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Fishery Bulletin 103(1) 



mal that has adequate food resources in nature; higher 

 loads are felt to be energetically significant. In this ex- 

 ample using a 15.5-kg cownose ray, the Drag as %TAX 

 is within acceptable parameters; however, at 0.60 m/s 

 the Total force as %TAX begins to exceed these guide- 

 lines. At this point, a researcher would have to consider 

 whether diving behavior at this speed would be a sig- 

 nificant factor in the animal's survival. 



Another application of this information would be to 

 determine the minimum reasonable size for a study ani- 

 mal of a particular species. Blaylock (1990) attempted to 

 address this issue for cownose rays by considering the 

 transmitter-to-ray mass ratio using dry weights. The 

 advantage of using metabolic rates is that it identifies 

 subtler but significant increases in energy requirement 

 to carry a PSAT. In his study, Blaylock examined two 

 age groups, a 0+ age group that had an average weight 

 of 1.8 kg and a 1+ age group that ranged in size be- 

 tween 4.3 kg and 7.8 kg. He concluded that the 0+ age 

 group was negatively impacted by the sonic tag but that 

 the 1+ age group was not effected. A PSAT is physically 

 smaller than the sonic tags used in his experiment; in 

 addition, it is attached to the animal at the nose-end 

 of the tag so that it is carried with the long axis of the 

 tag parallel to the long axis of the animal (Blaylock's 

 sonic tags were attached so that the long axis of the 

 tag was carried perpendicular to the long axis of the 

 animal). Both these factors — smaller physical size and 

 nose-end orientation in space — decrease the projected 

 surface area of the tag. As an example, consider the 

 metabolic cost of carrying a Wildlife Computers PAT 

 to each of these sizes (1.8, 4.3, and 7.8 kg) of cownose 

 ray (Table 3). For the 1.8-kg ray, only the exertion of 

 carrying the PSAT at 0.15 m/s horizontally was associ- 



ated with a %TAX of <5%; higher swimming speeds or 

 downward diving markedly increased the %TAX. It is 

 obvious why short-term effects of carrying a sonic tag 

 were evident. For the 4.3-kg ray, all swimming speeds 

 greater than 0.15 m/s, whether horizontal or diving, 

 required increased energy expenditures of >5%. For the 

 7.8-kg ray, %TAX was acceptable at 0.15 m/s, marginal 

 to slightly elevated for mid-range speeds, and was clear- 

 ly excessive at high speed. According to this analysis, 

 rays of these size classes would not be good candidates 

 for carrying a PSAT. As determined in this study, the 

 smallest cownose ray that ought to be considered for 

 PSAT tracking would be 14.8 kg. Drag as %TAX is s5% 

 for all speeds and only slightly >57c for Total force as 

 %TAX at 0.60 m/s. Because prolonged high speed div- 

 ing behavior is not likely a factor in this ray's ability 

 to survive, the minor elevation of %TAX for diving at 

 0.60 m/s can be disregarded. 



When applying this type of analysis to other species 

 that predominantly swim at speeds greater than 0.60 

 m/s, several caveats make unwise the extrapolation of 

 these data to higher velocities. Referring back to the 

 equations describing drag and power. Equation 1 and 

 Equation 2, respectively, drag is proportional to veloc- 

 ity squared and power is proportional to velocity cubed 

 provided that all other factors are constant. However, 

 in examining Figure 3, as velocity increases from 0.00 

 m/s to 0.60 m/s, all other factors are not constant. Spe- 

 cifically, the angle of deflection, 9, decreases from 90° 

 at 0.00 m/s to as low as 31.5° at 0.60 m/s. First, the 

 projected surface area, S, over which water flows de- 

 creases as velocity increases. Second, the orientation 

 (effective shape) of the object also effectively changes 

 as velocity increases. Hence the drag co-efficient, C D 



