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US Fish & Wildlife Service FieldNotes

STOCKTON FWO: Preliminary Results from Natural Marks on Delta Smelt

Region 8, September 24, 2012
Figure 1. Lateral view of adult delta smelt.
Figure 1. Lateral view of adult delta smelt. - Photo Credit: n/a
Figure 2. Dorsal view of adult delta smelt showing initial area of interest (rectangle) for natural marks.
Figure 2. Dorsal view of adult delta smelt showing initial area of interest (rectangle) for natural marks. - Photo Credit: n/a
Figure 3. Image acquisition for natural marks of delta smelt. Detail shows head of fish.
Figure 3. Image acquisition for natural marks of delta smelt. Detail shows head of fish. - Photo Credit: n/a
Figure 4. Flotarium used to obtain lateral view images and morphometric data for delta smelt.
Figure 4. Flotarium used to obtain lateral view images and morphometric data for delta smelt. - Photo Credit: n/a

By Gonzalo Castillo,  Fish Biologist

External body marks, such as external pigmentation patterns or scars, are potentially useful to identify individual animals. Natural marking is a non-invasive approach for identifying individuals within a group. For example, natural marking has been applied to identify individuals, including marine mammals and sharks. This approach could also have the potential to identify small fish for which more effective marking and tagging methods are still needed to track individuals.

Researchers from the Stockton Fish and Wildlife Office, the UC Davis Fish Conservation and Culture Lab, and Biopar, began to explore the use of natural marks in winter 2012. The selected species was the delta smelt (Hypomesus transpacificus), a small fish of environmental and management relevance in the upper San Francisco Estuary (Figure 1).

Researchers used laboratory raised adult delta smelt produced at the UC Davis Fish Conservation and Culture Laboratory to evaluate the use of melanophores as natural marks. External examination of adult delta smelt revealed several potential areas of interest (AOI) for natural marks (dorsal view of the head and caudal peduncle, ventral view of the mandible). The dorsal view of the head was selected as the preliminary AOI because melanophores are particularly common in this area (Figure 2).

To evaluate the short-term effectiveness of natural marks, researchers tagged fish with an individual alphanumeric code (VIA tags, Northwest Marine Technologies). They used a digital camera equipped with a macro lens to acquire images (Figure 3). A second digital camera and a flotarium were used to obtain lateral whole body images and preliminary morphometric measurements (Figure 4). Initial evaluation of natural marks involved manual (naked eye) matching of digital images. A qualitative matching-grade criteria was developed as a measure of confidence for the manual matching between two images (4: excellent; 3: good; 2: fair and 1: poor). Preliminary results using manual evaluation of the images taken of the dorsal view of the heads in ten delta smelt showed: 1) high density of pigments and pigmentation patterns, 2) variation in the size and shape of individual pigments over time, 3) a 100 percent correct matching of images taken one month apart and 4) excellent to good matching-grade for 80 percent of images and fair matching for the remaining images.

These preliminary results suggest that further evaluation of natural marks may facilitate the development of automated methods (algorithms) to match the pigmentation patterns of individual delta smelt from digital images taken over time. Thus, potentially enabling faster automated recognition of naturally marked fish. Eventual applications of natural marking method for delta smelt could include more detailed laboratory experiments and field mark-recapture experiments. Hence, facilitating improved research and management for this species.

Contact Info: Gonzalo Castillo, 209-334-2968x323, gonzalo_castillo@fws.gov