Document Sample
pBuckley02_comparing-zooplankton Powered By Docstoc
					                                                                                           Comparing Zooplankton Consumed by Walleye Pollock                                                                                                                                                                                                                                                                                                                                      BO45H-10
                                                                                           and Caught by Nets in the Eastern Bering Sea
                                                                                           Troy W. Buckley, Todd TenBrink, Jeffrey Napp, and Kerim Aydin
                                                               photo credit: Matt Wilson   Alaska Fisheries Science Center, NMFS, NOAA....................7600 Sand Point Way NE, Seattle, WA

                                                                                                                                                                                                                                                                                                                                                     175°E            180°            175°W        170°W        165°W         160°W         155°W        150°W

                                                                                                                                                                                                                                                                                                                                                                                                                                                                            Figure 1. Locations
Introduction                                                                                                                                                                                                                                                                                                                              60°N

                                                                                                                                                                                                                                                                                                                                                                                                                                                                            in the eastern
                                                                                                                                                                                                                                                                                                                                                                                                                                                                            Bering Sea where
The Alaska Fisheries Science Center’s bottom trawl survey of the eastern Bering Sea continental shelf provides a platform to sample walleye pollock                                                                                                                                                                                                                                                                                                                         pollock stomach
(Theragra chalcogramma) stomach contents and the zooplankton community as collected by Bongo net tows at nearly the same location and time (Figure 1).                                                                                                                                                                                                                                                                                                                      contents and
                                                                                                                                                                                                                                                                                                                                                                                                                                                                            zooplankton were
Relationships between pollock and their zooplankton prey are being investigated to support parameterization of the FEAST model (Forage/Euphausiid                                                                                                                                                                                                                                                                                                                 60°N      sampled
Abundance in Space and Time) for the North Pacific Research Board’s Bering Sea Integrated Ecosystem Research Program (BSIERP).                                                                                                                                                                                                                                                                                                                                              immediately
                                                                                                                                                                                                                                                                                                                                                                                                                                                                            afterward in 2006,
                                                                                                                                                                                                                                                                                                                                                   Year                                                                                                                     2007, and 2008.
                                                                                                                                                                                                                                                                                                                                                             2008                                                                                                 55°N

Results                                                                     2006                                                                              2007                                                                                          2008                                                                          50°N
                                                                                                                                                                                                                                                                                                                                                       175°W                  170°W               165°W               160°W               155°W

                                                                                                                                                                                                                                                                                                              Figure 2. Pie charts                                  Bongo Catch, Zooplankton (%N) | Pollock Diet, Zooplankton (%N)
               small pollock

                                                                                                                                                                                                                                                                                                              showing the composition                                                         Copepods          Copepods
                                                                                                                                                                                                                                                                                                              of the zooplankton in the                                           Amphipods                                Amphipods
                                                                                                                                                                                                                                                                                                              Bongo catch (left side)
                                                                                                                                                                                                                                                                                                              and in the pollock diet                                        Larvaceans                                         Larvaceans

                                                                                                                                                                                                                                                                                                              (right side) by location for
                                                                                                                                                                                                                                                                                                              small (FL ≤ 45 cm) and                                             Mysids                                          Mysids
                                                                                                                                                                                                                                                                                                              large (FL ≥ 46 cm) pollock.
                                                                                                                                                                                                                                                                                                                                                                                Chaetognaths                               Chaetognaths

                                                                                                                                                                                                                                                                                                                                                                                               Euphausiids      Euphausiids

                                                                                                                                                                                                                                                                                                                          • In most cases, the numerical composition of zooplankton in the Bongo net catch
                                                                                                                                                                                                                                                                                                                          and the pollock diet was very different, except along the outer continental shelf, or
                                                                                                                                                                                                                                                                                                                          sometimes for the diet of smaller pollock over the middle shelf.

                                                                                                                                                                                                                                                                                                                          • The Bongo net captured mostly copepods and sometimes relatively large
                                                                                                                                                                                                                                                                                                                          numbers of chaetognaths, especially in 2007. Euphausiids and larvaceans exceeded
                                                                                                                                                                                                                                                                                                                          5% of the numerical composition of the zooplankton at only two stations each.
               large pollock

                                                                                                                                                                                                                                                                                                                          • In the pollock diet, copepods tended to be a greater percentage of the consumed
                                                                                                                                                                                                                                                                                                                          zooplankton along the outer edge of the continental shelf for small and large pollock,
                                                                                                                                                                                                                                                                                                                          and copepods were generally more important in the diet of smaller pollock than
                                                                                                                                                                                                                                                                                                                          larger pollock. Euphausiids were a dominant component at most stations, especially
                                                                                                                                                                                                                                                                                                                          for larger pollock at the middle and inner shelf. But mysids, larvaceans, and
                                                                                                                                                                                                                                                                                                                          amphipods were the numerically dominant group at some stations for small and large

                                                                                                                                                                                                                                  Pollock FL ≥ 46 cm
                                     10000                                                                                                                                                               1000
                                                                                                                                                                       Euphausiids eaten (number/fish)

