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K.A. Lefebvre ÆM.W. Silver ÆS.L. Coale
R.S. Tjeerdema
Domoic acid in planktivorous fish in relation to toxic
Pseudo-nitzschia
cell densities
Received: 5 April 2001 / Accepted: 28 August 2001 / Published online: 20 October 2001
Springer-Verlag 2001
Abstract In at least two mass mortality events in
Monterey Bay, California, planktivorous fish were im-
plicated as vectors of the neurotoxin domoic acid (DA)
from diatoms to sea birds and marine mammals.
Whereas the transfer of DA from planktivorous fish to
piscivorous predators has been well established, the re-
lationship between toxin levels in plankton-feeding fish
and the regional abundance of DA-producing diatoms
has not been documented. Here we present data from an
extensive field study in which cell densities of toxic
Pseudo-nitzschia species and DA levels in anchovies and
sardines were measured from samples collected weekly
throughout Monterey Bay from 8 October 1999 to 18
October 2000. Four distinct blooms were documented
with cell densities ranging from 3.2·10
3
to 5.0·10
5
cells
l
–1
. DA was detected in fish viscera samples whenever
toxic diatom densities reached ‡10
3
cells l
–1
in surface
waters, suggesting that anchovies and sardines regularly
consume toxic diatoms when present. Fish contained
DA levels above the regulatory limit (20 lgDAg
–1
whole fish) only when toxic cell densities exceeded 10
4
cells l
–1
. DA was only detected in fish when toxic diatom
species were also present in the water, suggesting that the
toxin is quickly depurated and that fish are only dan-
gerous vectors during the bloom period. Anchovies ap-
pear to be more potent vectors than sardines as they
consistently contained more DA than sardines collected
simultaneously. Maximum DA levels detected in fish
were 1,815 lgDAg
–1
in anchovy and 728 lgDAg
–1
in
sardine viscera samples. In fish with high viscera levels of
DA, corresponding body tissues contained 0.2–2.2 lg
DA g
–1
(0.2±0.1% of the viscera level), suggesting that
DA is not accumulated in edible body tissues to levels
that threaten human consumers. Results from this study
suggest that anchovies may be a valuable indicator
species for assessing the risk of DA intoxication to pis-
civorous sea birds and mammals during toxic Pseudo-
nitzschia blooms in Monterey Bay.
Introduction
Toxins associated with harmful algal blooms have long
been recognized as a serious threat to public health,
wildlife, and fisheries (Taylor and Seliger 1979; Shum-
way 1990; Anderson et al. 1993). Toxin accumulation in
filter-feeding organisms can result in devastating effects
on higher-level predators such as humans, sea birds, and
marine mammals. One such toxin, domoic acid (DA), is
of particular concern in Monterey Bay, California where
DA-producing diatoms commonly occur (Horner et al.
1997; Scholin et al. 2000). DA is naturally produced by
some species of the diatom genus Pseudo-nitzschia and is
responsible for a neurotoxic illness called amnesic
shellfish poisoning (ASP) (Subba Rao et al. 1988; Bates
et al. 1989; Wright et al. 1989; Garrison et al. 1992). ASP
was first recognized in Canada in 1987 when over 100
people became ill and at least 3 died after consuming
DA-contaminated blue mussels (Mytilus edulis) (Quil-
liam and Wright 1989; Perl et al. 1990). Since that event
there has been extensive research focusing on DA ac-
cumulation in bivalve vector species (Wohlgeschaffen
et al. 1992; Wekell et al. 1994; Jones et al. 1995; Douglas
et al. 1997) and neurotoxic effects in mammals (Grim-
melt et al. 1990; Tasker et al. 1991; Truelove and Iverson
1994; Scholin et al. 2000), as well as the establishment of
Marine Biology (2002) 140: 625–631
DOI 10.1007/s00227-001-0713-5
Communicated by P.W. Sammarco, Chauvin
K.A. Lefebvre (&)
Biology Department, University of California at Santa Cruz,
Santa Cruz, CA 95064, USA
E-mail: Kathi.Lefebvre@noaa.gov
M.W. Silver ÆS.L. Coale
Institute of Marine Sciences,
University of California at Santa Cruz,
Santa Cruz, CA 95064, USA
R.S. Tjeerdema
Department of Environmental Toxicology,
University of California at Davis,
Davis, CA 95616-8588, USA
safety standards for harvested seafood (20 lgDAg
–1
mussel tissue is the upper limit for shellfish consumption
established by Health and Welfare Canada; Todd 1990).
