Nursery Area Utilization by Turbot (Psetta Maxima) and Flounder (Platichthys Flesus) at Gotland, Central Baltic Sea

Nursery area utilization by turbot (Psetta maxima) and flounder (Platichthys flesus) at Gotland, central Baltic Sea

Jesper Martinsson1)2) and Anders Nissling1)

1) Ar Research Station, Department of Biology, Gotland University, SE-621 67 Visby, Sweden 2) Department of Systems Ecology, Stockholm University, SE-106 91 Stockholm, Sweden

Received 21 Dec. 2009, accepted 18 May 2010 (Editor in charge of this article: Outi Heikinheimo)

Martinsson, J. & Nissling, A. 2011: Nursery area utilization by turbot (Psetta maxima) and flounder (Platichthys flesus) at Gotland, central Baltic Sea. Boreal Env. Res. 16: 60–70.

To reveal the temporal and spatial utilization of a preferred nursery ground habitat by 0-group turbot and flounder in the Baltic Sea, sampling was conducted in six sandy bays in early–mid-July to early–mid-September 2003–2005 off the coast of Gotland (ICES SD 27 & 28-2). Settlement and peak abundance of turbot occurred from late July–early August to early September and from mid-August to early September, respectively. Settlement of flounder occurred from early–mid-July with decreasing numbers over time, except in 2005. Peak abundance of 0-group flounder occurred in late July–early August to mid-September, suggesting a considerable temporal overlap with 0-group turbot. 0-group turbot and flounder also overlapped in respect to depth with preference for 0.2 and 0.6 m over 1 m. The spatial and temporal overlap of the species was verified by a logistic regression analysis; the probability of sampling 0-group flounder when 0-group turbot was caught in a haul was 0.84 (0.80–0.87, 95% CI).

Introduction Miller et al. (1991), factors controlling flatfish recruitment vary due to differences in life history Conclusions concerning the factors that affect traits between species and over species range of the variability in recruitment of flatfishes, as distribution. summarized by Van der Veer et al. (2000), have The life history traits of flounder (Platich- in general been derived from studies of com- thys flesus) and turbot (Psetta maxima) in the mercially important species such as plaice (Pleu- Baltic Sea differ from those in e.g. the North ronectes platessa) and sole (Solea solea) in the Sea in accordance with abiotic conditions. Due North Atlantic, mainly the North Sea area. The to brackish water, with salinity as a strong evo- findings stress that variability in recruitment lutionary force, populations with specific repro- is generated during the egg- and larval stages ductive strategies have evolved (Bagge 1987, under the control of large scale abiotic fac- Nissling et al. 2002). Two genetically distinct tors, and that density-dependent mortality after (Florin et al. 2005, Hemmer-Hansen et al. 2007) settlement dampens recruitment variability as flounder populations inhabit the Baltic Sea, one the early juveniles concentrate in the nursery spawning offshore in the Baltic deep basins grounds (Beverton 1995). However, according (ICES SD 24–26 and 28-2) with highly buoyant to the “species range hypothesis”, proposed by pelagic eggs (enabling neutral buoyancy in the Boreal Env. Res. vol. 16 • Nursery area utilization by turbot and flounder 61 brackish water), and one in coastal areas and on was to assess the temporal and spatial distribu- off-shore banks (ICES SD 25–30) with demer- tion of the two species within typical nursery sal eggs (Bagge 1981, Nissling et al. 2002). In grounds in terms of (i) settlement patterns and contrast, turbot in the Baltic Sea only spawn in peak abundances, (ii) depth distribution, and coastal areas and on off-shore banks (ICES SD additionally (iii) to examine the overlap between 24–29) and produces demersal eggs (Nissling et 0-group turbot and flounder at a small-scale to al. 2006). reveal the potential for species interaction even- In the North Sea, variation in hydrodynamic tually affecting survival of turbot. The study is and wind conditions have been shown to control a part of a research programme at Ar Research the transportation of plaice larvae to the nursery Station, Gotland University, aiming to reveal grounds and therefore works as factors generat- mechanisms affecting recruitment of turbot and ing variability in recruitment (Pihl 1990, Van der flounder in the Baltic Sea. Veer et al. 1998). This might be of less impor- tance in controlling the recruitment of turbot and flounder in the Baltic Sea since they spawn Material and methods along the coast, i.e. close to the 0-group nursery grounds. Further, predation by the brown shrimp Sampling (Crangon crangon) and shore crab (Carcinus maenas) have been shown to dampen recruit- Juvenile flounder and turbot were sampled in ment variability after settling and thereby regu- shallow sandy bays around Gotland, central late the year-class strength of plaice in the North Baltic Sea (ICES SD 27 and 28-2) (Fig. 1) in Sea (Van der Veer and Bergman 1987, Pihl 1990, 2003–2005. Sampling was conducted in early– Van der Veer et al. 2000). However, the shore mid-July to mid-September with two (2003– crab does not inhabit the central Baltic Sea and 2004) to four (2005) sampling occasions per the abundance of brown shrimp within the nurs- month. A total of six bays were examined, but ery areas is low during the period when 0-group only bay B and C were sampled every year turbot and flounder are present (A. Nissling (Table 1). The fish sampled were also used in unpubl. data). Hence, other controlling and regu- another study (Nissling et al. 2007). lating factors will come into play in affecting A total of 1094 samples were taken, using a the recruitment of flounder and turbot in the beach seine (mesh-size 4 mm and 2 mm in the central Baltic Sea. Flatfishes in general normally wings and cod end respectively), at 0.2, 0.6 and display low variability in recruitment (Bever- 1 m depths. On each sampling occasion, gener- ton 1995, Iles and Beverton 2000). In contrast, ally five samples were taken from each depth. In recruitment of flounder and in particular turbot each sample the number of flounder and turbot in the Baltic Sea is known to vary (Florin 2005). juveniles were counted and the length of each

