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Temporal Changes in Abundance of Harbor Porpoise (Phocoena

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Temporal Changes in Abundance of Harbor Porpoise (Phocoena 242 Abstract—Abundance of harbor por- Temporal changes in abundance of harbor poise (Phocoena phocoena) was es- timated from data collected during porpoise (Phocoena phocoena) inhabiting the vessel surveys conducted through- out the inland waters of Southeast inland waters of Southeast Alaska Alaska. Line-transect methods were used during 18 seasonal surveys Marilyn E. Dahlheim (contact author)1 spanning 22 years (1991–2012). Es- 1, 2 timates were derived from summer Alexandre N. Zerbini surveys only because of the broader Janice M. Waite1 spatial coverage and greater number Amy S. Kennedy1 of surveys during this season than during other seasons. Porpoise abun- Email address for contact author: [email protected] dance varied when different periods were compared (i.e., 1991–1993, 1 2006–2007, and 2010–2012); how- National Marine Mammal Laboratory ever, persistent areas of high por- Alaska Fisheries Science Center poise densities occurred in Glacier National Marine Fisheries Service, NOAA Bay and Icy Strait, and off the town 7600 Sand Point Way NE of Wrangell and Zarembo Island. Seattle, Washington 98115-6349 Overall abundance of harbor por- 2 Cascadia Research Collective poise significantly declined from the 218 ½ West Fourth Avenue early 1990s (N=1076, 95% confidence Olympia, Washington 98501 interval [CI]=910–1272) to the mid- 2000s (N=604, 95% CI=468–780). This downward trend was followed by a significant increase in the early 2010s (N=975, 95% CI=857–1109) when abundance rose to levels simi- Harbor porpoise (Phocoena phocoena) the Southeast Alaska stock—occur- lar to those observed 20 years ear- are distributed throughout Alaska ring from Dixon Entrance (54°30′N; lier. Potential factors that could con- waters (Fiscus et al.1; Leatherwood 134°00 W) to Cape Suckling (60°00 N; tribute to the downward trend were ′ ′ 2 examined. The 2 regions with high and Reeves, 1978; Leatherwood et al. ; 144°00′W), 2) the Gulf of Alaska 3,4 densities of harbor porpoise (i.e., Lowry et al. ; Dahlheim et al., 2000, stock—occurring from Cape Suck- Glacier Bay and Icy Strait as well as 2009; Hobbs and Waite, 2010), com- ling to Unimak Pass, and 3) the Ber- Wrangell and Zarembo islands), that monly inhabiting waters less than ing Sea stock—occurring throughout were consistently occupied by this 100 m deep (Barlow, 1988; Carretta the Aleutian Islands and all waters species, and the different trend val- et al., 2001; Hobbs and Waite, 2010). north of Unimak Pass (Allen and An- ues of these 2 regions indicate that Currently, 3 stocks of harbor por- gliss, 2012). The boundaries of these some fine-scale population structur- poise are recognized in Alaska: 1) 3 stocks are based on geography and ing may exist for harbor porpoise in- habiting the inland waters of South- 1 Fiscus, C. H., H. W. Braham, R. W. Mer- 3 Lowry, L. F., K. J. Frost, and J. J. Burns. east Alaska. cer, R. D. Everitt, B. D. Krogman, P. D. 1982. Investigations of marine mam- McQuire, C. E. Peterson, R. M. Sonntag, mals in the coastal zone of western Alas- and D. Withrow. 1976. Seasonal dis- ka during summer and autumn. Annu- tribution and relative abundance of ma- al report on contract NA 81 RAC0050, rine mammals in the Gulf of Alaska, 238 submitted to NOAA, Outer Continental p. Northwest Fish. Sci. Cent. Processed Shelf Environmental Assessment Pro- Rep., [Available from Alaska Fish. Sci. gram, Juneau, Alaska, 37 p. [Available Cent., Natl. Mar. Fish. Serv., 7600 Sand from the Natl Mar. Mamm. Lab., Alaska Manuscript submitted 14 May 2014. Point Way NE., Seattle, WA 98115.] Fish. Sci. Cent.,7600 Sand Point Way Manuscript accepted 25 March 2015. 2 Leatherwood, S., A. E. –Bowles, and R. NE., Seattle, WA 98115.] Fish. Bull 113:242–255 (2015). R. Reeves. 1983. Endangered whales 4 Lowry, L. F., K. J. Frost, D. G. Calkins, Online publication date: 14 April 2015. of the eastern Bering Sea and Shelikof G. L. Swartzman, and S. Hills. 1982. doi: 10.7755/FB.113.3.2 Strait, Alaska. Results of aerial surveys Feeding habits, food requirements, and April 1982 through April 1983 with notes status of Bering Sea marine mam- on other marine mammals seen. Hubbs mals. Vol. 1. Final report submitted The views and opinions expressed or Sea World Research Institute Technical to the North Pacific Fishery Manage- implied in this article are those of the Report No. 83-159, 315 p. [Available ment Council, Anchorage, Alaska, 292 p. author (or authors) and do not necessarily from the Natl. Mar. Mamm. Lab., Alas- [Available from the Natl. Mar. Mamm. reflect the position of the National ka Fish. Sci. Cent.,7600 Sand Point Way Lab., Alaska Fish. Sci. Cent.,7600 Sand Marine Fisheries Service, NOAA. NE., Seattle, WA 98115.] Point Way NE., Seattle, WA 98115.] Dahlheim et al.: Temporal changes in abundance of Phocoena phocoena inhabiting the inland waters of Southeast Alaska 