Baltic herring larvae appear earlier and grow faster due to climate change

Herring is commercially the most important species in Finland, and an important part of the Baltic Sea ecosystem. Temporal changes in the life cycle of Baltic herring can become critical for the survival of species when there is a mismatch in timing between prey and consumer, especially during early life stages. Using long-term and broad-scale survey data
along the whole coast of Finland, BlueAdapt postdoctoral researcher Benjamin Weigel and colleagues from the University of Helsinki and Luke, found that Baltic herring larvae have advanced their development on average by 7.7 days per decade in response to environmental change. In their study published in Marine Ecology progress Series, they highlight that the advancement in phenology is reflected in earlier occurrences of small larvae and in faster developments to bigger larvae. Rising water temperature was one factor that led to increasingly earlier appearances and faster growth of this key species. With little earlier knowledge on how timings of early life stages of fish are affected by climate change over multiple decades, the reported findings advance our understanding on the reproduction ecology of fish.

What is phenology and how do we measure it?

Timing in life history events of many organisms is linked to the seasonal climatic conditions that vary from year to year. During years with earlier warming in the season, one can for example observe also flowers blooming earlier. Such seasonally reoccurring biological events as e.g. the blooming time of flowers is called phenology. With progressing climate change, scientists have observed that such phenological events have shifted their timing during the past decades, predominantly as a response to increasing temperatures, and are now appearing on average earlier. But to study such changes we need high resolution data on the exact timing on when a certain event is occurring. For a flowering plants phenological events are easy to observe and can be recorded in parks and even your garden.

But how does one investigate such changes in species that are difficult to reach, as for example fish in the Baltic Sea? There are fish larvae surveys for some species, but are always conducted at the same time, and usually when there are larvae present already. This is because one doesn’t want to go out sampling and have empty nets. Hence, such surveys can’t answer if fish are spawning earlier in response to changing environmental conditions over long periods of time.

Baltic herring larvae phenology in the northern Baltic Sea is governed by changing
climatic conditions

The researchers from the University of Helsinki and Luke came up with a method to use a detailed data set of herring larvae surveys and reveal responses of herring phenology to climate change. They used data collected over 22 years from a herring larvae survey that was conducted in several areas along the whole Baltic coast of Finland. With no exact dates of first larvae hatching available, they modelled changes in phenology based on relative abundances of different size classes of fish larvae. They predicted the day of the year when the smallest larvae had a high probability of occurrence, and when largest larvae made up a significant percentage of all larvae. Earlier dates in high occurrence probabilities of the smallest larvae indicate relatively earlier spawning, and earlier dates in proportions of the largest larvae would indicate faster larvae growth.

The results highlight that both developmental stages in Baltic herring larvae, small and large, have shifted their timing to earlier dates, suggesting that herring spawn earlier and larvae grow faster, by about 7.7 days per decade. Water temperature and the amount of chlorophyll a in the water, serving as an estimate for the larval food resources, were strong drivers of this change. Advancements in phenology can become critical for the survival of species when there is a mismatch in timing between prey and consumer, especially during early life stages. Such results are important also from fisheries management point of view because herring is an economically important species and an important part of the Baltic marine ecosystem. Survival and growth of herring larvae have an impact on the adult herring population and on the prey and predator species. Hence, the conditions during herring spawning may have cascading effects on the whole Baltic ecosystem.

With increasing temperatures having positive effects on earlier occurrences and abundances, a continued advancement of Baltic herring larval phenology can be expected under future climate projections. Therefore, it is important to maintain long-term data series, such as the one used in this study, and to broaden data collection efforts so that in the future we can also include lower trophic levels to better understand how the resulting predator−prey dynamics are affected by changing species phenology in the Baltic Sea and elsewhere.


The study was published in Marine Ecology Progress Series (, by Benjamin Weigel, Jussi Mäkinen, and Jarno Vanhatalo from the University of Helsinki, and Meri Kallasvuo from the National Resource Institute Finland (Luke).

Weigel B, Mäkinen J, Kallasvuo M, Vanhatalo J (2021) Exposing changing phenology of fish larvae by modeling climate effects on temporal early life-stage shifts. Mar Ecol Prog Ser 666:135-148.

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