Harbor seal haul-out habitat and the feasibility of shifting haul-out locations at Dosewallips State Park, Washington

Publications >> Harbor seal haul-out habitat and the feasibility of shifting haul-out locations at Dosewallips State Park, Washington


Calambokidis, J., G.H. Steiger, B.D. McLaughlin, and J.R. Evenson. 1990. Harbor seal haul-out habitat and the feasibility of shifting haul-out locations at Dosewallips State Park, Washington. Report to the Washington State Parks and Recreation Commission, Olympia, Washington. 70pp. 

Executive summary

High fecal coliform concentrations that have been found at the Dosewallips River delta on the Hood Canal appear to originate from harbor seals that use the sloughs and marsh on the southern portion of the delta to haul out (Calambokidis et al. 1989, Calambokidis and McLaughlin 1988, DSHS 1988). In other areas in Puget Sound, harbor seals regularly haul out on human-made platforms, such as recreational floats and log booms (Calambokidis et al. 1978, 1985, Johnson and Jeffries 1977). Harbor seals used to haul-out on a log boom that existed at the north end of the Dosewallips River Delta through the mid 1970s. Because seal-related contamination is closely associated with haul-out locations, it may be possible to alter the affected areas by shifting the areas used by seals to haul out.

This study examines the feasibility of shifting the seal haul-out habitat at Dosewallips delta from the salt marsh areas used currently to human-made structures that would be located just off the delta. We summarize relevant findings about the biology of harbor seals at this location, harbor seal haul-out requirements, what steps would need to be taken to shift the location of haul-out, and limitations of trying to conduct such a project. This study was funded by the Washington State Parks and Recreation Commission.

Counts made on 49 days from December 1989 to August 1990 at Dosewallips Delta ranged from 11 to 390 harbor seals and averaged 140 (n=49, s.d.=85). A number of factors were identified that were statistically associated with the variations in the number of seals counted at the Dosewallips Delta. These included time of day, temperature, and tide height and time to high tide.

A major surprise in the results of this study were the significant decrease in seal numbers at the Dosewallips River Delta in recent years. This trend data should be viewed with caution, especially because it does not cover some critical seasons. There were not significant increases or decreases in seal numbers observed at Duckabush River Delta or Quilcene Bay.

At the Dosewallips River delta, harbor seals haul out along the three main southern sloughs and along or on logs along the river. We found significant seasonal and annual changes in the use of different portions of the Dosewallips River Delta.

A trial float placed at Dosewallips River delta on 16 May 1990 was not used by seals until 15 June 1990 and not regularly until late July. Females with pups occassionally hauled-out on the float in late August, towards the end of the pupping season. Exploratory behavior by seals in the water of the trial float was seen frequently.

The amount of haul-out space required per seal was similar among locations and habitats and averaged about 2 m2/seal. The area used by seals at human-made habitats tended to be slightly larger than natural habitats, though this difference was small. The mean shoreline distance per seal (the distance of shoreline or water access for a group of seals) was generally under 1 m/seal at natural habitats and greater than 1 m/seal at other human-made habitat.

Human-made haul-out areas appear to provide some advantages over natural habitats, especially in their accessibility at all tides and their rapid access to deep water. The height above water for human-made structures used by seals, including log booms, floats, and a fish net pen, ranged from 12 to 54 cm with log booms lowest in the water. The height above the water of many floats used by seals appear to prevent newborn seal pups from getting out on the float for the first 1-2 weeks.

An alternate haul-out structure able to accommodate the maximum seals seen using the current haul-out area (470) would need to be 1,175 m2 using 2.5 m2 per seal. Shoreline space would need to be at least 1 m per seal or about 500 m in total. This is less of a limiting factor since the haul-out area designs discussed below and human-made haul-out areas in general, easily meet this requirement.

Two human-made structures, log booms and floats, represent the best choices for alternate habitat at Dosewallips and a combination of both structures would provide the best alternative habitat for harbor seals. To allow segregation between groups we recommend use of multiple small structures as opposed to one or two large structures.

The ideal log-boom structure consists of three logs secured together by crosspieces with total width of 1.5 m. For the float design, the most suitable and compact structure would be a series of open-celled concrete floats. A fairly compact 12 x 17 m structure would be suitable for up to 60 harbor seals. A marked barrier around the structures would be necessary to keep boaters away from hauled seals and restrict them from using the structures for recreational purposes. The best location for an alternate haul-out site is along the steep slope marking the edge of the delta just north of the main river channel.

Seals may not readily adapt to new structures, as indicated by the delay in the use of the trial float. We propose actions that can be taken to encourage use of an alternate location. Additionally, options to discourage seal use of current areas include: 1) changes that elicit a behavioral avoidance by seals such as placement of a 'scarecrow' type object, 2) placement of physical barriers on marsh areas that restrict seal access to some or all haul-out areas, 3) placement of physical barriers at slough entrances to restrict access of seals but not smaller animals, and 4) disturbance of seals using haul-out areas. We recommend only the first two of these options be considered and only after seal use of alternate structures has begun, indicating these structures can meet seal haul-out requirements.

To evaluate the success of the project, we developed (in cooperation with personnel from the Department of Wildlife and Department of Health) a recommended monitoring program to be conducted if the relocation project is undertaken. The primary goals of the monitoring program would be to evaluate the success of the project to shift seal haul-out locations from marsh areas to new structures and to monitor trends in water quality for improvements as a result of the relocation activities.

A number of legal requirements exist for conducting this type of project. Discussion with personnel at the National Marine Fisheries Service, indicated that the information we have provided appear to be adequate to justify shifting haul-out locations as long as the method was non-lethal and alternate habitat were provided. A number of other permits are required, however, these all seem obtainable if public support of this project continues.

We do not expect significant impacts of seals at a new structure because they will be in deep, well-circulated water that is away from the intertidal zone where there would be more impacts. The large size of the proposed structures and their relative proximity to shore could pose an aesthetic impact, though this did not appear to be a concern expressed by residents of the area. Harbor seal predation on fish species would not likely be altered by the relatively minor shift in haul-out locations.

The experimental nature of this action makes it impossible to provide assurance of success even if all the procedures identified here are followed. The change in fecal coliform levels will probably be a slow process that may take years. The increased visibility of seals on a structure would make them susceptible to those who want to harm them. Additionally, the initial cost estimates for the artificial structures are high (over $100,000).

The project would have little chance for success should there be strong opposition from groups oriented towards protecting marine mammals or the local residents concerned about seal population increases. It is essential to maintain cooperation with these groups by including them in project evaluation.

Identified alternatives to the relocation option include no action (continue to prohibit shellfish harvesting in the state park), re-evaluation of the water quality standards, or elimination or reduction of the seal population. These alternatives are not feasible or would maintain the status quo.

We recommend that alternate structures be phased in gradually. This would allow evaluation of the success of the project prior to a full financial commitment and better identification of successful and unsuccessful approaches for shifting seal use. A series of steps are proposed for how to proceed on the project should State Parks decide to do so. The initial steps recommended before a final decision on implementation of the full project include: 

  • Experimental closing of one slough (#1) to seals to examine effects on water quality
  • Evaluation of engineering questions related to structures and water currents in area of planned float placement
  • Obtain more precise cost estimates on construction and installation of haul-out structures
  • The experimental closing of one slough would allow evaluation of how quickly seal relocation would result in improvements in water quality.

This experiment was recommended by the technical advisory committee for this project and should be conducted over at least a one year period. The slough chosen for this experiment is the most northern of those regularly used by seals.