Fish kills from the harmful alga Heterosigma akashiwo

in Puget Sound: Recent blooms and review

November 14, 2007

Prepared by:

J.E. Jack Rensel Ph.D.   

Rensel Associates Aquatic Sciences

Arlington, Washington, USA 98223

jackrensel@att.net

A Technical Report sponsored in part by:

National Oceanic and Atmospheric Administration

Center for Sponsored Coastal Ocean Research (CSCOR)

Prepared in cooperation with:

American Gold Seafoods, LLC

               

Heterosigma bloom (to right) passing southward in Bellingham Channel, North Puget Sound past

Deepwater Bay, Site 3 fish farm.  Note white spot in each pen from airlift pumps in operation.  
Fish kills from Heterosigma akashiwo in Puget Sound: recent blooms and review                 ii

Table of Contents

List of Figures............................................................................................................................. iii

List of Tables............................................................................................................................... v

Overview.....................................................................................................................................1

Introduction................................................................................................................................4

Background.................................................................................................................................4

Mechanisms of mortality.................................................................................................. 8

Fish Farms as Possible Causes of Blooms .......................................................................11

Prior Blooms, Initiation Sites and Transport...................................................................14

North Puget Sound ......................................................................................................... 15

Central Puget Sound ....................................................................................................... 17

June 2006 Bloom Chronology of Events...................................................................................17

North Puget Sound and Cypress Island ..........................................................................17

Port Angeles Harbor, Western Strait of Juan de Fuca and Pacific Ocean ......................20

Other Areas..................................................................................................................... 20

Factors Associated with Late June 2006 Heterosigma Bloom........................................21

Snowpack and River Discharge....................................................................................... 21

Water Temperature........................................................................................................ 22

Air Temperature.............................................................................................................. 23

Tides................................................................................................................................24

Winds.............................................................................................................................. 25

Time of Year.................................................................................................................... 25

Water Quality Sampling.................................................................................................. 26

August 2006 Bloom Chronology of Events...............................................................................30

Management of Farmed Fish ................................................................................................... 33

Cell Shapes, Sizes and Motility Relative to Fish Mortality........................................................35

Recommendations.................................................................................................................... 35

Wild Fish.......................................................................................................................... 36

Etiology of Fish Mortality................................................................................................ 36

Modeling:........................................................................................................................ 37

Response to Blooms........................................................................................................ 37

Remote Sensing Monitoring ........................................................................................... 37

Early warning of all HABs................................................................................................ 38
Fish kills from Heterosigma akashiwo in Puget Sound: recent blooms and review                 iii

Fish Farm Bloom Management....................................................................................... 39

Literature Cited......................................................................................................................... 40

Appendix A.   Report on June‐July 2006 Heterosigma Bloom ‐ Cypress Island and Port Angeles

..................................................................................................................................................44

Appendix B.  Harmful Algal Blooms and Finfish Resources in Puget Sound ............................50

List of Figures

Figure 1.  Mean and SD of ten separate DIN measurements at Puget Sound fish farms

performed in September during the 1980s and 1990s (Rensel 1991 and unpublished

manuscript)............................................................................................................................... 12

Figure 2.  Calculated dissolved nitrogen loading from oceanic inflow versus selected rivers,

treatment plants and different levels of fish farm development (13 to 100 farms) from WDF

1991.  Presently there are eight farms in Puget Sound but two are smolt production sites

with smaller fish only................................................................................................................ 13

Figure 3.  Vicinity map of Puget Sound showing subareas and place names referred to in text.

..................................................................................................................................................15

Figure 4. Vicinity map of probable or possible bloom initiation areas for Heterosigma in

North and Central Puget Sound in relation to clusters of fish farms.  (Port Angeles Harbor and

north Skagit Bay net pens not shown). ....................................................................................16

Figure 5.  Aerial survey results of June 27th 2006 conducted by Kevin Bright, AGS Biologist.   

Note that the survey was limited to the areas within the survey circle and part of the Eastern

Strait of Juan de Fuca, i.e., no observations were recorded from further North. ...................18

Figure 6.  General vicinity of the bloom in the early stages in late June 2006 in red, inferred

from survey vessel and aerial survey maps..............................................................................19

Figure 7.  General vicinity of the bloom during later stages as it flowed with surface waters

out the Strait of Juan de Fuca and into the Pacific Ocean. ......................................................19

Figure 8.  Deepwater Bay at Cypress Island, actually more of a bight than a bay, is subject to

very strong tidal currents and has large natural populations of invertebrates, fish and

seabirds (Rensel and Forster 2007).......................................................................................... 20

Figure 9.  Recent discharge flow data from the Fraser River at Hope, B.C. versus 2006 flow in

red.............................................................................................................................................21

Figure 10.  Mean high and low air temperatures compared to hourly air temperatures at

Seattle‐Tacoma Airport during June and early July 2006.........................................................23

Figure 11. Mean high and low air temperatures compared to hourly air temperatures at

Bellingham airport during June and early July 2006. ...............................................................23

Figure 12.  Mean high and low air temperatures compared to hourly air emperatures at Port

Angeles Airport during June and early July 2006. ....................................................................23
Fish kills from Heterosigma akashiwo in Puget Sound: recent blooms and review                 iv

Figure 13.  Mean high daily precipitation for period of record compared to hourly

precipitation at Seattle‐Tacoma Airport during the subject period. .......................................24

Figure 14.  Eagle Harbor, Cypress Island tidal plots from June 25th to July 8th, 2006. .............24

Figure 15.  Algal density versus depth from vertical profiles at the farm site in Deepwater

Bay. ...........................................................................................................................................26

Figure 16.  Algal density versus depth from vertical profiles near Cypress Head about 2 ......27

Figure 17.  Algal density versus depth from vertical profiles about 2 km south of the farm site

near the entry to Bellingham Channel and nearer Anacortes, Washington............................27

Figure 18. Algal density versus depth for vertical profiles near the north end of Bellingham

Channel about 10 km north of the farm site............................................................................27

Figure 19.  Salinity versus depth for 29 June 2006 near Cypress Island at fish farm sites 1 and

3 (both in Deepwater Bay) compared to conditions near the north end of the island by the

Cone Islands.............................................................................................................................. 28

Figure 20.  Nitrate, in vivo chlorophyll a, and water temperature for Deepwater Bay on the

west side of net pen Site 3 on 29 June 2006............................................................................28

Figure 21.  Vertical profiles of salinity, chlorophyll, temperature and density from station P26

(see Fig. 22) the eastern Strait of Juan de Fuca, west of Whidbey Island and south of

Deception pass from the morning of June 28, 2006. ...............................................................29

Figure 22.  University of Washington research vessel sampling locations.  Data from stations

P21 though P26 from June 28th, 2006 were inspected for this review....................................30

Figure 23.  Aerial survey results of August 3, 2006 from Central Puget Sound conducted by

Kevin Bright, AGS biologist. ...................................................................................................... 31

Figure 24 .  In vivo chlorophyll a profiles from 3 August 2006 in central Puget Sound. ..........32

Figure 25.  Sigma T (density) of same stations shown in Figure 23. ........................................32

Figure 26.  Dissolved inorganic nitrogen (DIN) mean and standard deviation and components

of nitrate, nitrite and ammonium from 5 locations shown in Figure 23 and 24, August 2006.

..................................................................................................................................................32

Figure 27.  Aerial photograph of Heterosigma bloom front in central Puget Sound near Elliot

Bay (Seattle).............................................................................................................................. 33

Figure 28.   Upwelling water inside a pen a minute after a compressor and airlift assembly

was turned on........................................................................................................................... 34

Figure 29.  Strands of distinctive purplish‐red Heterosigma akashiwo as visible from a small

plane while flying over backwaters on the east side of Guemes Island, Puget Sound (Photo by

K. Bright). ..................................................................................................................................35

Figure 30.  Atlantic salmon with secondary lamellae of gills visible after having been killed

during a Heterosigma bloom.................................................................................................... 37

Figure 31.  Greenish‐yellow colored bloom of Heterosigma seen in June 2006 in Deepwater

Bay and throughout the region. These blooms are sometimes green colored when viewed

from a boat, but curiously appear a shade of purple‐red when viewed from airplanes.........38
Fish kills from Heterosigma akashiwo in Puget Sound: recent blooms and review                 v

List of Tables

Table 1.  Summary of documented Heterosigma akashiwo blooms in Puget Sound since

1976............................................................................................................................................5

    

List of Acronyms and abbreviations

CPS Central Puget Sound

Heterosigma Heterosigma akashiwo

NPS North Puget Sound

PNW Pacific Northwest (of the contiguous 48 U.S. states) + British Columbia

SST Sea surface temperature (satellite derived)

Strait Strait of Juan de Fuca

the Sound Puget Sound

    

                                                 Acknowledgements

The National Oceanic and Atmospheric Administration, Center for Sponsored Coastal Ocean

Research (CSCOR) provided support for some of the activities described herein and together

with the Coastal Ocean Institute, Woods Hole Oceanographic Institution, helped initiate a

listserve internet communication and notification service known as SoundHAB.  Cover

picture and some other photos by Kevin Bright, American Gold Seafood, LLC.   Staff and

managers of American Gold Seafood LLC provided data and advice as well as in‐kind support.

Laboratory analysis by Kelley Bright, Shannon Point Marine Center and Kathy Krogslund,

University of Washington Routine Chemistry Laboratory.  Rita Horner, University of

Washington, reviewed a final draft.    

The opinions and interpretations expressed herein are those of the author and do not

necessarily reflect those of cooperating partners, agencies or reviewers.  
Overview

Heterosigma akashiwo (Raphidophyceae) is a widely‐distributed, fish‐killing

phytoplankton species that may under some conditions kill or injure farmed and wild fish

and possibly other aquatic organisms in Puget Sound, Washington state (“the sound”) and

several other locations around the world.   

This report briefly reviews background information on this alga, conditions that allow it

to bloom, how it may kill fish and occurrence of past blooms in Puget Sound. The report also

summarizes a small portion of the extensive literature regarding nutrients and

phytoplankton in Puget Sound.  Within this tidally‐active region that differs profoundly from

most other coastal regions of the U.S., light (or advection and grazing), not nutrient supply,

controls phytoplankton production in all areas except for some distal, poorly flushed bays or

fjords.  Another exception is for short periods near river mouths when freshets create

vertical stratification that limits mixing.   

It is evident that fish farms do not cause Heterosigma akashiwo blooms in marine waters

of Western Washington and the data suggests it is unlikely they exacerbate blooms.  Rather,

there is substantial evidence to the contrary, that Heterosigma blooms are large‐scale events

that initiate from remote cyst germination and are advected by tides, winds and estuarine

circulation into other areas including those with fish farms. Heterosigma blooms have

occurred in Pacific Northwest waters well before net‐pen fish farms were installed in Puget

Sound.  The few fish farms currently operating are clustered in North Puget Sound, Central

Puget Sound and Port Angeles Harbor in the Strait of Juan de Fuca.  All commercial fish farms

are required to locate in areas of naturally high dissolved inorganic nitrogen levels (from

Pacific Ocean upwelling) but in this area the industry is relatively small and the effluent

accounts for less than 0.1% of the  natural background flux of nitrogen from the Pacific

Ocean.

Germination of Heterosigma cysts is known to be successful above the 15°C bottom

temperature threshold in many locations worldwide. The conditions for development and

spread after cyst germination in Puget Sound are not exactly known, but clearly are linked to

warm, sunny weather in the May through September period.  In North Puget Sound the

linkage is also strongly related to annual peak discharge of the Fraser River and resulting

vertical stratification of the water column. Fish growers use this knowledge to assess risk and

elevate their monitoring when these factors coincide.  Data from the large‐scale blooms of

2006 and 2007 presented herein to add to the 17 year record of monitoring and

demonstrate the importance of the above cited linkages.  Blooms can occur in late spring to

early fall and biannual blooms have occurred in some previous years.  

In North Puget Sound, the brackish Fraser River plume creates suitable conditions for

blooms to form near the U.S.‐B.C. border or in shallow bays from Bellingham to Samish Bays.  

These blooms are then advected westerly and southwards into the main channels of North

Puget Sound, particularly those associated with the Fraser River plume.  Advection is

provided by estuarine circulation (the well‐known surface outflow toward the Pacific Ocean
Fish kills from Heterosigma akashiwo in Puget Sound: recent blooms and review                 2

via the Strait of Juan de Fuca) that often occurs in fortnightly pulses related to tidal cycle

variation as well as fair weather, northerly winds.   

On its way through North Puget Sound in late June 2006 a Heterosigma bloom killed two

million dollars worth of farmed salmon that were being reared in this high tidal energy

environment.  Another subsequent bloom occurred in May 2007 that followed nearly the

same pattern of events.  Timing of Heterosigma blooms in Northern Puget Sound does not

repeat annually as it has in some other areas of the world but is typically a late spring event

coinciding with periods of hot weather and basin‐wide vertical stratification (from Fraser

River discharge).  As an exception to the rule, a massive late summer or early fall bloom has

occurred in the past in this area (e.g., 1989) when river flows are at annual minimums and in

that case neap tides and hot weather appear to be the strongest correlating factors.

A separate bloom occurred in early August 2006 in central Puget Sound that appeared to

start in backwaters and spread over much of the basin.  Unlike the North Puget Sound

blooms of 2006, this bloom was associated with only with highly elevated surface water

temperatures as has been observed in the past in this subarea.  Blooms in this area are often

not unialgal as they are in the late spring North Puget Sound events, similar to a seasonal

pattern observed in Korean coastal waters.  No fish in the fish farms were killed as the bulk

of the bloom remained in the main (central) basin from Seattle northward or backwaters of

Kitsap County (west parts of the central basin) and the fish farm area, in a well‐mixed

channel (Rich Passage) and associated bight (Clam Bay) remained remarkably free of

Heterosigma cells.  Relatively high levels of ammonium were found throughout the sampled

surface waters in other areas, possibly from bloom senescence and decay.  A subsequent

bloom recurred in August 2007 of nearly the same timing and spatial distribution as the 2006

bloom but flood tide waters from the main basin brought increased numbers of Heterosigma

cells into the fish farming region and about 6% of the farmed fish were lost at commercial

fish farms.   

From a broader ecosystem and societal viewpoint, Heterosigma blooms in Puget Sound

may be doing much more damage than killing farmed fish.  Wild salmon and other fish as

well as invertebrates and plankton may be adversely affected by sublethal effects or killed.  

The most vulnerable are juvenile fishes, invertebrates and plankton that are restricted to the

surface and near‐surface waters where the blooms persist.  Near‐surface migrating fishes

such as sockeye salmon that pass through the surface waters of the area in potential bloom

periods may also be affected.  The extent of past mortality is unknown as these fish typically

sink and are rapidly consumed by predators or are transported by the typically strong tides

of this region.  It is likely that some Central Puget Sound blooms are restricted to the

immediate surface waters and are not as harmful to wild fish.  However, the June 2006

North Puget Sound bloom extended to 30m depth or more and the subsequent August 2006

bloom was relatively deep too.  These depths include those that many larval, juvenile and

other fishes typically inhabit and these fishes are not behaviorally or physiologically capable

of swimming to the depths necessary to escape the blooms. North Puget Sound areas are

more susceptible as the surface (“mixed”) brackish layer is often much more extensive and

deeper than in central Puget Sound, as a result of massive Fraser River discharge. The author

and others including agency staff have observed dead wild fish during previous Heterosigma
Fish kills from Heterosigma akashiwo in Puget Sound: recent blooms and review                 3

blooms in North Puget Sound and the Strait of Juan de Fuca while conducting their surveys.

Most interestingly, a pen of 400 subadult sablefish (aka black cod, Anoplopoma fimbria) held

at NOAA’s Manchester Laboratory nearby were not killed by the August 2007 bloom while all

Atlantic salmon (Salmo salar) held in a nearby research pen were lost.  These observations

provide yet another tantalizing but inexplicable clue about the etiology of fish mortality from

Heterosigma exposure.  

Fish farmers have a working system that adequately allows them to detect, manage and

mitigate the blooms to some extent.  But if bloom frequency increased, due to climate shift

or oscillation, the periodic losses presently incurred could become a major burden.  For the

broader society, the question of wild fish mortality remains a potentially major problem (to

be added to other, larger problems such as global warming, ocean acidification, human

overpopulation, etc.).  Ultimately, biology is a much more sensitive indicator of the diversity

and stability of our coastal oceans than water quality and the occurrence of species like

Heterosigma could well be an indicator of incipient and undesirable change. Such change is

certainly possible if climate change occurs due to global warming or even known regional

oscillation of weather patterns become more pronounced.

The report concludes with recommendations to help understand these blooms, their

nature and the extent of their effects.   
Fish kills from Heterosigma akashiwo in Puget Sound: recent blooms and review                 4

Introduction

The initial purpose of this report was to document what was learned about bloom

dynamics during the 2006 Heterosigma blooms in Puget Sound in order to further our

understanding of the factors responsible for bloom initiation and spread.  While interacting

with other harmful algal bloom specialists and reviewing a technical report (GEOHAB 2006)

that was inaccurate regarding this alga’s relationship with fish farms, at least in the Pacific

Northwest US, it became clear to me that there was a need to more fully document the

history of previous Heterosigma blooms in Puget Sound as well as the basic nature of

nutrients and phytoplankton dynamics in the area.  Additional blooms occurred in 2007 and

general observations from those events were included as they mirrored events of the prior

year.  The report concludes with recommendations for further improvements of monitoring,

management, research and modeling of these fish killing algae blooms.    

