A PRELIMINARY-STUDY OF THE SPATIAL VARIATION IN GROWTH OF RAFT-CULTURED BLUE MUSSELS MYTILUS-TROSSULUS IN NORTHERN PUGET-SOUND, WASHINGTON

The effects of culture rope density (high and low), position (inside, middle, and outside), and depth (1, 3, and 5 m) on growth rates, expre ssed as mean shell length increase (mm) per day, were quantified in ra ft-cultured blue mussels Mytilus trossulus in northern Puget Sound dur ing the fall of 1993. Growth rates differed by as much as 20% after th e 90-d experimental period, and varied significantly according to cult ure rope position, but not according to culture rope density or depth. Furthermore, a three-way analysis of variance revealed a significant first-order interaction effect, culture rope density X depth. The sign ificant culture rope position effect was due to the higher growth rate (mean +/- SEM) of mussels on outside culture ropes (0.199 +/- 0.009 m m/d) compared to inside culture ropes (0.168 +/- 0.007 mm/d). The inte raction effect was due to the significantly higher growth rates of mus sels at 1 and 3 m (0.197 +/- 0.009 and 0.204 +/- 0.011 mm/d, respectiv ely) compared to 5 m (0.162 +/- 0.014 mm/d) under low culture rope den sity conditions, and the significantly higher growth rate of mussels a t 3 m under low culture rope density conditions (0.204 +/- 0.011 mm/d) compared to high culture rope density conditions (0.168 +/- 0.012 mm/ d). Practical raft management is affected by such variation in growth. Commercial mussel growers may find it necessary to alter their cultur e methods to insure a more uniform product from seed to harvest.