A Sighting of the Indian Ocean Surgeon Fish Paracanthurus hepatus in Eastern Provence (North-Western Mediterranean Sea)

Abstract

Two Indian Ocean surgeon fish Paracanthurus hepatus individuals were observed near Saint-Raphaël (Provence, France, north-western Mediterranean Sea) in the late summer of 2024 in Posidonia oceanica seagrass and reef habitats. This species is very popular among aquarium hobbyists in Europe, and a growing number of mega-yachts, such as those which moor in the Saint-Raphaël marina, have seawater aquariums on board. Accidental or deliberate release from one such aquarium is the most probable origin of these individuals. The first individual was speared and the second one was no longer sighted after a September storm. Their establishment is unlikely; however, in the future, with the warming of Mediterranean waters and the rapid increase in the number of mega-yachts, this could change. Yacht owners and their staff should be informed of the risk posed by aquarium discharges.

1. Introduction

The surgeon fish Paracanthurus hepatus (Linnaeus, 1766) (Acanthuridae: Perciformes) is native to the Indo-Pacific Ocean, e.g., the Philippines, Indonesia, southern Japan, northern Australia, New Caledonia, the Samoa Islands, Sri Lanka, and eastern Africa [1,2,3]. It has not been reported in the Red Sea [4,5]. In its native area, it thrives between (2) 10 and 40 m of depth [1,2,3]. A single individual was photographed underwater in Israeli waters in the Mediterranean Sea, in 2015 [5,6]. In the Indo-Pacific, P. hepatus thrives 1–3 m above the seabed, in areas where the currents are perceptible, and feeds on planktonic invertebrates and benthic algae [1,5,7,8].

The Mediterranean Sea is a hotspot of marine epsilon species diversity, with 12,000 to 17,000 taxa [9,10,11]. It is also a hotspot of marine biological invasions [11,12,13,14,15]: more than 1000 non-indigenous species (NIS), including 188 teleosts, have been reported [5,16,17]. Among these NIS, at least 751 species are definitely established [17], i.e., they are self-reproducing without human assistance [17,18,19,20].

The main pathways of NIS arrival in the Mediterranean Sea are the Suez Canal, shellfish aquaculture, fouling on ship hulls, and ballast waters [21,22,23,24,25,26,27]. Escape from aquariums is regarded as a minor pathway illustrated by the well-known case of the green seaweed Caulerpa taxifolia, likely released from the public marine aquarium of Monaco [28,29,30].

Biological invasions are regarded worldwide as a major threat to biodiversity [31,32,33,34]. In the Mediterranean Sea, they constitute one of the ‘big three’, together with coastal development and overexploitation by fisheries; at the moment, the impact of biological invasions is greater than that of global warming [35,36,37,38].

Here, we report the in situ observation of two individuals of Paracanthurus hepatus in summer 2024 in eastern Provence (France, north-western Mediterranean).

2. Materials and Methods

The first individual (individual No. 1) was observed opportunistically in July 2024 by snorkeling during a spearfishing trip below the Dramont semaphore, near Île d’Or (municipality of Saint-Raphaël; 43°24.561′ N, 6°50.767′ E) (Figure 1); it was speared (Figure 2). A second individual (individual No. 2) was observed (43°24.666′ N, 06°51.402′ E) from 14 July to 5 September 2024, during scuba dives organized almost every day with the specific aim of searching for and observing individual No. 2 and other possible individuals.

Saint-Raphaël is a seaside tourist resort with a population of 35,000, with four marinas. The main marina, the Vieux Port, harbors a few fishing vessels and more than 260 recreational boats, including ~50 belonging to the mega-yacht category (over 30 m long).

The taxonomic identification of the sighted specimens was carried out using the available literature, e.g., [1,2,3].

3. Results and Discussion

Individual No. 1 was speared in July 2024 at around 5 m of depth above a reef covered with small-size macroalgae (Figure 3). It was ~20 cm long.

Individual No. 2 was tracked by scuba diving for around twenty days between mid-August and early September 2024. It is plausible to assume that it was the same individual at each sighting, with the same size and at the same site. Its length was estimated to be 20 cm. It was relatively sedentary, between 1 and 5 m deep, remaining during the study period within a perimeter 40 m in diameter, above a Posidonia oceanica (Linnaeus) Delile seagrass meadow, a Mediterranean endemic species, or a rocky reef (Figure 4 and Figure 5). During observations, the fish did not seem to be feeding. It showed no sign of fear, allowing itself to be approached (and photographed) from 1 m away. Tracking stopped due to severe weather. Afterward, the fish was not seen again: it could have moved, been fished, or died due to stormy conditions or low temperatures. Summer storms mix the heated sea surface water with deeper cold water, resulting in the rapid and severe cooling of the sea surface water [39,40,41,42].

Paracanthurus hepatus is very popular among aquarium hobbyists, is one of the most widely traded species of ornamental fish worldwide, and is one of the top ten species imported into the European Union [43]. The individual photographed in Israel in 2016 was probably released from a public or private aquarium: a Lessepsian origin (i.e., a migration from the Red Sea to the Mediterranean through the Suez Canal) has been excluded by Marcelli et al., as the species does not occur in the Red Sea [5,6]. The sighting we report here is the second one in the Mediterranean Sea and the first one in the western Mediterranean.