Background

In this background section I discuss a few literature references of local or wider

importance necessary to understand Heterosigma bloom dynamics in Puget Sound.  There is

a huge volume of quality literature dealing with this species; no attempt is made to review it

all.   Smayda (1998) provides an excellent review of the ecophysiology of the species.  

The first recorded Heterosigma bloom in Puget Sound was at the Lummi Indian Tribal

mariculture “Seapond” in Lummi Bay in 1976 (Gaines and Taylor 1986, Table 1).  This ~750

acre pond was created in an intertidal embayment of North Puget Sound and its substantial

environmental impact (seawater heating, eel grass community and associated invertebrate

extirpation) was studied and reported by Rensel (1972).   At that time, only a few small net

pen farms (both public and private) that reared Pacific salmon were present in other parts of

Puget Sound.  In south Puget Sound I reported on a fish kill at the Weyerhaeuser Co. net pen

project in Henderson Inlet due to the dinoflagellate Ceratium fusus (Rensel and Prentice

1979) but Heterosigma was not seen in that 1974‐1976 project.  Later the pens were

removed and no commercial fish culture is present in marine waters of South Puget Sound.  

Widespread blooms of Heterosigma in Puget Sound or nearby waters recur about every

5 to 10 years or more on average but the time series (Table 1) reveals an interesting

temporal distribution.  Truly massive blooms occurred in sequential years (e.g., 1989‐1990

and 2006‐2007) separated by a 16 year interval when several blooms occurred, but were

limited in distribution or intensity.  Fish losses occurred in 1997, but mostly that involved

escaped fish as a large set of pens in central Puget Sound was damaged during a towing

operation to avoid the threat of a developing bloom.  Although cells were often seen in

water samples during late spring or early summer in non bloom years, large‐scale blooms

apparently could not develop because of weather or physical forcing conditions.  Fish

farmers routinely monitored the water from spring through fall, and sometimes it looked as

if a bloom was commencing, but a Pacific cold front and/or some other physical factor would

act to attenuate the bloom.  With many years of experience, the fish farmers have  
Fish kills from Heterosigma akashiwo in Puget Sound: recent blooms and review                 5

Table 1.  Summary of documented Heterosigma akashiwo blooms in Puget Sound since 1976.   

            Location Date      Fish Species           Estimate Loss Source‐Comments

Lummi (Bay) Seapond, N.E. of

Bellingham, Wa. Large dike area

with small fish enclosure.  

1/

1976, date

uncertain

Cultured fish,

ostensibly native

coho salmon  

Unknown.  For area

description see Rensel

(1972)

Jefferson 1976,

Gaines and Taylor

1986

Cypress Island, North Puget

Sound, 4 commercial fish farms 2/ September 1989 Atlantic salmon 364,000 subadult fish First bloom to kill net

pen farmed fish

NOAA Manchester Research

Laboratory, Central Puget Sound

Peaked near

4 July 1990

Chinook, sockeye,

coho, Atlantic

salmon

1,910

Rensel/Horner

collected cell & CTD  

vertical profile data

Clam Bay, Orchard Rocks, Fort

Ward, three separate fish farms

in Central Puget Sound

Peak near 4 July

1990 Atlantic salmon 649,544 subadult fish Same as above &

Rensel 1995

Western Central Puget Sound in

Sinclair Inlet, Port Orchard &

Brownsville

Mid July 2003 None affected None known Rensel 1995

Budd Inlet, Olympia, South Puget

Sound   Summer 1993 Unknown   Unknown Eisner and Newton1997

Northern Case Inlet in Southern

Puget Sound September 1994

Coho, chum &

chinook salmon ,

marine fish

35 collected, many

more observed dead

Hershberger et al.

1997

NOAA Manchester Research

Laboratory, Central Puget Sound July 1997 Coho, chinook andsockeye salmon

737 (100% mortality

of coho)

Connell and Jacobs

1999

Clam Bay, Orchard Rocks, Fort

Ward, three separate fish farms

in Central Puget Sound

July 1997 Atlantic salmon

401,639 (not  killed,

accidental release

while towing pens to

avoid a bloom)

Connell and Jacobs

1999

Port Angeles Harbor August 1997 Atlantic salmon 62,000 Associated with above

bloom?

Hood Canal Mid September

2000

Low D.O. thought

to  be responsible Not applicable   Intense surface bloomConnell et al. 2000

North Puget Sound and Strait of

Juan de Fuca  

Late June &

early July 2006  

Atlantic salmon

Wild salmon

smolts and forage

fish 3/

Cypress Island ~140K

adult & sub adult fish.

Port Angeles 128,000

sub adult fish

NPS‐Strait bloom

occurred separate

timing from the CPS

bloom 4/   

Central Puget Sound early August

2006 None observed

The bloom generally did not affect the fish

farming area of CPS but was almost

everywhere else.  (this report)

North Puget Sound   May  2007 Atlantic salmon

Cypress Island ~4,000

adults and subadults

fish lost.   

NPS‐Strait bloom

occurred separate

timing from the CPS

bloom and repeated

timing of 2006 Central Puget Sound August

2007

Atlantic

salmon

~ 60,000

subadult fish lost  

1/ A bloom occurred in 1976 at the Manchester NOAA laboratory too, specimen collected by Lee Harrell, identified by

Richard Norris, Univ. of Wa. Botany Dept. (R. Horner, pers. comm.)

2/ Tailfin Inc. fish farm subsequently closed as a result of the bloom.  It was near Eagle Harbor and northeast Cypress Island

where higher cell concentrations were common as it was nearer the Fraser River plume origin. The entire Strait of Georgia

was involved with this same bloom (Taylor and Harrison 1993) that flowed southward into North Puget Sound.

3/ Kevin Bright, pers. comm.   He observed these stressed fish in North Puget Sound during the bloom.  

4/ ~ $2 million in total loss for the entire 2006 NPS and CPS blooms (J. Bielka, AGS manager, Port Angeles WA).
Fish kills from Heterosigma akashiwo in Puget Sound: recent blooms and review                 6

developed routine methods to detect, react and minimize damage from the blooms,

although the risk of significant losses still is possible.   

Much of the initial work on Heterosigma bloom dynamics in the Pacific Northwest was

done by Professor Max Taylor, his students and colleagues in British Columbia where fish‐

killing blooms were noticed before those in Puget Sound.  Taylor and Haigh (1993) reported

factors associated with prior blooms to the north in British Columbia where fish kills first

occurred at fish farms in 1986.  Factors such as circulation and vertical stratification are

different than Puget Sound and variable among subareas. Taylor and Harrison (2002) report:   

“Cells abruptly appear in the water when the temperature reaches 15°C (usually in June),

apparently due to excystment from shallow sediments. Blooms are most extensive in the

Strait of Georgia during summers with strong, shallow stratification due to high runoff from

the Fraser River, in turn resulting from a higher than normal snow‐pack the previous winter.

A drop in salinity in English Bay to 15‰ coincides with 15°C in severe bloom years in this

area. A prerequisite to strong blooms is that the spring diatom bloom must be over (usually

by late May) and the surface water depleted of nitrate”.   

       In the same work they report: “Blooms of Heterosigma in the Strait of Georgia are

apparently strongly linked to river runoff and stratification...”

As will be shown later, a similar conceptual model applies to North Puget Sound but the

river plume is more deeply mixed which in some cases results in a deeper bloom.  Strong

river‐induced vertical stratification is sometimes not important to midsummer or later

Central Puget Sound blooms as discussed herein.

Recent studies from Delaware on the U.S. east coast (D.A. Hutchins pers. comm., K.J.

Portune, MS in development) suggests that Heterosigma cysts germinate continually

throughout the spring and early summer and only “appear” in the water column in sufficient

numbers to be visible to microscopic examination when conditions are right for bloom

initiation.  In many cases authors have pinpointed the threshold of excystment at 15°C, a

subsurface temperature that rarely if ever occurs at fish farm sites in Puget Sound but is

probable at tentative bloom initiation sites identified herein.  Water temperatures are

important for bloom initiation but established blooms are often transported seaward

through the fast moving channels of North Puget Sound where water temperatures are

almost always less than 15°C.   

Heterosigma cells in the water column have been observed in various locations of the

Pacific Northwest almost every year since at least the 1960s (Horner et al. 1997) which

predates salmon or marine fish aquaculture anywhere in the Pacific Northwest.  During most

years, cell numbers are either very low (< 103 cells/L) or too low to cause fish mortality (ca. <

5 to 7.5 x 105 cells/L).  In British Columbia there have been reports of non harmful blooms of

Heterosigma too, but to the best of my knowledge, all blooms that exceed the above

benchmark at Puget Sound fish farms have resulted in at least some salmon mortality.  As

mentioned above, basin‐wide blooms that occur over much of Puget Sound and adjacent

waters are relatively uncommon, occurring approximately every 5 to 10 years or more in

1989, 1990, 2006 and 2007.  Many other blooms began (Table 1) but in most cases weather

was not suitable and the blooms were restricted in time and space.      
Fish kills from Heterosigma akashiwo in Puget Sound: recent blooms and review                 7

Monetary losses for fish farmers can be large during the major blooms, but not

disastrously so and growers have insurance that covers losses beyond some deductable

threshold of loss such as 10%.  Smaller scale blooms occurred in other years (Table 1) and

probably other years that were not observed as fish farms either do not exist or are not

permitted in other areas.  Farms were once installed at Hood Head in North Hood Canal but

the area is subject to occurrence of Chaetoceros spp. (subgenus Phaeoceros, Rensel et al.

1989) that fish kills both farmed and probably wild fish (Rensel Associates and PTI

Environmental Services 1991) and the farm was removed many years ago on this account.   

These are cryptic events, likely not to be noticed as the diatom kills fish at very low, non‐

bloom concentrations and cells may be distributed to 50 m or more depth.  

Prior to the 1960s Heterosigma blooms may have occurred but were not documented or

possibly even noticed by biologists as there was no routine monitoring of phytoplankton or

marine fish farming and as stated above, wild fish kills are difficult to detect in most

temperate waters.  A more extensive review of bloom history in Puget Sound is provided in

Anderson et al. (2001, available online) for Heterosigma blooms in Puget Sound.  

Rensel (1995) advanced a conceptual model of bloom development for Puget Sound and

adjacent waters (after major blooms in 1990 and 1991) that factored in weather, tidal

conditions and timing relative to snow pack melt and river flow (see appendix A).  This model

was a local adaptation of the work of Max Taylor and his colleagues in British Columbia

mentioned above.  The importance of temperature or salinity‐induced vertical stratification

in bloom formation was highlighted and even at that time it was apparent that blooms in

North Puget Sound probably originate near the U.S.‐Canada border and are advected

southward.  Hydrographic data were limited but the blooms were monospecific and

observations from fixed wing aircraft indicated at one point in 1990 the bloom covered most

of central Puget Sound from Tacoma to Port Townsend and beyond into the Strait of Juan de

Fuca toward the Pacific Ocean.  Several reports of dead and dying wild fish including

cutthroat trout were received by the Washington Department of Fisheries including several

from the Port Townsend area (Dick Allen, WDF, pers. comm. 1991).   

Some of us believed that there may have been some internal clock or cyst germination

mechanism that causes blooms to occur interannually within a tight time frame of early July

and September most every year similar to what Li and Smayda (2000) reported for the East

Coast U.S. waters of Narraganset Bay.  The coincidence of bloom timing for the first several

blooms in Puget Sound seemed strikingly similar on both coasts.  But subsequently

experience in Puget Sound has indicated the importance of physical forcing factors as

discussed herein, i.e., weather conditions are paramount for both North and Central Puget

Sound blooms and likely Hood Canal too. There are no commercial fish farms in Southern

Puget Sound and thus no extensive database for that area, but there have been blooms of

Heterosigma there, specifically in eutrophic Budd Inlet near Olympia (Eisner and Newton,

1997).  The Squaxin Island fish farm several miles north used in the spring for releasing

smolts (Rensel et al. 1988) has been infrequently affected in the past by plankton‐related

fish kills, but no definitive analysis of the causative species has been conducted.    

Connell and Jacobs (1997) further documented blooms and contributed to bloom

initiation conceptual modeling and reported on localized bloom of Heterosigma in the
Fish kills from Heterosigma akashiwo in Puget Sound: recent blooms and review                 8

western portion of central Puget Sound.  This bloom was associated with much above

average water temperature and reduced salinity of about 2 psu at the surface versus

subsurface during the bloom initiation period.  The bloom reportedly was first observed near

the east side of Bainbridge Island and spread quickly to > 30,000 km2 area and had mostly

subsided after about a week and was gone after two weeks. Later the same author and

others (Connell et al. 2000) reported a dense bloom of Heterosigma from mid September in

Southern Hood Canal.  Many wild fish died, but the authors concluded this was due to low

dissolved oxygen at that time, not the alga.  Although very dense, the bloom may have been

restricted to the immediate surface waters.  Literature was cited (Hard et al. 1999) that

salmonids may actively avoid Heterosigma blooms but there is scant evidence of that for

species cited (chinook salmon).  In the appropriate weather and hydrographic conditions,

blooms of Heterosigma may concentrate on the immediate surface (to several cm deep)

where adult salmonids are not to be found except occasionally they must imbibe air to

maintain their buoyancy.  Juvenile salmonids including cutthroat trout, chums, pinks and

subyearling chinook are, however, often found in spring or early summer in shallow depths

and nearshore where algae may be concentrated from winds and bathymetric influences.  

As discussed below, Heterosigma blooms in Puget Sound can be separated into late

spring‐ early summer blooms associated with riverine‐induced vertical stratification versus

midsummer to fall blooms that correlate with hot weather‐induced vertical stratification.   

The former are always unialgal or very nearly so, the latter are sometimes mixed with other

species, typically dinoflagellates (with some notable exceptions such as the September 1989

and September 2000 blooms).   A similar situation has been reported in Korean coastal

waters by Kim et al. (2007) where the seasonal variation is pronounced.

Mechanisms of mortality 

Heterosigma is harmful in many different regions of the world including the Pacific

Northwest, Chile, New Zealand, and Scotland but is not harmful to fish in broad areas such

as the entire coast of China and in other regions, such as near Japan, it is much less harmful

than in the above mentioned areas.  Previously it was thought not to be harmful to fish on

the U.S. east coast, but recent fish kills in eutrophic, shallow marine waters of Delaware have

correlated with Heterosigma blooms (but apparently not monospecific blooms so there is

some chance another species is involved).     

Heterosigma does not produce any known, persistent toxin but apparently kills fish

through an effect on the gills or a labile neurotoxin, both of which could produce blood

hypoxia but neither is well‐described or understood.  The mechanism(s) of fish killing remain

unknown or partially‐described at best and confusing.  As pointed out by Clough and Strom

(2005) the alga:  

“may well have multiple modes of toxicity that affect different types of aquatic organisms

in different ways.  At present, however, there is no accepted chemical measure of toxin

content in this species”.   

Indeed, there may be no apparent persistent toxin to be found in tissue of Heterosigma‐

killed fish as pointed out by Rensel and Whyte (2003) and in some cases fish killed by the

alga may have been consumed by humans without apparent ill effect.  Even the physiological
Fish kills from Heterosigma akashiwo in Puget Sound: recent blooms and review                 9

damage or effects of the alga on fish is poorly understood.  I conducted histological

examinations of moribund fish from a central Puget Sound site (Fort Ward) during the 1990

bloom (using very rapid dissection and preservation techniques) and found highly damaged

epithelial cells of all the sampled subadult or adult Atlantic salmon gills.  Epithelial

disruption, edema, and several other maladies stereotypically seen when gills are damaged

by a chemical or abrasion were noted.  Previously Black et al. (reported in 1991) conducted

studies with juvenile Chinook salmon that showed mortality in the field to fish flown into an

affected area.  However, in this case only, rapid sampling and fixing of gill tissue showed no

histological damage whatsoever. Larger fish usually die more quickly when the gills are

damaged by any mechanism compared to smaller fish due to reduced surface area of gills to

body mass ratios, so it is reasonable to assume that detectable damage to the small fish gills

would have been observed if that was the primary etiology of fish mortality.  Surprisingly,

there have not been many other observations of affected fish gill or other organ histology in

affected areas worldwide.  Others raphidophyte species such as those that kill fish in Japan

produce similar effects (Oda, 2007).

There are species specific differences of sensitivity within the salmonids and some

marine fishes may be immune to at least some strains of the alga as was the case in blooms

in Puget Sound in 2007 and sablefish held at the NOAA Manchester Research facility.   In this

case, a pen of about 400 sablefish (Anoplopoma fimbria) was totally unaffected while

Atlantic salmon (Salmo salar) in an adjacent pen did not survive (K. Massee, NOAA

Manchester Lab., pers. comm.).  The pens were within a few meters of each other so the

difference was likely not due to differing doses. The implications of this observation are

potentially very significant but somewhat mystifying as we have documented mortality of

other marine fish such as flatfish and sculpins.  

