Annual Progress Report – Oil Spill Recovery Institute
Date: Jan 16, 2007
Name of Awardee: Richard E. Thorne, Prince William Sound Science Center
OSRI Contract Number: 06-10-05
Project Title: Biological Monitoring of Spring Zooplankton and Nekton in Prince William
Sound
Dates this progress report covers: January 1-December 31, 2006
Abstract:
A program to monitor the pink salmon food supply and predators in Prince William Sound
was initiated by the Oil Spill Recovery Institute (OSRI) in 2000. This report describes the
results of monitoring during 2006, the seventh and final year of the study. Three cruises
were completed: April 29-May 1, May 12-14 and June 8-9. The survey coverage consisted
of the same six areas that were initially established in 2000, and covered each successive
year. Only two-thirds of these areas were covered during the second and third cruises of
2006 due to limited vessel availability. Data acquisition included acoustic backscatter at two
frequencies and vertical plankton tows. Copepods dominated the zooplankton net catches, as
has been the case in all years. Large copepods were 6.1% in 2006, compared with a sevenyear average of 6.6%. The percent of euphausids in the net catches was below average at
0.4%. Abundance of large copepods in the net catches in 2006 was slightly above the sevenyear average (1081 per tow compared to 1029 average), while euphausid catches were below
average (73 compared to 113). Overall the acoustic scattering from the zooplankton in 2006
was above average. The acoustic backscattering was well correlated with the
macrozooplankton catches. The fish densities observed during 2006 showed some of the
general tendency to move from the main basin to the corridor that has been observed in
previous years. However, the overall fish densities observed during 2006 were well below
average, so analysis of the details of the distribution was limited. Pink salmon spend slightly
over one year at sea before returning as adults. Observations during the first four years of
this program showed strong correlation between pink salmon returns and both large copepod
and euphausid abundance during the nursery year. However, those relationships have
weakened with results of the subsequent two years. Pink salmon returns do show positive
correlations with large copepod abundance and percent catch and with euphausid abundance,
but the correlations are relatively weak (r2 values of 0.20, 0.31 and 0.14 respectively).
Instead, the returns appear to be dominated by a two-year cycle.
Review of objectives:
The overall goal of the OSRI zooplankton-monitoring program was to develop and apply a
cost effective approach to estimation of pink salmon food supply and predator abundance.
The specific objectives were to measure the abundance of zooplankton as food supply for
juvenile pink salmon and the abundance of predators.
Problems encountered:
From 2000 to 2002, SERVS provided the Oil Skimmer Valdez Star for these surveys.
Security issues ended that service just prior to the 2003 effort, and subsequent surveys were
conducted using chartered fishing vessels. The change added about $12,000 to the annual
costs of the $75,000 per year program, which had to be taken from other categories. Impacts
included reductions in coverage. The reductions were partly the result of the funding
limitations and partly the result of conflicts with on-going commercial fisheries for vessel
availability during the second and third cruise periods. Even prior to this reduction, there
were concerns that the 3-cruise level of effort was marginal, based on the high variability that
was observed among the cruises. The impact was especially severe on the fish component of
the effort, since the fish were even more patchily distributed than the zooplankton, and trends
were not always consistent. Coverage in 2006, the most limited of the series, was not
adequate to describe fish densities with useful accuracy. These issue of limited sampling
effort may have contributed to the lower than expected strength of the correlations between
pink salmon survival and large copepod abundance.
Highlights:
We completed collection of the seventh year of distribution and abundance information on
zooplankton and fish during spring in Prince William Sound. These results provide a
reasonable endpoint for a relatively long monitoring program that should contribute
substantially to our understanding the Prince William Sound ecosystem. A paper from this
program entitled, “Pollock and the Prince William Sound Ecosystem” was given at the
Lowell-Wakefield Symposium. A poster presentation entitled, “Acoustic monitoring of the
juvenile pink salmon food supply and predators in Prince William Sound, Alaska”, was given
at the American Fisheries Societies annual meeting in Lake Placid, N.Y. It is anticipated that
at least one more refereed journal publication will result from the program.
