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ICES Journal of Marine Science: Journal du Conseil 2004 61(6):966-971; doi:10.1016/j.icesjms.2004.06.008
© 2004 by ICES/CIEM International Council for the Exploration of the Sea/Conseil International pour l'Exploration de la Mer
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Spatial feeding patterns of herring (Clupea harengus L.), sprat (Sprattus sprattus L.), and the three-spined stickleback (Gasterosteus aculeatus L.) in the Gulf of Finland, Baltic Sea

Heikki Peltonena,*, Mika Vinnib, Antti Lappalainenc and Jukka Pönnic

a Finnish Environment Institute PO Box 140, FIN-00251 Helsinki, Finland
b Department of Biological and Environmental Sciences PO Box 65, FIN-00014 University of Helsinki, Finland
c Finnish Game and Fisheries Research Institute PO Box 6, FIN-00721 Helsinki, Finland

*Correspondence to H. Peltonen: tel: +358 (0)9 40 300 236, fax: +358 (0)9 40 300 291. e-mail: heikki.peltonen{at}ymparisto.fi.

The pelagic-fish fauna in the Gulf of Finland, Baltic Sea was sampled by trawling and hydroacoustics in September 2002. Spatial and size/age-dependent patterns in the diets of herring (Clupea harengus), sprat (Sprattus sprattus), and the three-spined stickleback (Gasterosteus aculeatus) were explored. At night the fish concentrated at thermocline depth but at dawn they scattered over a larger depth range. All three fish species fed on mesozooplankton but nektobenthos, for example, was scarce. In the eastern Gulf of Finland, where there is a strong freshwater inflow, the cladoceran Bosmina longispina was the dominant prey item, but it was also abundant in the diets of young-of-the-year (total length <10 cm) clupeids in the western Gulf of Finland. In these more saline western areas, calanoid copepods, especially Eurytemora affinis, were the most important prey for large (≥10 cm) clupeids. The large clupeids in particular, also fed on Temora longicornis. The diet of three-spined stickleback overlapped with that of the clupeids. However, Cercopagis pengoi, a recent arrival to the area, was much more abundant in the diet of stickleback than in the diet of clupeids.

Keywords: Baltic Sea, Gulf of Finland, feeding, herring, sprat, three-spined stickleback

Received 12 August 2003; accepted 15 June 2004.


   Introduction
Top
 Introduction
Material and methods
 Results
 Discussion
 References
 
Several studies conducted during the 1980s and 1990s have indicated the importance of copepods, especially Pseudocalanus elongatus and Temora longicornis in the diets of herring (Clupea harengus) and sprat (Sprattus sprattus) in the Southern and Central Baltic Sea (Szypula, 1985; Patokina and Feldman, 1998; Möllmann and Köster, 1999). Cladocerans (e.g. Bosmina longispina maritima, Evadne nordmannii, and Pleopsis polyphemoides) are also found in the diets of both species especially during summer (Möllmann and Köster, 1999), and have been found to be more important for sprat than for herring (Szypula, 1985). Sprat feeds almost solely on copepods and cladocerans whereas herring may also utilize nektobenthos and zoobenthos (Szypula, 1985; Raid and Lankov, 1995; Patokina and Feldman, 1998). Three-spined stickleback (Gasterosteus aculeatus) is another abundant planktivorous species in the Baltic Sea (Jurvelius et al., 1996). However, feeding, abundance, and the ecology of this species in the pelagic areas is poorly known. In the Bothnian Sea in the northern Baltic Sea stickleback were observed to feed mainly on cladocerans and copepods i.e. B. longispina maritima and Eurytemora affinis (Leinikki, 1995).

In the Baltic Sea, the zooplankton assemblage is a mixture of marine, brackish water, and freshwater species. Thus, salinity is a key factor regulating the dynamics of the assemblage (e.g. Viitasalo et al., 1995). Changes in the hydrography are also reflected in the species composition of zooplankton and in other biota. In the Gulf of Finland, there was a shift between 1985 and 1991 in herring diets from neritic copepods to smaller and less energetic prey items. This change was linked with changes in the zooplankton assemblage following a decrease in salinity and changes in the stratification of the water column (Flinkman et al., 1998).

