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ICES Journal of Marine Science: Journal du Conseil Advance Access originally published online on June 2, 2007
ICES Journal of Marine Science: Journal du Conseil 2007 64(5):956-962; doi:10.1093/icesjms/fsm068
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© 2007 International Council for the Exploration of the Sea. Published by Oxford Journals. All rights reserved. For Permissions, please email: journals.permissions@oxfordjournals.org

Vertical distribution of Baltic sprat larvae: changes in patterns of diel migration?

Rüdiger Voss, Jörn O. Schmidt and Dietrich Schnack

Leibniz-Institute of Marine Sciences, Düsternbrooker Weg 20, 24105 Kiel, Germany

Correspondence to R. Voss: tel: +49 431 600 4557; fax: +49 431 600 4553; e-mail: rvoss{at}ifm-geomar.de

Voss, R., Schmidt, J. O., and Schnack, D. 2007. Vertical distribution of Baltic sprat larvae: changes in patterns of diel migration? – ICES Journal of Marine Science, 64: 956–962.

Ontogenetic and diurnal vertical migration patterns of Baltic sprat larvae were investigated for the periods 1989–1990 and 1998–2002. Comparison of the results led to the hypothesis that the diel vertical migration behaviour of sprat larvae >10 mm has changed. In 1989 and 1990, sprat larvae migrated to the surface at night, whereas they stayed 30–50 m deep by day. From 1998 to 2002, sprat larvae showed no signs of diel vertical migration, remaining in warmer, near-surface water by day and night. This behavioural change coincided with a more general change in the Baltic ecosystem, i.e. an increase in near-surface temperature and a general increase in abundance of the major prey organism (Acartia spp.) of Baltic sprat larvae, with more pronounced aggregation in surface waters.

Keywords: Baltic Sea, behavioural change, diel migration, prey availability, sprat larvae, vertical distribution

Received 14 September 2006; accepted 9 April 2007; advance access publication 2 June 2007.


   Introduction
Top
 Introduction
Material and methods
 Results
 Discussion
 References
 
Variation in the vertical distribution of planktonic organisms has long been recognized as an important factor controlling the structure and dynamics of marine foodwebs (Russell, 1927; Cushing, 1951; Banse, 1964; Longhurst, 1976; Lampert, 1989). The impact of such variation is amplified when there are strong vertical gradients, such as in the Baltic Sea. There, vertical distributions of different developmental stages and species are strongly affected by fluctuating hydrographic conditions (Grønkjaer and Wieland, 1997; Hansen et al., 2006; Schmidt, 2006), and predator–prey interactions are influenced by variable vertical overlap (Neuenfeldt, 2002; Köster et al., 2003; Neuenfeldt and Beyer, 2003; Möllmann et al., 2004).

In the Baltic Sea, sprat (Sprattus sprattus) are an ecologically important pelagic fish species (Rudstam et al., 1994; Kornilovs et al., 2001), being prey for top predators (e.g. cod, harbour porpoise) and predator on zooplankton and fish eggs (Arrhenius and Hansson, 1993; Bagge et al., 1994; Köster and Schnack, 1994; Möllmann and Köster, 1999; Köster and Möllmann, 2000a, b). Currently too, sprat represent the most abundant, commercially exploited species in the Baltic (ICES, 2006), and optimal management is challenged by large fluctuations in stock size.

The spawning of sprat and the distribution of its planktonic eggs is restricted to the central part of the deep basins in the Baltic, with vertical concentration in the upper part of the halocline, typically between 45 and 70 m. The Bornholm Basin in the central Baltic Sea is an especially important spawning ground for sprat (Köster et al., 2001). During the main spawning season in spring, the Bornholm Basin is characterized by a seasonal thermocline at ~20–30 m deep and a permanent halocline at 50–75 m, which separates less-saline surface waters (salinity 7–8) from more-saline bottom waters (salinity 10–18) (Kullenberg and Jacobsen, 1981; Møller and Hansen, 1994). Renewal of the bottom waters follows irregular saline water inflows into the Baltic Sea (Matthäus and Lass, 1995).

From the early 1980s to the late 1990s, there was a regime shift in the Baltic Sea, as shown by a profound change in fish and zooplankton abundance and species composition (Alheit et al., 2005). The copepod Pseudocalanus acuspes, major prey for cod larvae (Voss et al., 2003), generally decreased in abundance (Möllmann et al., 2000). The copepod Acartia spp., major prey of sprat larvae (Voss et al., 2003), increased in abundance likely because of an increase in water temperature (Möllmann et al., 2000). The sprat stock gained substantially from decreased predation pressure by the declining cod stock (Köster and Möllmann, 2000b), and from improved recruitment success, though there was increased variability too.

