Subject: Bering Sea Report (fwd)

Mike Williamson (pita@www1.wheelock.edu)
Tue, 20 Apr 1999 19:34:48 -0400 (EDT)

---------- Forwarded message ----------
Date: Tue, 20 Apr 1999 08:19:45 -0700
From: Pamela Rosenbaum <rosenbau@zoology.ubc.ca>
Reply-To: Marine Mammals Research and Conservation Discussion
     <MARMAM@UVVM.UVIC.CA>
To: MARMAM@UVVM.UVIC.CA
Subject: Bering Sea Report

A report titled "Ecosystem change and the decline of marine mammals in the
Eastern Bering Sea: testing the ecosystem shift and commercial whaling
hypotheses" was recently completed.  It is authored by A.W Trites, P.A.
Livingston, M.C. Vasconcellos, S. Mackinson, A.M. Springer, and D. Pauly.
 A pre-publication copy of the final report can be obtained from
http://www.marinemammal.org/pdfs/bering.pdf.


REPORT SUMMARY

Some species in the Bering Sea underwent large changes between the 1950s
and the 1980s.  Among the best documented are the declines of Steller sea
lions and northern fur seals, and the possible increase and dominance of
ground fish - pollock and large flatfish.  A frequently proposed
explanation is that human exploitation of top predators and/or a shift in
the physical oceanography altered the structure of the eastern Bering Sea
ecosystem.
We employed two inter-related software packages (Ecopath and Ecosim) to
describe quantitatively the eastern Bering Sea ecosystem during the 1950s,
before large-scale commercial fisheries were underway, and during the
1980s, after many marine mammal populations had declined.  We grouped the
hundreds of species that make up the Bering Sea ecosystem into 25
functional groups.
Our mass-balance ecosystem models showed that most of the top predators
(trophic level IV) declined from the 1950s to the 1980s.  They included
Steller sea lions, seals, sperm whales, deep-water fish and other demersal
fishes.  The only top predators to increase were large flatfish such as
arrowtooth flounder.   At the mid-trophic level (III), baleen whales and
pelagic fishes declined, while small flatfish, pollock, and walrus and
bearded seals increased.  Based on our model assumptions, pollock
contributed over 50% of the total flow of energy at the mid trophic levels
during the 1980s compared to only 10% in the 1950s model.  In contrast,
pelagic fishes contributed nearly 50% of the flow in the 1950s.  At trophic
level IV, no one species dominated the flow of energy during the 1950s.
 However, large flatfish contributed over 60% of the total energy flow in
the 1980s model. Large flatfish and adult pollock that dominate the Bering
Sea in the 1980s appear to be significant competitors of seals. Large
flatfish are also competitors of Steller sea lions and there are large
overlaps in the diets of pollock and baleen whales.
Changes in the biomass of marine mammals appear to have little effect on
the biomass of other groups in the Bering Sea.  Reductions in prey
abundance can quickly reduce marine mammal populations, but marine mammals
are unable to quickly recover when abundant food becomes available.   Our
models suggest that Steller sea lion populations would be larger if adult
pollock and large flatfish were lower in abundance due to competitive
release of important prey.
Most impacts on the modeled ecosystem can be associated with changing the
biomass of lower trophic levels. Total catch in the eastern Bering Sea rose
from 0.33 to 2.62 t.km-2 between the 1950s and the 1980s. Exploitation
during the 1950s used 47% of the net primary production, with most of it
flowing through the harvested whales.  Shifting the emphasis from
exploiting marine mammals in the 1950s to catching fish in the 1980s
lowered the amount of primary production required to sustain harvests to
6%.
Some ecosystem indices derived from our ecosystem models indicate that the
eastern Bering Sea was more mature in the 1950s than in the 1980s. However,
we are less certain about the actual state of the Bering Sea in the 1950s
due to the relative paucity of data from that time.  The ecosystem indices
for both the 1950s and 1980s models suggest that the Bering Sea is
relatively resilient and resistant to perturbations. Removing whales from
the 1950s ecosystem had a positive effect on pollock by reducing
competition for food.  However, whaling alone is insufficient to explain
the 400% increase in pollock biomass that may have occurred between the
1950s and the 1980s.  Nor can commercial fisheries account for these
observed changes.  The magnitude of changes that occurred in the biomass of
all the major groups in the eastern Bering Sea cannot be explained solely
through trophic interactions.  We suggest that other factors comprising a
regime shift, such as changes in water temperature or ocean currents may
have been at play.


Pamela Rosenbaum <rosenbau@zoology.ubc.ca>







Pamela Rosenbaum
Marine Mammal Research Unit
University of British Columbia
Room 18, Hut B-3, 6248 Biological Sciences Road
Vancouver, B.C.  Canada  V6T 1Z4

Phone:  (604) 822-8181, Fax:  (604) 822-8180
http://www.marinemammal.org