Döring, Julian (2012) Fecundity aspects in Baltic sprat - spatial variations in reproductive investment. (Diploma thesis), Christian-Albrechts-Universität zu Kiel, Kiel, Germany, 67 pp.

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Abstract

Sprat (Sprattus sprattus balticus S.) is one of the ecologically and commercially most important fish species in the Baltic Sea. Within the Baltic, it is living at the edge of its ecological range, especially with regard to ambient temperature and salinity. To investigate spatial differences in sprat reproductive investment along a gradient in these environmental features, batch fecundity, fish condition, gonadosomatic index, as well as the oocyte dry weight, protein content, lipid content, and fatty acid composition were measured in the western and central Baltic Sea. Sampling took place in five major spawning areas of the Baltic Sea in April and May 2012 (from West to East): Kiel Bight, Arkona Basin, Bornholm Basin, Gdansk Deep, and Gotland Basin. In the Kiel Bight additional samples were collected in March, enabling a seasonal analysis of sprat reproductive traits in this area. Relative batch fecundity, fish condition, and oocyte protein content were significantly decreasing from West to East within the Baltic. Consequently, a positive relationship between these characteristics can be assumed. Spawners in Kiel Bight are of a better nutritional condition compared to spawners in the more eastern parts of the Baltic. Further, female sprat sampled in the Kiel Bight produced up to twice as many oocytes, each with twofold higher protein content, when compared to spawners in the Gdansk Deep. Significant differences in essential fatty acid composition of sprat oocytes, in particular docosahexaenoic acid (DHA), eicosapentaenoic acid (EPA), and arachidonic acid (ArA) were detected among areas. Total amounts as well as ratios of essential fatty acids can be used as indicators for the female’s diet and nutritional supply, but may also be due to genetic differences between stocks. Lipids from Kiel Bight sprat oocytes had lower fractions of DHA and ArA, as well as a lower DHA/EPA ratio compared to spawners from the eastern areas of the Baltic. In addition to these regional differences, data from the three different time points observed in Kiel Bight sprat indicate that both egg production and investment into individual eggs vary over the spawning season: while relatively few oocytes are produced in March, these are heavy and rich in lipids. Over the course of the spawning season, fecundity increases while egg dry weight and lipid content decrease. The obtained data are important for a better understanding and evaluation of Baltic sprat reproductive potential. In particular, results from the present study suggest that the western Baltic sprat spawning in Kiel Bight are featuring drastically different reproductive traits compared to sprat spawning in all other areas, and might comprise a separate stock. Further, these data may serve as important input parameters for individual based and bioenergetic modelling of Baltic sprat reproductive output and population dynamics.

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