Journal of Plankton Research Vol.22 no.9 pp.1649–1662, 2000
The consumption of euphausiids by the pelagic fish community off
southwestern Vancouver Island, British Columbia
Clifford L.K.Robinson
Northwest Ecosystem Institute, Box 513, Lantzville, British Columbia, Canada
Present address: Parks Canada, Western Canada Service Centre, 300–300 West
Georgia St, Vancouver, British Columbia, V6B 6B4, Canada
Abstract. The consumption of adult euphausiids was estimated for the seven most abundant fish
species found on the continental shelf off southwestern Vancouver Island, British Columbia, Canada.
Empirical data on fish feeding rates, diets and biomass were used to estimate euphausiid consumption in August for each year from 1985 to 1997. The fish community in August was estimated to
consume an average 297 kt of euphausiids (124 t km–2). Pacific hake (Merluccius productus) and
Pacific herring (Clupea harengus) accounted for 60% and 30%, respectively, of the total euphausiids
consumed by fish. The total estimated consumption of euphausiids per tonne of fish averaged 0.93,
and there was no significant trend in the time series. In addition, there was no significant relationship
between the total fish community consumption or euphausiids consumed per tonne of fish and
euphausiid biomass, and there was no change in the fraction of euphausiids in the diet of small or
large Pacific hake. Thus, it appears that predation pressure from the pelagic fish community was relatively constant and not responsible for the apparent fivefold decline in adult euphausiid biomass
observed in the 1990s.
Introduction
The summer fish community on the continental shelf off southwestern Vancouver Island, British Columbia (Figure 1) is dominated by relatively few
pelagic and demersal fish species (Ware and McFarlane, 1995), namely: Pacific
hake (Merluccius productus), Pacific herring (Clupea harengus), spiny dogfish
(Squalus acanthias), coho salmon (Oncorhynchus kitsutch), Pacific cod (Gadus
macrocephalus), sablefish (Anoplopoma fimbria) and chinook salmon
(Onchorhychus tshawytscha). Also characteristic of the La Perouse region during
summer are wind-induced upwelling, and high biomasses and production of
phytoplankton and zooplankton on the continental shelf and along the shelfbreak. McFarlane et al. provide a detailed overview of this coastal upwelling
system (McFarlane et al., 1997).
The plankton and fish production dynamics of the La Perouse region have been
studied using an environmentally forced trophodynamics simulation model
(Robinson and Ware, 1994, 1999). One important component of the La Perouse
production system identified through model simulations, and confirmed by
observation, is the temporal variability in euphausiid biomass and production.
The euphausiids, Thysanoessa spinifera and Euphausia pacifica, it appears are the
key to understanding the interannual variability in fish production in the La
Perouse region. Recent studies have demonstrated that the typical summer distribution of hake in the La Perouse area strongly overlaps the horizontal distribution of euphausiids (Ware and McFarlane, 1995; Mackas et al., 1997). Mackas
et al. further analysed shelf-edge spatial distributions and found that the location
© Oxford University Press 2000
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C.L.K.Robinson
was of euphausiid–hake aggregations were best explained with reference to
flow-field indices (e.g. doming of isopycnals, intensity of cross-isobath flow and
upwelling at the depth of the euphausiid scattering layer) (Mackas et al., 1997).
These observations suggest that euphausiid biomass dynamics are tightly coupled
to regional upwelling, and that fish predators target the aggregations of euphausiids. Other studies in the La Perouse region have confirmed the importance of the
spatial overlap between fish and euphausiids by quantifying the importance of
euphausiids in fish diets. Tanasichuk et al. (Tanasichuk et al., 1991) and Ware and
McFarlane (Ware and McFarlane, 1995) have shown that euphausiids, on average,
constitute a high proportion by weight of the diet of the Pacific hake, spiny
dogfish and Pacific herring. Euphausiids also appear to be important in the diets
of the other abundant fish species, including sablefish, Pacific salmon and Pacific
cod.
