RESOURCE ECOLOGY AND ECOSYSTEM MODELING PROGRAM:
Fish Stomach Collection and Lab Analysis
Laboratory analysis was performed on 3,063 groundfish stomachs from the
eastern Bering Sea and 200 from the Aleutian Islands. No stomachs were
collected this quarter.
By Troy Buckley and Mei-Sun Yang.
Ecosystem Considerations for 2004
The Ecosystem Considerations Section for 2004 was completed as part of
the Stock Assessment and Fishery Evaluation (SAFE) Reports which are provided
to the North Pacific Fishery Management Council (NPFMC). A summary of
the first draft of the Ecosystem Considerations section was presented to
the NPFMC groundfish plan teams in September and updates/changes to the
draft were presented to the plan teams in November. The chapter content
and format is very similar to last year, primarily providing information
on the status of the environment. The final document is available on the
Alaska Fisheries Science Center web site at:
A summary of some of the main ecosystem indicators is presented in Table 1 of the document.
New to the Ecosystem Considerations section this year is the addition of
status and trend information pertaining to salmon, herring, crabs, and
zooplankton. Also new to the chapter is the addition of several community
indicators, such as community size spectrum and k-dominance curves, indices
of biodiversity, recruitment, and survival. Data gaps still include lower
trophic levels, such as phytoplankton, additional zooplankton, and nutrient
Stock assessment scientists continued to use indicators from the Ecosystem
Considerations section to assess ecosystem factors such as climate, predators,
prey, and habitat that might affect a particular stock. Also, information
regarding a particular fisherys catch, bycatch and temporal/spatial distribution
was used to consider the possible impacts of that fishery on the ecosystem.
We are still in early stages of using this type of information in stock
assessments. However, we anticipate the information could be used to modify
allowable biological catch recommendations or spatial/distribution of the
catch due to ecosystem concerns, or to target further research that would
be needed to quantify ecosystem impacts.
Another new addition to the Ecosystem Considerations section is the Ecosystem
Assessment. The primary intent of this contribution is to summarize historical
climate and fishing effects on the shelf and slope regions of the eastern
Bering Sea/Aleutian Islands, and the Gulf of Alaska from an ecosystem perspective
and to provide an assessment of the possible future effects of climate
and fishing on ecosystem structure and function. This is the first year
that this assessment strategy was used and not all of the modeling tools
are ready for use in projections. Future development of modeling tools
will enable scientists to provide advice on management strategies that
are robust to a wide range of future ecosystem states. The assessment
could be used to evaluate aggregate effects of groundfish fisheries on
ecosystem and habitat and could result in advice regarding changes in aggregate
catch levels (OY cap), species mix of the catch, and discard amounts.
By Jennifer Boldt.
Feeding Habits of Demersal Fish
Longitudinal gradients in the feeding habits of Pacific cod, Atka mackerel,
pollock and Pacific ocean perch (POP) were assessed from stomach collections
during AFSC trawl surveys in the Aleutian Islands and Western Gulf of Alaska
region in order to determine the presence of biophysical boundaries. Stomachs
were also collected by observers during commercial fishing operations.
Samples were taken between 1982 and 1999, and a total of 20,985 stomachs
were included in this analysis. The degree of taxonomic resolution varied
depending on prey type. Most fish prey were identified to species, if
possible, with exception of myctophids which were mostly identified only
to family (Myctophidae). Invertebrate prey species were likewise rarely
identified to species; euphausiids and copepods were identified to Order.
Data were pooled into two-degree longitude areas, from 164oW
to 170oE. Sample sizes per species, per two-degree block ranged
from 11 to 3,588. Diet composition (percent by weight) by two-degree area was
calculated for all four species; the major prey items (>approx. 80%) are presented
in detail, the remaining approx. 20% is summarized as other species.
On average, euphausiids make up 35%-50% of the diets of walleye pollock,
Atka mackerel, and POP. However, east of Samalga Pass, the portion of
euphausiids consumed ranged from 50% to 90% while west of the pass it generally
made up 10%-50% of the diets. Besides a step-decrease in the proportion
of euphausiids at Samalga Pass, there was a continuing declining trend
in the diets of POP and Atka mackerel from Samalga Pass to the far Aleutian
Islands. Copepods and myctophids contributed most to the remaining portion
of the diets to the west for POP, Atka mackerel, and pollock. Additionally,
these last two species seem to diversify their diet towards the west as
well, the portion of other species increasing in step fashion. The diet
composition of Pacific cod likewise showed a shift from east to west.
East of Samalga Pass, pollock predominates as the main prey item; whereas
west of the pass, Atka mackerel became an increasingly abundant component.
Squid and other fish prevail in the remainder of cod diet west of Samalga
Pass, in contrast to east of the pass where their joint contribution is
less than 20%.
Other than the shift in diet composition at Samalga Pass, there seemed
to be changes in diet occurring further west. Gradients were not necessarily
unidirectional all the way to the far Aleutians, which suggests the presence
of another biophysical boundary in the west. In the diet of POP, for example,
the proportion of myctophids increased substantially west of Buldir Island.
The diet of walleye pollock and Atka mackerel likewise showed an increased
importance of shrimp, amphipods and polychaetes (not shown; shrimp and
polychaetes aggregated within other species). And finally, shrimp also
increased its importance in the diet of cod sampled west of Buldir. Whereas
there is no oceanographical data to confirm this, these changes in diet
composition throughout the Aleutians can be viewed as biological evidence
of biophysical processes yet undescribed for this region.
By Ivonne Ortiz and Kerim Aydin.
quarterly Oct-Dec 2003 sidebar
Auke Bay Lab