Mortality and growth of 0-group flatfish in the brackish dollard (Ems Estuary, Wadden Sea)

119

Netherlands Journal of Sea Research 34 (1-3): 119-129 (1995)

MORTALITYAND GROWTH OF 0-GROUP FLATFISH IN THE BRACKISH DOLLARD (EMS ESTUARY,WADDEN SEA) Z. JAGER, H.L. KLEEF and P. TYDEMAN National Institute for Coastal and Marine Management / RIKZ, RO. Box 207, 9750 AE Haren, The Netherlands

ABSTRACT The population dynamics of three 0-group flatfish species, plaice (Pleuronectes platessa L.), flounder (Platichthys flesus L.) and sole (Solea solea L.) in the Dollard (Ems Estuary, Wadden Sea) were investigated in 1992. The instantaneous rate of decrease in catch density of plaice was 0 . 0 1 1 ' d , which corresponded with other calculated mortality rates of ~olaice in the western Wadden Sea. Catch densities of 0-group flounder decreased at a rate of 0.018.d". The rate of decrease in catch density of 0-group sole was estimated at 0.011"d "1, but was less accurate and probably reflected migration. The rate of increase in mean length of 0-group sole was in agreement with experimental growth studies under excess of food. The observed rate of increase in mean length of plaice and flounder appeared to decline from the beginning of June onwards in comparison with simulated growth in length. A number of factors that may be responsible for the observed differences are discussed.

Key words: plaice, flounder, sole, population dynamics, growth, nursery

1. INTRODUCTION The Ems-Dollard estuary is part of the Wadden Sea, which is an important nursery for flatfish (Zijlstra, 1972; Stam, 1984; Van Beek et aL, 1989). Following indications of Stam (1984) that the Dollard forms a nursery area for juvenile flatfish, a long-term research project was initiated to study the importance and quality of the Ems-Dollard estuary as a nursery area for the three flatfish species that enter the Wadden Sea as larvae. The main factors that affect the quality of the nursery are food, predators and temperature because they can affect growth and survival of a cohort. Both quality and quantity of flatfish nursery grounds can have an effect on recruitment (Gibson, 1994). In an earlier paper, the distribution in relation to abiotic factors of 0-group flatfish on the tidal flats of the estuary was described (Jager et aL, 1993). The present paper describes and compares mortality and growth of 0-group plaice, flounder and sole. The observed growth of the species is compared with growth models obtained from laboratory experiments at different temperatures under excessive food conditions. By comparing the observed with the expected length growth, conclusions can be drawn with respect to the existence of growth limitation under field condi-

tions. Several factors may affect growth. Intraspecific, density-dependent competition for food is one of them. However, growth limitation by food may also be caused by interspecific competition. A number of factors that may be responsible for differences between observed and expected length growth will be discussed. Also differences between the species with respect to their use of the area are considered. With this, a contribution is made to the ongoing debate whether growth of juvenile flatfish is (food) limited or not (Kuipers, 1973; Zijlstra etaL, 1982; Van der Veer, 1986; Van der Veer & Witte, 1993; Van der Veer etaL, 1994; Berghahn, 1987; Bergman et al., 1987; Gibson, 1994). 2. MATERIAL AND METHODS 2.1. STUDY AREA The Ems-Dollard estuary is situated in the eastern part of the Dutch Wadden Sea. The Dollard is part of this estuary and is a brackish intertidal area of about 100 km 2 (Fig. 1). Sediments are fine sand to silt and there are differences in the elevation level of the tidal flats throughout the area. Freshwater input originates from the river Ems (ca 110 m3.s q) and the Westerwoldsche Aa (ca 15 m3.s-1). Salinity in the Dollard

120

Z. JAGER, H.L. KLEEF & R TYDEMAN

5" NorthSea

53°3G ~ 53. A

6°

7~

plaice are distributed randomly on the tidal flats (Kuipers, 1973). The sampling gear used was a 2-m _~ beam trawl, mesh size 0.5 cm and one tickler chain, 53°30, which was towed at approximately 50 m.min -1 at a minimum fishing depth of 80 cm. The net differed from that used by Kuipers (1975, 1977), Zijlstra e t al. (1982) and Van der Veer (1986) by the shorter and ,,.~ TheNetherlands ~EEm~ lighter footrope (length 230 cm, plumb-rope 4.5 mm) 53° and a light tickler chain (length 220 cm) that had to be applied because of the soft substratum in the Dollard. The total weight of the net was about 15 kg. Duplicate hauls were made at each sample site and the length ' / ~ L of a haul was about 500 m. A meter wheel was used River Ems to check the distance fished. Each sample thus covered about 1000 m 2 of the tidal flat. Dollard Samples were deep-frozen and sorted out in the Groote Gat HZ• laboratory. All flatfish were counted and measured to LZ1 the mm below. Numbers per sample were converted kZ2• into catch densities (numbers per 1000 m2), without corrections for gear efficiency. All flatfish were LP HPlo Olo grouped in length classes of 5 mm without a correcMP1 MP2 • tion for possible shrinkage or gear efficiency and W• HP2• length-frequency distributions were calculated for Z4 / each species per week. Every week the mean length • 02 7W of the sample was calculated for each species. Water temperature and salinity were measured with a WTWconductivity meter simultaneously with the sampling.

