The survival of herring larvae threatened by climate stress in the Baltic Sea

27/01/2025

6 minutes

oceans and climate

Current climate upheavals are severely affecting marine ecosystems. Herring larvae, a key link in the ocean food chain, are particularly vulnerable, facing a combination of environmental stresses that compromise their development and survival.

By Laurie Henry

The planet’s biodiversity is fragile, and marine ecosystems rely on a delicate balance in which each species plays a key role. Among them, the herring is a small fish of crucial importance. It occupies a central place in trophic chains, since its larvae are essential food for many marine predators. They are also an important source of protein for puffins (Fratercula arctica), seabirds that depend on these small fish to feed their chicks. Among marine mammals, grey seals (Halichoerus grypus) regularly consume herring, including larvae, to maintain their body mass and energy levels. Among carnivorous fish, the Atlantic cod (Gadus morhua) is a major consumer of herring and its larvae in particular.

Yet climate change is threatening the survival of these vulnerable herring larvae. A team of researchers – led by Dr. Andrea Franke of the Helmholtz Institute for Functional Marine Biodiversity in Oldenburg, in collaboration with the GEOMAR Helmholtz Center for Ocean Research Kiel and the University of Kiel – has studied the combined effects of climatic and biological stresses on Baltic Sea herring larvae. Their study, published in Science of the Total Environment, reveals alarming biological failures and raises vital questions about the ability of marine ecosystems to withstand increasing anthropogenic pressures.

Larvae in distress: an impeded molecular reaction

To capture the impact of environmental stresses on herring larvae, the team designed a rigorous experiment, exposing larvae to three distinct scenarios – a temperature rise simulating a heat wave, the presence of pathogenic bacteria, and a combination of these two stresses – with the aim of analyzing how these conditions influence the larvae’s fundamental biological mechanisms.

Graphical summary of the experimental protocol © A. Franke et al., 2024

The scientists focused on three types of analysis. First, they studied the gene expression that regulates the production of proteins needed to respond to stress. Secondly, they examined microRNAs, short molecules that play a key role in activating or inhibiting cellular processes. Finally, they analyzed the larval microbiome using genetic techniques to identify the microbes present on and in the specimens. Together, these methods enabled a comprehensive assessment of the larvae’s biological responses to environmental changes.

The results revealed a striking contrast: when exposed to a single stress (such as heat or bacteria), larvae showed a significant reduction in gene expression, a mechanism interpreted as a cellular response aimed at minimizing the damage induced by climatic stress; in the presence of combined stresses, this reaction disappeared altogether. The inability of larvae to activate these protective mechanisms could lead to irreversible damage to proteins and DNA, compromising their development.

Number of differentially expressed genes (DEGs) in comparisons with more than 50 DEGs. Genes with increased expression are shown in red, while those with decreased expression are shown in blue (Hc : high control – Nc : normal control – N val : high V. alginolyticus – N val : normal V. alginolyticus) © A. Franke et al., 2024

The fragility of marine ecosystems in the face of acidification and global warming

In addition, the vulnerability of herring larvae to ocean acidification and global warming was studied. The increase inCO2 in the atmosphere, which is responsible for the acidification of marine waters, seriously disrupts larval development by altering their ability to form calcified structures. These structures (such as the otolith, essential for fish balance and navigation) become fragile, making the larvae more exposed to predation and limiting their chances of survival. This phenomenon is in addition to the thermal stress already observed, which disrupts the biological regulation and growth mechanisms of young individuals.

The cascading effects of these disturbances go far beyond the larvae themselves, since a significant reduction in their populations could unbalance entire trophic chains. Moreover, as a key species in the marine nutrient cycle, their decline would also compromise productivity in the oceans, by limiting the distribution of nutrients necessary for the growth of phytoplankton. These microscopic plants are at the base of the food chain and play a crucial role in carbon sequestration and maintaining ecological balance.

In addition, the authors also show in this study that experimental conditions (high temperature, exposure to Vibrio anguillarum or both combined) significantly modify the bacterial communities of herring larvae. For example, high temperature favors the expansion of certain bacterial genera to the detriment of others, leading to imbalances in the larvae’s natural microbiome. The authors conclude that these bacterial alterations could weaken larvae by compromising key functions such as immunity and metabolism, thus increasing their vulnerability in a marine environment already weakened by climate change.

The different bacterial communities associated with larvae, depending on the treatments applied © A. Franke et al., 2024

By linking these data, the study highlights the complexity of interactions between climatic stresses. Each factor, whether acidification, warming or infection, contributes to weakening an ecosystem already under stress.

A global response to preserve marine ecosystems

The challenges posed by climate stress on herring larvae require a global mobilization that goes beyond isolated actions. As Dr. Andrea Franke points out, “ responses to multiple environmental stresses require integrated approaches that take into account the complexity of ecosystem interactions ”. The implementation of ambitious climate policies must be accompanied by local solutions, such as protecting spawning grounds and reducing coastal pollution, which act as levers to improve the resilience of marine species.

Scientific collaboration is also essential to anticipate the long-term consequences of climate change on herring populations and trophic chains as a whole. New technologies, such as the genetic analysis of marine microbiomes, offer invaluable tools for monitoring the state of ecosystems in real time and adjusting conservation measures. By strengthening the dialogue between scientists and decision-makers, it becomes possible to prioritize targeted interventions, adapted to each region and to the specific needs of threatened species.

Beyond the ecological stakes, this issue raises profound questions about our natural resource management models. Protecting herring, and by extension marine ecosystems, is not just about responding to an environmental crisis, but about rethinking our cohabitation with nature. If the necessary measures are not rapidly put in place, the imbalances observed today could spread to other vital ecosystems. This global awareness could mark the beginning of a new era of ecological responsibility, guided by decisions based on sound science and a determination to ensure a sustainable future for generations to come.

Source : Andrea Franke et al., “Climate challenges for fish larvae: Interactive multi-stressor effects impair acclimation potential of Atlantic herring larvae”, Science of The Total Environment (2024).

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