Juvenile Salmon Use in Lake Union in Relation to Floating Home Complexes

 

Juvenile Salmon Use in Lake Union in Relation to Floating Home Complexes
Seattle, Washington

EXECUTIVE SUMMARY

During the spring and summer of 2009, Pentec Environmental conducted a juvenile salmon study in Lake Union for the Floating Home Association (FHA).

The principal objective of the study was to conduct site-specific surveys to provide the City of Seattle with improved scientific data with which to develop reasonable guidance and regulations for floating homes; these data were intended to clarify the actual significance, if any, of these structures to outmigrating juvenile salmon.

Observational data were collected around 9 floating home complexes by fisheries biologists at above water and snorkel transect stations using standardized methodologies. To augment findings, the results from recent acoustic tagging studies conducted by the U.S. Fish and Wildlife Service (USFWS) were also included in this report.

Results and conclusions are as follows:

• Based on above water and snorkel observation events, juvenile salmonids do not appear to be associated with the Lake Union nearshore either at floating home communities or at the Gas Works Park reference site. Only 1 of 97 observations events conducted between late-April and early-July revealed the presence of juvenile Chinook salmon proximal to floating homes.
• These findings are consistent with juvenile Chinook salmon acoustic tagging studies conducted during the 2007 and 2008 outmigratory period. These studies showed a pattern of use in Lake Union and throughout the Ship Canal characterized by a general offshore use of aquatic habitats. Juvenile salmonids largely selected against nearshore habitats in South Lake Union and Gas Works Park.
• Above water and snorkel results showed that nearshore habitats in the vicinity of floating home complexes were used by several fish species, most abundant was the small forage species threespine stickleback. Several other warm water resident species were present in low to moderate numbers including smallmouth bass, yellow perch, prickly sculpin and sunfish. Most warm water residents were juveniles that would not pose a predatory risk to juvenile salmon.
• No large adult smallmouth bass, northern pikeminnow, or other predatory fish were observed during observation events near or beneath the structures of floating home complexes.
• The prevalent offshore behavior of juvenile salmon in Lake Union is likely influenced by a number of physical and biotic factors including prey abundance, predator avoidance, and the high level of nearshore development already present in the lake.

 

Juvenile Salmon Use in Lake Union in Relation to Floating Home Complexes
Seattle, Washington

INTRODUCTION

Background

During the spring and summer of 2009, Pentec Environmental, the natural resources group of Hart Crowser, Inc., conducted a juvenile salmon study in Lake Union for the FHA.  The FHA consists of approximately 500 homes in the Lake Washington Ship Canal between the Fremont Bridge and Portage Bay (Figure 1).

The City of Seattle regulatory community has expressed concerns that floating home complexes in Lake Union and other portions of the Lake Washington Ship Canal have the potential to affect the outmigration and survival juvenile salmonids.  The principal objective of this study was to conduct site-specific surveys to provide the City of Seattle with improved scientific data with which to develop reasonable guidance and regulations for floating homes; these data were intended to clarify the actual significance, if any, of these structures to outmigrating juvenile salmon.  Specific objectives of this work included:

• Collect observational data on juvenile salmon around several floating home complexes to determine behavior of fish in the vicinity of floating homes and to assess fish abundance in the several microhabitats provided by floating homes.
• Collect observational data on juvenile salmon in a reference area of Lake Union to qualitatively compare relative habitat use and behavior in areas of known use with that seen in the floating home communities.
• Collect observational and census data of piscivorous fish and waterfowl species in the vicinity of floating home communities to determine if these areas provide substantial habitat for potential predators on juvenile salmon.
• Gather and review existing data and reports of juvenile salmon movements and behavior within the Lake Washington basin, particularly two studies conducted by the USFWS that conducted acoustic tracking in 2007 and 2008. 
• Prepare a scientific report that discusses the results of the Pentec and USFWS data and assesses the significance of juvenile salmon use near floating homes.