F

t

2.3. DATA ANALYSIS

Fig. 1. The Dutch Wadden Sea, showing the Dollard (A) and the positions of the sample locations. Sediment contours for 20% mud (---) are indicated.

The instantaneous rate of decrease in catch density was calculated following the method described by lies & Beverton (1991), by calculating the regression of the natural logarithm of the mean catch density against time (weeks), according to: N t = NO e-zt

ranges from about S=6 in winter to S=25 in summer. In the past, discharges of industrial organic sewage with a high Biological Oxygen Demand (BOD) from the Westerwoldsche Aa caused anoxic events in the Dollard every year in September-October. Recently, discharges of organic sewage have been reduced. Water turbidity is extremely high in the Dollard, suspended matter concentrations amounting to an annual average level of c a 100 mg.dm -3. The tidal amplitude of the area is 3 to 3.5 m (Anonymous, 1983). 2.2. SAMPLING Fifteen locations were selected on the tidal flats in the Dollard, ranging in elevation from -10 to +110 cm above Mean Sea Level (Fig. 1). The locations were sampled twice a month from April to September, and once a month from September to November in 1992. Sampling took place around high water (HW plus or minus two hours), because during that period young

in which N t is numbers or density at time t, N O is the initial numbers or density, t is time interval in days between N O and N t, and Z is the instantaneous 'mortality rate (d-l). In May and at the beginning of October, sampling was incomplete and therefore these data were not included in the analysis of the rate of decrease in catch numbers. The rate of decrease in catch density of sole was calculated until the end of October. This included a zero catch density in the last sampling week, which was needed to obtain enough data for the regression, and therefore the natural logarithm of (catch density + 1) was used. For each species the difference in mean length between two sample weeks was divided by the difference in time (in days), to calculate the daily increase in mean length during that time interval (mm-d-1). For plaice between 50 and 150 mm the observed increase in mean length was compared with simulated growth using an experimentally established growth model under excess of food developed by Fonds e t a l . (1992):

0-GROUP FLATFISH IN THE BRACKISH DOLLARD

dL = 0.0136.T 15 - 6.10-9"7.6 (r=0.99; n=7)

where dL is daily growth (mm.d 4) and T is mean water temperature (°C). For flounder between 35 and 80 mm a similar regression model was calculated, based on the experiments described in Fonds et al. (1992) and complementary data that were provided by M. Fonds (pers. comm.):

~ ~

5 4 3 2

,

.

.

.

_

.

,

.

.

.

.

.

121

.

,

. . . . .

.

.

,

.

.

,

.

.

.

,

. . . .

.

.

,

p
.

~o 1 "~ 0 -1

dL = 0.0028"T21 - 6.10-9"T6 (r=0.99; n=6)

.

- - . °

5 _ : ' ~

where dL is daily growth (mm.d -1) and T is mean water temperature (°C). The mean water temperatures were calculated using measurements that were taken during sampling. The simulation was started in the first week that the species was caught and the mean length in that week was taken as the starting length of the simulation.

.

4

: .~ ~, 3 ~ 2 o ~ 1 0 -= -1 b.

.

.

.

.

.

.

.

~

5

'

"

.

.

.

.

.

.

.

~

'

.

.

s l o 4 e = _ ~ ) . 0 1 "

.

3.1. DENSITY

4 3

'

" "

"

~

"

""

'

"

'

"

"

"

'

. . . .

"

--..

"

..c o

1

" =

'

"

"

"

'

- 0 . 0 1 1

p
The linear regressions of the natural logarithm of mean catch density on time were significant for all three species (Fig. 2). The slopes could therefore be taken as an estimate of the rate of decrease in catch density of the populations. The residuals of the regressions indicated that the rate of decrease in catch density may in reality not have been linear. The catch densities of the juvenile flatfish species of which the natural logarithms were calculated are presented in Table 1. Depending on the period of time that is taken into consideration, different estimates of the instantaneous rate of decrease in catch density were obtained (Table 2). The rate of decrease in catch density of plaice ranged from 0.011 to 0.020.d -1, that of flounder from 0.018 to 0.028-d q, and that of sole was 0.011 -d-1 .