Sound scientific explanation for the increased variability is missing. Spawning-stock biomass is a poor predictor of recruitment success of sprat (Köster et al., 2003; MacKenzie and Köster, 2004). Recent research has shown that recruitment depends to some degree on temperature conditions influencing gonad development and egg survival (MacKenzie and Köster, 2004). The critical periods in the sprat life cycle are, however, the larva and the early juvenile stage (Köster et al., 2003; Voss et al., 2006). Mechanisms influencing survival of sprat larvae are only poorly understood, at least partly because of missing knowledge of vertical distribution patterns of larvae, so hampering process-orientated research.

Here, we investigated the vertical distribution and migration of sprat larvae and zooplankton in the Bornholm Basin, based on samples obtained from 1998 to 2002 and a decade earlier, in 1989 and 1990. The purpose of the studies was to document the vertical distribution of sprat larvae and their potential prey. Differences in results between the two periods led to a hypothesis that there have been gradual changes in the diel vertical migration behaviour of sprat larvae >10 mm, perhaps linked to changes in the Baltic Sea ecosystem.


   Material and methods
Top
 Introduction
 Material and methods
Results
 Discussion
 References
 
Sampling and laboratory analysis
Vertically resolved sampling was conducted during seven spring cruises between May 1989 and June 2002 (Table 1) in the central part of the Bornholm Basin, central Baltic Sea (Figure 1). The earliest samples used were collected in 1989 with a MOCNESS (Wiebe et al., 1976); from 1990, a BIOMOC was used instead. Both gears are multiple opening/closing nets operating with nine nets and a mouth opening of 1 m2. The BIOMOC is a MOCNESS system modified similarly to the BIONESS (Sameoto et al., 1980), but still using vertically operated opening/closing bars. In both cases, the nets had a mesh size of 335 µm and were equipped with calibrated flowmeters (for a detailed description, see Wieland, 1995). The samplers were towed at a fixed depth for ~3 min at a speed of 3 knots. Two combined hauls of the gear in use (up to 17 nets) allowed investigation of vertical distribution in the water column with a resolution of 5 (10) m depth intervals, resolving the water column down to a maximum of 5 m above the sea floor.


Figure 1
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  		Figure 1. Map of the central Baltic Sea. Open circles indicate the positions of vertically resolved zooplankton sampling in June 2002, and the black dot the position of sampling on all other dates.

 


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  		Table 1. Sampling for sprat larvae and zooplankton in terms of number of profiles investigated and larvae analysed, type of sampling device, and vertical resolution.

 
For sampling of small zooplankton (e.g. nauplii, N), 50-µm liners mounted inside the multiple opening/closing nets were used, a technique already successfully applied to analyse vertical distribution patterns of zooplankton (Hansen et al., 2004, 2006). Alternatively (in June 2002), a vertically towed 0.25 m2 multinet was used. Samples were immediately fixed in borax-buffered formaldehyde–seawater solution (4% final concentration) for later analysis in the laboratory.

Sprat larvae were sorted from the samples and measured to the nearest 0.1 mm, applying no correction for shrinkage. Several zooplankton subsamples were taken for subsequent microscope identification (magnification 50x) until at least 500 individuals in total were counted. All individuals were identified to developmental stage [grouped in nauplii, copepodites (C1–C3), copepodites (C4 and C5), adult males (C6–m) and females (C6–f), and species]. Abundances of sprat larvae and copepods per m3 were calculated based on counts and filtered volumes.

In parallel with plankton sampling, vertical profiles of temperature, salinity, and oxygen were taken, using a conductivity, temperature, depth (CTD) probe with calibrated oxygen sensor mounted on a water-rosette sampler [Meerestechnik Elektronik (ME), Kiel, Germany].

Data analysis
As an index of vertical location of larvae, weighted mean depths (WMD) (Bollens and Frost, 1989) were computed as WMD =({sum}ni di)/{sum}ni, where ni is the abundance of individuals in depth stratum i with midpoint depth di.

Additionally, relative abundances per 5 m depth were calculated based on daylight sampling averaged over sampling dates. Sprat larvae were classified into three length groups (2 to <5; 5 to <10; ≥10 mm) according to morphological criteria, relevant to their swimming ability (Bartsch and Knust, 1994). Vertical distribution profiles are presented for copepods and size groups of sprat larvae for the daylight period 08:00–18:00 (local time) and for the period of darkness 23:30–04:00, i.e. for the sampling times with potentially most pronounced day–night differences.

