460,000-year fossils suggest twilight zone fish were sensitive to a warm ocean

A team of researchers analyzed a 460,000-year fish fossil record from the warmest area in the Pacific Ocean, called the Pacific Warm Pool. 
Mrigakshi Dixit
Examples of fossil fish otoliths from ODP Hole 1115B.
Examples of fossil fish otoliths from ODP Hole 1115B.


Ancient fish communities that thrived in the twilight zone were likely sensitive to warm ocean temperatures, as per a new study. 

A team of researchers analyzed a 460,000-year fish fossil record from the warmest area in the Pacific Ocean, called the Pacific Warm Pool. 

The findings indicate that twilight zone fish may have encountered warm temperature tipping thresholds, which, when exceeded, resulted in diminishing species as well as their populations. 

This thousand-year-old situation might have implications for current fish communities residing in the mesopelagic zone (twilight zone). These marine creatures that live in the Pacific Warm Pool and other comparable locations may be vulnerable to rising ocean temperatures caused by cascading climate change. 

The mesopelagic zone is a dark ocean ecosystem that ranges in depth from 200 to 1000 meters and is home to a diverse range of marine organisms. These fish and other animal species found here play an important part in the marine food webs that exist between the surface and deep seas. 

“Their ecological importance, alongside exploratory interest in mesopelagic fishes as a possible fishery resource, has led some scientists to question whether these communities could be particularly vulnerable to rising ocean temperatures and, thus, in need of better protection,” added the official release

This new study was conducted by an international team of experts from the Biodiversity Research Center Academia Sinica and National Taiwan University.

The fossil examination

The authors emphasize that a lack of historical data has made it difficult to have a deeper understanding of the marine populations that live in this zone. 

For this new study, the team examined “1,130 carbonate inner-ear fossils called otoliths from fish spanning four glacial cycles.” 

These fossils were collected from over 343 sediment samples discovered around the Pacific Warm Pool's southern edge. 

They identified over 28 taxa and 12 families of mesopelagic fish among them. The lanternfish (Myctophidae) family dominated 73.7 percent of the samples. 

This data was used to estimate two major factors: marine community abundance and species diversity over time. 

They then matched the collected data to ocean temperature records from the same time period and location. 

“The researchers found that diversity peaked during transitions from glacial to interglacial periods, and abundance peaked during interglacial periods – but both declined after temperatures surpassed their respective tipping points, particularly during warmer interglacial periods when temperatures exceeded 28 degrees Celsius,” explained the release. 

This assessment record can be regarded as crucial ecological baseline data with evident and pressing consequences for deep-sea ecosystems. 

The study has been published in the journal Science Advances.

Study abstract:

Field observations and theoretical modeling suggest that ongoing anthropogenic ocean warming will lead to marine ecosystem degradation. Mesopelagic fish are a fundamental component of the pelagic ecosystem, and their role in linking the surface- and deep-ocean ecosystems is essential for the biological carbon pump. However, their response to a warmer ocean is unconstrained due to data scarcity. Using extraordinarily well-preserved fish otoliths, we reconstruct a continuous mesopelagic fish community record in the Pacific Warm Pool region over 460,000 years. Fish production and diversity followed hump-shaped temperature gradients, with lower tipping point temperatures for the diversity than the production by ~1.5-2.0°C. During warmer-than-present interglacial periods, both production and diversity declined drastically. Our findings imply that the temperature-sensitive mesopelagic fish community at the southwestern margin of the Pacific Warm Pool, and possibly other hydrographically similar regions, may be especially affected if ocean warming continues unabated in the future.

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