Hydrogen peroxide or some other reactive oxygen species of chemical(s) are produced

by the alga that possibly affect the fish gills (Yang et al. 1995, Oda et al. 1997) although more

recent work provides evidence that it may not be possible as the amount of hydrogen

peroxide was insufficient to cause gill damage (Twiner et al. 2001).  Mucus production has

also been cited as a possible cause of this alga causing clogging of fish gills and blood‐

hypoxia from suffocation, but I personally have inspected many fresh, moribund fish during

Heterosigma blooms and find little or no mucus, compared to fish killed by the diatom

Chaetoceros (subgenus Phaeoceros) which was the subject of my dissertation and related

work (Rensel 1993).  The observation that Heterosigma may cause copious mucus

production by fish gills is possibly an error, rooted in casual observations of fish that have

been dead for some short (~5 minutes) or longer period of time. Fish gill mucus cells are

holocrine cells and are stimulated to discharge upon fish death when the nervous system

shuts down and this reaction is noticeably quicker in warm weather of course.   The cells

arise from subdermal tissues and are stimulated by irritation or other cues and new ones

must regenerate.  In the case of fish exposed to harmful algae or simply dense blooms of

non harmful algae these cells can be stimulated if exposure is rapid and intense such that

the fish may suffocate as gas exchange cannot be performed as rapidly as normal at the gills.  

The mucus does not prevent gas exchange, it merely slows it down proportionate to its
Fish kills from Heterosigma akashiwo in Puget Sound: recent blooms and review                 10

thickness.  Affected fish may be observed “coughing” which is a means to expel excess

mucus.  

The possibility of a brevetoxin cannot be ruled out (Khan et al. 1997, Ono et al. 2000,

Haque and Onoue 2002) but the evidence is not compelling because the detected levels are

low or undetectable and Heterosigma killed fish have been consumed by humans in the past

with no apparent ill effects.  It is possible the toxin(s) are highly labile so that detection is

difficult.   

No one has yet killed fish in the laboratory with clean (axenic, i.e., bacteria‐free

monospecific) cultures of Heterosigma except possibly at extremely high concentrations that

are unrealistic compared to that which is seen to kill fish in the field.  Thus a bacterial or viral

cofactor or interaction seems possible or even likely in my opinion.  My colleagues and I at

the University of Washington conducted numerous bioassays during the early 1990s that

involved laboratory exposure of Atlantic salmon (Salmo salar) to axenic (bacteria‐free)

Heterosigma cultures grown under a variety of nutrient conditions but not one of hundreds

of Atlantic salmon died or even showed signs of stress (K. Banse, F. Taub, R. Horner, J. Postel,

R.A. Cattolico, J. Rensel, unpublished data, University of Washington).  At the same time in

British Columbia, cultures in the laboratory of others were killing fish, but they were not

axenic.  We did not report our findings in the peer‐reviewed literature but it was later

reported from other similar observations by Carrasquero‐Verde (1999).   

Other explanations for Heterosigma‐caused fish mortality are possible too, including

elevated levels of DOC (Smayda 1997),  low levels of iron that stimulate exoenzyme

production by the alga and light‐activated haemolysin (Trick et al. 2007) and complex

combinations of nitrogen‐rich nutrients (Herndon and Cochlan 2006)  

The stage of cell development or population phase may be important in altering the cells

from harmless to harmful.  As discussed below, in the 2006 North Puget Sound bloom

smaller, non‐motile cells seemed to produce higher rates of mortality at pen sites.  This is a

condition long known in British Columbia by growers and interested scientists (Nicky Haigh,

pers. comm. on SoundHAB listserve).  She also reports that fish mortality may be more

common at night, than during the daylight but the reason(s) is unknown.  

Some authors and agency personnel have previously consider Heterosigma to be a fish

killer of farmed fish only.  This is unlikely for the Sound.   Hershberger et al. (1997) recorded

losses of various sizes of wild fish in Case Inlet, south Puget Sound as a result of a

Heterosigma bloom. There were many reports of dead fish during the 1990 bloom including

several in Port Townsend Bay.   In other prior or subsequent blooms in Puget Sound wild fish

losses likely occurred too but in temperate areas, wild and farmed fish, both salmonids and

marine fish, tend to sink to the bottom when killed by Heterosigma blooms or other causes

such as a bacterial septicemia.  As a result, only a few dead fish will be observed on the

surface or shorelines and those sinking to the sea bottom are likely preyed on by crabs, birds

or other predators.  For decades it was thought that the alga did not kill fish on the east

coast either, but we now know this is apparently untrue, as fish in a eutrophic estuary in

Delaware have been killed by Heterosigma blooms.   

Keppler et al. (2005) documented sublethal, adverse effects of Heterosigma on oyster

physiology on the U.S. Atlantic coast that shows that harmfulness is not restricted to fish.  
Fish kills from Heterosigma akashiwo in Puget Sound: recent blooms and review                 11

Some blooms have been found to be distributed very near the surface, others such as the

one discussed herein, were found throughout the upper water column to depths of 30 m or

more.  Cough and Strom (2005) conducted laboratory assays to demonstrated mortality of

large ciliates (zooplankton) that fed on Heterosigma.  These are often a major component of

coastal microzooplankton communities.  No dissolved, active fraction was found but

mortality was linked to ingestion. The cells’ ability to kill zooplankton is perhaps an

important element in forming and maintaining some blooms.  However, in Puget Sound, all

recorded blooms to date have been relatively short‐lived events of just a few days to less

than a week in any one area, before tides and winds push the bloom into other areas or the

blooms dissipate with weather changes.      

Cell counts in excess of 5 x 105 to 7 x 105 cells per liter are thought to be likely to lead to

fish death, although farmers know that “toxicity” of the cells varies greatly (Rensel and

Whyte 2003).  It is known, however, that much lower concentrations of cells, as low as 3x

105 cells per liter can cause mortality in some cases and is more often a problem at night

when a bloom is transported through a fish farm (N. Haigh, B.C. Harmful Algae Monitoring

Program, email July 10, 2006).  

Cell development as evidenced by shape and size is thought to be related to degree of

harmfulness, for example smaller cells, that are often irregularly shaped, are thought to be

more toxic than larger cells as many farmers in British Columbia have expressed (E. Black,

Canadian DFO, email, July 11, 2006).  In recent Puget Sound blooms discussed herein, a

significant portion of the cells were small, non‐motile cells that could have been cysts or

precursor of cysts.  I personally have never seen these in culture, but suggest that they could

be induced in the laboratory by cold shocking cultures which is what may be occurring in

North Puget Sound as the blooms move from warm northern and shallow waters to

relatively cool, deep channel waters near the San Juan Islands and into the Strait of Juan de

Fuca.   

The literature suggests that Heterosigma is mostly surface oriented (e.g., < 10 m depth)

but experience with this bloom in 2006 and a few prior blooms in Canada shows this is not

always the case.   Much of the literature on Heterosigma comes from Japan or the U.S. east

coast and is therefore not necessarily applicable to the Northwest.

Fish Farms as Possible Causes of Blooms 

Production of ammonium and smaller amounts of urea by salmon farms would seem

a possible link to initiation or exacerbation of Heterosigma blooms and may in fact be so in

some other areas of the world.  Certainly fish farms located in vertically stratified, poorly

flushed bays or inlets with low surface nutrient levels during the growing season are prime

candidates to exacerbate blooms of any microflagellate or dinoflagellate species that may be

present.  However, all evidence in Puget Sound is to the contrary.  This topic has been raised

before, for example by Taylor and Harrison (1993):

“Taylor et al. (1994) and Taylor and Horner (1994) concluded that there was no evidence for

links between HABs and salmon aquaculture in B.C. waters. The sites usually chosen for

salmon farming are deep and the large tidal range flushes the surface waters well.”
Fish kills from Heterosigma akashiwo in Puget Sound: recent blooms and review                 12

0.0

1.0

2.0

3.0

4.0

5.0

In Pen 6 m Downstream 30 m Downstream

DIN Increase (uM)

However, the reasons are more numerous and persuasive for Puget Sound.  All commercial

fish farms in the Sound are required to locate in areas replete in nitrate nitrogen (SAIC 1986,

Rensel Associates and PTI 1991, Washington Dept. of Natural Resource lease requirements)

that are relatively well and flushed. This simply means that there is typically plenty of

nitrogen available in these locations (e.g., Mackas and Harrison 1997) and other factors such

as light, horizontal and vertical advection of cells or other factors are more likely to limit

microalgal productivity of any kind.  Measurements of nitrate during the 2006 bloom in

North Puget Sound indicate about 4.0 μM during the peak of the bloom and fish mortality, as

discussed below which is much less than normal concentrations, but high relative to the

needs of Heterosigma cells.   

Salmonids excrete mostly ammonium and low levels of urea (Brett and Zala 1976) and it

is possible that ammonium could fuel blooms of Heterosigma or other algae if nitrate and

other sources of N are locally depleted.  I personally collected ten years of dissolved

inorganic nitrogen measurements and current meter data to calculate the flux up and

downstream of numerous fish farms in Puget Sound.  Slightly elevated ammonium can be

measured in the pens (1 or 2 μM) but by 30 m downstream the concentrations are

diminished to 0.1 to 0.2 uM total ammonium in most cases but dissolved inorganic nitrogen

(DIN) is about one uM , or ~ 5% of the

normal background levels (Fig 1).   

Figure 1.  Mean and SD of ten separate DIN

measurements at Puget Sound fish farms

performed in September during the 1980s

and 1990s (Rensel 1991 and unpublished

manuscript).  

Heterosigma had a minor, non‐

statistically elevated preference for

ammonia over nitrate and urea in studies by Herndon and Cochlan (2007) on the US west

coast.  Urea and ammonium affinity was greater than that for nitrate in other studies on the

US east coast (Johns and Glibert 2007). There are not extensive measurements of

background N concentrations during Heterosigma blooms in Puget Sound except for the few

I have collected or that of Connell and Jacobs (1999) and a few coincidental measurements

made during routine monitoring by the Washington Department of Ecology.  Thus it is

possible that some localized enhancement of the blooms occurs by fish farms, but the

evidence to date suggests not. Moreover, fish are not fed prior to and during blooms so as to

limit oxygen demand for digestion particularly when the gills may be damaged.  Their DIN

excretory products are therefore diminished by a factor of ~3x when unfed and at all times

they produce very little urea (e.g., Brett and Zala 1976).  As blooms initiate at other

locations, the nitrogen discharge of these pens is not a factor in blooms initiation.  Figure 2

gives some perspective on the relative importance of fish farm nitrogen versus that from

oceanic upwelling and other sources.   
Fish kills from Heterosigma akashiwo in Puget Sound: recent blooms and review                 13

The nets, floats and lines that compose a net pen farm hold a rich assortment of

colonizing organisms (> 100 species of invertebrates and macrophytes, Rensel and Forster

2007) and no doubt a large population of bacteria that allows for nitrification (see Rensel

unpublished reports to Washington Dept. of Natural Resources 1986‐1995). While increases

in nitrate and ammonium levels are seen a few meters downstream of farms a few meters,

the increases are very small and all the observed ambient concentrations were always  well

above the 1⁄2 saturation constants of uptake or growth for virtually all types of phytoplankton

and in a range likely to be not limiting to Heterosigma .  

Figure 2.  Calculated

dissolved nitrogen loading

from oceanic inflow versus

selected rivers, treatment

plants and different levels

of fish farm development

(13 to 100 farms) from WDF

1991.  Presently there are

eight farms in Puget Sound

but two are smolt

production sites with

smaller fish only.  

Recent experimental work by the Washington State Department of Ecology (Newton and

Van Voorhis 2003) indicated that some stations in the main basin of Puget Sound were

potentially nutrient sensitive at times during the spring.  This was attributed to diatom‐

bloom‐caused nitrogen drawdown of the surface waters in the selected areas. The

interpretation was from a bottle experiment where 30 μM ammonium was added to

seawater from a number of locations in or around the fringes of the main basin of Puget

Sound. It should be noted that the level of enrichment was huge relative to any probable

source and the authors of the study stated that the bottle experiments did not allow for the

process of zooplankton grazing which could be a major control to phytoplankton

production. It should also be noted that managers have long known that there is

considerable temporal and spatial variation in nutrient sensitivity of Puget Sound and that

some subareas have to be protected from all anthropogenic discharges.  Also, some of the

few test areas were highly vertically stratified from nearby river run off, which can create a

nutrient sensitive situation that dissipates when seasonally peak river discharge declines.

None of the findings repudiate the basic concept that if naturally occurring levels of DIN are

above some moderate threshold (highly variable depending on algal species but in all cases <

10 μg at/L N) then no amount of additional nitrogen will lead to additional productivity by

the phytoplankton in that area as long as some other important nutrient is not limiting (e.g.,
Fish kills from Heterosigma akashiwo in Puget Sound: recent blooms and review                 14

silica for diatoms, which is replete in Puget Sound).  Degree of nutrient depletion of surface

waters in combination with dissolved oxygen depletion have been the basis of salmon net

pen “zoning”  for over 20 years (e.g., SAIC 1986) and subsequent evaluations of Puget Sound

nutrient sensitivity (Rensel Associates and PTI 1991 and subsequent Dept. of Ecology

publications).  It is arguable that a day or two of nutrient limitation in a typical river mouth

area of Puget Sound with shallow surface layer would have little or no effect on fast

vertically migrating flagellates like Heterosigma, as the alga is able to rapidly (1 m/h) migrate

to obtain nutrients from deepwater even if the surface waters are nutrient depleted.  

Blooms of Heterosigma can be treated and flocculated to the bottom with small amounts

of phosphatic clay slurry (Sengco and Anderson 2004).  The technique has been tried on a

pilot scale at a fish farm with no apparent adverse environmental effect (Rensel and

Anderson 2004). The clay causes the cells to sink and may kill them too, and since there is no

persistent toxin, it is not a matter of transferring toxicity from one component of the food

web (pelagic) to another (benthic). Nevertheless, the method has never been tested on a

large scale during a fish‐killing Heterosigma bloom.  It is of course impractical to treat entire

areas of Puget Sound with clay, the technique was envisioned for use only in the cages.  

Prior Blooms, Initiation Sites and Transport  

The first major Heterosigma bloom that killed aquaculture fish in Puget Sound was in

1989 and another bloom followed in 1990 (Rensel 1995 see appendix B).  These were both

major events that caused relatively large losses of farmed, net‐pen fish as methods were not

available to mitigate the effects of the blooms.  At the time local scientists were relatively

poorly prepared or funded to document such large blooms.  Some hydrographic data were

collected but much of the evidence was anecdotal such as visual observations from

floatplanes that I made with State Fish and Wildlife staff or the verbal communications from

fish farm staff.   

During the 1990 bloom dead cutthroat trout and other wild fish were reported to the

State agencies from several areas of central Puget Sound (Fig. 3) including Port Townsend

Bay.  No fish tissue samples were collected or analyzed except for gill histology samples I

collected rapidly from moribund fish at the central Puget Sound sites.  

No major blooms occurred in the following interim until the present except the localized

1997 bloom in CPS discussed herein and reported by Connell and Jacobs (1999) and the year

2000  bloom in Hood Canal where no fish farms exist (Connell et al. 2000).  Many other

Heterosigma blooms undoubtedly have occurred in Puget Sound in disparate locations or

before humans started tracking such occurrences.   
Fish kills from Heterosigma akashiwo in Puget Sound: recent blooms and review                 15

Some misunderstanding of bloom origination and cause has surrounded Heterosigma

blooms in Puget Sound and probably elsewhere.  As farmed fish have sometimes been killed

by these events in Puget Sound or elsewhere, some have assumed that bloom

commencement or embellishment

must be linked with fish farms (e.g.,

GEOHAB 2006).   Having worked with

these blooms and the fish farming

industry in Puget Sound for nearly 20

years, interested scientists and local

fish farmers know with certainty that

blooms originate in areas remote

from the fish farm sites and are

advected by wind and tidally driven

currents into farm sites. Fish farms

are the stakeholders most interested

in harmful algae in many areas as

they are at risk and the fact that

blooms are discovered by fish farmers

relates to their routine monitoring

programs.   Evidence from the most

recent blooms is presented below and

once again confirms these findings as

does the literature mentioned above.

Figure 3.  Vicinity map of Puget Sound

showing subareas and place names

referred to in text.   

Heterosigma blooms in Puget

Sound may be divided into two spatial areas that often have differing temporal patterns and

causal factors: North Puget Sound (i.e,. inland marine waters of Western Washington north

of Admiralty Inlet including the Strait of Juan de Fuca) and Central Puget Sound (i.e., inland

marine waters of Western Washington south of the Strait of Juan de Fuca including areas

from Admiralty Inlet to the Tacoma Narrows.   

North Puget Sound  

The primary factor associated with North Puget Sound blooms is the Fraser River plume

which is most extensive in late spring and early summer when the mountain snowpack is

most rapidly melting.  The Fraser River is by far the largest river entering the coastal

waterways of Puget Sound and the Strait of Georgia and has a pronounced effect on the

physics and biology of the region as documented (see Thomson 1981 for an overview).  The

Fraser River plume initiates near the river mouth and flows into the Southern Strait of

Georgia, marked as “probable bloom initiation area 1” in Figure 4.    