Salmonids in the Lake Washington Ship Canal Basin

Lake Union is a part of the Lake Washington Ship Canal Basin, a 13.8 kilometer artificial waterway that allows navigation between Lake Washington and Puget Sound.  The basin consists of five sections starting from Lake Washington: Montlake Cut, Portage Bay, Lake Union, Fremont Cut, and Salmon Bay, ending at the Ballard locks.  The largest part of the basin is Lake Union, which is 235 hectares (ha) in size and has a mean depth of 9.8 meters (m).  The shoreline of Lake Union is highly developed with numerous marinas, commercial shipyards, floating home communities, and dry docks (Celedonia et al. 2009; Figure 1).  Over 90 percent of the Lake Union Shore is composed of vertical bulkhead with most of the remainder riprap.  Very little natural riparian vegetation is present, though there are some areas of landscaping for park settings.  Over 90 percent of the immediate upland is impervious (Toft et al. 2003).

Upstream of Lake Union is Lake Washington, a large monomictic lake with a total surface area of 9,500 ha and a mean depth of 33 m.  Nearly 80 percent of the shoreline is comprised of residential land use.  The largest tributary to Lake Washington is the Cedar River, which discharges to the south end of the lake.  Another major tributary is the Sammamish River, whose watershed includes Bear Creek, Lake Sammamish, Issaquah Creek, and several small tributaries. (Celedonia et al. 2009) 

Anadromous salmonids in the basin include sockeye salmon, Chinook salmon, coho salmon, and steelhead trout.  Sockeye are by far the most abundant anadromous salmonid in the basin with adult returns exceeding 350,000 fish in some years (Fresh and Lucchetti 2000).  The major source of natural production is the Cedar River with additional production from smaller tributaries draining to north Lake Washington.  Chinook salmon spawn in the Cedar River and several other Lake Washington tributaries.  Coho salmon spawn in numerous small tributaries as well as all major streams draining to Lake Washington (Tabor et al. 2009).  Juvenile Chinook and coho salmon are also released from the Issaquah Creek Hatchery in May and early June, reaching Lake Union in approximately two weeks (Roger Tabor, USFWS, personal communication, April 20, 2009).

Of the Pacific salmon species, juvenile Chinook salmon are found in Lake Union and the Ship Canal over the longest period, beginning in early-May and a presence through much of July (Celedonia et al. 2009; DeVries et al. 2005; Johnson et al. 2004).  Juvenile Chinook spend more time in Lake Union than in other areas of the Ship Canal, with individual residence times ranging from a few days to over 2 weeks (Celedonia et al. 2009; SPU and USACE 2008).  Juvenile sockeye salmon typically spend at least 1 year rearing in Lake Washington before outmigrating.  Sockeye do not use nearshore areas in the basin, preferring limnetic habitats deeper than 20 m in Lake Washington, ascending to shallow offshore waters at night, where they feed (Woodey 1972; Eggers 1978).  Outmigration occurs from mid-May to late-June, but sockeye typically pass through Lake Union and the Ship Canal over a few days or less (Johnson et al. 2004).  Similarly, coho salmon typically spend at least 1 year rearing in natal stream habitats before quickly outmigrating through Lake Washington and the Ship Canal (Johnson et al. 2004).  Very little information is available regarding steelhead trout use of Lake Union, but this species typically spends 2 years or more rearing in stream environments before outmigrating.  Given the larger size of outmigrating juvenile steelhead, it is believed that outmigration through the Ship Canal would be relatively rapid.

 

Juvenile Salmon Use in Lake Union in Relation to Floating Home Complexes
Seattle, Washington

METHODS

Floating Home Communities

During June and July of 2009, bi-monthly observations of fish presence and behavior, targeting juvenile salmon, were made at selected floating home complexes in Lake Union, Portage Bay, and the Lake Washington Ship Canal. Observations were made by trained field biologists using accepted and standardized observational techniques.  Observational stations were established and biologists recorded observations both from above water on floating home floats and docks, and below water by surface snorkelers.  Inventories of piscivorous fish and waterfowl, particularly avian piscivores, were also collected.

A site visit to several floating home communities on Lake Union, Portage Bay, and the Lake Washington Ship Canal was conducted on April 30, 2009 to determine the most appropriate study areas for juvenile salmon and salmon predator observations.  Criteria used to select communities to be studied and stations included access to the area for both above water observations and snorkeling, the presence of suitable aquatic habitats for juvenile salmonids, areas that were representative of the different types of community configuration, and areas broadly spaced throughout the Lake Union/Lake Washington Ship Canal.

