Thermal affinity as the dominant factor changing Mediterranean fish abundances

Area of Science:

• Marine ecology
• Climate change impacts
• Fisheries science

Background:

• Marine ecosystems face significant changes in species abundance and community composition due to anthropogenic drivers.
• Key drivers include exploitation, invasion by nonindigenous species, and climate change, though their relative importance is debated.
• The Eastern Mediterranean is particularly affected by Red Sea invasions, extreme climate change, and high fishing pressure.

Purpose of the Study:

• To empirically estimate the relative importance of climate change, species invasion, and fishing pressure on fish abundance changes in the Eastern Mediterranean.
• To correlate fish abundance changes with species' sensitivity to these major anthropogenic stressors.
• To predict future shifts in Mediterranean fish communities due to ongoing environmental changes.

Main Methods:

• Utilized two fish trawl surveys over a 20-year period to assess changes in fish abundance.
• Estimated species sensitivity to invasion using trait similarity between indigenous and nonindigenous species.
• Assessed sensitivity to fishing using a life-history composite index and sensitivity to climate change using climatic affinity from occurrence data.
• Employed meta-analytical and random forest analyses to determine the drivers of abundance changes.

Main Results:

• Sensitivity to climate change was the most significant driver of population size changes for shallow-water species.
• Species with warm-climate affinity increased in abundance, while cold-climate-associated species decreased, correlating with rising sea temperatures.
• Nonindigenous species invasion and fishing pressure played minor roles in explaining observed abundance changes.
• The decline of cold-water species at the warm edge of their distribution was a notable finding.

Conclusions:

• Ongoing ocean warming is driving a major restructuring of Mediterranean fish communities.
• Warm-adapted species are increasing in abundance, while cold-adapted species are declining.
• The long-term impacts of these community shifts on ecosystem function remain uncertain and require further investigation.