The effects of fishing on deep-water fish species to the west of Britain.

Basson, Gordon, Large, Lorance, Pope and Rackham

JNCC Report 324 (2002)

1991-2001

Funded by JNCC, Cefas, Ifremer, SAMS

JNCC Contract Hebridean Slope

Ecological approach - pre and post-exploitation

Assessment approach - single species assessments Vb, VI, VII.

Ecological approach

Database of trawl survey data from 1973 -1999

FRS, SAMS, IFREMER, CEFAS and German RV surveys (35)

Long-term trends in standardised CPUE - GLMs and GAMs -unbalanced data - analyses only possible for ‘exploited’ and ‘unexploited’ species and roundnose grenadier.

Presence/absence analyses

Size spectra studies

Diversity studies.

Exploited populations

0.00

0.50

1.00

1.50

2.00

2.50

3.00

1970 1975 1980 1985 1990 1995 2000 2005

rela

tive

cpue glm(2)

glm(3)

gam

Unexploited populations

0.00

0.50

1.00

1.50

2.00

2.50

3.00

1970 1975 1980 1985 1990 1995 2000 2005

rela

tive

cpue

glm

gam

Greater change in relative biomass of ‘exploited species’ than the ‘unexploited species’ group.

Large amount of unexplained variation.

Gear effects?

Roundnose grenadier

0

0.5

1

1.5

2

2.5

1970 1975 1980 1985 1990 1995 2000 2005

rela

tive c

pue

GAM

GLM

SAMS surveys - presence/absence analyses

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

1970 1975 1980 1985 1990 1995 2000

Pro

po

rtio

n o

f h

au

ls

C. coelolepis

C. squamosus

Concern regarding gear and depth factors

Results more reliable for Portuguese dogfish

Comparison of size-spectra for the southern North Sea.

Comparison of size spectra for the Hebridean slope, pre- and post-exploitation, as estimated from a CEFAS Granton trawl survey in the early 1970s and a single French arrow trawl survey in 1999, respectively.

Slope in 1970s similar to N.Sea in 1904

Steepening of slope – may be confounded by gear and depth effects

Slopes of size-spectra, calculated from all available trawl surveys since 1973, plotted against time.

May again be confounded by gear and depth effects

ANCOVA results inconclusive

375-624m

y = 0.065x - 126.43R2 = 0.27

0

1

2

3

4

5

1970 1980 1990 2000

Nega

tive

regr

essi

on s

lope

Thalassa- Arrow traw lChallenger - OTSB (P)Challenger - OTSB (S)

Challenger - Granton (L)Challanger - Granton (S)Walter Herw ig (BT 200)Cirolana - Granton (78')

1375-1624m

y = 0.0247x - 46.142R2 = 0.05

0

1

2

3

4

5

1970 1980 1990 2000

Neg

ativ

e re

gres

sion

slo

pe Thalassa- Arrow traw l

Challenger - OTSB (P)

Challenger - OTSB (S)

Challenger - Granton (L)

Challanger - Granton (S)

Walter Herw ig (BT 200)

Cirolana - Granton (78')

625-874m

y = 0.017x - 31.57R2 = 0.04

0

1

2

3

4

5

1970 1980 1990 2000

Negative

regre

ssio

n s

lope

Thalassa- Arrow traw l

Challenger - OTSB (P)

Challenger - OTSB (S)

Challenger - Granton (L)

Challanger - Granton (S)

Walter Herw ig (BT 200)

Cirolana - Granton (78')

1125-1374m

y = 0.0736x - 143.44R2 = 0.50

0

1

2

3

4

5

1970 1980 1990 2000

Ne

ga

tive

re

gre

ss

ion

slo

pe Thalassa- Arrow traw l

Challenger - OTSB (P)

Challenger - OTSB (S)

Challenger - Granton (L)

Challanger - Granton (S)

Walter Herw ig (BT 200)

Cirolana - Granton (78')

Slopes of size-spectra, calculated from all available trawl surveys since 1973, plotted against time by depthband

1625-1874m

y = 0.0747x - 144.51R2 = 0.40

0

1

2

3

4

5

1970 1980 1990 2000

Negative

regre

ssio

n s

lope

Thalassa- Arrow traw l

Challenger - OTSB (P)

Challenger - OTSB (S)

Challenger - Granton (L)

Challanger - Granton (S)

Walter Herw ig (BT 200)

Cirolana - Granton (78')

875-1124m

y = 0.0223x - 41.602R2 = 0.06

0

1

2

3

4

5

1970 1980 1990 2000

Negative

regre

ssio

n s

lope

Thalassa- Arrow traw l

Challenger - OTSB (P)