Nine floating home communities were selected for the study-5 communities on the east shore of Lake Union, 1 community on the west shore of Lake Union, 2 communities in Portage Bay, and 1 community in the Lake Washington Ship Canal, as shown in Table 1 and Figure 1.  Photographs of observation areas at most floating home sites are presented in Appendix A.  In addition, a reference station in the Gas Works Park area was also selected for observation.  This area has recently been studied (Celedonia et al. 2008; Celedonia et al. 2009; Tabor et al. 2009) and is known as a juvenile salmon outmigratory corridor.  Comparison of relative juvenile salmon use and behavior between the floating home communities and a known area of use was intended to provide information regarding the relative numbers of fish that are in contact with or affected by floating home communities and the general strength of the outmigrating year class of juveniles.

Juvenile Salmon Surveys

Above water surveys were conducted bi-monthly and snorkel surveys monthly from May 13 to July 2, 2009 to provide observations during the entire juvenile salmon outmigratory period (Table 1).  When above water and snorkeling was conducted during the same day, above water observations were made first so that fish would not be disturbed by snorkel activities.

This period was also selected to take advantage of hatchery releases of juvenile Chinook salmon from the Issaquah Hatchery (on Issaquah Creek, draining to Lake Washington), which took place in May and June.  Juvenile salmon were expected to take approximately 2 weeks to reach Lake Union after release from the hatchery (Roger Tabor, USFWS, personal communication, April 20, 2009).

Above Water Field Observations

At each station, above water observations were made for a minimum of 15 minutes for a distance of 8 to 10 feet across the water surface.  If fish were found, observations would be extended to completely document reactions and behavior of juvenile salmon as they encounter an overwater structure.  Each station was monitored twice during each day of monitoring.  Observations at each site included species (if discernable), school size, approximate fish size, behavior, water depth, fish depth, and surface conditions.  Particular attention was made to describe behavioral reactions to floats and shadow thresholds and whether fish swam underneath or around shadows and overwater structures.  In addition, measurements of temperature, oxygen, and turbidity were collected and recorded at each station.  The schools size classes were as follows:

• 1 to 10 salmonids;
• 11 to 100 salmonids; and
• >100 salmonids.

The behavioral categories were as follows:

• Passive milling:  no net movement
• Active milling:  slow net movement
• Polarized:  fast net movement; and
• Reversal:  movement direction reversed

A census of waterfowl, particularly avian piscivores and predators was also collected.  Any observations of predation on juvenile salmon or other fish species was carefully noted.  Baited minnow traps were set at identified locations to determine the presence of salmon or small fish predators such as prickly sculpin.  Traps were set in the morning and retrieved at the end of the field day.

Surface Snorkeling

Surface snorkeling was conducted at each floating home community where above water observations were made to provide observations on fish that were not visible to above water monitors.  Snorkeling permitted biologists to identify piscivorous fish such as small and largemouth bass, and northern pike minnow.  Snorkelers slowly traversed one to three transects, and all juvenile salmon, other fish species, and potentially piscivorous fish were recorded on a waterproof slate.  Transects extended from one end of the floating home complex to the other or between houses and floats.  Transects also extended out to approximately 6 m beyond structures.  When possible, the snorkeling biologist made free dives to make observations under structures and within the complex of floatation objects (logs, barrels, etc.).  Fish presence and behavioral data were collected in the same manner as outlined above.  When possible, actual counts of juvenile salmon and other species were made along each transect.

 

Juvenile Salmon Use in Lake Union in Relation to Floating Home Complexes
Seattle, Washington

RESULTS

Habitat Characteristics

The floating home complexes selected for study were chosen to encompass the widest possible range of nearshore habitat characteristics within the floating home communities in the Lake Washington Ship Canal basin.  Complexes were chosen on both shores of Lake Union, Portage Bay, and the Ship Canal, with emphasis on the eastern shore of Lake Union, where the majority of floating homes are located (Figure 1).  Where possible, at each floating home complex selected, observation stations were located along the shoreline next to the complex, in the middle of the complex, and along the waterward edge of the complex.  Both above water and snorkel observations were made during all daylight hours, including early morning (0500 to 0700) and at dusk (2000 to 2130).  Habitat characteristics at each complex are presented in Table 2 and photographs are presented in Appendix A.

