Climate change and the ocean: What future for fish?

Since the establishment of the Intergovernmental Panel on Climate Change (IPCC) and the emergence of their reports assessment of the global climate system, the alteration of environmental physico-chemical parameters is a certainty.

Indeed, the various emissions from anthropogenic activities (greenhouse gases, waste disposal, chemical and/or biological pollutants, etc.) disrupt ecosystems. By acting as a buffer, the oceans and more specifically surface waters regulate the consequent supply of atmospheric carbon dioxide (CO₂). These carbon sinks thus limit the terrestrial impact of the continuous emissions of CO2, one of the major players in this global change.

Currently, the carbon dioxide concentration (expressed in partial pressure pCO₂) is estimated at 400 µatm, which corresponds to a pH in the marine environment of 8.

According to IPCC projections, pCO2 of the surface ocean will reach values ​​of 1 µatm by 200, which will cause (among other things) a significant decrease in the pH of seawater (pH estimated at 2100). These variations in the physico-chemical properties of the world's surface waters lead to the weakening of marine ecosystems.

ocean acidification

Indeed, the modification of the parameters of the oceans (warming, acidification, hypoxia, salinity) can have an impact on the physiological functions of aquatic organisms, dependent on variations in their environment. Fish are particularly sensitive to environmental variables that can generate changes in their behavior and physiological functions (growth, feeding, maturation, reproduction).

The study of the effects of long-term global change on the physiology of fish is the subject of numerous research projects within LEMAR-Ifremer (MARin Environmental Science Laboratory et French research institute for the exploitation of the sea), who has gained significant experience studying the effects of ocean acidification, specifically by studying seabass. The exposure of marine fish to the environmental conditions predicted for the near future is essential to assess the physiological acclimatization capacities of aquatic organisms and thus to understand the vulnerability of these populations.

The European bar (Dicentrarchus labrax), is an ideal model species for experimental studies in a controlled environment. In fact, it combines several interesting characteristics: a species of commercial interest (aquaculture and fishing), having been the subject of numerous studies making it possible to capitalize on a large amount of information on its physiology, biology and ecology. Knowledge of this species (growth, reproduction, life cycle) and its ease of breeding make it possible to understand, in the laboratory, the effects and consequences of modification of environmental parameters (temperature, pH, food, oxygen) on its physiological functions. at different stages of its life cycle.

Adult European sea bass (Dicentrarchus labrax).
O.Mouchel

Among the projects carried out on the different life stages of sea bass, the larval stage being the most fragile and sensitive to environmental changes is often studied further. However, no effect on growth has been demonstrated in the face of an acidification of the environment, reflecting a resilience capacity of the larva in the face of this phenomenon. However, the olfactory system seems to be impacted at different levels of its functioning, as suggested by molecular biology studies. Against all expectations, these studies show a positive stimulation of the genes involved in the recognition of pathogens present in the olfactory system of juveniles, opening a new question on the potential effects of resistance of sea bass to viral and bacterial attacks. These effects could also be observed in their offspring, which may affect their perception and interaction with the environment and in the long term. impact their survival.

Available oxygen is decreasing in the oceans

The increase in CO2 aquatic also leads to a decrease in the oxygen available in the water. However, fish subjected to hypoxia (decrease in the concentration of oxygen in the water), even slight, during the early stages of development, develop respiratory malformations and their growth is reduced. This effect on growth is attributable to a dysfunction of the digestive proteins which seem less effective under these conditions. Moreover, the exposure of the larval stages to a phenomenon of hypoxia decreases the tolerance of the juvenile stages to low oxygen concentrations, which increases the vulnerability of the species facing hypoxia scenarios.

European sea bass (Dicentrarchus labrax) larvae after hatching.
O.Mouchel

Currently, the study of the effects of ocean acidification on this species in this laboratory focuses on adults. According to the results of the laboratory and on the basis of other scientific studies, it has been observed that changes in environmental variables such as temperature, salinity, acidification or hypoxia have an impact on behavior and reproductive physiology. from the bar that seems reduce adult reproductive success.

European sea bass roe (Dicentrarchus labrax).
O.Mouchel

These studies of the effects on the three major life stages of sea bass (larvae, adult juveniles) are currently being continued and deepened in the laboratory using an integrated approach making it possible to understand the entire life cycle of sea bass in its continuity and possible effects intergenerational. For this, an original and unique experiment for this type of species is in progress on individuals maintained in a rearing tank from the larval stage (two to four days post hatching) to the adult stage (4 years) under conditions of pH corresponding on the one hand to the current natural conditions and on the other hand to those anticipated by the most severe scenario of the IPCC for the 2100 horizon. The growth and reproduction of these fish are studied during several cycles of reproduction and on two generations. To our knowledge, the intergenerational effects of acidification on fish reproduction have never been studied before.

In previous experiments, differences were highlighted between the two experimental conditions with respect to various aspects of reproduction, such as maturation in both sexes, gamete quality and spawning period.

Based on these initial results, new study projects are being developed around multi-stress approaches aimed at combining different stresses of global change in order to highlight the combined impact of these disturbances on the physiological resilience of organisms. Thus, recently, the multi-stress approach (temperature, acidification and chemical contamination of the xenoestrogenic, EE2) was initiated in the laboratory on the threespine stickleback (Gasterosteus aculeatus) a new species with a short life cycle, allowing all life stages to be observed in one year. The first results show that, subjected to multi-stress, the eggs tend to be smaller and the larvae have a weaker growth. These early clues suggest early effects of environmental change.

Threespine stickleback embryo (Gasterosteus aculeatus) at four days of development.
J. Devergne

The continuation of these research projects to understand and anticipate the effects of climate change on fish is a topical subject and a major challenge for developing the strategy for the management and conservation of marine ecosystems.

Jimmy Devergne, PhD student in Biology, Cellular and Molecular Biochemistry, Ifremer; Arianna Servili, researcher in endocrinology and fish physiology in the face of changing environments, Ifremer; Cristina Garcia Fernandez, Post-doc in fish ecology and physiology, Ifremer et Veronique Loizeau, Researcher on the Bioaccumulation of Organic Contaminants, Ifremer

This article is republished from The Conversation under Creative Commons license. Read theoriginal article.

Image credit: Shutterstock.com / Johan Holmdahl

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