Marine Debris
Marine debris, specifically in the plastic form, has been found in every ocean basin and have a wide range of impacts on the marine world. One of the most critical issues is ingestion of plastic debris, specifically microplastics. Mesopelagic species migrate to the surface waters to feast on their main prey species, Zooplankton, who are mixed with microplastics in the surface waters. Additionally, research has shown that even zooplankton are consuming the microplastics themselves. These fish, play a key role in energy dynamics, meaning they provide food for a number of predators including birds, fish and marine mammals.
The concentration of these plastics has the potential to increase, meaning most economically important species could be contaminated as well. Concentration of plastic debris in mesopelagic populations can vary depending on geographical location and the concentration of marine debris located there. In 2018, approximately 73% of over 200 fish sampled in the North Atlantic had consumed plastic.
Bioaccumulation
The process of bioaccumulation (a buildup of a certain substance in the adipose tissue) and biomagnification (the process in which the concentration of the substance grows higher as you rise through the food chain) are growing issues in the Mesopelagic zone. Mercury in fish, which can be traced back to a combination of anthropological factors (such as coal mining) in addition to natural factors. Mercury is a particularly important bioaccumulation contaminant, as it’s concentration in the mesopelagic zone is increasing faster than in surface waters.
Inorganic mercury occurs in anthropogenic atmospheric emissions as it’s gaseous elemental form, which can be oxidized and then deposited in the ocean. Once there, the oxidized form can be converted to methylmercury, which is its organic form. Research suggests that current levels anthropogenic emissions will not equilibrate between the atmosphere and ocean for a period of decades to centuries, which means we can expect current mercury concentrations in the ocean to keep rising. Mercury is a potent neurotoxin, and poses health risks to not only the mesopelagic species that consume it. Many of the mesopelagic species such as myctophids make their diurnal migration to the surface waters, where they are consumed by pelagic fish, birds and mammals.
Fishing
Historically, there have been few examples of efforts to commercialize the mesopelagic zone, due to low economic value, technical feasibility and environmental impacts. While the biomass may be abundant, fish species here generally smaller in size and slower to reproduce. Fishing with large trawl nets poses threats to a high percentage of bycatch as well as poses potential impacts to the carbon cycling process. Additionally, getting to the mesopelagic requires fairly long journeys offshore.
In 1977, a Soviet fishery opened but closed less than 20 years later due to low commercial profits, while a South African purse seine fishery closed in the mid-1980’s due to processing difficulties from the high oil content of fish. As the biomass in the Mesopelagic is so abundant, there has been an increase in interest to whether or not these populations could be of economic use in areas other than direct human consumption. For example, it has been suggested that the high abundance of fish in this zone could potentially satisfy a demand for fishmeal and nutraceuticals.
With a growing population, the demand for fishmeal in support of a growing aquaculture industry is high. There is a potential for economically viable harvest, for example, 5 billion tons of mesopelagic biomass could result in the production of circa 1.25 billion tons of food for human consumption. Additionally, the demand for nutraceuticals is also rapidly growing, stemming from the consumption of Omega-3 Fatty Acids in addition to the aquaculture industry requiring a specific marine oil for feed material. Lanternfish are of much interest to the aquaculture market, as they are especially high in fatty acids.
Climate Change
The Mesopelagic region plays an important role in the Global Carbon Cycle, as it is the area where most of the organic matter that is travelling down the water column is respired. Mesopelagic species acquire carbon during their migration to feed in surface waters, and transport that carbon into the deep sea when they die. It is estimated that the mesopelagic cycles between 5 and 12 billion tons of carbon dioxide from the atmosphere per year, and until recently, this estimate was not included in many climate models. It is difficult to quantify the effects of climate change on the mesopelagic zone as a whole, as climate change does not have uniform impacts geographically. Research suggests that in warming waters, as long as there is adequate nutrients and food for fish, then mesopelagic biomass could actually be higher due to increases in traffic efficiency and temperature driven metabolism.
However, because warming will not be uniform throughout the global mesopelagic zone, it is predicted that some areas may actually decrease in fish production, while others increase. Also, with warming waters there is a higher likelihood of increased stratification. This increase reduces the input of nutrients to the euphotic zone through an increase in nutrient limitation, and can lead to decreases in both net primary production and sinking particulate matter[31]. Additional research suggests shifts in the geographical range of many species could also occur with warming, with many of them shifting away from the equator. The combination of these factors could potentially mean that as global ocean basins continue to warm, there could be areas in the mesopelagic that increase in biodiversity and species richness, especially moving farther from the equator.