'

s ......

s l o p e

+ >, -~

8"

~
.......

J~

3. RESULTS

.

......

=

6 7 %

-

~o 0 -C.

""--.

-1 -. . . . . . . . . . . . . . . . . . . . . . . . . . A M J J A

S

O

"':-.... N (1992)

Fig. 2. Rate of decrease in catch density of 0-group flatfish as calculated from regression of the natural logarithm of mean catch densities against time. The dotted lines indicate 95% CL. a. plaice; b. flounder and c. sole. 3.2. MEAN LENGTH The length-frequency distributions of plaice were more or less symmetrical from the beginning of April

TABLE 1 Date and number of sampled locations, catch densities (N.1000 m-2) of 0-group plaice, flounder and sole and their standard errors (SE) in the Dollard in 1992. sampling period

20/04--24/04 04/05--08/05 18/05---22/05 01/06--05/06 15/06--19/06 29/06--03/07 13/07--17/07 27/07--31/07 10/08--14/08 24/08--28/08 28/09--02/10 26/10--30/10 23/11--27/11

n samples 18 16 12 30 30 30 28 27 29 30 18 30 30

plaice N. lOOO m -2 45.5 31.7 38.7 23.5 18.5 14.1 17.2 15.2 17.1 7.6 10.9 1.6 0.4

SE

9.9 9.7 9.4 6.7 4.5 3.1 3.8 3.3 5.7 2.1 3.4 0.4 0.2

flounder N. lOOO m -2 0 0.06 0.6 5.2 26.2 14.5 17.4 8.7 4.1 4.3 1.9 2.1 1.3

SE

0 0.06 0.3 1.7 6.9 4.8 4.1 3.3 1.7 1.8 0.5 0.4 0.3

sole N. lOOO m -2 0 0 0 0 2.3 2.9 1.9 2.6 4.4 1.5 0.3 0 0

SE

0 0 0 0 0.6 0.6 0.6 1.1 2.5 0.8 0.1 0 0

122

Z. JAGER, H.L. KLEEF & R TYDEMAN

TABLE 2 Rate of decrease in catch density of plaice, flounder and sole in the Dollard in 1992. The slope of the calculated regression indicates the rate of decrease (slope) in the natural log of catch density of the flatfish ~opulations. The coefficient of determination of the model (R") and the significanceof the fitted regression (P) are given for the three flatfish species and different time intervals. species time interval slope R2 P st. error (d-1) slope (d-1) plaice April- Dec -0.020 0.89 <0.001 0.002 April- Sept -0.011 0.80 <0.01 0.002 flounder June- Dec -0.018 0.92 <0.001 0.002 June- Sept -0.028 0.90 0.004 0.005 sole July- Oct -0.011 0.67 0.046 0.004 to the beginning of September. Thereafter numbers dropped (Fig. 3). Length-frequency distributions of flounder became skewed after the middle of August and numbers dropped after September (Fig. 4). The length-frequency distributions of sole were irregular, which may have been caused by the low numbers measured (less than 100, except in the middle of August; Fig. 5). Despite occasional deviations from a normal distribution, the mean lengths were estimated.

Mean length of plaice increased from 21 mm (sd=4.0) at the end of April to 78 mm (sd=14.6) at the end of September. Mean length of flounder increased from 29 mm (sd=5.2) in June to 71 mm (s.d.11.0) at the end of September, and mean length of sole increased from 24 mm (sd=4.7) in mid-June to 117 mm (sd=15.0) at the end of September (Fig. 6). The observed rate of increase in mean length of the 0-group plaice cohort was highest in Ma.y1 (0.7-0.8 mm.d-'), gradually decreased to 0.3 mm.d- in June/ July, and finally dropped to 0.1 mm.d -1 in mid-August. The rate of increase observed in mean length of the juvenile flounder gradually declined from 0.7 mm.d -1 in June to about 0.2 mm-d -1 from August to November. Sole showed the highest rate of increase in mean length, 1.0-2.0 mm-d -1 from July to October (Table 3). Fig. 7 shows that at the prevailing water temperatures the expected growth rate, based on the laboratory growth experiments of Fonds et al. (1992), was higher for flounder than for plaice. The patterns in the rate of increase in mean length of plaice and flounder are very similar with a time lag of about one month. The deviation from the expected growth rate of 0group plaice occurred at the beginning of June and coincided with the appearance of the 0-group floun-

I July

12 August

0.4

0.4

0.2

0.2

0.2

42

3 June 03

n=423

0.4

52

62

72

82

92

47 57 67 77 87 97 107

n=138

0.4

0.4

0.4

0.2

0.2

0.2

CT

n=450

26 August

n=202

28 October

q=48

32 37 42 47 52 57 62

17 June

n-468

29 July

n=347

0.4

0.4

0.4

0.2

0.2

0.2

o

0 32

42

52

62

72

87

47 57 67 77 87 97

length class (mm)

Fig. 3. Length-frequencydistributions of plaice in the Dollard in 1992. Date (top left) and number of measured fish (top right) are indicated in each figure.

