bottoms of the shclf, long-liners mainly on the shelf break and gillnets
on rocky grounds near the coast or on the shelf break, too. Pelagic fisherics arc seasonal and catch anchovy (seiner) and tunas (troll and bait
boats) during their trophic migrations in spring and summer.
The last estimates by ICl-5 assessment working groups showed that the
southcrn stocks of hakc and monkflsh wcrc outside safc biological limits
and that the southern stocks of mcgrims and horsc mackerel werc overfished. Landings of hakc (the main demersal commercial species) in the
last four years havc been below the agrced tacks and the spawning biomass has rcachcd a historical minimum. ( )nc of thc main management
measurcs adopted by Spanish Fisherics Administration is the protection
of juveniles. Restricted areas to trawl have been established in zones where
hake recruits conccntrate each year. Kecpingin mind that the continental shelf is vert' narrow (from 10 to 60 km), these measures havc limited
the work area of the trawl fleet, causing a great uneasincss in the sector
becausc of the strong cconomic repercussions of thesc measures. Traditional stock assessment tools using the single-species approach do not allow us to ask questions about spatial management options, nor about
thc ecological impacts of policy altcrnatives. This work is a first study of
thc Cantabrian Set ecosystcm using a mass-balance model of trophic interactions (Ecopath), trying to know the possible impacts of the different
fisheries that operate in the arca and to cstimate the consequcnces of
some management measures, such as closed areas, on the ecosystem.
Material and methods
Thc Ixopath (vcrsion 4) modcl has been applied in ordcr to produce tt
balanccd steady-state dcscriptiftn (tf thc Cantabrian Sca shclf ecosystem.
The Fxopath model combines an approach for thc estimation of biomass and food consumption of thc various elements (species or groups
of specics) in an aquatic ecosvstcm with an approach for analvsis of
flows between the elements of ecosystems (Polovina, 1984; (ihristensen &
Pauly, 1992, 1993). The energy balance of each trophic group is given bt’
the basic equation:
Consumption =Production + Respiration + Unassimilated food
The production of each trophic group is balanced by its predation by
othcr trophic groups in thc svstem, its exports from thc svstcm and
mortality. Thc ccost’stem is modelled using a sct of simultaneous linear
equations (one for each group / in the system), i.e.:
Production by (i) - all predation on (i) - non-predation losses of (i)
- export of (i) - biomass accumulation of (i) = 0, for all (i).
This can also be put as:
Bi Pi/ Bi -1 Bj Qj /Bj DCji, – Pj/Bj (1 - EEj ) – EXi = O
where,
Bi is the biomass of (i);
188
on rocky grounds near the coast or on the shelf break, too. Pelagic fisherics arc seasonal and catch anchovy (seiner) and tunas (troll and bait
boats) during their trophic migrations in spring and summer.
The last estimates by ICl-5 assessment working groups showed that the
southcrn stocks of hakc and monkflsh wcrc outside safc biological limits
and that the southern stocks of mcgrims and horsc mackerel werc overfished. Landings of hakc (the main demersal commercial species) in the
last four years havc been below the agrced tacks and the spawning biomass has rcachcd a historical minimum. ( )nc of thc main management
measurcs adopted by Spanish Fisherics Administration is the protection
of juveniles. Restricted areas to trawl have been established in zones where
hake recruits conccntrate each year. Kecpingin mind that the continental shelf is vert' narrow (from 10 to 60 km), these measures havc limited
the work area of the trawl fleet, causing a great uneasincss in the sector
becausc of the strong cconomic repercussions of thesc measures. Traditional stock assessment tools using the single-species approach do not allow us to ask questions about spatial management options, nor about
thc ecological impacts of policy altcrnatives. This work is a first study of
thc Cantabrian Set ecosystcm using a mass-balance model of trophic interactions (Ecopath), trying to know the possible impacts of the different
fisheries that operate in the arca and to cstimate the consequcnces of
some management measures, such as closed areas, on the ecosystem.
Material and methods
Thc Ixopath (vcrsion 4) modcl has been applied in ordcr to produce tt
balanccd steady-state dcscriptiftn (tf thc Cantabrian Sca shclf ecosystem.
The Fxopath model combines an approach for thc estimation of biomass and food consumption of thc various elements (species or groups
of specics) in an aquatic ecosvstcm with an approach for analvsis of
flows between the elements of ecosystems (Polovina, 1984; (ihristensen &
Pauly, 1992, 1993). The energy balance of each trophic group is given bt’
the basic equation:
Consumption =Production + Respiration + Unassimilated food
The production of each trophic group is balanced by its predation by
othcr trophic groups in thc svstem, its exports from thc svstcm and
mortality. Thc ccost’stem is modelled using a sct of simultaneous linear
equations (one for each group / in the system), i.e.:
Production by (i) - all predation on (i) - non-predation losses of (i)
- export of (i) - biomass accumulation of (i) = 0, for all (i).
This can also be put as:
Bi Pi/ Bi -1 Bj Qj /Bj DCji, – Pj/Bj (1 - EEj ) – EXi = O
where,
Bi is the biomass of (i);
188