Buoyancy compensation of juvenile chinook salmon implanted with two different size dummy transmitters

We investigated the effect of two different sizes of surgically implanted t ransmitters on the buoyancy compensation of juvenile chinook salmon Oncorhy nchus tshawytscha. We determined buoyancy by measuring the density of fish with a filled air bladder in graded salinity baths. In addition, we examine d the effect of pressure changes on buoyancy by measuring the pressure redu ction (P-R) at which fish became neutrally buoyant. We found no significant difference between the density of control and tagged groups, indicating th at fish were able to compensate for the transmitter by filling their air bl adders. However, both groups of tagged fish had significantly lower P-R tha n control fish. Regression analysis of fish density on P-R indicated that d ensity of the tagged groups changed at a higher rate than that of the contr ols. As a result, tagged fish attained neutral buoyancy with less pressure reduction even though the tagged and control groups exhibited similar densi ties. This relation was confirmed by using Boyle's law to simulate buoyancy changes with change in depth. Although fish compensated for the transmitte r, changes in depth affected the buoyancy of tagged fish more than that of untagged fish. Reduced buoyancy at depth may affect the behavior and physio logy of tagged juvenile salmonids, and researchers should be aware of this potential bias in telemetry data. In addition, there was little difference in P-R or the slope of the density -P-R regression lines between tagged gro ups. This was caused by the small difference in excess mass (i.e., weight i n water) of the two transmitters. Thus, although two transmitters may not w eigh the same, their effects on buoyancy may be similar depending on the ex cess mass.