In general, most of the floating home complexes were situated within 5 m of shore, extending out variable distances, ranging from 50 to 130 m from shore.  Each complex was composed of between 5 and 20 homes.  Open water areas between homes and between rows of homes were generally relatively narrow, usually between 3 and 15 m.  Shorelines and nearshore habitats range from moderate to steep grades; most riparian areas were narrow but well vegetated.  Aquatic substrates at most complexes were composed of a layer of silty sand underlain by gravel, cobbles, and rubble (Table 2).

Exceptions to this general layout were at Complexes 2 and 4.  Complex 2 lies adjacent to a shoreline park, which has a natural, low gradient beach composed of sand and gravel, with some rubble or small riprap pieces in the nearshore.  The beach is approximately 45 m wide; considerably wider then most of the open water channels between floating homes.  Complex 4 is a smaller, less dense floating home community of five floating homes, situated further from shore (approximately 22 m).  Shoreline habitats had been enhanced with a cobble gravel beach, natural low gradient and a revegetated riparian buffer.

The Gas Works Park reference area was located along the eastern portion of the park, composed of a low gradient beach with a sand gravel substrate.  The beach was relatively long, approximately 60 m extending in to the nearshore at a low gradient.  Snorkel surveys could observe bottom substrates up to 15 m offshore.

Water Quality

Temperature, dissolved oxygen, and turbidity measurements are presented in Table 3.  Surface water temperatures increased steadily over the course of the study period, from a mean of 11.5o C in mid-May, peaking to over 20o C by mid-June.  Temperatures were consistently over 20o from early June to the end of the study period in early July.  Despite the increased temperatures, dissolved oxygen levels remained relatively high ranging from 7.87 to 11.9 milligrams per liter (mg/L).  Turbidities increased over the course of the study period, but remained relatively low.

Fish Presence

Juvenile Salmonids

Only one sighting of juvenile salmonids was made during the overwater and snorkel observation period of mid-May through early-July (Table 4).  A moderately large school (~200 individuals) of juvenile Chinook salmon (Oncorhynchus tshawytscha) was observed during overwater observations at Complex 2, adjacent to Lynn Street Park.  This site was composed of a relatively natural, low gradient beach of about 45 m in length between a houseboat complex to the south and a condominium to the north.  No artificial lighting, except those present at the floating homes and condominiums was present.  Fish were observed to be actively milling and surface feeding approximately 5 to 6 m from shore in the evening at approximately 2115 hours.  Fish were observed for approximately 25 minutes until too dark to see.  Fish did not appear to be associated with the adjacent floating home complex, but appeared to be feeding with a slow net movement offshore in shallow water (0.5 to 1 m).

No juvenile salmonids were observed during any snorkel observation events at floating home complexes.  No juvenile salmonids were observed at Gas Works Park.

Other Fish Species

Several other fish species, including the small forage species threespine stickleback (Gasterosteus aculeatus), smallmouth bass (Micropterus dolomieui), yellow perch (Perca flavescens), sunfish (Lepomis spp.), and prickly sculpin (Cottus asper) were found by overwater and snorkel observations at most of the floating home complexes in the study area (Table 4).  Threespine stickleback were by far the most abundant species, with over 1,700 specimens observed during the study period.  Most were adults between 60 and 70 millimeter (mm) found in small loose schools to moderately large schools of over 100 individuals.  Most occupied the surface layer from a few centimeters beneath the surface to a maximum of about 1 m in depth.  Stickleback occupied nearly all habitats around floating home complexes from nearshore beach environments to surface layers offshore in deeper water.  The species was readily observed by above water monitors relative to snorkelers because of their presence primarily in surface waters.  Many gravid females were observed throughout the study period, with schools of juveniles appearing in late-June.

Juvenile and subadult smallmouth bass were second in abundance with 232 specimens, observed at all but one complex.  At least two cohorts-juvenile fish in the 70 to 80 mm range and larger fish up to 150 mm were observed.  Smallmouth bass generally traveled in small loose schools, particularly smaller fish under 100 mm.  Larger fish were often solitary and associated with structures.  Larger fish were often observed by snorkelers beneath or on the edge of floating home structures and near floatation logs.  No predation was observed during any of the observation periods.

