Changes in Abundance of Harbor Seals in Maine, 1981–2001

Abstract

Aerial counts of harbor seals (Pboca vitulina concolor) on ledges along the Maine coast were conducted during the pupping season in 1981, 1986, 1993, 1997, and 2001. Between 1981 and 2001, the uncorrected counts of seals increased from 10,543 to 38,014, an annual rate of 6.6 percent. In 2001 30 harbor seals were captured and radio‐tagged prior to aerial counts. Of these, 19 harbor seals (six adult males, two adult females, seven juvenile males, and four juvenile females) were available during the survey to develop a correction factor for the fraction of seals not observed. The corrected 2001 abundance estimate was 99,340 harbor seals. Productivity in this population has increased since 1981 from 6.4% pups to 24.4% pups. The number of gray seals (Halichoerus grypus) counted during the harbor seal surveys increased from zero in both 1981 and 1986 to 1,731 animals in 2001.

Genomic signatures of population bottleneck and recovery in Northwest Atlantic pinnipeds

“…The model was initially seeded with observed SNP frequencies at all r80 loci from the 1973 to 1974 cohort and run for 45 years, sampling allele frequencies at time points that corresponded to the sequenced cohorts. Similarly, to assess rates of change in allele frequency among harbor seal cohorts, the model was parameterized as follows to represent the time period 1995 to 2015: starting population size = 67,500; r  =   .066 (Gilbert et al., 2005); and K  =   100,000 (Waring et al., 2016). The model was initially seeded with SNP frequencies at all r80 loci from the 1992 to 1998 cohort and run for 20 years, sampling allele frequencies every 10 years.…”

“…At that point, gray seals were considered rare in both eastern Canada and the Northeast United States (Davies, 1957), and harbor seal pupping colonies had been extirpated south of Maine (Katona, Rough, & Richardson, 1993). Following the cessation of bounties, enactment of local protection (Lelli & Harris, 2006), and passing of the US Marine Mammal Protection Act of 1972, population surveys have documented the rapid return of these seals over the past several decades (Bowen, den Heyer, McMillan, & Hammill, 2011; Gilbert, Waring, Wynne, & Guldager, 2005; Waring, Josephson, Maze‐Foley, & Rosel, 2016). In the Northeast United States, estimates of harbor seal population size have grown from 5,000 in the early 1970s (Richardson, 1976) to over 75,000 today (Waring et al., 2016), and gray seals have returned from essentially absent until the early 1990s (Gilbert et al., 2005) to 30,000–50,000 today (Moxley et al., 2017).…”

“…Today, growth at the largest worldwide gray seal population on Sable Island in Canada is slowing potentially as a result of nearing carrying capacity (den Heyer, Lang, Bowen, & Hammill, 2017), while gray seal populations in the United States continue to increase (Waring et al., 2016), likely representing a population size‐mediated range expansion from Canadian source populations (Wood et al., 2011). Following protection, the harbor seal population in the Northeast United States also experienced steady growth through the 1990s (Gilbert et al., 2005), but more recently this growth has slowed and potentially reversed (Waring et al., 2016). Even more dramatic, on Sable Island the rapid growth of gray seals has been coincident with a reduction to near absence of the local harbor seal population (Bowen, Ellis, Iverson, & Boness, 2003; Bowen, McMillan, & Mohn, 2003).…”

Status of harbour seals (<i>Phoca vitulina</i>) in mainland Norway

“…Harbour seals were counted along the entire Norwegian coast at known moulting haulout sites in the period mid-August to early September 2003-2006-2005, almost all known moulting areas from Finnmark to Vestfold counties were covered by aerial photo surveys flown at altitudes of approximately 800-900 ft (243-274 m), and at low tide (± 2 hours). Surveys in the Østfold County were flown in 2003-2006 at 300 ft (91 m), and the small tidal amplitudes permitted counts to be carried out irrespective of the tidal cycle.…”

“…1990). Harbour seals are usually counted during pupping and/or moult, when most of the species haul out (Heide-Jørgensen and Härkönen 1988, Thompson and Harwood 1990, Reijndeers et al 1997, Huber et al 2001, Gilbert et al 2005. A first harbour seal survey that included the entire Norwegian coast, primarily based on aerial photographic surveys, but supplemented with boat based visual counts in a few areas, was conducted during moult (August-September) in 1996-1999 and resulted in a point estimate of 7,465 harbour seals for the entire Norwegian coast (Bjørge et al 2007).…”