Wind-induced transport of plaice (Pleuronectes platessa) early life-history stages in the Skagerrak-Kattegat

In Collaboration with the Netherlands Institute for Sea Research

ELSEVIER

Journal of Sea Research

39

( 1998) I l-28

Wind-induced transport of plaice (Pleuronectes platessa) early life-history stages in the Skagerrak-Kattegat Else Nielsen *, Ole Bagge, Brian R. MacKenzie Danish Institute for Fisheries Research, Received

Charlottedud

2 November

Castle. DK-2920.

1996: accepted

25 February

Chnrlottenlund,

Dermurk

1997

Abstract

Identifying mechanisms of exchange between adjacent fish populations is important to understanding causes of fluctuations in abundance. This study addresses the hypothesis that the abundance of settled O-group plaice along the Danish coast of the Kattegat depends on transport from the Skagerrak. Abundance data are derived from annual ( 19571994; lo-15 years missing depending on sample site) juvenile fish surveys conducted at four sites by the Danish Institute for Fisheries Research. The O-group abundance measured in July-August is significantly higher in years when wind conditions during the larval development period (March-April) were moderate to strong. Meristic variation (number of anal fin rays) depends on wind conditions in a manner consistent with the role of wind on abundance. In years with strong winds, meristic variation along the coast is low due to influx of progeny from the Skagerrak; in years with weak winds (when influx of Skagerrak progeny is low), regional variation in meristic counts is larger. These abundance and meristic patterns are consistent with historical observations of stock distribution and mixing in the area, and demonstrate the potential for physical processes to mediate exchange of eggs and larvae between areas. Abundances showed no evidence of long-term changes, even after allowing for the significant role of wind on abundance, and despite eutrophication of the Kattegat. 0 1998 Elsevier Science B.V. All rights reserved. Keywords: O-group plaice; abundance; distribution; Kattegat; wind; eutrophication

1. Introduction

this possibility for plaice, Pleuin the Kattegat-Skagerrak region, and consider in particular the influence of wind on transport of the pelagic stages from the spawning site to O-group nursery sites. An important spawning site for plaice in this region is located west of Skagen (northern Denmark) along the Danish coast and some spawning is also believed to occur in the western deeper parts of the Kattegat (Poulsen, 1938). Since plaice eggs and larvae are pelagically distributed (Coombs et al., 1990) their distribution and settling sites will partly depend on water mass transport. Poulsen (1938), Nielsen and Bagge (1985) We evaluate

ronectes platessa,

Recruitment variability in flatfish is believed to be coarsely determined during the pelagic phase with some further adjustment of year-class strength after settlement (Van der Veer et al., 1990; Leggett and DeBlois, 1994; Rijnsdorp et al., 1995). If this sequence of events is true, then the interannual abundance of settled O-group stages may be associated with processes affecting distribution, growth and survival of the egg and larval stages. * Corresponding

author. E-mail: [email protected]

1385-l 101/98/$19.00 0 1998 Elsevier Science B.V. All rights reserved. PII Sl385-I 101(97)00014-2

12

E. Nielsen et al. /Journal of Sea Research 39 (1998) II-28

and Pihl(l990) have shown that O-group plaice nursery areas are located along the Danish and Swedish coasts of the Kattegat and in the Belt Sea. The large-scale circulation pattern in the northern Kattegat depends mainly on the interaction between Baltic runoff and local variations due to wind stress (Poulsen, 1991; Jakobsen et al., 1994; Rodhe, 1996; Gustafsson and Stigebrandt, 1996). The circulation is generally characterized by a brackish surface outflow from the Baltic and a saline deeper inflow from the Skagerrak and the North Sea (Rodhe, 1996; Gustafsson and Stigebrandt, 1996). In addition to this circulation pattern, there is a major salinity front at the juncture of the Kattegat and Skagerrak. The location of the front is highly variable (Richardson, 1985; Poulsen, 1991) and, depending on wind conditions, can be situated much farther north into the central Skagerrak or to the south (Gustafsson and Stigebrandt, 1996). We hypothesise that variations in the circulation pattern and frontal position may be responsible for fluctuations in the transport of plaice eggs and larvae and in the abundance of settled O-group plaice in Kattegat nursery areas. Poulsen (1938), Nielsen and Bagge (1985) and Pihl (1990) have shown that Ogroup plaice can be found in shallow-water (< 2 m) nursery areas along the Danish and Swedish coasts of the Kattegat and in the Belt Sea. Abundances in these areas vary about lo-20 fold between years and sites (Nielsen and Bagge, 1985; Pihl, 1990). Nielsen and Bagge (1985) found that temperature and salinity fluctuations, as proxy indicators for water mass movements, often coincided with fluctuations in Ogroup abundance along the Danish coast of the Kattegat during the years 1950-1984. Pihl (1990) found that in Gullmar Fjord (Swedish Kattegat), O-group plaice abundance during 1978-1988 was higher in years when there were frequent onshore winds during spring. These two studies suggest therefore that regional water mass movements, and their dependence on wind conditions, may be important factors responsible for supplying nursery areas with metamorphosing larvae. In addition to the possible influence of windrelated transport on O-group abundance, eutrophication of the Kattegat may have contributed to longterm variations in abundance. During the period of our study, phytoplankton primary production in the Kattegat has increased nearly 3-fold (Richardson and

Heilmann, 1995) the area of nursery sites annually covered by filamentous green algae has greatly increased (Bagge and Nielsen, 1989; Isaksson and Pihl, 1992; Rosenberg et al., 1996), and benthic biomass has increased nearly 2-fold (Josefson, 1990). These ecosystem changes could affect O-group plaice abundance due to enhanced food supply for the settled stages, or to changes in sediment structure and vulnerability to shrimp predation (Isaksson and Pihl, 1992; Pihl and Van der Veer, 1992). We have evaluated how both wind and eutrophication may have affected abundance of O-group plaice using historical O-group survey and wind data for the Kattegat-Belt Sea region. Our results, based on statistical analyses of data sets containing 23 to 28 annual observations spanning a period of 38 years, show that wind conditions during the egg and larval drift period have a positive effect on O-group abundance and are associated with up to 20-fold variability in O-group abundance at specific sites along the Danish coast of the Kattegat. However, we found no evidence of a long-term overall increase or decrease in abundance during the study period which coincided with regional eutrophication. 2. Materials and methods

2.1. General Spawning stock biomass exerts a major influence on the abundance of recruits in several marine fish populations (Myers and Barrowman, 1996). As a prerequisite to considering how transport might affect O-group abundance, we first considered whether the abundance of O-group plaice in summer could be related to the winter-spring abundance of spawning adults in the Kattegat-Skagerrak regions. The residual variation from such a relationship could then be used to assess the influence of environmental variables on O-group abundance. However, there was no significant effect (P > 0.05) of either the Kattegat (1968-1992) or combined Kattegat-Skagerrak (1979- 1994) spawning stock sizes (ICES, 1993, 1996 respectively) on O-group abundance for any of the four areas where O-group abundance estimates were available. We then evaluated whether wind conditions were associated with variability in O-group plaice abun-

E. Nielsen et al. /Journal of Sea Research 39 (I 998) 1 l-28

dance. Scatterplots and correlation analyses (Pearson r) between O-group plaice data and wind variables identified associations between wind conditions and O-group abundance; significance levels were set to P < 0.01. Recently compiled (1985-1994) meristic and hydrographic data assisted in our interpretation of the wind-abundance results. To assess whether eutrophication affected abundance, we used regression and general linear models to evaluate long-term trends in plaice abundances. We compared abundances before and after the recent increase in benthic macrovegetation and hypoxia events during the 1980s (Bagge et al., 1990; Isaksson and Pihl, 1992; Pihl, 1994) because some of the major ecosystem changes in the Kattegat have occurred recently. Lastly, we used residual variation from the most significant wind-abundance relations to check whether temporal trends were obscured by interannual wind variability. 2.2. Plaice early life-history

in Kattegat

2.2.1. Spawning dates and egg/larval development periods The seasonality of spawning activity, egg or larval abundance, or timing of settlement of O-group plaice in the Kattegat, or in nearby areas like the Belt Sea or Skagerrak is poorly known. However, historical spawning and settling date information is necessary to configure wind variables appropriately. The length of settled O-group plaice in the Danish part of the Kattegat averaged ca. 45 mm in July during the years 1950-1973 (Nielsen and Bagge, 1985). The size at settlement of O-group plaice at four other European nursery sites, including Gullmar Bay (Swedish coast of Kattegat) is ca. 15 mm (Van der Veer et al., 1990). One would expect the mean date of settlement to be approximately 5 June, if growth of settled O-group plaice is determined mainly by water temperature, and is not limited by food availability (as is the case in some other O-group nursery areas: Van der Veer et al., 1990). This estimate is based on a mean sampling date of mid-July (Nielsen and Bagge, 1985). and assumes that growth of Kattegat O-group plaice is consistent with the growth model of Glazenburg (1983) in Van der Veer (1986), and that water temperature in the Danish nursery area is similar to that in Swedish bays (Pihl and Van der Veer, 1992).