North P uget Sound

Central 

Puget 

Sound

South Puget Sound

Case Inlet

Fraser River

Mouth Area

Padilla Bay

Hood Canal
Fish kills from Heterosigma akashiwo in Puget Sound: recent blooms and review                 16

As it flows out of the river mouth it partially mixes with seawater and forms a stable

brackish surface layer that allows a very productive phytoplankton production area that

otherwise might be mixed out of the photic zone into deeper waters.  During fair weather,

the plume is advected southward with the fair‐weather north winds.  Most of the flow

eventually flows out through the Strait of Juan de Fuca to the Pacific Ocean fueling a classical

estuarine circulation.  

While I believe the most likely Heterosigma bloom initiation area for North Puget Sound

blooms is in soft sediments of areas near the Fraser River mouth (labeled bloom initiation

area 1 in Fig. 4), I cannot exclude other potential areas to the south such as Bellingham Bay

to Padilla Bay (labeled as bloom origination area 2 in Fig. 4) where Heterosigma blooms are

sometimes prevalent.   

Figure 4. Vicinity map of probable or

possible bloom initiation areas for

Heterosigma in North and Central

Puget Sound in relation to clusters of

fish farms.  (Port Angeles Harbor and

north Skagit Bay net pens not

shown).   

Without more observation and

research, it is difficult to

differentiate between these two

areas and it is academic to fish

farmers who are more concerned

with waters nearer to the North

Puget Sound fish farming sites. The

innermost waters of East Sound, a

sheltered and isolated bay at Orcas

Island in the San Juan Islands,

appear to have early summer

blooms of Heterosigma on an

annual basis.  But it is unlikely that

a bloom could emerge intact due

to the strong tidal mixing in

surrounding channels and Straits

and the very long entry way to the

bay.  Individual cells from there

could, however, survive transport out to other areas to possibly initiate a bloom but the

aerial observations suggest that it is not a likely source of blooms in north Puget Sound

where blooms appear to move down large, relatively less‐well‐mixed channels such as

Rosario Strait from the U.S.‐Canada border.  
Fish kills from Heterosigma akashiwo in Puget Sound: recent blooms and review                 17

Fish farmers routinely sample and conduct aerial surveys of waters of north Puget Sound,

central Puget Sound and areas near Port Angeles in May or early June particularly if the

weather is very warm and the Fraser River flow is large. They prepare maps of estimated

bloom distribution based on visual observations from planes.  Heterosigma blooms have a

characteristic purplish‐red coloration that is distinctively obvious to an experienced observer

and as discussed herein, many blooms are nearly monospecific which makes visual detection

from airplanes easier. The aerial observations are then ground truthed with water samples

for cell counts using a microscope if the risks appear moderate or high and as blooms

approach a fish farming area.  

Central Puget Sound 

A similar set of circumstances is involved in controlling blooms of Heterosigma in central

Puget Sound.  In this area the three fish farms are clustered on the west side of Puget Sound

west of Seattle and south of Bainbridge Island (Fig. 4).  This area has no rivers and only a few

small streams so freshwater runoff and subsequent surface vertical stratification is typically

not experienced to the degree seen in North Puget Sound or even other parts of central

Puget Sound.  It does, however, experience relatively strong vertical stratification later in the

summer when the weather is unusually hot.  At such times, surface waters of the shallow

backwater bays and inlets experience temperatures in excess of 18°C.   

Nearly two decades of monitoring in the backwater to the west of these farms in areas

such as Port Orchard, Sinclair Inlet and all marine waters on the west side of Bainbridge

Island indicate that blooms in central Puget Sound originate there. These areas include

relatively calm, sheltered areas with mud and silt bottoms suitable as cyst beds and initial

bloom development.  Shallow bays on the east side of Bainbridge Island especially Blakely

Harbor are also apparent bloom initiation areas as they are often the first areas to be

observed with bloom conditions and have silty mud bottoms suitable as cyst beds for the

alga.  Conversely, the fish farm areas discussed above have mostly fine sand to cobble

bottom sediments, so it is unlikely that cysts are either deposited or survive for any time

period beneath or adjacent to the farm sites.  Initial blooms are never seen adjacent to the

fish farms at any location in Puget Sound and in all cases appear to be advected by currents,

winds and circulation patterns into the fish farm areas as discussed in the next section.  

June 2006 Bloom Chronology of Events

The late June 2006 and early July Heterosigma bloom was monitored by American Gold

Seafoods staff and myself and is the principal focus of this report.   I assisted by collecting

CTD casts throughout areas near Cypress Island, performing cell counts and performing

related tasks.  Kevin Bright of American Gold Seafoods prepared a chronology of events that

was modified and presented below.  I summarize this information in brief as follows:  

North Puget Sound and Cypress Island  

Increasing cell counts of Heterosigma at the Cypress Island fish farms were noted

beginning on Sunday June 25th which invoked a high risk level of awareness and response.  

By June 28th significant levels of fish morality were occurring at the Cypress Island fish farm
Fish kills from Heterosigma akashiwo in Puget Sound: recent blooms and review                 18

sites.  Aerial flights and ground surveys were mounted on June 26th and all available

evidence suggests that this bloom, as was the case for prior blooms in North Puget Sound,

was advected into the fish farm area from areas much further to the north or from the

shallow bays to the east (Bellingham,

Samish, Padilla Bays).  Figure 5 is the

record of one of the aerial surveys,

showing a survey radius with the

focus of the survey circle being the

fish farm locations in Deepwater

Bay.  The brackish‐water Fraser River

plume is known to be the means by

which the cells are maintained in

surface waters of this area and the

density differences act to reduce

mixing out of the surface and photic

zones.  Tides and winds act together

to move the cells southward toward

the Strait of Juan de Fuca and the

Pacific Ocean (Figs. 6, 7).   

Figure 5.  Aerial survey results of June

27th 2006 conducted by Kevin Bright,

AGS Biologist.   Note that the survey

was limited to the areas within the

survey circle and part of the Eastern

Strait of Juan de Fuca, i.e., no

observations were recorded from

further North.  

Because of time and hired airplane costs, surveys are not conducted all over the North Puget

Sound, but in the past the general trends shown in Figures 6 and 7 are known to be more or

less accurate.  

Samples taken from shallow bays and passages some distance from Cypress Island

indicated peak cell counts of 3 to 4 million cells/L which is 3 or 4 times the highest values

seen at the fish farm at any time during this bloom. The evidence also includes maps of

bloom distribution prepared by an experienced observer (K. Bright) who flew in a small plane

over the area periodically during the pertinent time period.  Heterosigma blooms in the

Pacific Northwest exhibit a characteristic color that can be differentiated from blooms of

other algae.  This has been repeatedly verified by coupling aerial surveys with on the water

collection and analysis of samples.  Spread of the bloom from north to south and possibly

east to west is the typical scenario that has often been seen in the past in this area as

previously reported. Cell counts varied as shown in Appendix A but by Friday evening, June

30th there appeared to be a decline in cell counts and by Sunday, July 2nd the farm site and
Fish kills from Heterosigma akashiwo in Puget Sound: recent blooms and review                 19

adjacent usual “hot spots” were experiencing much lower numbers of cells.  As in the past,

this bloom was an intense but short lived event, lasting approximately a week throughout

the NPS region.  Tides and circulation in this area are very intensive and the blooms are

clearly subject to rapid transport and some degree of vertical mixing despite the river‐

induced vertical stratification.  

Figure 6.  General vicinity of the

bloom in the early stages in late June

2006 in red, inferred from survey

vessel and aerial survey maps.  

Figure 7.  General vicinity of the bloom

during later stages as it flowed with

surface waters out the Strait of Juan de

Fuca and into the Pacific Ocean.

During this bloom samples collected near Cypress Island showed a distinct bimodal size

distribution of Heterosigma cells. Relatively large numbers of small cells (i.e., ~ 10 um or

less) were mixed with larger, more typical cells (i.e., ~ 20 to 25 um).   An ocular micrometer

was not available at the time so these sizes represent estimates only, but clearly there were

two size classes.  Typically Heterosigma cells seen in PNW waters are motile and relatively

similar in size and shape as discussed herein. British Columbia growers have suggested that

the smaller cells are more often associated with increased rates of fish mortality.   The

reason for this is unknown.  Phytoplankton including other microflagellates, dinoflagellates

and some diatoms that produce toxins do so under a variety of growth stages and

environmental stressors, so it is not possible to generalize.  Deficiency or abundance of

nutrients (nitrogen or phosphorus or micronutrients) is sometimes a factor in toxin

production of some other fish‐killing species.  It may just be a coincidence, but Washington

Dept. of Ecology surface water data from Admiralty Inlet was depleted of nitrate and other

forms of DIN to below detection limits a week prior to the truly massive 1990 bloom

(Admiralty Inlet station).  That area is usually rich with DIN and Heterosigma is a strong
Fish kills from Heterosigma akashiwo in Puget Sound: recent blooms and review                 20

vertical migrating alga that can

prosper when many other

phytoplankton especially diatoms are

unable to survive. A caution with the

use of this observation because the

Department of Ecology’s DIN

measurements at that time may have

been influenced by a relatively high

detection limit.  This came to light

after our comment in Hershberger et

al. (1997) about no detectable

nitrogen occurring in the (surface)

water of the area during a large, prior

bloom (1990 CPS bloom).  

Figure 8.  Deepwater Bay at Cypress Island, actually more of a bight than a bay, is subject to very

strong tidal currents and has large natural populations of invertebrates, fish and seabirds (Rensel

and Forster 2007).  

During this bloom dead fish from natural populations were reported in small numbers

from North of Cypress Island to shoreline areas as far west as Freshwater Bay in the Strait of

Juan de Fuca (SoundHAB list serve entries).   It is worth repeating that dead marine and

salmonid fishes typically sink in our temperate marine waters, which makes observation of

effects very difficult and typically only a few dead fish are observed in these blooms.  Dead

fish will rapidly be preyed on by scavengers such as crabs and certain fishes and dispersed by

tides and currents therefore not always notice on the surface.  

Port Angeles Harbor, Western Strait of Juan de Fuca and Pacific Ocean  

A few cells first appeared on Thursday, June 29th and by Sunday July 2nd cell counts were

high and fish were dying.  The timing was just a few days later than the North Puget Sound

bloom and there was a noticeable bloom on the U.S. side of the Strait during this time period

too.  By July 5th cell counts began diminishing concurrent with a weak low pressure weather

front movement into the area. The following day the weather continued to be less suitable

for bloom maintenance with greatly reduced air temperature, clouds and thunderstorms.  

Cell counts at 1 to 10 m were low but at 35 m fairly high concentrations (470,000 cells per

liter) remained.  The remaining cells soon dissipated.   It is of interest to note that blooms of

Heterosigma were spotted off the coast of Washington shortly after this time period, by

Brian Bill of NOAA‐NWFSC who collected samples.  The sequence of timing suggests that the

bloom was an extension of the initial North Puget Sound bloom, but it is difficult to prove

that exactly.  

Other Areas 

The actual timing and extent of the 2006 or 2007 blooms in other areas remote from the

fish farms is not fully known except for other observations cited herein and from some
Fish kills from Heterosigma akashiwo in Puget Sound: recent blooms and review                 21

general observations from airplane flights in the areas nearer the farms or on transit lines

between the subareas as one plane did all the observations. These maps are retained and

may actually be a valuable research tool for those interested in the timing and spread of the

blooms.  For example, satellite imagery from similar time periods could be compared to the

maps to begin testing the use of such images for management and risk assessment.  

Factors Associated with Late June 2006 Heterosigma Bloom 

Here I review some of the conditions that preceded and accompanied the bloom late

June‐July 2006.  This is done to add to the body of knowledge regarding conditions that may

initiate or sustain blooms, as they occur relatively rarely and without recording the data

there is no means to modify conceptual or numerical models for bloom prediction in the

future.  The sites initially at risk in the 2006 bloom were Sites 1, 2 and 3 at Deepwater Bay at

Cypress Island in North Puget Sound near the San Juan Islands.  

Snowpack and River Discharge 

The late June Heterosigma bloom commenced in Northern Puget Sound associated with

the major freshwater source for the entire area, the Fraser River plume. River discharge of

this very large river peaks this time of year and has a profound influence on surface and

near‐surface water quality and algal growing conditions throughout much of North Puget

Sound and even the Strait of Juan de Fuca.  In previous Puget Sound Heterosigma blooms

where water quality profiles were collected a reduced salinity surface layer coincided with or

persisted through the bloom (Rensel 1995, Connell et al. 1997).  For other blooms, no data

are readily available.    

Although snow pack from the winter of 2005‐2006 was among the largest in several

decades, river discharge was below median long term historical levels during the later

periods of June 2006 (Fig. 9) having peaked a bit earlier in the month and in late May and at

a level exceeding the maximum median discharge.   

Figure 9.  Recent discharge flow data from

the Fraser River at Hope, B.C. versus 2006

flow in red.

However, freshwater does not flow

out of North Puget Sound into the Strait in a linear fashion, but rather pools up during larger

tides and pulses out during neap tidal series, as discussed in the next paragraph.  Figure 9

indicates that river discharge is typically greater in late spring and early summer, peaking

near June 15th and 2006 was an exception with a much earlier run off peak.  As discussed
Fish kills from Heterosigma akashiwo in Puget Sound: recent blooms and review                 22

below, we measured relatively low salinity to relatively great depth over broad areas of

North Puget Sound that indicates the effects of rivers were pronounced despite the timing

records of total discharge.   

Data from a concurrent University of Washington PRISM research cruise (J. Newton, UW

APL unpublished data) in late June suggests that the Eastern Strait of Juan de Fuca was not

experiencing a significant reduction of surface salinity which is evidence that the Fraser River

water may have been “pooling” near the Lummi Island to North Cypress Island areas and

associated backwaters.  So either the river plume waters had not reached those areas at that

point or mixing was sufficient to attenuate the signal.   

     Estuary to ocean exchange in South Georgia Strait, north Puget Sound and the Strait is

modulated by tidal mixing and wind forcing that increases export of freshwater during neap

versus spring tidal periods (Griffin and LeBlond 1990).  Monthly or bimonthly pulses of

relatively warm fresh water have been documented traveling seaward from the western

entry of the Strait of Juan de Fuca (Hickey et al. 1991).  This is the opposite of what many

would think, but the relationship is well established.  Rensel et al. (2007) reported a mid

Strait of Juan de Fuca cold water anomaly that persist throughout most of the year that may

or may not have been related to this circulation pattern as it persisted in all tidal phases

(suggesting it was not related) but measurably increased during spring tides (when

freshwater export was reduced, hence vertical mixing possibly enhanced due to decreased

density differences).   

Water Temperature 

Elevated water temperature has generally been associated with some dinoflagellate and

flagellate blooms in Puget Sound.  In the past literature and grey literature from our area

and elsewhere notes that temperatures of ~15°C are associated either with cyst germination

or the growth and spread of vegetative (live, normal) Heterosigma cells.  However, in the

PNW these conclusions were largely based on observations during blooms and not

experimental evidence or monitoring of cyst beds.  Studies of growth of Heterosigma in the

laboratory show that maximum growth occurs at 25°C but toxicity was very low (Ono et al.

2000).  The same workers found the highest toxicity at 20°C, a temperature that is

encountered in back bays and shallows of Puget Sound but is never encountered or even

approached at Cypress Island at the net pen site in Deepwater Bay.   

Surface to near surface water temperatures during the June‐July 2006 bloom at the

Cypress Island net pen site were in the range of about 11.5 to 13.5°C but further north with  

an increase in vertically‐stratification, temperatures were consistently nearer to 13.8°C both

in my vertical profiles and data records collected by Kevin Bright.  We can only speculate

about water temperatures further north in the U.S.‐Canada border areas, but most likely it

was warmer there as typically seen in SST images in summer.  In summary, water

temperatures were suboptimal for growth but did not prevent harmful effects at the fish

farm site during this bloom.  Reduced water temperature at the fish farm sites could be

causing cyst germination which in turn could have some effect on toxicity to fish.  
Fish kills from Heterosigma akashiwo in Puget Sound: recent blooms and review                 23

Air Temperature 

June weather in Western Washington is historically relatively cool and cloudy, with

occasional showers while in contrast, July is often hot and sunny.  In 2006 prior to and during

to the fish kill in June we experienced unusually sunny days with temperature in the 80 to

90°F range.   Figures 10, 11 and 12 are air temperature records for Seattle‐Tacoma Airport

(central Puget Sound), Bellingham Airport (North Puget Sound) and Port Angeles Airport

(Strait of Juan de Fuca) during the subject period.  Note that high and low air temperatures

exceeded long term period of record average highs and lows for each hourly observation.   

This was particularly true for Seattle Tacoma airport and for Bellingham Airport where

temperatures exceed the daily averages by up to 20°F but less so for Port Angeles.   

Figure 10.  Mean

high and low air

temperatures

compared to

hourly air

temperatures at

Seattle‐Tacoma

Airport during

June and early July

2006.  

