Dr. Chien-Hsiang Lin of our Center, in collaboration with an international research team, analyzed fish fossil otolith records from deep-sea sediments in the Eastern Mediterranean spanning the past 10,000 years, revealing how ancient marine ecosystems responded to a major deoxygenation event. The study found that during this ancient deoxygenation period, lanternfishes inhabiting the Mediterranean mesopelagic zone nearly disappeared, while at the same time, surface-dwelling species such as the European anchovy (Engraulis encrasicolus) and Mueller's pearlside (Maurolicus muelleri) increased dramatically, indicating starkly different sensitivities and responses to oxygen depletion among fishes occupying different ecological niches.

This deoxygenation event corresponds to the well-known “Sapropel” in the geological history of the Mediterranean. These are organic-rich black sediment layers that serve as important geological markers for studying deep-water anoxia, water-column stratification, and ecosystem change in the region. The formation of sapropels is linked to the Earth’s orbital parameters (Milankovitch cycles), specifically the ~21,000-year precession cycle, which influences the strength of the African monsoon. Intensified monsoons increase freshwater input to the Mediterranean, altering ocean circulation patterns and ultimately leading to the deposition of these dark sediment layers.

Using high-resolution otolith identification and counts, the team reconstructed fish community composition before, during, and after the deoxygenation event, revealing a clear niche replacement phenomenon: oxygen stress drove the decline or disappearance of mesopelagic fishes, while some epipelagic species expanded their distribution and population size. After the Sapropel event when oxygen returned to the mesopelagic zone, lanternfish populations not only recovered but exploded to more than 20 times their pre-event abundance, demonstrating both a strong dependence on oxygenated midwater habitats and an extraordinary capacity for rapid population rebuilding. This finding underscores the sensitivity of mesopelagic fishes to oxygen availability and highlights their potential role in the global carbon cycle, as they are key drivers of the “biological carbon pump” that transports carbon from surface waters to the deep ocean.

This research represents the first long-term quantitative analysis of fish community dynamics during an ancient deoxygenation event in the Mediterranean, offering a new perspective for paleoecology and a warning for modern marine conservation. Today, oxygen minimum zones in the ocean are expanding rapidly, and future mesopelagic fish communities may face survival challenges similar to those of 10,000 years ago, with cascading effects on marine food webs and carbon cycling.

In this study, Dr. Lin was responsible for otolith taxonomy and fish community reconstruction. The paper was published on July 28, 2025, in Communications Earth & Environment.
Full article: https://doi.org/10.1038/s43247-025-02568-8


A. High mesopelagic fish production in a well-ventilated water column during the Late Holocene.
B. Enhanced/low epipelagic/mesopelagic fish production under expanded ocean hypoxia during Sapropel deposition.
C. Ocean data view bathymetry map of study area and core locations of M144 KC5-6 (this study) and SK1

Inner view of fossil fish otoliths from kasten lot core M144 KC5-6 (central western Aegean Sea) belonging to family Myctophidae.
A-E: Benthosema glaciale; F-H: Diaphus holti; I-J: Notoscopelus elongatus;
K-N: Ceratoscopelus maderensis; O-P: Electrona risso. Scale bar 1mm.

Chien-Hsiang Lin
Associate Research Fellow

Marine Paleontology