Contents lists available at ScienceDirect Fisheries Research journal homepage: www.elsevier.com/locate/ fishres Spatial and seasonal variability in reproductive investment of Baltic sprat Julian Döring a,b, ⁎ , Helena Hauss b , Holger Haslob c a Leibniz Centre for Tropical Marine Research (ZMT), Fahrenheitstr. 6, 28359 Bremen, Germany b GEOMAR Helmholtz Centre for Ocean Research Kiel, Düsternbrooker Weg 20, 24105 Kiel, Germany c Thünen-Institute of Sea Fisheries (TI), Palmaille 9, 22767 Hamburg, Germany ARTICLE INFO Handled by George A. Rose Keywords: Fecundity Reproductive potential Fatty acids Spawning energetics Clupeidae Oocyte ABSTRACT Fecundity of marine fish species is highly variable, but trade-offs between fecundity and egg quality have rarely been observed at the individual level. We investigated spatial differences in reproductive investment of in- dividual European sprat Sprattus sprattus (Linnaeus 1758) females by determining batch fecundity, condition indices (somatic condition index and gonadosomatic index) as well as oocyte dry weight, protein content, lipid content, spawning batch energy content, and fatty acid composition. Sampling was conducted in five different spawning areas within the Baltic Sea between March and May 2012. Spawning sprat from the Kiel Bight were in a better nutritional condition compared to sprat from the Arkona Basin, Bornholm Basin, and the Gdansk Deep. These females were also producing up to twice as many oocytes, and invested significantly more energy in reproduction, than their counterparts sampled in the eastern part of the Baltic Sea. Still, oocytes produced by Kiel Bight sprat contained significantly lower fractions of the essential fatty acids 20:4 (n-6) and 22:6 (n-3). A seasonal trade-off between oocyte weight/lipid content and fecundity was found for Baltic sprat, albeit the gross energy invested into spawning remained constant. Observed spatial and seasonal differences in sprat re- productive investment may be linked to hydrographic conditions and food availability and will impact the survival probability of yolk-sac and first feeding larvae. These findings indicate that Baltic sprat is able to adapt its reproductive tactics to the highly variable pelagic habitat of the Baltic Sea. 1. Introduction The European Sprat Sprattus sprattus (Linnaeus 1758) is a key spe- cies in the pelagic food web of the Baltic Sea (Rudstam et al., 1994). Within the Baltic Sea sprat occur from the Belt Sea and Kiel Bight in the West to the Gulf of Finland in the East (Parmanne et al., 1994). Due to the hydrographic characteristics of the Baltic, sprat encounter a large range of environmental conditions within their pelagic habitat. The Baltic is a semi-enclosed brackish sea with a steep temperature and salinity gradient decreasing from West to East (Janssen et al., 1999). It is further characterized by a stable thermohaline stratification in the deep basins. The depths of the halocline and the oxycline depend pri- marily on the frequency of inflow events of oxygenated, saline water from the adjacent North Sea through the shallow Danish straits (Leppäranta and Myrberg, 2009). Recruitment in sprat is salinity de- pendent; salinities below 8 impair sprat egg survival (Petereit et al., 2009). Thus spawning mainly takes place in the Western Baltic and the deep basins of the central Baltic (Aro 1989; Karasiova and Zezera 2005; Baumann et al., 2006). Like many other clupeid fish species, sprat is an indeterminate batch spawner releasing a number of successive egg batches during a pro- tracted spawning season (Alheit, 1988; Heidrich, 1925). The spawning season of sprat within the Baltic generally ranges from February to August (Grauman et al., 1987; Ojaveer and Kalejs, 2010), but spawning individuals have been observed in the South-Central Baltic as early as January (Haslob et al., 2013). Peak spawning typically occurs between May and June (Heidrich, 1925; Karasiova, 2002), although extremely cold winter temperatures may cause a delay (Karasiova, 2002). Rechlin (1975) showed significant differences in morphology and life history traits of sprat from the Bight of Mecklenburg and sprat from the central part of the Baltic. Thus, the western Baltic sprat population may be more closely related to the Kattegat-Skagerrak population. While, Debes et al. (2008) did not observe differences in the mi- tochondrial DNA of Baltic and North Sea sprat populations, Limborg et al. (2009) was able to detect genetic differences among sprat sampled in the Baltic Proper using a DNA microsatellite approach. Limborg et al. (2009) showed that sprat populations from the Arkona Basin, the Gdansk Deep, and the Bornholm Basin are all genetically distinct. However, the migration behaviour of sprat leads to an extensive mixing of different stock components in some areas, which hampered clear https://doi.org/10.1016/j.fishres.2018.02.002 Received 17 March 2017; Received in revised form 2 February 2018; Accepted 5 February 2018 ⁎ Corresponding author at: Leibniz Centre for Tropical Marine Research (ZMT), Fahrenheitstr. 6, 28359 Bremen, Germany. E-mail addresses: julian.doering@leibniz-zmt.de, julian.doering@me.com (J. Döring). Fisheries Research 204 (2018) 49–60 0165-7836/ © 2018 Elsevier B.V. All rights reserved. T