13

Larval development time for settled Wadden Sea O-group plaice was estimated to be 50 to 70 days (Karakiri et al., 1991). Given an egg development time of ca. 30 days for a mean temperature of 2°C (Ryland and Nichols, 1975), we estimate that the long-term mean spawning date for plaice in Kattegat is late February to mid-late March. This estimate coincides with the gonad maturity data of Ulmestrand (1992), who showed that some Kattegat adult plaice were mature in February 1990, 1991 and 1992. In addition, plaice larvae have been captured in the northern Kattegat in March 1959 (ICES, 1989) and in early April 1983 (Christensen et al., 1983). Hence plaice eggs and larvae are most likely to be in the water column during February to May. We therefore restricted analyses to these months. 2.2.2. Meristic variation Poulsen (1938) found that plaice along the North Sea coast of Denmark, in the Skagerrak and in the northern part of the Kattegat have more anal fin rays than those in the southern Kattegat and Belt Sea. In general, variation in meristic characters is partly determined by environmental conditions (e.g. temperature) during development and by genetics (Lindsey, 1988; Frank, 1991). In plaice, meristic counts become fixed during the larval stage and by the end of metamorphosis (Molander and MolanderSwedmark, 1975; Brooks and Johnston, 1994). As a result they can be useful in determining environmental conditions during egg and larval development and the geographic origins of different plaice populations (Poulsen, 1938; Dannevig, 1950). We investigated whether the meristics data vary with wind and hydrographic conditions in ways consistent with the results of our abundance-wind analyses. Additional details of the hypotheses tested are given in the results section. The meristic information available is the number of anal fin rays estimated in approximately 100 fish at each of the sampled sites per year. The time series of meristic variation covers the years 1985-1994. 2.3. Data sets 2.3.1. Plaice O-group plaice have been sampled almost annually along the Danish coast of the Kattegat since

14

E. Nielsen et al. /Journal

1950 using a Petersen young-fish trawl at water depths of I to 1.5 m (Nielsen and Bagge, 1985; Bagge and Nielsen, 1993). The ground rope of the trawl is 7 m long and fitted with a 4 kg lead, and the head rope is 7 m long and fitted with cork. Stretched mesh size in the belly and cod-end are 10 and 6 mm, respectively. Total length of the net is 9 m. Its mouth is held open during deployment with 12.5 kg otter-boards (68 x 34 cm) which are separated by a distance of 4.1 m. The vertical opening of the net between head-rope and ground-rope is 65 cm as measured by a diver. The gear is typically towed for 10 min at 1.5 knots (45 m min-’ ) to produce a sampled area of approximately 1900 ml. Lewy et al. (1982) compared the efficiency of this gear with a 2 m beam trawl in 1980 and 1981. These comparisons showed that the young fish trawl caught 6.3 times more O-group plaice than the beam trawl. Sampling was discontinued in 1974 and resumed again in the early 1980s. All samples were collected in mid-late July and early August at several sites in the Kattegat and the Belt Sea (Fig. 1). Within a site, several lo-min tows were made in a direction parallel to the shore in water depths of ca. 1 to 1.5 m. In total, 1051 hauls have been made in the Kattegat over the 38-year period (mean no. of hauls per year was 38). The abundance of plaice per IO-min haul within each site was calculated and then natural log-transformed (Hennemuth et al., 1980) for comparison with wind conditions and for time series analyses. All plaice were measured fresh for total length immediately after capture, and then preserved in formalin. A random sample of 100 plaice per site, or the entire sample if less than 100 individuals were available, was later withdrawn for enumeration of anal fin rays. 2.3.2. Wind We used wind speeds (m SC’) and directions compiled by Danmarks Meteorologiske Institut (D.M.I.) in Nautisk-meteorologisk yearbooks. Recording stations used in our analyses were located at Skagen and Skagen Reef, Kattegat Southwest-Gniben, and Anholt North and Anholt (Fig. 1). Wind measurements were made at noon, 0 to 2 m above sea surface for the lightships, and 4 to 12 m for the lighthouses. The raw wind data have been recorded by calibrated instruments since the late 1950s (Kristensen

of Seu Research 39 (1998) 11-28

* *

Wind Stations light Vmsels

x Sample sites (Belt Sea)

56”-

55”-

I

I

i 14

Fig. 1. Map of Kattegat and Skagerrak showing sampling locations and wind recording sites. The arrow represents the longshore wind velocity component discussed in the text; winds blowing in the direction of the arrow were configured to have negative values and winds blowing in the opposite direction were configured to have positive values. Note that areas 6 and 7 were combined for statistical analyses; hereafter they are referred to as area 67.

and Frydenholm, 1991). Prior to this period, observations were made visually and according to the Beaufort scale. Since these data are more likely to be biased than the instrumented recordings (Kristensen and Frydenholm, 1991), years prior to 1957 were excluded. The wind data after 1957 are not continuous and have not been recorded at exactly the same location. In years when no wind data were available, that year’s plaice abundance was excluded from analyses. Between 1971 and 1979 the sea-based lightships were withdrawn from service. However, wind data were also recorded for long periods at nearby land-based instrumented lighthouses (Skagen: 19601994+, Anholt 1963-1994+, and Gniben: 19611994+). To develop a continuous time series of wind observations for each site and to intercalibrate the measurements for differences in methodology (e.g. instrument height), we developed regression mod-

15

E. Nielsen et 01. /Journal of Sea Research 39 (1998) I I-28

els for the periods of overlap between the lightship and lighthouse observations (dependent variable was the lightship observation). We used these regression models (see Appendix A) to develop a proxy time series of recent wind data for the Skagen Reef, Anholt North and southwest Kattegat lightships. 2.3.3. Wind variables We first compared O-group abundances with the mean monthly wind speed. However, field studies show that short-term wind events in which winds are unusually strong or calm, or originate from particular directions, have major impacts on coastal water mass movements (Rydberg et al., 1996) and transport of fish eggs and larvae (Taggart and Leggett, 1987; Heath, 1992). Since such events can be obscured by monthly-averaging, we developed a simple wind index to represent strong wind occasions, and situations when winds from given directions exceeded velocities of a specified threshold. The first wind index chosen was the number of days per month when wind speed exceeded 7.5 m s-‘. This value was chosen because it exceeds the long-term mean wind speed for the region (Moller and Hansen, 1994) during the late winter-spring period, and therefore may be a reasonable proxy estimate of the potential for strong winds to produce above-average current speeds and water mass movements. In addition, we developed wind indices to represent the longshore vector component of wind velocity. This component was calculated as WLoNo = -W x cos(A + 20), where W is the wind speed (m s-’ ) and A is the direction (degrees) from which the wind is originating (Schneider and Methven, 1988). The principal axis of the Kattegat was considered to be 20 degrees west of due north. Positive and negative longshore wind components are assumed to induce a flow of surface water in the Kattegat approximately northwestward and southeastward respectively (Fig. 1). We assumed that longshore velocities exceeding the 75th percentile of the long-term (1957-1993) velocity distribution would be of sufficient strength to generate a significant flow northwards. We hypothesised that in such situations the transport of water and plaice into the Kattegat would be reduced. Similarly for the longshore component in the negative direction, we assumed that velocities less