Furthermore, Molander (1964) observed, when individual (Lt, 0.5 mm accuracy) was measured. focusing on turbot recruitment, that peaks in 0-group juveniles were separated from 1-group the year class strength of turbot coincided with juveniles by comparing length distributions, weak year classes of flounder in the Baltic Sea. which were bimodal from the time at settlement This suggests an interaction between the species onwards (Fig. 2). The mean lengths (± SD) of that may occur during the juvenile stage, which 0-group individuals sampled following the first affects 0-group turbot negatively, as both species recorded settlement were 28 ± 4 and 19 ± 5 mm utilize shallow sandy bays as nursery grounds for turbot and flounder, respectively. Thus, floun- (Kostrzewska-Szlakowska 1990, Aarnio 1996, der ≤ 20 mm and turbot ≤ 30 mm were regarded Florin et al. 2009). as newly settled fish. In total, 4760 flounders and To assess potential controlling and regulat- 1028 turbots were sampled. The length of each ing mechanisms affecting recruitment of turbot haul together with the width (4.5 m) of the beach and flounder in the Baltic Sea, information about seine was used to calculate the area covered (on nursery ground utilization of the two species average 120 m2). The densities of 0-group floun- is needed. Thus, the aim of the present study der and turbot were calculated from the area cov- 62 Martinsson & Nissling • Boreal Env. Res. vol. 16

ered in each haul and expressed as the number of individuals 100 m–2.

Statistical analyses

The general depth distribution of 0-group turbot and flounder was analysed using the PER- MANOVA software (Anderson 2001, McArdle and Anderson 2001), which uses permutational analysis of variance. The Euclidian distance was used and Monte-Carlo p values presented (Anderson and Robinson 2003). Using densities per se in the analysis misleading results might be attained as the abundance could vary greatly between sample occasions. Hence, to maintain the relative differences (or similarities) in density between depths, the relative abundances of 0-group turbot and flounder in each haul on each occasion was calculated by dividing the density of each haul by the total density of all hauls on the occasion. Further, although several hundred of hauls were sampled at each depth, only six independent observations per depth could be used in the analysis, i.e. the mean relative abun- Fig. 1. The Baltic Sea with ICES subdivisions (SD) dance at each bay. The depth distribution was and location of sampling areas (bays A–F) at the coast analysed using the model: of Gotland, central Baltic Sea (ICES SD 27 & 28-2) (based on GIS layers from the Baltic GIS Portal and ¥ ICES). y = α + Sp + De + Sp De + ε (1)

Table 1. Arrival to (week), peak abundance (week), and the mean density (ind. 100 m–2) at peak abundance for 0-group turbot and flounder for baysA –F.

