The numbers and kinds of animals in midwater can be 

 quantified by any number of standard visual or two-dimensional 

 optical techniques. For example, for animals that are not 

 particularly abundant in midwater (e.g. large medusae) we 

 routinely obtain information on density by making horizontal 

 transects at specific depths and recording the number of 

 individuals of particular species that pass through a premeasured 

 area in front of the submarine. Knowledge of the size of this 

 area, speed of the submersible, and duration of the transect 

 gives density. When the animals of interest are more common, one 

 can obtain instantaneous measurements of density by estimating 

 nearest neighbor distances ( Hamner and Carleton, 1979; Mackie and 

 Mills, 1983). Unfortunately, measurement of density alone 

 constitutes only a small part of the quantitative information 

 that we wish to record. Density is a mean figure for the number 

 of objects within a given volume; it tells us nothing about the 

 actual location of the objects within that volume. Furthermore, 

 zoologists invariably are interested in the behavior of the 

 animals that they observe, and they need information not only on 

 "how many" or "where, " but also on "what" the animals do within 

 the observational arena over time. 



In order to extract quantitative information from video 

 tapes on the location of animals and their behavior through time 

 within a three-dimensional volume, optical information must be 

 recorded stereoscopically in order to calculate x-, y-, and z- 

 coordinates. Use of 2-D optics has repeatedly been demonstrated 

 as futile for quantitative investigations of objects or organisms 

 in a 3-D medium such as air or water unless the optics are 

 supplemented with additional information on depth. For example, 

 in the laboratory depth information on the structure of fish 

 schools has been obtained via the use of shadows in conjunction 

 with standard 2-D photography (Cullen et al . , 1965; Partridge et 

 al., 1980). But in the field the only way to collect 3-D optical 

 information (besides laser ranging) is via stereoscopic 

 photography (Dill et al . , 1981; Potel and Wassersug, 1981; 

 Klimley and Brown, 1983). 



THE ROLE OF 3-D OPTICS IN MIDWATER RESEARCH 



Because of the enormous expense and limited time available 

 for exploration of the midwater realm via manned submersibles or 

 remote vehicles, it is absolutely necessary to obtain as complete 

 a record as possible of each dive on film or on video tape for 

 subsequent analysis in the laboratory. The footage obtained must 

 contain 3-D information and it must be in a form that permits 

 subsequent comfortable viewing by the investigator for long 

 periods of time. The format must be compatible with frame-by- 

 frame computer-automated image analysis because of the immense 

 amount of optical information that is encoded on the film or 

 video tape for any given dive. 



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