The empirical studies of hake–euphausiid spatial aggregations and fish
stomach-content analyses in the La Perouse region indicate intensive predator–prey interactions. Coupled with these interactions is the observation that
adult euphausiid biomass in Barkley Sound, near the La Perouse region, has
steadily declined during the 1990s to about 20% of the peak biomass estimated
in 1991 (Tanasichuk, 1999). Based on food-web theory, the large decline in adult
euphausiid biomass might be attributed to one of two processes. Environmental
conditions in the coastal ocean may have changed sufficiently to result in reduced
growth or survival of larval euphausiids (‘bottom-up’ processes), or the impact of
size-selective fish predators may have resulted in reduced survival of large adult
euphausiids [‘top-down’ processes (McQueen et al., 1989)].
The main objectives of this paper are to (i) assemble empirical data describing
the feeding rates and biomasses of dominant fish species in the La Perouse region,
(ii) calculate consumption of euphausiids by the fish community in August and
(iii) determine if the observed decline in adult euphausiid biomass is linked to
predation by the pelagic fish community.
Method
Two methods were used to calculate the consumption of euphausiids by the fish
community in the La Perouse region. For hake, dogfish and sablefish, the
consumption of euphausiids in August of each year was calculated using Bi as the
estimated biomass (tonnes) of species i, and Ri as the maximum ration expressed
as percentage of body weight per day (%BWD) of species i.
C = Bi * Ri * 31d
(1)
In this case, the daily ration of euphausiids by a fish was calculated using the
Elliot–Persson equation [see (Tanasichuk et al., 1991)], where R = DR*
MSFIE*24h, and DR is the hourly digestive rate for euphausiids of 0.13% body
weight h–1 (Tanasichuk et al., 1991). The MSFIE represents the average content
of euphausiids in a fish stomach, expressed as a percentage of fish body weight,
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Euphausiid consumption by pelagic fish
and was estimated in the field from fish randomly taken from multiple midwater
or bottom tows (Tanasichuk et al., 1991; Ware and McFarlane, 1995).
For herring, cod, coho and chinook salmon, the estimated consumption of
euphausiids in equation 1 was modified by including F, which is the fraction
(percentage by weight) of euphausiids in the diet of fish species i.
C = Bi * Ri * Fi * 31d
(2)
The sources of the species-specific consumption parameters are summarized in
Table I, and their values are discussed below.
Biomass estimates
Hake biomass in the La Perouse region during mid summer was estimated from
hydroacoustic surveys conducted by the Department of Fisheries and Oceans
Canada in late summer (Ware and McFarlane, 1995; Ware, unpublished data).
Hake biomass from 1985 to 1997 averaged 215 kt, and ranged from 58 kt to 387
kt. Robinson and Ware assumed that adult herring foraging in the La Perouse
region originate primarily from the southwestern coast of Vancouver Island
(Barkley Sound), and secondarily from the southern Georgia Strait (Robinson
and Ware, 1999). Herring stock biomass (spawners and catch) for each region for
each year from 1985 to 1997 were taken from Schweigert et al. (Schweigert et al.,
1998). The adult herring biomass averaged 40 kt. The biomass of sablefish in the
La Perouse region was available for only a few years (1989–1991) from data in
Stocker (Stocker, 1994). The 1989–1991 average sablefish biomass of 5.3 kt was
assumed to occur in the La Perouse region from 1985–1988 and 1992–1997. The
biomass of Pacific cod for shelf areas near, and including, the La Perouse region
was also taken from Stocker (Stocker, 1994). The average cod biomass calculated
from 1985 to 1990 was about 15 kt, which is about 5 kt higher than the summer
estimate discussed in Ware and McFarlane (Ware and McFarlane, 1995). The
biomass of chinook and coho salmon foraging in the La Perouse region in August
was taken from Ware and McFarlane (Ware and McFarlane, 1995), and was
assumed constant at 11 kt and 15 kt, respectively.