Conclusions:
The seventh year of data collection was reasonably successful despite the struggle for
available vessel time. The sixth year of adult returns adds to a growing data base that
appears to provide some predictability. The adult returns in 2007 will provide the final point
for correlation in this relatively long monitoring program. The results indicate that a better
understanding of the recently manifested two-year cycle in the PWS pink salmon returns is
needed. The overall results provide a firm basis on which to design any future monitoring
programs.
Considerable data were obtained that were beyond the original scope of the program. These
include the indications of the importance of euphausids for pink salmon survival and a better
understanding of the dynamics and complexity of the zooplankton and fish distributions.
There is potential for further use of both the zooplankton and acoustic samples for
investigations beyond the scope of this program, including analysis of the deeper euphausid
scattering layers. It is also possible that aspects of this monitoring program will be continued
by other investigators, since the data have some relevance to concerns about herring
restoration.
A detailed report on the results of this project is given as Appendix 1 (pages 3-16).
Appendix 1-Monitoring the Juvenile Pink Salmon Food Supply and Predators
in Prince William Sound During 2006
Richard E. Thorne
Introduction
Research conducted by the Sound Ecosystem Assessment (SEA) program in the 1990s
indicated that the survival of pink salmon fry (Oncorhynchus gorbuscha) in Prince
William Sound (PWS) is dependent on the zooplankton food availability and predator
abundance. Large calanoid copepods, mainly of the genus Neocalanus, typically consist
of more than 50% of the biomass of PWS zooplankton in April and May. They are a
valuable source of food for many fishes, including pink salmon fry, because of their
relatively large size and high energy content (Cooney 1986). Willette et al. (2001)
showed that both survival and early growth rates of pink salmon were correlated with the
duration of the Neocalanus spring bloom. Cooney et al. (2001a) also showed that most
pink salmon fry rearing in PWS are consumed by predators during their initial 45-60 days
of early marine residence. The major predator is walleye pollock. Adult pollock feed on
Neocalanus, thus are competitors of juvenile pink salmon for this food source. However,
when Neocalanus abundance is low, pollock become piscivorous and are the dominant
pelagic predator of pink salmon fry (Willette et al. 2001). Pacific herring (Clupea pallasi)
exhibit a similar prey switching behavior.
Subsequent to the SEA program, the Prince William Sound Science Center (PWSSC),
with support from the Oil Spill Recovery Institute (OSRI) and in cooperation with the
Alaska Department of Fish and Game and the Ship Escort/Response Vessel System
(SERVS), initiated a program in FY00 to begin monitoring the spring predator and prey
densities of juvenile pink salmon (Thorne 2000; Thorne and Thomas 2001; Thorne 2002;
Thorne 2003; Thorne 2004a; Thorne 2004b; Thorne 2005a; Thorne 2005b). The PWSSC
program was built on the findings of SEA. Specifically, the goals were to monitor the
abundance of zooplankton and predators. Multi-frequency acoustic systems in
conjunction with plankton nets provided estimates of zooplankton and fish abundance
and distribution. The program has completed seven years of fieldwork, associated with
six subsequent years of pink salmon returns. This report details the results of the seventh
year of monitoring and compares the results among the seven years, including adult
salmon returns that resulted from nursery conditions during the first six years.
Methods
Three cruises were completed in 2006: April 29-May 1, May 12-14 and June 8-9. The
survey coverage consisted of the same six areas that were initially established in 2000,
and covered each successive year. Three of the areas extend north/south through the
main basin of PWS, and three areas from Perry Island Passage out through Knight Island
Passage (Fig. 1). The latter region is referred to as the “outmigration corridor” as it is a
well-documented nursery area of pink salmon juveniles. All six areas were successfully
sampled during the first cruise in 2006, but only four areas were surveyed during the
second and third cruises because of limited vessel availability.
Figure 1. Location of transects for zooplankton surveys in Prince William Sound
The acoustic system was a two-frequency BioSonics DT5000 at 120 kHz and 420 kHz.
The acoustic data acquisition consisted of volume backscatter measurements from the
upper 50 m at the 420 kHz frequency and the upper 200 m with the 120 kHz system.
Both transducers were calibrated with standard targets following procedures of Foote et
al. (1987).
The zooplankton sampling was a 50 m vertical tow using a 0.335-mm 0.5 m-ring net,
following procedures of Cooney et al. (1995). Samples were preserved in the field in
10% formalin. Data collection was limited to daytime hours for consistency.
