STUDY OF FREE-RANGING SHARKS 439 



weight and the extreme messiness when disassembling the unit for adjust- 

 ment or battery replacement. An example of this type of packaging is the 

 Mark III shark transmitter described by Ferrel et al. (1974). 



Air filling— An example of this type is the current Mark V shark 

 transmitter shown in Figures 10 and 11. A relatively thick-walled, pressure- 

 resistant tube is terminated on both ends by either glued or O-ring sealed 

 caps. Various material may be used; the author's preference is for plastic 

 because of its light weight, ease of machining, and incorrodibility. Ordinary 

 PVC pipe (sp gr = 1.4) is convenient, but ABS is better, being stiffer and 

 lighter (sp gr = 1.1). Acrylic or polycarbonate tubing (sp gr = 1.2) may be 

 used if transparency is desired. In air-filled housings, the acoustic transducer 

 element must be well coupled to the tube, such as by cementing it in with 

 epoxy as shown in Figure 11. 



The main advantage of an air-filled housing is ease of assembly and dis- 

 assembly. To facilitate easy removal of circuit sections, multipin plugs may 

 be used, as in the Mark V unit. Disadvantages include an operating depth 

 limit and a slight overall size increase because of the wall thickness of the 

 tube and end caps. However, submerged weight will generally be less than for 

 an equivalent oil-filled or potted unit. The probability of O-ring leakage is 

 very low, provided that the sealing surfaces are reasonably smooth and clean. 



If relatively few disassemblies are anticipated, it may be most convenient 

 to use simple glue-bonded butt joints. Flat joints (without internal shoulders) 

 are strong enough and can be cleanly separated with lathe and parting tool 

 with relatively little loss of material. Since glue-bonded end caps take less 

 tube length than O-ring sealed caps, an overall transmitter length reduction 

 can result, even if some extra material is left to allow for several possible 

 lathe partings. 



Sensors 



If information other than the animal's location is desired, one or more 

 sensors must be included in the transmitter package. Many different param- 

 eters are measurable with the available technology. The following parameters 

 are those for which sensors have been built or are planned for the CSULB 

 shark transmitters. These sensors are of the variable-resistance type, having at 

 least a 2X resistance change in the general range of 50-300 kfi . Some sensors 

 used in past shark transmitters are diagramed in Figure 12 and discussed by 

 Ferrel et al (1974). 



Temperature— Temperature can be directly sensed with a thermistor, 

 a device that changes resistance with temperature. In ordinary thermistors, as 

 temperature increases, resistance decreases, a typical coefficient being 

 4.6%/° C for a unit of 100 kft at 25° C. 



Light— Light also can be sensed directly by a single component, a 

 cadmium-sulfide photoresistive cell. However, the range of ambient-light in- 

 tensities found in the natural environment from day to night is much too 

 great to be covered by a single photocell. Instead, several cells of different 



