dc.description.abstract | 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 1000 m, 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. | es_ES |