Development of a new standardised method for sustainable monitoring of the vulnerable pink sea fan Eunicella verrucosa

Abstract

The aim of this survey was to test a standardised method to follow the demographic evolution of a dense aggregative 'forest' population of the temperate gorgonian Eunicella verrucosa (Octocorallia, Gorgoniidae) using in situ photographic recordings. Distribution, density, growth and demographic evolution of the colonies was compared along two parallel transects. Computer treatment allowed the estimation of the total branch fan surface area, and the individual growth of tagged colonies was determined by measuring the increase in this surface area, using consecutive photographs taken at two-year intervals. To integrate the potential bias of branch overlapping, we proposed a correction factor between the in situ photographic surface area and the surface area of the gorgonian calculated from ex situ photographic surface area with all branches deployed. The surface-frequency distribution of colonies was converted to estimated-age-frequency distribution using an estimated growth curve based on the net growth rate. The technique used revealed significant differences in population structure and the dynamics of gorgonian colonies, as the two transects appeared to be influenced by different environmental conditions. The recruitment also seemed to vary according to year and transect, resulting in different densities. Our work showed clear results in characterising the variations of gorgonian demographic evolution at a small spatial scale; thus, it is assumed that this method could be a sustainable tool for coastal environmental managers.

Size/Age Models for Monitoring of the Pink Sea Fan Eunicella verrucosa (Cnidaria: Alcyonacea) and a Case Study Application

“…We considered two different models for the correlation of size and age in E. verrucosa. Model A [33] was developed based on 70 colonies tagged and surveyed between 2006 and 2008 in the Gulf of Morbihan (northwest Atlantic coast of France), at a depth of 8-20 m. The authors measured the colonies with in situ photographic recordings, and then used imaging analysis to estimate the total Branch Fan Surface Area (BFSA, cm 2 ), namely, the measure of the surface that is occupied by the organism on photographs in two-dimensional (2D) projection ( Figure 1). The BFSA was calculated considering both height and width, and linearly correlated to the BFSA with different coefficients (Figure 2a,b).…”

“…A correction factor for branch overlapping was introduced considering that E. verrucosa colonies could exhibit a three-dimensional (3D) morphology. Individual growth was assessed by measuring the increase in the BFSA of tagged colonies during the three years of study, and the following model that correlated the age of the colony with the BFSA was proposed (Figure 2a) [33]: BFSA = 1.25 × age 1.73 .…”

“…Individual growth rate was assessed by measuring the increase in the height of tagged colonies during the nine years of study, and the following Von Bertalanffy growth function that correlated height (h) and age of the colonies was proposed ( Figure 3) [ For both models, the behavior of height as a function of age was analyzed to compare the performance of the two models. In particular, the following relationship was obtained from the modeling proposed in [33]:…”

“…The standard error of regression coefficients, their 95% confidence intervals, and p-values were evaluated by means of a t-statistic. The linear regression was compared with a scatter plot for the population off Sanremo and with a regression illustrated in [33] relevant to model A.…”

“…For both models, the behavior of height as a function of age was analyzed to compare the performance of the two models. In particular, the following relationship was obtained from the modeling proposed in [33]:…”