Larval group differentiation in Atlantic cod (Gadus morhua) inside and outside the Gullmar Fjord

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Fisheries Research 90 (2008) 9–16

Larval group differentiation in Atlantic cod (Gadus morhua) inside and outside the Gullmar Fjord Vidar Øresland a,∗ , Carl Andr´e b a

b

Institute of Marine Research, Swedish Board of Fisheries, P.O. Box 4, SE-453 21 Lysekil, Sweden Department of Marine Ecology, G¨oteborg University, Tj¨arn¨o Marine Biological Laboratory, SE-452 96 Str¨omstad, Sweden Received 2 January 2007; received in revised form 30 August 2007; accepted 14 September 2007

Abstract The spatial and temporal occurrence of pelagic fish stages and their biological variability may affect their dispersal and survival, and ultimately fish recruitment. We collected Atlantic cod larvae at one station inside and at one station outside the Gullmar Fjord, eastern Skagerrak, in order to investigate small-scale larval group differentiation. Rectangular midwater trawl hauls were taken every 6 h (during 24 h) from three separate depth intervals between the surface and 70 m depth. About 80% and 20% of all larvae occurred above the halocline at the Fjord station and the Coastal station, respectively. Hatching (at both stations) occurred from the 3rd week in February to the 1st week in May, indicating that cod larvae were present for at least 5 months (from late February to early August). The length and hatch date frequency distributions of larvae from the surface layer were unimodal inside the fjord but bimodal outside the fjord. Analyses of seven microsatellite DNA loci indicated that larvae collected inside the fjord (where local spawning occurs) were genetically distinct from larvae sampled on the outside (FST = 0.0026). The two age cohorts outside the fjord were not, however, genetically different, nor were larvae collected at different depths. We conclude that small-scale variability of vertical concentration and larval life history variability should have consequences for interpreting models of larval dispersal and survival, and subpopulation structure analyses. © 2007 Elsevier B.V. All rights reserved. Keywords: Cod; Gadus morhua; Larvae; Spawning; Hatch dates; Genetic; Otoliths; Vertical distribution; Dispersal

1. Introduction Fish reproductive condition, spawning locations and spawning periods, as well as time periods of the planktonic eggs and larvae, may vary temporally within a species distribution area. This may, together with variation in dispersal and survival of the planktonic stages, affect the temporal settlement success within different nursery areas (see Lough et al., 1994; Pepin and Helbig, 1997; Trippel et al., 1997, 2005; Palumbi, 1999; Bradbury et al., 2000; ICES, 2005). A specific nursery area may thus receive larvae of different age, size, condition, concentration, and spawning origin, within one season and between years. Such variability of larval sources can affect recruitment, population dynamic, and genetic composition of marine fish on both local and regional scales (see Marteinsdottir et al., 2000; Smedbol and Stephenson, 2001; Beacham et al., 2002; Stenseth et al., 2006; Cowen et al.,

∗

Corresponding author. Tel.: +46 523 18734; fax: +46 523 13977. E-mail address: [email protected] (V. Øresland).

0165-7836/$ – see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.fishres.2007.09.008

2006). Investigations using microsatellite DNA (Knutsen et al., 2004; Stenseth et al., 2006) indicate that the coastal Skagerrak may receive settlers originating from both local spawning and non-local spawning, while Nielsen et al. (2005) provided support for the hypothesis that the population structure of cod in the area between the North Sea and the Baltic Sea, including the Skagerrak was maintained by retention of locally produced juveniles. Different hypotheses regarding mixing and origin of cod in the Skagerrak–Kattegat area have been discussed for a long time (Poulsen, 1931 and cited references). However, the knowledge is still limited regarding both spawning sites offshore (Hagstr¨om et al., 1990) and in the fjords (Knutsen et al., 2007), spawning and larval periods, as well as the vertical distribution and dispersal patterns of eggs (Knutsen et al., 2007) and larvae (Munk et al., 1995, 1999). The oceanographic current system in the eastern Skagerrak is highly complex (Svansson, 1975; Rodhe, 1992; Danielssen et al., 1997) with dynamic front, eddy, and upwelling systems, and water mass transport from the Baltic Sea (mixing with North Sea water) causing strong vertical and horizontal