Although bivalves were the vector in the first ASP
event, as is the case with many algal toxins, subsequent
DA poisoning events have revealed that fish can also be
prominent vectors of DA. In fact, it appears that filter-
feeding fish are more potent vectors of DA than bivalves
in Monterey Bay. These fish link primary algal pro-
ducers directly to higher-level consumers. Two species of
planktivorous fish, northern anchovy (Engraulis mor-
dax) and Pacific sardine (Sardinops sagax), dominate
surface waters of the California Current and Monterey
Bay, posing a serious threat to secondary consumers
during toxic Pseudo-nitzschia blooms. In two docu-
mented cases of ASP in Monterey Bay, transfer of the
toxin occurred when Pseudo-nitzschia cells accumulated
in the gastrointestinal (GI) tracts of planktivorous fish
and were then passed to piscivorous predators. In 1991,
the deaths of several hundred brown pelicans (Pelecanus
occidentalis) and Brant’s cormorants (Phalacrocorax
penicillatus) were attributed to the consumption of an-
chovies containing toxic Pseudo-nitzschia (Fritz et al.
1992; Work et al. 1993). The second event occurred in
1998, when DA intoxication killed hundreds of Cali-
fornia sea lions (Zalophus californianus) and left many
seizuring on beaches. Anchovies were, again, identified
as the vector species for the transfer of DA from primary
diatom producers to higher trophic levels (Lefebvre et al.
1999; Scholin et al. 2000).
The most well studied vectors of algal toxins are
bivalve mollusks (Kvitek 1991; Wohlgeschaffen et al.
1992; Cembella et al. 1994; Douglas et al. 1997), whereas
little is known about the movement of algal toxins
through other members of the marine food web such as
filter-feeding herbivorous fish. For the Monterey Bay
region, understanding the dynamics of DA movement
and accumulation in planktivorous fish is paramount to
understanding the risks associated with Pseudo-nitzschia
blooms to various members of the local marine food
web. Here we present a comprehensive field study in
which toxic Pseudo-nitzschia densities and DA levels in
anchovies and sardines were measured over a 1-year
period from samples collected weekly throughout
Monterey Bay. Our results detail the presence of DA in
one level of the food web under ‘‘actual’’ bloom condi-
tions.
Materials and methods
Phytoplankton density samples
Water samples were collected in Monterey Bay from October 1999
through October 2000. Weekly water samples, taken within ap-
proximately 30 cm of the surface with a 1-l bottle, were collected
from the Santa Cruz Municipal Wharf. Other sites were opportu-
nistically sampled in the same way in response to reports of phy-
toplankton blooms. All sites are shown in Fig. 1. In the laboratory,
water column abundances of toxic Pseudo-nitzschia species,
P. australis and P. multiseries, were determined using whole-cell
hybridization with species-specific large subunit (LSU) rRNA-tar-
geted fluorescent probes. Based on the unique LSU rRNA nucle-
otide signatures of different species of Pseudo-nitzschia,
fluorescently labeled oligonucleotide probes were developed that
recognize these unique sequences (Miller and Scholin 1998). The
whole-cell hybridization method used here is described in detail in
Miller and Scholin (1998) and a brief description follows. Five
aliquots (10–30 ml) from surface sea water samples were filtered
onto 1.2-lm Isopore polycarbonate filters (Millipore Corp., Bed-
ford, Mass.) and preserved with a saline ethanol solution for at
least 1 h. After rinsing with hybridization buffer, the samples were
incubated with species-specific probes for P. australis and P. mul-
tiseries. After 1 h, filters were rinsed and placed on microscope
slides. Intact cells that retained the fluorescein-labeled probe were
then counted on a Zeiss Standard 18 compound microscope,
equipped with epifluorescence. The entire area of each filter was
counted. P. australis and P. multiseries counts were combined to
give the total number of toxic Pseudo-nitzschia cells present per liter
of surface seawater at each collection date.