than the 25th percentile of the long-term distribution would advect water masses towards the Belt Sea and via Coriolis effects towards nursery areas along the Danish coast, and perhaps weaken the surface outflow from the Baltic. We hypothesised that during these situations the transport of water and plaice into the Kattegat would be enhanced. Variables were then developed which tallied the number of days per month when the longshore velocity component exceeded the 75th percentile or were less than the 25th percentile (WLONG75% = 3.6ms’) and W~~~o25% = -3.4 m s’ ). 2.3.4. Hydrographic data We compared the meristic variation with hydrographic survey data (e.g., temperature, salinity) contained in the ICES data base. These data were used to calculate monthly averages for the northern, central and southern regions of the Kattegat corresponding approximately to the former locations of the lightship positions (Skagen Reef-Kattegat North: 57”40’57”50’; Anholt North-Kattegat Central: 56”45’-57 20’; Kattegat Southwest-Kattegat South: 55”55’56”45’; see also Fig. 1). The survey data are mean temperature and mean salinity recorded at depths of 0, 15 and 35 m, respectively. These depths were chosen because they correspond to the relatively fresh surface layer due to outflow of brackish water from the Baltic Sea (Rodhe, 1996), the thermocline, and the deep saline layer due to North Sea/Skagerrak water (Heilmann et al., 1994). 3. Results 3.1. Plaice abundance:

spatial comparisons

The overall raw abundance (mean &2 standard errors) for all sites and years was 18.7 h4.8 per 1000 m*. Plaice were most abundant in area 1 (Fig. 2; 3 1.3 f 11.6) and least abundant in area 67 (4.5 f 2.0; see also Appendix A, Table A4). Areas 2 and 5 had intermediate abundances (28.4% 10.8 and 12.7f7.2, respectively). The site-specific coefficients of variation (100 x SD/mean) of abundance among years was 37 to 82%, with areas 1 and 67 having the smallest and largest CVs, respectively. Correlations in abundance between sites showed that abundances at adjacent sites tended to co-vary, with the covari-

E. Nielsen et al. /Journal

16

Area 2

of Sea Research 39 (1998) II-28

I

decrease over the period of our study (Fig. 2). Probability levels relating log(abundance) to year in linear regression models were 0.07 for area 1 and 0.6 to 0.8 for areas 2, 5 and 67. Since abundance may have increased in a step-like fashion after eutrophication during the 197Os, we compared abundances before and after 1979 in a two-way analysis of variance using time (i.e. before or after 1979), site and a time x site interaction term as class factors. However, only the site term was significant (P < 0.0001, R* = 37%, P = 0.18 and 0.45 for the time and interaction terms, respectively). 3.3. Plaice abundance: wind effects The northernmost site had stronger winds than the southernmost site in nearly all years of our time series (Fig. 3). However, all three sites tended to have the same amount of variability, as expressed by 12 11

1960

1970

1960

A

T

1990

Fig. 2. Abundances of O-group plaice at four sites along Danish coast of the Kattegat during 1957- 1994.

the

ante decreasing with increasing distance between sites (Table 1). Mean plaice size was 40 to 70 mm.

I

I

25

/

,

1970

1960

I

I

I

I

1990

1980

2000

,

3.2. Plaice abundance: long-term time trends The abundance of O-group plaice in our four sampling areas showed no systematic increase or Table 1 Correlation coefficients, r, between the abundance of O-group plaice in different areas of the Kattegat and Belt Sea Area 1

Area 2

0.5540 b 0.4429 a 0.3416 0.3747

1.00 0.4445 0.3343 0.5488

Area 5

Area 67 I

Area Area Area Belt

2 5 67 Sea

a Significant b Significant

at 5% level; at I % level.

1960

1.oo 0.7397 b 0.4435

1.00 0.7850 =

1

I

1970

I

1980

I

I

1990

I

2000

Fig. 3. (A) Mean wind speed in April during 1957-1994 at Skagen and Kattegat Southwest. (B) Number of days in April when wind speed at 12 noon exceeded 7.5 m s-t during 1957-1994 at Skagen and Kattegat Southwest. Circles: Skagen: squares: Kattegat Southwest.

17

E. Nielsen et al. /Journal of Sea Research 39 (1998) 1 l-28

,62

r=.69

c..

01

3

2 6 10 14 18 22 Number of days with wind speedz7.5

4 5 6 7 10 Mean wind speed (m+ec-l)

D

T

12

‘65 ‘67

‘66

‘73

2

z g1 C l-

‘80

0

I 2

I 6

I

r=.57

‘62 I I 10

I I 14

I I 18

0

r=.51 I

22

Number of days with wind speed>7.5

I

0

2

I

4

I

6

I

8

I

I

I

10

12

14

Number of wmd days WY<-3.4

Fig. 4. (A) Abundance of O-group plaice in area I relative to number of days in April when wind speed exceeded 7.5 m s-’ at Skagen. (B) Abundance of O-group plaice in area 67 relative to mean wind speed in April at Kattegat Southwest. (C) Abundance of O-group plaice along the Danish coast of the Kattegat relative to the number of days in April when wind speed exceeded 7.5 m s.-’ at Skagen. (D) Abundance of O-group plaice in area 67 relative to the number of days in April when the longshore wind velocity in the negative direction was less than -3.4 m s-’ at Kattegat SW. See Table 2 for regression statistics for panels A-C. For panel D, the relationship is y = 0.18x + 0.37 (r = 0.51, P < 001). All abundances are expressed as I+ numbers per IO-min haul: one haul samples an area of = 1900 m2. Symbols on panels denote sample years.

the coefficients of variation of mean monthly wind speed (CV = 20.3-21.4). Comparison of O-group plaice abundances (natural log-transformed) with the wind data showed that abundances in the northern Kattegat (area 1) and southern Kattegat (area 67) were significantly higher in years when wind speeds (expressed as monthly means, or number of days per month with wind speed > 7.5 m s-‘) were higher (Fig. 4A, B; Tables 2 and 3); in fact, in all cases involving a significant correlation, the sign of the correlation coefficient was positive (Tables 2 and 3). Abundances at areas 2 and 5 varied independently of all wind variables (P > 0.01). When the mean abundance for the entire Danish coast of the Kattegat was used in analyses, abundance was also higher in years when wind speeds were higher (Fig. 4C, Tables 2 and 3).

For the two areas (areas 1 and 67) where plaice abundance co-varied with wind speed, the correlations were almost always strongest with the wind conditions measured at the closest recording site. Thus in the northern Kattegat (area 1). abundance varied with winds measured at Skagen, but not at Anholt or Kattegat SW. Similarly, winds at Kattegat SW or Anholt explained significant amounts of variation in plaice abundance at the southern Kattegat site (area 67) whereas winds at Skagen did not (Tables 2 and 3). March and April had the strongest associations between abundance and wind conditions: there were also strong correlations for May, but none for February (Tables 2 and 3). Nearly all of the significant wind-abundance correlations involved the mean monthly wind speed, or

18

E. Nielsen et al. /Journal

Table 2 Monthly correlations

between O-group plaice abundance

of Sea Research 39 (1998) II-28

at different

sites in the Kattegat

and number of days with wind speed > 7.5 m

SK’

Sampling

site

1 67 67 67 Kattegat Kattegat Only relationships

Wind station

Month

R

P

Model

Skagen Kattegat SW Kattegat SW Anholt Skagen Kattegat SW

April March April May April March

0.69 0.53 0.54 0.52 0.57 0.54

0.0003 0.0036 0.0027 0.0054 0.0014 0.003 1

Y =0.18x y = 0.10.x y = 0.14.x y = 0.18.x y = 0.1 Ix y = 0.08.~

of ca. 2 (Fig. 6). Areas 67 and the Belt Sea were more variable (range 5 1 to 55) than areas I, 2 and 5 (range 54 to 55). Since meristic variation in fish can be influenced by environmental conditions (Lindsey, 1988), we evaluated whether temperature and salinity near the nursery site were associated with meristic variation. This analysis assumes that eggs and larvae are produced near O-group nursery sites and that conditions here determine meristic variation. However, meristic variability was independent of both surface temperature and salinity during MarchApril at the recording station nearest to each O-group sampling site. For example, fin ray counts were stable across years (P > 0.05) for areas in the northern Kattegat (i.e. areas 1, 2 and 5; Fig. 6) despite a 6 degree range in temperatures. In contrast, counts were more variable in area 67 (southern Kattegat) and the Belt Sea (Fig. 6) even though temperatures varied by only 3 degrees at these sites; however, there was no significant correlation (P > 0.05). Similarly, no significant effects of local surface salinity on fin ray counts were seen within any of the sites (P > 0.05). These observations, together with the effects of both wind speed and the longshore wind velocity

