, 1979; Berghahn, 1987) and are absent from the tidal flats of the Scheldt estuary (Wolff et aL, 1981). This paper summarizes information on juvenile dab in the open North Sea, in the Wadden Sea and along the Dutch North Sea coast in the past decades. The purpose was to identify the nursery areas and to assess their relative importance.

c3e'/~

North

Sea

Egmondaan Zee

l~

~/ijk aan Zee

;muiden ~

oordwijk aen Zee

2. MATERIAL AND METHODS 2.1. SAMPLING Four different data sets have been analysed: North Sea survey (1972-1980); Dutch coastal survey (19781982); Wadden Sea survey 1 (1971-1972); Wadden Sea survey 2 (1986). These surveys were conducted to estimate densities for several species, not specifically for dab. The total sampling area, covered by all these surveys together, represents all possible habitat types for dab to a depth of 75 m.

Fig. 2. Position of sampling stations of the coastal survey 1978-1982.

NURSERY GROUNDS OF DAB IN THE SOUTHERN NORTH SEA

Wadden Sea survey 1 Two subtidal stations in the western Dutch Wadden Sea were sampled weekly, from September 1971 to August 1972 (Fig 3). The stations were located between the low-water spring tide mark (LWS) and LWS-1 m. Samples were collected during a 1-hour period around high tide. Fishing was done with a 2-m beam trawl with one tickler chain and a 5 x 5 mm mesh size net, towed from a rubber dinghy (Kuipers, 1975). Two hauls were completed per station with each haul covering 400 m 2. Wadden Sea survey 2 Nineteen stations were sampled bi-monthly throughout 1986. The stations were distributed over tidal flats, subtidal areas and tidal channels in the western Dutch Wadden Sea (Fig. 3). At each station one to three samples were collected, during a 3-h period around high tide. In the intertidal area, samples were taken with a 2-m beam trawl with one tickler chain and a 5 x 5 mm mesh size net, towed from a rubber dinghy (Kuipers, 1975). Haul distance was recorded 40 •

50"

5o

301

by an odometer fitted to the frame. In the subtidal area (between LWS and LWS-5 m) and the tidal channels (deeper than LWS-5 m) fishing was done by RV 'Navicula' using a 3-m beam trawl with 1 tickler chain and a 1 x 1 cm mesh size net. Haul distance was estimated using Decca Track plots. 2.2. DATA ANALYSIS Catch samples were sorted immediately and all dab were measured to the nearest 5 mm total length interval. Age groups were distinguished by analysis of size frequency distributions, using 1 January as a birth date. Fig. 4 presents examples of size frequency distributions for data collected during the North Sea survey and the coastal survey. Nearly all dab (99%) caught in the Wadden Sea were in their first year of life. The arithmetic mean length of 0-group dab was used, since the size frequency distributions proved to be normally distributed. Catch numbers were converted to density estimates (ind'1000 m2), without correcting for net effilO ,

20"

30"

20"-

20"

I0:

10"

5~.

53°

Dq

40"

50"

10"

20"

30"

Fig. 3. Positions of sampling stations of the Wadden Sea survey 1971-1972 (A) and Wadden survey 1986 (O).

302

L.J. BOLLE, R. DAPPER, J.IJ. WlTTE & H.W. VAN DER VEER 251

60-

July 1972 (n=568)

/ 20-

[]

July (n=379)

50-

age 0

age 0 •

age >_1

4030-

~ lO 20-

u?_

100

0

>,

5

10

15

20

25

15-

O c t o b e r 1972 (n=818)

12-

[]

i

~

30

o

f

i

J

i-i-i-i

5

i

i

10

i

i

i

1

i

15

i

r

i

i

i

20

40-

i

i

i

t

i

25

i

i

30

O c t o b e r (n--2041)

age 0

[]

age 0

•

age _>1

9

~E

o"

6

Ii

3-

i

~

i

llllnl0 ,illll,,,,,,, ,

t

,

i

i

,

5

r

.

J

i

i

,

10

i

i

,

i

~

15

15-

i

,

,

i

9-

o"

6-

li i er

,

i

_, . J

25

i

0

30

[]

age 1

•

age _>2

J

i

i

,

"

i

J

1

J

i

~ r. .r .i . ,

0

i

,

10

t

t

i

1

i

15

i

i

,

,

i

l

20

30-

J a n u a r y 1973 (n=771)

12>,, ,j

i

20

i

i

i

25

i

i

30

J a n u a r y (n=367)

25-

age t •

20-

age _>2

LL 3-

0

T

0

I

[

I

I

I

5

I

[

r

I

I

~

I

10

15-

I

I

I

15

I

~

I

I

I|-|..