The changes in zooplankton abundance and community structure in the Southern and Central Baltic Sea may, at least, be partially attributed to the changes in the abundance and predation rates of planktivorous fish (Rudstam et al., 1994; Möllmann et al., 2000; Kornilovs et al., 2001). On the other hand, the changes in prey resources have been reflected in a considerable decrease in the growth rates and condition of clupeids during 1980s and 1990s, especially in the northern parts of the Baltic Sea (Cardinale and Arrhenius, 2000; Stephenson et al., 2001; Cardinale et al., 2002). While in the Gulf of Finland three-spined stickleback may in large areas be as abundant as herring (e.g. this study), and sprat abundance exceeds the biomass of herring (ICES, 2003), little is known about diet overlaps and resource partitioning between herring, sprat, and stickleback. This paper compares the feeding of the three major planktivorous fish species, herring, sprat, and three-spined stickleback in different locations in the Gulf of Finland.


   Material and methods
Top
 Introduction
 Material and methods
Results
 Discussion
 References
 
The fish were collected from 10 pelagic-trawl hauls during 2–6 September 2002 in the Finnish and Estonian economic zones in the Gulf of Finland (Figure 1). Trawling was conducted with the research vessel "Muikku", which has a length of 28 m and a propulsion power of 540 kW. The hauling speed was about 2 knots. The trawl height at the headrope was 15 m, maximum width about 35 m, and the codend mesh size was 7 mm (stretched mesh). The hauls were targeted at depths where most of the fish biomass was observed with hydroacoustics just before trawling, and the duration of the trawl hauls was adjusted using approximate estimates of cumulative catches during trawling derived from real-time, hydroacoustic, areal backscattering-strength values (e.g. MacLennan and Simmonds, 1992). The headrope depth of the trawl was 10 to 25 m in different hauls, and the haul duration ranged from 5 to 30 minutes. At each sampling station, salinity, oxygen concentration, temperature, and light-intensity profiles of the water column were recorded with a Seabird-19 CTD profiler to explore the dependence of the vertical distribution of fish on environmental factors.


Figure 1
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  		Figure 1 The fish-biomass estimates (metric tonnes per squared nautical mile) of herring, sprat, three-spined stickleback, and other species at the stations within the hydroacoustics and trawling survey area. In the diet analyses per se Stations 1–3, 4–8 and 9–10 were combined as western, central, and eastern subareas, respectively.

 
Fish biomass and the vertical distribution of fish were estimated with hydroacoustics at each trawling station with the procedures set down in ICES (2000). The hydroacoustic instruments comprised a Simrad EY500 echosounder equipped with a 38-kHz transducer (beam angle 12° at –3 dB level). The transmitting power was 250 W and a pulse rate of 1 s–1 was used. The location information from a DGPS location sensor was recorded together with the acoustic measurements. The areal-backscattering strength (Sa, metre2 per squared nautical mile) was analyzed in 10-m layers. "Blind zones" 5 m below the surface and 1 m above the bottom were excluded. The length frequency distribution and species composition of catches were assumed to be unbiased samples of the fish community at the sampling stations. The acoustic target strength (TS) in each 5-mm fish-length group (L) was estimated as TS = 20 log L – 71.2. The acoustic cross-section of each fish was estimated as {sigma} = 4{pi} 10(TS/10), and it was averaged over the whole catch at each station to estimate the mean acoustic cross-section Formula . The fish density per squared nautical mile was estimated as Formula . The biomass of each fish species at a station was estimated as Formula , where pi is the proportion and Formula is the average weight of species i in a catch.

The species composition in every trawl catch was determined onboard, and the fish were measured in 5-mm classes to determine the length frequency distribution of each species. The herring, sprat, and three-spined stickleback intended for diet analyses were frozen immediately after capture to preserve their stomach contents. In the laboratory, stomach fullness was examined and the total length and weight were measured with 1-mm and 0.01-g precision, respectively. The ages were determined from otoliths to separate the length distributions of young-of-the-year and older clupeids. The stomach contents were identified and fullness was estimated using a volumetric points method (Windell, 1971). Prey species were grouped into six categories: Bosmina longispina maritima, Cercopagis pengoi, Eurytemora affinis, Temora longicornis, Acartia sp., and other prey items (Pseudocalanus elongates, Centropages hamatus, Limnocalanus macrurus, Mesocyclops leuckarti, Mysidae, Daphnia sp., Evadne normanni, Pleopsis polyphemoides, Amphipoda, Balanus improvisus cypris larva). Altogether, the stomach contents of 401 herring, 415 sprat, and 197 stickleback were analysed. To analyse spatial patterns in diets, the research area in the Gulf of Finland was divided into three subareas i.e. western (Stations 1–3), central (Stations 4–8), and eastern (Stations 9–10) (Figure 1). Sprat and herring were divided into young-of-the-year (YOY, <10-cm total length) and older. Diet composition was compared between these age/size classes within a species, and between different species.