Challenger - OTSB (S)

Challenger - Granton (L)

Challanger - Granton (S)

Walter Herw ig (BT 200)

Cirolana - Granton (78')

Challenger 3 1985 875-1124m

0%

20%

40%

60%

80%

100%

1 3 5 7 9 11 13 15

Weight-class (log2 w eight (g))

Perc

ent of

tota

l catc

h w

eig

ht

in e

ach w

eig

ht cla

ss p

er

haul

All others

Rabbitf ishes

Synaphobranchidae

Alepocephalids

Sharks

Moridae & Gadids

Macrouridae

Challenger 143 1999875-1124m

0%

20%

40%

60%

80%

100%

1 3 5 7 9 11 13 15

Weight-class (log2 w eight (g))

Perc

ent of

tota

l catc

h w

eig

ht

in e

ach w

eig

ht cla

ss p

er

haul

All others

Rabbitf ishes

Synaphobranchidae

Alepocephalids

Sharks

Moridae & Gadids

Macrouridae

Same gear - fewer larger fish in 1999.

In 1999, fewer sharks, more rabbitfish.

Only 15 (1985) and 5 (1999) hauls – sampling error

0

1

2

3

4

500 750 1000 1250 1500 1750 2000

GTL 1976-79 (pre)GTS 1975-76 (pre)OTSB(S) 1983-87 (pre)OTSB(P) 1984-87 (pre)ARO 1996-99 (expl.)OTSB(S) 1999 (expl.)

Depth (m)

0

1

2

3

4

500 750 1000 1250 1500 1750 2000

Depth (m)

Shannon Index

Delta index

Results inconclusive

0

1

2

3

4

500 750 1000 1250 1500 1750 2000

Depth (m)

0

1

2

3

500 750 1000 1250 1500 1750 2000

Depth (m)

1983-87 (pre)

OTS(85) (pre)

1999 (post)

Shannon index

Delta index

Results inconclusive – sampling error?

Single species exploratory assessments DeLury depletion constant recruitment model - requires estimate of M.

M estimated using Annala and Sullivan (1996):- M = Ln(100) maximum age

Non-equilibrium Schaefer production model - time-lag 0 used because time-series catch and CPUE too short to explore the effect of time-lag over a range of years commensurate to age of recruitment. Assumed that growth rather than recruitment was main contributor to production.

CEDA - sensitivity analysis - effect on results (goodness of fit, residual plots, model parameter and population estimates) of range of error models. Confidence limits about model parameters and population estimates calculated by bootstrapping.

The general procedure

adopted was to use sensitivity analysis to evaluate the effect on results (goodness of

fit, residual plots, model parameter and population estimates) of a range of error

models and assumptions for stock size in the first year as a proportion of virgin

biomass. Confidence limits about model parameters and population estimates were

calculated by bootstrapping

Estimates of Bo from German trawl surveys 1974-80 – roundnose grenadier, blue ling & orange roughy

Beddington and Cooke procedure – estimates of MSY assuming some knowledge of Bo from above methods.

B0 estimates using the swept area method - German trawl surveys 1974-80 (Dupouy and Lorance, 2001) for roundnose grenadier, blue ling and orange roughy in ICES Sub-area VI and Divisions VIIb,c over a depth range of 400 to 1400m.

Information on q not available - assumed to be 1 and therefore values of B0 are minimum estimates.

Estimates of MSY using Beddington and Cooke method (1983), where the ratio MSY/B0 is calculated taking into M, the growth rate (k) and size (or age) at recruitment to the fishery.

Based on equilibrium calculations assuming constant recruitment.

Dependent on availability of estimates of B0 .

Roundnose grenadier in VI,VII and Vb Schaefer model - data for 1990-98

Discards assumed to be 30% of TIL

Fit was similar for a range of error models (R2=0.86).

log error model gave most realistic estimates of r ( 0.1 compared to 0.5 etc).

Estimated parameter values were reasonably robust for range of ratios of initial stock biomass to population carrying capacity

Swept area method using German trawl survey data from the 1970s gave a B0 of circa167,000t (cf Schaefer around 130kt (CIs : 54,000-230,000t).

Beddington and Cooke, assuming B0 = 167,000t, gave MSY = 6,800t cf Schaefer circa 4,000t (CIs : 10-230,000t).

Conclusions:- results should be treated with caution due to the wide confidence limits about estimates.

Results from Schaefer analysis indicate that exploitable biomass at the end of 1998 was below 50% of B0 and may have been close to 20% of B0

The same conclusion can be drawn by considering the CPUE indices directly.