Diet and feeding strategies of mesopelagic fishes in the western Mediterranean

Myctophids, gonostomatids and sternoptychids are the most abundant teleosteans worldwide and constitute an important assemblage of the mesopelagic ecosystem, functioning as vehicles of energy and matter through trophic webs. This study concentrates on the trophic ecology of the most abundant mesopelagic fishes of the western Mediterranean (WM) based on stomach content analysis. The myctophids (in this study: Benthosema glaciale, Ceratoscopelus maderensis, Lobianchia dofleini, Myctophum punctatum, Hygophum benoiti, Hygophum hygomii, Lampanyctus crocodilus, Lampanyctus pusillus and Notoscopelus elongatus) perform extensive diel migrations across the water column, between the surface to as deep as 1000m, interacting with plankton and micronekton at multiple depths, and generally feeding in the epipelagic layers at night. In contrast, the gonostomatids Cyclothone braueri, Cyclothone pygmaea, and the sternoptychid Argyropelecus hemigymnus remain below epipelagic layers, feeding at different times throughout the day and night. The diet composition, trophic niche breadth and prey selectivity of 11 of these fish species were determined for juvenile and adult individuals from two surveys performed in December 2009 and July 2010 in the western Mediterranean Sea. The number of prey items varied among species, e.g. Myctophum punctatum was the species with the highest feeding intensity, reaching ca. 700 prey items in a stomach, whereas the mean number of prey in Cyclothone braueri was low (usually 1 or 2 prey per stomach). A dietary shift towards larger prey was evident from juveniles to the largest and oldest adult individuals, despite trophic niche breadths did not increase with body length for any of these mesopelagic species. The diets of the small gonostomatids, sternoptychid and early juveniles of myctophids were dominated by non-calanoid copepods, ostracods, and other small zooplankton, whereas medium-sized myctophids, e.g. L. dofleini or H. benoiti, preyed mainly on calanoids. The oldest stages of L. crocodilus and N. elongatus fed mostly on macrozooplankton and micronekton. There was high diet overlap among mesopelagic fish species and strong heterogeneity in diet composition at intraspecific level. Nevertheless, some species showed certain degree of segregation of food resources, determined by the developmental stage or spatial distribution, and positive selection towards particular prey items. The Chesson’s electivity index showed that L. dofleini, N. elongatus, L. crocodilus and L. pusillus preyed selectively on euphausiids; B. glaciale was selective on the calanoid genus Pleuromamma, and C. maderensis preferred to feed on larvaceans in autumn. The two congeneric species of Hygophum consumed a high number of food items, but H. hygomii showed positive selection for euphausiids, whilst H. benoiti preferred small corycaeid copepods. Overall, the main trophic difference among mesopelagic fishes in the WM was observed between the small non-migratory species that do not evidence a diel rhythm, feeding during both day- and night-time on small zooplankton, and the largest-sized myctophids, which fed on meso- and macrozooplankton and, more occasionally, on small fishes. Mediterranean midwater fishes can be characterised by the adoption of mixed feeding strategies, with varying degrees of specialisation on different prey types that allow flexibility in a changeable environment.