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Population-based indicators to assess the status of the Gulf of Alaska and Bering Sea ecosystems
Franz J. Mueter1 and Bernard A. Megrey2
1 Joint Institute for the Study of the Atmosphere and the Oceans,P.O. Box 354235, University of Washington, Seattle, WA 98115, USA. E-mail: fmueter@alaska.net
2 National Marine Fisheries Service, Alaska Fisheries Science Center, 7600 Sand Point Way NE, Seattle, WA, USA 98115
Ecosystem-based approach to fisheries management
Goal: “Maintain ecosystem health and sustainability”Objectives
maintain characteristic structure and function
maintain productivity and diversity
achieve sustainable exploitation of living resources
Ecosystem-based approach to fisheries managementProcess to achieve objectives
Assess ecosystem status (& health?)Physical environment (climate, habitat)Biological components (populations, assemblages, communities)Human impacts (fishing)
Define relevant indicators and reference points or reference states
Manage human actions to maintain system “near” target reference points or within desired reference states
Assessment of demersal community
2002 NMFS report to congress:186 of 219 listed fish stocks off Alaska have “unknown status”
Full stock assessment of commercial + other “important” populations
Comprehensive assessment should include target and non-target species
Assessment of demersal community
Approaches
Extend full stock assessment to all populations
Monitor indicator species (abundance, presence/absence)
Monitor community characteristics (food-web metrics, diversity, multi-variate indicators, etc.)
Species-based indicators to monitor trends in maximum number of populations
Species-based indicators
Potential indicatorsPopulation growth rate (trend in abundance)Trends in presence / absenceTrends in average size (length / weight)Trends in total mortality / exploitation rateTrends in size at maturity
Evaluate average trends and distribution of trends across many populationsFirst step from single-species to community assessment
(based on Rochet & Trenkel 2003. CJFAS 60: 86-99)
Simple species-based indicators based on bottom trawl surveys
Population growth rate = Trends in Catch-per-unit-effort (CPUE in kg/km2) over time
CPUE as index of abundanceEstimates of average CPUE across survey area by taxonand year
Trends in frequency of occurrence (FO) over timeMore robust index of abundanceEstimates of proportion of hauls that caught a given taxon across years (spatial distribution)
Trends in average weight over timeIndex of average weight of individuals across survey area by taxon and yearReflecting changes in size-at-age and/or average age
Bottom Trawl surveys
Gulf of AlaskaStandardized surveys, triennially 1984-1999, 2001 (2003)Shelf & slope stations < 500 mSpecies groupings
65 fish taxa29 invertebrate taxa
Trend analysis 1993-99 (east), 1993-2001 (west)
Eastern Bering SeaStandardized survey, annually 1982-2003Shelf stations, 10-180 mSpecies groupings
47 fish taxa32 invertebrate taxa
Trend analysis 1993-2003
ALASKA
Gulf of Alaska717 stations sampled
in 1999 survey
Eastern Bering Sea306 stations sampled
1983-2003
170 W 160 W 150 W 140 W
Eastern GoA
Western
GoA
Anchorage
Junea
Kodiak
50 N
55 N
60 N
Population growth rate: Regression of CPUE vs. year
Simple linear regression
Regression with auto-correlated errors:
CPUEi = α + β yeari + εi
where εi = φ εi+1 + νI
and νi ~ N(0, σ2)
ExampleChionoecetes sp. (tanner & snow crab) in the Bering Sea
1990 1992 1994 1996 1998 2000 2002
-1.0
0.0
0.5
1.0
1.5
Stan
dard
ized
CPU
E
Year
Rajidae
1994 1998 2002
Atheresthes.evermanni Atheresthes.stomias
1994 1998 2002
Reinhardtius.hippoglossoidesHippoglossus.stenolepis
1994 1998 2002
Hippoglossoides.elassodonHippoglossoides.robustus
-2-1012
1994 1998 2002
Glyptocephalus.zachirus
-2-1012
Limanda.aspera Limanda.proboscidea Limanda.sakhalinensis Platichthys.stellatus Lepidopsetta.sp Isopsetta.isolepis Pleuronectes.quadrituberculatusLeptagonus.frenatus
Podothecus.acipenserinus Aspidophoroides.bartoni Occella.dodecaedron Ammodytes.hexapterus Anarhichas.orientalis Anoplopoma.fimbria Bathymasteridae
-2-1012
Clupea.pallasi
-2-1012
Gymnocanthus.sp Hemilepidotus.jordani Hemilepidotus.papilio Triglops.sp Myoxocephalus.sp Dasycottus.setiger Hemitripterus.bolini Icelus.sp