243 perceived areas of low porpoise density, but to date height of 5.9 m. A line-transect method was employed there has been no analysis of genetic or individual to survey predetermined tracklines. At the start of this movement to assess the validity of these designations. study, distribution, habitat preferences, and seasonal The preference of harbor porpoise for shallower wa- occurrence of harbor porpoise within the study area ters makes them highly vulnerable to incidental cap- were unknown. Tracklines were designed throughout ture during net-fishing operations (Jefferson and Curry, the study area with either a zig-zag or straight-line 1994; Read, 1994; Barlow et al., 1995). The nature and path, depending upon the size of the different areas. magnitude of incidental takes are currently unknown The survey was designed to include all major water- but could be significant in some gill-net and purse- ways and a selection of smaller bays and inlets to ex- seine fisheries targeting Alaska salmon (Oncorhynchus amine both deepwater and nearshore habitats through- spp.) and Pacific herring (Clupea pallasi). out the entire study area. The same trackline design Obtaining abundance estimates for harbor porpoise, was employed for all surveys completed between 1991 a small, inconspicuous cetacean species, is challenging. and 1993, although alterations were made during some For example, the ability to detect harbor porpoise is surveys depending on weather and other unforeseen highly sensitive to environmental conditions; surveys circumstances (e.g., mechanical breakdowns or engage- should be limited to relatively calm sea states and ment in rescue operations). good lighting conditions. Despite such challenges, es- During line-transect surveys, sighting data were col- timates of both density and abundance for this species lected by a team of 3 observers, with 1 observer at each do exist for Alaska waters. Taylor and Dawson (1984) of 3 stations: starboard, port, and recorder station. In reported on a shore-based study that yielded density the early 1990s, the total number of biologists partici- estimates for Glacier Bay National Park and Preserve. pating in the survey was 6; therefore, a full observer In 1991–1993, and again in 1997–1999, aerial surveys rotation took 2 h, with each observer spending 40 min of coastal waters in Alaska, ranging from the south- at each station or watch, followed by a 2-h rest period eastern Bering Sea to Dixon Entrance, yielded more for each observer after each full rotation of watches. recent abundance estimates (Dahlheim et al., 2000; Schedules for observer rotations were selected random- Hobbs and Waite, 2010). ly each day. In this study, we report the results from dedicated Port and starboard observers used 7×50 Fujinon5 line-transect surveys conducted to determine the den- binoculars (model 56A2, Fujifilm Holdings Corp., To- sity and abundance of harbor porpoise in Southeast kyo) to search from 0° (at the ship’s bow) to 90°. Scan- Alaska over a 22-year period from 1991 through 2012. ning techniques were standardized with nearly 32 min The objectives of these surveys were 1) to obtain rela- (or 80%) of the 40-min watch spent scanning with the tive abundance estimates of harbor porpoise within binoculars and about 8 min spent scanning with the the inland waterways of Southeast Alaska, 2) to in- naked eye. To reduce observer fatigue, binoculars were vestigate porpoise density and abundance by different supported by adjustable metal poles that were either strata (i.e., smaller regions), 3) to establish a baseline handheld or rested on the observer’s hips. When not for detecting changes in harbor porpoise abundance entering data, the recorder searched for porpoise by through time, and 4) to report on significant insights scanning both sides of the ship from the bridge with on this species as a result of these investigations. the naked eye. Binoculars were only used by the re- corder to confirm sighting identifications and numbers. Sightings made by the officers, crew, and off-watch ob- Materials and methods servers were recorded as “off effort” and were not used in calculations of density estimates. Study area A GPS unit was connected directly to a portable computer on the bridge. The date, time, and position The study area included the inland waters of Southeast of the ship were automatically entered into a data file Alaska (Fig. 1). Surveys covered all major channels or every 10 min and whenever data were entered by the bays from Juneau to Ketchikan: Lynn Canal, Icy Strait, recorder. Search effort was recorded on the computer Glacier Bay, Cross Sound, Chatham Strait, Stephens by marking the beginning and end of each transect. Passage, Frederick Sound, Sumner Strait, and Clarence Beaufort sea state, a weather description (rain and Strait. When time permitted or weather precluded the fog), a visibility index, and observer positions (port, re- surveying of major channels, many adjacent smaller corder, and starboard) were also entered.
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