Jurica Jug-Dujaković

Cannibalism as a factor affecting the survival of intensively cultured sea bass (Dicentrarchus labrax) fingerlings

Abstract The large percentage of intensively cultured sea bass fingerlings apparently lost to cannibalism indicates that this source of mortality probably contributes significantly to poor survival. The growth rate of ungraded fingerlings produces increasing differences in size which run parallel with increasing cannibalism, especially after the weaning period. Sea bass fingerlings, if not fed early in the morning, showed increased cannibalistic activities; 37% of the larger fish filled their stomachs with smaller siblings. The predator must be twice the length of the victim for ingestion. The extent of cannibalism is found to depend on feeding frequency.

Preliminary studies on reproduction and larval rearing of common dentex, Dentex dentex (Linnaeus 1758)

Abstract The paper describes preliminary results on maturation, artificial spawning, embryonic development and larval rearing of common dentex, Dentex dentex (L.), in captivity. The spawning season of fish maintained under controlled conditions lasted from the beginning of May until the end of June. Since all smaller fish were either males or immature, and bigger fish were females, it can be stated that common dentex is a proterandrous hermaphrodite. Both sexes matured spontaneously in captivity, giving small quantities of gametes. A positive response to hormonal treatment (HCG) was observed. Average number of eggs per injected female was 97 000 per kg of wet weight. The maturation of oocytes is not synchronized and so eggs were produced in several spawnings over 13 days. The functional mouth opening of larvae varied between 98 and 124 μm. However, feeding with rotifers, even though selected through a 120-μm net, was not very successful. In the larval stages, pronounced euryhalinity was not observed. Survival of larvae was best at an ambient temperature of 19°C and salinity of 38‰.

First evidence of establishment of the rayed pearl oyster, Pinctada imbricata radiata (Leach, 1814), in the eastern Adriatic Sea

The Mediterranean Sea is increasingly under threat from invasive species that may negatively affect biodiversity and/or modify ecosystem structure and function. The bivalve mollusc Pinctada imbricata radiata is listed among the 100 most invasive species in the Mediterranean. A first finding of an established population of P. imbricata radiata in the coastal waters of the eastern Adriatic Sea, is presented in this paper. Six and then 30 live specimens were collected in 2015 and in 2017, respectively, at depths of 5 to 15m from the island of Mljet, Croatia. DNA sequencing of the mitochondrial cytochrome c oxidase I gene (COI) revealed three different haplotypes. All samples showed greatest similarity (98 to >99%) to P. radiata COI sequence records in GenBank (=P. imbricata radiata as used in this paper). A Neighbour Joining tree placed all Croatian samples within the 100% bootstrap supported clade for P. imbricata radiata.