Jocolyn Kearney
11/12/13
4:00 PM Lab
Research Paper
“Using Grizzly Bears to Assess Harvest – Ecosystem Tradeoffs in
Salmon Fisheries”
INTRODUCTION:
Salmon, specifically sockeye salmon, is a hot commodity not only to us humans
for consumption, but also to many different wildlife species, such as “orcas, salmon
sharks, pinnipeds, and grizzly bears”. “Salmon carcasses, distributed primarily by bears
during spawning events, contribute annual pulses of marine-derived nutrients to
freshwater systems that propagate through food webs and influence primary producers,
invertebrates, fish and wildlife”. However, it seems that stocking fisheries with a lot of
salmon are having great effects on the ecosystem in multiple ways. In order to address
these problems the ecosystem-based fisheries management (EBFM) is trying to come up
with ways that the tradeoffs won’t harm the fishery yields or the ecosystem too much.
However, “designing EBFM approaches requires an assessment of the tradeoffs inherent
to balancing ecosystem protection and economic costs”, so they haven’t exactly found a
solution that works the best for everything quite yet.
The study decided to use grizzly bears “as a surrogate of salmon-influenced
ecosystem function because 1) bear population dynamics are strongly linked to salmon
abundance; 2) bears are the terminal predator, consuming salmon in their final life history
phase…3) bears are the dominant species mediating the flow of salmon-derived nutrients
from the ocean to the terrestrial ecosystem”. “The fundamental challenge with
implementing EBFM in this bear-salmon-human system (and others) is to determine how
much of the fished resource to allocate to fisheries verses the ecosystem”.
MATERIALS, METHODS PROCEDURES:
A “multi-stage analysis to predict how bear population density would respond to
variation in spawning salmon abundance as influenced by harvest management” was
used. They studied the diets of 18 grizzly bear populations from 1995-2003 and used
their hair to determine the amount of salmon in their diets. They used an equation that
determined the percent of salmon in the diet as a function of salmon biomass density,
which also included how quickly the bear’s diets respond to the availability of salmon.
To measure fishery yields, they took the “recruitment minus escapement” to
determine the expected salmon harvest. However, many fisheries have a lot of
uncertainties and variability. “Fisheries with adequate stock-recruitment data can be
managed by targeting a biologically based escapement…other fisheries are managed
between lower and upper target escapements that have provided adequate yield in the
past” even though this is not the optimal way to manage them.
Another important thing they needed to calculate is “the bear density at a
particular escapement relative to the bear density at the stock-specific maximum
escapement (i.e. no fishery”. It’s important to know what the benefits are of having a
fishery and being able to compare the results to each other.
RESULTS:
“In all systems, bear diets would respond considerably to increases in salmon
abundance (i.e. escapement)”. However, “the presence and degree of conflict between
fisheries yields and bear densities is stock-specific”. In the Chilko and Quesnel stocks,
they found that “recruitment declines as the number of spawners increases”. However,
comparing the upper goals of Ugashnik, Egegik, Nushagak, and Rivers Inlet “expected
increases in yield are proportionately much greater than increases in bear densities”. In
conclusion, “increasing escapement beyond EMSY (Maximum sustainable yield) leads to
conflict between fishery yields and bear density, with the former decreasing and the latter
increasing”.
DISCUSSION:
“Our goal hear was to assess quantitatively the expected impact to fisheries and
grizzly bears--a surrogate for salmon ecosystem function--if status quo management was
adjusted to increase escapement across a range of contexts”.
In looking at four of the systems they studied they found a win-win situation
where both the bear density and the fishery yield both “benefit from increasing
escapement”; however, year-to-year the recruitment varies and when it gets too high the
fishery could close if the escapement goals aren’t met. One solution to this is to set lower
goals so that they are more likely able to meet the goals of the escapement numbers.
One thing that was noted when looking at all six systems with the MSC
certification is that it is clear that the “bear densities can increase substantially with
increased escapement from current management levels”, meaning that the fisheries are
competing with not only the bears but also with numerous “other ecosystem recipients”.
CONCLUSION:
In conclusion, increasing the number of spawning salmon greatly increases the
density of salmon in bear diets. It is also “associated with positive ecological responses
across a broad array of taxa, including aquatic primary productivity, terrestrial vegetation
growth, invertebrate density, songbird density and growth rates of resident fish”.
However, the numbers and cost effectiveness varies with each stock. The best thing to do
is a stock-by-stock comparison and set up goals so that the needs of the specific stocks
are met. Those with high numbers of other salmon species don’t need to have as much
sock-eye salmon fed into the fisheries, however, those who rely heavily on the sock-eye
may need more escapement into their bodies of water. On-the-other-hand, the
management needs to be aware of how much salmon is getting distributed, because if it
exceeds their EMSY it may cause fisheries to close, which would be detrimental to the
economy.