I

I

I

I

T

_I l

l

25

[]

I

0

30

~

~

I

~

I I I

0

10

5

15

20

25

20-

30

April (n=50)

age 1 15-

[]

age 1

•

age >_2

9-

o¢-

o"

I

April 1972 (n=903)

12>,

I

20

711lllll]ll,,,.age-2 lo5

6"

LI3-

,, ,-,~ . . . . . . . . . . . . . . . . .

0 0

5

10 15 20 Length (cm below)

," ;,~,"," ," . . . . 25

o 30

i

i

i

i

i

i

5

i

i

100,,I,,

r

~

i

i

E

i

i

J

i

i

i

i

i

10 15 20 Length (cm below)

i

i

i

i

L

25

1

i

i

i

,

30

Fig. 4. Size frequency distributions of dab caught in July, October, January and April. North S e a survey (left); coastal survey (right).

NURSERY GROUNDS OF DAB IN THE SOUTHERN NORTH SEA

o

5C

,

3°

,

4°

5°

(

56°

303

•1000 m-2) were observed in the subtidal areas and the tidal channels of the Wadden Sea in autumn and early winter. Very few dab (less than 1 ind-1000 m "2) were caught on the tidal flats (not shown). 3.2. SEASONAL VARIATION IN DENSITY

55°-

• 55 °

• --

.::.:

Fig. 7 presents monthly averages of density estimates for the North Sea, the Dutch coast and the Wadden Sea subtidal (1971-1972). Mean densities of juvenile dab in the subtidal and in the tidal channels estimated during the 1986 Wadden Sea survey

1oo]

5 4~

40-

530-

20-

•53°

0

0

-

i

1

2

3

4

5

6

1

2

3

4

5

6

100052 ~

F

800-

- 52 °

3°

4

6°

E

o

Fig. 5. Geographic distribution of 0-group dab in the North Sea: presence (O) or absence (-) in the period June-December. ciency. Monthly averages of log(d+1) transformed density estimates were calculated, because of skewed distributions.

g

600400-

£3

2000

3. RESULTS 3.1. DISTRIBUTION OF 0-GROUP DAB

0

7

10C

Fig. 5 shows the distribution of 0-group dab in the southeastern North Sea. Since not all stations were sampled every month, a quantitative presentation of the geographic distribution would be biased. Fig. 5 only indicates the presence or absence of 0-group dab per block of 10 x 10 miles. 0-group dab were widely distributed in the North Sea. They ranged from coastal waters to offshore waters and from 1 m to a depth of 70 m (Fig. 6). Highest densities occurred at a depth of 3 m in summer (Fig. 6a) and of 5 m in autumn (Fig. 6b). Density estimates in the open North Sea were one to two orders of magnitude lower than in the shallow coastal zone (Fig. 6c). 0-group dab were absent in the western Wadden Sea until late summer. Large numbers (up to 100 ind

8-

420

0

1'0

2'0

3'0 4'0 5'0 Depth (m)

6'0

7'0

8'0

Fig. 6. Depth distribution (mean density _+SE) of 0-group dab. a. coastal survey, June-September; b. coastal survey, October-December; c. North Sea survey, June-December.

304

L.J. BOLLE, R. DAPPER, J.IJ. WlTTE & H.W. VAN DER VEER

4a

32-

"r

r

t

i

r

i

i

i

J

i

M

J

J

A

S

O

N

D

J

F

M

t

J

A

M

4-

b E O O O

O_

32

c

_o

{3) O

M

J

J

A

S

O

N

D

J

F

M

A

M

more abundant in coastal waters. Density increased from 0.5 ind.1000 m -2 in June to approximately 25 ind'1000 m "2 in July, August and September. Peak abundance, 850 ind.1000 m "2, was reached in October and densities declined steadily afterwards (Fig. 7b). A similar autumn peak was observed in the Wadden Sea subtidal and tidal channels. After the arrival of 0-group dab in August, density increased to 12 ind.1000 m -2 in October and remained at this level until January. After a strong decline in density in February, few dab returned to the Wadden Sea in March, April and May (Fig. 7c). 3.3. MEAN LENGTH Mean lengths for 0-group dab in the open North Sea, the coastal zone and the Wadden Sea subtidal are presented in Fig. 8. Similar patterns could be observed in North Sea coastal and offshore waters (Figs 8a and b). Mean length increased from 3.1-3.3 cm in June to 6.2-6.6 cm in November. 0-group dab did not appear in the Wadden Sea until August, when they had already attained a mean length of 5.0 (+0.9) cm, which was not significantly different from the mean length of the 0-group in coastal waters at that time (4.8+0.4 cm). In all areas, mean length increased to 7.0 cm in November-December (Fig. 8c). 4. DISCUSSION

