Study Describes Foodweb Dynamics of Predatory Deep-Sea Fishes in the Gulf of Mexico

Trophic level (TL) estimates for each species.  Letters represent significant differences in trophic level, with like letters being similar and non-like letters different.  Dashed lines represent the Nitrogen isotope threshold values of TL 3 and 4 when using primary consumers to set the baseline and dotted lines using primary producers to establish the baseline. Provided by Travis Richards

Trophic level (TL) estimates for each species. Letters represent significant differences in trophic level, with like letters being similar and non-like letters different. Dashed lines represent the Nitrogen isotope threshold values of TL 3 and 4 when using primary consumers to set the baseline and dotted lines using primary producers to establish the baseline. Provided by Travis Richards

Researchers provided some of the first descriptions of the feeding habits of eight deep-sea fishes using dietary tracers (stable isotopes), offering insight into the trophic structure of deep-sea ecosystems and informing ecosystem-based modeling. The fishes, which had contrasting vertical migration habits, ranged from 200 – 3000 m depth and were largely supported by food sources within the upper 200 m of the ocean. Trophic level estimates placed all species between the third and fourth levels; however, the trophic position of some species were found to shift. As body size increased, so did changes in nitrogen isotope values, which can be attributed to size-based patterns in feeding or to spatial and temporal changes in the isotopic signature of primary producers at the base of the foodweb. Species with the highest relative trophic positions also tended to have the largest isotopic niches, suggesting utilization of a wider range of food sources.

The researchers published their findings in the ICES Journal of Marine Science: Trophic ecology of meso- and bathypelagic predatory fishes in the Gulf of Mexico.

Fishes living in the ocean’s deep-pelagic zone (200 m depth to just above the seafloor) provide many important ecosystem services, such as nutrient regeneration and carbon cycling, but much of its food web structure remains poorly understood. As highly abundant mid-level consumers, deep pelagic fishes help regulate zooplankton populations and serve as trophic links between zooplankton and higher-order consumers such as epipelagic fishes and marine mammals. Due to their sheer numbers and vertical migration behavior, which can exceed 1000 m in vertical extent, it is increasingly being recognized that these fishes play key ecological and biogeochemical roles in open-ocean ecosystems.

To reduce the current knowledge gap in deep-ocean foodweb structure, this study’s researchers analyzed stable isotopes (variants of chemical elements that can be traced in foodwebs) in muscle tissues collected from 212 specimens across eight fish species: Anoplogaster cornuta (the common fangtooth), Chauliodus sloani (Sloane’s viperfish), Coccorella atlantica (Atlantic sabretooth), Gigantura chuni and G. indica (telescopefish), Omosudis lowii (hammerjaw), Photostomias guernei (Loosejaw), and Stomias affinis (Günther’s boafish). They identified differences in isotopes among species and between 154 particulate organic matter samples from different depths and estimated their relative contribution to each species.

The carbon isotope value (d13C) was similar among predator species, suggesting that a similar carbon source supported them. Mixing model analysis supported this finding, showing that all of these deep-living predators received the majority (>73%) of their carbon from food sources in the epipelagic zone (where there is enough light for photosynthesis). Carbon isotope values in particulate organic matter did not significantly differ across depth zones, but nitrogen isotope values (d15N) increased in mesopelagic (200 – 1,000 m depth) and bathypelagic (1,000 – 4,000 m depth) zones.

There were species-specific differences in carbon and nitrogen isotope values. Carbon was enriched in G. chuni and G. indica and depleted in O. lowii and S. affinis and correlated with latitude and longitude. Nitrogen was enriched in A. cornuta, G. chuni, and G. indica and depleted in C. sloani and P. guernei but was not correlated with latitude and longitude.  Piscivorous fishes (A. cornuta, G. chuni, G. indica, and S. affinis) occupied the highest trophic positions; species preying on cephalopods and fishes (C. atlantica and O. lowii) occupied the intermediate trophic positions; and species preying on mainly macrocrustaceans (P. guernei) occupied the lowest trophic position among the predators.

The authors noted that their findings suggest that that the species they analyzed may consume migratory prey that feed within food chains supported by production from surface-derived carbon. These results provide insights into the extent that spatially distinct consumers are connected in the northern Gulf of Mexico.

Data are publicly available through the Gulf of Mexico Research Initiative Information & Data Cooperative (GRIIDC) at doi:10.7266/N7ZS2V04.

The study’s authors are Travis M. Richards, Emily E. Gipson, April Cook, Tracey T. Sutton, and R.J. David Wells.

************

This research was made possible in part by a grant from the Gulf of Mexico Research Initiative (GoMRI) to the Deep-Pelagic Nekton Dynamics of the Gulf of Mexico (DEEPEND) consortium.

The Gulf of Mexico Research Initiative (GoMRI) is a 10-year independent research program established to study the effect, and the potential associated impact, of hydrocarbon releases on the environment and public health, as well as to develop improved spill mitigation, oil detection, characterization and remediation technologies. An independent and academic 20-member Research Board makes the funding and research direction decisions to ensure the intellectual quality, effectiveness and academic independence of the GoMRI research. All research data, findings and publications will be made publicly available. The program was established through a $500 million financial commitment from BP. For more information, visit http://gulfresearchinitiative.org/.

By Nilde Maggie Dannreuther and Stephanie Ellis. Contact maggied@ngi.msstate.edu with questions or comments.

© Copyright 2010-2018 Gulf of Mexico Research Initiative (GoMRI) – All Rights Reserved. Redistribution is encouraged with acknowledgement to the Gulf of Mexico Research Initiative (GoMRI). Please credit images and/or videos as done in each article. Questions? Contact web-content editor Nilde “Maggie” Dannreuther, Northern Gulf Institute, Mississippi State University (maggied@ngi.msstate.edu).