Growth of juvenile milkfish Chanos chanos in a natural habitat

Abstract

A population of juvenile milkfish, Chanos chanos (Forsskdl) was studied in a small mangrove lagoon in Naburut Island, central Philippines. Several size groups of milkfish occurred in the lagoon as a result of its periodic connection with the sea. Some 903 specimens were collected in Mar to Nov 1979, covering a fork length range of 12 to 180 mm. Body-we~ght to fork-length relation was: log W = -5.2991 + 3.2388 log L, similar to that of pond-cultured specimens. In Naburut lagoon, juvenile milkfish take in primarily blue-green algae, as well as mangrove and seagrass debris, diatoms and detritus. Variations In condition factor and gut weights of samples taken at different times of day and night indicate diurnal feeding. The condition factor of fish caught during the day from May to Nov stayed constant, indicating that lagoon conditions for growth in terms of food did not change markedly during the year. The monthly size-frequency distribution shows that juvenile milkfish in the lagoon grew at a rate of 7 to 9 mm wk-' in 1979. Compared with pond-cultured specimens, their growth rate was lower during the first month but higher during the second month in the nursery. Specimens larger than 150 mm fork length were almost absent from the lagoon, indicating that they leave after a stay of 4 to 5 mo. The limited area and depth of Naburut lagoon probably set the limit to the size of juvenile milkfish; these can be sustained there to just 150 to 180 mm fork length.