C

4.1. NURSERY GROUNDS

0

,~ M

~ J

J

A

S

O

N

D

J

F

M

A

M

Month

Fig. 7. Monthly average population density (mean density + SE) of juvenile dab. a. North Sea survey; b. coastal survey; c. Wadden Sea survey. revealed a pattern similar to Fig. 7c. Dab were virtually absent in the intertidal area of the Wadden Sea with densities declining from 1 ind.1000 m °2 in September to 0 in January. 0-group dab were first observed in the North Sea in June (Fig. 7a and b). In offshore waters, density increased from 0.06 ind.1000 m "2 in June to 5 ind.1000 m -2 in September. Thereafter densities fluctuated between 1 and 5 ind.1000 m -2 (Fig. 7a). These fluctuations could be a bias because samples were collected in different years. Juvenile dab were far

The results of surveys in the southeastern North Sea indicate large-scale settlement of dab in both inshore and offshore waters, from 1 m to a depth of 70 m. Post-settlement migration as an explanation for the wide distribution of 0-group dab cannot be ruled out completely. However, it is considered unlikely because the distribution of the smallest size class retained by the nets (2-3 cm) showed the same general pattern. As in previous studies in Scottish bays (Edwards & Steele, 1968; Poxton et aL, 1982), highest densities of 0-group dab were observed between a depth of 3 and 5 m. Density estimates for coastal waters were one to two orders of magnitude higher than for offshore waters. Care has to be taken when comparing abundance data of the 0-10 m depth zone and the 1070 m depth zone, because different beam trawls were used and the data cannot be corrected for net efficiency. Assuming the efficiencies of both beam trawls were of the same order of magnitude, shallow coastal waters appear to be the optimal nursery ground for dab. However, the contribution of offshore nursery grounds to the overall recruitment of North Sea dab may be higher, due to the much larger area of this habitat. In order to evaluate the relative importance of open sea versus coastal nursery grounds precisely,

NURSERY GROUNDS OF DAB IN THE SOUTHERN NORTH SEA

not reach the Wadden Sea or whether this area is unsuitable for settlement. The former seems more likely, since data from plankton samples collected in the Wadden Sea have never recorded larval dab (Creutzberg et aL, 1978; Rijnsdorp et aL, 1985; Van der Veer, 1985).

an inshore-offshore gradient should be sampled within the same year(s) with the same gear, so that differences in year-class strength and net efficiency will not be a problem. Dab do not settle in the estuarine Wadden Sea, not on the tidal flats, nor in the subtidal areas and the tidal channels. The occurrence of dab in the Wadden Sea in autumn is due to migration rather than settlement as indicated by the mean length at time of arrival (about 5.0 cm). It is unclear whether larvae do

4.2. SEASONAL VARIATION IN DENSITY

10-

8-

64-

, 10-

E

8.

t-t-

6

¢-

O

4

+

÷ ,

10-

++

8

6 4¸

305

t+ + ' Month

Fig. 8. Monthly average mean length (+ SE) of 0-group dab. a. North Sea survey; b. coastal survey; c. Wadden Sea survey.