0-GROUP FLATFISH IN THE BRACKISH DOLLARD

"17June

['29July

n=692

0.4

0.4

0.2

0.2

123

n=186 1

]0.20I [30 0j September . ~ 4 n=31

22 32 42 52 62 b" C

O ET

1July

R=435

I"12 / August

n=106

'28October

0.4

04f

0.4

0.2

0.2

0.2

27 37 47 57 67 15July

47 57 67 77 87 97

n=484

26AUgUSt

n=63

52 62 72 82 02 102112

n=109

"25November n=33

0.4

0.4

0.4

0.2

0.2

0.2

length class (mm)

Fig. 4. Length-frequency distributions of flounder in the Dollard in 1992. Date (top left) and number of measured fish (top right) are indicated in each figure. der in the Dollard (Table 3, Fig. 7). From June onwards the observed mean length of plaice was lower than the expected length at the observed water temperatures based on experimental studies with excessive food supply (Fig. 8). The growth rate of flounder was always lower at the observed water temperatures than expected from experimental studies with excessive food supply, except at the beginning of November (Fig. 7). Within the Dollard, significant differences in mean lengths were found between flounders caught in the central Dollard and flounders caught in the elevated nearshore mudflats (Table 4). However, even the higher mean length of the latter was not the maximum possible at the observed water temperatures (Fig. 9). Water temperatures did not differ significantly between the two groups of locations during sampling (Fig. 10). 4. DISCUSSION The decrease in catch numbers could be the result of an interaction of three different processes: changing catch efficiency due to increasing fish length and changing water temperature during the sampling season, -

- migration, and - mortality. Kuipers (1975) indicated that the efficiency of his 2-m beam trawl was 100% for plaice <7 cm, which is the mean length that plaice in the Dollard reach at the end of summer. However, the catch efficiency for juvenile flounder and sole is unknown, and so is the effect of variable water temperature and substratum on the efficiency of the beam trawl. Until more is known about the catch efficiency of the trawl for the three species under different circumstances, it will be difficult to translate the rate of decrease in catch density into a precise estimate of mortality and growth. 4.1. MORTALITY In the Balgzand, a tidal flat area in the western Wadden Sea, juvenile plaice leave the area in AugustSeptember to migrate towards deeper water, the larger individuals first (Kuipers, 1977; Zijlstra et aL, 1982). The same pattern was found in the German Wadden Sea (Berghahn, 1986), and in a Scottish bay (Gibson, 1973). In the Dollard, plaice catch density decreased until the end of August, coinciding with a reduction in the rate of increase in mean length to 0.1

124

Z. JAGER, H.L. KLEEF & P. T Y D E M A N

17 June

n=59

29 July

0.4

0.4

0.2

0.2

n=55

TABLE 3 Rate of increase in mean length of plaice, flounder and sole, and mean water temperatures in the Dollard in 1992. The date indicates the middle of the time interval during which the increase in length took place or during which the water temperatures were measured.

date

plaice (mm.d -1)

0 42 52

1 July

n=87 ]

0.4 [12 August

n=108

0.4

0.2

0.2

o 12

22

32

15 July

42

57

52 67 77 87 97 t07 117

n=53

0.4

0.4

0.2

0.2

0 77

87

97 107 117 132

length class (mm)

22/04 29/04 06/05 13/05 20/05 27/05 03/06 10/06 17/06 24/06 01/07 08/07 15/07 22/07 29/07 05/08 12/08 19/08 26/08 12/09 30/09 14/10 28/10 11/11

flounder (mm.d -1)

sole (mm.d -7)

temperature (C °) 9.8

0.39 14.0 0.68 17.4 0.83 19.8 0.52

0.70

0.47

0.65

0.40

0.32

0.53

1.25

0.34

0.43

1.95

0.34

0.21

1.06

0.10

0.21

1.00

0.04

0.12

0.39

-0.05

0.33

0.32

0.15

21.0 22.3 18.4 19.3 20.6 17.6 16.1 6.5

Fig. 5. Length-frequency distributions of sole in the Dollard in 1992. Date (top left) and number of measured fish (top right) are indicated in each figure. I(mm) 120 dL (mm-d -1)

100

1.5 80

/

~"

7" °-

60

--~3

°-[3-o-

-o-

. _

0.5

40 J 20 o

06/04 I

I

I

I

I

I

I

A

M

J

J

A

S

O

I

N

i

i

i

26/05

15/07

03/09

i

23/10 12/12 date (1992)

month (1992) Fig. 6. Mean length of plaice (---), flounder (...) and sole (--) versus time in 1992. Vertical bars indicate 95% CL.

