Fish Carbon: Exploring Marine Vertebrate Carbon Services

5. TWILIGHT ZONE CARBON

avoidance behaviour, which reduces their accidental capture in current fishing gears (Irigoien et al. 2014). Twilight Zone Carbon may be under-valued in current estimates of oceanic carbon cycling, as recent research suggests that the total biomass of mesopelagic fish may be between 1,000 to 10,000 megatons; ten times higher than previous estimates (Irigoien et al. 2014). Twilight Zone Carbon, possibly the most intact biological mechanism of marine vertebrate oceanic carbon cycling (Irigoien et al. 2014), appears to provide a direct two-step route from the ocean surface to the deep sea and sediment, where carbon can be stored for millennia or longer (Lutz et al. 2007).

Mesopelagic fish that live in deep waters undertake a vertical migration at night to feed on zooplankton in the surface waters of the ocean. During the day, to avoid predation, these fish descend back to the ocean’s ‘twilight zone’ at depths of 200 to 1000 meters, transporting substantial quantities of organic carbon away from the surface and ultimately releasing it as faeces, which sink further into the depths (Figure 2, service 5) (Davison et al. 2013). Through this mechanism, carbon is effectively transported below the upper thermocline, the depth zone in which most carbon remineralization occurs (Davison et al. 2013).

Commercial fisheries do not currently target mesopelagic fish and it has been suggested that these fish undertake net-

Vertical migration of mesopelagic fish transports carbon away from surface waters to depths of 200-1000m

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