•
.
..
Not to bc quotCd without prior reference to the authors
International Council for the
Exploration of the Sea
CM 19921G:67
Demersal Fish Committee
RefH
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<,
MULTISPECIES FORECASTS FOR NORTH SEA FISH STOCKS,
A PRESENTATION OF FURTHER EXPLOITATION SCENARIOS
by
S T Forbes and P A Kurizlik
SOAFD Marine Laboratory
PO Box 101, Victoria Road
Abcrdeen AB9 8DB
Scotland, UK
ABSTRACT
The Multispecies Assessment Working Group has reported . thc catch projectioris for
various scenarios regarding the exploitation pattern and fishirig iritensity of specific fleet
groupings. A further range of scenarios are presentedhere and discussed in relation to
previous analyses.
.
INTRODUCTION
To date, the determination of~ "key-run" ofthe North Sea multispecies virtual population
arialysis (MSVPA, see Gislason and Sparre, 1987) has been an important aspect of each
meeting ofthe Multispecies Assessment Working Group (MSWG). Onee established, the
key-nm. provides a foundation for the examination ofthe multispecies model, for examplc
through sensitivity analysis, and it also defiries thc currcnt status of thc stockS for
prediction purposes. Predictions CaD. be made assumirig no change in the statüs quo and
carried out for short- medium- or long-term purposes. These enn then be used as a
baseline for sensitivity analysis or against which predictioris \Inder. alternative
exploitation scenarios cari be measured.
At its meetings so far, the MSWG haB examined changes in bcith thc pattern and level of
exploitation for variety of scenarios rehiting to vanous "fisheries" defiried within the
multispeCies model (see Anon., 1986). Examples ofthe "fleets" defining such fisberies Me
those for roundfish, saithe, mackerel, hemng, flatfish and industnal (both pelai;c arid
deinersal).
a
AIthough Pope (1991) gives a concise history of thc MSWG and discusses some of thc
conclusions it has reached conceinirig various exploitation strategies, many details of thc
investigations remain acccssible only in the repcirts of the Working Group. Due to thc
1
scope and amount of work coriducted by the MSWG at its meetings it is not, therefore,
surprising that sometimes even quite struightforward rcsults of thc prcdicted behaviour
of the multispecies system c3.n become submerged by the sheer volume of the Group's
output.
The pUrPose of this work is to provide and discuss the realisation of same simple iongterm exploitation scenarios, the results ofwhich may not be explicitly or readily apparent
elsewherc.
.
.
simulations
The MSFOR program (Anon;,1987) was used for prcdiction. Esscntially this prevides a
multispecies solution of thc catch equation and exponentiUl decay of cohorts forWards in
time arid across Ullspecics Within the MSVPA modet It utilises the stock mimbers,
fishing mortality rates, predation parameters, consumption estimates, residual natural
mortality levels and menn weights. at age which are available from output of thc
retrospective MSVPA cUlculations. The input parameter values used here were taken
from thö key-run of the 1990 MSWG (Allen.; 1991).
For the purpose of pre<Üction, constant ieveis of rccruitment were selectcd far all species;
equal to their mean values over the time series of the MSVPA; and a realisation of thc
model at constant (current) levels offishing eITort in a11 fisheries provided baseline results
against which others could be contrasted. Lang-term forecasts wäre then made over a
range of fishing riiortality mUltipliers (0.0 to 2.0) to siinulate eITort changes in each fleet,
assriming direct proportionality between efforl and fishing riiartality. The multipliers
wero applied to a single fishery whilst the other fisheries were.held constant ut current
levels. This process was repeated for each fishery in turn änd the results are presented
as the percentage deviation of yield and spuwning biomass from thei:- baseline values.
The roundfish fleet had an 85 mm minimum mesh sizc in the most recerit data year afthe
MSVPA (1989) and the results ofthe forecasts assume no change in selectiVity sinco then,
eITectively ignoring thc subsequent minimuni mesh size increases to 90 mm arid; most
recently, to 100 mm (with certain derogations) within that fishCIj.
RESULTS.
