Abundance Monitoring

Boat Surveys for Abundance Trend Estimation

Gray whale survey in Laguna San Ignacio

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Boat surveys are conducted to document seasonal trends in gray whale abundance and to estimate the minimum number of gray whales within the primary gray whale winter aggregation and breeding lagoons along the Pacific coast of Baja California.  Each survey utilizes a hand-held Global Position System (GPS) device to follow a predetermined survey track line that passes through the deep-water areas utilized by gray whales in each lagoon. Observer and sighting protocols are specified for each lagoon's unique characteristics and are used to record whales' counts along each track line. This method allows comparison within-year survey counts along identical survey tracks in each lagoon area, and for comparison with historical counts from previous years.

Whale counts are conducted from 7-8-m long outboard-powered boats (Pangas) at a speed of 11-km/hr. Speed and course along the track line are continuously verified using a hand-held GPS. This survey speed minimized the likelihood that whales (which typically travel at 7 to 9-km/hr) do not move ahead of the survey boat and are counted more than once. It also allows observers sufficient time to detect surfacing whales along the track line.

Foreach survey, two pairs of observers (one pair searching to the left and one pair searching to the right sides of the boat) note the number of whales seen they pass abeam of the survey boat, thereby documenting the number and distribution of whale sightings along the track line. A fifth person records each sighting on printed survey forms, noting: the time of each sighing, the number of whales in each group, their direction of movement, and whether they are single whales or female-calf pairs.

Gray whale survey route in Laguna San Igancio
Gray Whale survey track line in Laguna San Ignacio.

 

 

 

Boat surveys in Laguna San Ignacio began in the winter of 2006 and continue to the present. Each survey follows a 30-km long GPS track line that begins at the north end of Isla Garzas in the northernmost end of the lagoon (North End Basin) to the west end of Isla Ana at Punta Holcombe at the lagoon entrance. The survey track line is divided into five "zones" or segments to record the whales' distribution within the lagoon. Whales in the "North End Basin" (north of the survey track line) are counted from the center of this area by observers searching in 360-degrees around the stationary boat.

 

 

 

 

 

 

Gray Whale Survey tracks in Bahia Magdalena
Gray whale survey tracks in Bahia Magdalena.

 

 

Gray whales in the Bahía Magdalena lagoon complex included three different areas where gray whales gather in winter: Canal de Santo Domingo in the north from Boca la Soledad south to La Florida; in Bahía 

Magdalena's center region, west and southwest areas; and in Bahía Almejas in the south from a point in the center of the bay south of Puerto el Dátil north to the north-east of Puerto Cortés on Isla Santa Margarita.

 

 

 

 

Examples of the seasonal counts of gray whales in Laguna San Ignacio.

Counts of single adult whales in Laguna San Ignacio

 

Counts of female-calf pairs in Laguna San Ignacio

 

 

 

Boat survey track-line for estimating minimum abundance of gray whales in Laguna San Ignacio. Counts of gray whales in the “North End Basin” portion of the lagoon are obtained from a 360⁰ scan of the area. The GPS survey track line continues 30 km south from Isla Garzas (Zone 1) over the deepest central portions of the lagoon to Punta Holcombe on the west end of Isla Ana at the entrance of the lagoon (Zone 5).

Boat survey track-lines for estimating gray whale abundance in the Bahía Magdalena lagoon complex in three areas where gray whales aggregate:  Canal de Santo Domingo in the north; Bahía Magdalena’s center, west and southwest areas; and in Bahía Almejas in the south.