3.4. Meristic variation and wind-hydrography effects

The mean number of anal fin rays in O-group plaice was ca. 51 to 55 with a standard deviation

Sampling

sites

1 67 Kattegat Kattegat Kattegat Only relationships

1.43 0.28 0.56 0.73 + I .32 + 1.81

with P < 0.01 are presented.

the monthly tally of days having wind speed > 7.5 ms -‘. The longshore component of wind direction was not significantly associated with variability in plaice abundance at sites 1, 2 or 5. However, there was a strong and significant association between plaice abundance in area 67 and the negative longshore wind velocity vector (Fig. 4D); i.e. years with strong winds blowing approximately from northeast to southwest tended to be years with more plaice at the southern Kattegat location (area 67). Residuals from our most significant wind-abundance relationships for areas 1 and 67 were analyzed for time trends. Linear regression analysis showed that O-group plaice abundances were stable over the whole time period of our study (Fig. 5A, B; P = 0.46 for area 1 and 0.36 for area 67). In addition, mean residual abundance within each of these sites did not differ before and after 1979 (t-tests; P = 0.79 and 0.13 for sites 1 and 67, respectively).

Table 3 Monthly correlations

+ + + +

between the mean wind speed and abundance

at different sample sites in the Kattegat

Wind station

Month

R

P

Model

Anholt Kattegat Skagen Anholt Anholt

April April May April May

0.55 0.60 0.55 0.51 0.59

0.0063 O.OOO8 0.0022 0.0059 0.0011

y y y y y

SW

with P < 0.01 are presented.

= = = = =

0.73.x 0.54.x 0.65.x 0.50x 0.67x

+ -

0.62 1.75 1.36 0.18 0.74

E. Nielsen et al. /Journal of Sea Research 39 (1998) 11-28

(area 67: r = 0.90, and March (Belt Sea: r = 0.69, P < 0.10, y = 0.41~ + 48.1). The corollary to this hypothesis is that if the meristic variation at area 67 and in the Belt Sea depends on wind-induced transport, differences in meristic variation along the entire Danish Kattegat coast within a year should be related to wind conditions. This hypothesis was tested by comparing the annual difference in mean fin ray count between (1) the northern Kattegat (here considered to be areas 1, 2 and 5) and area 67 and, (2) the northern Kattegat (areas 1, 2 and 5) and the Belt Sea with the threshold longshore wind velocity tally for the month and site that had the strongest association with meristic variation in the Belt Sea or at area 67. For area 67 this was April and Skagen, and for the Belt Sea, this was March and Skagen. As hypothesised, fin ray counts throughout the Danish part of the Kattegat and Belt Sea region tended to be most similar in years when wind conditions are strongest in the direction Skagerrak to Belt Sea (Fig. 7). As an additional test of the possibility that wind-induced transport from north to south may have been associated with the meristic variability, we compared surface salinities in the northern Kattegat during March with the March wind conditions at Skagen for the years covered by our meristic variation time series (1985 to 1994). We hypothesised that surface salinity would be higher in years when wind speeds were higher and when the longshore wind speed vector was most negative because such winds might advect saline Skagerrak water further to the south and also cause more vertical mixing of deep saline water throughout the water column. This hypothesis is supported by the available data (Fig. 8). Surface salinities were significantly higher in years when wind speeds were stronger at Skagen and oriented in the direction Skagerrak-Belt Sea. Similar but weaker patterns were also evident during April (r = 0.48, P - 0.20 and r = 0.42, P - 0.20, respectively, for number of days when negative longshore wind was less than -3.4 m s-’ and number of days in April with winds exceeding 7.5 m s-l).

north Area 1

-3 I.,,.,,,,,,...,,.,, 1960

1970

,, ,,I,,., 1960

.,,.,,,,, 1990

Fig. 5. (A) Time series of variation in abundance of O-group plaice at area 1 after accounting for interannual differences in wind conditions using regression analysis. The regression model used was Mindex + 1) = 0.18 x WIND + 1.43 (Table 2), where WIND is the number of days in April when wind speed exceeded 7.5 m SC’ at Skagen. (B) Time series of variation in abundance of O-group plaice in area 67 after accounting for interannual differences in wind conditions using regression analysis. The regression model used was ln(index + 1) = 0.54 x WIND + 1.75 (Table 3). where WIND is the mean wind speed at Kattegat Southwest.

vector on plaice abundance, suggested that meristic variation in sites where variability was greatest (i.e. area 67 and the Belt Sea) could depend more on hydrographic input of eggs or larvae from other (northern) areas, than on local production of eggs and larvae. We tested this hypothesis by comparing the fin ray counts in area 67 and the Belt Sea with one of our wind indices. The wind index chosen was the one which our previous analyses (Fig. 6D) suggested was most likely to advect eggs and larvae along the north-south axis of the Kattegat (i.e. the negative longshore wind vector). In accordance with our hypothesis, fin ray counts were significantly higher in years when the Skagen wind site had a higher frequency of strong longshore winds from

19

to south

P < 0.005,

during

April

y = 0.66x + 46.71)

20

E. Nielsen et al. /Journal

E "x

60

60

59

.c

56

sz 59 E c 56

;

57

E

5

56

2 m

z ?I r)

55

B

55

54

$

54

53

E 2

53

E 2 c 8 I

52 51

%

60

E

59

56

60

* p

59

5

56

3'

57

c j_ 5

56

i%i 56

z ki

55

5

55

2

54

rl 5

54

58 57

2

53

$

53

2

52

c

52

r"

51

B I

51

50 1966

variation

1

57

52 s aJ 51 I 50

50

Fig. 6. Interannual

of Sea Research 39 (1998) II-28

1990

1994

50 1966

1990

in the number of rays in the anal fin of O-group plaice collected

4. Discussion 4.1. The role of wind in O-group plaice abundance Long-term mean abundances recorded in our surveys are similar to those observed in other areas. For example, the abundance estimates (size range = 40 to 70 mm) for area 1 (31 per 1000 m*) and area 2 (28 per 1000 m*) of the Danish Kattegat are comparable to the abundance of 47 to 86 mm and 80 to 100 mm plaice in the Dutch Wadden Sea, where abundances were 1 to 18 per 1000 m2 (Jager et al., 1993) and 5 to 120 per 1000 m* (Van der Veer et al., 1994). Plaice in area 67 of the Danish Kattegat are much less abundant than in the Dutch Wadden Sea. The differences in abundances between areas could be due to various factors (e.g. spawning stock biomass, predation mortality, substrate type). In addition, the Danish abundance estimates are much lower than abundances observed along the Swedish coast of the Kattegat by Pihl and Van der Veer (1992). This difference could be due to differences in the timing of sampling relative to the timing of settling. The Swedish estimates were made during the period of peak settlement, and the Danish estimates were made about two months later.

1994

in the Kattegat

and the Belt Sea.

Our interannual abundance comparisons with wind conditions demonstrate significant associations involving wind speed and the abundance, distribution and meristic variation of O-group plaice in the Kattegat and Belt Sea. The most important finding of our analysis is that all of the significant relationships involving wind speed and abundance were positive. This pattern is unlikely to have occurred by chance, in which case there should have been some significant negative correlations. However, our results show that, in general, (1) years with strong winds were those in which O-group plaice were most abundant, and (2) this co-variance was strongest for March and April, when eggs and larvae are most likely to be in the water column (Poulsen, 1938; Christensen et al., 1983). A possible explanation for this pattern is the role of wind in transporting eggs and larvae from spawning areas to O-group nursery sites. Several investigations have now demonstrated that exchange through the SkagerraWKattegat frontal region and along the main axis of the Kattegat (Jensen, 1940; Poulsen, 1991; Jakobsen et al., 1994; Rydberg et al., 1996; Rodhe, 1996; Gustafsson and Stigebrandt, 1996) is a frequent phenomenon and at least partly winddriven. Variations in regional circulation patterns can

21

E. Nielsen et al. /Journal of Sea Research 39 (1998) 11-28

32 IT. 2 30> .c 28 z? E 262 a 24 m” %J 22 cn 20

-

-

-

6

Number of wind days (wyc-3.4 in April by Skagen)