The resuits given here ar~ represcnted by pairs ae flgures for eITort chariges in each
fishery. The firstshows the perccntage deviatians of Yield potentially attributable to
eITort changes in the respective fislieries and the seeond indicates the pereentage deviation
from baseline of spawrnng stock estimates. Results are shoWn in Figures 1~16. Thc yield
for eaCh species is sumnl.ed over Ull fisheries and, for the roundfish fishery, both human
consümption landings and discards ofthe target species (eod, hriddock, whiting and smthe)
have been grouped together by species. Only species which are affected by tlie eITart
changes eit.her directly as target specics or indircctly througb biological interactions are
.
·representcd in thc figures.. '
2
Industriill Dcmersal (Figs 1 and 2)
Inereased efrort: the major effeets are on Norway pout arid sandeel yields arid to a lesser
extent on those of 'whiting arid herring where all show inereases. The spaWriing
biciinasscs of sandeeI; Norway pout and whiting diminish.
Deereascd effort: YieldS ofthe target speeies, Noi-way pout and sundeei, demonstrate the
greatest reductions. LOsses ofyield clsewherc
trivial at worst. Tbe yieldS ofhaddock
in particular, but also of eöd, rire improved. This oecurs despitC thc mcrease in spaWning
biomass ofwhiting, a major preda.tor. The spawning biomasses ofNorway pout, sandeel
and haddoek also ineroase as does that of eod rilbeit to a lesser extent. .
are
indiisti-iw. Pelagic (Figs 3 and 4)
Increased effort: thc main effec:t is tri increase thc yield of sprat but to diminish thc Yield
of herring and the spaWnirig biomasses of both herring and sprat.
.
Deereased effort: DO mamatic Yield inereases oceur although those of herring and haddoek
rise.. However, this is set agwnst dramatic inereases in the spaWriing biomasses of
heriirig and sprat.
Total InduStl-iai (Flgs 5 and 6)
rncreased effort: the target species' Yields are inereüsed (Norwny pout, sandeei arid sprat)
as is that of whiting, a by-eatch species in the industrial fisheries. Thc spaWning
biomaases of these species decline alcing with that of herring.
Decreased eITort: thc yieldS of thc. target species fall as does that, to a lesser eiterit, of
whiting. Those of eod, haddock rind herring increase. These ehanges are set agamst some
rather mamatie increases in the spaWning bicimasses of most speeies.
8aithe (Figs7 and 8)
Inereased eITort: the yield imd 'spaWning biomass of saithe dimiriishes with inereases in
the yields and spawrnng biomasses of haddoek arid Norway
pciilt.
.
.
•
Decreased eITort: the yield und spaWning biomass of saithe increases whilst the reverse
is true for haddoek; Norwriy pout aIid, tci a lesser extent, eod.
.
Roundfish CFigs 9 alu! 10)
Inereased erforl: Norway pout und hriddock yields al"e inererised as Die these. of whiting
und herring. The catch of saithe declines aIong with dramritic declines in the spaWriing
biomasses ofeod and smthe. The spawnirig bicimasses ofwhiting and haddock also decline
but those ofNorWay pout and herring increase.
Decreased effort: no greatly iriereased yields are apparent. Those of eod, haddock and
whiting are all rcduced as Ure these of Norway pout and hei-ring. The latter two also
show a decline in spawnmg biomass as do whiting arid hriddoek at extreme reduetions of
eITort.'
.
3
•
.
"
Increased cffort: no signÜicant gairis or losses accroe cxcept io thc spawrung biomaSs of
mackerel which diminishes.
Decreased effort: thc principaI effect mi yield is, a declme for mackereI.' However, its
spawning biomass increases greatly whilst yields arid spawning biomasses of the oUier
species, notably sandeeI, heiTirig and sprat decline.
'
Flatfish (Figs 15 ami 16)
Increased eITort: the yields from both plalce andsoie Ure diminished as are their leveis of
spawning biomass. Very little change in the total North Sen yield or biomass occurs.
Decreased eiTort: Yields increase to a maximum far bath plaice and sole and then fall
away to zero as effort declines further. Large increases in spawning biciinass occur for
both species for relatively modest reductions in effort. For both Yield arid biomass. these
changes are large eriough to be reflected by changes in total North Sea values.
.
'
,
DISCUSSION
It is clearlypossible in forecastS such. as these. thatthe predicted state ofthc system may
be moved significantly away fram that under whichthe retrospective MSVPAmadel was
parameterised. It has been suggested that simulations which farce stock size estimates
away from their recent histoncal levels by more than a factor of 1.5 ure likely to be
unrealistic and must be taken only aS illustrative (Anon.• 1984). Given thc brarid range
of effort mwtipliers used here it is worth emphasising thrit }lornt onee more. From the
bounds at which stock sizes chringe markedly from their baseline vwues. the results can
be interpreted cinly as being iridicative of the behaviour of the model rind doos not
.
necessarily represent the likely dynamies of the flsh' stockS or fisheries.'