We applied the Paul–Banerjee statistic (Paul and Banerjee, 1998) to test for significance of differences in vertical distribution. The method considers patchiness and requires replicate samples. It is a complement to the test of Solow et al. (2000), which is used in cases when replicate samples are not available. The null hypothesis tested states that the shapes of the depth profiles of mean abundance are the same under all different conditions (Beet et al., 2003). Test statistic B has an approximate {chi}2 distribution with (T – 1) x (D – 1) degrees of freedom at T different conditions and D depths.

As sprat larvae feed almost exclusively on Acartia spp. (Voss et al., 2003), the vertical distribution of that taxon is presented (g wet weight per m3). Biomass values for the different developmental stages were taken from Hernroth (1985). For a simple illustration of change in the vertical distribution of Acartia spp., the surface fraction (SF) was calculated by dividing Acartia spp. biomass in the 0–20 m stratum by total Acartia spp. biomass in the 0–50 m stratum. The vertical axis was restricted to 50 m in all cases to preclude influence from differences in vertical extension of the zooplankton sampling.


   Results
Top
 Introduction
 Material and methods
 Results
Discussion
 References
 
Ontogenetic migration
Ontogenetic changes in vertical distribution of sprat larvae were studied on the basis of daylight sampling between May 1998 and June 2002. Yolk-sac larvae, which mainly depend on endogenous energy reserves (size class 2 to <5 mm), had WMDs between 43 and 70 m (Figure 2). Mean relative abundance per 5-m depth layer was greatest at around 70 m. An ontogenetic migration to upper water layers was found for larger size classes: larvae of 5 to <10 mm length were more widely spread through the water column; mean relative abundance peaked in two different layers, deep in the water column, at around 70 m, and near the surface, at around 10 m. Large larvae (≥10 mm) were concentrated exclusively near the surface (WMD 10–18 m); mean larval length per size class varied between samplings, but with only small absolute differences (Figure 2).


Figure 2
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  		Figure 2. Ontogenetic changes in vertical distribution of sprat larvae represented by three size classes. WMD (symbols, mean ± s.d.) for individual surveys is given as well as mean relative abundance per 5 m depth layer (shading), averaged over sampling dates and daylight periods. Additionally, length of larvae per size group (mean ± s.d.) is provided along with maximum size caught.

 
Diurnal vertical migration
Diurnal vertical migration was investigated for the largest size class of sprat larvae (≥10 mm), because this group is least restricted by limited swimming ability (Figures 3 and 4). In the period 1999–2002, these larvae stayed mainly in less-saline surface waters (7–8), in the depth range where the seasonal thermocline develops. Using Paul–Banerjee statistics, we found no significant day–night differences in vertical distribution in the period 1999–2002 (p > 0.05 for all cases; Figure 3), indicating the absence of any substantial diurnal migration. Hydrographic conditions showed the typical vertical structure of the deep, central Baltic basins in spring: a developing warm, low salinity surface layer, cooler intermediate waters below the seasonal thermocline, and higher salinity, sometimes oxygen-depleted water below the permanent halocline.


Figure 3
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  		Figure 3. Vertical distribution of sprat larvae compared with hydrographical profiles in spring of 1999–2002. By night (left) and day (right), the distribution of larvae ≥10 mm is shown, circles representing single measurements, and bars mean values. Larva length is given as mean ± s.d. and size range.

 


Figure 4
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  		Figure 4. Vertical distribution of sprat larvae in comparison to hydrographical profiles in spring of 1989–1990. By night (left) and day (right), the distribution of larvae ≥10 mm is shown, circles representing single measurements, and bars mean values. Larva length is given as mean ± s.d. and size range.

 
During the earlier sampling period of 1989 and 1990, larvae ≥10 mm were mainly near the surface at night (Figure 4), as in the more recent period. Daylight profiles, however, were significantly different between the two periods (Paul–Banerjee statistics, B = 39.32; p < 0.001), showing deeper distribution in 1989 and 1990, with abundance maxima in the 35–55 m depth range (Figure 4). Testing daylight against night-time distribution for the earlier period revealed statistically significant differences (Paul–Banerjee statistics, B = 67.77; p < 0.001) and therefore the existence of diurnal vertical migration by larvae ≥10 mm. Larvae experienced lower ambient temperature by day as well as slightly increased salinity. Day–night differences in the ambient temperature amounted to 3–5°C in May and 6–10°C in June. Mean size of larvae was always slightly bigger by night than by day, but the differences were small and the size ranges sampled did not indicate any change in the sampled cohort of larvae between day and night.