The plankton samples were analyzed to determine both size and frequency of the major
components following procedures detailed in Kirsch et al. (2000). Quantitative subsamples were taken using a Hensen-Stempel pipette. For purposes of this study, the term,
"large-bodied", is used to refer to stage IV and V Neocalanus, or equivalent size
copepods. In practice, this typically corresponds to copepods above 2 mm length,
although some stage III Neocalanus overlap with stage IV (Cooney et al. 2001b).
The acoustic data were analyzed using standard echo integration techniques (Thorne
1983a,b; MacLennan and Simmonds 1992). The BioSonics acoustic system stores the
raw digital echo information directly on a computer hard-drive. These data were
analyzed using BioSonics Echo Integration Analyzer Program Version 4.02. Both
frequencies were analyzed twice: once for total backscatter and a second time for fish
only. Zooplankton backscattering was then calculated by subtracting the fish backscatter
from the total backscatter. A generalized acoustic cross-section equivalent to –32 dB/kg
was used to estimate fish biomass from the fish backscatter (Thorne 1983b).
Results
Zooplankton Composition
Six zooplankton hauls were completed in each region (corridor and main basin) during
each cruise, a total of 36 samples. Copepods dominated the zooplankton net catches, as
has been the case in all years (Tables 1, 2). Small copepods were 75.9% numerically in
2006, compared to a seven-year average of 81.1%. Large copepods were 6.1% in 2006,
compared to a seven-year average of 6.6%. The percent of euphausids in the net catches
was below average at 0.4%. Relative abundances of pteropods and larvacea were above
average.
Table 1. Numerical composition (percent) of zooplankton catches during 2006 surveys
Cruise
1
Cruise
2
Cruise
3
2006
Small
copepods
Large
copepods
Corridor
Main
Average
84.9
84.7
84.9
4,2
10.8
6.3
Corridor
Main
Average
75.7
83.5
79.2
Corridor
Main
Average
Average
Larvacea
Pteropod
Euphausid
Other
9.8
2.5
7.5
0.1
0.6
0.3
0.2
0.4
0.3
0.8
0.9
0.8
12.9
9.1
11.2
8.7
3.9
6.6
0.5
1.5
1.0
0.9
0.8
0.9
1.2
1.3
1.2
54.4
67.2
63.6
1.9
0.4
0.8
16.3
6.9
9.6
25.4
23.5
24.0
0.2
0.1
0.1
1.9
1.9
1.9
75.9
6.1
7.9
8.4
0.4
1.3
Table 2-Comparison of overall average numerical compositions, 2000-2006
Year
2006
2005
2004
2003
2002
2001
2000
Small
copepods
75.9
75.5
79.3
86.0
84.3
81.0
85.5
average
81.1
Large
copepods
6.1
8.7
8.9
3.0
7.5
4.2
8.1
6.6
Larvacea
7.9
2.9
3.1
6.1
1.8
4.0
4.3
4.3
Pteropod
8.4
10.2
2.0
1.0
1.9
4.5
0.6
4.1
Euphausid
0.4
0.7
0.6
1.4
1.4
0.8
0.6
0.8
Other
1.3
2.0
6.1
2.5
3.1
5.5
0.9
3.1
Relative Abundance From Net Catches
Catch rates of large copepods were greatest in the corridor during the second cruise
(Table 3). Large copepod catches declined substantially for the third cruise. Euphausid
catches showed a similar pattern. Pteropods increased over the three cruises,
dramatically in the third. Overall net catches in 2006 were above the seven-year average
for small and large copepods, pteropods and larvacea, but below average for euphausids
(Table 4). Abundance of large copepods in the net catches was 1081 per tow in 2006
compared to 1029 average, while euphausid catches were 73 compared to a 113 average.