,
,
t
0;'
,
Ir expressed aS absolute vaIues rather than percontage deViations. thc results given here
could be thought of as niwtispecies yield-per-recruit rind biomass~per-recruit curves.
However. this is in the partiewar case of constarit recruitrOent in
fishenes rit speciflcd
mean levels. GislaSon (1992) discusses a broader multispecies analogue 10 the sirigle, species per-recruit model which takes account ofchanges in the mean level ofrecrWtment
across all species and from which more gerieralised results Can be identifled.
Nevertbeless. by treating thc reswts giveri here as mwtispecies equivaIent tri singlespecies jrield-per-recruit. it is clear that one objection tO the single-species formulaÜon has
been addrcssed. Namely, that smgle-species models do not ,take account of biological
interactions. That the curves may, demonstrate UIirealistic responses
large
perturbatioris iIi eITort is a further cnticism but not one thrit is uniquely associated with
all
to
4
l
thc multispecies frarriework. Single-species per-recruit models orten demonstrate
unrealistically high stock biomasses if. for example. biomaSs-per-recruit values at zero or
low effort multipliers are scaled to the mean level of recruitment ooserVed in the stOck.
That a mulÜspecies analogue is also prone to unrmllism should not be surprising. In the
former rose biologicaI interactions are not accounted for, whilst; in the latter. insufficicnt
information on the response of predator and prey spccies to largc sCale changes in
abundance suggest that they may not be accounted for correcUY. Whether of not results
should oe presented for "urirealistic" eitrripolaticins of the, model is a. question which
applies irrespective of the framework. either single- or multispecies. .The i-esults given
here follow leES tradition in that they extend over a Wide range of effort multipliers.
Appropririte Cri.ution iri their interpretation is required.
•
At its 1985 meeting. the Working Groul> ealcuiriied yield and biomass curves along tbe
lines of thosepresented here.Thc rcsults givcn therc (Anon.• 1986) show chariges in thc
logantlunic valuc of yield arid spawning biomass derived from thc "Shepherd" forecast
model (Shcphcro. 1984).' Thc different stylc of presentaticiri makes them difficult tri
compare with thosc given here although differences do seem to exist. For example; as
effort is iricreased in the industrlril demersal fishcry, the' earlier resUltS suggest thrit
Norway pöut yield
increase but that of sarideel will remain relative!)" stable whilst
thc spawning biomriss of sandeel will decreaSc and that of Norway pout remmns stable.
However, the current res\ilts suggest that as effort iricreases in the iridustrial demersal
fishery both Will undergci increases in Yield but declines in spawrung biomass. It is, of
course, possible that these differences may rellett changes in the current status of the
fisberiesand stocks compared to tbe period in the carly 1980s arid these confounding
effects make comparisons even more difficult.
will
.On a more general basis. the results ofmultispecies forecasting have dearly demoristratcd .
that. whereas \inder single-species assumptions stock biomasses will31ways decrease With
increased fishing mOrlality, the same is not tme under multispecies assumptions (Anon.,
1986). In terms of Yield it is apparent,that single-species and multispecies predictions
can. siniilarly, be rit variance. , In fact. the variety of scenarios exaniined by the MSWG
at its meetings have suggested that, despite already high fishing mortality rates Within
the NOJ:th Sea, further increasing the fishing pressure on important predators .will
genemlly increase the yield (oüt not necessarily the value) oftotal hiridings (Anon.• 1988).
Irideed, multispecieS forecasts suggest that if mesh sizes are increased in the fraction of
the roundfish fleet thrit fishes for cod arid, thrit, iri the remriinirig fraction and iri thc
industrial fisheries, the fishing mortaIityon whitirig is doubled, then substaritial increascs
in the spawning stockS of ead, haddock and herring are likely compared to those cxiJccted
from ä mesh size increase alone (Anon.• 1989).. In other wordS, in a cui-rently heavily
exploited system a meaSure to reduce fishing pressurc in the cod fisheri will perform
"better" in terms of yields and biomasses of selected species if it is coupled tO an increase
in fisbing pressure on predators in other fishriries.