Anchovy and sardine samples
Anchovies and/or sardines were obtained weekly from landings of
commercial boats at Moss Landing harbor (Fig. 1). The catches
were made in various sectors within Monterey Bay from October
1999 through October 2000. Because fish were opportunistically
obtained from commercial boats and other collectors, fish samples
Fig. 1 The nine sample sites in Monterey Bay, California where
surface water samples used for toxic Pseudo-nitzschia counts were
collected. Anchovies (Engraulis mordax) and sardines (Sardinops
sagax) were caught in various sectors of Monterey Bay by
commercial boats and landed at Moss Landing harbor. Fish catch
sites included directly off the Santa Cruz pier, M
1
, within Moss
Landing harbor, and within 3.7 km of the coast off Monterey,
Moss Landing, and Marina, which lies halfway between Moss
Landing and Santa Cruz
626
were not collected at the exact time and location as the surface
water samples. As a result, there are spatial and temporal differ-
ences between fish and water samples collected. Fish were caught in
the following regions of Monterey Bay: directly off the Santa Cruz
pier, at M
1
, within Moss Landing harbor, and within 3.7 km of the
coast off Monterey, Moss Landing, and Marina, which lies halfway
between Moss Landing and Santa Cruz (Fig. 1). At the dock, five
to ten live fish of each type were removed from bait wells, packaged
in Ziplock bags, and placed directly into a –20C freezer at the site.
Fish viscera samples (n=66) were analyzed for the presence of DA
via HPLC-UV (Hatfield et al. 1994; Lefebvre et al. 1999). In fish
with high viscera DA levels, body tissues were also analyzed for the
presence of DA. Anchovy and sardine samples collected simulta-
neously at seven distinct time points were examined for the pres-
ence of Pseudo-nitzschia in gut contents using a compound
microscope and diatoms were identified to genus level only.
The established seafood safety level of 20 lgDAg
–1
refers to
whole organisms. In fish, most of the toxin is found in the gut
(Lefebvre et al. 1999; Vale and Sampayo 2001). To determine which
viscera DA levels detected in this study resulted in whole fish DA
levels above the regulatory limit, mean percent viscera (viscera
weight/whole fish weight·100) was determined for anchovies
(n=39) and sardines (n=12).
Chemical reagents
DACS-1C certified DA standard (National Research Council of
Canada, Institute for Marine Biosciences, 1411 Oxford Street,
Halifax, Nova Scotia) and 90% pure DA reagent (Sigma Chemical
Co., St. Louis, Mo.) were obtained for calibration standard prep-
aration and spike/recovery calculations. Trifluoroacetic acid
(TFA), analytical grade sodium chloride (NaCl), and Optima grade
methanol (MeOH) and acetonitrile (MeCN) were obtained from
Fisher Scientific (Pittsburgh, Penn.). Nanopure water was used for
solution preparation. Standards were kept refrigerated and in the
dark when not in use.
DA detection in fish viscera and body tissue samples
via HPLC-UV
Fish viscera and body tissue samples were analyzed for the presence
of DA using an isocratic elution profile on a Hewlett-Packard 1090
HPLC equipped with a diode array detector set at 242 nm with a
bandwidth of 10 nm. The reference signal was set at 450 nm with a
bandwidth of 10 nm. A reverse phase Vydac C
18
column (catalog
no. 201TP52, 2.1 mm·25 mm, Separations Group, Hesperia,
Calif.) equipped with a Vydac guard column (particle size 5 lm)
was used. The mobile phase (90/10/0.1, water/MeCN/TFA) was
degassed with helium for 10 min prior to analysis. A calibration
curve was generated using DACS-1C DA standards of 0.15, 0.3,
0.5, 1.0, 2.0, 4.0, 8.0, and 16 lgml
–1
(r=0.99). The lowest detect-
able standard was 0.15 lgDAml
–1
. The instrument detection limit,
which was equivalent to the concentration that corresponded to
3 times the standard deviation of the signal from the lowest de-
tectable standard (n=3), was 0.2 lgDAml
–1
. Injections were 10 ll
or 25 ll with a flow rate of 0.3 ml min
–1
.