   Results
Top
 Introduction
 Material and methods
 Results
Discussion
 References
 
Herring dominated the catches especially in the eastern and northern parts of the research area while sprat was dominant in the southwestern region (Figure 1). Three-spined stickleback was abundant in samples taken from the southernmost stations. The proportion of YOY herring and sprat was highest in the southwestern Gulf of Finland while in the easternmost stations there were fewer young clupeids. Of the other species, smelt (Osmerus eperlanus) and nine-spined stickleback (Pungitius pungitius) were the most abundant, with maximum percentages 4% and 2%, respectively of the trawl catches. The trawl catches were made up of small fish, as the average weight of the sampled fish at the stations ranged from 3 to 18 g. Few herring in the catches exceeded 18 cm in total length. Stickleback length was 2–7 cm. Fish abundance at the trawling stations was highest in the southwestern part of the Gulf of Finland where fish biomass reached 530 metric tonnes per squared nautical mile.

At the different stations, surface temperature during the survey varied between 15 and 20°C. The warm water layer at the surface extended to about 10-m depth in the northern part of the Gulf of Finland and to 25–30-m depth near the southern coast (Figure 2). At the surface, the salinity was of the order of five at all stations, although a slight increase from east to west was found. However, the vertical salinity gradient was considerably larger at the deep stations at the southern and western areas than at the shallow northern stations. At Stations 1 and 3 in the western Gulf of Finland, a steep halocline occurred near the bottom at approximately 70-m depth. At the same sites and also at stations 5 and 6, low oxygen concentrations near the bottom were found. Usually, fish were sparse below the thermocline where the temperature was below 3°C.


Figure 2
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  		Figure 2 Hydroacoustic estimates of the vertical distribution of fish biomass, and oxygen concentration, salinity, and temperature profiles at the sampling stations. The horizontal lines indicate the trawling depth.

 
However, at Stations 1, 4, and 5 a narrow layer of fish was found at the halocline depth where the water was warmer than in the layer above. During the dark hours, the fish were mainly in a narrow layer at the thermocline and a large proportion of the total was at the trawling depth. In the daytime, fish scattered over a greater depth range and were abundant at the 5–10-m depth at Stations 1, 5, 6, and 8, where the light-intensity in the water was highest.

The number of samples in the different species and age/size groups ranged from 27 to 67 (Figure 3). Of the analysed fish, some 69% contained identifiable food items. The YOY herring and sprat fed mainly on B. longispina. Copepods were abundant in the diet of large herring and, in the western Gulf of Finland, they were the most important prey for the large-size category of both clupeids (Figure 3). The areal pattern of the diets of both clupeids resembled each other, i.e. the share of cladocerans increased from west to east. Of the copepods, E. affinis was the most abundant, but the diet of especially large herring contained high percentages of T. longicornis. The abundance of T. longicornis and Acartia sp. did not have clear areal patterns. B. longispina was the principal prey for stickleback although E. affinis and C. pengoi were abundant, too. As with clupeids, E. affinis was more important for stickleback in the western than in the eastern sampling area. The proportion of C. pengoi in the diet of stickleback was larger than in the diets of the other species studied. However, C. pengoi was also abundant in the diet of large herring in the eastern Gulf of Finland, but was virtually absent from the diet of sprat. Mysids were scarce in the diets of the analyzed fish, and the overall proportion of nektobenthic organisms was very low.


Figure 3
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  		Figure 3 Composition of the diets of herring, sprat, and three-spined stickleback in the western, central, and eastern Gulf of Finland. The herring and sprat were grouped into young-of-the-year (total length <10 cm) and older. The numbers on top of the bars indicate the number of fish stomachs examined in each case.

