Comparison of growth and gonad development of farmed green mussel Perna viridis L. in three habitats

The green mussel Perna viridis was farmed in three different ecosystems, viz., a semienclosed bay (Kollam Bay), estuary (Ashtamudi Lake) and open sea (off Narakkal) along Kerala coast during the period 2003-2004. Mussels of the length range 25 to 32 mm were seeded on to nylon ropes and suspended from off bottom structures. Their growth in length, width, depth, total weight and meat weight were measured at monthly intervals. The condition index (CI) and gonad development were also observed. The farm ecology was monitored and the variations in salinity, temperature, nutrients and productivity during the culture period were recorded and correlated with biological parameters. The instantaneous growth rates (IGR) in length, width and depth were the highest at 0.0124, 0.010 and 0.012 mm day -1 respectively in mussels grown in the sea and these were significantly different (p < 0.05) from those of mussels grown in the bay and estuary. The IGR in total weight was also the higher for the mussels grown in the sea (0.028 mg day -1 ) and bay (0.022 mg day -1 ) but were lower and significantly different from those grown in the estuary (0.013 mg day -1 ). In the bay and open sea farm, all the reproductive stages were observed indicating that mussels were reproductively active at these sites. However, in the estuarine farms, completely spent mussels were not observed. The ecological conditions and growth rates indicate that it is possible to have two crops from bay (August/ September to December/January; February/March to May/June) and sea (September/October to December/January; January/February to March/April) while in the estuary only one crop (November/ December to May/June) is possible due to the low saline conditions from June to October. The observations on the reproductive stages of mussels indicate that it is also possible to collect seed mussels from bay and sea farms during the spawning period of mussels by placing suitable spat collectors.