Table I. Summary of data sources used to calculate the consumption of euphausiids by the seven most
abundant fish species in the La Perouse region in summer. Abbreviations: WM 95: (Ware and
McFarlane, 1995); SH 98: (Schwiegert et al., 1998); ST 94: (Stocker, 1994); T 95: (Tanasichuk, 1995);
WT: (Ware and Tanasichuk, unpublished data); M: (Morris, unpublished data); T: (Tanasichuk,
unpublished data)
Fish species
Biomass
MSFI or daily ration
Fraction euphausiids in diet
Pacific hake
Pacific herring
Spiny dogfish
Sablefish
Pacific Cod
Chinook salmon
Coho salmon
1985–96 (WM 95)
1985–96 (SH 98)
1985–96 (WM 95)
1985–91 (ST 94)
1985–90 (ST 94)
1985–92 (WM 95)
1985–89 (WM 95)
1985–96 (WT)
1985–96 (T)
1985–96 (WT)
1985–89 (T 95)
1985–90 (W 96)
1987–90 (M)
1987–90 (M)
1985–96 (WT)
1985–96 (T)
1985–96 (WT)
1985–91 (WM 95)
1985–90 (WM 95)
1985–91 (WM 95)
1985–89 (WM 95)
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C.L.K.Robinson
The biomass of spiny dogfish in the La Perouse region in summer was estimated by Ware and McFarlane (Ware and McFarlane, 1995), from swept-area
data from trawl surveys, to be about 38 kt during the 1980s. The relative change
in abundance in dogfish in the La Perouse region from 1985–1992 was determined
from the January to June commercial trawl catch per unit effort (CPUE) data in
Stocker (Stocker, 1994). There was a significant positive correlation between the
winter commercial dogfish CPUE and the DFO research cruise standardized
catch rate of dogfish in August (r = 0.91; P < 0.01). Because the change in catch
of dogfish from two independent studies varied synchronously, it most likely
reflects the relative interannual abundance of dogfish in the La Perouse region.
The CPUE data indicate that dogfish abundance increased from 1985 (1.830) to
a peak in 1989 (5.018) before dropping to an 8 year low in 1992 (1.154). It is
assumed that the highest dogfish CPUE in 1989 corresponded to the 38 kt
reported in Ware and McFarlane (Ware and McFarlane, 1995), and that the
lowest CPUE corresponded to a dogfish biomass of 8.7 kt (1992).
Ration estimates
Stomach fullness index data for euphausiids (SFIE) were required for sablefish,
dogfish and hake. It is well documented for the La Perouse region that large fish
predators like hake, sablefish and dogfish exhibit a decrease in the proportion of
euphausiids (and an increase of fish) in their stomachs with increasing predator
length [e.g. (Tanasichuk, 1995)]. To account for this change in diet with predator
length, the SFIE of major fish predators for each August was calculated by
weighting the diet samples by the proportion of a size class to the total number
of predators sampled. Size-dependent SFIE data for sablefish were available for
1985–1989 from Tanasichuk (Tanasichuk, 1995). Size-dependent SFIE data for
dogfish and hake for August of each year from 1985 to 1997 were calculated from
field data (Ware and Tanasichuk, unpublished data).
The calculation of an average August SFIE for each year for hake and dogfish
also required that the diet samples were weighted by the relative abundance of
dogfish or hake caught in trawls in the La Perouse region. For example, the
number of research trawls conducted in the La Perouse region and the biomass
of hake or dogfish caught in each trawl varied from year to year. To ensure that
trawls with low biomass but high SFIE did not skew the average yearly August
SFIE, individual SFIEs were weighted in relation to the total biomass of a species
collected from all trawls set [see (Ware and McFarlane, 1995)].
The daily ration of herring for 1985–1996 was estimated from field data (Tanasichuk, DFO, Nanaimo, unpublished data), while the daily ration for Pacific cod
was assumed constant at 1% BWD, as determined for fish caught near Barkley
Sound in July 1975 (Westrheim, 1977). The maximum daily ration for chinook
and coho salmon in the La Perouse region was estimated from fish sampled
from the troll fleet in May of 1989 and 1990 (J. Morris, Department of Fisheries
and Oceans, unpublished data); data for August were not collected. Of the coho
salmon sampled (40–60 cm) in 1989 and 1990, about 56% and 48% of the fish
with stomach contents had euphausiids present, while about 12% and 15% of the
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Euphausiid consumption by pelagic fish
coho fed exclusively on euphausiids, respectively. Coho salmon feeding on
euphausiids were estimated to have an average ration of about 1% BWD. About
50% and 37% of the chinook (50–90 cm) sampled in May of 1989 and 1990 were
found to have euphausiids in their stomachs, while about 12% and 16% fed exclusively on euphausiids. An average daily ration of 0.5% BWD was calculated for
chinook feeding on euphausiids (Table II).