Table 3. Average catch rates for various zooplankton categories during 2006
Cruise
1
Small
copepods
Large
copepods
Larvacea
Pteropod
Euphausid
Corridor
Main
average
24555
11530
18043
1200
1480
1340
2825
345
1585
35
75
55
65
60
63
235
120
178
2
Corridor
Main
average
13541
11960
12751
2316
1300
1808
1556
555
1055
97
210
153
168
115
142
214
185
200
3
Corridor
Main
average
3485
11030
7258
120
70
95
1045
1140
1093
1630
3860
2745
10
20
15
120
305
213
Corridor
Main
average
13860
11507
12684
1212
950
1081
1809
680
1244
587
1382
984
81
65
73
190
203
197
all
Location
Other
Table 4-Average catches by category for seven-year period, 2000-2006
2000
2001
2002
2003
2004
2005
2006
Average
Small
Copepods
17055
11071
14062
9926
8924
11143
12684
12123
Large
Copepods
1869
442
1545
284
837
1146
1081
1029
Larvacea
1113
403
438
596
356
314
1244
638
Pteropods
75
689
425
114
227
2429
984
706
Euphausids
109
114
220
127
76
71
73
113
Acoustic Backscattering
Acoustic scattering at 120 kHz from zooplankton layers in the upper 50 m was generally
higher in the corridor region than the main basin (Table 5; Fig. 2). Scattering in the
corridor region peaked during the second cruise, while that in the main basin showed a
decreasing trend throughout the spring. Scattering during the third cruise was
considerably lower for both locations. Overall the 120 kHz scattering in 2006 was above
average (Fig. 3). Overall average acoustic backscattering is well correlated with the
average macrozooplankton catch (Fig 4), although the acoustic backscattering does not
seem to be overly influenced by any single component. The 420 kHz backscattering
collected during 2006 was relatively noisy. Since the 120 kHz frequency has been the
most useful, further analysis of the 420 kHz data was concluded.
Table 5. 120 kHz area backscattering from zooplankton (0-50 m) and fish (0-200 m) for
the main basin and corridor locations, three cruises.
Cruise
1
2
3
Location
Main
Corridor
Main
Corridor
Main
Corridor
Zoo
4.98E-06
9.21E-06
4.54E-06
1.24E-05
8.53E-07
5.45E-07
Fish
9.69E-07
4.26E-08
6.93E-08
3.17E-07
6.20E-07
2.57E-07
1.40E-05
1.20E-05
Area Backscatter
1.00E-05
Main
8.00E-06
Corridor
6.00E-06
4.00E-06
2.00E-06
0.00E+00
1
2
3
Cruise
Figure 2. 120 kHz area backscatter (0-50 m) from zooplankton in the main basin and
corridor during three cruises in 2006
9.00E-06
Average 120 kHz Area Backscattering
8.00E-06
7.00E-06
6.00E-06
5.00E-06
4.00E-06
3.00E-06
2.00E-06
1.00E-06
0.00E+00
2000
2001
2002
2003
2004
2005
2006
Year
Figure 3. Variation in 120 kHz area backscattering (0-50 m) from zooplankton from 2000 to
2006.
Fig. 4. Comparison of average annual net catches of macrozooplankton with average annual
120 kHz acoustic area backscattering (0-50 m) for 2000 to 2006.
Fish Abundance and Distribution
The general fish pattern seen in previous years has been a shift from most abundant in the
main basin during cruise 1 to most abundant in the corridor during cruise 3. The fish
distribution in 2006 showed this characteristic between the first two cruises. Then the
biomass in the main basin increased while that in the corridor decreased slightly (Fig. 5).
120 kHz area backscatter (0-200 m) from fish in the main basin and corridor
during three cruises in 2006
1.20E-06
Area Backscattering
1.00E-06
Main
Corridor
8.00E-07
6.00E-07
4.00E-07
2.00E-07
0.00E+00
1
2
3
Cruise
Fig. 5. Estimated densities of fish in the main basin and corridor for the three cruises during
2006.
The fish distributions are very patchy, and values are definitely influenced by
opportunistic encounters with large schools. Overall the observed fish densities were
very low in both the main basin and corridor for all three cruises during 2006. The low
densities precluded further analysis of distributional details.
Comparisons of Zooplankton Abundance and Subsequent Harvests
Pink salmon spend slightly over one year at sea before returning as adults. Observations
during the first four years of this program showed strong correlation between pink
salmon returns and both large copepod and euphausid abundance during the nursery year
(Thorne 2004a, 2005b). However, those relationships have weakened with results of the
subsequent two years. Pink salmon returns do show positive correlations with large
copepod abundance and percent catch and with euphausid abundance, but the correlations
are relatively weak (r2 values of 0.20, 0.31 and 0.14 respectively; Figs 5-7). Instead, the
returns appear to be dominated by a two-year cycle (Fig. 8).