Time at settlement Peak abundance (week) Peak abundance density (week) (week) (ind. 100 m–2) (mean ± SE)

Year Bay Flounder turbot Flounder turbot Flounder turbot

2003 a 28 31 31 38 6.75 ± 1.69 3.69 ± 1.44 B 28 30 30 31 7.07 ± 1.42 10.41 ± 2.57 C 28 33 35 35 4.29 ± 1.29 3.06 ± 0.88 E 28 33 33 33 1.72 ± 0.43 1.67 ± 0.34 2004 a 29 31 35 35 3.04 ± 0.95 1.33 ± 0.30 B 29 33 33 37 4.22 ± 1.11 0.94 ± 0.32 C 31 33 35 37 0.32 ± 0.13 0.95 ± 0.37 D 29 33 35 33 1.72 ± 0.50 0.75 ± 0.22 E 29 32 31 37 3.30 ± 2.01 0.26 ± 0.12 2005 B 28 30 36 33 8.37 ± 1.86 2.49 ± 0.95 C 28 30 33 33 7.06 ± 1.51 3.19 ± 1.45 F 28 30 35 33 30.56 ± 5.32 2.11 ± 0.63 Boreal Env. Res. vol. 16 • Nursery area utilization by turbot and flounder 63 where y is the mean relative abundance in a bay, 15 α is the intercept, Sp is the factor Species with two levels (0-group turbot and flounder), De is the factor Depth with three levels (0.2, 0.6 and 10 1 m) and ε is the error term. y The sample occasions from first settlement of the species were used until last week in August. Frequenc Samples taken in September were excluded 5 since these might have biased the results as the species starts to migrate towards deeper waters in autumn. 0 Significant results using PERMANOVA may 20 40 60 80 Length (mm) be due to differences in either the mean or the variance between groups (Anderson 2001). Fig. 2. Length distribution of flounder at first encounter of 0-group individuals (week 28) in bay B in 2003 at the Therefore, tests of homogeneity of variance coast of Gotland, central Baltic Sea (ICES SD 27 and were performed using PERMDISP. The results 28-2). revealed no significant difference in variance between the groups. Consequently, the dataset was not transformed in the analysis. containing only turbot were assigned the value 0. PERMANOVA was also used to analyse the In this way, the analysis estimated the probabil- general size distribution of 0-group turbot and ity of finding flounder when turbot was present flounder stratified according to depth. As for the in a sample. This procedure was undertaken as previous analysis, the observations consist of 0-group turbot was less abundant as compared six independent observations per depth, i.e. the with 0-group flounder. mean length at each bay. The size distribution was analysed using the model: Results y = α + Sp + De + Sp ¥ De + ε (2) Temporal distribution where y is the mean length in a bay, α is the intercept, Sp is the factor Species, De is the The settling pattern varied somewhat between factor Depth and ε is the error term. years (Table 1 and Fig. 3). 0-group flounder No 0-group turbot was caught at 1 m depth arrived earlier to the nursery grounds than in bay D. As PERMANOVA requires balanced 0-group turbot. For flounder, a peak of newly datasets, a dummy calculated by using the mean settled individuals was recorded in early–mid- of the observations at 1 m in the other bays was July (week 28–30) in 2003 and 2005 and in mid– added to the dataset. The mean square for the late-July (week 29–31) in 2004, levelling off and residuals was therefore recalculated using the resulting in few arriving individuals onwards. In appropriate number of degrees of freedom. No 2005, however, settlement continued with new significant difference in variance among groups cohorts arriving throughout the sampling period, were found using PERMDISP, thus no transfor- e.g. shown as a second peak in September in bay mation was applied to the dataset in the analysis. C (Fig. 3). For turbot the main settling period Moreover, a logistic regression with a binary lasted from late July-early August to early Sep- response variable (present/absent) was con- tember (week 31/32–36) in 2003 and 2005, but ducted in SPSS 17.0 using a generalized linear was somewhat later in 2004. model with the binomial distribution and the For 0-group fish in general (all sizes pooled) logit link function to assess the spatial overlap the highest abundances of flounder were of 0-group flounder and turbot on a small scale, recorded from late July-early August to mid- i.e. within each sample. Samples containing both September (week 31/32–37). For turbot peak species were assigned the value 1 and samples abundance occurred in mid-August–early Sep- 64 Martinsson & Nissling • Boreal Env. Res. vol. 16

