Guiet, J. | bioRxiv Science

bioRxiv2022

Patterns and drivers of the diving behavior of large pelagic predators

Many large pelagic predators, including shark, tuna, and billfish, periodically dive to deep oceanic layers, connecting the surface and mesopelagic ecosystems. However, the patterns and drivers of deep dives across species remain poorly understood. To close this gap, we conduct a meta-analysis of the diving behavior of 24 pelagic predator species from the global ocean, resulting in 671 independent diving depth estimates from 87 tagging studies. Our analysis reveals consistent large-scale patterns in diving depths, with predators diving deeper offshore and during the day, and shallower closer to the coast and during the night. Deep diving species show similar diving depths during the night, with deeper dives for sharks, and shallower dives for tuna and swordfish. These patterns are reversed during the day, widening the gap between day and night vertical ranges for these groups. In contrast, shallow diving species show smaller variations between day and night dives, with sharks diving slightly deeper on average, followed by tuna and billfish. Correlations with co-located environmental variables suggest an important predictive role for proxies of prey abundance and light availability, as well as variables that influence physiology, such as oxygen and temperature. These relationships are more robust for deep divers during the day, and shallow divers at night. Our analysis highlights the value of tagging observations for the development of a mechanistic, quantitative characterization of vertical habitat use of large marine predators and its environmental constraints.

ecology↗

bioRxiv2021

Movement shapes the structure of fish communities along a cross-shore section in the California Current

Pelagic fish communities are shaped by bottom-up and top-down processes, transport by currents, and active swimming. However, the interaction of these processes remains poorly understood. Here, we use a regional implementation of the APex ECOSystem Model (APECOSM), a mechanistic model of the pelagic food web, to investigate these processes in the California Current, a highly productive upwelling system characterized by vigorous mesoscale circulation. The model is coupled with an eddy-resolving representation of ocean currents and lower trophic levels, and is tuned to reproduce observed fish biomass from fisheries independent trawls. Several emergent properties of the model compare realistically with observations. First, the epipelagic community accounts for one order of magnitude less biomass than the vertically migratory community, and is composed by smaller species. Second, the abundance of small fish decreases from the coast to the open ocean, while the abundance of large fish remains relatively uniform. This in turn leads to flattening of biomass size-spectra away from the coast for both communities. Third, the model reproduces a cross-shore succession of small to large sizes moving offshore, consistent with observations of species occurrence. These cross-shore variations emerge in the model from a combination of: (1) passive offshore advection by the mean current, (2) active swimming towards coastal productive regions to counterbalance this transport, and (3) mesoscale heterogeneity that reduces the ability of organisms to return to coastal waters. Our results highlight the importance of passive and active movement in structuring the pelagic food web, and suggest that a correct representation of these processes is needed for realistic simulations with marine ecosystem models.

ecology↗