Fig. 7. Expected (open symbols) and observed (closed symbols) growth rates of plaice (square) and flounder (circle) at the prevailing water temperatures in the Dollard in 1992.

0-GROUP FLATFISH IN THE BRACKISH DOLLARD

125

mean len( th (mm) 200 a.

mean length (mm) 2o0

150

150

o

os 100

100

Q" o

•

50

Y

50

i

06/04

26}05

15/07 '

03/09 '

23/10

12/12

06104

date (1992)

i

i

i

i

26/05

15•07

03/09

23/10

date (1992)

Fig. 8. Comparison of observed mean length (--) with simulated length (---) of 0-group plaice.

mm.d -1, but the length-frequency distribution remained symmetrical. This might suggest that in the Dollard, plaice left the area at the end of August. In that case it would be more appropriate to calculate the rate of decrease between April and the end of August, leading to an instantaneous rate of decrease in catch density of juvenile plaice of 0.011-d -1. This value is identical to the mean mortality rate of 0.011.d -1 that has been estimated for plaice in the western Wadden Sea over the years 1973 to 1982 (lies & Beverton, 1991; Beverton & lies 1992). The rate of decrease in catch density of juvenile plaice, from the moment of peak density to December, was 0.020.d q. The estimated instantaneous rate of decrease in the catch density of flounder from June to December was 0.018.d q. There were no indications that flounder left the Dollard in September. A comparison of the rate of decrease of plaice and flounder during the

mean length (mm) 2o0

150

100

50

06/04

26 05

15/07

03/09

23/10

central Dollard (ram)

28.7 38.7 47.8 54.9 60.4 63.1 66.8 69.6 77.9 84.0

sd

5.2 6.5 7.2 8.7 8.9 10.4 10.3 9.8 15.0 16.9

12/12

date (1992) Fig. 9. Comparison of observed mean length (--) with simulated length (-o-) of 0-group flounder for a. the whole Dollard (0) and b. for the central part (A) and the nearshore part (~) of the Dollard separately.

TABLE 4 Mean length (mm) of flounder in the central Dollard (< 14% mud) and in the nearshore elevated area (>14% mud). date 22/04 06/05 20/05 03/06 17/06 01/07 15/07 29/07 12/08 26/08 30/09 28/10 25/11

12/12

nearshore Do~lard(rnm)

32.6 39.5 53.8 61.4 69.6 70.4 72.8 83.3 89.5 99.5

sd

9.8 8.0 12.1 10.2 11.2 10.3 10.2 13.1 12.4 19.1

Oollard (all) (mm)

28.9 38.8 47.9 55.3 61.3 64.3 67.1 71.4 81.0 84.9

sd

5.7 6.6 7.4 8.9 9.5 10.7 10.4 11.1 15.2 17.2

126

Z. JAGER, H.L. KLEEF & R TYDEMAN

"C

ppt ,

,

,

,

,

,

,

,

,

,

,

,

..428

20

425

16

~22

12

-419 a.

J

a.

,

24

--'--'--------/t._._____ River Ems

.!

Dollard

Groote Gat

. -416

+'+"

.+

;b

"413

-110

April

June

July

iiiiiiiiMiiiilM+ ,

A u g u s t Octobe~November 1992

iiiii~iiiiiiiiii! '

Fig• 10. Mean water temperatures (a) and salinities (b) of the sampled locations in the Dollard in 1992, measured on the sample locations• Vertical bars indicate 95% CL.