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density gradients (Rodhe, 1998). Cod larvae and juveniles may occur both above and below the vertical density gradient and individuals can be found down to 70 m or more (Poulsen, 1931; Lough and Potter, 1993). Consequently, the retention, dispersal, and mixing of eggs, larvae and juveniles (of different ages, sizes, conditions, spawning origin and perhaps genetic composition) can be expected to vary at any time and at different horizontal and vertical scales along the Skagerrak coast. At the local scale, this may, possibly, give rise to the temporal and spatial appearance of different modes of larvae, with different characteristics, contributing differently to local cod recruitment and population dynamics. The aim of this study was, for the first time, to describe cod larvae group differentiation inside the Gullmar Fjord, where local spawning occurs (Sved¨ang et al., 2004), and at one location outside the fjord, possibly representing larvae also from other sources. We tested hypotheses of vertical concentration, length and age distributions, and genetic differentiation, among different groups of cod larvae. We provide estimates of hatch and larval periods, and discuss the relevance of our results to studies of larval dispersal and genetic structure of cod in this area. 2. Methods 2.1. Field sampling Cod larvae were caught at one station inside the Gullmar Fjord (Fjord station) and at one offshore station (Coastal station) approximately 15 km off the coastline (Fig. 1). These stations were chosen as to represent local spawning locations (i.e. inside the fjord) as well as areas with different hydrographical regimes that could possibly influence group differentiation among cod larvae. The cod larvae were collected using a rectangular midwater trawl (RMT) (Clarke, 1969). The modified RMT (500 ␮m mesh size, 8.5 m long, 5.7 m2 net opening, and 102 kg lead inside the lower iron pipe) was equipped with an angle meter and a CTD model SD204 (both from SAIV A/S, Bergen, Norway). Two calibrated General Oceanic flowmeters (model 2030) were mounted inside the net. A third flowmeter was mounted above the net in order to estimate filtration efficiency during tows in the upper depth interval. A net (knotless, 8 mm mesh size) that prevented scyphozoans from being caught was fastened to the front of the RMT.

Fig. 1. The positions of the two stations and their approximate bottom depth in the eastern Skagerrak and in the Gullmar Fjord in Sweden. Fjord station: N 58◦ 20 , E 11◦ 33 , ∼110 m; Coastal station: N 58◦ 12 , E 11◦ 4 , ∼109 m. The line shows the 100 m depth contour.

Multi-oblique hauls were taken between the surface and just below the halocline, below the halocline and 50 m, and between 50 and 70 m depth (Table 1). We hypothesised that larvae occurring above and below the halocline could differ regarding concentration and larval group differentiation, due to different dispersal and biotic and abiotic conditions. These three hauls were then immediately repeated (replicate hauls) one more time in the same order. This sampling schedule was repeated every 6 h (during 24 h, starting at 12.00 h) at both stations between 2–3 May (Fjord station) and 7–8 May (Coastal station). Mean haul time was 14 min and speed through water was between 2–3 knots. Since haul depth and net angle were monitored in real time every 2 s during the hauling we could sample all depths within a sampling interval equally (by continuously adjusting wire and ship speed). The RMT was lowered and raised vertically to and from the second and third depth interval, while the research vessel was not moving, without filtrating the water. In a performance test, where the RMT was lowered vertically down to 80 m depth and taken up vertically, less than 100 copepods and no fish larvae (of any species) were found inside the net. The complete zooplankton samples were immediately preserved in 95% ethanol that was exchanged within 30 h.

Table 1 Mean concentration (and S.E.) of Gadus morhua larvae Location

Fjord Station

Coastal Station

Depth interval (m)

No. of hauls

No. of larvae

n/1000 m3

n/m2 Over depth interval

Upper 50 m

0–20 20–50 50–70

8 8 8

239 50 16

17.29 (2.412) 3.01 (0.574) 0.83 (0.441)

0.346 (0.048) 0.090 (0.017) 0.017 (0.009)

0.436 (0.046)

0–10 10–50 50–70

8 7 8

194 138 11

16.57 (3.337) 18.40 (4.177) 1.43 (0.478)

0.166 (0.033) 0.736 (0.167) 0.098 (0.071)

0.823 (0.170)

Upper 70 m

0.453 (0.045)

0.920 (0.167)