.
It is difficult to conccive that the current search to dett~rminc the feasibiiity within the
North Sea of a directed whiting fishery with minimal technical interactions .With other
species and fisheries has not been strongly inlluenced by such results. Thc diVide
between exploratOry model anaIysis and the potential provision ofnianagcment ridVice has
been Crossed. Nevertheless. as weIl as the Caution necessary when interpretirig the
results of the multispecies forecrists it maybe considered precipitate to implement such
resultS iD. mariagement without fIrSt evaluating other options either separately or joiritly.
In parÜculrir, it is not derir wh~lt potential exists for niodifying fishery pressure ori prey
5
spedes. For example, ",hEin aSked to "advise on the consequences for other fisheries of
fishing large quantities of prey. species, in prirticUlar, Norway pout and sandeel in the
.North Sea", the MSWG responded by simulating increased fishing mortalities of 50% in
these fisheries (Allon., 1991) despite the frict that over one million tonnes of sandeel alone
were cilUght in 1989 (Arion., 1992). This waS ameliorated tri some extent by fam8l
sensitivity analyses where changes tO the order of ±10% were applied arid thc response
surface extrapolrited to represent changes of ±30% (Anon.; 1991). Unfortunately, it is not
clear whether the reswts presentcd for 30% chimges in fishing pressure npply to effart
chimges in the industnal demersal fishery or thc roundfish fleet. Tbe results shoWn here
, suggest thrit beneficial changes iIi thc yield and biomass of ether fish stocks are possible
if fishing mort8lity rates on prey fish in the iiidustrial fisheries are reduced.
.'
,
Thc range of potential exploitation scenarios is enormous and, in conscquence, only a
limited set can be investigated. However, if the move towardS the provision of lcing-term.
management advice under multispecies assuInptions is to be consolidated theri a brand
spectrum of possibilities must be covered. It is towards thrit erid thai a number simple
, sections through the overall yield rind biomass surfaces are presented here along With the
suggestion that the possibility exists that in an already heavily exploited system it may
be possible to red~ce eITort iri order.to hnprove the status ofNorth Sea fish stocks.
.
"
.
~
~.
e
,
ACKNOWLEDGMENT
Thanks are due to meinbers of thc Multispecies Assessment Working Group' who
discussed an earlier drrift of this paper at its most recent meeting. Nevertheless the
opinions mqiressed here are entirely those of thc riuthors and are not necessanly shri.red
. by other members of the Working Group
".
REFERENCES
Anon. 1984. Report of the ad hoc Multispecies Assessmerit Workirig Group~ ICES, Doc.
CM 19841Assess:20.
Arion. 1986. )ieport of thc ad hoc Multispecles Assessment Working Group. ICES,
CM 1986/Assess:9..
nOt.
Ahon~ 1987. 'Report ofthe ad hoc Multispecies Assessmeht WorkiDg GroUp. ICEs, nOt.
CM 1987/Assess:9.
Anon. 1988.' Report
1988/Assess:23.
oe the Multispecies Assessment Woridng Group. ICES, Dac. CM
Anon~ 1989. Report of the Muitispecies Assessment Woaclng Graup.
1989/Assess:20.,
ICEs, Doc.
Anon. '199L Reporl efthe ~fultispecies Assessment Working Group. ICES,
19911ASsess:7.
CM
noc.' C~f
r
. Anon. 1992. Report of the Industriril Fisheries Asses~merit Working Group. ICES
CM 1992/Assess:9. '
6
nOt.
•
Gislason, H. 1992. The multispecies equivalent to the yield per recruit model ofBeverton
and Holt. Working Paper No 10, ICES Multispecies Assessment Working Group,
June 1992.
Gislason, H. and Sparre, P. 1987. Some theoretical aspects of the implementation of
multispecies virtual population analysis in ICES. ICES, Doc. CM 1987/G:51.
Pope, J.G. 1991. The ICES Multispecies Assessment Working Group: evolution, insights,
and future problems. lCES Mar. Sei. Symp., 193: 22-33.
Shepherd, J.G. 1984. A promising method for the assessment of multispecies fisheries.
ICES Doc. CM 1984/G:4.
•
7
FIGURES
Percentage deviations from the baseline forecast are shown for yields and spawning stock
biomasses. These are attributable to effort changes in the fishery with others held at
current levels.
