Healthy assemblages of Isidella elongata unintentionally protected from trawling offshore of Asinara Island (northwestern Sardinia, NW Mediterranean Sea)

Abstract

Deep-sea coral assemblages are marine biodiversity hot spots. Because of their life history traits, deep-sea corals are highly vulnerable to the impacts of human activities such as fishing. The critically endangered "bamboo coral" Isidella elongata is a key structuring species of deep muddy bottoms that is susceptible to habitat destruction, particularly from trawling. A shallow population of this species was recently discovered by a multibeam and ROV survey offshore of the Asinara Island marine protected area (MPA) (northwestern Sardinia, NW Mediterranean Sea). This vulnerable marine assemblage has been found under healthy conditions at depths ranging from 110 to 298 m. Isidella elongata occurs on a muddy seafloor locally characterised by boulders associated with black coral species (Parantipathes larix and Antipathes dichotoma). The lush colonies of I. elongata seem to be related to natural protection from bottom trawling activity; nevertheless, the presence of lost fishing artisanal nets has been observed in the study area. These structuring species are indicators of vulnerable marine ecosystems, and their conservation is essential for preserving marine biodiversity. Therefore, enlarging the perimeter of the Asinara Island MPA into its deeper western waters is suggested to ensure the protection of these valuable and vulnerable marine ecosystems.

Figure 1

Investigated area. Upper left insets: Italy and the Sardinia region. The red rectangles indicate the sites where high-resolution multibeam surveys were carried out (except for Fp) and the locations of the eight remotely operated vehicle (ROV) dives (M1–M7, Fp). The blue polygon indicates the perimeter of the Asinara Island MPA. Background bathymetry data were obtained from EMODnet (https://emodnet.ec.europa.eu/en/bathymetry). The map was generated using ESRI's ArcGIS 10.3 for Desktop software (http://www.esri.com), version 10.3.0.4322.

Figure 2

Megabenthic species richness (%) per phylum in each dive.

Figure 3

Megafauna in the study area. (A) Phakellia sp.; (B) Paramuricea hirsuta with Tritoniidae nd. specimen (white arrow); (C) Eunicella cavolini and E. verrucosa; (D) Callogorgia verticillata; (E) Dendrophyllia cornigera; (F) Parantipathes larix; (G) Funiculina quadrangularis; (H) Palinurus elephas. Scale bar: 15 cm.

Figure 4

Isidella elongata facies. (A) I. elongata and Parantipathes larix; (B–E) Colonies of I. elongata; (F) Close-up image of I. elongata polyps; (G) Close-up image of Leptometra phalangium branches on an I. elongata colony; (H) Macroramphosus scolopax swimming near a bamboo coral colony; (I) Healthy and tall colonies of I. elongata. Scale bar (A–E, H–I): 15 cm; Scale bar (F–G): 0.5 cm.

Figure 5

High-definition bathymetric maps (except for dive Fp) with eight explorative remotely operated vehicle (ROV) transects (M1–M7, Fp) and the spatial location of the most relevant megabenthic species identified in the area.

Figure 6

Bathymetric distribution and colony densities of Isidella elongata. (A) The grey rectangles represent the bathymetric range explored for each dive, and the fuchsia rectangles represents the bathymetric range where I. elongata was found. (B) Boxplot of the colony density of I. elongata in each dive: boxes indicate the first and third quartiles; bold lines indicate the median value; lines indicate the range between the minimum and maximum values; and dots indicate outliers.

Figure 7

Population structure of Isidella elongata colonies. (A) Percentage of total small (< 20 cm height) and large (> 20 cm height) colonies at each site. (B) Size distribution of colony height on a subsample of I. elongata colonies in the study area.

Figure 8

Anthropogenic impact in the study areas. (A,B) Epibionted Isidella elongata colonies; (C–E) Broken and eradicated colonies of I. elongata; (F) An eradicate I. elongata colony (white arrow) by a rope; (G) Farrea sp. growing on a lost rope; (H) Polystyrene box; (I) A lost fishing pot; (J) Parantipathes larix and a can lying on the seafloor; (K) Funiculina quadrangularis and P. larix near a lost trammel net in the working position; (L) A lost trammel net entangled on rocks; (M) A lost trammel net hanging from an outcrop. Scale bar: 15 cm.

Figure 9

Plots of the multivariate analysis. (A) Principal component analysis (PCA) of seafloor descriptors for each dive; (B) Canonical correspondence analysis (CCA). Red arrows indicate environmental variables. Points represent sampling unit ordinations based on species abundance and environmental variable constraints; (C) Close-up of the central area of the CCA plot. Por = Porifera; P_spp = Phakellia spp.; A_spp = Axinella spp.; P_mon = Pachastrella monilifera; P_com = Poecillastra compressa; Alcy = Alcyonacea; C_med = Chironephthya mediterranea; B_mol = Bebryce mollis; C_ver = Callogorgia verticillata; E_cav = Eunicella cavolini; P_hir = Paramuricea hirsuta; S_dub = Swiftia dubia; D_cor = Dendrophyllia cornigera; A_dic = Antipathes dichotoma; P_lar = Parantipathes larix; F_qua = Funiculina quadrangularis; I_elo = Isidella elongata; V_beb = Villogorgia bebrycoides; B_vir = Bonellia viridis.

Figure 10

Composition of litter in the study area. (A) Top 10 most common litter types according to the Marine Strategy Framework Directive Joint List; (B) Percentage frequency of litter macro categories in each site.