Population biology and life strategies of Chlorophthalmus agassizii Bonaparte, 1840 (Pisces: Osteichthyes) in the Mediterranean Sea

The population biology and life strategies of Chlorophthalmus agassizii were studied in the Ionian Sea (eastern-central Mediterranean) using the data collected during the experimental trawl surveys carried out from 1995 to 2000. Depth-related trends of both density and size were found. With depth the former decreased while the latter increased. A typical bigger-deeper phenomenon was detected: young-of-the-year individuals occur on the shelf during autumn-winter months and move towards bathyal bottoms as they grow. The sampled population was made up of several size-age groups. The maximum age of 8 years was identified by means of otolith readings. The Von Bertalanffy growth parameters were estimated from the age-length key (L (infinity)=189.04 +/- 5.401 mm; k=0.24 +/- 0.021; t(o)=-1.20 +/- 0.132; phi'=3.94) and modal progression analysis (L infinity=218.33 +/- 18.397 mm; k=0.164 +/- 0.028; t(o)=- 1.694 +/- 0.171; phi'=3.89). Reproduction of this monoecious fish was observed during summer-early autumn. Considering the female portion of the gonad, the size at attainment of 50% maturity was 115 mm TL. The corresponding age is within the third year of life. The simultaneous occurrence of oocytes in different development stages was shown in the ovary. Both the asynchronous ovary and oocyte size distribution indicate that C. agassizii spawns more than once during a reproductive season (batch spawner). Functional fecundity (on average 3,018 hydrated oocytes) was between 37 and 69% of the absolute fecundity and increased significantly with the individual size. Since adult specimens are mostly distributed on the slope, eggs and larvae develop in epipelagic waters and migrate in-shore where juvenile forms recruit on the shelf. Juveniles migrate ontogenetically towards deeper bottoms and after 2-3 years start to reproduce annually within a life span greater than 10 years.