Sample extraction and solid-phase extraction
The entire visceral mass (digestive tract and all internal organs
except gonads) was dissected from two to ten fish from each date,
then pooled and homogenized with a mortar and pestle. Sixteen
milliliters of 50% MeOH was added to a 4-g aliquot of homoge-
nized viscera, vortexed for 30 s, and sonicated for 4 min in a water
bath. Following sonication, the sample was vortexed for 30 s, ho-
mogenized for 1 min with a homogenizer probe on ice, and vort-
exed again for 30 s. The resulting slurry was centrifuged for 20 min
at 4,000 rpm and the supernatant passed through a 1.0-lm filter
(Millipore Corp., Bedford, Mass.). Two milliliters of filtrate were
then passed through a strong anion exchange solid phase extraction
column (JT Baker) that was preconditioned with 6 ml nanopure
water, followed by 3 ml 100% methanol, and finally 3 ml 50%
MeOH (Lefebvre et al. 1999). After washing the column with 5 ml
10% MeCN, DA was eluted with 5 ml 0.5 MNaCl in 10% MeCN
at a rate of one drop per second. The column was not allowed to
run dry at any time during solid phase extraction. Analysis of body
tissue samples followed the same procedure as viscera samples ex-
cept that 2 g tissue was homogenized in 8 ml 50% MeOH, and 6 ml
filtrate was eluted with 5 ml 0.5 MNaCl. Efficiency of extraction
was 95±9.1% (n=5) and 90±4.5% (n=4) for viscera and body
tissue samples, respectively. Method detection sensitivities were
0.5 lgDAg
–1
for viscera and 0.2 lgDAg
–1
for body tissues.
Results
Toxic Pseudo-nitzschia densities and DA levels
detected in fish viscera
Toxic Pseudo-nitzschia species reached bloom conditions
(>10
3
cells l
–1
) at four distinct times in Monterey Bay
over the 1-year sampling period (Fig. 2A). DA was de-
tected in fish viscera samples only at times when Pseudo-
nitzschia blooms were also observed, suggesting that the
two are tightly temporally coupled (Fig. 2A, B). The
strong correlation between DA levels in fish and toxic
cell densities (Fig. 2B) suggests that spatial and temporal
differences in sample collection between fish and surface
water samples were minor. As mentioned in the meth-
ods, both types of samples were taken at least once a
week (more often during blooms) and from sites near
Fig. 2 A Number of toxic Pseudo-nitzschia cells l
–1
in surface water
collected from Monterey Bay from 8 October 1999 to 8 October
2000. BDomoic acid concentrations in viscera from anchovies
(E. mordax) and sardines (S. sagax) collected in Monterey Bay
from 8 October 1999 to 8 October 2000
627
each other (Fig. 1). Although algal blooms are often
patchy in distribution, our density samples (n=140)
appear to have given a good representation of toxic di-
atom bloom activity in Monterey Bay and of diatom
availability to fish. The highest toxic cell densities
counted during each bloom period were 3.4·10
4
cells l
–1
on 17 December 1999, 6.8·10
4
cells l
–1
on 24 March
2000, 3.2·10
3
cell l
–1
on 16 June 2000, and 5.0·10
5
cells
l
–1
on 12 September 2000 (Fig. 2A). The highest DA
levels detected in viscera samples during each bloom
period were 23 lgDAg
–1
on 17 December 1999, 200 lg
DA g
–1
on 13 April 2000, 4.3 lgDAg
–1
on 22 June
2000, and 1,815 lgDAg
–1
on 10 September 2000
(Fig. 2B). Maximum fish viscera DA levels detected
during each bloom period were positively correlated
with maximum total toxic cell densities counted at cor-
responding bloom periods.