B

-

2 _. E

g

8

9

10

11

12

13

Number of wind days (wyc-3.4 March by Skagen)

Fig. 7. Geographical variation in meristic variation of O-group plaice compared with wind conditions. (A) Difference in number of anal fin rays in O-group plaice collected in the northern Kattegat (areas 1, 2 and 5) and in the southern Kattegat (areas 6 and 7) relative to number of days in April when longshore wind velocities were less than -3.4 m s-t (y = -0.46x + 5.90; r = 0.78, P < 0.05) (B) Difference in number of anal fin rays in O-group plaice collected in the northern Kattegat (areas 1, 2 and 5) and in the Belt Sea relative to the number of days in March when longshore wind velocities were less than -3.4 m SC’ at Skagen t.v = -0.45x + 6.88; r = 0.94, P < 0.05). Symbols represent years.

be expected to affect the supply of larvae to Kattegat nursery areas since a major plaice spawning site is located west and north of Skagen. Two pieces of evidence supported this concept. First, geographic variation in meristic characters of O-groups was related to wind conditions during the egg and larval stages. The number of anal fin rays

7

8 9 10 11 12 Number of wind days-z-3.4

13

14

Fig. 8. Interannual variation in March surface salinity at Skagen relative to the number of days in March when longshore wind velocities at Skagen were less than -3.4 m SC’ (I’ = 1.41.x + 10.2; r = 0.71, P < 0.05).

was larger and less variable in the northern areas than in the southern areas. This trend (see also Bagge and Nielsen, 1993) is consistent with the geographic distribution of meristic variability reported by Poulsen (1938). However, between-site differences in meristic variation are associated with wind conditions, and these same wind conditions are similar to those which are significantly associated with variability in O-group abundance. This result was most evident for the southernmost site in the Kattegat where abundances and meristic variation were most variable. Second, wind variables associated with both meristic variation and abundance are also associated with variations in salinity. During March in the northern Kattegat, surface salinities increased in years when winds at Skagen were stronger, and oriented northwest to southeast. Similar windhydrography patterns were reported earlier, and are probably due to advection of Skagerrak water and the SkagerraWKattegat front into the northern Kattegat, and by vertical mixing of deeper saline water into the brackish surface layer (e.g. Jensen, 1940; Rasmussen, 1995). 4.2. Other factors affecting O-group abundance We are aware that our results are based on statistical associations and therefore may not be indicative of true cause-effect relationships; other interpre-

22

E. Nielsen et al. /Journal of Sea Research 39 (1998) 11-28

tations of these same data may also be valid. In addition, residual variation from even our most significant relationships is large (ca. 50%), and could be associated with many other variables. It is quite possible therefore that the relationships identified here may not be applicable in future if other ecosystem components vary and interact in ways not seen previously. Below we consider some alternative explanations for temporal variability in abundance of O-group plaice in the Kattegat. 4.2.1. Plaice spawning stock biomass and stock structure For flatfish populations in general, there is little evidence of a relationship between spawning stock biomass and recruitment (Iles, 1994; Rijnsdorp, 1994). However, other evidence shows that recruitment in many other fish species is related to spawning stock biomass (Myers and Barrowman, 1996). We therefore checked whether spawner biomass influenced O-group abundance in our statistical analyses, but found no evidence of a linkage. However, the available time series of spawning stock biomasses for the Kattegat and combined Kattegat-Skagerrak region start only in 1968 and 1979, respectively, and therefore exclude most years covered by our O-group time series. Variations in spawner abundance before 1979 in either the Kattegat or Skagerrak, and the relative contributions of these two populations to O-group abundances in the Kattegat, could have contributed to the variability in our time series which extends back to 1957. Finally, it is difficult to make any definitive statements about the possible effect of spawning stock biomass because of the doubtful quality of input data to stock assessments (ICES, 1996). Hence it cannot be excluded that variations in spawner abundance may have been an important factor determining Kattegat O-group abundances. 4.2.2. Wind as an index of water mass transport Our wind indices are coarse approximations of the area’s complex hydrographic processes, which include at least three different currents (outflowing Baltic water, inflowing deep Skagerrak water, and the Jutland coastal current) and a dynamic frontal zone (see details in Jakobsen et al., 1994; Rodhe, 1996; Gustafsson and Stigebrandt, 1996; Rydberg et

al., 1996). These indices should therefore only be regarded as coarse indicators associated with major hydrographic changes, and not as direct quantitative estimates of the effect of wind on water mass movements. As such they could probably be improved with more detailed physical oceanographic information; if this were done, the variation in plaice abundance and meristics associated with wind and hydrographic conditions could perhaps be higher. A more sophisticated approach to describe such fluxes would be to use a regional 3-D circulation model to simulate drift trajectories of particles like plaice eggs and larvae. Such approaches have been useful in defining drift patterns in the Southern Bight plaice population (Van der Veer et al., 1998) and for other fish larvae (e.g. Werner et al., 1996; Hinckley et al., 1996) and may be appropriate for the Skagerrak/Kattegat plaice population as well. The application of satellite imagery to detect major water mass movements could also be useful. 4.2.3. Small-scale turbulence and larval plaice feeding rates Turbulent water motion at cm-mm spatial scales can increase the encounter rate of larval fish with their prey (Rothschild and Osborn, 1988; MacKenzie and Kiorboe, 1995). If plaice larvae grow at foodlimited rates in the Skagerrak and Kattegat, as they often do in neighbouring areas (e.g. southern North Sea; Hovenkamp, 1990), then years with stronger winds might be beneficial to larval feeding, at least up to some optimal level of turbulence (MacKenzie et al., 1994; Kiorboe and Saiz, 1995). Hence a secondary effect of winds may have been an increase in feeding (Sundby et al., 1994) and growth (Gallego et al., 1996) rates. Alternatively, turbulence is known to disperse aggregated prey distributions (Owen, 1989). Had this process been a major factor affecting interannual variability in O-group abundance, we should have observed a negative relation between wind speed and abundance. However, there were no significant negative correlations in our analyses. Moreover, neither a positive nor negative mechanism involving turbulence can easily explain the patterns between meristic variation, wind and salinity. We therefore do not consider turbulence effects on feeding rates (e.g. via increased encounter rates, or patch dispersal) to be the prin-

E. Nielsen et al. /Journal of Sea Research 39 (1998) II-28

cipal explanation for our results, although have been contributing factors.

they may

4.2.4. Water temperature Water temperature has been shown to affect both the abundance of stage V plaice eggs (Zijlstra and Witte, 1985) and recruitment of O-group plaice (Van der Veer et al., 1990; Pihl, 1990). The mechanisms by which temperature affects egg abundance and recruitment are unclear, but seem to be related to conditions on the spawning grounds and predation by the brown shrimp (Crangon crangon) on the newly settled O-groups (Zijlstra and Witte, 1985; Van der Veer et al., 1990; Pihl, 1990). These effects may also have influenced the abundance of O-group plaice observed in our surveys. To evaluate this possibility, we conducted two additional statistical analyses. The first evaluated the hypothesis that temperatures in the surface layer affect O-group abundance via a detrimental effect on survival of eggs. For this test, we used temperatures recorded at Skagen because (i) this site is located between the two main spawning sites in the Skagerrak and Kattegat; and (ii) eggs and larvae appear to be advected past this location during their drift period (see Section 3). The months used for this analysis were February and March which corresponds to the period when eggs are most likely to be in the water column. However, the temperature time series for the period 1957-1994 is not continuous due to the withdrawal of lightships in the 1970s and a lack of data from research vessels in the earlier decades. It is not possible therefore to construct an intercalibration temperature time series as was done for the wind data (see Section 2). For these reasons we divided the plaice time series into two periods (i.e. 19571973 and 1985-1994) for separate analyses. The result of this analysis showed that there was no significant association (P > 0.05) between surface temperature and O-group abundance in any of our four sampling areas, nor for the average abundance along the entire Danish coast of the Kattegat. This result was seen for both periods of our time series. The second temperature-related hypothesis we tested was related to the effect of temperature on predators of O-group plaice. The main predator on