Fig. 5. Comparison of pink salmon returns with average catch of large-bodied copepods
during the nursery year for nursery years 2000 to 2005.
Fig. 6. Comparison of total PWS pink salmon harvest with the percentage of large
copepods in the net catches during the nursery year, for nursery years 2000 to 2005.
Fig. 7. Comparison of total PWS pink salmon harvest with the abundance of euphausids
in the net catches during the nursery year, for nursery years 2000 to 2005.
Comparison of Large Copepod Catch and Subsequent Returns of Pink Salmon
60.0
2000
Catch
1800
Harvest (millions)
50.0
1400
40.0
1200
30.0
1000
800
Pink Salmon Returns
Large Copepod Catch
1600
20.0
600
400
10.0
200
0.0
0
2000
2001
2002
2003
2004
2005
Nursery Year
Figure 8. Comparison of Large Copepod Catch and Subsequent Returns of Pink Salmon.
Note apparent 2-year cycle in pink salmon returns.
Discussion and Conclusions
The pink salmon returns have shown a strong 2-year cycle, even though most of the fry
are of hatchery origin and annual releases have been consistent. Returns from 2000-2003
were well correlated with large copepod and euphausid abundance from net catches.
Large copepod abundance also alternated in abundance during this four-year period,
while euphausid abundance was exceptional in 2002, corresponding to a record pink
salmon survival. However, during the past two years large copepod abundance has been
only average and euphausid abundance low. Yet the pink salmon returns from these
nursery conditions have maintained the two year cycle. Thus while macrozooplankton
abundance appears to have some impact on juvenile pink salmon survival, it is clear that
some other parameter is having a more dominant effect.
A caution must be given to this conclusion, especially relative to euphausids. A
correlation between pink salmon survival and large-bodied copepods would have been
expected given the results from the SEA program. There had been no previous indication
that euphausids might play a role as a prey-sheltering mechanism for juvenile pink
salmon, but it is well documented that euphausids are a favored prey of both walleye
pollock and herring. The lack of higher correlation values for euphausids may reflect
limitations of the approach. Euphausids are vertical migrators, and are undoubtedly
much more abundant than indicated in the net tows. Although pink salmon survival was
positively correlated with euphausids in net catches, euphausids were a relatively minor
component of the daytime, near-surface zooplankton and did not make a major
contribution to the near-surface acoustic backscatter. The sampling effort for
zooplankton in this program was focused on the upper 50 m on the assumption that the
large-bodied copepods were the primary object of interest. The 120 kHz frequency is
capable of extending the measurement to the daytime depths of euphausids, but the 0.5 m
ring net is not an effective sampler for larger zooplankton, especially during daytime.
It is also likely that pink salmon survival is impacted by more complex spatial and
temporal variation than were captured in the PWSSC study design (Thorne 2005b).
There was substantial patchiness in the zooplankton distributions (Fig. 9). There was
also substantial annual variation in the timing of the large-copepod appearance in the
surface waters of PWS, which may reflect annual variation in the exchange processes
with the GOA. Variation among cruises has been relatively high, and the middle cruise
has seldom had the highest abundance of large-bodied copepods.
Fig. 9. Echogram from first half of transect across Hinchinbrook Entrance during 2005
cruise 3, illustrating complexity of scattering.
A related concern is the variability in the fish distributions and averages. The fish are
even more patchily distributed than the zooplankton, and trends are not always consistent.
Some additional information will become available in late 2007 with the seventh and last
corresponding pink salmon return. However, even a seven-year monitoring program may
not been adequate to resolve these patchiness problems.
Literature Cited
Cooney, R.T. 1986. Zooplankton. In the Gulf of Alaska, physical environmental and biological
resources, D.W. Hood and S.T. Zimmerman (eds.) Anchorage: U.S Depart. of
Commerce, Minerals Management Service, pp. 285-304.
Cooney, R.T., T.M. Willette, S. Sharr, D. Sharp and J. Olsen 1995. The effect of climate on
North Pacific pink salmon (Onchorynchus gorbuscha) production: examinling some
details of a natural experiment. In: R.J. Beamish (ed) Climate Change and Northern Fish
Populations. Can Spec. Publ. Fish Aquat. Sci. 121:475-482.