Figure 11. Mean

high and low air

temperatures

compared to

hourly air

temperatures at

Bellingham

airport during

June and early

July 2006.

Figure 12.  Mean

high and low air

temperatures

compared to

hourly air

emperatures at

Port Angeles

Airport during

June and early

July 2006.
Fish kills from Heterosigma akashiwo in Puget Sound: recent blooms and review                 24

Precipitation far exceeded normal levels in early June 2006 as shown in Figure 13 but the

effects of this on bloom formation and spread are unknown and not likely to be pronounced

compared to the Fraser River discharge.  Although it could have influenced conditions in

bays that had Heterosigma cysts emerging at that time from soft sediments.

Figure 13.  Mean high

daily precipitation for

period of record

compared to hourly

precipitation at Seattle‐

Tacoma Airport during

the subject period.  

Tides 

In the past in Western Washington the onset of Heterosigma blooms or even increased

background concentrations of cells has often coincided with neap (minimal) tidal exchanges.  

During the June 2006 bloom the bloom was killing farmed fish at Cypress Island during a

period of declining tidal amplitude (Fig. 14).  However, the bloom appeared to have formed

in areas further to the north including the southern Strait of Georgia during the prior minus

tide series.   

    

  6/25/06                                    7/1/06                        7/4/06                                7/8/06             

        Figure 14.  Eagle Harbor, Cypress Island tidal plots from June 25th to July 8th, 2006.     

   Horizontal lines mark mean sea level and zero datum (mean lower low water).  

                               

As bulk transport of brackish surface water tends to exit North Puget Sound and the

Strait during neap periods, tidal amplitude may have as much to do with allowing cells to

congregate near the surface as well as be advected south and east out of the two areas,

respectively.   Whatever the relationship, fish farmers use decreasing tidal amplitude as one

measure of increased bloom risk and it has proved to be useful.  
Fish kills from Heterosigma akashiwo in Puget Sound: recent blooms and review                 25

It is probable that the effects of river discharge in creating a stable surface layer for

Heterosigma outweighed the likely bloom‐deterring effects of large tidal exchange prior to

late June 2006.  When the bloom was approaching the fish farming area, the tides were

tending toward neap exchanges that would have allowed the bloom to escape some of the

effects of tidal mixing that would occur during spring tides.  We know this to be true as the

salinity probe on the CTD (multiprobe) I used to take vertical profiles of water quality

showed a relatively deep, brackish surface layer throughout much of the area surveyed near

Cypress Island.  Additionally, Heterosigma has an amazing attraction to tiny amounts of fresh

water when added to tall laboratory columns (Hershberger et al. 1997), which is an

important clue to the understanding of its vertical migratory behavior.     

Winds 

No regional or subarea‐specific wind data were acquired for the preparation of this

report.  In general, winds were typical of what is usually seen for the Cypress Island area in

June, with some afternoon breezes that in some cases produced whitecap conditions in

open areas such as the Clark Point of north Guemes Island.   

In the eastern Strait of Juan de Fuca and Port Angeles Harbor, however, winds are

typically strong in the afternoon in spring and summer fueled by the land breeze

phenomenon. Yet during this Heterosigma bloom they were atypically strong and persistent.   

Estimated wind speeds of 20 to 30 mph from the west1 occurred during the bloom period

and unlike the normal conditions in the Strait in spring, did not diminish in the evening but

continued through the night hours.  The fact that the bloom continued during such strong

and persistent wind conditions was remarkable as many microflagellate or dinoflagellate

blooms are often linked to calm, warm conditions.  It is possible that the upper water

column by that time (several days after the PRISM cruise data was collected) were

sufficiently brackish to restrict mixing to depth by the winds but limited antecedent water

quality data, discussed below, suggests that was not the case.   

Time of Year 

Occurrence of Heterosigma vegetative cells in the water column is known to be most

prevalent biannually in the summer (Tomas 1980) and during the late June or early July

period and again in mid September in decades of sampling conducted in Narraganset Bay in

Rhode Island (Li and Smayda 2000). There is no systematic sampling and recording of

phytoplankton occurrence in Puget Sound and approaches but from my own observations I

believe that blooms do indeed occur more regularly at those times.  Dr. Max Taylor at the

University of British Columbia observed late spring blooms of Heterosigma in English Bay

near Vancouver, B.C. most every year for several decades.  The most severe blooms in the

                                                      

1 Westerly winds near Port Angeles were observed but wind direction is remarkably divergent in the Strait

during spring, with a divergence in direction often occurring near Port Angeles.  Easterly winds in the outer

Strait would have enhanced bloom transport to the ocean.   
Fish kills from Heterosigma akashiwo in Puget Sound: recent blooms and review                 26

-35

-30

-25

-20

-15

-10

-5

0

0 5 10 15

Farm Site Chlorophyll a (ug/L) 

Depth (m) 

Puget Sound in the past have always been near the 1st of July or in mid September2.   The

most likely forcing factor is water temperature that drives cyst germination, but other co‐

factors may contribute.  It is reasonable to assume that the year after a basin‐wide or larger

Heterosigma bloom there would be many more viable cysts, which increases the likelihood

of a bloom in sequential years.  This is what we have seen in Puget Sound in 1989‐1990 and

2006‐2007.  Thus there is an increased risk of blooms in the area in 2008, but as in all years,

the physical forcing of vertical stratification, water temperature, tidal amplitude and hot

weather periods dictate if the cells can survive and bloom.   

Water Quality Sampling 

Principal measurement of interest included chlorophyll a (a measure of phytoplankton

density via pigment concentration), water temperature, salinity, dissolved oxygen, dissolved

inorganic nitrogen and turbidity.  In my work, many of the profiles I collected were

accompanied by collection of cell counts. A Hydrolab 4a multiprobe with a variety of probes

and integrated Turner SCUFA in vivo chlorophyll sensor with 100 m cable on a continuous

reel was used to record vertical profiles at a number of locations around Cypress Island and

approaches on June 28th, 29th and 30th.   Calibration of the Hydrolab was conducted for all

the available probes as per previously cited protocols and the chlorophyll sensor was

separately calibrated by concurrently measuring and collecting water samples with a

“Scotty” 2L water bottle sampler for laboratory extraction and analysis using standard

protocols.  Water from different depths with varying chlorophyll a readings that spanned the

range from low (about 2 ug/L) to high (about 15 ug/L) was used and a correction factor was

derived with very low (0.1 ug/L) standard deviation from the mean.  

Figures 15 to 18 show the distribution of chlorophyll a measurements relative to depth.  

Normally we would expect a general trend of more cells near the surface, particularly during

the daylight hours when these

measurements were collected.  The farm

site (Figure 15) however shows no linear

trend line (the dashed line) because algal

biomass was more or less equally

distributed at all measured depths.   

Figure 15.  Algal density versus depth from vertical profiles at the farm site in Deepwater Bay.  

                                                      

2 A Heterosigma bloom occurred in early May 2007 in north Puget Sound during an especially warm period

but after killing just a few fish at the Cypress Island site it dissipated.  As usual, most of the bloom was

concentrated in areas north of Cypress Island and not adjacent to the fish farms.  
Fish kills from Heterosigma akashiwo in Puget Sound: recent blooms and review                 27

-35

-30

-25

-20

-15

-10

-5

0

0 5 10 15

Bellingham Channel South Entry Chlorophyll a (ug/L) 

Depth (m) 

-35

-30

-25

-20

-15

-10

-5

0

0 5 10 15

Cypress Head Chlorophyll a (ug/L) 

Depth (m) 

-35

-30

-25

-20

-15

-10

-5

0

0 5 10 15

Northern Cypress (Cone Islands) Chlorophyll a (ug/L) 

Depth (m) 

Figure 16.  Algal density versus depth from

vertical profiles near Cypress Head about 2  

m north of the farm site in a well mixed

area.  

North of the farm site in Bellingham Channel (Fig. 16), the few available data also show

the unexpected general increase in algal density with increasing depth. South of the farm

site at the entry to Bellingham Channel (Fig. 17), a more clear trend of declining chlorophyll

concentration with depth, similar to the far north end of the channel (Fig. 18).  

Figure 17.  Algal density versus depth from

vertical profiles about 2 km south of the

farm site near the entry to Bellingham

Channel and nearer Anacortes, Washington.

Figure 18. Algal density versus depth for

vertical profiles near the north end of

Bellingham Channel about 10 km north of

the farm site.  

NPS surface waters had reduced salinity from the normal 28.5 to ~26 psu or less and the

mixed layer extended to relatively great depths of 30 to 50 m or more in many areas.  For

comparison, full strength seawater in the North Pacific near Washington State is ~32 psu or

more.  Interestingly, there is evidence that the optimum salinity for Heterosigma growth is ~

25 psu (Haque and Onoue 2002) or very near the salinity observed in areas just north of

Cypress Island during the June‐July 2006 bloom.  Of course Heterosigma grows faster than

this in warmer waters (> 1 d‐1

), but how the growth relates to harmfulness to fish is

unknown.  
Fish kills from Heterosigma akashiwo in Puget Sound: recent blooms and review                 28

Salinity (psu) vs Depth (m) 29 Aug 06

-35

-30

-25

-20

-15

-10

-5

0

25.0 26.0 27.0 28.0 29.0

Site1 D1

Site 3 D1

Site 1 D1 slack water

Site 3 D1 PM

North Cypress D1

An example of the range of salinity recorded during the bloom is shown in Figure 19.  

Note the reduced surface salinity to about 15m depth from the station near the north end of

Cypress Island.  This same water ebbs through Bellingham Passage to the farm site but is

mixed by turbulence when flowing around headlands and over irregular bathymetry.  A

relatively large concentration of Heterosigma cells was found in this sample (both from in

vivo fluorescence and cell counts) at 5 m depth especially.   Note too how salinity changed

within a few hours at fish farm Site 1 from 27.2 to 28 psu.  This illustrates the dynamic

nature of these waters and the patchiness of conditions that make it difficult to generalize.  

Figure 19.  Salinity versus depth for 29

June 2006 near Cypress Island at fish

farm sites 1 and 3 (both in Deepwater

Bay) compared to conditions near the

north end of the island by the Cone

Islands.  

Some nutrient data were collected,

including nitrogen samples during the peak of

fish mortality in Deepwater Bay at Site 3.  

Unfortunately, the laboratory that analyzed

the data did not conduct nitrite or

ammonium analysis, but the nitrate results

are of interest. Figure 20 shows these data

compared to chlorophyll a and water

temperature profiles.  For reference, the 1⁄2

saturation constant for Heterosigma growth

in saturating light conditions is between ~ 1

and 2 uM for nitrate or ammonium and there

apparently is a slight preference for the latter

(Herndon and Cochlan 2007, Johns and

Glibert 2007).

Figure 20.  Nitrate, in vivo chlorophyll a, and

water temperature for Deepwater Bay on the

west side of net pen Site 3 on 29 June 2006.   

Thus Heterosigma cells at the peak of the fish kill were subject to over twice the

calculated 1⁄2 saturation rate concentration, which means they may not have been growth
Fish kills from Heterosigma akashiwo in Puget Sound: recent blooms and review                 29

limited (if there was some ammonium or urea available) or very marginally growth limited by

N supply (if not).  Note as discussed in this report, these levels are much lower than normal  

for the fish farm sites which is  a short term product of the Fraser River plume that was

dominant throughout the area at this time.   As an aside, in Figure 20 note relatively high

algal abundance with only modest decline to depth of 30m.  

Data from a University of Washington cruise of the oceanography vessel RV Thompson at

Station 26 (just south of Deception Pass in the far eastern Strait of Juan de Fuca (herein “the

Strait”) is shown in Figure 21 from the early morning of June 28, 2006.  These data indicate

relatively cool waters and the presence of a surface layer of reduced salinity waters.   Also

the presence of a modest chlorophyll signal in the top 15 meter of water of ~4 ug/L was

noted but these are not bloom quantities typical of Heterosigma.  It is impossible to say, of

course, what species those alga(e) might have been but conditions were generally suitable

for Heterosigma survival, although significantly cooler than is preferred.  Observations from

fixed wing aircraft confirmed the presence of the bloom in U.S. waters of the eastern Strait

but nearer Port Angeles.  No surveys were recorded on the Canadian side of the Strait that I

am aware of during this bloom.  

These data and many others from the

PRISM cruise (not shown here for brevity,

Fig. 22) as well as aerial surveys in the

central Strait, observations in and near

Port Angeles Harbor and the central Strait

by HAB technicians working for the fish

farm suggest that the bloom had not

begun moving into the Strait of Juan de

Fuca or the Pacific Ocean at this time but

that some cells were present, as

previously discussed.    

Figure 21.  Vertical profiles of salinity,

chlorophyll, temperature and density from

station P26 (see Fig. 22) the eastern Strait of

Juan de Fuca, west of Whidbey Island and

south of Deception pass from the morning of

June 28, 2006.
Fish kills from Heterosigma akashiwo in Puget Sound: recent blooms and review                 30

Figure 22.  University of Washington research vessel

sampling locations.  Data from stations P21 though

P26 from June 28th, 2006 were inspected for this

review.  

4It is possible that Heterosigma cells were

being transported through the eastern Strait in

late June, but tidal and wind mixing may have

reduced their concentrations in vertical mixing.  I

believe this is entirely plausible as there were

persistent winds of relatively high velocity during

this time period.  At the time we were greatly

impressed that the bloom managed to continue

under such strong winds, but no data except

aerial observations were collected from the Strait

during the peak of the bloom (~July 2nd) to

quantify the extent and intensity of the vertically

stratified surface layer. By July 2, 2006 the bloom

had commenced in earnest in Port Angeles Harbor and adjacent areas of the Strait of Juan

de Fuca.  Several days later NOAA HAB field scientists reported observations of the bloom

offshore of the Olympic Peninsula in the Pacific Ocean.  It may have been a coincidence, but

most likely the bloom was riding the reduced salinity surface layer out of the Strait, which is

the normal estuarine circulation pattern.    

August 2006 Bloom Chronology of Events

A second Heterosigma bloom occurred later in the summer, but this one was isolated in

Central Puget Sound and was not seen in North Puget Sound or the Strait of Juan de Fuca.  It

was also not monospecific, but mixed with other phytoplankton cells.  It is likely that this

bloom was a result of the extremely warm, calm weather that occurred at that time.  It is

possible that cells from the North Puget Sound bloom were present in this area to initiate

the bloom, but regular monitoring during the May through July period did not indicate the

presence of cells or any north to south transport, which would have been against the

outward surface flow to the Strait.  A remarkable feature about this bloom was that the least

affected area was the Clam Bay location which is where two major fish farms were

operating.  Very few fish mortalities occurred, and they may not have been due to

Heterosigma.  At the same time waters both to the east (Sinclair Inlet) and to the west

(central Puget Sound main basin and east shore of Bainbridge Island) had significant

quantities of Heterosigma and other phytoplankton. The bloom extended into North Hood

Canal and an indeterminate distance to the south in the main basin.  

I made limited observations around Bainbridge Island (backwaters near Bremerton WA

and to the north) and in the main basin of Puget Sound all the way to Seattle on August 2nd

and 3rd, 2006.   I circumnavigated Bainbridge Island, taking water samples and CTD casts at

numerous locations.  An aerial survey was performed by Kevin Bright on the 3rd of August
Fish kills from Heterosigma akashiwo in Puget Sound: recent blooms and review                 31

too, and the results showed patches of the alga throughout Central Puget Sound with major

concentrations east and west of Bainbridge Island but not near the fish farm, similar to other

observations by myself and repeated cell counts by fish farm technicians (Figure 23).

Concurrently, an Alexandrium

catenella bloom that had initiated

about three weeks before the

Heterosigma bloom was still in

progress.  Subsequently, this turned

out to be a major PSP event for

Puget Sound.  The fact that fish

farmer technicians clearly identified

the fact that there was a A.

catenella bloom of major magnitude

prior to the closure of shellfish beds

indicates the potential power of

routine monitoring.  Some kind of

formal or informal link between the

fish farmers and the Washington

Dept. of Health to aid in early

warning is discussed in the

recommendations section below.  

(see inset news item, this page)  

Figure 23.  Aerial survey results of August 3,

2006 from Central Puget Sound conducted by

Kevin Bright, AGS biologist.  

Extensive cell count and bloom

distribution data were collected during

this bloom by the fish growers and I

collected vertical profiles of water

quality as in the June bloom in NPS.   

The bloom was generally more

restricted to near surface layers than

the June 2006 NPS bloom (Figure 24)

and as it was not unialgal, no

conclusions about vertical distribution

can be made (although the fish growers

collected and recorded such data).  

Closure of area shellfish beds prompts worries

over health  

By Curt Woodward  The Associated Press August 26, 2006:   

“OLYMPIA ‐ The worst red tide in perhaps a decade

has shut down shellfish beds all along Puget Sound and

prompted serious public‐health worries, state officials

said Wednesday. The state Health Department said the

newest round of closures means virtually the entire

shoreline from Everett south to the Nisqually River just

north of Olympia is off‐limits for shellfish harvesting. The

eastern Kitsap Peninsula also has been affected, along

with areas near Port Gamble, Port Ludlow and along the

Strait of Juan de Fuca, said Frank Cox, a Health

Department marine‐biotoxin coordinator. ‘I don't think

we've ever had anything quite to this scale," Cox said

Wednesday.  “I'm concerned if people ignore these

warnings, we could wind up with people with illness, if

not worse."
Fish kills from Heterosigma akashiwo in Puget Sound: recent blooms and review                 32

Figure 24 .  In vivo chlorophyll a profiles from 3

August 2006 in central Puget Sound.  