Trichodon.trichodon Gadus.macrocephalus Boreogadus.saida Eleginus.gracilis Theragra.chalcogramma Hexagrammos.sp Liparidinae
-2-1012
Thaleichthys.pacificus
-2-1012
Mallotus.villosus Stichaeidae Lycodes.raridens Lycodes.palearis Lycodes.brevipes Zoarcidae Sebastes.sp Scyphozoa
Anthozoa Pennatulacea Actiniaria Polychaeta barnacle Pandalidae Crangonidae
-2-1012
other.shrimp
-2-1012
Cancer.sp Oregonia.gracilis Chionoecetes.sp Hyas.sp Telmessus.cheiragonus Paguridae Paralithodes.camtschaticus Paralithodes.platypus
Erimacrus.isenbeckii nudibranch snails Pectinid Bivalvia Octopoda squid
-2-1012
Asteroidea
-2-1012
1994 1998 2002
Echinoida Gorgonocephalus.eucnemis
1994 1998 2002
Ophiuroidea Holothuroidea
1994 1998 2002
Porifera Bryozoa
1994 1998 2002
tunicate
Year
CP
UE
CPUE by year for 79 EBS taxa1993-2003, with linear trends
Stan
dard
ized
CPU
E
Year
Distribution of slopesBering Sea CPUE, 1993-2003 (79 taxa)
-0.4 -0.2 0.0 0.2 0.4
0.0
0.5
1.0
1.5
2.0
2.5
3.0
Slope of standardized CPUE vs. year, 1993-2003
Pro
babi
lity
53 taxa decreasing 26 taxa increasing
Simulated distribution of trends
Simulate time series of CPUE with characteristics of observed series
Same autocorrelation
Same correlation structure (var-cov matrix)
Estimate trends (slopes of CPUE vs. year)Plot distribution of simulated slopesCompare to observed slopes
Graphical comparison
Statistical comparison (Simulation envelope)
Actual and simulated slopesBering Sea CPUE, 1993-2003 (79 taxa)
Observedslopes
Simulated slopes(average)
-0.4 -0.2 0.0 0.2 0.4
01
23
45
6
Slope of standardized CPUE on year, 1993-2003
Prob
abilit
y
Actual and simulated slopesBering Sea CPUE, 1993-2003 (79 taxa)
Observedslopes
Simulated slopes(average)
-0.4 -0.2 0.0 0.2 0.4
01
23
45
6
Slope of standardized CPUE on year, 1993-2003
Prob
abilit
y
Simulationenvelope
Actual and simulated slopesBering Sea FO, 1993-2003 (79 taxa)
Observedslopes
01
23
45
Pro
babi
lity
Simulationenvelope
Simulated slopes(average)
-0.4 -0.2 0.0 0.2 0.4Slope of frequency of occurrence on year, 1993-2003
Slope of CPUE vs. year, 1993-2003
Slo
pe o
f FO
vs.
yea
r, 19
93-2
003
-0.4 -0.2 0.0 0.2 0.4
-0.3
-0.1
0.0
0.1
0.2
Raj
Aev
Ast
RhiHst
Hel
Hro
Gza
Las
LprLsa
Pst
Lsp
IisPqu
Lfr
Pac
Aba
Odo
AheAor
Afi
Bat
Cpa
Gsp
Hjo
Hpa
Tsp
Msp
Dse Hbo
IspTtr
Gma
Bsa
Egr
TchHex
Lip
Tpa
Mvi
Sti
Lra
LpaLbr
Zoa
Ssp
Scy
AntPen
Act Pol
bar
PanCra
Shr
Csp Ogr
ChiHya
Tel
PagPca
PplEis
Nud
SnaPecBiv Octsqu
Ate
Ech
GeuOph
Hol
Por
Bry
Tun
Density0.5 1.0 1.5 2.0 2.5
Den
sity
0.5
1.0
1.5
2.0
2.5
0.3
Slope of CPUE vs. year, 1993-2003
Slo
pe o
f FO
vs.
yea
r, 19
93-2
003
Density
Den
sity
-0.4 -0.2 0.0 0.2 0.4
-0.3
-0.1
0.0
0.1
0.2
0.3
Raj
Hro
Lfr
Pac
Aba
Odo
AheAor
Bat
Gsp
Hjo
Hpa
Tsp
Msp
Dse Hbo
IspTtr
Bsa
Egr
Hex
Lip
Tpa
Mvi
Sti
Lra
LpaLbr
Zoa
Scy
AntPen
Act Pol
bar
PanCra
Shr
Csp Ogr
Hya
Tel
Pag
Eis
Nud
SnaPecBiv Octsqu
Ate
EchGeu
OphHol
Por
Bry
Tun
Aev
Ast
RhiHst
Hel
Gza
Las
LprLsa
Pst
Lsp
IisPqu
Afi
CpaGmaTch
Ssp
Chi
Pca
Ppl
non-target species
Target species
0.5 1.0 1.5 2.0 2.5
0.5
1.0
1.5
2.0
2.5
56
58
60
62
-175 -170 -165 -160
1993 1994
-175 -170 -165 -160
1995 1996
1997 1998 1999
56
58
60
62
2000
56
58
60
62
2001-175 -170 -165 -160
2002 2003
Longitude
Latit
ude
Polychaeta
56
58
60
62
-175 -170 -165 -160
1993 1994
-175 -170 -165 -160
1995 1996
1997 1998 1999
56
58
60
62
2000
56
58
60
62
2001-175 -170 -165 -160
2002 2003
Longitude
Latit
ude
Rajidae
Observed trends (target / non-target)
Bering Sea Gulf of Alaska
Population growth (CPUE)
- / - n.s.Spatial distribution (FO)
(+) / + n.s. / +Average size (weight)
(+) / n.s. n.s.
Summary & conclusions (1)
Survey-based population characteristics provide simple indicators for assessing trends in a communityStatistical tests can be constructed to identify deviations from expected distribution of trends across speciesSimulation envelopes provide reference state for “balanced” community“Global” test for directional, unexpected changes in a community
Summary & conclusions (2)
Groundfish community in the Bering Sea has undergone significant directional change, 1993-2003
Decrease in CPUE of many species
Spatial “expansion” of many non-target species
Reasons for observed changes currently unknown
Warming trend
Indirect effects of fishing
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