100 B C

60 2003 40

100 ) 80

60 2004 40

20 Relative abundance (% 0

100

60 Fig. 3. Relative abun- 2005 40 dance (percentage of total density), in bays B and C 20 for all years (2003–2005), of newly settled individu- 0 als of flounder (≤ 20 mm) and turbot (≤ 30 mm) at 28 29 30 31 32 33 35 36 37 38 28 29 30 31 32 33 35 36 37 38 the coast of Gotland, cen- Week tral Baltic Sea (ICES SD Flounder Turbot 27 and 28-2). tember, week 33–36 (Figs. 4 and 5). Due to later abundance varied considerably between years settlement, peak abundance of 0-group turbot (Table 1). e.g. between 7.06 ± 1.51 (SE) and occurred somewhat later in 2004 as compared 0.32 ± 0.13 (SE) ind. 100 m–2 in 2005 and with the other years, as shown for bay B and C 2004, respectively, for 0-group flounder in bay (Figs. 3 and 4). Comparisons of peak abundance C and between 3.06 ± 0.88 and 0.95 ± 0.37 reveal a considerable temporal overlap in habitat ind. 100 m–2 in 2003 and 2004, respectively, for utilization between the two species. 0-group turbot in bay C. One-year old individuals were found on the nursery grounds during the entire period. The numbers decreased over the season for flounder, Spatial distribution whereas the largest amounts of 1-group turbot were recorded in early August–early September 0-group flounder and turbot expressed the same (week 32–37). However, the share of 1-group depth distribution and showed preference for 0.2 fish differed significantly between turbot and and 0.6 m over 1 m depth. No significant interac- flounder, 3% (33 ind.) and 25% (953 ind.), tion between the factors Species and Depth was respectively, showing that flounder uses the nurs- found (F2,30 = 1.63, p = 0.21), whereas there was ery ground to a higher extent the second year. a significant effect of the factor Depth (F2,30 = For both species, mean density at peak 26.95, p < 0.001). The following pairwise com- Boreal Env. Res. vol. 16 • Nursery area utilization by turbot and flounder 65