73

newly-settled O-group plaice is the common sand shrimp (Van der Veer and Bergman. 1987; Van der Veer et al., 1990; Pihl and Van der Veer, 1992; Gibson et al., 1995). In cold winters these predators move to deeper water and their return to the shallow inshore areas is delayed (Pihl and Rosenberg, 1982; Pihl, 1990; Van der Veer et al., 1990). Hence, O-group plaice which settle in cold years enter a relatively predator-poor habitat and should have a higher probability of surviving. We evaluated this hypothesis for Danish areas of the Kattegat using bottom temperatures (to which benthic predators should be most sensitive) during February and March (the coldest months of the year) recorded at Skagen. The analysis was conducted for the months of February and March for the two separate time periods. However, bottom temperatures explained none (P > 0.05) of the interannual variation in O-group abundance for any of our four sampling areas, nor for the average of the four areas. These two analyses indicate that temperature variations may be less important regulators of O-group abundance than other factors or that temperature interacts in more complex ways than can be revealed by our simple correlation analyses. A temperature-related shrimp predation effect on abundance of newly settled plaice could nevertheless exist for the Danish part of the Kattegat since shrimps are common (Nielsen and Bagge, 1985; Pihl, 1990). Unfortunately quantitative annual estimates of shrimp abundance are not available for our region and its role cannot yet be established. 4.2.5. Eutrophication As outlined in Section 1 the Kattegat ecosystem has been altered in many ways by eutrophication. A change in plaice abundance with eutrophication could perhaps have been expected because O-group plaice abundance in the Swedish Kattegat is maximal when sediment organic matter content is 1 to 2% (Pihl and Van der Veer, 1992) and vulnerability to shrimp predation would probably change with a change in sediment structure (Pihl and Van der Veer, 1992; Isaksson and Pihl, 1992). Alternatively, the increased supply of organic matter to the benthos could have benefited growth and survival of O-group plaice by increasing prey availability.

24

E. Nielsen et al. /Journal of Sea Research 39 (1998) II-28

However, we found no evidence that O-group plaice abundance in any of the four areas along the Danish Kattegat coast has increased or decreased over the period of our study. In addition, when the significant wind effects on abundance in areas 1 and 67 are removed using our regression models, the remaining variation in abundance showed no evidence of a significant time trend. These observations suggest that O-group abundances have been robust to increases in organic matter production and decomposition. 4.3. O-Group abundance as a recruitment

index?

It is useful to know whether the O-group indices can be used as an early estimate of recruitment. In the Kattegat-Skagerrak region, recruitment is determined by ICES (1996) as numbers of 2-year-old fish. We compared our O-group abundance estimates with the available time series of VPA-based recruitment estimates for the combined Kattegat-Skagerrak region. For these comparisons we used the annual mean O-group abundance for the entire Danish coast of the Kattegat (i.e. areas 1,2, 5,67). However, there was no significant correlation between the O-group and 2-group log-transformed abundance indices (P > 0.05). This is probably partly due to poor quality of input data which prevents working groups from making reliable stock assessments (ICES, 1996). In addition, other processes that determine the abundance of 2-year olds in the Kattegat and Skagerrak may occur in the period between our O-group surveys and the time of recruitment. This observation suggests that transport is a necessary condition for good plaice recruitment in the Kattegat, but that other processes subsequent to settlement are also important in determining the abundance of 2-year-old recruits. 5. Conclusions We have compared the abundance of O-group plaice in the Danish part of the Kattegat with regional wind conditions for the period 1957 to 1994. Our results show that years with strong winds during the egg and larval drift period, particularly when oriented along the main axis of the Kattegat, co-

incide with years of high plaice abundances in Danish Kattegat nursery sites. The main points of the plaice-wind-hydrography interaction are as follows. Plaice spawning to the west and north of Skagen introduce their eggs and larvae to a saline surface layer during March to April. These plaice have at metamorphosis a natural biological tag (high fin ray counts) that distinguishes them from plaice spawning in the Belt Sea and western Baltic (Poulsen, 1938). As with most meristic characters, this tag will be genetically controlled but susceptible to environmental influences during ontogeny (Lindsey, 1988). The movements and mixing of higher salinity Skagerrak water depend partly on wind conditions (Jensen, 1940; Poulsen, 1991; Gustafsson and Stigebrandt, 1996; Rydberg et al., 1996), with strong winds, particularly from the north-northwest, inducing mixing and advection towards the east and south. During the pelagic phase, first-feeding plaice larvae can be expected to be distributed relatively high in the water column because laboratory studies show that feeding incidence is highest at light intensities of 100-1000 lux (Huse, 1994) [by comparison, first-feeding cod larvae have a peak feeding incidence at light intensities of l- 10 lux (Huse, 1994)]. Our inferred vertical distribution for plaice larvae increases the likelihood that they will be susceptible to wind-induced advection of water masses. The advection of water from the Skagerrak is likely to carry plaice eggs and larvae into the Kattegat where they settle in shallow nursery areas (Nielsen and Bagge, 1985; Pihl, 1990). Areas closest to the Skagerrak spawning site have O-group plaice whose meristic counts are most similar to the Skagetrak spawning population, whereas O-groups further to the south have meristic counts more similar to adults found in the Belt Sea. However, in windy years, the entire Danish Kattegat coast is more likely to be supplied with offspring produced in the Skagerrak than in calm years. This will reduce the geographic variation in meristic variability. We conclude that the contribution of progeny originating from the Skagerrak significantly increases the abundance of O-group in the Danish Kattegat, and probably also the Swedish Kattegat (Pihl, 1990). In particular, our results suggest that the southern part

25

E. Nielsen et al. /Journal of Sea Research 39 (1998) 11-28

ing hydrographic data, Andy Visser for physical oceanographic discussions, and our reviewers for their suggestions to improve an earlier version of the manuscript.

of the Danish Kattegat (area 67) would have very few O-group plaice without wind-induced transport of eggs and larvae from the north. Acknowledgements

We thank Martin Scherfig for assistance in the field and laboratory, 0. Vagn Olsen for extract-

Appendix A Table A 1 Correlation (R’) between speed and direction Correlation coefficient February March April May

two overlapping

time series (Skagen

Skagen

1960-1978.

Kattegat

Kattegat

SW 1961-1973.

Anholt

1963-1978)

of daily wind

Anholt

SW

Wind speed

Direction

Wind sneed

Direction

Wind speed

Direction

0.8830 0.8676 0.7502 0.7829

0.6681 0.6288 0.6344 0.6220

0.6650 0.8014 0.7736 0.7093

0.7450 0.75 I I 0.7629 0.6368

0.6635 0.7236 0.6721 0.6330

0.6144 0.6291 0.5681 0.4716

In all cases, the regression model is OLD = a x NEW + b, where OLD is the predicted daily wind speed or direction based on the NEW value for wind speed or direction measured by instrumented lighthouses. All correlations are highly significant (P < 0.01).

Table A2 Slopes of linear regression Slope

February March April May

models relating wind speed and direction

Skagen

Kattegat

for two overlapping

time series of daily wind speed and direction

SW

Anhoh

Wind speed

Direction

Wind speed

Direction

Wind speed

Direction

0.9753 0.9785 0.9722 0.9586

0.7986 0.7607 0.7769 0.7581

0.6539 0.6846 0.6959 0.6015

0.8496 0.8445 0.8554 0.765 1

0.6247 0.7137 0.7436 0.7027

0.8084 0.8289 0.7705 0.7068

In all cases, the regression model is OLD = a x NEW + b, where OLD is the predicted daily wind speed or direction value for wind speed or direction measured by instrumented lighthouses. See also Table Al.

