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