The fraction of euphausiids in the diet of sablefish, cod, chinook and coho
salmon from 1985–1991 was taken from Ware and McFarlane (Ware and McFarlane, 1995). Adult herring caught in the La Perouse region in August appear
to feed exclusively on euphausiids (personal observation and unpublished data,
R. Tanasichuk, DFO, Nanaimo, BC), and thus the fraction of euphausiids in the
herring diet was assumed to be 1.
Euphausiid biomass patterns
Reliable time series of euphausiid biomass estimates are difficult to obtain
because of changes in sampling methods, time of sampling (day versus night) or
in stations monitored (Shaw and Robinson, 1998). Fortunately, there is a 7 year
index of euphausiid biomass for Barkley Sound, near the La Perouse region
(Figure 1), collected by the same person using a consistent sampling method and
the same series of four stations. Tanasichuk estimated the biomass of adult
euphausiids (>10 mm: T.spinifera and E.pacifica) by sampling with bongo nets
several times during the upwelling season, and he estimated a fivefold decline
from 1991 to 1997 (Tanasichuk, 1999). Although bongo net sampling is likely to
underestimate euphausiid biomass and gives biased size frequency distributions,
this is a relative index of euphausiids available for comparison with fish feeding
data from the La Perouse region.
Results
Pacific hake, the most abundant fish species in the La Perouse region, have the
longest available and most reliable data for estimating euphausiid consumption.
The estimated average consumption of euphausiids by hake in August ranged
from a low of 37 kt in 1985, to a high of about 453 kt in 1997 (Figure 2). Over the
1985 to 1997 time series, euphausiid consumption by hake averaged about 191 kt,
Table II. Calculated ingestion rate (% body weight day; BWD) for mature chinook and coho salmon
caught in the La Perouse Bank region in 1989 and 1990 (unpublished data, J. Morris, DFO)
Number stomachs with
Ingestion rate (%BWD)
Year – species
Number stomachs
(empty)
Euph + fish
Euph only
Euph + fish
Euph only
1989 – Coho
1990 – Coho
1989 – Chinook
1990 – Chinook
197 (60)
253 (100)
647 (131)
365 (119)
77
74
262
92
16
23
61
41
0.8
0.9
0.5
0.5
1.1
1.2
0.5
0.5
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C.L.K.Robinson
Fig. 1. Location of the La Perouse region on the continental shelf off south-western Vancouver Island,
British Columbia, Canada.
Fig. 2. The estimated consumption of euphausiids (kt) by small (<50 cm) and large (>50 cm) hake in
the La Perouse region each August from 1985 to 1997.
and had a coefficient of variation of 74%. There was no trend in the euphausiid
consumption time series (Figure 2).
Over the whole time series, hake <50 cm consumed about 72% (138 kt) of the
estimated average total euphausiid consumption, while from 1991 to 1997, small
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Euphausiid consumption by pelagic fish
hake consumed about 89% (Figure 2). Small hake had higher euphausiid
consumption because they had a significantly higher fraction of euphausiids in
their diet compared with hake >50 cm (71% versus 38%, respectively, P < 0.001),
and a higher daily euphausiid ration (3.1 versus 2.4 % BWD; P = 0.14). Size class
data also indicated that from 1985 to 1997, small hake made up about 69%
(149 kt), on average, of the total hake stock estimated by hydroacoustic sampling.
From 1991 to 1997, small hake made up about 73% of the hake stock.
How does the estimated August euphausiid consumption by hake compare
with the rest of the fish community? The available data allowed for a comparison of euphausiid consumption among all seven fish species during the 1985–1989
period only. The total consumption of euphausiids in the La Perouse region in
August by the dominant fish community is estimated to average 268 kt, and it
ranged from 452 kt in 1987 to 125 kt in 1985 (Table III). This converts to an
average euphausiid consumption of about 3.6 t km–2 day–1 or 0.24 g C m–2 day–1
in August (31 days), assuming that the major fish–euphausiid aggregations occur
in about 2400 km2 of the La Perouse region, and that 6.75% of wet weight is
carbon (Robinson, 1994). Hake were the most important euphausiid fish predator during the 1985–1989 period and consumed about 58% of the total fish
community euphausiid consumption, while herring consumed about 30% (Table
III). There were sufficient data available to extend the estimated consumption of
euphausiids by hake and herring to 1997. Herring exceeded hake consumption of
euphausiids by 10–30% in 3 years, while hake exceeded herring consumption of
euphausiids by >50% in 5 years and >30% in 7 years (Figure 3).