Figure 25.  Sigma T (density) of same

stations shown in Figure 23.   

Figure 25 shows sigma T density vertical profiles for the same CTD casts discussed above.  

These data indicate that the fish farm site (Clam Bay in this case) was much less vertically

stratified than other stations around Bainbridge Island or in the main basin of Puget Sound.  

Surface water temperatures were 1 to 3°C cooler and slightly more saline than the other

stations, probably as a result of vertical mixing in adjacent channel areas of Rich Passage.  

Similar conditions recurred throughout the August 2006 bloom which may have protected

the fish farm area, although in the past this has not always been the case.  Connell and

Jacobs (1999) report significant water temperature and salinity layering in the same area

during the July 1997 bloom.  In discussions with Dr. Connell and looking at river data, there

was a very unusual and large mid‐summer discharge of the distant Skagit River that

exceeded 20,000 cfs just before the bloom3

.   Even the density differences created by the

temperature layering could have created a highly suitable bloom condition even without the

effects of salinity induced vertical

stratification but it was likely that both

were pronounced.  

Figure 26.  Dissolved inorganic nitrogen

(DIN) mean and standard deviation and

components of nitrate, nitrite and

ammonium from 5 locations shown in

Figure 23 and 24, August 2006.  

                                                      

3 I detected probable systematic error in the typical and bloom period profiles (figure 3) of that publication

but the relative difference (drop in salinity and increase in temperature) were undoubtedly correct.  Profiles of

salinity from several stations in CPS collected by the Washington Department of Ecology about the same time

period from their routine monitoring program show > 25.9 psu at the surface in all cases, typically ~ 27 psu.
Fish kills from Heterosigma akashiwo in Puget Sound: recent blooms and review                 33

Skagit River peak discharge has been attributed as the primary cause of Alexandrium

catenella bloom spread to CPS by Nishitani et al. (1988).

During the August 2006 period I collected vertical profile samples for DIN analysis (University

of Washington Routine Chem. Lab.) for the same stations discussed above.  Figure 26 shows

that DIN averaged about 5 μM with about equal amounts of nitrate and ammonium.  The

averages for nitrate would have been higher, but one station (confluence of Port Orchard

and Sinclair Inlet) was in a very strong tidal mixing area and mixing was in progress during

sample collection.   Ammonium was, however, similar there despite the active mixing.  

Connell and Jacobs (1999) reported a significant drop of nitrate (no other N species

reported) for the 1997 bloom in central Puget Sound to 1.4 μM for several days from the

normal levels of about 15 μM.  They attributed this drawdown to the bloom, not to a prior

diatom bloom as had been stated as a bloom requirement by Max Taylor and his colleagues

in British Columbia.   As discussed above, nitrate was undetectable in surface waters of

Admiralty Inlet about a week prior to the 1990 Heterosigma bloom and fish kill, but

detection limits were high for the available agency data.   Cumulatively, these data do

indicate that dissolved nitrogen concentrations usually are much less than normal during

Heterosigma blooms for at least short periods.  Urea was not measured in any case, but with

regard to fish farm effects, salmonids produce very little urea compared to ammonium

(Brett and Zala 1976).   

Figure 27.  Aerial photograph of

Heterosigma bloom front in

central Puget Sound near Elliot

Bay (Seattle).    

Management of Farmed Fish  

As a scientist with some experience in harmful bloom studies and fish farming, I offer the

following observations about how fish farmers managed their fish during the bloom.  I

restrict my observations to the Cypress Island site only, where I was present for several days

during the peak of the June 2006 bloom.    
Fish kills from Heterosigma akashiwo in Puget Sound: recent blooms and review                 34

American Gold Seafood LLC (AGS) staff had anticipated the late June 2006 bloom, as cells

had been seen in the water but the first clue was that the fish seemed to behave unusually.  

AGS immediately increased its normal sampling frequency on Sunday June 25th when it

became apparent that a few cells were present in the water.  Sampling by boat in adjacent

regions and aerial surveys were carried out over the following 10 days to monitor the

bloom’s vertical and horizontal distribution so that appropriate management measurements

could be taken.   

In the past towing of the cages away from the blooms has been successful both here and

internationally (Anderson et al. 2001) and is considered one of the most effective measures

to reduce fish loss due to algal blooms.  But in 1997 the towing caused pen damage and

escapes of fish from one site so at present towing is not allowed in Washington State.   To be

effective there needs to be a refuge area that will remain bloom free.  No such area exists

near Cypress Island although for central Puget Sound pens the Colvos Passage area west of

Vashon Island has filled this need in the past.  Strong currents and vertical mixing apparently

reduce or diminish the concentration of algal cells in surface waters there.      

Fish farmers were challenged with several tasks during the 2006 blooms including

attempts to mitigate the effects of the bloom on living fish and the need to quickly remove

dead fish from the net pens at the same time.   For the former, fish feeding was suspended

early on after the onset of the bloom to reduce digestive demand for oxygen that could be in

short supply due to damaged gills.  Cell counts were taken from discrete water samples from

several depths to judge if upwelling of water would be warranted.  If deep water (>10 to 12

m) had lower than surface concentrations of the alga, diesel compressors were started to

pump large volumes of air to depth to create an “airlift” of deepwater to the surface that

would displace the alga‐rich surface waters (Fig. 28).   There are potential drawbacks to this

method, such as the creation of vertical convection cells of circulating water that some

authors have argued exist at some times.  But in many cases upwelling that is coupled with

some sort of vertical profiling of algal cells seems to be a useful mitigation method.  

Figure 28.   Upwelling water inside a pen a

minute after a compressor and airlift

assembly was turned on.   

As plankton may be patchy in their

vertical distribution too, a cell count

conduced 10 or 20 minutes in the past

may not be indicative of conditions that

exist at a later time.  To deal with this

during daylight hours, farm managers

watch the color of the upwelling water

to judge real time cell concentrations.   

I conducted a few lateral sweeps with

my fluorometer‐equipped CTD to verify that upwelling water was indeed lower in algal

concentrations and in one case, where it was not, the farm manager had already noted this
Fish kills from Heterosigma akashiwo in Puget Sound: recent blooms and review                 35

visually (with no cue from me whatsoever) and shut down the system.   Visual observation

systems are useful but do not offer any means of operation at night and often fish mortality

in British Columbia increases at night (N. Haigh, Harmful Algae Monitoring Program, email

July 10, 2006) so the availability of an in situ fluorometer for unialgal bloom monitoring is

recommended.   

After a few minutes the extent of the upwelling covered at least 1⁄2 of the surface area of

the cages.  Managers visually inspect the color of the water to determine if deep water is

better or worse than surface water and if upwelling should be continued.   

Cell Shapes, Sizes and Motility Relative to Fish Mortality

During this bloom technicians and managers noticed that there were different types of

Heterosigma cells as previously noted by researchers and fish farmers in British Columbia.  

These included non‐motile (sessile) cells that did not move but otherwise appeared healthy

in all other aspects.  On one occasion during the peak of the bloom at Cypress Island we

observed one such cell actually dividing (asexual reproduction) into two daughter cells.   It is

possible that some or all of the non‐motile cells were “cyst, or non‐motile round cells” as

noted by Connell et al. 1997 towards the end of the 1997 bloom in CPS.  However, Han et al.

(2002) report such resting cells as:  

“completely immobile, although both flagella remained attached. Heterosigma resting

cells did not require a maturation period before successful activation to the vegetative state

could occur. Cell division and motility were impacted sequentially during both the induction

and activation phases of resting cell development”.   

Figure 29.  Strands of distinctive purplish‐

red Heterosigma akashiwo as visible

from a small plane while flying over

backwaters on the east side of Guemes

Island, Puget Sound (Photo by K. Bright).  

Recommendations

The following recommendations are made to assist in regional understanding of

Heterosigma bloom dynamics, fish mortality, monitoring and mitigation.  
Fish kills from Heterosigma akashiwo in Puget Sound: recent blooms and review                 36

Wild Fish   As discussed above, we know very little about the effects of Heterosigma

blooms on wild fish.  It is clear that wild fish are killed in some cases by the alga and that

some blooms are not restricted to the immediate surface water and again, the fish sink

when killed in our cool temperate waters, thus being less visible to detection.  A simple

review of fish ecology and distribution of Puget Sound using the extensive data developed by

University of Washington, state agency and NOAA workers would identify where juvenile or

migrating fish would be expected during the risk periods and surveys with video drop

cameras or camera sleds could be mounted during periods when the farmed fish are being

killed.  In this manner the farmed fish have been and continue to be a “canary in a

mineshaft”.  Agency and public interest as well as research funding for adequate

Heterosigma research work could well pivot on this issue.  Few are concerned about

aquaculture fish in the Pacific Northwest; many are interested in the fate of wild fish.     

Etiology of Fish Mortality   Several researchers in North America remain interested in

Heterosigma and the mystery of how it kills fish or other aquatic organisms.  I believe there

is enough evidence to suggest that bacteria or some other interacting factor in non‐axenic

populations of the alga may be the key to understanding the cause of fish mortality. There

are a number of likely explanations discussed above, but so many are potentially viable and

may possibly co‐occur that to probe them all adequately would require a massive research

effort.  Whatever the ultimate cause(s), I believe that we have hardly begun to link field and

laboratory approaches to helping unlock the mystery.  While the basics of bloom initiation

location, transport and timing are better understood now than 20 years ago, we have not

begun to tap existing resources to more explicitly characterize these blooms and the cause

of fish death, as noted below.  For example, side by side exposures of different species of

fish with sampling of fish tissue upon death or distress would also aid in this effort.   To date,

such work has been very limited and pathology work very incomplete in my opinion.  The

fish gill histology studies and results are very limited in scope and conflicting in conclusions.

There also has been a very limited attempt to collect and isolate toxins or causative

chemicals from actual fish kills in the field in North America.  With a algal species that seems

to “lose” its harmfulness when isolated in axenic cultures, this seems like an obvious

approach that could easily yield results.  Insight may be gained from other simple laboratory

approaches, such as treating field samples of cells that have freshly killed fish with

antibiotics or antiviral compounds and compare results of fish bioassays of untreated

samples to see if harmfulness can be manipulated.  We know that cultures held for long

periods in the laboratory are not harmful to fish at reasonable concentrations, but how they

lose their harmfulness is perhaps a valid way to assess the etiology of fish mortality.  

Another example would be to separate small, non motile cells from larger, swimming cells

and test for differences in harmfulness.  This could be done through established methods

that utilize the alga’s response to very small amounts of freshwater (Hershberger et al. 1997)

or by other methods.

Solutions may not come from Asian researchers, as Heterosigma is not an important fish‐

killing species in China and Japan (although it does so in New Zealand, Chile, and Scotland).

Much of the North American work on this species is in the laboratories of Drs. Trick, Cochlan,
Fish kills from Heterosigma akashiwo in Puget Sound: recent blooms and review                 37

Glibert, Cattolico and Strom.  There is a need for government funding for these laboratories

to continue this difficult but promising work.  

Figure 30.  Atlantic salmon with

secondary lamellae of gills visible after

having been killed during a

Heterosigma bloom.

Modeling:  the existing conceptual model of bloom initiation and spread could relatively

easily be parameterized and modeled if support were available.   The repeating nature of

these blooms suggests some significant physical forcing factors, as discussed above, and the

occurrence during clear weather would allow the use of satellite sea surface temperature

and chlorophyll a sensor data to map blooms and correlate distribution with existing and

developing hydrodynamic models, routine and special monitoring and other forms of remote

or in situ monitoring.   As discussed herein, the conceptual model of bloom development

and spread is rooted in years of experience and provides relatively accurate guidance for fish

growers, but is not adequate for further scientific and Puget Sound wide use at present.  

Response to Blooms:  It is clear that a very rapid response to blooms is necessary to

derive useful information and data. This involves having people and equipment ready to go

on a moment’s notice during the bloom risk period.  Awareness about the blooms, both with

agencies and the public should also be heightened.  In the first documented blooms there

was more of a response from state and federal agencies, but little or no state agency interest

has been shown in recent years and NOAA‐NWFSC staff are interested, but their funding

does not include Heterosigma specifically at present.  Some sort of coalition of interested

parties could be assembled to delegate tasks and coordinate sampling.  The SoundHAB list

serve provided by NOAA and WHOI are steps in the right direction in this regard to maximize

coordination and minimize expense, although this is a volunteer service I provide.  

Remote Sensing Monitoring:  Presently little use is being made of available remote

sensing resources for bloom detection and monitoring in Puget Sound.  Both Heterosigma

and PSP causing Alexandrium catenella blooms are closely linked to fair weather patterns as

described above, so satellite data could be very useful for detecting onset of blooms even

though A. catenella does not usually form monospecific blooms in our waters and is usually

present in relatively low numbers.   For subsurface waters, there is an urgent need for a

Puget Sound wide buoy sensing system to vertically profile conditions real time.  Presently

buoys are being used in Hood Canal, but without source water buoys in the Strait or more
Fish kills from Heterosigma akashiwo in Puget Sound: recent blooms and review                 38

frequent monitoring; water and nutrient budgets for such threatened areas will remain the

subject of guesswork and speculation because of the dynamic nature of source waters4

.   

Figure 31.  Greenish‐yellow colored bloom of

Heterosigma seen in June 2006 in Deepwater Bay and

throughout the region. These blooms are sometimes

green colored when viewed from a boat, but curiously

appear a shade of purple‐red when viewed from

airplanes.  

In the future an integrated monitoring system

could house molecular‐based HAB detection

systems, which are presently operational but

require refinement in terms of longevity of

operation and reduction of bulk size.  

Early warning of all HABs:  Presently the

system that the fish farmers operate is self‐

sufficient but participating in a regional‐wide

program might offer additional capabilities that

would improve bloom forecasting and response

management capability.  I believe that NOAA

scientists are aware of the importance of having fish farm participation in a regional wide

monitoring system. The fact that fish farmers know and routinely sample phytoplankton

populations throughout the growing season in three major basins of Puget Sound is

potentially a major societal and scientific contribution.  No other routine monitoring

program in Puget Sound exists or is likely to in the near future.  As mentioned herein, in the

future there may be molecular‐based automated monitoring, but until then it would be

helpful to our society to be able to utilize the efforts of the fish farmers who are willing to

share their data.  The fish farmers are in the business of growing fish, so they are not able to

devote the time to quality assurance and distribution or maintenance of their data base.   It

would seem that this is a role for government.  I strongly believe that biology is a better,

more sensitive indicator of the health of the environment compared to water quality

measurement, a belief rooted in 35 years of experience with both.  Ecosystem based

management of our coastal waters cannot be accomplished if we have little idea about the

status and trends of the species responsible for primary productivity!

Through the SoundHAB list serve, I voluntarily distribute important HAB data acquired by

the fish farmers, but more formal linkages may be in order.  For example, detection of the

massive summer 2006 PSP toxin event in the central basin of Puget Sound was made several

weeks before it became evident in shellfish toxicity testing.  Such early warning could be

                                                      

4 The Strait of Juan de Fuca is highly dynamic, monthly monitoring is inadequate to capture its nature, see

review by Rensel 2002 at http://www.wfga.net/SJDF/reports/2001annualrep.pdf
Fish kills from Heterosigma akashiwo in Puget Sound: recent blooms and review                 39

shared with State Department of Health officials in the future once communication channels

are made.  Similarly, the threat of domoic acid poisoning remains in Puget Sound and

observations of Pseudo‐nitzschia spp. diatom blooms could aid in this effort.  There is no

direct benefit to the fish growers for such a service but if it was facilitated by government in

some fashion it could be a societal benefit.  It may be possible to gain more than HAB

warnings from the system, this is a topic that should be raised and discussed by responsible

agencies.   

In this report I have identified likely Heterosigma bloom initiation areas, based on where

blooms are first noticed.  A survey for detection of viable cysts could be conducted to more

precisely locate these areas using techniques worked out in Japan (Imai and Itakura 1999).  

Fish Farm Bloom Management   The performance of fish farm staff and management

was exemplary during the 2006 and 2007 bloom event and demonstrated how far the

monitoring and mitigation has come in the past 20 years.  I offer only a few

recommendations to improve the ability to predict and deal with future blooms should they

recur. Most of the monitoring is either visual from airplanes or by cell counts.   As the

blooms are often monospecific in North Puget Sound particularly, use of a chlorophyll sensor

probe to monitor algal cell concentrations and could reduce the need to perform cell counts.  

