How a deepwater oil spill provided a new perspective on fish egg vertical distribution

The Deepwater Horizon (DWH) was a major catastrophe of this century, and we are still trying to understand its environmental impacts. This oil spill's characteristics (an offshore, deepwater well in a highly hydrodynamic area) led to consider the impact of oil on pelagic habitats rather than only coastal ones. Pelagic fish embryos are globally assumed to be floating at the surface, where external stressors are the highest. Yet, the actual fine scale vertical distribution of early life stages, and therefore their exposure to surface stressors (e.g. damaging UV radiation) has hardly ever been tested. Here, we used data collected after DWH on mahi-mahi (Coryphaena hippurus; hereafter "mahi"), a highly valuable but poorly known species, to explicitly address this shortcoming. We used a Lagrangian, individual-based biophysical model, with a thermal and an irradiance module, to simulate the vertical distribution of mahi embryos throughout the Gulf of Mexico. The model was parameterized based on in situ tagging data on spawning behavior and laboratory experiments on embryos' terminal velocity. Virtual embryos were released throughout the year, above the thermocline (i.e. between the surface and 30 m depth), and the hourly measurements of terminal velocity were used to determine the embryos' buoyancy through time. Each virtual embryo was tracked for 48 h, the maximum time before hatching, considering the minimum surface water temperature in the Gulf of Mexico (around 20°C). The results show that, when assessed in realistic environmental conditions, pelagic fish embryos are actually distributed in the subsurface, and that this slightly-deeper-than-expected vertical distribution may be sufficient counter UV and thermal stress. The DWH therefore participated in shedding light on a potentially universal protection mechanism in the early life stages of pelagic fishes. This research was made possible by the RECOVER and CIMAGE grants from The Gulf of Mexico Research Initiative (No: SA-1520).