DA levels detected in anchovy and sardine viscera
On 12 occasions throughout this study, anchovies and
sardines were collected simultaneously from the same
location. A comparison of DA levels detected in an-
chovy and sardine viscera is shown in Table 1. When
DA was detectable, anchovy viscera contained more DA
than sardine viscera in eight of nine cases (Table 1). A
nonparametric two-tailed binomial test confirmed that
anchovies had significantly higher DA viscera values
than sardines (P<0.05). Viscera DA levels detected in
anchovies were up to 39 times higher than those detected
in sardines for the same date. Pseudo-nitzschia frustules
were observed in five of seven anchovy and in five of
seven sardine stomach content samples analyzed. In
these samples, DA was not always detected when Psu-
edo-nitzschia frustules were present in viscera and in
some cases DA was detected and frustules were not. This
discrepancy may be due to the fact that although the
corresponding fish used for gut analysis were from the
same batch as those used for DA analysis, they were not
the same exact fish.
DA levels detected in fish body tissue
In anchovies containing viscera levels ‡288 lgDAg
–1
,
five out of eight contained detectable levels of DA in
body tissue (Table 2). In seven sets of sardine samples
containing viscera levels ‡169 lgDAg
–1
, all contained
detectable levels of DA in body tissue (Table 3). Viscera
DA levels in anchovy and sardine samples were 250–
1,800 times higher than corresponding body tissue DA
levels, suggesting that DA accumulation in body tissue is
minimal (Tables 2, 3). All body tissue DA levels detected
in fish (n=12) were less than 3 lgDAg
–1
, far below the
established seafood safety limit of 20 lgDAg
–1
, sug-
gesting that DA is not accumulated to dangerous con-
centrations in edible fish tissue.
Mean visceral masses were 9.4±1.2% and 8.9±1.3%
of total fish weight for anchovies (n=39) and sardines
(n=12), respectively. In the 12 fish in which DA was
detected in tissue (Tables 2, 3), body tissue contained
0.2±0.1% of the DA measured in corresponding viscera
samples. Using 9% to estimate percent viscera for both
species and assuming that 99.8% of the toxin remains in
the gut, the FDA regulatory limit of 20 lgDAg
–1
whole
fish can be converted to 222 lgDAg
–1
for viscera
samples. Although moderate DA levels were detected in
fish viscera samples several times throughout the year,
DA levels exceeding seafood safety standards (20 lg
Table 1 A comparison of domoic acid (DA) levels in anchovies
(Engraulis mordax) and sardines (Sardinops sagax) collected si-
multaneously from Monterey Bay, California. DA values are in
micrograms DA g
–1
viscera wet weight. ND Not detected
Date collected Anchovy viscera Sardine viscera
(lgDAg
–1
)(lgDAg
–1
)
7 December 1999 3.3 ND
23 May 2000 ND ND
5 June 2000 ND 1.0
16 June 2000 ND ND
20 June 2000 2.5 ND
22 June 2000 4.3 ND
9 July 2000 ND ND
20 July 2000 23 ND
24 August 2000 288 28
8 September 2000 1,175 279
10 September 2000 1,815 46
3 November 2000 30 1.2
Table 2 DA levels in viscera and body tissues from anchovies
(E. mordax) collected during toxic Pseudo-nitzschia blooms in
Monterey Bay. DA values are in micrograms DA g
–1
viscera or
body tissue wet weight. ND Not detected
Date collected No. of
anchovies pooled
Viscera
(lgDAg
–1
)
Body tissue
(lgDAg
–1
)
13 April 2000 1 131 ND
5 July 2000 2 128 ND
19 August 2000 2 507 ND
24 August 2000 8 288 1.1
26 August 2000 2 288 ND
2 230 ND
8 September 2000 2 1,050 1.2
2 1,050 0.6
2 1,175 1.1
10 September 2000 2 1,815 ND
2 1,600 1.0
Table 3 DA levels in viscera and body tissues from sardines
(S. sagax) collected during a toxic Pseudo-nitzschia bloom in
Monterey Bay. DA values are in micrograms DA g
–1
viscera or
body tissue wet weight
Date
collected
No. of
sardines pooled
Viscera
(lgDAg
–1
)
Body tissue
(lgDAg
–1
)
1 September 2000 2 169 0.2
2 216 0.8
6 September 2000 2 558 0.5
7 September 2000 2 588 0.5
2 551 2.2
8 September 2000 2 279 0.2
2 386 0.5
628
DA g
–1
whole fish or 222 lgDAg
–1
viscera) were de-
tected only during the largest bloom, which occurred in
August and September of 2000.