The estimated total consumption of euphausiids by the fish community in the
La Perouse region during August from 1985 to 1997 is shown in Figure 4. Note
that for those fish species and years for which observational data did not exist the
average consumption estimated from earlier years was assumed. There is no
significant correlation between total fish community consumption and the
observed euphausiid biomass (r = –0.32; P = 0.48 ). To remove the affect of higher
hake biomass in years of low euphausiid biomass [migratory hake biomass is
dependent on water temperature (Ware and McFarlane, 1995)], an estimate of
the total consumption of euphausiids per tonne of fish was also calculated (Figure
4). The average total euphausiid consumption per tonne of fish from 1985–1997
Table III. Estimated consumption of euphausiids (kt) by the La Perouse fish community in August of
each year from 1985 to 1989
1985
1986
1987
1988
1989
Average
Standard
deviation
Coefficient of
variation (%)
Coho
Chinook
1.8
2.7
2.7
4.6
1.8
2.7
1.1
41
Dogfish
Sablefish
Herring
Hake
P. cod
Total
0.7
0.6
0.2
0.2
0.7
0.5
26.5
17.8
27.1
19.5
25.1
23.2
1.8
12.5
8.0
0.2
3.2
5.1
57.4
5.3
69.4
73.3
194.3
80.0
37.4
235.8
341.1
53.5
105.6
154.7
0.0
5.9
3.3
0.4
1.8
2.3
125.6
280.7
451.7
151.8
332.4
268.4
0.3
4.3
5.0
69.5
130.1
2.4
134
60
19
98
87
84
104
50
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C.L.K.Robinson
Fig. 3. The estimated consumption of euphausiids (kt) by Pacific hake and Pacific herring each
August from 1985 to 1997.
was 0.93, ranging from 1.5 in 1989 to 0.5 in 1993. The consumption per tonne
of fish was significantly positively correlated with herring (r = 0.68) and hake
(r = 0.6) consumption. There was no overall trend in the pattern of relative
euphausiid consumption (Figure 4).
Discussion
A major objective of this study was to assemble fish ration, diet and biomass data
and to estimate the consumption of euphausiids by seven of the most common
fish species in the La Perouse region in August. The euphausiid consumption estimates presented in this paper are considered low because data on other key
euphausiid predators were not available. For example, during the warm 1990s,
high biomasses of Pacific mackerel (Scomber japonicus) occurred in the La
Perouse region. Few data were also available regarding other important
euphausiid predators, for example the Cassin’s Auklet (Ptychoramphus aleuticus)
and baleen whales such as the humpback whale (Megaptera novaeangliae).
Previous studies conducted in this region have focussed on documenting the
occurrence and quantity of euphausiids in fish stomachs, and how they change
over space and time [e.g. (Outram and Haegle, 1972; Tanasichuk et al., 1991; Ware
and McFarlane, 1995)]. In other regions, the literature is also replete with studies
describing the importance of euphausiids in fish diets, but there are only a few
studies that have estimated the consumption of euphausiids by the fish
community. This is quite surprising given the keystone role of euphausiids as prey
in most coastal food webs. Payne et al. (Payne et al., 1987) and Punt et al. (Punt
et al., 1992) calculated prey consumption by Cape hake off the west coast of South
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Euphausiid consumption by pelagic fish
Fig. 4. The estimated total consumption of euphausiids by the La Perouse fish community and the
euphausiid consumption per tonne of fish in each August from 1985 to 1997.
Africa. About one third of the estimated total annual ration of Cape hake
consisted of crustaceans (euphausiids and amphipods). Other studies conducted
in the northern Benguela Current Upwelling region have estimated that macrozooplankton (i.e. euphausiids) constitute about 40% of the total estimated
consumption of four categories of planktivorous fish (Hewitson and Cruickshank,
1993). The results from the South African studies are not directly comparable
with this study because consumption in the former was calculated annually, while
this study has focused on one month in mid summer.