same time interval is biased by differences in size, because flounder were smaller than plaice at any one time due to their later arrival in the Doltard (Jager et al., 1993). Size-selective mortality has been found to occur in juvenile flatfish (Van der Veer & Bergman, 1987; Van der Veer et al., 1994). In order to compare overall mortality in the nursery it might be more appropriate to compare the rate of decrease in catch density of plaice between April and September with that of flounder between June and December. The rate of decrease in the catch density of flounder would than be 1.6 times that of plaice. This difference is smaller than observed by Van der Veer et al. (1991) in the western Wadden Sea, where flounder mortality was about three times that of plaice. Van der Veer et al. (1991) explained the higher rate of decrease of flounder by migration to freshwater. Occasional sampling with the 2-m beamtrawl in the adjacent freshwater habitats of the Dollard revealed no substantial numbers of 0-group flounder in the Westerwoldsche Aa and only small numbers were found in the river Ems (Jager et al., unpubl.). Therefore, it does not seem likely that the higher rate of decrease of flounder in the Dollard than of plaice was caused by migration of flounder to freshwater. The instantaneous rate of decrease in the catch density of sole between July and October was 0.011 .d -1. However, the regression is marginally significant at the 5% level (Table 2). This is due to the small number of data points because sole was caught during a shorter period than the two other flatfish species. Sole were more abundant in the tidal channels (Stam, 1984; Jager, unpubl.) and the individuals that

Wemterwoldmche Aa

j

b.

,

-'-----------J L River Ems

S

Dollard

Groote Gat

•

,:: :i:i:i:i:ii:idAi : :i:i~,

• ::::::::::::::::::::::::::::::::::::

.

,, : : : : : : : : : : : : : : : : : : : : : :

+

•

Westerwoldsche

J

C.

Groote Gat

Aa

--'----------J L ~ __ River Ems

Dollard

/ /

-

•

+~iiiiiiii:::::i7+i~ ~iiii:/:iii::i!~;i ii::i!i •

+iiiY!ii!i;

::::::::::::::::::::

•

•

::::::::::::::::::::::

iiii~ii~' Fig. 11. Centre of distribution (averaged over the sampling weeks) of 0-group flatfish in the Dollard in 1992. a. plaice; b. flounder and c. sole. Darker shadings are the most dense concentrations. ---- sediment contour 20% (<16 ,u.m). Westerwoldsche

Aa

0-GROUP FLATFISH IN THE BRACKISH DOLLARD

127

growth rates under field conditions. The different flatfishes have their species specific growth characteristics. Flounder have a higher meta25O 40 bolic rate, they eat more and grow faster at higher temperatures than plaice (Fonds et aL, 1992). This is 200 reflected in the difference in expected growth rate 30 between plaice and flounder at the temperature conditions in the Dollard in 1992. Neither of plaice nor 150 flounder did the observed rate of increase in mean 20 length during 1992 seem to be the maximum possible given the observed water temperatures. For 0-group 100 sole Fonds (pers. comm.) found experimental mean growth rates of 1.3 to 1.7 mm.d q at temperatures of 10 50 18 to 22°C. The rate of increase in mean length of sole was at a constant high level (1.0-2.0 mm-d -1) during most of their presence in the Dollard. This is F A M J J A S O N D approximately the experimental range and therefore month (1974-1978) the increase in mean length of 0-group sole appeared to be maximal at the observed water temperatures. As was stated earlier, sole may not have been samFig. 12. Pattern of mean catch densities of shrimp (left axis) pled efficiently in June because of their small sizes. (--) and gobies (---) in the Dollard (1974-1978) after Stam Until the catch efficiency is optimal, this leads to an (1984, 1989). underestimation of the increase in mean length, which might explain the low value of 0.4 mm.d q during June. In September, sole left the Dollard, probably were caught on the tidal flats probably represent an the larger individuals first as is the case with plaice, overflow from the population in the tidal channels, which led to a relatively low rate of increase in mean which might also explain the irregular length-fre- length of 0.39 mm.d q in that month. The mean length quency distributions. If true, the rate of decrease of of 11 cm of 0-group sole in September was higher sole probably reflects migration rather than mortality. than found earlier in the Dollard (7 cm, average of No comparable data for sole are available from other 1974-1978; Stam, 1984). The value is comparable studies. In comparing the three flatfish species, the with the mean size of 0-group sole in August and instantaneous rate of decrease in mean catch densi- September in the Bay of Vilaine, where summer temperatures were 17-23°C (Dorel et aL, 1991; Marties was in the same order of magnitude. chand, 1991). The water temperatures in the Dollard in 1992 reached the same levels as in France. 4.2. GROWTH As mentioned before, a number of growth limiting When the observed growth rate of a flatfish species is factors can be responsible for the observed difference compared with the experimental growth rate at differ- between expected growth and actual growth. Sizeent temperatures under excess of food ('maximal selective migration of 0-group plaice and size-selective catch efficiency were discussed earlier and are growth') there are three possible outcomes: observed growth = maximal growth. This means most likely to have contributed to the apparent subthat food was not limiting in the field situation, and maximal growth rates by causing underestimation of the mean lengths of 0-group plaice and flounder. that growth was determined only by temperature. No information is available to draw conclusions on observed growth < maximal growth. In this case a number of factors may have caused an apparent intraspecific competition, which acts in a densitygrowth limitation: size selective migration of juve- dependent way. Density-dependent growth should not nile flatfish, size selective catch efficiency for juve- be confused with food limitation. Interspecific compenile flatfish, intraspecific competition, interspecific tition might occur in the Dollard. Plaice and flounder competition, food availability, food quality or low may be competitors for food: the main food item of both species in the Dollard is Corophium volutator salinity. - observed growth > maximal growth. This means (Jager et al., 1993), and plaice and flounder also parthat there have probably been growth restrictions in tially overlap in spatial distribution (Fig. 11). Fonds the experimental situation, and that the growth (pers. comm.) suggests that the increasing densities of competitors for food, such as shrimp (Crangon model should be revised. The validity of the experimental growth models is crangon), crab (Carcinus maenas) and gobies crucial in such a comparison. More frequent water (Pomatoschistus microps and R minutus), might be temperature measurements may improve the accu- responsible for the apparent growth limitation. In the racy of the estimates of the expected juvenile flatfish Dollard, these species can become very abundant n1OOOm #