Subadult and adult yellow perch and sunfish (100 to 150 mm) were observed during the mid-June and early-July periods, most often associated with shoreline areas.  These species were not as common as smallmouth bass in the study area. Five prickly sculpin were observed by snorkelers, all on the bottom often associated with larger rocks in shallow water.  This species was relatively rare, but may be under represented in the observational census given their association with bottom structure and body mottling that provides effective camouflage.  Several were captured in minnow traps allowing definitive identification.

Only prickly sculpin and threespine stickleback were captured in minnow traps; no smallmouth bass were captured despite their abundance.

The same non-salmonid species, with the exception of prickly sculpin, were observed at the Gas Works Park reference station.  Proportionally, more threespine stickleback were observed by snorkelers, indicating that more fish occupied areas further from shore.  Only 4 smallmouth bass were observed, all by snorkelers (Table 4).

 

Juvenile Salmon Use in Lake Union in Relation to Floating Home Complexes
Seattle, Washington

DISCUSSION

Juvenile Salmon
Use of Lake Union

Observational data collected in the vicinity of floating homes, as well as other studies conducted in the Lake Washington Ship Canal were assessed to determine the behavior and movements of juvenile salmon in the basin and the possible interactions with floating homes complexes in Lake Union.  Previous juvenile salmonid studies conducted in the Lake Washington basin indicate that the most likely species to rear and feed in Lake Union would be Chinook salmon.  Other salmonid species present in the basin quickly outmigrate through the Ship Canal to Puget Sound (Johnson et al. 2004).

Over the course of the study period, 76 overwater and 21 snorkel observation events were conducted at the nine floating home complexes and the reference station at Gas Works Park.  Of these events, only one observation of juvenile salmonids occurred on the evening of May 30th, representing an observation rate of 1.03 percent.  During this observation, juvenile Chinook salmon were present on one occasion in habitats in the vicinity of floating homes at Complex 2, a shoreline park with a relatively wide natural beach situated between the complex and a condominium.  Fish meandered within the approximate center of the channel formed by the complex and condominium approximately 20 meters from structure edges.  No other observations of juvenile salmon were made by overwater observers, around the floating homes or at Gas Works Park. 

Similarly, snorkelers found no juvenile salmonids in transects run beneath, along, and up to about 6 m beyond the waterward edges of the floating homes.  Along the waterward edges of the complexes, it is unlikely that snorkelers could see any juvenile salmon at depths beyond about 3 meters in open water, but numerous dives were conducted beneath the edges of all floating home complexes.  These data indicate that no juvenile salmonids occupied areas associated with complex edges.  In contrast, numerous observations of the small forage species threespine stickleback and juvenile to subadult smallmouth bass were made throughout the study period.

Recent acoustic tracking studies conducted by USFWS show a pattern of habitat use and behavior in Lake Union and throughout the Ship Canal, characterized by a general offshore use of aquatic habitats (Celedonia et al. 2009).  Tagged Chinook salmon were broadly distributed throughout offshore, deep water areas in all parts of the Ship Canal that were studied.  Overall, data indicate that juvenile Chinook salmon travel relatively quickly within the Ship Canal, and spend a substantial amount of holding time in Lake Union.  Holding times in Lake Union ranged from 1 to 2 days, upwards to 2 weeks; mostly in offshore waters.  USFWS acoustic tagging studies emphasized two areas of Lake Union-Gas Works Park, an area relatively free of overwater structures, and South Lake Union, an area where overwater structures are highly abundant (Celedonia et al. 2009).  Though USFWS did not study any areas near floating home complexes, the south Lake Union site can represent an area occupied by overwater structures similar to our areas studied near floating homes.  Gas Works Park represents an area where both our studies overlapped.  Comparison of results at these areas and others can provide an illustration of the behavior of juvenile salmonids in Lake Union in areas of overwater structures and those areas free of structures.

South Lake Union

As reported, no floating home complexes are present in south Lake Union where the USFWS established an acoustic monitoring site, but the site is an area of dense overwater structures and as such represents an area where juvenile salmonids may interact with nearshore structures (Celedonia et al. 2009).  Acoustic monitoring studies showed that tagged Chinook salmon had movement patterns consistent with holding fish as opposed to active migration.  Fish were often observed on numerous days milling and meandering throughout the area and/or in specific areas of the site.  Fish made considerable use of areas 20 to 25 m offshore of structure edges, but were rarely observed directly beneath structures.  Fish occasionally wandered between habitat structures, particularly where the channel was widest (Figure 2).  Nearshore areas (within 10 m of shore) were rarely used, and only during the early outmigratory period when temperatures were below 18o C.  Habitat selection calculations suggested that fish often selected for offshore, open water areas, although much less regularly than fish at Gas Works Park.  Areas with deeper water columns were also selected by fish at South Lake Union, but again, not as frequently and consistently as fish at Gas Works Park.  Areas less than 2 meters in depth were selected against by most all fish.