                                                                                                                     The Bongo net catches large                                                                                                                         However, at stations where euphausiids occur in
                                                                                                                                                                                                                                                                                                                                                                                                 Conclusions & Directions
    Pollock catch (number of fish)

                                                                                                                     numbers of copepods, but the                                                                                                                        the Bongo net catch, the average number of
                                                                                                                     density of copepods at a station                                                                                                                    euphausiids eaten by larger pollock may have a
                                                                                                                     appears to have no relationship to                                                                                                                  weak negative relationship to euphausiid                                                                                    Prey selectivity of the zooplankton
                                                                                                                     the number of pollock locally                                                                                                                       density in the water. In other words, when                                                                                  community is very different between Bongo
                                       100                                                                           available to the Bottom Trawl Survey.                                                                                                               more euphausiids are found in pollock                                                                                       net sampling and pollock feeding. Strong
                                                                                                                                                                                                                                                                         stomachs, there appears to be fewer in the                                                                                  relationships between the pollock
                                                                                                                                                                                                           10                                                            water.
                                                                                                                                                                                                                                                                                                                                                                                                     zooplankton diet and the zooplankton
                                                                                                                                                                                                                                                                         This may suggest that pollock can rapidly graze                                                                             community were not found in the samples
                                                                                                                                                                                                                                                                         down the local abundance of euphausiids.                                                                                    we have available at this time even though
                                        1                                                                                                                                                                   1                                                                                                                                                                                        they were collected in close temporal and
                                             1      10         100                    1000       10000       100000                                                                                         0.01           0.1           1             10         100
                                                                                                                                                                                                                                                                                                                                                                                                     spatial proximity.
                                                 Copepod density (number/cubic meter)                                                                                                                                Euphausiid density (number/cubic meter)
      Figure 3. The number of pollock caught at a station plotted                                                                                                        Figure 5. The average number of euphausiids eaten per fish by larger pollock plotted
                                                                                                                                                                                                                                                                                                                                                                                                     As more sampled stations become available,
      against the copepod density found at that station.                                                                                                                 against the euphausiid density found at that station.
                                                                                                                                                                                                                                                                                                                                                                                                     we will be able to refine our analyses to
                                                                                                                                                                                                                                                                                                                                                                                                     tighter subsets of stations (geographically),
                                                                                                                                                                                                                                                                                                                                                                                                     of pollock (narrower size ranges), and of
                                                                                                                                                                                                                                 Pollock FL ≤ 45 cm                                                                                                                                                  consumed zooplankton (digestion state).
                                     10000                                                                                                                                                               10000
                                                                                                                     Although our Bongo net sampling
                                                                                                                                                                          Copepods eaten (number/fish)

                                                                                                                                                                                                                                                                         For smaller pollock, when copepods are
 Pollock catch (number of fish)

                                                                                                                     catches relatively few euphausiids,                                                                                                                                                                                                                                             In addition, we will compare catches from
                                                                                                                                                                                                                                                                         eaten at the station, there may be a weak
                                     1000                                                                            when euphausiids are caught, it                                                                                                                     positive relationship between number of                                                                                     nets that target euphausiids and larger
                                                                                                                     appears there may be a positive
                                                                                                                                                                                                                                                                         copepods eaten per fish and local copepod                                                                                   zooplankton with pollock diets where we
                                                                                                                     relationship between euphausiid                                                      100
                                                                                                                                                                                                                                                                         density in the water.
                                      100                                                                            density and the number of pollock                                                                                                                                                                                                                                               are able to identify a close temporal and
                                                                                                                     locally available to the Bottom                                                       10
                                                                                                                                                                                                                                                                                                                                                                                                     spatial match.
                                                                                                                     Trawl Survey.

                                        1                                                                                                                                                                                                                                                                                                                                                 Acknowledgments
                                                                                                                                                                                                           0.1                                                                                                                                                                            We appreciate the dedication of AFSC’s eastern Bering Sea survey
                                        0.01             0.1               1                    10             100                                                                                               1    10           100       1000       10000   100000
                                                                                                                                                                                                                                                                                                                                                                                          personnel and the captain and crew of the F/V Arcturus for making the
                                                 Euphausiid density (number/cubic meter)                                                                                                                             Copepod density (number/cubic meter)                                                                                                                                 collection of these samples possible. Financial support for this project
          Figure 4. The number of pollock caught at a station plotted against the                                                                                            Figure 6. The average number of copepods eaten per fish by smaller pollock plotted                                                                                                                           is provided by the North Pacific Research Board and the Alaska
          euphausiid density found at that station.                                                                                                                          against the copepod density found at that station.                                                                                                                                                           Fisheries Science Center.
                                                                                                                                                                                                                                                                                 The recommendations and general content presented in this poster do not necessarily represent the views or official position of the Department of Commerce, the National Oceanic and Atmospheric Administration, or the National Marine Fisheries Service.

Shared By: