Large-Scale Simulation of a Shipping Speed Limitation Measure in the Western Mediterranean Sea: Effects on Underwater Noise

Underwater noise from shipping activity can impact marine ecosystems in the long term and at large scale. Speed limitation has been considered to reduce noise emission levels. In this article, the effects of speed limitation on shipping noise levels are investigated at high spatial resolution (5 arc-min) in the Western Mediterranean Sea. Scenarios of maximum speed limits of 10 kt and 15 kn are computed. The impact of a speed reduction is time-dependent and tends to redistribute sources of noise temporally, smoothing the contrasts existing in marine traffic at the daily scale. The effectiveness of the measure is evaluated over short successive time windows (6 h), allowing for capture of the dynamic of the effect of speed reduction. Several metrics are proposed to evaluate the effectiveness of speed reduction as a mitigation measure according to its temporal stability. This study illustrates complex phenomena related to (1) the increased vessel density in the speed limitation area due to longer navigation time and (2) deep-water and shallow-water propagations. The bathymetry and the local distribution of traffic are two elements of importance with respect to the effectiveness and the stability of the measure, whereas the traffic properties seem to impact the stability of the effect in particular, and deep waters seem to increase the effectiveness. This research shows the areas in which the proposed measure would be the most effective

Using passive acoustic monitoring to assess humpback whale occurrence and breeding activity around La Réunion Island

Humpback whales use the coastal waters of La Réunion Island seasonally from June to October. Their distribution is relatively well-known on the west coast, which provides suitable breeding habitat, however little is known about their use and movement along the south and east coast of the island. Three autonomous acoustic recorders were deployed during the breeding season of 2016 off the west, south and east coasts to investigate spatial and temporal variations in humpback whale occurrence around the island. The dataset consisted of 10 minute acoustic recordings every hour, continuously from mid-June to early September from these three locations. Song and social call events were discriminated and their total durations were calculated and expressed as a percentage of the total recording duration per day. Off the west and south coast, social calls were distributed fairly homogenously over the season, while songs occurred over a shorter time period, with two significant peaks observed in mid-July and early-August. Off the east coast very few vocalizations were detected. These results demonstrated that humpback whales preferentially use the west coast of the island for breeding. In this area, daily variations in singing activity were observed. Singers appeared to be more active at night and during the morning, when less boat traffic was detected.

Analyse de scénarios de réduction de vitesse du trafic maritime pour la mitigation de la pollution sonore sous-marine en Mer Méditerranée occidentale

Le bruit généré par le trafic maritime est un contributeur important du paysage sonore sous-marin. Il est identifié comme une pollution par la communauté scientifique et pris en compte au sein des textes réglementaires et des politiques publiques. La Directive Cadre Stratégie Milieu Marin (DCSMM 2008/56/CE) vise à garantir un équilibre entre protection de l’environnement marin et développement socio-économique par l’atteinte et le maintien du Bon Etat Ecologique (BEE). Le BEE est défini par onze descripteurs, et notamment par le critère D11C2 (bruit continu), évalué sur des cartes du bruit généré par le trafic maritime. <par> La mesure principale considérée afin de limiter le bruit de trafic est la réduction de la vitesse des navires. L’efficacité d’une telle mesure est cependant difficile à anticiper, d’une part parce qu’un ralentissement des navires entrainerait une densification plus importante du trafic, et d’autre part, parce que la propagation acoustique est dépendante de l’environnement. <par> Ce cas d’étude vise à quantifier l’impact de différents scénarios de réduction de vitesse de navigation sur le bruit de trafic. L’analyse est menée en Méditerranée Occidentale, dans une zone d’habitat préférentiel du rorqual commun, où le trafic maritime est dense. <par> La plupart des navires sont équipés d’émetteur AIS (Système d’Identification Automatique), communiquant les informations relatives au navire (identifiant, activité, longueur, etc.) et à sa navigation (position, vitesse, cap, etc.). Ces informations sont utilisées pour modéliser le bruit rayonné. Les zones de réduction de vitesse sont simulées en modifiant les dates d'émission des AIS. Les niveaux sont propagés dans l’environnement pour réaliser des cartes de bruit stratégiques. <par> Les résultats sont analysés en fonction des activités principales du trafic maritime et des zones d’habitats des rorquals communs. L’efficacité du ralentissement pour la réduction du bruit est discutée au regard de l’évaluation du D11C2, avec une attention particulière sur la maîtrise des incertitudes

Habitat availability and geographic isolation as potential drivers of population structure in an oceanic dolphin in the Southwest Indian Ocean

International audienceDelphinid populations show highly variable patterns of genetic diversity and population structure. Previous studies indicate that habitat discontinuities and geographic isolation are major drivers of population division in cetaceans. Spinner dolphins (Stenella longirostris) are distributed in all tropical oceans, but they are particularly common around islands and atolls. This species occurs in shallow waters at daytime to rest and socialise, and feeds on offshore mesopelagic prey overnight. Here we investigated the genetic population structure of spinner dolphins in the Southwest (SW) Indian Ocean along a west-east geographical gradient, from eastern Africa to the Mascarene archipelago. We combined analyses of 12 microsatellite loci, mtDNA control region sequences, and sighting data to assess genetic differentiation and characterise habitat preferences of these populations. Significant genetic structure among the three sampled sites (Zanzibar, Mayotte and La Réunion) was observed using both types of molecular markers. Overall, our results indicate that geographic isolation, and potentially other factors such as shallow water habitats to rest and socialise, may be important drivers of the genetic population structure of insular spinner dolphins in this region