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

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 saltmarsh creeks in Aiguillon Bay (French Atlantic coast) were studied to determine if they use saltmarshes 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 saltmarsh. Isotopic signatures of G1 (spring: δ13C: −14.8‰, δ15N: 14.1‰) and G3+ mullets (spring: δ13C: −16.9‰, δ15N: 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.