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2.2. Otolith preparations and age analyses All fish larvae were sorted and identified from the zooplankton samples under stereomicroscopes (Auditore et al., 1994; Hunt von Herbing et al., 1996). Standard length was measured in tap water (after 2 min) to the nearest mm. The otoliths (lapillus) were dissected out using a micro-scalpel and tungsten needles, and mounted in thermoplastic cement (TC) (Buehler, USA). Polarized light was useful only when the head was damaged or when dealing with newly hatched larvae. The otoliths were photographed under an inverted microscope (Leica DM IRB) using a digital camera (Leica DC 300) and the Leica IM 500 software (including the image compare module, which was used when checking that increments were not double counted or missed). The lapillus otolith that appeared easiest to analyse for number of increments was chosen for grinding using 3 and 1 ␮m 3M Imperial lapping film. Counted increments were assumed to be daily (see e.g. Dale, 1984; Campana, 1989; Radke, 1989; Geffen, 1995). However, counting daily increments for age and hatch date estimates assumes that the day of hatch can be identified (see Campana, 2001). The distinct increment, found normally between 8 and 10 ␮m from the otolith centre in this study, was assumed to be the hatch increment. This increment was not seen in otoliths from un-hatched larvae. However, one or two weak increments could sometimes be seen between the hatch increment and the centre in otoliths from hatched larvae. Such increments were also seen in lapillus from un-hatched larvae supporting the use of the distinct increment as the first increment after hatching. Sub-daily rings in the middle and outer area of larger otoliths were observed, but they normally disappeared by either moving somewhat out of focus or using lower magnification (see Stevenson and Campana, 1992). Differences in concentrations and length distributions among groups of larvae were tested using the StatXact (V. 6) statistical software for exact nonparametric inference. 2.3. Genetic methods DNA for microsatellite analysis was extracted from the tails of individual larvae using the DNEASY animal tissue kit (Qiagen Inc.). Seven microsatellite loci were amplified with PCR following published protocols with minor modifications: Gmo2 (Brooker et al., 1994); Gmo19, Gmo34, Gmo35, Gmo36 and Gmo37 (Miller et al., 2000); and Tch13 (O’Reilly et al., 2000). The microsatellite DNA fragments were then separated on an ALF express II automatic sequencer (Amersham Pharmacia Biotech). Analysing two control individuals spanning the anticipated allelic ranges on all gels, in addition to internal and external size ladders, ensured scoring consistency among runs. Two persons scored genotypes independently and any inconsistent scorings were noted and the individual was screened again. Genotyping success was 0.988. Reliability was further examined by using MICROCHECKER 2.2.1 (van Oosterhout et al., 2004) to test for technical errors or the presence of null alleles. Amounts of genetic variation within cod larval groups were estimated as expected heterozygozity, He (Nei, 1987). Departure

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from Hardy–Weinberg proportions was estimated as FIS , and tested (one-sided) for each combination of locus and sample. Population differentiation, FST , was estimated per sample pair and overall using the unbiased estimator θ (Weir and Cockerham, 1984). Statistical significance of FIS and FST was examined using permutation test (10,000 randomizations) implemented in FSTAT (Goudet, 2001). Significance for differentiation was also tested using permutation tests in GENETIX (www.univmontp2.fr/∼genetix/genetix/genetix.html); these results were consistent with those provided by FSTAT and are not reported further. For all multiple tests, significance levels adjusted using the sequential Bonferroni method (Rice, 1989) are provided for comparison. ViSta 5.6.3 (Young, 1996) was used to perform multi-dimensional scaling (MDS) analysis of pair-wise FST values and to visualize genetic relationships among samples. The specific hypothesis of genetic difference between fjord and coastal samples was tested using hierarchical analysis of molecular variance (AMOVA). Statistical significance in the AMOVA was obtained from 1000 permutations in ARLEQUIN (Schneider et al., 2000). 3. Results 3.1. Larval concentrations Table 1 shows the mean concentration of cod larvae in the different depth intervals during the 24 h sampling periods. Note that the surface interval at the Coastal station is only 10 m since the halocline was closer to the surface outside the fjord. The halocline did not change much within stations during the sampling periods and standard sampling depths were therefore applied within stations. The median concentration of cod larvae (n/1000 m3 ) was highest in the surface interval (15.6), compared to the intermediate interval (3.0), at the Fjord station (Wilcoxon–Mann–Whitney-test, p < 0.001). In contrast, at the Coastal station the concentrations in the surface and the intermediate intervals were similar. However, the number of larvae per m2 , over a depth interval (Table 1), indicates that about 80% and 20% of all larvae occurred above the halocline at the Fjord station and the Coastal station, respectively. Concentrations were generally low below 50 m depth. The mean volume of water filtered per haul was 2024 m3 at the Fjord station and 1075 m3 at the Coastal station. The mean net filtration efficiency of all hauls in the upper layer was 79% at the Fjord station and 85% at the Coastal station. 3.2. Length and hatch date distributions The fjord larvae were shorter (median 10 mm) than the coastal larvae (median 12 mm) below the halocline (Fig. 2B) (Wilcoxon–Mann–Whitney, p < 0.001). No test of surface (Fig. 2A) medians were done due to bimodality of the Coastal station data; however, the difference between the stations in surface layer length distribution is obvious. Within the Fjord station there was no difference in median length above and below the halocline (Wilcoxon–Mann–Whitney, p = 0.10). Within the Coastal station it appears that the larvae in the surface layer were

