Is climate change affecting the spawning phenology of Baltic sprat?

Rising water temperatures associated with ongoing climate change can significantly affect fish reproductive cycles. This study examined whether such environmental changes influence the reproduction of Baltic sprat (Sprattus sprattus balticus). The species is one of the most abundant pelagic fish in the Baltic Sea and an important component of the regional marine ecosystem.

The research was based on a long-term analysis of variability in Baltic sprat spawning phenology in the southern Baltic Sea. Data from ichthyological analyses collected regularly between 1970 and 2025 were used. Gonad development stages were determined macroscopically according to the modified Maier’s scale. Stages VI and VII were considered to represent individuals actively participating in spawning. The analyses covered three major Baltic sprat spawning grounds: Bornholm Deep, Słupsk Furrow, and Gdańsk Deep.

The results indicate noticeable long-term changes in spawning activity. In recent decades, the onset of spawning has gradually shifted toward earlier periods of the year. These changes have coincided with increasing water temperatures observed in the Baltic Sea during the same period. The analyses also revealed a relationship between water warming and the proportion of spawning individuals within the population.

Understanding how climate-related environmental changes affect fish reproduction is essential for predicting future changes in population dynamics and ecosystem functioning. This study highlights the importance of long-term monitoring data in understanding biological responses to changing environmental conditions and provides additional insight into the ecological consequences of climate change in the Baltic Sea.