-

-

M

0

128

Z. JAGER, H.L. KLEEF & R TYDEMAN

during summer (Fig. 12), as was shown by Stam (1982). No data are available for crab. Interspecific competition cannot be excluded as a factor restricting length growth of 0-group plaice and flounder in the Dollar& Food quality is suggested as a possible growth-limiting factor by Van der Veer & Witte (1993) who concluded that tail tips of the lugworm Arenicola marina are more suitable as food for juvenile plaice than Crustacea. However, in the Dollard Arenicola are not abundant, and juvenile plaice and flounder mainly feed on Crustacea. Stam (1984) also attributed the reduced length growth of 0-group plaice in the Dollard to lack of (suitable) food. Food availability may also be restricting in the Dollard. Juvenile plaice and flounder use tidal migration as a feeding strategy (Kuipers, 1973; Wolff et aL, 1981). In the Dollard the elevation of the tidal flats is generally high, the sampling locations ranging from -10 to +110 cm relative to mean sea level (msl). The mean high tide level of +135 cm msl (Delfzijl) and the tidal difference of 3.5 m result in a relatively short inundation of the tidal flats. In addition to this, juvenile flatfish have high digestion rates (Fonds, pers. comm.) so even stomach filling might cause restrictions to growth. The high turbidity in the Dollard may interfere with feeding of plaice and flounder, who are considered to be visual feeders. Although salinities were low in the Dollard, it is not very likely that this factor was responsible for the observed reduced length growth. The salinities were lowest in April, when growth rates of plaice were relatively high, and increased from May onwards to reach a maximum in July-August when the plaice growth rates became reduced. The higher mean lengths of 0-group flounder in the nearshore area than in the central Dollard may be an effect of less competition and higher food abundance in the nearshore area. However, it may also be that only the larger and stronger individuals are able to take the risk of feeding on these more elevated tidal flats. A similar difference in mean length between (shallow) muddy and sandy tidal flats was described and discussed for plaice in the North Frisian Wadden Sea by Berghahn (1987). The theory that food is not limiting growth and that the variability in growth of 0-group plaice between years and between areas is not caused by intraspecific density-dependent processes but only by differences in prevailing water temperatures (Kuipers, 1977; Zijlstra etal., 1982; Van der Veer & Witte, 1993) needs a more differentiated approach. The data in the present paper suggest that length growth of plaice and flounder may have been limited in the Dollard, whereas length growth of sole does appear to have been determined by temperature. Sole differed from plaice and flounder in their shorter stay in the area, their distinct spatial distribution (a preference for the deeper locations), a higher rate of increase in mean length, as well as a larger mean length attained at the

