STUDY OF FREE-RANGING SHARKS 437 



Monolithic integration— The maximum in microminiaturization is 

 achieved by full integrated-circuit (IC) construction. The multilayered circuit 

 is produced on a silicon wafer, with conductor lines and spaces down to 

 about 0.0025-mm widths. All components are laid down by photomasking 

 and diffusion deposition of the desired circuit materials. The resulting IC 

 chips, although minute, cannot be used alone; they must be either incor- 

 porated in hybrid circuits or mounted in other relatively large packages to 

 facilitate making standard connections to them. 



The production of an entire special-purpose circuit (such as a multi- 

 channel UST) on a single IC chip would be economical only for quantities of 

 more than about 20 000 to 50 000 units and would be prohibitively costly 

 in the small quantities normally used for fish-tracking experiments. 



Overall Size, Density— When considering how much to miniaturize a 

 transmitter circuit, it should be kept in mind that there are usually certain 

 components that cannot be miniaturized, e.g., battery, large capacitors, 

 transducer, and transformer (Figure 8). Total transmitter size can therefore 

 be reduced only so much by techniques such as hybridization and, for some 

 simple USTs, these may not yield a significant overall size reduction. Thus, 

 for the purpose of size reduction alone, hybridization may be worthwhile 

 only for rather complex circuits. However, another benefit of hybridizing 

 circuits is in the labor saved during final transmitter assembly. Having the 

 hybrids produced by an outside company (as is usually done) makes for a 

 simpler final wiring job. This is especially helpful to the researchers if they 

 are doing this job themselves. 



Another important consideration is the density (specific gravity) of the 

 final transmitter package. Small transmitter size is obviously desirable, but 

 the unit should also be close to neutral density, so as to neither weigh down 

 nor lift up the animal. Many USTs turn out to be considerably heavier than 

 water, especially those using relatively large mercury batteries. In such cases, 

 it is questionable whether further size reduction (allowed by smaller circuits) 

 is good or bad if it means an increase in submerged weight. It may be that a 

 small, heavier-than-water unit may upset a fish's behavior more than a unit 

 of the same mass that is somewhat larger, but closer to neutral density. 



Packaging— There are several ways to house the transmitter compo- 

 nents, the choice depending on factors such as cost, operating depth, and 

 expendability. If the unit is to be recovered for repeated use, it should have a 

 convenient means of entry for changing batteries or adjusting signal param- 

 eters. Also, since the more complex units are more likely to require 

 troubleshooting, readjusting, etc., it is important that such units allow easy 

 access to their various sections. 



Potting— Transmitters and their batteries are often cast (potted) into 

 one solid block of material such as wax, urethane, epoxy, or dental acrylic. 

 While this method is very simple and yields minimum package size (but not 