A chlorophyll a to cell count ratio was developed during the 2006 bloom that had little

variance over short time scales of days.  We performed side‐by‐side chlorophyll sensor casts

along with cell counts in a number of locations during that bloom.  Although still preliminary,

it appeared that each unit of chlorophyll a (in ug/L = ppm) was equivalent to about 100,000

cells/L of the alga (after post‐calibration of the unit with laboratory extraction

measurement).  Regardless of the relationship, it would be useful to be able to rapidly

predict where cells are concentrated to avoid pumping up water that might, in some cases,

be worse than surface water.  Such a probe could also be of use during a harmful

Chaetoceros event too, although they tend to be mixed in with other species.   

Several different manufacturers produce chlorophyll sensors.  Some, like the Turner Co.

SCUFA unit I use can be used with a multiprobe CTD or alone with a cable connecting it to a

personal computer or data logger.  A single sensor could be purchased and used at different

sites as blooms often do not co‐occur at varying areas of Puget Sound at the same time.  

They are relatively easy to calibrate and hold their calibrations well with use of a solid

reference standard.   Recently the fish farmers have started recording water quality

continuously, and the combination of temperature, salinity and in vivo chlorophyll a will

provide a strong suite of measures to assess risk and variance from normal conditions.  

      There are molecular tests for Heterosigma presence such as the sandwich hybrid assay

(e.g., Tyrrell et al. 2002) that has been recently improved and miniaturized (Marin et

al.2007).    The test takes some equipment and supplies and about an hour to assay 8

samples.  Since live, wet mount identification or enumeration with a microscope is relatively

easy and quick for an observer with minimal training5

, I see no advantage to a relatively

complex test for Heterosigma detection in Puget Sound as long as live samples are used, but

                                                      

5 Fish growers use 0.1 ml on a wet mount slide which allows them to slow down the live cells to allow easy

counts.  Sometimes the sample can be chilled slightly that slows down the alga’s swimming.  
Fish kills from Heterosigma akashiwo in Puget Sound: recent blooms and review                 40

for preserved samples the method has definite advantages (O’Halloran et al. 2006).  The

assays seem more appropriate for detection of Heterosigma in other areas that may be at

risk that have mixed assemblages of phytoplankton where other HABs may need to be

monitored too.  HABs that occur in mixed phytoplankton assemblages in Puget Sound

include A. catenella (PSP) and Pseudo‐nitzschia spp. (ASP).  Undoubtedly other HAB species

will eventually bloom in Puget Sound, e.g., Cochlodinium polykrikoides and a fish killing

Chrysochromulina (possibly C. polylepis) have killed fish in British Columbia in recent years.

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Fish kills from Heterosigma akashiwo in Puget Sound: recent blooms and review                 43

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Fish kills from Heterosigma akashiwo in Puget Sound: recent blooms and review                 44

Appendix A.   Report on June­July 2006 Heterosigma Bloom ­ Cypress

Island and Port Angeles  

Prepared By Kevin Bright, American Gold Seafoods, July 20, 2006  

Cypress Island Sites Chronology

Monday, June 26th – Cypress sites see first signs of Heterosigma. Evening shift employee,

Svein Wiese Hansen, observed some “jumpy” behavior in fish stocks and performed a

plankton count prior to doing the evening feed. Cell counts are approximately 150,000

(150K) cells per liter of Heterosigma at 4:30 PM. Kevin Bright and Site Manager, Bill Clark,

were notified by phone and a decision is made to not feed the fish this evening. Counts were

zero earlier in the day and this sample was taken on a flood tide which typically brings clear

water from the south end of Cypress Island. Heterosigma counts go down as flood tide

continues into the evening. Plankton counts later that night are in the 20K to 30K cells/liter

range.

Tuesday, June 27th – Fish still off feed from previous night. ‐ Kevin Bright takes plankton

flight (11: 00 AM) looking at immediate area around Cypress Island, Guemes Channel and

north of Sinclair Island. Red streaks and brown water seen in Fidalgo Bay, Samish Bay and

the eastern side of Guemes Island, water on west side of Cypress and to south of Cypress

look clean with no strong colors in that sector. Data from cell counts starting at 6:00 AM. All

cell counts are from Site 1 location unless otherwise noted. Cell counts start out low in the

morning hours but continually climb through out the day. Peak counts are 1.1 million cells

per liter at 3:45 PM. Counts go down as flood pushes in cleaner water.  

0600 – 80K cells @ 1.5 m (ebb tide)

0930 – 140K cells @ 1.5 m  

1100 – 370K @ 1.5 m (tide begins flooding)

            510K @ 10.0 m

1545 ‐ 1.1mill @ surface (fish showing some stress signs)

            870K @ 10.0 m

1900 – 60K @ surface  

            50K @ 1.5 m

Wednesday, June 28th – Fish still off feed. Plankton flight (at 10:30am) to look further

north at waters during ebb tide. Reddish browns streaks continue on east side of Guemes

Island, water north of Sinclair brownish, but no strong reddish browns. Water further north

toward Clark and Barnes Island, west side of Lummi appears milky with Fraser River fresh

water at surface and patches of reddish/brown color indicating plankton colonies thriving in

brackish water. Brownish, brown and red colors appearing in these patches appear to be
Fish kills from Heterosigma akashiwo in Puget Sound: recent blooms and review                 45

dinoflagelate blooms (Heterosigma, Noctiluca and Alexandrium) Water appears stratified.

Weather sunny and hot. Northwesterly winds 10 knts.

0600 – 70K cells

0830 – 180K @ 1.5 m

            180K @ 10.0 m

1130 – 130K @ 1.5 m (ebbing)

1600 – 150K @ 5 m  

1620 – 860K @ 1.0 m (flood tide)

1645 – 300K @ 8.0 m

1710 – 1.8 M @ 1.0 m (fish stressed and some roll overs seen)

1900 – 980K @ 1.0 m

2230 – 110K @ 1.0 m (water cleaned up at end of flood)

As can be seen from the plankton data, Heterosigma counts started out the day low and

remained low through out the ebb tide. During the strong ebb tide, reddish/brown bloom

water was pulled south down through Guemes Channel and around the north end of

Guemes Island. This water was then pushed into the Deepwater Bay area and into the

Cypress net pen sites during the flood tide. The start of the flood tide brings water from a

south easterly direction toward the farm and into Deepwater Bay. At the end of flood tide,

the current velocity strengthens and the water comes from a more southerly direction.

Counts peaked at 1.8 million cells per liter around 5:00 PM which was toxic to the fish. It was

noted by Bill Clark, Kevin Bright and Dr. Jack Rensel that a large number of the cells of

Heterosigma appeared smaller and less motile than previously seen. The “resting” cells were

still enumerated in the samples, as they were positively identified as Heterosigma under the

microscope. It should also be noted that there were high number of cells even at 10 to 12

meters in depth. As the flood tide progressed and the currents strengthened, cleaner water

replaced the bloom water and the cell counts subsided.  

Thursday, June 29th  

Fish still off feed.  Fish showing signs of stress and mortality. Floaters in some pens.

Heterosigma counts started out high in the morning at 800K cells per liter. As the day

progressed the cell counts peaked again at 1.8 million cells per liter around 6:30 PM. Fish

were visibly stressed through out the day, with fish gasping and rolling over in the corners of

the pens. Mortality increased significantly in the afternoon with increasing cell counts

recorded during the beginning of flood tide. Water in Guemes Channel, Bellingham Channel

had high counts in patches and mixed bodies of water. High counts recorded in Guemes

Channel (approx., 4.0 million cells/liter) and Clark Point (2.2 million), Bellingham Channel

(1.3 million). As the flood strengthened toward the end of the tidal cycle, water

temperatures dropped and plankton levels flushed out of Deepwater Bay, being replaced by

deeper and clearer water from the south end of Cypress Island. Turn over of the water

column occurred rapidly, and was easily visible from the surface and by the behavior of fish

stocks. Fish began schooling and circling in pens, their coloration returned to normal.
Fish kills from Heterosigma akashiwo in Puget Sound: recent blooms and review                 46

0700 – 810K @ 1.0 m

            510K @ 10.0 m

1000 – 950K @ 1.0 m  (SE Cypress Head)

1030 – 250K @ 1.5 m

            350K @ 10.0 m

1230 – 1.7M @ 5.0 m (Clark Point 4 miles NE of fish pens)  

            2.2M @ 10.0 m (Clark Point 4 miles NE of fish pens)

1430 – 750K @ 1.5 m (Site 1)

            650K @ 10.0 m (Site 1)

1445 – 1.26M @ surface (Site 3)

1630 – 1.3M @ 1.5 m  

            320K @ 320K

1830 – 1.8M @ 1.5 m

            640K @ 10.0 m  

1845 – 1.1M @ 1.5 m (Site 3)

            350K @ 10.0 m (Site 3)

2015 – 750K @ 1.5 m  

            220K @ 10.0 m

2030 – 170K @ 2.0 m (end of flood)  

            160K @ 10.0 m  

Friday, June 30th – Fish still on starve. Fish seemed to be handling plankton levels better

today. Significantly less mortality found in pens that had been cleaned out on the previous

day by divers. Concentrations at the farm did not reach the peaks of earlier two days. By

9:00 PM water appeared to be clearing up with the flood and fish were visibly better in

appearance.

0630 – 360K @ 1.5 m

            460K @ 10.0 m

0900 – 900K @ 1.5 m

            550K @ 10.0 m

1120 – 1.3M @ 2.0 m

1530 – 800K @ 2.0 m

1800 – 1.0M @ 2.0 m

            750K @ 10.0 m

2100 – 800K @ 5.0 m

Saturday, July 1st   Plankton counts going down and numbers of motile cells appear to

be increasing. Samples from Guemes Channel are still high (2.5 million). Water samples from

areas to the immediate north, east and south of farm appear to be diminishing quickly. Farm

counts down and fish appear to be doing better. Mort numbers way down.
Fish kills from Heterosigma akashiwo in Puget Sound: recent blooms and review                 47

0950 – 300K @ 1.0 m

1500 – 110K @ 2.0 m

              80K @ 10.0 m

1600 – 200K @ 1.0 m

Sunday, July 2nd   Fish doing better. Samples clearing up around known hot spots.  

0930 – 110K @ 2.0 m

             80K @ 10.0 m

1710 ‐  40K @ 1.5 m

Port Angeles Site Chronology:

Thursday, June 29th   First signs of Heterosigma. Plankton counts pick up 10K cells/liter at

the surface and 40K cells/liter at 5 meters. Fish feed in the morning, but taken off feed in the

afternoon by Site Manager Randy Hodgin. Weather sunny, hot and strong westerly winds

blowing through out the day.  

Friday, June 30th   Fish still off feed. Plankton counts show 40K cells at 1.0 and 5.0 meters

and 10K at 10 meters. Sunny, west winds.

Saturday, July 1st   Fish off feed. Plankton levels begin climbing through out the day.  

0900 – 160K @ 1.0 m  

            230K @ 5.0 m

            130K @ 10 m   

      1600 ‐ 500K @ 1.0 m

       360K @ 5.0 m

   180K @ 10 m

Sunday, July 2nd   Fish off feed. Plankton starts out at 1.0 million at 6:00 AM and climbs

to 2.7 million cells at 15 meters by 12:15 PM. Site manager begins seeing fish mortality by

the afternoon of the day in most pens. Airlifts being run on and off through out the day. The

same type of resting cells that were seen at Cypress are seen in Port Angeles.  

0600 – 1.0M @ 1.0 m

1.3M @ 5.0 m

500K @ 10 m

850K @ 15 m

1215 – 2.0M @ 1.0 m

1.6M @ 5.0 m

2.2M @ 10 m

2.7M @ 15 m

1615 – 1.8M @ 1.0 m

2.3M @ 5.0 m

2.1M @ 10 m
Fish kills from Heterosigma akashiwo in Puget Sound: recent blooms and review                 48

1.6M @ 15 m

1700 – 4.6M @ 1.0 m at Francis Street location south east of farm near shore.

Monday, July 3rd    Plankton counts start out high in the morning (2 million at 15 meters)

and peak at 3.5 million cells per liter at 1.0 meter by 4:21 Pm. Fish are extremely stressed

and rolling over rapidly. Farm is in full mortality clean up mode. The F/V Harvestor is

pumping fish into holds assisted by 2 to 3 divers. Weather continues sunny and hot, with

strong westerly winds blowing all day (15 to 25 knts). John Bielka and Kevin Bright charter

plane and fly along shoreline from Port Angeles eastward to Pt. Townsend, then south down

Admiralty Inlet to Rich Pass sites. The entire Harbor of Port Angeles is visibly a coffee color.

Water outside of Ediz Hook is brown, and water along shoreline to Pt. Townsend is brick red

to brown. Stronger colors near shore then dissipating as you travel north away from

shoreline. Discovery Bay and Sequim Bay are reddish/brown color. Bloom extends offshore

for several miles in some areas along the Strait. By Pt. Townsend color disappears. Water

looked good through Admiralty Inlet except for West Point area was brownish red in a very

localized area.  

0630 – 1.0M @ 1.0 m

1.5M @ 5.0 m

1.6M @ 10 m

2.0M @ 15 m

1045 – 2.0M @ 1.0 m

1.8M @ 5.0 m

2.1M @ 10 m

900K @ 15 m

1621 – 3.5M @ 1.0 m

2.7M @ 5.0 m

2.1M @ 10 m

1.1M @ 15 m

Tuesday, July 4th   Farm counts still high. Weather still sunny after morning fog burns off.

Still loosing fish through out the day. Harvestor continues pumping off pens.

1000 – 1.9M @ 1.0 m

2.0M @ 5.0 m

2.0M @ 10 m

3.0M @ 15 m

1330 – 4.4M @ 1.0 m

2.9M @ 5.0 m

1500 – 2.5M @ 1.0 m

2.8M @ 5.0 m

Wednesday, July 5th  Counts begin to go down at farm. Mortality rate drops. Weather

conditions begin to break down as weak system moves in.  
Fish kills from Heterosigma akashiwo in Puget Sound: recent blooms and review                 49

0700 – 400K @ 1.0 m

850K @ 5.0 m

1.0M @ 10 m

800K @ 15 m

1040 – 240K @ 1.0 m

130K @ 5.0 m

140K @ 10 m

140K @ 15 m

1300 ‐   30K @ 1.0 m

   80K @ 5.0 m

   70K @ 10 m

210K @ 15 m

1715 – 250K @ 1.0 m

530K @ 5.0 m

920K @ 10 m

850K @ 15 m

Thursday, July 6th   Water returns to normal and weather continues to cool down. Farm

cell counts return to low levels. Samples in morning show cells are sinking. Low levels at

surface (30K), and counts down to 35 meters of depth are nearly 16 times higher (470K)  

0700 – 30K @ 1.0 m

20K @ 5.0 m

60K @ 10 m

       160K @ 15 m

       470K @ 35 m

1330 – 200K @ 1.0 m

180K @ 5.0 m

   10K @ 10 m

   10 K @ 15 m  
Fish kills from Heterosigma akashiwo in Puget Sound: recent blooms and review                 50

Appendix B.  Harmful Algal Blooms and Finfish Resources in Puget Sound   

Reprinted from:    Rensel, J. 1995.  Harmful algal blooms and finfish resources in Puget

Sound. In R.M. Strickland (ed.): Puget Sound Research '98; Proceedings of the Water

Quality Action Team. Puget Sound. Olympia, WA, p. 422‐429.  

(Note: Heterosigma species name has reverted to akashiwo since this was published).  

INTRODUCTION

Several harmful algal blooms (HAB) species have economic and biological importance to

finfish resources by causing biotoxin accumulation in the food‐web or fish kills.  These

problems are apparently less prevalent for wild finfish in Puget Sound than in other U.S.

coastal areas, but there is uncertainty regarding their significance here.  Worldwide at least a

dozen genera of microalgae have been involved in mortality of wild or aquaculture fish

(Hallegraeff 1991).  Several occur in Puget Sound, although only three types have often been

implicated in fish kills locally (Table 1).  The table shows that little is known regarding

exposure concentrations of most microalgae that kill fish; the causes of physiological harm

are also poorly documented in several cases.  Table 1 should not be considered all inclusive,

as there may be other microalgae in Puget Sound that can cause fish kills.  Unexplained

losses of hatchery‐raised and wild fish have been reported from Puget Sound in recent years.  

For example, an unidentified dinoflagellate was responsible for losses of salmon smolts in a

state‐operated net pen in Hood Canal during 1993 (R. Horner and J. Rensel, unpublished

data).  Two types of harmful microalgae problematic in Puget Sound, Chaetoceros spp. and

Heterosigma carterae, are discussed below.

CHAETOCEROS SPP.

The marine diatom genus Chaetoceros is separated into 2 subgenera by the presence

(Phaeoceros) or absence (Hyalochaete) of long, partly hollow setae or primary spines that

contain chloroplasts.  Phaeoceros species, such as C. concavicornis  and C. convolutus have

more robust setae and frustules than Hyalochaete species and the setae are armed with

short secondary spines that point toward the distal ends of the primary setae (Cupp, 1943).  

With few exceptions, the chain‐forming Phaeoceros species are responsible for mortality of

fish, although dense blooms of Hyalochaete species were implicated in the poor survival of

one group of salmon smolts in Port Angeles Harbor net pens in 1988 (R. Elston and J. Rensel,

unpublished data).   