Prey distribution
The SF of Acartia spp. biomass, the preferred prey for sprat larvae, ranged from 0.46 to 0.78 depending on sampling dates and stages included (Figure 5). In 1989, the SF of Acartia spp. was significantly lower in the water column than in 1999 or 2002 (Student's t-test, t = 4.15, p < 0.01 excluding adults; t = 3.26, p < 0.01 including adults).


Figure 5
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  		Figure 5. Vertical distribution of Acartia spp. in May 1999, June 2002, and May 1989. Results are given for all stages inclusive (left) and for adult stages excluded (right). Dots indicate single measurements, bars indicate mean values. SF of zooplankton (mean ± s.d.). The number of profiles is given in Table 1.

 

   Discussion
Top
 Introduction
 Material and methods
 Results
 Discussion
References
 
Ontogenetic migration
In the Bornholm Basin, the gravity of sprat eggs and the hydrographic characteristics confine late-stage eggs to deeper water (Wieland and Zuzarte, 1991; Nissling et al., 2003), where younger larvae were also found. The gravity of the eggs and their vertical distribution shows seasonal and annual variation (Nissling et al., 2003), along with corresponding fluctuations in the WMD of newly hatched larvae. The results presented here show that yolk-sac and feeding larvae are spread throughout the water column in a non-uniform manner. The bimodal distribution might be the result of either or both of the late-stage egg distribution and the necessity to migrate to the surface for optimal feeding.

Ontogenetic vertical migration of sprat larvae is expected to be coupled to feeding, as already shown for Baltic cod larvae (Grønkjær and Wieland, 1997). Alternatively, an upward ontogenetic vertical migration might reduce predation risk, because adult sprat and herring concentrate to feed in depths >50 m (Köster and Schnack, 1994; Köster and Möllmann, 2000b).

The fraction of larger larvae found below the halocline probably consisted to a great extent of starving larvae with reduced condition and swimming ability, which were not able to perform a first-feeding migration or to maintain their vertical position in optimal feeding conditions. These assumptions are supported by short-latency proxies of larval condition, showing depth dependence (A. Dänhardt, pers. comm.).

Vertical migration patterns
Larger larvae showed clear vertical aggregation patterns in all profiles sampled. Vertical aggregation of feeding larvae is often determined by a combined effect of food abundance and suitable light level (Munk et al., 1989; Gilbert et al., 1992; Ponton and Fortier, 1992; Grønkjær and Wieland, 1997). Light level is important because sprat larvae are visual predators. Unlike the more dusk and/or dawn feeding behaviour of cod larvae (Last, 1978; Kane, 1984), the main daily feeding period of sprat larvae is around midday, i.e. when light levels are greatest (Voss et al., 2003). The effect of light intensity on feeding of sprat larvae has not, however, been investigated to date. The biomass of Acartia spp. was always greatest at depths of 5–15 m, and because they do not perform diurnal vertical migrations in the Baltic Sea (Schmidt, 2006), the combined effect of light level and prey abundance should favour sprat larval growth and survival near the surface. However, the hypothesis of larvae primarily seeking depths according to optimal feeding conditions, as determined by an optimal light for feeding and optimal densities and sizes of prey (Fortier and Leggett, 1983; Lough and Potter, 1993), does not explain the downward migration by day in 1989 and 1990. We therefore suggest a combined effect of a changed vertical distribution pattern of Acartia spp. and temperature on larval migration. Since the beginning of the 1990s, the abundance of Acartia spp. has generally increased in the eastern Baltic. Greater abundance has been related to an overall increase in water temperature (Möllmann et al., 2000). Our data demonstrate a significantly greater SF of Acartia spp. in 1999 and 2002 than in 1989, which might be linked to an increase in the temperature of surface waters in the Bornholm Basin. However, the data do not confirm an increase in total Acartia spp. biomass, so we propose further research into temperature and prey as potential drivers of behavioural change in larval Baltic sprat, because this might considerably impact larval survival probability and therefore recruitment.


   Acknowledgements
 
We thank all those involved in sampling and analysing the material, especially the crew of RV "Alkor" and all participants of the GLOBEC Germany project for creating a convivial working atmosphere. Three anonymous referees considerably improved the manuscript with pertinent comments. The work was conducted under the framework of the GLOBEC Germany project, with financial support from BMBF.


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 Discussion
 References
 

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