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Fig. 2. Length (mm) frequency distribution (three data points per moving average) of cod larvae (Gadus morhua).

longer (due to the bimodality) than the larvae from below the halocline. The surface larval length frequencies at the Coastal station (Fig. 2A) and their corresponding hatch date frequencies (Fig. 3A) show parallel trends of bimodality. The two surface hatch modes at the Coastal station were divided (by eye, see arrow in Fig. 3A) for separate genetic analyses (see below), with the youngest group starting from 22nd of March. The hatch date frequencies for larvae taken below the halocline at the Fjord station (Fig. 3B) were irregular (few larvae taken) and difficult to interpret. The minor differences between number of larvae collected (Table 1) and number of larvae aged and measured to length are due to head or tail damages. The range of the hatch date period, which was similar for larvae from both stations and depth intervals, was over 8 weeks long and started before March 1 and ended after May 1. The low proportion of newly hatched larvae in this study indicates that the sampling dates in early May were close to the end of the 2001 hatch period. The youngest and oldest larvae found in this study were 1 and 69 days old (Fjord station) and 4 and 83 days old (Coastal station). 3.3. Genetic differentiation Subsets of the larvae collected at the Fjord and Coastal stations were analysed for microsatellite DNA variation. Based on the larval hatch date distributions together with sampling location and depth we identified five different groups of cod larvae (Table 2). The surface larvae at the Coastal station were divided into two age groups corresponding to larvae older than 42 days versus 42 days or younger (Fig. 3A). In order to compare similar

Fig. 3. Hatch date frequency distribution (three data points per moving average) of cod larvae (Gadus morhua). The arrow shows where the two hatch modes at the Coastal station were divided for separate genetic analyses so the youngest mode starts 22nd of March.

age groups among stations and depths, we only analysed larvae 42 days or younger from below the halocline at the Coastal station and from both the surface and below the halocline at the Fjord station. Three out of the in total 35 locus/larval group – specific FIS –values indicated significant deviation from Hardy–Weinberg expectations (Table 2). Two of these (Gmo34, Fjord deep, and Gmo37, Coast Surface old) were highlighted for having heterozygote deficiency by MICROCHECKER. For the groups Fjord deep and Coast Surface old, there was also an indication of overall deviation from Hardy–Weinberg expectations. However, neither the single nor the group FIS -values remained significant after applying Bonferroni correction for multiple testing (Table 2). Four out of the seven individual microsatellite loci showed significant genetic differentiation among samples (Table 3). The overall genetic differentiation, between samples was low (FST = 0.0028) but indicate that the different groups of larvae did not originate from a single spawning population (p = 0.002). Pair-wise comparison of genetic differentiation revealed that the coastal samples are differentiated from the fjord samples (Table 4 and Fig. 4). This pattern is supported by the hierarchical AMOVA showing that differentiation is higher between locations (fjord versus coastal) than between samples within locations (Table 5).

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Table 2 Sample sizes, n, expected heterozygosity, He , and deviation from Hardy–Weinberg expectancy, FIS , for single loci and all seven loci combined in five samples of cod larvae Locus

Gmo2 Gmo19 Gmo34 Gmo35 Gmo36 Gmo37 Tch13 All loci

Sample Fjord Surf Y (n = 133)

Fjord Deep Y (n = 47)

Coast Surf Y (n = 69)

Coast Surf O (n = 47)

Coast Deep Y (n = 98)

He

FIS

He

FIS

He

FIS

He

FIS

He

FIS

0.87 0.92 0.60 0.84 0.50 0.85 0.92 0.78

0.06 0.00 −0.07 0.06 0.03 0.03 −0.01 0.02

0.85 0.92 0.59 0.81 0.53 0.84 0.91 0.78

0.00 −0.01 0.24 0.05 0.20 −0.02 −0.02 0.04

0.85 0.91 0.63 0.80 0.57 0.85 0.93 0.79

0.05 −0.02 −0.09 −0.06 0.02 0.07 0.06 0.01

0.81 0.92 0.65 0.82 0.50 0.83 0.91 0.77

0.10 0.02 0.01 −0.01 0.14 0.15 0.05 0.06

0.87 0.92 0.60 0.84 0.50 0.85 0.92 0.78

0.06 0.00 −0.07 0.06 0.03 0.03 −0.01 −0.00

Bold denotes FIS values significantly different from zero (p < 0.05). No significances remained after Bonferroni correction (corrected α = 0.0014 at k = 35 for single loci, and 0.01 at k = 5 for all loci combined).

