Trends and Status of Harbor Seals in Washington State: 1978-1999

Abstract

In the first half of the twentieth century, harbor seal ( Phoca vitulina richardsi ) numbers were severely reduced in Washington state by a state-financed population control program. Seal numbers began to recover after the cessation of bounties in 1960 and passage of the Marine Mammal Protection Act (MMPA) in 1972 . From 1978 to 1999 , aerial surveys were flown at midday low tides during pupping season to determine the distribution and abundance of harbor seals in Washington. We used exponential and generalized logistic models to examine population trends and size relative to maximum net productivity level (MNPL) and carrying capacity (K). Observed harbor seal abundance has increased 3 -fold since 1978 , and estimated abundance has increased 7 to 10 -fold since 1970 . Under National Marine Fisheries Service (NMFS) management, Washington harbor seals are divided into 2 stocks: coastal and inland waters. The observed population size for 1999 is very close to the predicted K for both stocks. The current management philosophy for marine mammals that assumes a density-dependent response in population growth with MNPL >K/ 2 is supported by growth of harbor seal stocks in Washington waters. MANAGEMENT 67(1):208-219 JOURNAL OF WILDLIFE

The status of harbour seals (<i>Phoca vitulina</i>) in the United Kingdom

“…The observed growth rates in several, apparently closed, harbour seal populations have approached 12-13% p.a. (Heide-Jørgensen and Härkönen 1988;Reijnders 1992b;Boveng et al 2003;Härkönen et al 2002;Jeffries et al 2003). This probably represents the intrinsic rate of increase of an undisturbed and unconstrained harbour seal population in the absence of density dependent effects.…”

Shifts in the estuarine demersal fish community after a fishery closure in Puget Sound, Washington

“…Demersal fi sh species in Puget Sound are consumed by elasmobranchs, such as Spiny Dogfi sh (Squalus acanthias) and Bluntnose Sixgill Shark (Hexanchus griseus), and marine mammals, such as the harbor seal (Phoca vitulina; Bromaghin et al, 2013;Howard et al, 2013) and California sea lion (Zalophus californianus), but we are unaware of any abrupt changes in predator densities in Port Madison to explain these patterns. Jeffries et al (2003) reported a monotonic increase in harbor seal abundance in Puget Sound from the late 1970s to the 1980s that was followed by little change in abundance during the mid-to late 1990s. Fourth, groundfi sh densities can be sensitive to water quality, especially to oxygen concentrations at the seafl oor that result in distributional shifts to normoxic conditions (Breitburg et al, 2009;Essington and Paulsen, 2010), and chronic hypoxia exposure could diminish productivity of groundfi sh prey (Diaz and Rosenberg, 1995).…”

The status of the harbour seal (<i>Phoca vitulina</i>) in Ireland

“…It is critical that surveys are repeated as frequently as is financially viable to maximize our power to detect trends. Replicate counts within a survey region have been used to obtain more precise population estimates and periodic replicate surveys have been used to examine trends (Pitcher 1990, Frost et al 1999, Adkison et al 2003, Jeffries et al 2003. Studies on trend analyses of harbour seal count data suggest that even surveying annually (with 2-3 replicate surveys) it will take at least 5 years to robustly estimate annual trends greater than 5%, but as changes of more than 5% per annum are unusual in stable seal populations then realistically it will take even longer than 5 years (Adkison et al 2003, ICES 2003.…”

“…However, these counts have varied in location, consistency, timing and methodology and could not provide complete national or island-wide perspectives on population size and distribution. , 1997, Huber 1995, Jeffries et al 2003, Adkinson et al 2003, Hayward et al 2005. While breeding season counts provide reliable estimates of abundance as well as valuable pup production data, Härkönen et al (1999) concluded that in nonstable age-structured populations the influence of the differential haulout behaviour on estimating abundance is likely to be greater during the breeding period than during the moult period.…”