USFWS studies also observed apparent influences of artificial lighting on night time movement and habitat use of tagged Chinook salmon at the south Lake Union site, as well as sites at the University Bridge and Ballard Locks (Celedonia et al. 2009).  At the south Lake Union site, there were several instances of fish spending prolonged periods adjacent to artificial lights where light sources were approximately 1 meter above the water surface (Figure 3).  In all cases, the prolonged use was observed near the edge of marina structures.

Gas Works Park

The area off Gas Works Park was used by tagged Chinook salmon as a holding area during the migration process.  Fish appeared most frequently in a very broad offshore area southwest of the park (Figure 4).  Most fish roamed broadly in offshore areas with no apparent general direction of travel.  Fish often left and reentered the site repeatedly, presumably because the tracking area was part of a larger area they were using.  Tagged fish rarely occupied areas closer than approximately 40 m from shore.  In 2007 tagging studies (n= 92 tagged fish off Gas Works), habitat selection calculations showed that all fish selected for offshore open areas.  In 2008 (n=16 tagged fish off Gas Works), preferences were more variable, but fish consistently selected for offshore vegetation-edge areas and selected against inshore open areas and inshore vegetation edge areas.  Fish consistently selected deeper depths occupying areas ranging from 4 to 14 m deep.  Habitat selection calculation indicated that all fish selected against surface waters between 0 and 2 m deep.

Predator Use of Lake Union

Piscivorous Fish

Two predatory fish species known to prey on juvenile salmonids, smallmouth bass and prickly sculpin were observed by above water observers and snorkelers during the study period.  Yellow perch, another freshwater resident species, was also observed in the study area.  This species, while sometimes piscivorous, is not known as a predator of juvenile salmon, but does reach a size where they may opportunistically prey upon them.  Overwater and snorkel results showed that the majority of bass and perch were juveniles under 100 mm in length and would not likely pose a predatory risk to juvenile salmon.  However, 47 smallmouth bass and 9 yellow perch were estimated to be between 100 and 150 mm, which is of suitable size to prey on juvenile salmonids.  In addition, 5 adult prickly sculpin were also observed in the study area and may present a predatory risk to juvenile salmon.  As reported, the number of sculpin observed in the study area may be underestimated because of difficulties in spotting the fish.  No large adult smallmouth bass were observed under floating home structures.

The number of potentially piscivorous fish in the vicinity of the floating home complexes greatly exceeded the number observed at the Gas Works Park reference area, perhaps due to the underwater structures and greater habitat complexity at the floating homes.  These results are not surprising since smallmouth bass, and to a lesser extent, yellow perch are known to occupy areas of dense underwater structures (Celedonia et al. 2008; Wydoski and Whitney 2003).

Tabor et al. (2009) tagged small- and largemouth bass (S. micropterus), and northern pike minnow (Ptychocheilus oregonensis) with acoustic tags and tracked them within the Lake Washington Ship Canal in order to determine their potential relationship to juvenile salmon survival within the basin.  The study concentrated on two areas-Gas Works Park, and an area around the I-5 and University bridges.  Smallmouth bass inhabited a wide range of depths between 0 and 10 m with the most used depth interval of 2 to 4 m.  The bridge habitats were used to a greater extent than Gas Works Park.  Smallmouth inhabited deeper areas at the I-5/University Bridge due largely to the use of the deep support structures of the University Bridge.  These structures appeared to allow smallmouth to inhabitant offshore, deeper waters and probably have a greater overlap with juvenile salmonids.  These fish had a strong selection for sparse vegetation and vegetated edge habitats.  Most smallmouth also showed some positive selection for nearshore overwater structures, which was particularly evident in bass between 150 and 349 mm.