end of their stay in the Dollard. Acknowledgements.--The assistance of J. Eppinga and E.E. Welling in analysing the samples is very much appreciated. We are grateful to H. Mulder, who calculated a flounder growth equation from the data that were kindly provided by M. Fonds. We would like to thank A.D. Rijnsdorp, N. Daan, F. Colijn and anonymous referees for critical reading of the manuscript. 5. REFERENCES Anonymous, 1983. Biologisch onderzoek Eems-Dollard estuarium. BOEDE publicaties en verslagen 1983-1: 1267 (in Dutch). Berghahn, R., 1986. Determining abundance, distribution, and mortality of 0-group plaice (Pleuronectes platessa L) in the Wadden Sea.--& appl. Ichthyol. 2"11-22. , 1987. Effects of tidal migration on growth of 0-group plaice (Pleuronectes platessa L.) in the North Frisian Wadden Sea.--Meeresforsch. 31: 209-226. Bergman, M.J.N., A. Stam & H.W. Van der Veer, 1987. Abundance and growth of 0-group plaice (Pleuronectes platessa L.) in relation to food abundance in a coastal nursery area. ICES C.M. 1987/L:10. Beverton, R.J.H. & T.C. lies, 1992. Mortality rates of 0-group plaice (Pleuronectes platessa L.), dab (Limanda fimanda 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. Dorel, D., C. Koutsikopoulos, Y. Desaunay & J. Marchand, 1991. Seasonal distribution of young sole (Solea solea (L.)) in the nursery ground of the Bay of Vilaine (northern Bay of Biscay).--Neth. J. Sea Res. 27: 297-306. Fonds, M., R. Cronie, A.D. Vethaak & P. Van der Puyl, 1992. Metabolism, food consumption and growth of plaice (Pleuronectes platessa) and flounder (Platichthys flesus) in relation to fish size and temperature.-Neth. J. Sea Res. 29:127-143. 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. --, 1994. Impact of habitat quality and quantity on the recruitment of juvenile flatfishes.--Neth. J. Sea Res. 32: 191-206. lies, T.C. & R.J.H. Beverton, 1991. Mortality rates of 0-group plaice (Pleuronectes platessa L.), dab (Limanda fimanda 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. Jager, Z., H.L. Kleef & P. Tydeman, 1993. The distribution of 0-group flatfish in relation to abiotic factors on the tidal flats in the brackish Dollard (Ems Estuary, Wadden Sea).--& Fish Biol. 43 (Suppl. A): 31-43. Kuipers, B.R., 1973. On the tidal migration of young plaice (Pleuronectes platessa) in the Wadden Sea.--Neth. J. Sea Res. 6; 376-388. - - , 1975. On the efficiency of a two-metre beam trawl for juvenile plaice (Pleuronectes platessa).--Neth. J. Sea Res. 9: 69-85. - - , 1977. On the ecology of juvenile plaice on a tidal flat in

0-GROUP FLATFISH IN THE BRACKISH DOLLARD

the Wadden Sea.--Neth. J. Sea Res. 11: 56-91. Marchand, J., 1991. The influence of environmental conditions on settlement, distribution and growth of 0-group sole (Solea solea (L.)) in a macrotidal estuary (Vilaine, France).--Neth. J. Sea Res. 27:307-316. Stare, A., 1982. De vissen, krabben en gamalen van bet Eems-Dollard estuarium+ I1. Kwantitatieve inventarisatie van de garnalen. BOEDE Publicaties en verslagen 1982 4:1-37 (in Dutch). ,1984. De vissen, krabben en garnalen van het EemsDol/ard estuarium. Ill. Kwantitatieve inventarisatie van de platvissen. BOEDE Publicaties en verslagen 1984 2:1-16 (in Dutch). --, 1989. De vissen, krabben en garnalen van het EemsDollard estuarium. V. Kwantitatieve inventarisatie van de rondvissen. BOEDE Publicaties en verslagen: 1- 45 (unpubl., in Dutch). 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.-Helgo]b,nder Meeresunters. 43: 461-477. Van der Veer, H.W., 1986. Immigration, settlement and density-dependent mortality of a larval and early postlarval 0-group plaice (Pleuronectes platessa) population in the western Wadden Sea.--Mar. Ecol. Prog. Ser. 29: 223-236. Van der Veer, H.W. & M.J.N. Bergman, 1987. Predation by crustaceans on a newly settled 0-group plaice (Pleu-

129

ronectes platessa L.) population in the western Wadden Sea.--Mar. Ecol. Prog. Ser. 35: 203-215. Van der Veer, H.W. & J.IJ. Witte, 1993. The 'maximum growth/optimal food condition' hypothesis: a test for 0group plaice Pleuronectes platessa in the Dutch Wadden Sea.--Mar. Ecol. Prog. Ser. 101" 81-90. Van der Veer, H.W., MJ.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. Van der Veer, H.W., R. Berghahn & A.D. Rijnsdorp, 1994. Impact of juvenile growth on recruitment of flatfish.-Neth. J. Sea Res. 32:153-173. Wolff, W.J., M.A. Mandos & A.J.J. Sandee, 1981. Tidal migration of plaice and flounders 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. Lond. 29" 233-258. Zijlstra, J.J., R. Dapper & J.IJ. Witte, 1982. Settlement, growth and mortality of post-larval plaice (Pleuronectes platessa) in the western Wadden Sea.--Neth. J. Sea Res. 15: 250-272,