B C 100

60 2003 40

100 ) 80

60 2004 40

Relative abundance (% 0

100

60 Fig. 4. Relative abun- 2005 dance (percentage of total 40 density), in bays B and C 20 for all years (2003–2005), of 0-group flounder and 0 turbot (all sizes pooled) at the coast of Gotland, cen- 28 29 30 31 32 33 35 36 37 38 28 29 30 31 32 33 35 36 37 38 tral Baltic Sea (ICES SD Week 27 and 28-2). Flounder Turbot parisons showed that the mean relative abun- present in each haul when 0-group turbot was dance did not differ between 0.2 and 0.6 m depth caught was 0.84 (0.80–0.87, 95% CI). Moreover, (p = 0.1), but was significantly lower at 1 m (p when co-occurring in samples 0-group floun- < 0.001) as compared with that at other depths der was on average 3.2 ± 0.41 (SE) times more (Fig. 6). abundant than 0-group turbot. When analysing the size distribution over the Both species were found in all bays examined depths both species shared a tendency of larger and in all years, but densities varied considerably size at greater depths. No significant interac- among locations (Table 1), e.g. in 2004 estimated tion between the factors Species and Depth was densities of 0-group flounder varied between 0.32 –2 found (F2,29 = 0.37, p > 0.05), whereas there was ± 0.13 (SE) and 4.22 ± 1.11 (SE) ind. 100 m in a significant effect of the factor Depth (F2,29 = bay C and B, respectively. Similarly, for 0-group 26.95, p < 0.01). The following pairwise com- turbot estimated densities in 2004 varied between parisons showed a significant difference between 0.26 ± 0.12 (SE) and 1.33 ± 0.3 (SE) ind. 100 m–2 0.2 and 1 m (p < 0.01), and almost a significant in bays E and A, respectively. difference between 0.6 and 1 (p = 0.05), and between 0.2 and 0.6 m (p = 0.06) (Fig. 7). The logistic regression showed a strong spa- Discussion tial overlap of 0-group turbot and flounder (Wald χ2 = 126.94, df = 1, p < 0.001) within a sample. Flounder utilize the nursery grounds for a longer The probability of 0-group flounder being time, both as 0- and 1-group, as settling started 66 Martinsson & Nissling • Boreal Env. Res. vol. 16

Settlers 0-group 100 80 60 40 A 20 0 100 80 60 ) 40 B 20 0 100 80 60 Relative abundance (% 40 C 20 0 100 Fig. 5. Relative abun- 80 dance (percentage of total 60 density), in bays A, B, C E 40 and E in 2003, of newly 20 settled- and 0-group indi- 0 viduals of flounder and turbot (all sizes pooled) at 28 30 31 33 35 38 28 30 31 33 35 38 the coast of Gotland, cen- Week tral Baltic Sea (ICES SD Flounder Turbot 27 and 28-2).