At Gas Works Park, the natural beach shoreline on the east side of the site was selected by smallmouth bass over the bulkhead and riprap, though some use of riprap shorelines also occurred.  Gravel and sand was the most used substrate type (Tabor et al. 2009).

Smallmouth bass displayed a strong seasonal migration presence between the Ship Canal and Lake Washington, moving to the canal in early-March to mid-April and moving back to Lake Washington from June to October.  All tagged smallmouth in Portage Bay displayed these movements, while 73 percent from Lake Union moved back to the larger lake.  Tabor et al. (2009) surmised that these movements corresponded to initial warming in the spring and may be directly related to spawning behavior, rather than the following juvenile salmonids, since the salmonid outmigratory period occurs later in the spring in May and June (Tabor et al. 2009).

Of the northern pike minnow that were tagged, most left the Ship Canal and moved to Lake Washington shortly after release, though some movement in and out of the Ship Canal occurred.  Results suggest that the species spends most of its time in Lake Washington and overall does not appear to be a major predator of juvenile salmonids in Lake Union (Tabor et al. 2009).

Piscivorous Birds

During the 97 overwater and snorkel observations events conducted between mid-May and early-July, no piscivorous birds were observed in the vicinity of the floating home communities.  The most commonly known waterfowl species that prey on salmonids and other fish species within the Puget Sound area, such as western grebe (Aechmophorus occidentalis), red-breasted merganser (Mergus serrator), double-breasted cormorant (Phalacrocorax auritus), and great blue heron (Ardea herodias) were not found during the study period in the vicinity of floating home complexes.  The most common species observed were mallard duck (Anas platyrhynchos) and Canada geese (Branta canadensis); several nesting pairs of mallards were observed with young.

Interactions Between Juvenile Salmonids and Floating Homes

Observational data collected during the 2009 outmigratory period showed very limited interaction between juvenile Chinook salmon and floating home structures in Lake Union and other portions of the Ship Canal.  The only observed usage of habitats near floating homes was in a wide area of natural beach (approximately 45 m) in a waterfront park between a floating home complex and an overwater condominium.  These fish were observed between the complex and condominium, approximately 20 m between the overwater structures and 3 to 5 m offshore.  Notably, no observations of juvenile Chinook were made at Complex 4, which was specifically designed to occupy waters farther away from shore to accommodate the shoreline orientation of juvenile salmonids observed in Lake Washington (Tabor and Piaskowski 2002; Tabor et al. 2004; Tabor et al. 2006; Sergeant and Beauchamp 2006).  A semi-natural low gradient beach environment with gravel substrates was also created at this complex.  Ironically, the highest incidence of piscivorous fish (smallmouth bass and yellow perch) was observed at this complex relative to the others.  Most of these potential piscivores were juveniles, but some subadult bass and adult perch were observed that would likely be capable of preying on juvenile salmon. 

Our findings were consistent with extensive acoustic tagging studies conducted within the Lake Washington Ship Canal in 2007 and 2008 (Celedonia et al. 2009).  In South Lake Union, juvenile salmon chose to rear and migrate most often in offshore waters, usually no closer than 20 to 25 m from overwater structures, though extensive periods were often spent along this outer edge of the structures.  Even in the absence of overwater structures near Gas Works Park, juvenile Chinook were most often observed in the acoustic tagging studies to be offshore.  During nighttime periods, there was some evidence that bright artificial lighting attracted juvenile Chinook to areas closer to overwater structures.

This behavior is notably different than the known early migratory paradigm of juvenile Chinook in lake, estuary, and marine areas throughout the Puget Sound basin.  Numerous studies, including Lake Washington but not in Lake Union, show that in areas studied, juvenile Chinook prefer low gradient, natural beaches in shallow surface waters close to shore (Tabor and Piaskowski 2002; Tabor et al. 2004; Tabor et al. 2006; Brennan et al. 2004; Simenstad et al. 1982).  Because of this nearshore orientation, numerous studies have investigated the potential for migratory delays and other effects caused by nearshore overwater structures in freshwater, estuaries, and the marine environment (Nightingale and Simenstad 2001; Carrasquero 2001; Toft et al. 2007). 

