Trophic ecology of mullets during their spring migration in a European saltmarsh: A stable isotope study

Abstract

Mullet populations are abundant in littoral waters throughout the world and play a significant role in organic matter fluxes. Mullets are opportunistic feeders: adults have frequently been shown to feed on primary producers (e. g. fresh or detrital plant material, microphytobenthos) but they may also feed on meiofauna. The population structure and stomach contents of mullets that colonize salt marsh creeks in Aiguillon Bay (French Atlantic coast) were studied to determine if they use salt marshes as a feeding ground in spring. Stable isotope analyses were carried out on mullets sampled to assess their diet during their spring migration. The mullet population was primarily composed of young-of-the-year (G0), 1-year old (G1) of both Liza ramada and Liza aurata species and 3-year-old or older (G3+) L. ramada individuals. G0 and G3+ population densities increased during the spring period: catch per unit effort (CPUE) increased from 0.22 to 1.49 ind.min-1 for the G0 age group; but stomach content analyses revealed that only G1 and G3+ feed in the salt marsh. Isotopic signatures of G1 (spring: δ 13 C:-14.8‰, δ 15 N: 14.1‰) and G3+ mullets (spring: δ 13 C:-16.9‰, δ 15 N: 13.8‰) indicate that mullet growth is supported largely by primary consumers, such as benthic meiofauna or small macrofauna. Mullets are thus positioned at a much higher trophic level than true primary consumers.

Trophic shift in young‐of‐the‐year Mugilidae during salt‐marsh colonization

“…Fish collection was carried out following the same procedure as in Lebreton et al . (), using a fyke net (4 mm mesh size, 5 m deep, 1·80 m high and 20 m long) set across the creek. The net was set for 20 min periods and then removed for 5 min to empty it.…”

“…This approach allows the determination of food sources actually assimilated by fishes (Pinnegar & Polunin, ) and is thus regularly used to assess changes in diet during migrations (Fry, ). The summer results were compared with those from spring (Lebreton et al ., ). The relationship between stable‐isotope signatures and fish length allows a determination of the size at which YOY shift from neritic to salt‐marsh food resources.…”

“…Along the European Atlantic Ocean coasts, two species of Mugilidae migrate into intertidal salt‐marsh creeks: the thinlip grey mullet Liza ramada (Risso 1827) and the golden grey mullet Liza aurata (Risso 1810) (Laffaille et al ., ; Parlier et al ., ; Lebreton et al ., ). In these habitats, they exploit benthic food resources by scraping mud surfaces and sorting food items using their gill rakers (Hickling, ).…”

“…In these habitats, they exploit benthic food resources by scraping mud surfaces and sorting food items using their gill rakers (Hickling, ). Most studies about L. ramada and L. aurata food sources focus on adults (Lam Hoï, ; Bruslé, ; Almeida et al ., ; Cardona, ; Laffaille et al ., ; Almeida, ), whereas there has been less focus on young‐of‐the‐year (YOY) (Albertini‐Berhaut, , ; Ferrari & Chieregato, ; Lebreton et al ., ). These two species reproduce on the continental shelf: from August to February for L. aurata and from autumn to winter for L. ramada , but the spawning period can vary depending on location and species (Keith et al ., ).…”

Is the meiofauna a good indicator for climate change and anthropogenic impacts?

“…They are the most abundant size class in the benthos and are closely connected to other compartments of the benthos (including prokaryotes and macrofauna; Baldrighi et al 2013). They play an important role in benthic food webs, not only as consumers (feeding on detritus, diatoms and algae, and preying on other small metazoans; Pergent et al 1994;Duarte and Cebrian 1996;Gee and Somerfield 1997;Mateo and Romero 1997;Gwyther 2003;Torres-Pratts and Schizas 2007;Mascart et al 2013Mascart et al , 2015, but also as producers (being a food source for macrofauna and fish; Gerlach 1978; Lebreton et al 2011;Vinagre et al 2012;Carpentier et al 2014). The abundance, diversity, distribution and functional properties of meiofauna can be affected by several abiotic factors, including temperature, salinity, hydrodynamic and sedimentary processes, sediment grain size, oxygenation level and food availability (Ingels et al 2011;Lizhe et al 2012;Ngo et al 2013;Zeppilli et al 2013;Górska et al 2014;Pusceddu et al 2014b), and also, by biotic factors such as predation and competition (Tita et al 2000;Danovaro et al 2007;Nascimento et al 2011;Braeckman et al 2013;Boldina et al 2014).…”