3D Photogrammetry Modeling Highlights Efficient Reserve Effect Apparition After 5 Years and Stillness After 40 for Red Coral (Corallium rubrum) Conservation in French MPAs

Abstract

Imaging the marine environment is more and more useful to understand relationships between species, as well as natural processes. Developing photogrammetry allowed the use of 3D measuring to study populations dynamics of sessile organisms at various scales: from colony to population. This study focuses on red coral (Corallium rubrum), as known as precious coral. Metrics measured at a colony scale (e.g., maximum height, diameter and number of branches) allowed population understanding and a comparison between an old (Cerbère-Banyuls reserve) vs. a new (Calanques National Park) MPA. Our results suggested a 5-year time step allows the appearance of a significant difference between populations inside vs. outside the Calanques National Park no-take zones. Red coral colonies were taller and had more branches inside no-take zones. A significant difference was still observable for the populations inside the Cerbère-Banyuls reserve after 40 years of protection, reflecting the sustainability and effectiveness of precautionary measures set by the reserve. The impacts at the local level (mechanical destruction) and those presumed to occur via global change (climatic variations) underline the need to develop strategies both to follow the evolutions of red coral populations but also to understand their resilience. Photogrammetry induced modeling is a time and cost effective as well as non-invasive method which could be used to understand population dynamics at a seascape scale on coralligenous reefs.

Testing Transplantation Techniques for the Red Coral Corallium rubrum

“…Its extensive harvesting resulted in a dramatic size and structural shift in shallow water populations from banks previously shaped as a "forest-like" structure, dominated by large branched colonies of 30-50 cm height (10-30 mm diameter) to populations shaped like a "grass plainlike" structure, dominated by small colonies of 3-5 cm height (5-7 mm diameter) [14,27,28]. A reversal trend has been documented in some French, Spanish, and Italian Marine Protected Areas, where red coral fishery has been definitively forbidden (Cerbère-Banyuls Marine Natural Reserve (1974), Scandola Natural Reserve (1975), Côte Bleue Marine Park (1982), Bouches de Bonifacio Natural Reserve (1999), Calanques National Park (2012), and Portofino marine Protected Area [28][29][30]). To prevent overfishing and ensure long-term yields of this resource in the Mediterranean Basin, a minimum diameter of 7 mm was established for harvesting purposes (1983) and a ban on dredge use (ingegno) at Mediterranean scale was implemented in 1994.…”

“…Nowadays, due to the depletion of shallow banks and the establishment of harvest regulations, the harvesting interest is turning towards deeper banks (50-200 m depth) despite the fact their ecology is still poorly understood [31][32][33]. An additional consideration has to be made regarding deep populations, which may be considered as a genetic refuge, enhancing species resilience in the case of thermal anomalies on the upper layers of the water column, and therefore should be also considered for conservatory measures [30,34,35].…”

Conservation of Long-Lived Marine Species: Some Hints for Adopting a Population Approach

“…1) ROV recordings for studies on deep-dwelling coral populations, as well as Seafloor multi-beam echo sound, multiparametric fibre-optic-cabled videos and other new technologies are emerging tools for standardized ecological monitoring and could allow collection of some important demographic data such as recruitment rate and population structure; 3D photogrammetry modelling may help greatly in determining population size structure and even to estimate the potential larval output of colonies in corals, and can allow for testing the effectiveness of enforcement measures (Rossi, P. et al, 2021;Richaume et al, 2021). Other population features not all directly related to demography have also been measured by ROV recordings: population size and area, colony size frequency distribution, occupancy, encounter frequency, orientation/exposure of colonies (e.g.…”

Structure and status of the Italian red coral forests: What can a large-scale study tell?

“…The "grass plain-like" condition may persist for a long span of time, because of intra-specific competition (Tsounis et al, 2006), and only in a few Marine Protected Areas has a reversion been recognized after a long-term protection (Bavestrello et al, 2015;Richaume et al, 2021). Moreover, besides harvesting, other factors are known to threaten red coral populations, both direct (e.g., fishing activities and mass mortalities putatively linked to episodic and highly energetic events, like heat waves and volcanic eruptions) and indirect (e.g., climate change-driven events) (e.g., Cerrano et al, 2013;Bavestrello et al, 2014;Lodolo et al, 2017;Cau et al, 2018;Galgani et al, 2018;Enrichetti et al, 2019a).…”