Multispecies Production Models:Past and Present, Examples at the NEFSC

ByW.J. Overholtz, J.S. Link, H.

Moustahfid, R.J. Gamble, M.C. Tyrrell, B.E. Smith

NEMOW-August 2007

2-Tier Surplus Production Analysis

• Brown et al 1977: analysis of the ICNAF bottom trawl fishery in SA 5+6 during 1961-1972.

• Mayo et al 1992: re-analysis with additional years from 1973-1987.

• Aggregate catch and effort data to estimate surplus production and BRPs.

ICNAF Effort and Catch 1961-1972

0

500

1000

1500

1961 1964 1967 1970

Year

Effo

rt (0

00s

days

) C

atch

(000

s m

t)

effort catch61-72

Brown et al. 1976

0

500

1000

1500

0 100 200 300 400

Standardized Days Fished (000s)

Cat

ch (0

00s

mt)

catch61-72 Avmsy

Brown et al. 1976 Catch and Effort 1961-1972

Aggregate MSY 900,000 mt

Summed Quota 1,300,000 mt

Effort and Catch 1960-1987

01000200030004000

1960 1970 1980 1990

Year

Effo

rt (0

00s

Hrs

) C

atch

(000

s m

t)

effort catch

Fogarty et al. 1992

0200400600800

1000

0 1000 2000 3000 4000

Standardized Hrs Fished (000s)

Cat

ch (0

00s

mt)

catch avMSY

Mayo et al. 1992- Catch and Effort 1960-1987

1960-1976

1977-1987

Aggregate MSY 700,000 mt

Conclusions• “…summing the MSYs from individual

assessments may be an overestimate of the total MSY.”

• The 2nd Tier Quota was 30-45% lower than the sum of the individual quotas (1.3 e6 mt).

• Similar results from LP analyses: weak stock management necessary!

Gulf of Maine-Georges Bank HerringSurplus Production

Accounting for Predation

0.00.20.40.60.81.01.21.4

1959 1964 1969 1974 1979 1984 1989 1994 1999Year

Mor

talit

y R

ate

M2 F

Atlantic Herring M2 and F during 1959-2002

PDkB

kBeMSYSP pfpf ln

Modified Fox model

predator SP-predator SP-fishery600 144 395800 192 347

1000 240 2991200 287 2511400 335 2041600 383 1561800 431 1082000 479 60

If predators increase, less fishery SP would be available

At Bmsy=1452 kt

0100200300400500600

500 1000 1500 2000

Predator Biomass (kt)

Surp

lus

Prod

uctio

n (k

t)

SP-pred SP-fishery

Predator vs Fishery Tradeoff

2007 1200

2026 1400

At Bmsy=1452 kt

Conclusions• Predation mortality (M2) should be

included in stock assessments of prey fish.

• SS assessments of prey fish are generally optimistic relative to BRPs

• If the fishery and predators utilize the same size spectrum of prey, then tradeoffs are probably warranted.

Guild-Based Biomass Guild-Based Biomass Production ModelProduction Model

• We examined a suite of ecological and harvest scenarios.

• The ecological scenarios: modify competition and predation parameters (nsh).

• The harvest scenarios: modifying harvest rates.

• The goal was to investigate general properties and responses in a Guild System

p

iipipG

G

GiG

G

g

gig

G

iii

i NHNKK

N

K

N

KNNr

dtdN

1

11 ))(

1(

Modified Schaefer Model

N =Biomass B = Competition Coefficient I = species

R = Growth Rate a = Predation Coefficient p = predator (on species i)

H = Harvest Rate K = Carrying Capacity g = Guild Member

G = Guild

DEMERSALS OVERFISHEDDEMERSALS OVERFISHED

1. total system biomass increased. 2. Piscivores and Shrimp-Fish declined.3. Planktivores increased and k increased for this guild.4. Benthivores increased due to a higher growth rate, and a reduction in competition and predation.5. Reduction in predation caused an overall biomass increase.

PISCIVORES OVERFISHEDPISCIVORES OVERFISHED

1. Total biomass of the system increased, 2. Rankings within the Planktivore guild changed. This was because more predation occurred on PL1 and PL2 than PL4. 3. When predation effects were lowered, PL1 and PL2 were able to out compete PL4.

Results for this scenario show a strong similarity to the historical situation in the Northeast U,S, Atlantic Ecosystem . Starting in the early 1990’s, the Piscivore biomass began to decline, resulting in an increase in Planktivore biomass. Additionally, increasing trends in Benthivore and Shrimp-amphipod biomasses occur in both the real world situation and this scenario.

0

20

40

60

80

100

120

140

160

180

200

1960 1965 1970 1975 1980 1985 1990 1995 2000 2005 2010

Estim

ate

of S

urve

y Bi

omas

s (k

g/to

w)

AMPSHR

BENTHI

PISC

PLANKT

SHRFIS

Total all guilds

1. Total biomass of the system decreased.

2. Most of this decrease can be attributed to the much lower biomass of Planktivores.

PELAGICS OVERFISHEDPELAGICS OVERFISHED

CONCLUSIONSCONCLUSIONS1. Predation appears to be the dominant biotic term within the model. 2. Overall biomass of a guild tends to be stable. even though individual species biomasses may change greatly. 3. Changes in one biotic factor may cause unexpected results due to predatory and competitive interactions.4. A demonstration copy can be obtained by contacting Robert Gamble (Robert.Gamble@noaa.gov).

General Conclusions• Rich History of Past and Current Use.• Sum and Parts are Important• Addresses the Issue of Biological and

Technological Interactions• Could this Approach to Simplify

Management.