Table 3 Overall genetic differentiation among larval cod samples, FST , for single loci and all seven loci combined Locus

FST

p

Gmo2 Gmo19 Gmo34 Gmo35 Gmo36 Gmo37 Tch13 All loci

0.0020 −0.0010 −0.0006 0.0041 0.0167 0.0042 −0.0012 0.0028

0.049 0.815 0.323 0.045 0.000 0.024 0.265 0.002 Fig. 4. Multi-dimensional scaling plot based on pair-wise estimates of genetic differentiation (FST ) between cod larval samples. Dimension 1 explains 54% and dimension 2 explains 24% of the genetic variation among samples.

Table 4 Pair-wise genetic differentiation, FST , between cod samples (below diagonal) and corresponding p-values (above diagonal)

4. Discussion

Sample

Fjord Surf Y Fjord Deep Y Coast Surf Y Coast Surf O Coast Deep Y

Fjord Surf Y

Fjord Deep Y

Coast Surf Y

Coast Surf O

Coast Deep Y

– 0.0013 0.0066 0.0035 0.0016

0.2204 – 0.0053 0.0059 0.0033

0.0003 0.4632 – 0.0034 0.0003

0.0307 0.0119 0.1735 – 0.0000

0.0900 0.6551 0.5786 0.5353 –

Bold denotes FST values significantly different from zero (p < 0.05). Italics denote FST value significant after applying sequential Bonferroni correction (corrected α = 0.0051 at k = 10).

Table 5 Hierarchical analysis of genetic differences between fjord and coastal samples of larval cod (AMOVA) Source of variation

Fixation index

p

Between locations (fjord, coastal) Among samples within locations Within samples

0.0026 0.0010 0.0036

0.0000 0.0020

The recognition of larval groups with different characteristics is important when investigating dispersal and recruitment mechanisms and ultimately subpopulation dynamics. Here we present evidence that cod larvae collected inside the Gullmar Fjord differed in several aspects compared to larvae collected off shore: fjord larvae were more common in surface waters, they were shorter, had unimodal length and hatch date distributions, and they differed in genetic composition compared to the larvae collected outside the fjord. Below we discuss the relevance this small-scale variability of larvae for studies of larval dispersal and genetic structure of cod. Modelling larval dispersal requires detailed knowledge of the vertical distribution of larvae throughout their development because of the high spatial and temporal variability of water mass movement. The importance of this was demonstrated here as 80% of the larvae found in the Gullmar Fjord occurred in the surface layer (which is most influenced by wind-driven transport) compared to only 20% at the offshore station. Knowledge of the planktonic time period is crucial for modelling both dispersal and survival. The simultaneous occurrence in May of both

