Fine-scale structures as spots of increased fish concentration in the open ocean

Abstract

Oceanic frontal zones have been shown to deeply influence the distribution of primary producers and, at the other extreme of the trophic web, top predators. However, the relationship between these structures and intermediate trophic levels is much more obscure. In this paper we address this knowledge gap by comparing acoustic measurements of mesopelagic fish concentrations to satellite-derived fine-scale Lagrangian Coherent Structures in the Indian sector of the Southern Ocean. First, we demonstrate that higher fish concentrations occur more frequently in correspondence with strong Lagrangian Coherent Structures. Secondly, we illustrate that, while increased fish densities are more likely to be observed over these structures, the presence of a fine-scale feature does not imply a concomitant fish accumulation, as other factors affect fish distribution. Thirdly, we show that, when only chlorophyll-rich waters are considered, front intensity modulates significantly more the local fish concentration. Finally, we discuss a model representing fish movement along Lagrangian features, specifically built for mid-trophic levels. Its results, obtained with realistic parameters, are qualitatively consistent with the observations and the spatio-temporal scales analysed. Overall, these findings may help to integrate intermediate trophic levels in trophic models, which can ultimately support management and conservation policies.

Open science resources from the Tara Pacific expedition across coral reef and surface ocean ecosystems

“…This was considered as a stadium shape with the two semi-circles centered on the starting and ending points of the transect, respectively. The radius of the stadium semi-circles was considered 0.1°, which is in accordance with previous studies 49, 53, 54 . The stadium was filled with virtual particles separated by 0.01°.…”

Global-Scale Environmental Niche and Habitat of Blue Shark (Prionace glauca) by Size and Sex: A Pivotal Step to Improving Stock Management

“…In mesotrophic areas, the daily detection of productive oceanic features (chlorophyll-a fronts) from ocean color satellite sensors (currently MODIS-Aqua) is a good generic proxy for food availability to fish populations (Druon et al, 2021). When productivity fronts are active long enough (from weeks to months) to allow the development of mesozooplankton populations (Druon et al, 2019), they were shown to attract epiand mesopelagic fish and top predators (Olson et al, 1994;Polovina et al, 2001;Briscoe et al, 2017;Druon et al, 2017;Baudena et al, 2021). After an initial development phase of 3-4 weeks, the mesozooplankton biomass reached substantial levels in the persistent and not necessarily stationary chlorophyll-a fronts, i.e.…”

“…Small and large chlorophyll-a fronts were derived from and refer to different levels of chlorophyll-a gradient values (see the SM for details on the chlorophyll-a gradient calculation). Based on the link described above between productivity fronts and biomass of low and high trophic levels (e.g., Olson et al, 1994;Polovina et al, 2001;Druon et al, 2019;Druon et al, 2021;Baudena et al, 2021), high chlorophyll-a gradient levels are assumed to correspond, when persistent, to productivity fronts with an important capacity to sustain well-developed food chains and foraging opportunities for predators. The five-day chlorophyll-a gradient values were extracted in a 25 km radius centered on the (day and location of) blue shark presence data for each size and sex class, and a minimum of 33% coverage was set to accept the mean gradient value (5-day and 25 km-radius mean value).…”

Drifting Speed of Lagrangian Fronts and Oil Spill Dispersal at the Ocean Surface

“…Mixing across frontal structures, including diffusivity even in its smallscale, might have an important impact on the dispersion of passive tracers in the ocean. Unlike horizontal stirring that reinforces the tracer gradients across fronts, small-scale diffusivity processes tend to reduce and smooth the gradients [14,65,66]. Nevertheless, in this study, we particularly highlighted the impact of the horizontal stirring in controlling the formation and the evolution of the transport patterns, shaping, in turn, the trajectory of the oil tracer.…”