Yamamura et al. examined the predation on E.pacifica by demersal fishes off
Sendai, northern Japan (Yamamura et al., 1998). Twenty-four fish species representing >90% of the demersal fish community biomass were found to be highly
dependent on E.pacifica. Although no estimates of euphausiid abundance were
available, the authors noted that the annual estimates of euphausiid consumption
by the demersal fish community represented 15–64% of the commercial catch of
E.pacifica, and they concluded that a euphausiid fishery would have a considerable impact on demersal fish assemblages. It is interesting to note that the majority of euphausiid consumption by fish in the La Perouse region was due to two
species, Pacific hake and herring, while more than 20 demersal fish species were
identified as important euphausiid consumers off northern Japan.
Bogstad and Mehl calculated the consumption of krill (Euphausiacea) and
other prey species by Atlantic cod in the Barents Sea from 1984 to 1995 (Bogstad
and Mehl, 1997). The authors found that the diet of age-1 cod was dominated by
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C.L.K.Robinson
krill, but that its dietary importance declined with cod age (size). In addition, krill
consumption was 1.5–15% of the total prey consumed by cod over the 12 year
study period. These results indicate that krill are relatively unimportant to Atlantic cod in the Barents Sea compared with the importance of euphausiids to Pacific
hake off the west coast of Vancouver Island.
Everson et al. calculated the consumption of krill by two major predators on
the South Georgia shelf in the Scotia Sea near Antarctica (Everson et al., 1999).
From these calculations, the authors determined that the relative abundance of
krill could potentially drive the trophodynamic interactions between fur-seals and
icefish. In years of low krill abundance, fur-seals increased their consumption of
icefish and during years of high krill abundance, fur-seals fed primarily on krill.
The interaction between icefish and fur-seals appeared to be dependent on the
timing of krill scarcity and the year-class strength of the icefish population.
Euphausiids appear to be a keystone prey species in the South Georgia shelf as
they are in the La Perouse region.
Rexstad and Pikitch estimated the consumption of T.spinifera by Pacific hake
on the Washington and Oregon shelf using size-specific prey consumption data
(Rexstad and Pikitch, 1986). The authors determined that hake <49 cm consumed
about 98% of the total hake population’s euphausiid consumption in August/
September 1983. This compares with an average euphausiid consumption of 72%
(range 26–95%) by hake <49 cm in the La Perouse region in August from 1985
to 1996. The difference between the studies can be accounted for because the
majority of the migratory hake stock on the Washington and Oregon shelf in
summer comprises smaller euphausiid-eating individuals, while larger and
smaller hake migrate further northward into British Columbia coastal waters
(Smith et al., 1990; Dorn, 1992).
Another objective of this study was to determine if predation by the pelagic
fish community caused the decline in euphausiid biomass observed by Tanasichuk
(Tanasichuk, 1998). Two results from the present study address this objective.
First, there was no significant relationship between the consumption of euphausiids per tonne of fish and euphausiid biomass (r = –0.21; P = 0.65), or between
total fish community consumption and euphausiid biomass (r = –0.32; P = 0.48).
Thus, pelagic fish community euphausiid consumption was relatively constant
during the 1990s, and the decline in euphausiid biomass cannot be ascribed to fish
predation. Relatively few marine studies have documented the direct impact of
fish predators on euphausiids, and those that do also indicate a lack of evidence
for predatory impacts. Mullin and Conversi, for example, hypothesized that the
initiation of the hake fishery along the Oregon and Washington coast from
1966–1969 would remove the euphausiids’ main predator, and thus euphausiid
biomass or individual size should increase compared with the pre-fishery period
1960–1965 (Mullin and Conversi, 1988). The authors analyzed data on size-fractionated zooplankton biomass and could not detect a significant increase in
euphausiid biomass after the initiation of the hake fishery. Thus, euphausiid
biomass did not appear to increase with reduced predation pressure from hake.
Second, from food-web theory it was also expected that if euphausiid biomass
declined substantially, then fish would ‘switch’ to alternative, more abundant
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Euphausiid consumption by pelagic fish
prey. Murphy et al. (Murphy et al., 1998) and Everson et al. (Everson et al., 1999),
for example, discuss how the diets of ice-fish and fur-seals changed in response
to krill abundance on the South Georgia shelf in the Scotia Sea. Brodeur and
Pearcy also noted that during years of strong upwelling along the Washington–
Oregon coast, euphausiids dominated the diets of fish predators and trophic
diversity was low (Brodeur and Pearcy, 1992). In years of reduced upwelling,
there was a reduction in euphausiids in fish diets and trophic diversity was high.