•
Figure 1. Effort changes applied to the industrial demersal fishery.
Relative catch forecast vs. fishing effort
1.4
1.2
•
•
1
eOD
OWHITING
•
SAITHE
o HADDOCK
0.8
0.6
*
HERRING
i::s.
SPRAT
X
N. POUT
)I( SANDEEL
-
TOTAL
0.4
0.2
o
o
0.2
0.4
0.6
0.8
1
1.2
1.4
1.6
1.8
2
Relative level of fishing effort for industrial demersal fishery
_______________1
Figure 2. Effort changes applied to the industrial demersal fishery.
Relative stock forecast vs. fishing effort
2.4
2.2
2
1.8
•
1.6
COD
o WHITING
•
1.4
SAITHE
o HADDOCK
*
1.2
HERRING
-/:::; SPRAT
X
1
N. POUT
)K SANDEEL
0.8
-
TOTAL
0.6
0.4
0.2
0-t----t---t----;1---t---t---t---+---l--f---1
o
0.2
0.4
0.6
0.8
1
1.2
1.4
1.6
1.8
2
Relative level of fishing effort for industrial demersal fishery
-------------
1
--------------
Figure 3. Effort changes applied to the industrial pelagic fishery.
Relative catch forecast vs. fishing effort
1.4
1.2
•
1
eGO
o WHITING
+ SAITHE
o HAOOOCK
0.8
.. HERRING
0.6
f:s.
SPRAT
X
N. POUT
)K SANOEEL
-
0.4
0.2
o
o
0.2
0.4
0.6
0.8
1
1.2
1.4
1.6
1.8
Relative level of fishing effort for industrial pelagic fishery
2
TOTAL
Figure 4. Effort changes applied to the industrial pelagic fishery.
Relative stock forecast vs. fishing effort
2.4
2.2
2
•
1.8
•
eOD
o WHITING
1.6
+ SAITHE
1.4
o HADDOCK
1.2
1
*
HERRING
6.
SPRAT
X
N. POUT
)f( SANDEEl
-
0.8
0.6
0.4
0.2
o
o
0.2
0.4
0.6
0.8
1
1.2
1.4
1.6
1.8
Relative level of fishing effort for industrial pelagic fishery
2
TOTAL
Figure 5. Effort changes applied to the total industrial fishery.
Relative catch forecast vs. fishing effort
1.4
1.2
•
•
1
COO
o WHITING
•
SAITHE
o HAOOOCK
0.8
-. HERRING
0.6
f:s.
SPRAT
X
N. POUT
)K SANOEEL
-
0.4
0.2
o
o
0.2
0.4
0.6
0.8
1
1.2
1.4
1.6
1.8
Relative level of fishing effort for all industrial fishery
2
TOTAL
Figure 6. Effort changes applied to the total industrial fishery.
Relative stock torecast vs. tishing effort
2.4
2.2
)K
2
•
1.8
•
1.6
eOD
o WHITING
•
1.4
SAITHE
o HADDOCK
1.2
1
•
HERRING
f:s.
SPRAT
X
N. POUT
)K SANDEEL
-
0.8
0.6
0.4
0.2
O-l---+---l---+---f---+--I--+---f---+---1
o
0.2
0.4
0.6
0.8
1
1.2
1.4
1.6
1.8
Relative level ot tishing effort tor all industrial tishery
2
TOTAL
Figure 7. Effort changes applied to the saithe fishery.
Relative catch forecast vs. fishing effort
1.4
1.2
•
• eoo
1
o WHITING
•
SAITHE
o HADDOCK
0.8
0.6
*
HERRING
t:s.
SPRAT
X
N. POUT
)K SANOEEL
-
004
X
0.2
o
o
0.2
0.4
0.6
0.8
1
1.2
1.4
1.6
Relative level of fishing effort for saithe fishery
1.8
2
TOTAL
Figure 8. Effort changes applied to the saithe fishery.
Relative stock forecast vs. fishing effort
2.4
2.2
2
1.8
•
1.6
COD
o WHITING
•
1.4
SAITHE
o HADDOCK
1.2
1
*
HERRING
-6
SPRAT
X
N. paUT
)K SANDEEL
0.8
-
0.6
0.4
0.2
o
o
0.2
0.4
0.6
0.8
1
1.2
1.4
1.6
Relative level of fishing ettort for saithe fishery
1.8
2
TOTAL
Figure 9. Effort changes applied to the roundfish fishery.
