Spatial and temporal extension of wild fish aggregations at Sparus aurata and Thunnus thynnus farms in the north-western Mediterranean

Abstract

Fish farms cause wild fish to aggregate nearby and thus act to modify their distribution, but the spatial and temporal extent of the attraction effect around farms is still poorly understood. We examined attraction using an integrated approach combining 3 factors: spatial extent, depth and season. We analysed an Atlantic bluefin tuna Thunnus thynnus (ABT) and a gilthead seabream Sparus aurata (SB) farm off the north-eastern coast of Spain which represent different fish farming models in the Mediterranean. Underwater visual counts were conducted from summer 2010 to spring 2011. We recorded 39 species at the SB farm and 17 species at the ABT farm. The analysis of fish abundance and biomass at 3 stations located along a distance gradient from the farm (SB: 0, 30, 200 m; ABT: 0, 100, 200 m) showed significantly higher values next to the cages than at more distant stations, revealing that the spatial extent of the attraction effect was mainly restricted to the cage rather than following a gradient. The intensity of the cage effect varied according to farm, season and depth, reflecting species-specific behaviours. At the SB farm, the cage effect persisted year round, varying with depth, whereas at the ABT farm, the effect was only detected in summer and spring. These distinct spatial and temporal aggregation patterns suggest that implementation of fishery regulatory measures to protect wild fish should be farm-specific. This study emphasizes the importance of using an integrated approach to study wild fish aggregations at farms.

First occurrence of Caprella scaura Templeton, 1836 (Crustacea: Amphipoda) on off-coast fish farm cages in the Mediterranean Sea

“…Additionally, fish abundance and species composition of aggregated fish assemblages around both kinds of fish farms are different (i.e. larger fish aggregation around sea bream/sea bass farms; Bacher et al 2012). This could result in lower invasibility of sea bream/ sea bass farms related to predation rates or competition with pre-existing detritivore species in the fouling community.…”

Aggregations of wild Atlantic Bluefin Tuna (Thunnus thynnus L.) at Mediterranean offshore fish farm sites: Environmental and management considerations

“…1). It has been frequently observed that ABT aggregates around tuna farm cages in Croatia and Spain, mainly in spring, summer and autumn season (Šegvić-Bubić et al, 2011;Bacher et al, 2012). Akyol and Ertosluk (2010) recorded ABT at Turkish sea bream and sea bass fish farms.…”

“…The most commonly recorded families observed aggregated common dolphinfish (Coryphaena hippurus), greater amberjack (Seriola dumerili), Atlantic bonito (Sarda sarda) and chub mackerel (Scomber japonicus), among others (e.g. Sanchez-Jerez et al, 2011;Segvić-Bubić et al, 2011;Bacher et al, 2012Bacher et al, , 2013. Moreover, larger species such as swordfish (Xiphias gladius), which is enlisted in the IUCN Red List of Threatened Species (category: least concern; see http://www.iucnredlist.org), have also been sporadically observed in the vicinity of the farms (Arechavala-Lopez et al, 2014b).…”

“…Other large marine pelagic fauna that have been recorded in the vicinity of fish farms include species of pelagic chondrichthyes, such as Blue Shark (Prionace glauca), which is listed as "vulnerable" (criterion A2cd + 3cd) (e.g. Bacher et al, 2012), as well as some marine mammals whose populations are deemed "critically endangered" in the Mediterranean, such as Bottlenose Dolphin (Tursiops truncatus) (criterion A3bcd; C1), and Monk Seal (Monachus monachus) (criterion A2abc; C2a(i); E) (e.g. Güç lüsoy and Savas, 2003;Díaz-López and Bernal-Shirai, 2007;Piroddi et al, 2011).…”

First report of a swordfish (Xiphias gladius Linnaeus, 1758) beneath open-sea farming cages in the Western Mediterranean Sea.

“…The assemblages of small wild fish commonly concentrated in large numbers around fish farms, such as members of the Clupeidae, Sparidae, Mugilidae and Carangidae families, also attract larger predatory fin fish species, due to the increase in foraging opportunities. For instance, it is highly frequent to observe aggregated species of both ecological and economic interest such as the common dolphinfish (Coryphaena hippurus), greater amberjack (Seriola dumerili), bluefish (Pomatomus saltatrix), common dentex (Dentex dentex) and Atlantic bluefin tuna (Thunnus thynnus) (SanchezJerez et al, 2011;Šegvić-Bubić et al, 2011;Bacher et al, 2012;. However, swordfish (Xiphias gladius) have never been reported before within the limits of a Mediterranean fish-farm facility.…”