Life in the Mesopelagic

The mesopelagic - or ocean twilight zone - is the dimly lit part of the ocean between the productive surface waters and the deep-sea. It is among the largest but also least explored and understood ecoystems of the planet. This vast midwater habitat is inhabited by fish, squid, jellyfish and zooplankton. Their ubiquitous presence is revealed by acoustic deep scattering layers found across all oceans.

Mesopelagic fish stocks are the largest and least exploited in the world, with a global biomass on the order of 1,000 to 10,000 million tons, maybe 10-20 times that of all other fishes combined. Many organisms in the mesopelagic rise to the surface at nighttime to feed, before migrating back to great depth before the sun returns. Therefore, mesopelagic organisms play an influential role in oceanic food webs, carbon sequestration, and biogeochemical cycling. There is also renewed interest in their commercial exploitation as feeds or food for human consumption.

Large knowledge gaps on these organisms and their role in and interactions with other parts of the marine ecosystem and global climate remain. Specifically, How much mesopelagic biomass is there and how to accurately estimate it? What controls species composition, population structure and distribution of mesopelagic communities? What is the importance of mesopelagic organisms in the diet of oceanic top predators and commercial fish stocks? What is the fraction of migrators and what controls their migration? What are the consequences of de-oxygenation and implications of harvesting of midwater organisms? How do migrating mesopelagic fishes shape pelagic ecosystems through top-down control and food-web cascades? We often also lack fundamental biological information e.g., physiological measurements of vital rates but also studies of life histories and behaviours are sparse.

Jump to section:

• Long-term research focus - Light, Vision, and Ecosystem Dynamics
• Ongoing work and emerging questions
• Mesopelagic fish models
• Early work - the mesopelagic on our doorstep
• Group members involved in mesopelagic research

 

Long-term research focus - Light, Vision, and Ecosystem Dynamics

From the outset, our research has focused on the role of light and visual foraging interactions in shaping behaviors, distributions, and life histories. While many research groups in visual ecology concentrate on the design and evolution of eyes, their specialization for specific tasks, and their integration with other senses for signaling, navigation, and prey detection, our work stands out for its emphasis on how light, through vision, influences ecosystem dynamics via top-down control. Our research spans ecosystems from the Arctic to the Mediterranean, the Red Sea, and the Southern Ocean, linking physics to biology by integrating field observations with detailed modeling studies.

Here are three recent papers:

 

and some of our "classics":

Ongoing work and emerging questions

Mesopelagic food-web cascades and their impact on ecosystem structure and predictive modeling

While several of our previous studies have explored how light penetration affects mesopelagic fish, we have recently become interested in how mesopelagic fish might affect light penetration through food-web cascades. Simulations with idealized microbial food-web models demonstrate that euphotic zone properties are strongly affected by migrating mesopelagic fishes. The emergent hypothesis is that strong predatory impact on mesozooplankton by mesopelagic fishes causes food-web cascades involving a diatom nutrient trap. This trap, being located at the bottom of the euphotic zone, controls the amount of nutrients available to the organisms of the upper euphotic zone. Although such food-web cascades might not be very pronounced in nature, they are very pronounced in models and demand far more attention in ecosystem and climate modeling.

 

Advection and trophic subsidies

Mesopelagic ecosystems have been primarily studied for their vertical connections between surface waters and the deep sea, focusing on sinking particles and vertical migration. Small mesopelagic fishes may account for up to 95% of the global fish biomass. What happens to these organisms as they drift with the currents, and how and where does this biomass enter the food chain? What is the role of ocean currents in driving source-sink dynamics in mesopelagic fish? We hypothesize that advection transforms ocean currents into food conveyor belts, transporting prey from the open ocean to subsidize local production in shelf seas on a much larger scale than currently acknowledged.

Trond Mohn Project: Advection and Trophic Subsidies
The world’s open ocean may appear barren, but is it its low productivity, transported by currents, that fuels the rich ecosystems along shelves and coasts? This TMF Starting Grant Project (2025-2028) will investigate where and when prey from the open ocean becomes accessible to predators, and how advected biomass influences production, life-history strategies, and species distributions in the receiving ecosystems.

 

Light limits distributions at high latitudes

Diel vertical migration, exhibited by many mesopelagic organisms including fishes, is an adaptive behavior evolved under a regular light cycle to avoid detection by visual predators in sunlit surface waters. However, this "hide and seek" strategy becomes challenging at higher latitudes where the light cycle is more extreme. During the midnight sun, zooplankton prey is abundant in surface waters, but it is never safe for mesopelagic migrants to ascend due to constant daylight. Conversely, during the polar night, the twilight zone extends to surface waters around the clock, but most copepod prey are in diapause at great depths, where it is too dark even for mesopelagic fish to detect them. As a result, seasonality in the diel light cycle emerges as a major constraint on the distribution of mesopelagic fishes at high latitudes in our models. What does the absence of an otherwise ubiquitous zooplanktivore mean for the ecosystem structure and function at high latitudes?

Mesopelagic fish models

Over the years, we have developed several models predicting mesopelagic fish behavior and the ensuing fitness consequences. Here are a few examples:

 

Early work - the mesopelagic on our doorstep

The Theoretical Ecology Group has been studying mesopelagic ecosystems since the 1980s. Initially, our research focused on the fjord environments near Bergen. Many of the insights that inform our work today were gained during these early efforts observing and modeling the mesopelagic environment using fjords as model systems.

News

 

Mesopelagics team