The importance of euphausiids in the fish diets was thought to reflect changes in
euphausiid abundance (Brodeur and Pearcy, 1992). In this study, the fraction of
euphausiids in the diet of small (< 50cm) and large (> 50 cm) hake, the most abundant fish predator, did not change relative to the 1991–1997 decline in euphausiid
biomass. Small hake maintained euphausiids at 75%, on average, of their total
prey selection by weight (Figure 5). Tanasichuk also indicated that during the
period of euphausiid biomass decline, hake showed no overall change in the size
or species of euphausiid preferred (Tanasichuk, 1999).
Although there is no evidence from this study to indicate that fish predators
have a significant impact on euphausiid biomass, other studies clearly support the
observation that euphausiids have an impact on fish. Tanasichuk demonstrated
that Pacific herring stocks in the La Perouse region exhibited poor growth in the
1990s, during the period of decline in biomass and production of their main prey
(euphausiids) (Tanasichuk, 1997). Robinson and Ware used an ecosystem simulation model to demonstrate a significant positive correlation between observed
hake condition factor and simulated hake production during the upwelling season
(Robinson and Ware, 1999). The authors found that the observed hake condition
factor was most strongly linked to changes in the August biomass of euphausiids.
Fig. 5. The fraction of euphausiids in the diet of small (<50 cm) and large (>50 cm) Pacific hake caught
in the La Perouse region each August from 1991 to 1997, compared with the observed decline in adult
euphausiid biomass.
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C.L.K.Robinson
Thus, there appear to be clear linkages between the well-being of major fish
predators and euphausiid biomass in the La Perouse region in August.
The decline in adult euphausiid biomass in the La Perouse region in the 1990s
may ultimately be related to changes in coastal upwelling currents, which result
in reduced aggregations of adult euphausiids and/or reduced biomasses of
euphausiid recruits. Ware and McFarlane, for example, have shown major differences in hake distribution in stronger versus weaker upwelling years (Ware and
McFarlane, 1995). More hake are generally found along the shelf-break and
northward of the La Perouse region in years of reduced upwelling, while hake are
more concentrated in the La Perouse region during stronger upwelling years.
Hake distributions have been linked to euphausiid distributions (Mackas et al.,
1997). The influence of oceanographic processes on the distribution of euphausiids is also well documented for other oceanic regions. Priddle et al., for example,
suggested that low krill abundance in the Southern Ocean (Antarctic) marine
ecosystem can be attributed to krill redistribution by physical oceanographic
processes (Priddle et al., 1988). More recently, Yamamura et al. (Yamamura et al.,
1998) found a negative correlation between water temperature and euphausiid
consumption of the demersal fish community, and concluded that it was related
to variability in the distribution of E.pacifica and the fish predators in response
to a changing physical environment.
The conclusion that fish community predation is not responsible for the decline
in euphausiid biomass may only be applicable to the mid-summer upwelling
period (July and August). It remains to be determined if this conclusion can be
robustly applied to earlier in the upwelling season (e.g. June) or later in the
upwelling season (e.g. September). The robustness of this conclusion is questionable because the data used in the analysis were collected in August only, and
because of the known variability in fish feeding rates and biomass, and in
euphausiid biomass, over the upwelling season (Robinson and Ware, 1999). The
main result of this study extends the conclusion drawn by Tanasichuk (Tanasichuk, 1998), who indicated that hake alone did not cause the observed
euphausiid biomass decline. The available evidence from this study suggests that
the fish community in August is also not responsible for the fivefold decline in
adult euphausiid biomass observed in the 1990s off southwestern Vancouver
Island. Ultimately, the relative interplay between predation and oceanic factors
on euphausiid populations, at various space and time scales, needs to be further
addressed before we will fully understand the mechanisms driving the biomass
dynamics of a keystone prey species in this highly productive coastal upwelling
system.
Acknowledgements
Drs D.Ware and R.Tanasichuk, Fisheries and Oceans Canada, Nanaimo, British
Columbia kindly provided data on hake, herring and dogfish diets. I also thank
them for their discussions concerning the potential influence of fish predation on
euphausiids. J.Morris, DFO, Nanaimo provided diet data for chinook and coho
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Euphausiid consumption by pelagic fish
salmon. Two anonymous reviewers made several suggestions that improved the
manuscript.
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Received on October 10, 1999; accepted on February 16, 2000
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