12 50 Flounder Turbot Flounder Turbot

) 45

8 40

4 Mean length (mm) Mean relative abundance (% 30 2

0 25

0.2 0.6 1 0.2 0.6 1 Depth (m) Depth (m) Fig. 6. Depth distribution (mean relative abundance in Fig. 7. Size-specific depth distribution (mean length ± a haul ± SE), all bays (A–F) and years (2003–2005) SE), all bays (A–F) and years (2003–2005) included, included, of 0-group flounder and turbot in nursery of 0-group flounder and turbot in nursery grounds at grounds at the coast of Gotland, central Baltic Sea the coast of Gotland, central Baltic Sea (ICES SD 27 (ICES SD 27 and 28-2). and 28-2). Boreal Env. Res. vol. 16 • Nursery area utilization by turbot and flounder 67 earlier in the season and a larger proportion of turbot, on the other hand, preferred food organ- 1-group individuals were found within the nurs- isms (juvenile fish in a certain size-range) when ery grounds as compared with turbot. Earlier set- reached > 55 mm (Nissling et al. 2007) are avail- tlement of flounder can be expected as flounder able only during a short period during the season. spawns earlier in the year, March–May, whereas Lower food availability for 0-group turbot is indi- spawning of turbot peaks in June–early July in cated by a considerable proportion of individuals the central Baltic Sea (Bagge 1981, authors’ pers. with empty stomachs (without gut content), i.e. a obs.). The longer settling period of flounder in lower feeding incidence as compared to 0-group 2005 may reflect that cohorts from both coastal- flounder (Nissling et al. 2007). and offshore-spawning populations were settling Analysis of depth distribution of 0-group fish in the bays. The latter population spawns in the revealed a significant preference of both spe- Gotland basin at > 70–80 m depth (ICES SD cies for 0.2- and 0.6-m depths over 1-m depth. 28-2), i.e. the larvae have to be transported to the Further, a size-specific depth distribution was coast and may consequently appear at the nurs- found, which indicated that smaller individuals ery grounds later in the season. The occurrence are found in shallower water. In contrast to the of larvae originating from the offshore spawning present study, Florin et al. (2009) found no sig- population will, however, vary due to varying nificant contribution of depth in explaining the salinity and oxygen conditions, influencing the abundance of 0-group flounder and turbot at a reproductive success (Drews 1999, Nissling et regional scale in the northern Baltic Sea. How- al. 2002), and varying wind and hydrodynamic ever, the design of this study was different and conditions affecting the transportation to nursery carried out in several types of substrates that may grounds along the coast. Hence, the appearance be more important as compared with depth in can be expected to vary significantly between explaining the abundances. Moreover, the results years, potentially reflecting the differences in of the present study is in agreement with findings settling patterns (Table 1 and Fig. 3). reported by Gibson (1973) in a sandy bay off The observed pattern of nursery ground uti- Scotland, where the centre of distribution was at lization may also be related to differences in < 0.5 m depth and < 1 m for 0-group turbot and food preferences. After an ontogenetic shift at flounder, respectively. Gibson (1973) also noted ≈ 40 mm length, endobenthic fauna as chirono- a significant relationship between fish size and mids, amphipods and additionally oligocha- depth (shown for juvenile plaice) with small fish etes are important food items of flounder up occurring in shallow water. The observed distri- to ≈ 85 mm (Aarnio et al. 1996, Nissling et bution may reflect either feeding conditions or al. 2007). Contrary, turbot successively feed on predation mortality, but also involve reduction of larger food. At lengths < 30 mm, turbot uses a intra-specific competition, as argued by Gibson mix of copepods, chironomids and amphipods. (1973). The brown shrimp is known to predate At 30–55 mm lengths they feed mainly on mysids heavily on in particular early-juvenile flatfish and additionally on amphipods, and at lengths (Van der Veer and Bergman 1987, Wennhage above 55 mm increasingly on fish (mainly juve- 2002). Although it usually occurs in low abun- nile Pomatoschistus spp. and Gasterosteus spp.) dances at nursery grounds in the central Baltic (Aarnio et al. 1996, Nissling et al. 2007). The Sea, it has been observed to be less abundant availability of endobenthic fauna is rather stable, at 0.2- as compared with 0.6- and 1-m depths. both in terms of abundance and size, through- Contrary to that of brown shrimp, abundance out the season (Aarnio et al. 1996). In contrast, of mysids was significantly higher at 0.2- than epibenthic organisms (such as juvenile fish) pre- at 0.6–1-m depths (A. Nissling unpubl. data). ferred by turbot, not only vary spatially and tem- Hence, survival probabilities of specifically small porally, but also vary in size over the season due turbot may be enhanced in shallow waters. Fur- to growth. Hence, flounder utilizes a food source ther, as derived from experiments on plaice and with low variability in abundance and in size, flounder, the optimum temperature for feeding i.e. allow settling during a long period and form and growth is negatively correlated with fish size a suitable food source for also 1-group fish. For (Fonds et al. 1985). Hence, newly settled juve- 68 Martinsson & Nissling • Boreal Env. Res. vol. 16 niles may benefit from normally higher tempera- both vision and chemical cues (De Groot 1971, tures at smaller