Relative catch forecast vs. fishing effort
2
1.8
1.6
1.4
• eoo
o WHITING
1.2
.. SAITHE
o HAOOOCK
1
0.8
*
HERRING
iJ.
SPRAT
X
N. POUT
)K SANOEEL
-
0.6
0.4
0.2
o
o
0.2
0.4
0.6
0.8
1
1.2
1.4
1.6
Relative level of tishing effort tor rf85 tishery
1.8
2
TOTAL
Figure 10. Effort changes applied to the roundfish fishery.
Relative stock forecast vs. fishing effort
2.4
2.2
2
•
1.8
•
1.6
COO
o WHITING
•
1.4
SAITHE
o HAOOOCK
.. HERRING
1.2
-6
SPRAT
\
X
1
N. POUT
>K SANOEEL
0.8
-
0.6
0.4
0.2
o
o
0.2
0.4
0.6
0.8
1
1.2
1.4
1.6
Relative level of fishing effort for rf85 fishery
1.8
2
TOTAL
Figure 11
Effort changes applied to the herring fishery.
Relative catch forecast vs. fishing effort
1.4
1.2
•
~~
1
*
~
0.8
0.6
• eoo
o WHITING
•
SAITHE
o
HAODOCK
*
HERRING
t::s.
SPRAT
X
N. POUT
)I( SANDEEL
-
0.4
0.2
O-t---t---t--1--+---I---f---1---+--t-------1
o
0.2
0.4
0.6
0.8
1
1.2
1.4
1.6
Relative level of fishing effort for herring fishery
1.8
2
TOTAL
r---------------~
-
----
Figure 12. Effort changes applied to the herring fishery.
Relative stock forecast vs. fishing effort
2.4
2.2
2
1.8
•
1.6
COD
o WHITING
•
1.4
SAITHE
o HADDOCK
1.2
1
*
HERRING
l5.
SPRAT
X
N. POUT
)K SANDER
0.8
-
0.6
0.4
0.2
O-t---t---t---t---t---+--+--+--1---f---1
o 0.2 0.4 0.6 0.8
1
1.2
1.4
1.6
1.8
2
Relative level of fishing effort for herring fishery
TOTAL
Figure 13. Effort changes applied to the mackerel fishery.
Relative catch forecast vs. fishing effort
1.4
1.2
•
1
COD
DWHITING
0.8
•
MACKEREL
o
HADDOCK
*
HERRING
-l::s. SPRAT
X
0.6
N. POUT
)K SANDEEL
-
0.4
0.2
o
o
0.2
0.4
0.6
0.8
1
1.2
1.4
1.6
Relative level of fishing eHort for mackerel fishery
1.8
2
TOTAL
•
Figure 15. Effort changes applied to the flatfish fishery.
Relative catch forecast vs. fishing effort
1.4
1.2
1
0.8
0.6
.0.4
0.2
o
o
0.2
0.4
0.6
0.8
1
1.2
1.4
1.6
Relative level of fishing effort for flatfish fishery
1.8
2
•
PLAICE
o
SOLE
•
TOTAL
•
Figure 16. Effort changes applied to the flatfish fishery .
Relative stock forecast vs. fishing effort
5
4.8
4.6
4.4
4.2
4
3.8
3.6
3.4
3.2
3
2.8
•
2.6
PLAICE
o SOLE
2.4
•
2.2
2
1.8
1.6
1.4
1.2
1
0.8
0.6
0.4
0.2
0
0
0.2
0.4
0.6
0.8
1 . 1.2
1.4
1.6
Relative level of fishing effort for flatfish fishery
1.8
2
TOTAL
.---------
--------
Figura 14. Effort changes applied to the mackerel fishery.
Relative stock forecast vs. fishing eftort
2.4
2.2
2
1.8
•
1.6
COO
o WHITING
•
1.4
MACKEREL
o HAODOCK
1.2
1
*
HERRING
-6.
SPRAT
X
N. POUT
)K SANOEEL
0.8
-
0.6
0.4
0.2
o
o
0.2
0.4
0.6
0.8
1
1.2
1.4
1.6
Relative level of fishing effort for mackerel fishery
1.8
2
TOTAL