Table A3 Intercepts of linear regression Intercept

February March April May

models relating

wind speed and direction

Skagen

Kattegat

for two overlapping

based on the NEW

time series of daily wind speed and direction Anholt

SW

Wind speed

Direction

Wind speed

Direction

Wind speed

Direction

4.579 2.878 1.278 -0.374

41.37 47.36 42.23 45.24

26.41 16.34 14.71 19.24

41.26 34.42 31.13 40.19

31.15 17.51 15.50 14.99

34.63 25.68 47.05 62.71

In all cases, the regression model is OLD = a x NEW + b, where OLD is the predicted value for wind speed or direction measured by instrumented lighthouses.

daily wind speed or direction

based on the NEW

26

E. Nielsen et al. /Journal of Sea Research 39 (1998) 11-28

Table A4 Abundance

indices for O-group plaice used in statistical

Area

1

Year

mean

std

N

mean

std

1.95 2.48

1.42 1.1

17 13

4.02 2.43 0.4 1

0.45 3.44

1.95 1.20 3.67 3.05 5.45 3.59

1.67 1.43 1.48 1.38 0.13 0.97

6 12 6 18 3 10

4.99 1.65 3.43 3.28 4.67 2.67

3.78 4.85 3.77 3.86 4.70 4.70

0.42 0.19 1.10 0.66 0.42 0.65

10 10 6 22 12 18

2.32 3.92

I .65 0.96

6 10

3.52 4.43 4.43 4.38

0.34 0.42 0.41 0.41

12 12 9 I2

4.39 3.21 4.68

0.53 0.37 0.27

12 2 I8

1957 1958 1959 1960 1961 1962 1963 1964 1965 1966 1967 1968 1969 1970 1971 1972 1973 1980 1984 1985 1986 1987 1988 1989 1990 1991 1992 1993 1994

All indices are presented

analyses

2

as ln(abundance+

5 mean

std

4 2 1

1.76 2.03 1.60

1.46 1.72 0.49

6 5 2

0.12 0.76 2.22 0.79 0.44 1.52

3 4 8 7 2 9

3.5 0.51 1.98 3.59 4.43 3.79

0.94 0.82 2.21 0.81 I .34 0.58

6 5 5 7 3 4

4.57 4.70 3.79 3.31 5 5.32 1.12 2.13 4.28

0.53 0.04 1.5 0.33 0.88 0.67 0.55 0.6 0.98

3 3 5 5 5 5 I6 32 16

2.94 1.94 4.48 1.52

0.07 0.78

4.93 1.01 1.62 0.66

0.86 0.67 0.7 0.63

3.58 4.12 3.30 3.18 3.85 3.71 3.80 2.35

0.63 1.04 0.71 0.98 0.75 0.41 0.58 0.80

12 26 25 24 24 9 24 22

0.68 3.43 2.53 I.88 3.73 2.48 1.37 4.07

0.88 1.17 0.51 0.92 0.55 0.35 0.86 0.21

1). where abundance

N

67 N

mean

std

N

7 10 8 IO

0 1.88 1.04 1.10 1.99 0.06 0.51 2.13 3.51 3.37 3.64 2.59 0.75 2.3 1.29 1.2 3.08 0 0.31 1.69

0 1.47 0.57 0.51 1.16 0.15 0.70 1.59 0.72 I .4 1.42 1.25 0.85 0.79 0.77 0.43 1.5 0 0.47 0.97

II 13 6 6 13 7 11 I4 16 10 10 8 6 7 6 7 21 6 15 14

8 I2 7 5 8 8 4 I5

0.35 0.63 0.33 1.22 2.22 2.46 0.84 2.79

0.38 0.69 0.44 0.83 0.57 1.19 0.65 1.01

6 16 I5 18 26 15 14 30

0.6

is number of plaice captured per tow. One tow covers an area of 1900 m?.

References Bagge, O., Nielsen, E.. 1989. The change in abundance and growth of plaice and dab in Subdivision 22 1962-85. Rapp. P.-v. R&n. Cons. Int. Explor. Mer 190, 183-192. Bagge, 0. and Nielsen, E., 1993. Abundance of O-group plaice in different areas in the Kattegat and in the Belt Sea in the period 1950-1992. ICES CM 1993/5:9: l-23. Bagge. O., Nielsen. E., Mellergaard, S. and Dalsgaard, I., 1990. Hypoxia and the demersal fish stock in the Kattegat (IIIA) and Subdivision 22. ICES. CM 199O/E:4: l-52. Brooks, S., Johnston, I.A., 1994. Temperature and somitogenesis in embryos of the plaice (Pleuronectes platessa). J. Fish Biol. 45,699-702. Christensen, V., Dahl, E., Danielssen, D.S., Hundahl, H., Ki@rboe, T. and Kullenberg. G.. 1983. A combined fish larval. phytoplankton. and oceanographic survey in the Skagerrak and the Kattegat in April 1983. ICES CM/L:26. Coombs, S.H., Nichols, J.H., Fosh, CA., 1990. Plaice eggs

(Pleuronectes piatessa L.) in the southern North Sea: abundance, spawning areas. vertical distribution, and buoyancy. J. Cons. Int. Explor. Mer 47, 133-l 39. Dannevig. A., 1950. The influence of the environment on number of vertebrae in plaice. Report on Norwegian Fishery and Marine Investigations Vol. IX, No. 9, 6 pp. Frank, K.T., 1991. Predicting recruitment variation from yearclass specific vertebral counts. An analysis of the potential and plan for verification. Can. J. Fish. Aqat. Sci. 48, 1350-1357. Gallego, A., Heath, M.R., McKenzie, E., Cargill, L.H., 1996. Environmentally induced short-term variability in the growth rates of larval herring. Mar. Ecol. Prog. Ser. 137, 1 l-23. Glazenburg, B., 1983. Een experimenteel onderzoek naar de groei van 0-groep schol en bot in relatie tot de watertemperatur. Inteme Verslagen Nederlands Instituut voor Onderzoek der Zee. Texel, 1983-5: l-19. Gibson, R.N., Yin, M.C.. Robb, L., 1995. The behavioural basis of predator-prey size relationships between shrimp (Crangon

E. Nielsen et al. /Journal of Sea Research 39 (1998) II-28 crangon) and juvenile plaice (Pleuronectes platessa). J. Mar. Biol. Ass. U.K. 75, 331-349. Gustafsson, B., Stigebrandt, A., 1996. Dynamics of the freshwater-influenced surface layers in the Skagerrak. J. Sea Res. 35. 39-53. Heath, M.R., 1992. Field investigations of the early life stages of marine fish. Adv. Mar. Biol. 28. l-174. Heilmann, J., Richardson. K., &tebjerg, G., 1994. Annual distrbution and activity of phytoplankton in the SkagemUKattegat frontal region. Mar. Ecol. Prog. Ser. 112, 213-223. Hennemuth, R.C., Palmer, J.E., Brown, B.E., 1980. A statistical description of recruitment in eighteen selected fish stocks. J. Northw. Atl. Fish. Sci. 1, 101-111. Hinckley. S., Hermann, A.J., Megrey, B.A.. 1996. Development of a spatially explicit, individual-based model of marine fish early life history. Mar. Ecol. Prog. Ser. 139, 47-68. Hovenkamp, F., 1990. Growth differences in larval plaice Pleuronectes platessa in the Southern Bight of the North Sea as indicated by otolith increments and RNA/DNA ratios, Mar. Ecol. Prog. Ser. 58, 205-215. Huse, I., 1994. Feeding at different illumination levels in larvae of three marine teleost species: cod, Gadus morhua L.. plaice, Pleuronectes platessa L., and turbot, Scophthalmus maximus (L.). Aquacult. Fish. Manag. 25, 687-695. ICES, 1989. Report of the Division IIIA demersal stocks working group. ICES CM 1989 Assess: 10. ICES, 1993. Working Group on the assessment of demersal stocks in the North Sea and Skagerrak. Copenhagen, 6-14 October, 1992. ICES CM 1993/Assess:5. ICES, 1996. Report of the Working Group on the assessment of demersal stocks in the North Sea and Skagerrak. ICES CM 1996/Assess:6. Iles, T.C.. 1994. A review of stock-recruitment relationships with reference to flatfish populations. Neth. J. Sea Res. 32, 399420. Isaksson. I.. Pihl, L., 1992. Structural changes in benthic macrovegetation and associated epibenthic fauna1 communities. Neth. J. Sea Res. 30, 131-140. Jager, Z., Kleef, H.L.. Tydeman. I!, 1993. The distribution of O-group flatfish in relation to abiotic factors on the tidal flats in the brackish Dollard (Ems Estuary Wadden Sea) J. Fish Biol. 43 (Supplement A), 31-43. Jakobsen, F.. artebjerg, G., Agger, CT., Hojerslev, N.K., Holt, N., Heilmann, J. and Richardson, K., 1994. Hydrografisk og biologisk beskrivelse af Skagerrak fronten. Havforskning fra Miljostyrelsen. Miljoministeriet, Copenhagen, Denmark No. 49: 106 pp. Jensen, A.G.C., 1940. The influence of the currents in the Danish waters on the surface temperature in winter, and on the winter temperature of the air. Meddr Kommn Danm. Fisk. -og Havunders., Ser. Hydrografi, Bind III, Nr. 2, 52 pp. Josefson, A.B., 1990. Increase of benthic biomass in the Skagerrak-Kattegat during the 1970s and 1980s effects of organic enrichment?. Mar. Ecol. Prog. Ser. 66, 117-130. Karakiri, M.. Berghahn, R., Van der Veer, H.W.. 1991. Variations in settlement and growth of O-group plaice (Pleuronectes