The prevalent behavior of juvenile salmon in Lake Union is a general selection against nearshore environments, perhaps due to the very high level of nearshore development already present in the lake and the occupation of those habitats by piscivorous fish.  Celedonia et al. (2009) surmised that differences in predator populations between Lake Washington and the Lake Washington Ship Canal may partially explain the differences in Chinook spatial differences between the two water bodies.  The authors had the general sense that predator populations were lower in the Ship Canal than in Lake Washington and the principal predators present were structure oriented.  Tabor et al. (2009) found that smallmouth bass in the Ship Canal were largely nearshore and structure oriented.  Fresh et al. (2001) found high abundance of smallmouth bass beneath and near (within 2 m) overwater structures in Lake Washington.  In contrast, offshore, open water predators such as cutthroat trout (O. clarkii) and northern pikeminnow are believed to be fewer in number in Lake Union relative to Lake Washington (Celedonia et al. 2009; Tabor et al. 2009).  Snorkel observations in our study found very few larger smallmouth bass near and under the floating home complexes, though bottom habitats could not be observed at most of the outer edges, because of water depth.

In tagging studies, juvenile Chinook appeared to congregate in areas offshore of overwater structures and rarely ventured near or beneath the structures.  Juvenile salmon with an innate ability to avoid such structures may exhibit higher levels of survival.  Similarly, if fewer predators are present in offshore areas, increased levels of survival would likely be gained by those fish occupying offshore habitats.  Further evidence for an offshore preference can be seen in zooplankton sampling data collected in the Ship Canal by Celedonia et al. (2009) in 2008.  This study found higher abundances of zooplankton, which are the preferred prey of juvenile salmon (Koehler et al. 2006), in offshore areas than in the nearshore. 

 

Juvenile Salmon Use in Lake Union in Relation to Floating Home Complexes
Seattle, Washington

SUMMARY AND CONCLUSIONS

The evidence presented in this and other studies conducted in Lake Union and the Ship Canal indicates that juvenile salmonids do not readily use nearshore aquatic habitats in close proximity to floating home complexes and are not affected by such complexes.  Rather, these fish tend to occupy offshore habitats of Lake Union during their outmigration from the Lake Washington basin to Puget Sound. 

Structured, site-specific observations made directly from floating home complexes and an assessment of recent juvenile salmonid studies in the basin lead to the following conclusions:

• Very little interaction of juvenile salmon with floating homes complexes is evident in Lake Union and nearby areas of the Lake Washington Ship Canal.  Only 1 of 97 observational events revealed the presence of juvenile Chinook salmon proximal to floating homes. 
• Nearshore habitats in the vicinity of the floating home complexes were used by several fish species, most abundant were the small forage species threespine stickleback.
• Several other warm freshwater resident species were present in low to moderate numbers including smallmouth bass, yellow perch, prickly sculpin, and sunfish.
• Most warm water resident species near floating homes were juveniles, though some larger subadult and adult fish that are potential predators on juvenile salmon were found.
• No large adult smallmouth bass, northern pikeminnow, or other predatory fish were observed during observation events near and beneath the structures of floating home complexes, though other studies show a near structure orientation for smallmouth bass in the Ship Canal.
• Observational monitoring indicates that piscivorous birds do not frequent floating home complexes.  Several pairs of mallard duck and Canada geese used nearby areas for nesting.
• Acoustic monitoring studies conducted in 2007 and 2008 by USFWS revealed that Lake Union is used by juvenile Chinook salmon as an extended rearing area as well as a migratory corridor.  Residence times of tagged fish ranged from a few days to up to 2 weeks.  The outmigratory period ranged from mid-May through early-July.
• Acoustic monitoring studies also show that juvenile Chinook tend to occupy offshore habitats and largely select against nearshore habitats in South Lake Union and Gas Works Park, consistent with the general lack of salmonids observed at floating home stations. 
• Studies show that juvenile Chinook do tend to congregate between 20 and 25 m off the edge of overwater structures, but do not commonly approach closer or travel beneath structures.  Movements between structures occur most often when widely spaced, consistent with our one observation of Chinook, which occupied a relatively wide beach adjacent to a floating home complex.
• A number of physical and biotic factors may be responsible for the juvenile salmonid preference for offshore habitats including prey abundance, predator abundance and spatial location, and the high level of nearshore development already present in the lake.

 

Juvenile Salmon Use in Lake Union in Relation to Floating Home Complexes
Seattle, Washington

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