Large mesopelagic fishes biomass and trophic efficiency in the open ocean

Abstract

With a current estimate of ~1,000 million tons, mesopelagic fishes likely dominate the world total fishes biomass. However, recent acoustic observations show that mesopelagic fishes biomass could be significantly larger than the current estimate. Here we combine modelling and a sensitivity analysis of the acoustic observations from the Malaspina 2010 Circumnavigation Expedition to show that the previous estimate needs to be revised to at least one order of magnitude higher. We show that there is a close relationship between the open ocean fishes biomass and primary production, and that the energy transfer efficiency from phytoplankton to mesopelagic fishes in the open ocean is higher than what is typically assumed. Our results indicate that the role of mesopelagic fishes in oceanic ecosystems and global ocean biogeochemical cycles needs to be revised as they may be respiring ~10% of the primary production in deep waters.

Figure 1. The Malaspina cruise.

(a) The surface-integrated estimated mesopelagic fishes biomass (g wet weight per m²) for the 200–1,000 m depth range along the Malaspina 2010 Expedition cruise transect (black circles) superimposed on a satellite-derived global map of PP (average in mg C m⁻² d⁻¹ for 2010–colour bar); (b) a daytime echogram from 0–1,000 m along the cruise track (measured in dB—colour bar), and (c) interpolated temperature profiles along the cruise track (measured in °C—colour bar). The black triangles in b and c indicate the border between oceanic basins. AT for Atlantic Ocean, IO for Indian Ocean, WP for Western Pacific and EP for Eastern Pacific.

Figure 2. Relationship between PP and acoustic backscatter.

The relationship between PP (2010 annual average mg C m⁻² d⁻¹) and the nautical area scattering coefficient (s_A, m² nmi⁻²) from 200–1,000 m during day time.

Figure 3. Sensitivity analysis of the assumptions used to estimate biomass.

Computations are based on the total backscatter estimated from the GWR regression (Table 1, 5.57E+17), contours are the ratio of our estimate to 1,000 million tons. Quartiles, maximum and minimum values of the db/weight ratios found in literature are indicated by the dotted lines.

Figure 4. Relation between PP and fish biomass.

The relationship between PP (2010 annual average mg C m⁻² d⁻¹) and the estimated mesopelagic fishes biomass using the median db/weight ratio (black circles) and the ECOTROPH model with a transfer efficiency of 0.1 and considering that 90% of the PP enters the food web (red line). The regression equation of the acoustic biomass estimate against PP is mesopelagic fishes biomass (g m⁻²)=0.185 PP (mg C m⁻² d⁻¹)—6.66, r²=0.46, P<0,001, n=209).

Figure 5. Trophic efficiency.

The estimated trophic efficiency from PP to MFP (the ratio of fishes production to PP) as a function of PP. The line is a 50 period running average for 50 contiguous estimates.