platessa L.) in the Dutch Wadden Sea as determined

27

by otolith

microstructure analysis. Neth. J. Sea Res. 27. 345-35 I. Kiorboe. T., Saiz. E., 1995. Planktivorous feeding in calm and turbulent environments, with emphasis on copepods. Mar. Ecol. Prog. Ser. 122, 135-145. Kristensen, L. and Frydenholm. K., 1991. Danmarks vindklima fra 1870 til nutiden. Havforskning fra Miljostyrelsen. Miljoministeriet, Copenhagen, Denmark. Nr. 2. 57 pp. Leggett, W.C., DeBlois. E.. 1994. Recruitment in marine fishes: is it regulated by starvation and predation in the eggs and larval stages. Neth. J. Sea. Res. 32, 119-I 34. Lindsey, C.C., 1988. Factor controlling meristic variation. Fish Physiol. 11 B, 197-274. Lewy, P., Hoffmann, E. and Nielsen. E.. 1982. Analyser af rodspaetteyngeltogter i Kattegat 1980-81. Internal Report No. 199 Danish Fishery Board. MacKenzie, B.R., Kiorboe, T., 1995. Encounter rates and swimming behaviour of pause-travel and cruise larval fish predators in calm and turbulent environments. Limnol. Oceanogr. 40, 1278-l 289. MacKenzie. B.R.. Miller, T.J., Leggett, W.C., Cyr, S., 1994. Evidence for a dome-shaped relationship between turbulence and larval fish ingestion rates. Limnol. Oceanogr. 39, 17901799. Molander, A.R. and Molander-Swedmark, M., 1975. Experimental investigations on variation in plaice (Pleuronectes platessa Linne). Report No. 7, Biology Series, Inst. Mar. Res. Lysekil, Fishery Board Sweden, 45 pp. Moller, J.S., Hansen, I.S., 1994. Hydrographic processes and changes in the Baltic Sea. Dana 10, 87-104. Myers, R.A. and Barrowman. J.N., 1996. Is fish recruitment related to spawner abundance? Fish. Bull. 94, 707-724. Nielsen, E. and Bagge, O., 1985. Preliminary investigations of O-group and l-group plaice surveys in the Kattegat in the period 1950-84. ICES CM 1985/G:l9, 34 pp. Owen, R.W., 1989. Microscale and finescale variations of small plankton in coastal and pelagic environments. J. Mar. Res. 47, 197-240. Pihl, L., 1990. Year-class strength regulation in plaice (Pleuronectes platessa L.) on the Swedish west coast. Hydrobiologia 195. 79-88. Pihl, L., 1994. Changes in the diet of demersal fish due to eutrophication-induced hypoxia in the Kattegat. Sweden, Can. J. Fish. Aquat. Sci. 5 I, 321-336. Pihl, L., Rosenberg, R., 1982. Production, abundance and biomass of mobile epibenthic marine fauna in shallow waters, western Sweden. J. Exp. Mar. Biol. Ecol. 57. 273-301. Pihl, L., Van der Veer, H.W., 1992. importance of exposure and habitat structure for the populations density of O-group plaice. Pleuronectes platessa L. in coastal nursery areas. Neth. J. Sea Res. 29, 145-152. Poulsen, E.M.. 1938. On the migrations and the racial character of the plaice. Report of the Danish Biological Station XLIII, 80 PP. Poulsen, O., 199 1. The hydrography of the Skagerrak and Kattegat: the dynamics of the Skagerrak front. Series Paper No.

28

E. Nielsen et al. /Journal of Sea Research 39 (1998) 11-28

54, Institute of Hydrodynamics and Hydraulic Engineering, Technical University of Denmark, Lyngby, Denmark, 164 pp. Rasmussen, B., 1995. Stratification and wind mixing in the southern Kattegat. Ophelia 42, 3 19-334. Richardson, K., 1985. Plankton distribution and activity in the North Sea/Skagerrak-Kattegat frontal area in April 1984. Mar. Ecol. Prog. Ser. 26, 233-244. Richardson, K., Heilmann, J.P.. 1995. Primary production in the Kattegat: past and present. Ophelia 41, 317-338. Rijnsdorp, A.D., 1994. Population-regulating processes during the adult phase in the flatfish. Neth. J. Sea Res. 32, 207-223. Rijnsdorp, A.D., Berghahn, R., Miller. J.M., Van der Veer, H.W., 1995. Recruitment mechanisms in flatfish: what did we learn and where do we go?. Neth. J. Sea Res. 34, 237-242. Rodhe, J., 1996. On the dynamics of the large-scale circulation of the Skagerrak. J. Sea Res. 35,9-21. Rosenberg, R., Cato, I., Forlin, L., Grip, K., Rodhe, J., 1996. Marine environment quality assessment of the SkagerrakKattegat. J. Sea Res. 35, l-8. Rothschild, B.J., Osborn, T., 1988. Small-scale turbulence and plankton contact rates. J. Plankton Res. IO, 465-474. Rydberg, L., Haamer. J., Lungman, O., 1996. Fluxes of water and nutrients within and into the Skagerrak. J. Sea Res. 35, 23-38. Ryland, J.S., Nichols, J.H., 1975. Effect of temperature on the embryonic development of the plaice, Pleuronectes platessa L. (Teleostei). J. Exp. Mar. Biol. Ecol. 18, 121-137. Schneider, D.C., Methven, D.A., 1988. Response of capelin to wind-induced thermal events in the southern Labrador Current. J. Mar. Res. 46, 105-I 18. Sundby, S., Ellertsen, B., Fossum, P., 1994. Encounter rates between first-feeding cod larvae and their prey during moderate to strong turbulent mixing. ICES Mar. Sci. Symp. 198, 393-405.

Taggart, CT., Leggett, W.C., 1987. Wind-forced hydrodynamics and their interaction with larval fish and plankton abundance: a time-series analysis of physical-biological data. Can. J. Fish. Aquat. Sci. 44,438-45 I. Ulmestrand, M.. 1992. The geographical distribution, size composition and maturity stages of plaice Pleumnectes platessa L. during spawning season in the Skagerrak and Kattegat. Medd. Havsfiskelab. No. 325. Lysekil, Sweden. Van der Veer, H.W., 1986. Immigration, settlement, and densitydependent mortality of a larval and early postlarval O-group plaice (Pleuronectes platessa) population in the western Wadden Sea. Mar. Ecol. Prog. Ser. 29, 223-236. Van der Veer, H.W.. Bergman. M.J.N., 1987. Predation by crustaceans on a newly settled O-group plaice (Pleuronectes platessa) population in the western Wadden Sea. Mar. Ecol. Prog. Ser. 35, 203-215. Van der Veer, H.W.. Pihl, L., Bergman, M.J.N., 1990. Recruitment mechanisms in North Sea plaice (Pleuronectes platessa). Mar. Ecol. Prog. Ser. 64, l-12. Van der Veer, H.W., Berghahn, R.. Rijnsdorp, A.D., 1994. Impact of juvenile growth on recruitment in flatfish. Neth. J. Sea Res. 32, 153-173. Van der Veer, H.W., Ruardy, I?, Van den Berg, A.J., Ridderinkhof, H., 1998. Impact of interannual variability in hydrodynamic circulation on egg and larval transport of plaice Pleuronectes platessa L. in the southern North Sea. J. Sea Res. 39,29-40. Werner. E.F., Perry, R.I., Lough, R.G., Naimie, C.E., 1996. Trophodynamic and advective influences on Georges Bank larval cod and haddock. Deep-Sea Res. II 43, 1793-1822. Zijlstra, J.J., Witte, J.IJ., 1985. On the recruitment of O-group plaice in the North Sea. Neth. J. Zool. 35, 360-376.