The release of NIS from aquariums is not infrequent [29,30,44,45,46]. The green macroalga Halimeda incrassata (J. Ellis) J.V. Lamouroux, which has recently invaded soft substrate habitats of Mallorca (Balearic Islands, Spain), is commonly used as an ornament in seawater aquariums [47,48]. The reports of the teleosts Abudefduf vaigiensis (Quoy and Gaimard, 1825) in Italy [49], Pomacanthus asfur (Forsskål, 1775) in Malta [50], Zebrasoma xanthurum (Blyth, 1852) in Sardinia [51], and Heteropriacanthus cruentatus (Lacepède, 1801) in Lebanon [52], also likely correspond to escape or deliberate release from aquariums. In the Florida coral reefs, 16 species of exotic teleosts occur that were likely released by marine aquariums, including the invasive lionfish Pterois volitans which escaped from the the Biscayne Bay Aquarium during Hurricane Andrew [53,54].

In most cases, discharge from an aquarium is no more than a highly probable hypothesis. A unique case is that of an aquarium hobbyist from the Athens region (Greece), recounted by Apostolis Ladopoulos, the owner of an aquarium shop (in Zenetos et al. [55]). This hobbyist bought a specimen of the snapper Lutjanus sebae (Cuvier, 1816) from him, a fish that does not occur in the Mediterranean Sea; as the snapper was growing quickly, and it was feeding on the other species in his aquarium, he decided to release it into the sea, at Lagonissi, in October 2009 (Gulf of Saronikos, Greece). He used to visit ‘his’ snapper, which stayed for some time near the release site; then, he lost sight of it. In December 2010, in the Gulf of Saronikos, an individual of Lutjanus sebae was caught by a spearfisher. There is no absolute proof that it was the same individual, but the probability is very high [55].

A growing number of mega-yachts, such as those that moor in the Saint-Raphaël marinas, have seawater aquariums on board; the risk that, during their maintenance or cleaning, species from warm seas are accidentally or deliberately released into the sea, must be considered [51,56]. This is undoubtedly the most probable origin of the two individuals of P. hepatus that we have observed near Saint-Raphaël.

Fortunately, these tropical fish do not seem likely to establish themselves in the Mediterranean, for several reasons. Although they survive there in the summer, winter temperatures are less favorable for them, especially in the north-western Mediterranean. In addition, the founding population is too small (Propagule Pressure Hypothesis). Their low genetic diversity is not favorable for long-term establishment, although some species have successfully naturalized from a very few founding individuals or with low genetic diversity (e.g., [57,58,59,60,61]). As far as P. hepatus is concerned, it is a gonochoric species, with males and females; during spawning, individuals gather in small numbers and take on a paler color; males chase females and, when a couple forms, the partners suddenly rise near the sea surface, releasing their gametes into the water before descending to the bottom [62]. We may presume that P. hepatus will not be successful in spawning and establishing in the north-western Mediterranean, like many other NIS (see, e.g., [17,63,64,65,66]).

However, in the future, the risk of tropical teleosts released from aquariums on board mega-yachts successfully establishing themselves in the Mediterranean Sea will increase. Firstly, the sea surface temperature is increasing, sometimes at a faster pace than elsewhere in the world, making conditions in these habitats closer to those of their native area [67,68]. Secondly, the Mediterranean harbors between half and a third of the world fleet of yachts over 24 m long, likely to be equipped with on-board marine aquariums [69,70]; in addition, the number of yachts over 30 m in length is rapidly growing worldwide: 966 in 1988 vs. 4950 in 2018 [71].

It would be useful if yacht owners and their staff were made aware of the risk posed by aquarium discharges. This could be the role of harbor masters and should constitute a criterion for the award of the ‘Blue Flag’ to coastal municipalities in Europe. The Blue Flag is an ecolabel awarded to beaches and marinas based on 27 specific criteria for beaches and 16 specific criteria for marinas. The requirements cover four aspects: water quality, environmental education and information, environmental management, and safety and services. The Blue Flag Campaign is owned and run by the independent non-profit organization Foundation for Environmental Education (FEE) [72,73,74].

4. Conclusions

Two individuals of the Indian Ocean surgeon fish Paracanthurus hepatus were observed near Saint-Raphaël in the north-western Mediterranean Sea. The first one was spearfished, and the second was observed by scuba divers for around 20 days. The most probable hypothesis is that they were accidentally or deliberately released from a marine aquarium on board a mega-yacht, such as those which moor in the Saint-Raphaël marinas.

The released population is probably too small, and winter temperatures are too cold, for this tropical species to be established in the north-western Mediterranean Sea. However, this could change with the ongoing warming of the Mediterranean.

Yacht owners and their staff should be made aware of the risk posed by aquarium discharges. They should be informed of this risk by harbor masters, and the provision of this information should constitute a criterion for the award of the ‘Blue Flag’ label to coastal municipalities.