depths (Widbom, unpublished Holmes and Gibson 1983) and search for food data) in nursery areas at Gotland during the by slowly swimming along the bottom (Holmes season. Maximum growth of 0-group flounder and Gibson 1983). Potentially, the prey of turbot has been reported to occur at 18–22 °C (Fonds et or turbot itself may be disturbed by flounder al. 1992). The corresponding values for 0-group searching along the bottom, thereby reducing the turbot is 20–23 °C (Imsland et al. 2000). feeding success of turbot. As opposed to Riley et al. (1981), who found Aiming at evaluating the timing of nursery that 0-group turbot dwell at exposed beaches ground utilization as a basis for future studies at salinities > 32 psu as compared with estuar- (e.g. growth and mortality rates) of juvenile ies at < 28 psu for 0-group flounder around the turbot and flounder in the Baltic Sea, the sam- coast of England and Wales, the present study pling period, i.e. from early–mid-July to mid- demonstrated considerable co-occurrence of the September, covered the main settling period as species. The co-occurrence suggests a potential well as peak abundance of turbot. Concerning for inter-specific interactions. This may explain flounder, however, settlement of 0-group fish the observed negative correlation between feed- was not fully covered as the abundance of fish < ing incidence of turbot < 30 mm and density of 20 mm peaked already in early mid-July (week 0-group flounder (Nissling et al. 2007), and that 28) and probably started earlier. Further, at least peaks in the year class strength of turbot were in some years (2005), settlement of flounder observed to coincide with weak year classes of may continue in September, i.e. 0-group abun- flounder in the Baltic Sea (Molander 1964). dance may peak later. Thus, for studies involving In an earlier study, Nissling et al. (2007) 0-group flounder the sampling period should be found that the diets of 0-group turbot and floun- extended by beginning somewhat earlier and der differed in general but overlapped to some finished later to cover “late settlers”. Consider- extent for turbot < 30 mm and flounder > 40 ing depth distribution, depth strata sampled seem mm. Flounder may have reached sizes > 40 mm to, in accordance with Gibson (1973), cover the at first arrival of turbot since flounder settlement main occurrence of 0-group fish; the vast major- occurs earlier. Therefore, turbot might have to ity occurred at < 1 m depth, 87.4% and 81.6% for compete for food at settling. However, the prey turbot and flounder, respectively. Regarding the items shared (amphipods and chironomids) dis- potential for studying ecology of 1-group floun- play stable abundances within the season and der appearing at the nursery grounds throughout occur in high densities (Bonsdorff and Blomqvist the season, the sampling scheme conducted is 1993, Nissling et al. 2007), i.e. competition for inappropriate; 1-group flounder occur in high the shared prey items is less likely to occur. This abundances already early in the season and prob- concurs with the general view that macrohabitat ably have a different depth distribution as larger overlap is inversely related to competition (Sch- individuals occur more offshore as observed by oener 1983). The association between habitat e.g. Gibson (1973). Moreover, the 1-group fish overlap and diet partitioning, i.e. differences appearing at the nursery grounds probably repre- in diet, have also been observed by Piet et al. sent only a part of the population. (1998) when studying flatfish assemblages in the In summary, turbot and flounder in the Baltic southern North Sea. Sea showed high temporal and spatial overlap There is a potential for 0-group turbot and within the shallow, sandy nursery grounds inves- flounder to interact by interference. Turbot preys tigated. However, flounder utilized the nursery on fast moving epibenthic prey by vision at the grounds for longer period of time both as 0- and bottom and in the water column. The feeding 1-group, which may be explained by the differing behaviour is characterized by burying into the feeding preferences of the species. The differing sediment waiting for prey and when detected a diets probably also reduces competition between short stalking behaviour is employed followed the species, thus enabling the observed coexis- by a rapid lunge (Holmes and Gibson 1983, D. tence. The results of this study can be used to Nygren pers. comm.). Contrary, flounder uses design studies aiming at assessing which control- Boreal Env. Res. vol. 16 • Nursery area utilization by turbot and flounder 69 ling and/or regulating mechanisms within typical Metabolism, food consumption and growth of plaice (Pleuronectes platessa) and flounder (Platichtys flesus) nursery grounds, i.e. shallow sandy bays, that in relation to fish size and temperature. Neth. J. Sea Res. affect variability in recruitment of the species. 29: 127–143. Gibson R.N. 1973. The intertidal movements and distribution Acknowledgements: This study was funded by The Carl of young fish on a sandy beach with special reference to Trygger foundation, the EU by ‘Financial Instrument for the plaice (Pleuronectes platessa L.). J. Exp. Mar. Biol. Fisheries Guidence (FIFG)’, the Rica City Hotels Envi- Ecol. 12: 79–102. ronmental Foundation and the Royal Swedish Academy of Hemmer-Hansen J., Nielsen E.E., Grønkjær P. & Loeschcke Agriculture and Forestry (KSLA). We thank M. Appelberg, V. 2007. Evolutionary mechanisms shaping the genetic O. Hjerne, H. Wennhage, A. 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