|
Article Excerpt
Introduction
In the mid 1960's, researchers began to photograph individual marine mammals with the express purpose of using the images to identify individual animals on the basis of natural markings. Over time, researchers began to develop photo catalogs of individuals as they were sighted and photographed in different years and areas (Hammond et al., 1990). As the number of photographs has increased, so did the need for computer assistance to help with the collation and integration of the large collections.
Starting in the mid 1980's, computer-assisted systems began to be developed to aid in the identification of individual marine mammals (Hiby and Lovell, 1990; Mizroch et al., 1990). The system developed by Hiby and Lovell use a scanned image and a 3-dimensional computer model to interpret the photograph and to develop an identification algorithm. Their system is considered semi-automated because the computer system measures some of the photograph's characteristics independent of the system operator. The system developed by Mizroch and colleagues is categorical and requires that identification photographs be classified visually by a trained observer. This system is based on a categorization scheme of natural marks and scars, and data related to each photograph are entered into a computer database. The system operator controls all of the matching information and uses a computer to query the database for possible matching choices.
The NMFS National Marine Mammal Laboratory (NMML) has been developing and curating a collection of humpback whale, Megaptera novaeangliae, tail flukes photographs taken in North Pacific waters since 1985. This collection has grown from about 750 images in 1986 to about 24,000 in 1999, representing contributions from over 18 research groups from all regions in the North Pacific (Table 1). Unique NMML identification numbers (NMMLID) are assigned only when there are at least 2 photographs of a particular individual whale in the database. As of April 1999, 3,093 unique NMMLID numbers had been assigned and 12,057 tail flukes photographs had been assigned a NMMLID; 11,156 tail flukes photographs had not yet been assigned a NMMLID. Overall, the 23,213 tail flukes photographs evaluated in this paper may represent the sightings and resightings of no more than 6,000 individual humpback whales.
When conducting certain numerical studies using photo-identification data (e.g. capture-recapture analyses), it is important to segregate the photographic data strictly on photographic quality only (Hammond, 1986: Hammond et al., 1990; Mizroch et al., 1990). Photographs in the NMML database are given two different ratings: one based on photographic quality (focus, angle, distance), and the other based on recognition quality (distinctive pattern, marks, or scars) (Mizroch et al., 1990, provide more details). The analysis conducted in this paper stratified the photographs by three levels of photographic quality (hereafter simply referred to as photo quality), examples of which are shown in Figure 1. Matching was conducted using the system described in Mizroch et al. (1990), except that the patterns in use today (Fig. 2) have been simplified and improved. The tail flukes map (Fig. 3) has not been modified.
[FIGURES 1-3 OMITTED]
Tests of the NMML system (i.e. stratified by recognition quality) were first presented in Mizroch et al. (1990), when the database contained 9,353 photographs. Here, we present test results for the NMML database when it contained 12,000 photographs (using ad hoc tests conducted from 1991 to 1995), and tests with the database at its current size of nearly 24,000 photographs.
Methods
Categorizing Whale Tail Flukes
Humpback whale tail flukes have black and white pigment patterns that can match one or several categories (Fig. 2). For each photograph, a selection of patterns that most closely resembled the tail flukes was chosen. In general, the user selected between one and six patterns for each photo being matched, depending on what characteristics were visible on the photograph to be matched. In addition to selecting patterns, the user evaluated locations of natural markings, scars, or other unique marks on the tail flukes (Fig. 3), and selected any or all sectors that contained the markings (e.g. a distinctive line in Sector 5 and an open circle in Sector...
|
