engleski

How much is overlap between sardine and anchovy applying stomach content and stable isotope analysis?

Sardine and anchovy are ecologicaly and economically importan pelagic fish species that are widly spread throughout the Mediterranean basin and its adjacent seas, as well as in the north-eastern Atlantic. Lead by the fact that those two small pelagic fish species are short- lived and fast-growing and inhabit the same ecological niche the idea of this study was to investigate their overlap in terms of diet and isotopic niche. Overall, 80 specimens of both species (40 individuals by species) were collected during the scientific survey MEDITS (30 June – 21 July, 2018) in the area of eastern Adriatic. Immediately after landing, the total length (TL, cm) and weight (W, g) of collected target individuals were measured. Then half of the individuals were used for stomach content analysis and their visceral cavity was carefully opened prior to preservation in 4% buffered formalin. In the laboratory, stomachs were removed and measured to an accuracy of 0.01 g before and after the prey items were carefully extracted. Prey items were identified under a stereomicroscope (Zeiss Discovery. V12 ; magnification 40-80x), to the taxonomic level of class or family, and counted in order to calculate its frequency of occurrence and abundance. Individuals used for stable isotope analysis were immediately put in deep-freeze (-20°C) and upon arrival in the laboratory a piece of dorsal-anterior muscle section was dissected and skin and bones were removed. To avoid inorganic carbon contamination, if it was necessary, acidification treatment was done. Standard procedure in stable isotopes were applied on collected tissues, which were sent at the Stable Isotope Facility at the University of California, Davis (USA). Tissue samples stable isotope ratios of carbon (13C/12C) and nitrogen (15N/14N), and elemental %C and %N analyses were conducted on a PDZ Europa ANCA-GSL elemental analyzer interfaced to a PDZ Europa 20-20 isotope ratio mass spectrometer (Sercon Ltd., Cheshire, UK). According to the stomach content analysis it was obvious that both small pelagic fish species preferred to feed on animal food items, precisely copepods, bivalvia larvae and pisces ova/ larvae although both of them also had a plant items within their stomach. Although their diet seemed qualitatively the same, quantitatively it was statisticaly different (ANOSIM, One-way, Global R=0.223, p<0.001). Stable isotope analysis complement observed difference. Namely, their isotopic niches slightly overlap, precisely anchovy niche was much narrow than sardine. Since the sampling time fitted within the anchovy spawning period (May-September) in Adriatic, its narrow niche and paticulare feeding (search for higher energy-content prey like pisces ova or larger crabs-decapods, amphipoda and mysidacea) here observed might be related to that. Bearing in mind, that this are first preliminar results on stable isotope analysis done in this area it is obvious that further investigation on higer number of samples collected throughout the whole year needs to be done if we tend to elucidate possible competition between this to small pelagic fish species.

small pelagic ; diet overlap ; stable isotopes ; Adriatic

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