 

   Discussion
Top
 Introduction
 Material and methods
 Results
 Discussion
References
 
Vertical migrations of herring and sprat may be adaptations to predator avoidance, bioenergetics, or feeding (Cardinale et al., 2003). It is possible that during the day, the vertical distribution of fish is controlled by feeding whilst the aggregations of fish near the thermocline at night may indicate the optimization of bioenergetics, i.e. the maximization of the net energy that can be allocated for growth. The latter is supported by the fact that the average stomach "fullness" was lower at night than during day. Fish obviously avoided the water layers with temperatures of 3°C and below. However, the somewhat warmer water found as a narrow layer at the halocline depth in the deep water may have been favorable for fish, but below the halocline the oxygen concentrations were too low. The vertical migrations in this study were different from those observed in the Southern Baltic Sea, where both in spring (Nilsson et al., 2003) and autumn (Orlowski, 2001) the clupeids migrated upwards at dusk and down at dawn. The horizontal distribution of fish species appears to be linked not only to environmental factors but also to the distance from spawning areas. YOY sprat were scarce, for example, in the easternmost stations which were far away from the spawning and nursery areas in the Baltic Main Basin. The small size of the herring is typical of the Gulf of Finland because there has been a considerable decrease in herring weight-at-age during the last two decades (Stephenson et al., 2001; ICES, 2003).

The scarcity of benthos and nektobenthos that was found in the diet of the herring may be due to the fact that this is specifically their autumn and winter food (Szypula, 1985; Raid and Lankov, 1995; Lankov, 2002). In some years, however, this group has been important for herring in the Main Basin during August and September (Möllmann and Köster, 1999). The scarcity in diets found in this study may also be linked to the smallness of the populations. In August 2002, benthos and nektobenthos were scarce in deep water areas and absent below the halocline in the western Gulf of Finland, where saline water intruded from the Baltic Main Basin in late summer (Pitkänen et al., 2003). The populations were also small on the extensive shallow areas (Pitkänen et al., 2003), where, via cultural eutrophication, large amounts of organic material settle, decompose, and consume oxygen (Lips et al., 2002).

The clear areal patterns in diets were principally in agreement with the environmental preferences and distribution pattern of the different zooplankton taxa (Viitasalo et al., 1995; Ojaveer et al., 1998; Vuorinen et al., 1998). For example, the increase in the abundance of copepods and the decrease in cladocerans from east to west could be assumed a priori based, for example, on studies of zooplankton ecology (Vuorinen et al., 1998, 2003). Similar gradients in zooplankton community from east to west were also found in the Gulf of Finland some two weeks before the current study (M. Verta, unpublished). The lack of any trend in the occurrence of the marine copepod T. longicornis is somewhat unexpected. However, Vuorinen et al. (1998) too did not find that the abundance of this species correlated significantly with salinity along the southwestern coast of Finland, and Vuorinen et al. (2003) suggested that it may, in fact, belong to the surface community.

The diets of the three fish species overlapped in large part. The results were in line with the observations that sprat have a higher preference than herring to feed on cladocerans (Szypula, 1985; Arrhenius, 1996), although the differences in diet were larger between different size categories than between clupeid species. The invasion of C. pengoi in the 1990s (Uitto et al., 1999) has added a new prey item to the diets of planktivorous fish. However, this predaceous species may also compete for prey with clupeids and three-spined stickleback, because it feeds on the same mesozooplankton taxa (Uitto et al., 1999). In the present study, C. pengoi was abundant especially in the diet of stickleback which also fed, however, on B. longispina and E. affinis. All these three crustacean species favor relatively high temperature and low salinity water (Viitasalo et al., 1995; Vuorinen et al., 1998; Uitto, et al. 1999). Consequently, stickleback may have been feeding closer to the surface than clupeids.

The overlap in diet between the stickleback and the clupeids, together with the high stickleback biomass, suggests that it may be an important competitor in the pelagic areas of the Gulf of Finland, and that it should be considered when estimating, for example, the total planktivory in this sea area.


   Acknowledgements
 
We thank all the participants of the research cruise during which the samples for this study were collected. The comments by Petra Tallberg and Outi Setälä enhanced the manuscript. The financial support from the Employment and Economic Development Centre for Southeastern Finland is appreciated.


   References
Top
 Introduction
 Material and methods
 Results
 Discussion
 References
 

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H. Peltonen, M. Luoto, J.-P. Paakkonen, M. Karjalainen, A. Tuomaala, J. Ponni, and M. Viitasalo
Pelagic fish abundance in relation to regional environmental variation in the Gulf of Finland, northern Baltic Sea
ICES J. Mar. Sci., April 1, 2007; 64(3): 487 - 495.
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