Sonar from ships are among the human-generated perturbation suspected to have an impact on marine life in general, cetacean in particular. The US Marine Mammal Protection Act prohibits injury or disturbance of marine mammals in US waters or by US citizens on the high seas (worldwide). Thus, every 5 years, the US Navy must estimate the number of individuals of each marine mammal “stock” that will be “taken” during testing and training activities, and seek a permit. To estimate numbers of cetaceans, aerial and ship surveys are organized, where they are counted by observers. From these observations, their density is modelled. Classical variables used in that modelling are mostly surface ones (Chlorophyll-A from satellites or primary production computed from it). However a number of cetaceans spend times in depth, since they feed there and, moreover, show a different behaviour over day or night time. Using abundance of micronekton predicted from an ecosystem model like SEAPODYM could fill in those gaps and help a better estimate of the real density of cetaceans in a given area — and thus the ones possibly impacted by human activities.

Left, 2010-2016 mean SEAPODYM lower mesopelagic micronekton biomass (g m-2), with beaked whale sightings (black dots). Right, 1998-2016 mean modeled beaked whale density (animals 100 km-2), with beaked whale sightings (black dots) (Credits Duke U.)

Left, 2010-2016 mean SEAPODYM lower mesopelagic micronekton biomass (g.m-2), with beaked whale sightings (black dots). Right, 1998-2016 mean modeled beaked whale density (animals 100 km-2), with beaked whale sightings (black dots) (Credits Duke U.)

Jason Roberts and his team have tested the Low and Mid Trophic Levels (LMTL) functional groups of the SEAPODYM, i.e., the zooplankton and micronekton, as new explanatory variables in their cetacean species habitat and density models. These variables yielded better fitting density models than lower-trophic covariates (chlorophyll or derived primary production) for 4 of 5 deep-divers groups or species tested (beaked whales, sperm whale, dwarf and pygmy sperm whales, pilot whales, and striped dolphin). Consistently, models for deeper divers selected Mid Trophic Levels functional groups in deeper layers.

References

  • Jason J. Roberts, Benjamin D. Best, Laura Mannocci, Ei Fujioka, Patrick N. Halpin, Debra L. Palka, Lance P. Garrison, Keith D. Mullin, Timothy V. N. Cole, Christin B. Khan, William A. McLellan, D. Ann Pabst & Gwen G. Lockhart (2016). Habitat-based cetacean density models for the U . S . Atlantic and Gulf of Mexico. Scientific Reports, 6, 1–12. https://doi.org/10.1038/srep22615