Discussion
Results from this year-long survey reveal that planktiv-
orous fish such as anchovies and sardines are regular
and consistent carriers of DA when toxic diatom blooms
are present in Monterey Bay. Despite spatial and tem-
poral differences in sampling, the observed relationship
between fish viscera DA levels and toxic cell densities
(Fig. 2) indicates that our density data represent an ac-
curate depiction of toxic diatom densities experienced by
anchovies and sardines during this study. It also suggests
that avoidance behavior does not occur and moreover,
that anchovies and sardines, which move freely about
the Bay, may actually track toxic diatom blooms to feed.
Fish viscera DA levels not only tracked cell densities of
Pseudo-nitzschia australis and P. multiseries, but DA was
detected in viscera every time toxic diatom populations
reached densities of ‡10
3
cells l
–1
in surface waters. The
fact that DA was only present in viscera when toxic
diatom species were present, and not at other times,
suggests that this water-soluble toxin is quickly depu-
rated and that fish vectors present a threat to higher-
level predators only as long as a bloom persists. In ad-
dition to being temporally coupled, the amount of toxin
detected in viscera samples was correlated with the
magnitude of cell density of toxic species. Although DA
was present in fish several times throughout the year, it
only reached levels at or above seafood safety limits
when cell densities exceeded 10
4
cells l
–1
, suggesting that
bloom densities at or below 10
4
cells l
–1
may not present
a threat to piscivorous sea birds and mammals.
However, toxic cell densities could have been greater in
adjacent patches that were not sampled and densities
even higher than 10
4
cells l
–1
may be required before
harmful DA levels are accumulated in fish viscera.
Stomach content analyses reveal that both anchovies
and sardines consume Pseudo-nitzschia and can therefore
act as DA vectors (McGinness et al. 1995; Lefebvre et al.
1999; Scholin et al. 2000; this study). However, our data
on viscera DA levels suggest that anchovies may be more
potent vectors than sardines. When both species were
collected simultaneously, anchovy viscera consistently
contained more DA than sardine viscera, indicating that
anchovies generally consume more toxic cells than sar-
dines (Table 1). Maximum DA levels detected in fish
over the entire study were 1,815 lgDAg
–1
in anchovy
and 728 lgDAg
–1
in sardine viscera samples. Previous
work, based on stomach content analysis, reveals that
both anchovies and sardines are omnivorous with the
ability to feed on phytoplankton and zooplankton via
filter feeding or particulate/selective feeding modes (Ra-
dovich 1952; Loukashkin 1970). The consistently higher
DA levels detected in anchovies suggests that during
Pseudo-nitzschia blooms in Monterey Bay anchovies may
filter feed more exclusively on diatoms, whereas sardines
may target zooplankton, thereby accumulating Pseudo-
nitzschia secondarily or in lower quantities as zoo-
plankton fulfill their dietary needs.
Dangerous levels of DA (‡222 lgDAg
–1
viscera) are
commonly detected in fish viscera samples, yet DA ac-
cumulation in body tissue of anchovies and sardines
appears to be negligible (Tables 2, 3). Although unsafe
DA levels (‡20 lgDAg
–1
body tissue) have been pre-
viously reported for anchovy body tissues (Quilliam et al.
1991; Work et al. 1993; Wekell et al. 1994; Lefebvre et al.
1999), those measurements are questionable due to
possible leakage of DA from the GI tract during sample
collection, storage, and processing procedures (Lefebvre
et al. 2001). In the present study, all DA levels detected
in body tissue samples (n=12) were £2.2 lgDAg
–1
,
whereas viscera levels ranged from 169 to 1,600 lgDA
g
–1
(Tables 2, 3). This indicates that DA is not accu-
mulated to levels that threaten consumers of fish body
tissue. However, even though DA uptake appears to be
low, it may be sufficient to induce neurotoxic symptoms
in the fish themselves (Lefebvre et al. 2001). If DA is not
accumulated to toxic levels in edible body tissues, then
fish that are gutted and cleaned may not be a threat to
human consumers even when caught during high-density
toxic diatom blooms. In all documented cases of ASP in
which fish were identified as the DA vector, entire fish
were consumed by affected sea birds and marine mam-
mals (Fritz et al. 1992; Work et al. 1993; Sierra-Beltran
et al. 1997; Lefebvre et al. 1999; Scholin et al. 2000).