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newly hatched and 70–80 days old larvae indicates that settling of larvae could occur at least until early August (assuming that some larvae developing during summer would be at least 80 days old before settling). It should be noted that all larvae in this study were shorter than 3 cm and that detailed knowledge of the size or age at settling is lacking for the Swedish west coast. However, H¨ussy et al. (1997) reported that juvenile cod began to consume benthic prey when between 40 and 50 mm in the Bornholm Basin (the Baltic Sea). Thus, settling may perhaps occur even later than suggested here. Most factors that influence larval dispersal and survival may vary considerably during the 5-month long pelagic time span, from late February to early August. Spawning areas are the natural starting points when modelling egg and larval dispersal or comparing genetic similarity between eggs, larvae and spawning adults. Unfortunately, spawning areas (or egg distribution) are still not well described ¨ for the Oresund (Westerberg, 1994), the Kattegat and the Skagerrak (Hagstr¨om et al., 1990; Sved¨ang et al., 2004), and some Norwegian fjords (Knutsen et al., 2007). The bimodal length and hatch date frequencies of cod larvae at the Coastal station indicate that larvae cohorts had different histories, both within the Coastal station water layers and between stations. The occurrence of newly hatched larvae in the fjord (both above and below the halocline) suggests that they originated from the fjord or nearby spawning areas in the Skagerrak or the Kattegat. The abundance of the 2001 year class of juvenile cod was high throughout the Skagerrak (Sved¨ang, 2003). Stenseth et al. (2006) suggested (using hydrodynamic modelling) that this high abundance was due to larval transport from spawning areas in the North Sea. However, both Stenseth et al. (2006) and Knutsen et al. (2004) assumed a short larval period, restricted to March–April, when modelling cod larval drift from the North Sea along the Norwegian Skagerrak coast. Such a limited time span, and lack of vertical distribution data, as well as data of different spawning areas, may not give a comprehensive picture of the actual dispersal variability in this area. Munk et al. (1995) dismissed the possibility of larval transport from North Sea spawning areas to the inner Skagerrak, for the years investigated, since an expected increase in larval mean length from the North Sea towards the inner Skagerrak was not evident. The origin of different larval groups can be investigated in large-scale studies by comparing larvae from different areas and spawning adults from different spawning sites (collected the same year). Such studies require data not yet available from the eastern Skagerrak. In this small-scale study there was a weak but significant genetic differentiation, FST , among the five larval groups. The overall FST of 0.0028 is low but similar to levels of FST reported earlier for cod in the Skagerrak (e.g. Knutsen et al., 2003). The locus having the highest value of FST , Gmo36, had also the highest value in the study by Knutsen et al. (2003). Neither this nor Knutsen’s study found any indications that this particular locus should be subjected to technical artifacts or null alleles. Another possibility is that the low heterozygosity in Gmo36 could explain a higher FST (Hedrick, 1999). This is, however, less likely since we found no strong relationship

between heterozygosity and FST (see also Knutsen et al., 2003). It cannot be ruled out, however, that this locus may be affected by selection as has been shown for the cod locus Gmo132 (not used in this study) (Nielsen et al., 2006). Allele frequencies within samples in general conformed to Hardy–Weinberg expectations (Table 2). This indicates that the larvae within each sample are likely to originate from single spawning groups, rather than being mixtures of larvae from different spawning populations. The heterogeneity among larval groups showed a stronger spatial than temporal pattern. Although one of the coastal samples contained older larvae (Table 2 and Fig. 3A), the different samples at each station grouped together genetically (Fig. 4). This suggest that similar transport mechanisms could be operating at different times. Small-scale genetic structure among larval size cohorts were also observed within a confined water mass off Nova Scotia (Ruzzante et al., 1996, 1999), suggesting that larvae of different origins may mix locally. A sill fjord, like the Gullmar Fjord, may constitute a barrier to gene flow, and Sarvas and Fevolden (2005) demonstrated the occurrence of two distinct genetic cod populations within the Ullfjord, northern Norway. In the inner part of the fjord cod was dominated by the Pan IA allele in the Pantophysin DNA locus, whereas outside the fjord sill both Pan IA and IB , common among offshore (north-east arctic) cod, occurred. While the scope of the present study was to investigate smallscale structure of cod larvae, Nielsen et al. (2005) reported in larger scale study that juveniles along the North Sea–Baltic salinity transition were locally recruited since juveniles grouped genetically together with adults collected in the same region. Knutsen et al. (2004) on the other hand suggested, based on genetic analyses and oceanographic modelling, an annually variable drift of North Sea cod larvae into coastal Skagerrak populations, and that cod juveniles in coastal Skagerrak were locally recruited in 2000 but not in 2001. The presence of juvenile cod of local as well as of external origin makes it difficult to estimate the recruitment strength of local populations, and to identify and monitor different life-history stages. In conclusion, our results suggest that modelling dispersal of larvae in this area would benefit from better knowledge of spawning areas and should take the variability of vertical distribution into account and not be to narrow in time. When larval groups with different history settle in different local habitats, or mix in the same habitat, such complexity will be of crucial importance for the representative sampling of juveniles for population structure analyses. Acknowledgements We thank R/V Ancylus crew and the Institute of Marine Research staff for help during field sampling, and P. Johnsson and M. Casini for sorting out fish larvae, and A.-C. Rudolphi for identifying cod larvae. B. J¨onsson, E. Norlinder and O. Tatarenko assisted with genetic analyses. C.A. received financial support from the Swedish research council FORMAS and from EU INTERREG IIIA. We thank P. Munk for helpful comments on an earlier draft.

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