Censuses and disturbance of harbor seals at Woodard Bay and recommendations for protection

Publications >> Censuses and disturbance of harbor seals at Woodard Bay and recommendations for protection


Calambokidis, J., G.H. Steiger, J.R. Evenson, and S.J. Jeffries. 1991. Censuses and disturbance of harbor seals at Woodard Bay and recommendations for protectionFinal report to the Washington Department of Natural Resources, Olympia. 44pp.

Executive summary

The Woodard Bay Natural Resource Conservation Area was purchased by the state of Washington. One of the important features of this site are the harbor seals that haul-out to rest and give birth to young on the log booms. This report summarizes research conducted by Cascadia Research and Washington Department of Wildlife on harbor seals at Woodard Bay (Henderson Inlet, Puget Sound) related to harbor seal abundance and disturbance and provides recommendations for management of the site.

Personnel with Cascadia Research and the Marine Mammal Investigations of the Washington Department of Wildlife (WDW) have studied harbor seals during 300 visits (681 hours) to Woodard Bay by land or air between 11 August 1977 and 31 December 1990. Most of the effort was in 1984 (131.5 hours during 65 visits) and 1990 (217 hours during 67 visits). Disturbances of harbor seals at Woodard Bay were recorded beginning in 1984. During 1990, we conducted observations to determine the distance at which seals entered the water in response to approaching vessels.

Harbor seals have used the log dump since at least the 1930s, though research on the seal numbers at this site began only in 1977. A number of factors were identified from the census data taken from 1977 to 1990 that influenced the number of seals at Woodard Bay in a statistically significant manner. These included: 1) time of day, 2) season, 3) year, 3) height of high tide, 4) rainfall, and 5) day of the week. These factors statistically accounted for over 50% of the variation in seal numbers observed throughout these historical censuses.

Counts were highest in August to October, coinciding with the latter half of the pupping season and the molt. Harbor seal numbers have increased dramatically at Woodard Bay since regular monitoring began in 1977. The pace of this increase was most pronounced between 1977 and 1985, probably because 1) seal populations were increasing more rapidly and 2) an increased use of the log booms as a result of the slowing and eventual elimination of commercial activity at the site. Though seal numbers appeared to stabilize between 1985 and 1989, the censuses in 1990 have revealed that seal use of this site is still increasing.

Pups are born at the Woodard Bay usually starting in early July, although viable pups were first seen in 1990 on 28 June. Births appeared to continue through early August when the maximum number of pups are seen. Births at Woodard Bay occurred on the log booms and especially on some of the areas that were covered with planks. The highest counts of pups made to date at Woodard Bay was 95 made on 7 August 1990. This high number indicates Woodard Bay is now approaching Gertrude Island as one of the most important pupping areas for harbor seals in Puget Sound.

Disturbances of seals (human activities causing seals to enter the water) were seen on 45% of our observation periods in 1990 at a rate of 0.33 disturbances per observation hour. Disturbance rates were highest during summer months, weekend days, and weekday evenings, reflecting the factors that influence the number of people out on the water and around the site. Disturbance rates were higher in 1990 than any other previous year at the site. The disturbance rate at Woodard Bay in 1984 as well as 1990 was more than twice as high as at other sites we monitored in Washington State.

The primary causes of disturbances of seals at Woodard Bay were people coming to the site in recreational motor boats, skiffs, and canoes or kayaks to observe the harbor seals. Most of the people who caused disturbances usually approached the seals without the apparent intention of disturbing or harassing the seals.

Seals entered the water on the approach of vessel at distances up to 246 m, although the average was 56 m (n=44, s.d.=44). The distances that seals were disturbed varied significantly by vessel type; seals entered the water at a greater distance in response to kayaks and canoes compared to recreational motorboats and skiffs. Seals entered the water in response to people on foot at up to 256 m although, on many occasions, we were able to pass less than 100 m from seals while maintaining a low profile without causing disturbance.

The impacts of disturbances on harbor seals have not been well studied though a number of possible effects have been identified by other researchers. These include: 1) change in behavior at site by altering haul-out times, 2) abandonment of preferred haulout areas, 3) mother-pup separation during bond formation, 4) interruption of nursing, 5) increased stress during the molt, 6) potential stress during other seasons (e.g. mating), and 7) interruption of rest resulting in lower fitness and health.

Due to a high rate of disturbance noted in July 1990, the following actions were taken in August 1990 to reduce disturbances: 1) signs were posted warning people not to disturb the seals, 2) newspaper articles were published describing the problem with disturbance at the site, 3) the National Marine Fisheries Service was provided with information on potential violators of the Marine Mammal Protection Act, and 4) buoys and lines were used to close off the entrances to the interior of the log booms. These actions were extremely successful and there was an immediate dramatic decrease in the rate of disturbances after they were put in effect.

As part of this project for DNR, we have provided recommendations for longterm preservation of harbor seals at Woodard Bay. The most important recommendations are:

1) The most critical period for harbor seals at the site is July and August when pupping is occurring at the site and September to October when animals are molting. Activities that might disturb harbor seals should be avoided during these periods. Activities that are disruptive to seals are best conducted in early morning hours and during rainy days.

2) For the maximum seal numbers using Woodard Bay (possibly approaching 500 seals within the next few years), a minimum haul-out area of 1,250 m2 and 500 m of water access is required. The current log boom area provides a surplus of space for seals using the site at this time. Two types of structures, log booms and floats, represent the best choices for replacement habitat at Woodard Bay because both are used commonly by harbor seals. Given the current surplus of haul-out space at Woodard Bay, replacement of haul-out structures do not need to be implemented until space available reached 2,000 m2.

3) Protecting seals from disturbance represents the major challenge for insuring that public access to the site does not threaten the use of the site by seals. Vessels (motorized and non-motorized) as well as people on foot should not be allowed to approach closer than 150 m from the seal haul-out area. A public viewing area closer than 150 m from the seals could be used if it was shielded from the seals.

4) The harbor seals at Woodard Bay do provide a unique opportunity for educating the public and providing them with access to observe and understand marine mammals in a wild setting. An essential aspect of protecting marine mammals at Woodard Bay and other areas is to use this opportunity to allow people to see and learn about seals. Attempts to avoid disturbance of harbor seals do not need to prevent human access. Informing people on proper ways to observe marine mammals without causing disturbances can be an important educational value of their experience at the site.

5) Future research and monitoring on seals should focus on the effectiveness of measures to prevent disturbance of seals as the human traffic at the site increases, future changes in seal numbers, seal food habits and their role in the ecosystem, and conflicts between seals with shellfish production at the site.

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