Cooney, R.T., J.R. Allen, M.A. Bishop, D.L. Eslinger, T. Kline, B.L. Norcross, C.P. Mcroy, J.
Milton, J. Olsen, V. Patrick, A.J. Paul, D. Salmon, D. Scheel, G.L. Thomas, S.L.
Vaughan and T. M. Willette 2001a. Ecosystems controls of juvenile pink salmon
(Onchorynchus gorbuscha) and Pacific herring (Clupea pallasi) populations in Prince
William Sound, Alaska. Fish Ocean 10 (Suppl 1):1-13.
Cooney, R.T., K.O. Coyle, E. Stockmar and C. Stark 2001b. Seasonality in surface-layer net
zooplankton communities in Prince William Sound, Alaska. Fish Ocean 10 (Suppl 1):97109.
Foote, K.G., H.P. Knudsen, G. Vestnes, D.N. MacLennan and E.J. Simmonds 1987. Calibration
of acoustic instruments for fish density estimation: a practical guide. Int. Coun. Explor.
Sea coop. Res. Rep. No. 144: 57 Pp.
Kirsch, J., G.L. Thomas and R.T. Cooney. 2000. Acoustic estimates of zooplankton distributions
in Prince William Sound, spring 1996. Fish. Res. 47: 245-260.
MacLennan, D. N., and E. J. Simmonds 1992. Fisheries Acoustics, Chapman and Hall, London.
Thorne, R.E. 1983a. Hydroacoustics. Chapt. 12 in L. Nielson and D. Johnson (eds)., Fisheries
Techniques. American Fisheries Society, Bethesda, MD.
Thorne, R.E. 1983b. Assessment of population abundance by echo integration. Proc. Symp. On
Assessment of Micronekton. Biol. Ocean. J. 2:253-262.
Thorne, R.E. 2000. Biological Monitoring of Spring Zooplankton and Nekton in Prince William
Sound. OSRI Annual Report, Contract 00-10-04, 48 p.
Thorne, R.E. 2002. Biological Monitoring of Spring Zooplankton and Nekton in Prince William
Sound. OSRI Annual Report, Contract 01-10-04, 34 p.
Thorne, R.E. 2003. Biological Monitoring of Spring Zooplankton and Nekton in Prince William
Sound. OSRI Annual Report, Contract 02-10-16, 60p.
Thorne, R.E. 2004a. Factors Governing Pink Salmon Survival in Prince William Sound, Alaska.
Pp 67-72 in Keong, V. (ed) Proceedings of the 21st Northeast Pink and Chum Salmon
Workshop, February 26-28, Victoria B.C., Pacific Salmon Commission, Vancouver, B.C.
Thorne, R.E. 2004b. Biological Monitoring of Spring Zooplankton and Nekton in Prince William
Sound. OSRI Annual Report, Contract 03-10-22, 22p.
Thorne, R.E. 2005a. Biological Monitoring of Spring Zooplankton and Nekton in Prince William
Sound. OSRI Annual Report, Contract 04-10-11, 16p.
Thorne, R.E. 2005b. Acoustic monitoring of the juvenile pink salmon food supply and predators
in Prince William Sound, Alaska. Proc. Oceans05, Washington D.C. Sept 20-23, 2005
Thorne, R.E. and G.L. Thomas 2001. Monitoring the juvenile pink salmon food supply and
predators in Prince William Sound. Pages 42-44, in R. Beamish, Y. Ishida, V. Karpenko,
P. Livingston and K. Myers, Workshop on factors affecting production of juvenile
salmon: comparative studies on juvenile salmon ecology between the East and West
North Pacific Ocean, Technical Report 2, North Pacific Anadromous Fish Commission,
Vancouver, B.C.
Thomas, G.L. and R.E. Thorne 2003. Acoustical-optical assessment of Pacific herring and their
predator assemblage in Prince William Sound, Alaska. Aquatic Living Resources
16:247-253.
Willette, T.M., R.T. Cooney, V. Patrick, D. M. Mason, G.L. Thomas and D. Scheel 2001.
Ecological processes influencing mortality of juvenile pink salmon (Onchorynchus
gorbuscha) in Prince William Sound, Alaska. Fish Ocean 10 (Suppl 1):14-41.