On the diet and reproduction of the oilfish Ruvettus pretiosus (Perciformes: Gempylidae) in the eastern Mediterranean

The oilfish, Ruvettus pretiosus, is an incidental by-catch of the drifting surface longlines targeting swordfish and Thunnidae in the Mediterranean and elsewhere. Despite its global distribution and frequent occurrence in the catch of many fishing gears, little is known about its biology. In this study we examined the dietary preferences and reproductive biology of this species from specimens caught in the eastern Mediterranean. The most numerous and frequently occurring prey items were benthopelagic fish, followed by cephalopods. The sex-ratio was highly skewed in favour of females (1:8.4), while histological examination of the collected ovaries displayed six developmental stages (perinucleolar, early and late lipid stage; early, middle and late vitellogenesis). Oocyte diameter ranged from 23 –69 mm at the perinucleolar stage to 224 –366 mm at the late vitellogenesis stage. Spawning is expected to occur during mid and late summer; however there were no individuals available from this period. The findings are discussed in relation to the ecology of the species.