The timing of settlement appears to be similar in all areas studied. Newly settled dab were found from June onwards in Red Wharf Bay (Macer, 1967), in Loch Ewe (Edwards & Steele, 1968), in Ardmucknish Bay (Gibson, 1973) and in Irvine and Ayr bays (Poxton et aL, 1982). The first appearance of 0-group dab in Dutch coastal waters and in the open North Sea was in June as well. 0-group dab, which have settled at greater depths, migrate into the coastal zone and the subtidal areas of the Wadden Sea in autumn. Dab appearing in the Wadden Sea in August were too large to be recently settled. If the enormous increase in densities in coastal waters in October was caused by a second period of settlement, then one would expect a decrease in mean length and bimodality in the size frequency distribution. However, the mean length increased and the size frequency distribution of 0group dab was normally distributed in October. Therefore, the increase in densities in autumn both in coastal and estuarine waters was probably due to migration. In February, most of these (by then called 1-group) dab had disappeared from the Wadden Sea and the coastal zone. This feature has also been reported by Pihl (1989) for 0-group dab in Laholm Bay, Sweden. Data presented by Creutzberg & Fonds (1971) show a similar migration pattern. In November 1963 a pronounced increase in abundance of juvenile dab (size range 5-7 cm) occurred in the Wadden Sea. By February all these dab had disappeared. This migration pattern might be triggered by water temperature, which undergoes faster changes in coastal and estuarine environments than in offshore waters. From September until February water temperature in the Wadden Sea is much lower, during summer much higher than in the open North Sea (Van der Veer et aL, 1990). Laboratory studies have shown that optimal growth rates for small dab occur at 15-18°C. At temperatures higher than 18°C feeding and growth decrease markedly and at 22°C no growth was observed (Fonds, unpubl, data). Water temperatures in the Wadden Sea in summer might be too high for optimal growth conditions. On the other hand dab avoid water temperatures lower than 2.5°C (Creutzberg & Fonds, 1971; Fonds, 1979). Bohl (1957) reports that fishermen have seen large groups of pelagic dab leaving the Wadden Sea after the first period of frost. These physiological constraints could explain the absence of dab in the Wadden Sea in summer and for a short period during winter.

306

L.J. BOLLE, R. DAPPER, J.IJ. WlTTE & H.W. VAN DER VEER 4.3. GROWTH

The present data set does not warrant a detailed analysis of differences in growth between the various areas. Sampling occurred in different years and some bias by migration cannot be excluded. However, for all areas the same general pattern of growth was observed with mean length increasing to 6-7 cm by the end of the first year of life. The similarity in growth between the various areas might suggest similar food conditions. However, temperature and food conditions are confounded. A detailed analysis of these effects is only possible when water temperature data are available in conjunction with growth model simulations for juvenile dab under optimal food conditions. This would permit a comparison of estimated optimal growth versus observed growth, as has been done for 0-group plaice (Zijlstra et aL, 1982; Van der Veer, 1986; Van der Veer et aL, 1990) and 0-group flounder (Van der Veer et aL, 1991). 4.4. EARLY LIFE HISTORY COMPARISON WITH PLAICE Both plaice and dab spawn in offshore waters. The spawning period of plaice is from December until March, whereas the peak in spawning activity of dab is from February until May (Harding & Nichols, 1987; Van der Land, 1991 ). The developing plaice eggs and larvae are transported to the coastal nursery grounds by residual currents (Talbot, 1976, 1978), followed by selective tidal behaviour (Rijnsdorp et aL, 1985; Bergman et aL, 1989). Post-larval plaice reach the nursery grounds in March-April, 2 to 3 months before dab do (Macer, 1967; Edwards & Steele, 1968; Gibson, 1973; Poxton et aL, 1982; Zijlstra et aL, 1982; Van der Veer, 1986; Van der Veer et aL, 1990). Plaice use shallow coastal and estuarine areas as nurseries. The Wadden Sea, especially the intertidal area, is the most important nursery ground for North Sea plaice (Zijlstra, 1972; Anonymous, 1985; Van Beek et aL, 1989; Van der Veer et aL, 1990). In this respect dab shows two profound differences. First, tidal flats are not used as nursery grounds by dab, nor are the deeper areas in the Wadden Sea. Dab settle and spend their first months in the North Sea. Second, 0-group dab are not restricted to shallow coastal nurseries. Use of the Wadden Sea tidal flats as a nursery ground is thought to reduce predation risks, resulting in lower mortality rates (Zijlstra et aL, 1982; Van der Veer, 1986; Van der Veer et aL, 1990). lies & Beverton (1991) re-analysed all published data on mortality rates of 0-group flatfish and found higher mortality rates for 0-group plaice in British and Scottish bays than in the Wadden Sea. Mortality rates of 0-group dab differed little from those of plaice derived from the same sampling locations (Beverton & lies, 1992). It would be interesting to compare mortality rates of 0-group dab in the open sea and along the