Since intoxication occurs from consumption of whole
fish, care should also be taken when utilizing small pel-
agic filter-feeding fish for nonhuman consumption. Fish
such as anchovies, sardines, and herring are often used
to feed captive animals including sea lions, seals, dol-
phins, and penguins in zoos, aquariums, and marine
laboratories and therefore could pose threats to these
animals if obtained from areas with dense blooms of
toxic diatoms. Furthermore, the processing of whole fish
for other purposes, such as for fishmeal, could result in a
product with DA levels exceeding the recommended
toxin limits for safe consumption.
Since anchovies have a central position in the marine
food web, contain DA levels that track Pseudo-nitzschia
blooms, and appear to be more potent vectors than
sardines, they may be the most valuable indicators of
potential DA intoxication of higher-level predators,
including sea birds and marine mammals. However, not
all toxic diatom blooms in Monterey Bay result in
obvious symptoms at higher trophic levels. Our data
suggest that many blooms occur throughout the year
with no observed adverse effects on wildlife. Many fac-
tors, such as bloom density, duration, per-cell toxicity of
Pseudo-nitzschia, and the presence or absence of filter-
feeding fish, are likely to influence the impact a partic-
ular bloom will have on various predators. Our data
suggest that the density of toxic cells is more influential
than the bloom duration, since fish contained unsafe
levels of DA within a few days of the onset of high toxic
629
cell densities in August 2000. Monitoring DA levels in
anchovies when toxic diatoms are present in Monterey
Bay will allow regulators to predict potential intoxica-
tion events involving sea birds or marine mammals, as
well as provide information for warning fisheries of
potential contamination in whole processed fish.
Conclusions
DA levels accumulated in fish viscera track toxic cell
densities in surface waters, confirming that anchovies
and sardines regularly consume toxic diatoms when
present in Monterey Bay. Although DA was detected in
fish viscera samples several times throughout the year,
fish appear to accumulate levels of DA harmful to pis-
civorous predators only when toxic cell densities exceed
10
4
cells l
–1
. It is likely that DA is quickly depurated and
that fish are only dangerous vectors during a bloom
period, since DA was only present in fish when toxic
cells were also present in the water. In addition,
anchovies appear to be more potent vectors than
sardines. Both species accumulate toxic levels of DA in
the GI tract, but neither appears to accumulate DA
in edible body tissues to levels that are unsafe for human
consumption. Although DA uptake into body tissue is
low, it may be sufficient to induce neurotoxic symptoms
in fish. Because whole fish can be potent vectors of DA,
attention should be paid to monitoring fish collected
during toxic diatom blooms to be used for feeding of
captive animals or for converting to fishmeal. Finally,
anchovies may be a valuable indicator species for
assessing the risk of DA intoxication to piscivorous sea
birds and mammals during toxic Pseudo-nitzschia
blooms in Monterey Bay.
Acknowledgements The authors would like to thank Victoria
Wellborn, Sibel Bargu, Shonna Dovel, Christine O’Halloran, and
Megan Coale of University of California at Santa Cruz, Karen
Osborne, Dr. Chris Scholin, Roman Marin II, and Kurt Buck of
Monterey Bay Aquarium Research Institute, Dr. Rikk Kvitek,
Judah Goldberg, Maria Ferdin, and Carrie Bretz of California
State University Monterey Bay, Julie Haws and Jeff Field of
Moss Landing Marine Laboratories, Sal Locatelli of Bay Fresh
Fish Co., Moss Landing, Calif., and Dr. David Marcinek for
helpful discussions, specimen collection, and/or laboratory assis-
tance. This project was supported by NOAA award no.
NA960P0476. All work performed in this study complied with
current laws of the USA. This paper is ECOHAB contribution
no. 29.
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