Identification of some common food items in the guts of fish larvae and juveniles in the Black Sea

Over the past decade the positive trends in the average annual number of fish larvae and in the copepod population dynamics in the coastal area of the Black Sea agree. The increased fish larvae abundance is hypothesized due to improvement of their nutrition associated with the drastic increase in number of introduced invasive cyclopoid copepod Oithona davisae. This assumption is difficult to be verified through fish gut content analysis in absence of methodology allowing prey species identification from their fragmentary residual remnants. Our paper offers an original approach to identification of several common copepod prey using specific distinctive features detected on their chitin fragments from guts of fish larvae and juveniles. To identify specific features of the common species from the coastal areas off Sevastopol (Acartia tonsa, Oithona davisae, Longipedia sp., Cyclopina sp.), alive copepods were isolated from the samples and reared as monospecific cultures in laboratory. Images of alive copepods of each species at successive stages of development and their moulted exoskeletons were compared with the images of chitin remnants found in the fish guts. This technique discloses relatively intact specific morphological features remaining undigested in chitin fragments of prey. These species-specific taxonomic features are suggested to be used for trophic analysis of the Black Sea fishes at early stages of development. Application of proposed method is helpful for assessment of qualitative and quantitative composition of consumed prey and selectivity of fish, especially during the changes in zooplankton community structure affecting significantly survival of fish generations.