Abstract
The direct and indirect impact of fish farms, shellfish aquaculture, and extensive forms of aquaculture such as seeding of juvenile sea urchins, on macrophytes (seaweeds and seagrasses), is reviewed in Mediterranean benthic ecosystems. Fish farms constitute a source of organic matter and nutrients (food and fecal pellets) that causes the extirpation of Posidonia oceanica seagrass meadows beneath and near to farm facilities. In addition to direct effects, the nitrogen enrichment of macrophytes tissues increases the grazing pressure by herbivorous fishes and sea urchins. In some cases, the impact can continue to increase several years after the cessation of farming activities. Natural restoration of extirpated seagrass meadows is generally unlikely at the human time scale. Shellfish aquaculture is the cause of the main flow of introduced macrophytes in the Mediterranean; the main vector is the importation of oyster spat from Japan and Korea. North-eastern Pacific seaweeds are now the dominant biotic component of some Mediterranean lagoons (e.g., Thau, Mar Piccolo and Venice lagoons). In addition to direct effects, mussel aquaculture can constitute a source of larvae that flow with currents, the adults of which can overwhelm seaweed forests (e.g., Carpodesmia mediterranea). Shellfish aquaculture is also a source of fecal pellets, resulting in changes in bottom macrophytes, and a vector of diseases of metazoans, the extirpation of which may change the functioning of recipient macrophyte ecosystems. The edible sea urchin Paracentrotus lividus is sometimes erroneously considered as in decline due to over-harvesting. However, its abundance in the second half of the 20th century was probably a consequence of human impact (overfishing of its predatory fish, organic pollution. This man-induced proliferation resulted in the extirpation of seaweed forests (e.g., Carpodesmia spp., Treptacantha spp. – formerly Cystoseira spp. – Sargassum spp.; many species are endemic), which play a key role in Mediterranean coastal ecosystems. Therefore, the attempts to restore sea urchin abundance, via seeding of juveniles from hatcheries, has further artificialized the habitats rather than contributing to the restoration of natural ecosystems. Good practices guidelines are proposed aimed at minimizing the impact of aquaculture on macrophytes.
Highlights
The P. oceanica ecosystem thrives between the sea level and 30–40 m depth, in the infralittoral zone (Molinier and Picard, 1952; Boudouresque and Meinesz, 1982; Boudouresque et al, 2012); for Mediterranean biotic zones and habitats, see Pérès and Picard (1964) and Pérès (1982). (i) There is a decrease in light availability under facilities, due to both increased turbidity and the shadow of the cages: 38% of the bottom light is lost beneath the cages in the Gulf of Ajaccio (Corsica)
The actual baseline of the population density of P. lividus in the Mediterranean is unknown, there are reasons to think that its abundance, in the second half of the 20th century, could be, at least partly, a consequence of human impact (Boudouresque and Verlaque, 2013); it is usually uncommon in No-Take Zones of Marine Protected Areas, i.e., zones where all types of fishing and harvesting, both amateur and commercial, are banned (Boudouresque et al, 1992), and proliferates in the vicinity of untreated domestic sewage outfalls (Harmelin et al, 1981; Boudouresque and Verlaque, 2013) and fish farms (Kušpilicet al., 2007)
Three species of bivalves constitute the bulk of the shellfish aquaculture in the Mediterranean Sea (Basurco and Lovatelli, 2003): the mussel Mytilus galloprovincialis, which is native to the Mediterranean, the Pacific oyster Magallana gigas (= Crassostrea gigas), native to the Pacific coast of Asia (Salvi and Mariottini, 2017), and the Manila clam Ruditapes philippinarum, native to the coasts of the Indian and Pacific Oceans, from Pakistan to Japan and the Kuril Islands (Goulletquer, 2005)
Summary
The aquaculture of marine organisms has undergone a spectacular development in the Mediterranean since the beginning of the 20th century, since the 1980s, especially shellfish culture in the western basin and fish farming in the eastern basin (Charbonnier, 1990; Belias and Dassenakis, 2002; Basurco and Lovatelli, 2003; Grigorakis and Rigos, 2011; Rountos et al, 2012; Massa et al, 2017). The actual baseline of the population density of P. lividus in the Mediterranean is unknown, there are reasons to think that its abundance, in the second half of the 20th century, could be, at least partly, a consequence of human impact (overfishing of its predatory fish, organic pollution) (Boudouresque and Verlaque, 2013); it is usually uncommon in No-Take Zones of Marine Protected Areas, i.e., zones where all types of fishing and harvesting (fish, custaceans and echinoderms), both amateur and commercial, are banned (Boudouresque et al, 1992), and proliferates in the vicinity of untreated domestic sewage outfalls (Harmelin et al, 1981; Boudouresque and Verlaque, 2013) and fish farms (Kušpilicet al., 2007) This man-induced proliferation of P. lividus has resulted in the extirpation of seaweed forests, e.g., Carpodesmia spp., Treptacantha spp. Rhodobionta (Archaeplastida) Aglaothamnion halliae Agardhiella subulata Ahnfeltiopsis flabelliformis Antithamnion nipponicum Chondria coerulescens
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