continental coasts with these data. Unfortunately, the density estimates presented in this paper are not suitable for estimations of mortality rates, because they are based on data collected in different years and probably biased due to annual variations. A difference in migration patterns of plaice and dab is apparent. 0-group plaice settle in shallow water and move into progressively deeper water as they increase in length (Gibson, 1973). 0-group dab show a reverse of this, they move from deeper water to shallow coastal areas and into the Wadden Sea in autumn. Both plaice and dab leave the Wadden Sea in winter, when water temperature drops below 2.5°C (Creutzberg & Fonds, 1971; Fonds, 1979). Whereas, 1-group plaice use selective tidal transport to re-enter the Wadden Sea in spring (De Veen, 1978), the majority of 1-group dab do not re-enter the Wadden Sea. Acknowledgements.---Thanks are due to the crew of RV 'Aurelia' for assistance, and we are very grateful to our colleagues from the Netherlands Institute for Fishery Research (RIVO-DLO) for putting their unpublished coastal survey data at our disposal. Furthermore we thank Niels Daan and Adriaan D. Rijnsdorp for critical reading of the manuscript. 5. REFERENCES Anonymous, 1985. Report of the 0-group North Sea Flatfish Working Group. ICES C.M./G:2: 1-46. Berghahn, R., 1987. The Wadden Sea as nursery for fish and crustacean species.--Fiskeri-og Sofartsmuseet Saltvandsakvariets Biologiske Meddelser 31: 69-85. Bergman, M.J.N., H.W. Van der Veer & J.J. Zijlstra, 1988. Plaice nurseries: effects on recruitment.--J. Fish Biol. 33 (Suppl. A): 201-218. Bergman, M.J.N., H.W. Van der Veer, A. Stam & D. Zuidema, 1989. Transport mechanisms of larval plaice (Pleuronectes platessa L.) from the coastal zone into the Wadden Sea nursery area.--Rapp. P.-v. R6un. Cons. perm. int. Explor. Mer 191: 43-49. Beverton, R.J.H. & T.C. lies, 1992. Mortality rates of 0-group plaice (Pleuronectes platessa L.), dab (Limanda limanda L.) and turbot (Scophthalmus maximus L.) in European waters II. Comparison of mortality rates and construction of life table for 0-group plaice.--Neth. J. Sea Res. 29: 49-59. Bohl, H., 1957. Die Biologie der Kliesche (Limanda limanda L.) in der Nordsee.--Ber. dt. wiss. Kommn Meeresforsch. 15: 1-58. Creutzberg, F. & M. Fonds, 1971. The seasonal variation in the distribution of some demersal fish species in the Dutch Wadden Sea.--Thalassia Jugosl. 7: 13-23. Creutzberg, F., A.Th.G.W. Eltink & G.J. Van Noort, 1978. The migration of plaice larvae Pleuronectes platessa into the western Wadden Sea. In: D.S. McLusky & A.J. Berry. Physiology and behaviour of marine organisms. Proc. 12th EMBS. Pergamon Press, New York: 243251. Daan, N., P.J. Bromley, J.R.G. Hislop & N.A. Nielsen, 1990. Ecology of North Sea fish.--Neth. J. Sea Res 26: 343386. De Veen, J.F., 1978. On selective tidal transport in the

NURSERY GROUNDS OF DAB IN THE SOUTHERN NORTH SEA

migration of North Sea plaice (Pleuronectes platessa) and other flatfish species.--Neth. J. Sea Res. 12:115147. De Vlas, J., 1979. Annual food intake by plaice and flounder in a tidal flat area in the Dutch Wadden Sea, with special reference to consumption of regenerating parts of macrobenthic prey.--Neth. J. Sea Res. 13:117-153. Edwards, R. & J.H. Steele, 1968. The ecology of 0-group plaice and common dab at Loch Ewe. I. Population and food.--J, exp. mar. Biol. Ecol. 2: 215-238. Fonds, M., 1979. The seasonal distribution of some fish species in the western Dutch Wadden Sea. In: W.J. Wolff. Ecology of the Wadden Sea. Vol. 2, part 5. Balkema, Rotterdam: 42-77. Gibson, R.N., 1973. The intertidal movements and distribution of young fish on a sandy beach with special reference to the plaice (Pleuronectes platessa L.).--J. exp. mar. Biol. Ecol. 12: 79-102. Harding, D. & J.H. Nichols, 1987. Plankton surveys off the north-east coast of England in 1976: an introductory report and summary of the results.--Fish. Res. Tech. Rep. 86: 1-56. lies, T.C. & R.J.H. Beverton, 1991. Mortality rates of 0-group plaice (Pleuronectes platessa L.), dab (Limanda limanda L.) and turbot (Scophthalmus maximus L.) in European waters I. Statistical analysis of the data and estimation of parameters.--Neth. J. Sea Res. 27: 217235. Kuipers, B., 1975. On the efficiency of a two-metre beam trawl for juvenile plaice (Pleuronectes platessa).-Neth. J. Sea Res. 9: 69-85. Lockwood, S.J., 1974. The settlement, distribution and movements of 0-group plaice Pleuronectes platessa (L.) in Filey Bay, Yorkshire.~. Fish Biol. 6: 465-477. Macer, C.T., 1967. The food web in Red Wharf Bay (North Wales) with particular reference to young plaice (Pleuronectes platessa).--Helgol&nder wiss. Meeresunters. 15: 560-573. Pihl, L., 1989. Abundance, biomass and production of juvenile flatfish in the southeastern Kattegat.--Neth. J. Sea Res. 24: 69-81. Poxton, M.G., A. Eieftheriou & A.D. Mclntyre, 1982. The population dynamics of 0-group flatfish on nursery grounds in the Clyde Sea Area.--Estuar. coast. Shelf Sci. 14: 265-282. Rijnsdorp, A.D., M. Van Stralen & H.W. Van der Veer, 1985. Selective tidal transport of North Sea plaice Pleuronectes platessa in coastal nursery areas.--Trans. Am. Fish. Soc. 114: 461-470. Rijnsdorp, A.D., A.D. Vethaak & P.I. Van Leeuwen, 1992. Population biology of dab Limanda limanda in the southeastern North Sea.--Mar. Ecol. Prog. Ser. 91: 19-35. Riley, J.D., D.J. Symonds & L. Woolner, 1981. On the factors influencing the distribution of 0-group demersal