The first reported case of Chaetoceros‐caused fish mortality involved wild lingcod

(Ophiodon elongatus) that were captured and held temporarily in fishermen's cages in

British Columbia (Bell, 1961).  Subsequently these diatoms have been involved in occasional

losses of net‐pen salmon, including a major loss near Cypress Island in 1987 (Rensel et al.,

1989; Horner et al., 1990).  It has been suggested that harmful Chaetoceros setae break off
Fish kills from Heterosigma akashiwo in Puget Sound: recent blooms and review                 51

the cells and enter the gill tissue butt‐end first;  the apically‐pointed secondary spines on the

primary setae may then act like barbs on fish hooks (Bell, 1961).  This hypothesis and

reference is often repeated in the literature but is based on only a few samples of wet‐

mounted gill tissue.  Rensel (1992, 1993a) used scanning electron microscope techniques to

show that penetration of the gill tissue by C. concavicornis was uncommon.  Rather, chains

of cells tended to lodge between the secondary lamellae and be present in the surrounding

gill mucus.  Blood‐gas studies showed that affected salmon had severe blood‐hypoxia as a

result of mucus production during acute exposure or physiological damage to the gills after

longer‐term exposure.  Longer chains of C. concavicornis caused significantly lower blood‐

oxygen partial pressures compared to fish exposed to shorter chains.  Longer setae

associated with longer chains were apparently more likely to become wedged in the gills and

stimulate mucus cell release, lesions and epithelial damage.  

Salmon respond to C. concavicornis exposure by an immediate and periodic cough

response that diminishes slightly in frequency over time (Rensel, 1992, 1993a).  This is

similar to fish coughing caused by many environmental irritants and chemicals (Heath,

1987).  Coughing and mucus production act in concert to help clear the gills of the diatoms.  

Long strings of mucus were seen trailing from the gills of live fish during the 1987 Cypress

Island fish kill.    

Short‐term laboratory exposure to as few as 10 cells per ml of C. concavicornis caused a

rapid increase in mucus cell discharge on the gills as well as a severe hypoxia and elevated

carbon dioxide content in the blood of Atlantic salmon (Rensel, 1992; 1993a).  Long‐term

exposure to   < 5 cells/ml of harmful Chaetoceros in net pens has been reported to increase

disease and mortality of farmed salmon (Albright et al., 1993), although sampling in this

study may not have been frequent or extensive enough to assure that higher concentrations

of the diatoms did not occur.  Nevertheless, it is clear that low concentrations of these

diatoms can kill salmon, hence the term "bloom" is not always appropriate when referring to

harmful Chaetoceros‐caused fish kills.

It is likely that harmful Chaetoceros have caused mortalities of wild and hatchery‐

released fish.  South Hood Canal and  Dabob Bay/North Hood Canal are areas where these

harmful diatoms often occur (Rensel et al., 1989; Rensel Associates and PTI Environmental

Services, 1991; J. Postel, unpublished data).  During the fall of 1990 a mixed water column in

north Hood Canal containing Chaetoceros concavicornis to at least 40 m correlated with the

otherwise unexplained mortality of migrating adult chinook salmon (Rensel Associates and

PTI Environmental Services, 1991).  In southern Hood Canal, mortality of juvenile salmon in

seawater pumped from a subsurface depth was due to harmful Chaetoceros; this suggests

the possibility of a recurring problem for wild smolts moving through the area.  Harmful

Chaetoceros often occur in late spring and again in September or October, although they

may be present at any time of year in Pacific Northwest waters (R. Horner, unpublished data;

Rensel, 1992).  Their geographic range and relationship to occasional fish kills in Puget Sound

is not well defined.    

HETEROSIGMA CARTERAE
Fish kills from Heterosigma akashiwo in Puget Sound: recent blooms and review                 52

This raphidophyte flagellate is known worldwide as a fish killer and has caused significant

mortality of net‐pen salmon in Puget Sound in 1989 and 1990 after several years of

problems in British Columbia waters.  H. carterae may have been present for at least several

decades in Puget Sound before the 1989 and 1990 blooms.  Taylor and Haigh (1993) note the

species has appeared with great regularity in late spring in the Strait of Georgia since 1967,

when surveys were first initiated.  Appearance there coincides with a rise in water

temperature above 15°C and a decline in surface salinity to less than 15 psu.  The 1989

bloom in Puget Sound was apparently restricted to the northern areas of Puget Sound and

the San Juan Islands, but the July 1990 bloom involved large areas of northern, central and

southern Puget Sound.  In 1993, a bloom of H. carterae was recorded in Budd Inlet, the

southern most extreme of Puget Sound.

Recent laboratory studies suggest that reduced salinity plays an important role in bloom

formation of H. carterae.  By simply adding a small amount of distilled water to the surface

of vertical columns containing the alga, large quantities of the cells accumulated near the

surface (Hershberger et al., in prep.).  The importance of freshwater‐induced vertical

stratification in field studies of H. carterae has been observed by Taylor and Haigh (1993).   

Heterosigma blooms apparently require quiescent weather conditions, as was

experienced immediately prior to and during the 1989 and 1990 blooms in Puget Sound.  

Strong vertical stratification of the water column due to freshwater and perhaps solar

heating also seems to be a prerequisite.  The September 1989 bloom was apparently

associated with the Fraser River plume in Northern Puget Sound.  The July 1990 bloom

occurred in an extremely calm, warm period in early July following a June when Puget Sound

lowland precipitation was 51% more than normal (NOAA, 1990).  Moreover, routine

monitoring in Admiralty Inlet approximately 10 days prior to the 1990 bloom confirmed that

the surface layer was warm (near 15°C) and of reduced salinity (19.5 ppt) (Janzen, 1992).  

This is significant given the mid Puget Sound location of this sampling station, remote from

creeks and riverine sources of freshwater.  Other regional stations had similar values.  

Macronutrient content in the surface layer was undetectable, but H. carterae is capable of

vertical migration, moving up to 1 m/hr to obtain nutrients from subsurface depths.   

In mid‐July of 1993, a bloom of H. carterae occurred in the Port Orchard/Brownsville

area east of Bainbridge Island.  Antecedent rainfall and quiescent weather conditions were

similar to those of the 1990 bloom.  The bloom appeared to increase in apparent size for

several days but an abrupt weather change, including strong south winds, correlated with

the rapid termination of the bloom.  Soft bottom sediments of this protected area may be a

seed bed for some of the blooms.  

The physiological cause of fish mortality from H. carterae exposure is unknown despite

several laboratory studies that used clones or cultures taken from local fish kills.  

Histopathology of gills of moribund subadult salmon from net‐pen kills usually show major

damage to the epithelium and mucus buildup, but not in one case with juvenile fish where a

labile icthyotoxin was suspected as the cause of mortality (Black et al., 1991).  However, no

toxin has been detected in the edible tissues of any affected fish.  Although H. carterae

grows well in culture, most laboratory bioassays here and abroad have yielded few fish

deaths even when fish are exposed to very high concentrations of cells or conditioned
Fish kills from Heterosigma akashiwo in Puget Sound: recent blooms and review                 53

medium.  Several authors have suggested that the fish‐killing mechanism may be similar to

that of the related alga Chattonella antiqua that occurs in Japan.  Superoxide anion radical

and hydrogen peroxide produced by the alga reportedly strips the fish gill of mucus and

leads to fatal osmoregulatory stress (Tanaka et al., 1994).  Yet this does not fit with current

knowledge of salmonid physiology because the normal, unstressed condition is to have little

or no mucus on the gills (Handy and Eddy 1991).  A possible advance regarding laboratory‐

induced toxin production by Heterosigma was reported in 1994,  although few details are

available at this time (R.A. Cattolico, pers. comm. 1994).   

H. carterae blooms may extirpate or outcompete virtually all other algal species in the

upper water column (Taylor and Haigh, 1993).  Its effect on larval or other fish frequenting

shallow depths has not been studied, but there is evidence that wild fish or hatchery‐

released fish have been harmed.  There were several reports of wild fish kills and wild fish

acting in a distressed manner in Port Townsend Bay and the Strait of Juan de Fuca during the

1990 bloom.  Limited aerial surveys failed to validate the reports (R. Allen, J. Rensel,  survey

of 12 July 1990), but the surveys were restricted to the Port Townsend area and there was a

lag of at least one day between the reports and the surveys.  Dead adult chinook salmon

were seen by local boaters in Bellingham Channel during the 1989 bloom, but no tissue

samples were collected.   

More recently, in the fall of 1994, chinook, coho and summer chum salmon as well as

several marine fish species were killed by a Heterosigma bloom in upper Case Inlet of

southern Puget Sound.  It is a shallow area where several streams enter Puget Sound and the

fish were apparently unable to escape the bloom.  The full extent of fish loss was unknown,

but numbered at least in the hundreds.  State agency and university researchers were not

notified of the kill until well past its commencement, reducing the ability to collect useful

data.   

DETECTION AND MITIGATION OF FISH KILLS AND ALGAL BLOOMS

Detection of wild fish losses due to HABs in Puget Sound is a difficult task. Affected wild

salmon most likely sink to the bottom, similar to HAB‐killed salmon in net pens.  Usually

shoreline residents or boaters do not notice fish losses or will notify state agencies too late

for adequate data collection during a bloom.  In some cases, residents have mistakenly

supposed that dead adult salmon are simply spawned‐out fish, particularly in areas with

creeks and rivers having salmon runs.  Fish mortalities can occur in areas with less shoreline

development, such as North Hood Canal, resulting in fewer people noticing dead fish.  

Migrating adult salmon do not necessarily travel in large schools (e.g., chinook salmon), so

an occasional dead fish does not seem significant.  A partial solution to these detection

problems is public and government agency education and resource agency planning prior to

the occurrence of HABs.  

Reducing non‐point and direct discharges of nitrogen into nutrient‐limited marine waters

may be viewed as one means of prevention or mitigation of some types of HABs.  A review

of several HAB case histories worldwide suggests that some, but certainly not all HABs, are

associated with increased coastal eutrophication (Rensel Associates and PTI Environmental

Services, 1991).  The connection may be more related to rates of tidal flushing and vertical
Fish kills from Heterosigma akashiwo in Puget Sound: recent blooms and review                 54

stratification in specific areas than to characteristics of a given HAB species.  For example,

nutrient discharge is an important factor controlling the extent of Heterosigma blooms in

the Seto Inland Sea of Japan (Honjo 1993), but there is no evidence of this in the main basins

of Puget Sound.  Yet, poorly‐flushed and nutrient‐sensitive areas of Puget Sound have

minimal nitrogen supply rates in clement weather that may limit algal production (Yake,

1981).  Therefore HABs can be stimulated by increased nitrogen loading.  Rapid urbanization,  

poor agricultural or logging practices and municipal discharges have in some cases resulted

in very large increases of nitrate loading to Pacific Northwest streams and rivers (e.g., Smith

et al., 1987; Harr and Fredriksen, 1988), but few nutrient discharge data have been analyzed

with regard to eutrophication or HAB trends in nutrient‐sensitive areas of Puget Sound.  

Monitoring and experimental data suggest that very low or undetectable concentrations

of nitrogen in surface and subsurface waters of nutrient‐sensitive areas of Puget Sound have

limited the geographic range or activity of paralytic shellfish poisoning (PSP) caused by the

dinoflagellate Alexandrium catenella (Rensel, 1993b).  Salmonid smolts and other

phytophagous finfish may be at risk from saxitoxins accumulated by zooplankton that have

consumed A. catenella.  This has been demonstrated in the laboratory (Erickson, 1988), but

not as of yet in Puget Sound.  It follows that phytophagous fish in these areas are at risk if

PSP activity increases due to increased nitrogen loading.  With regard to the occurrence of

harmful Chaetoceros, however, there are no data suggesting an association with nutrient

discharge or specific type of area, nutrient‐sensitive or otherwise.  For example, these

diatoms occur in both seasonally nitrogen‐limited South Hood Canal and nitrogen‐replete

North Hood Canal.

Harmful Chaetoceros are difficult for fish farmers to detect because a lethal

concentration of cells may have no effect on easily measured parameters such as water

transparency, dissolved oxygen, pH, etc.  The vertical and horizontal distribution of cells is

difficult to ascertain without extensive and time‐consuming microscopic cell counts.  

Automated cell counting techniques are not possible due to the large size and spines of

these diatoms.  A simple means to detect the presence of harmful Chaetoceros at fish farms

is microscopic examination of wet mounts from gill scrapings.  Other techniques such as

histopathology are slow, expensive, and not appropriate for routine detection on the gills

because the diatom cells occur in the interlamellar spaces and on the epidermis and may be

washed away during the extensive tissue preparations that are required.

Blooms of harmful microflagellates or dinoflagellates that occur in high concentrations

are often easier for fish culturists to identify and manage because visual surveys from boats

or airplanes can be used to define their geographic limits.  This facilitates towing of net pens

to unaffected areas such as in 1990 when pens were towed from Rich Passage into Colvos

Passage.  These blooms are often restricted to near surface depths allowing for the pumping

of algal‐free deep water into the pens equipped with surrounding skirts; onshore or upland

facilities may have water intakes at several depths to avoid HABs.  Because Heterosigma

blooms occur in surface waters during clear weather, sea‐surface temperature and

chlorophyll a sensing satellites that are scheduled to be launched may be useful to track and

predict the distribution of these blooms.  
Fish kills from Heterosigma akashiwo in Puget Sound: recent blooms and review                 55

RESEARCH NEEDS

There is a need to investigate wild fish kills involving HABs more completely and

expeditiously and to document accompanying hydrographic and meteorological conditions.  

These investigations can form a basis for understanding HAB dynamics, trends and possible

connections with land and riverine management practices.  Nitrogen loading trends for

streams, rivers and municipal discharges should be examined for nutrient‐sensitive areas of

Puget Sound.    

The physiological causes of fish mortality due to Heterosigma remain to be discovered.  

Larvae or juveniles of marine fish and salmonids that utilize shallow embayments during late

spring through fall may be at risk to the apparent recent spread of Heterosigma. Juvenile

and adult salmon and marine fish in Hood Canal and perhaps elsewhere in Puget Sound

appear to be at risk to harmful Chaetoceros occurrence, but this remains to be quantified.  

Finally, field studies are recommended to test the importance of saxitoxin accumulation

from the dinoflagellate Alexandrium catenella to zooplankton and throughout the food web

to salmon and other predators.  

ACKNOWLEDGMENTS

Some of the work described here was supported by Washington Sea Grant and NOAA

Saltonstall‐Kennedy grants to Professors K. Banse and F.B. Taub.  Special thanks to these

principal investigators and to R.A. Horner who contributed taxonomic, biological expertise

and review of an early draft of this manuscript.  J. Postel, A. Matter and P. Hershberger

contributed significantly in field and laboratory studies.  Global Aqua USA Inc., Stolt Sea Farm

Inc. and Scan Am Fish Farms Inc. provided boats and other support for field studies of some

of the blooms.

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________________________________________________________________________

Appendix Table 1.  Fish‐killing phytoplankton species known to be present in Puget

Sound; those with an asterisk have caused documented fish losses in Puget Sound.  The

second column relates the concentration suspected to be harmful to fish and the etiology of

harm.

___________________________________________________________________________

Category and Species

‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐ 

Harmful Concentrations & Etiology

‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐ 

References

‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐ 

DIATOMS   

Chaetoceros concavicornis*

and C. convolutus* and possibly

others of the subgenus Phaeoceros

such as C. danicus

> 2‐5 cells/ml for salmonids; depends on

chain length.  Cells lodge between gill

lamellae causing mucus production, irritation

& leads to blood‐hypoxia or anoxia

Bell, 1961;  

Rensel, 1992; 1993a

Albright, 1993;

DINOFLAGELLATES    

Alexandrium catenella   unknown; acute mortality to farmed fish not

well documented or prevalent; chronic, food

web problem with wild fish

White, 1980; Mortenson,

1985 (for A. tamarense);

Erickson, 1988

Ceratium fusus* unknown; gill irritation, poorly understood Rensel and Prentice, 1980

Noctiluca scintillans variable; unionized ammonia causes gill

damage and other problems for fish

Okaichi and Nishio, 1976

PRYMNESIOPHYTE FLAGELLATES   

Chrysochromulina polylepis unknown; causes gill damage and

osmoregulatory problems  

Estep and MacIntyre, 1989

Phaeocystis pouchetii unknown; irritant substances and the alga's

mucus can clog gills

Gaines and Taylor, 1986

Smayda, 1989

RAPHIDOPHYTE FLAGELLATES   

Heterosigma akishiwo* probably variable, in most cases >750 to

1,000 cells/ml; cause of fish death unknown,

may be similar to Chattonella

Black et al., 1991;

Taylor and Haigh, 1993;

Tanaka et al., 1994

SILICOFLAGELLATES    

Dictyocha speculum unknown;  siliceous skeleton may irritate

gills,  possible toxin action too.

Larsen and Moestrup, 1989

UNKNOWN ALGAL SPECIES    

Net pen Liver disease* unknown; chronic losses possibly caused by a

microcystin‐producing alga.   

Kent, 1990

unidentified dinoflagellate* mortality of delayed‐release net‐pen salmon

in Hood Canal in summer, 1993

R. Horner and J. Rensel,

unpublished data