307

fish in coastal waters.--Rapp. P.-v. R~un. Cons. perm. int. Explor. Mer 178: 223-228. Russell, F.S., 1976. The eggs and planktonic stages of British marine fishes. Academic Press, London: 1-524. Talbot, J.W., 1976. The dispersal of plaice eggs and larvae in the Southern Bight of the North S e a . ~ . Cons. perm. int. Explor. Mer 37: 221-248. ,1978. Changes in plaice larval dispersal in the last fifteen years.--Rapp. P.-v. R~un. Cons. perm. int. Explor. Mer 172:114-123. Van Beek, F.A., A.D. Rijnsdorp & R. De Clerck, 1989. Monitoring juvenile stocks of flatfish in the Wadden Sea and the coastal areas of the southeastern North Sea.-Helgol~,nder Meeresunters. 43: 275-293. Van der Land, M.A., 1991. Distribution of flatfish eggs in the 1989 egg surveys in the southeastern North Sea, and mortality of plaice and sole eggs.--Neth. J. Sea Res. 27: 277-286. Van der Veer, H.W., 1985. Impact of coelenterate predation on larval plaice Pleuronectes platessa and flounder Platichthys flesus stock in the western Wadden S e a . Mar. Ecol. Prog. Ser. 25: 229-238. - - , 1986. Immigration, settlement and density-dependent mortality of a larval and early post-larval 0-group plaice (Pleuronectes platessa) population in the western Dutch Wadden Sea.--Mar. Ecol. Prog. Ser. 29: 223236. Van der Veer, H.W. & J.IJ. Witte, 1993. The 'maximum growth/optimal food condition' hypothesis: a test for 0group plaice Pleuronectes platessa L. in the Dutch Wadden Sea.--Mar. Ecol. Prog. Ser, 101: 81-90. Van der Veer, H.W., L. Pihl & M.J.N. Bergman, 1990. Recruitment mechanisms in North Sea plaice Pleuronectes platessa.--Mar. Ecol. Prog. Ser. 64:1-12. Van der Veer, H.W., M.J.N. Bergman, R. Dapper & J.IJ. Witte, 1991. Population dynamics of an intertidal 0group flounder Platichthys flesus population in the western Dutch Wadden Sea.--Mar. Ecol. Prog. Ser. 73: 141-148. Wheeler, A., 1978. Key to the fishes of northern Europe. Frederick Warne, London: 1-380. Wolff, W.J., M.A. Mandos & A.J.J. Sandee, 1981. Tidal migration of plaice and flounder as a feeding strategy. In: N.V. Jones & W.J. Wolff. Feeding and survival strategies of estuarine organisms. Plenum Press, New York: 159-171. Zijlstra, J.J., 1972. On the importance of the Wadden Sea as a nursery area in relation to the conservation of the southern North Sea fishery resources.--Symp, zool. Soc. London 29: 233-258. Zijlstra, J.J., R. Dapper & J.IJ. Witte, 1982. Settlement, growth and mortality of post-larval plaice (Pleuronectes platessa L.) in the western Wadden Sea.--Neth. J. Sea Res. 15: 250-272.