Introduction to the model of the Guinea- Bissau Continental Shelf Ecosystem and an exercise on simulating the effects of fishing Patrícia Amorim 1, Gregório Duarte 2, Miriam Guerra 1, Telmo Morato 3 and Kim A. Stobberup 1 1: IPIMAR: Instituto de Investigação das Pescas e do Mar (Portugal) 2: CIPA: Centro de Investigação Pesqueira Aplicada (Guiné-Bissau) 3: DOP: Departamento de Oceanografia e Pescas (Portugal) Presented at the Symposium on “Marine fisheries, ecosystems, and societies in West Africa: half a century of change”, June 2002, Dakar, Senegal.

 Parameter estimation Production/Biomass Production/Biomass (P/B) and Consumption/Biomass (Q/B) Based on empirical equations Information from fisbase and surveys Information from other models  Diet matrix Fish Groups: Fish Groups: Published data and Fishbase information Non Fish Groups: Non Fish Groups: Other models with a similar division of functional groups (Opitz, 1993 and Arreguín-Sanchez et al, 1993) Model construction

Improvements on the “first” preliminary model:  incorporation of more reliable estimates on artisanal catches (D. Thiam, 1993) - artisanal catches were increased from to tons, which is almost a triplication in relation to the first version of the model (mainly mullets and Ethmalosa catches).  biomass estimates for benthos groups were included, giving a better definition of lower trophic level. These modifications lead to problems in balancing the model, particulary in relation to B and P/B parameters for demersal fish groups.

31 ecological groups: - Marine mammals - Turtles - Seabirds - Fish groups (15) - Squid - Benthos groups (7) - Zooplankton - Primary producers (2) - Detritus (2) Guinea Bissau Ecopath Model  Area covered by the model Extends along Guinea-Bissau shore, including the Bijagós Archipelago and the continental shelf down to a depth of 200 meters Inner waters such as the Geba River were not considered The total area considered was km 2.  Reference period The reference period of the model is from 1990 to 1993, because of the availability of fisheries statistics and biomass estimates from trawl and acoustics surveys.

CIPA-IPIMAR surveys: trawl stations  Surveys covered the same area  138 stations

Fish groups (including 166 species) - Biomass estimates  CIPA-IPIMAR trawl surveys average of 1990 and 1991 Billfish and Marlins* Tuna* Pelagic sharks* Pelagic predatores Benthic predatores* Demersal sharks Groupers/Snappers* Rays* Benthos/Fish feeders Sparids Flatfish* Benthic feeders Small pelagics Phytoplanktivores Mullets * Biomass estimated by model Estimates of total biomass Survey 1990  Ton Survey 1991  Ton  Great inter-annual variability of the total catch

Problems found: Groups poorly sampled by trawl surveys Published data Low values of biomass estimated from surveys data were in conflict with high catch estimates Biomass estimated by model Benthic predatores Groupers/Snappers Rays Flatfish Small pelagics Pelagic predatores Phytoplanktivores Mullets Fish Groups

CIPA-IPIMAR surveys: Benthic grab samples  91 stations

Benthos groups Cephalopods* Gastropods/Bivalves Shrimps* Crabs Small crustaceans Annelids Equinoderms - Biomass estimates Survey 1990  40 t/km 2 Survey 1991  59 t/km 2 Dominant groups: Annelids: B=21.7 t/km 2 Gastropods/Bivalves: B=16.1 t/km 2 Crabs: B= 9.1 t/km 2  These groups constitute an important part of the diet of many fish groups * Biomass estimated by model

Other Groups  Biomass estimates were not available for these groups.  Thus, biomass was estimated by model except for: - Zooplankton and Phytoplankton (Longhurst, 1983) Marine mammals Seabirds Turtles Squid Zooplankton Phytoplankton Benthic algae Discards Detritus

Fisheries Data Fisheries in Guinea-Bissau can be divided into three major categories: Industrial, Artisanal and Tuna. Guinea Bissau statistics present some problems of coverage and species identification. Therefore, it was necessary to estimate catch for some groups of species.

Catches by Fleet  Artisanal fishing dominated in terms of total catches Total catch estimate  ton. (D. Thiam 1993 )  Industrial fishing: Total catch estimate  ton. (CIPA,1996) - Shrimp trawlers are the most important component of the industrial fleet  Tuna fishery: catches estimated for EEZ of Guinea Bissau based on ICCAT data (1991). Total catch estimate  4800 ton

Catch species composition Artisanal Fishery: mullets and phytoplanktivores (Ethmalosa fimbriata) Trawl: multispecific fishery Purse seine: small pelagics Tuna Fishery: tuna

Box Diagram Results

Mixed trophic impacts Phytoplankton  positive impact on almost all groups Artisanal fishery  negative impact on mullets (intensive fishing) Shrimp and Fish trawl  negative impact on Pelagic sharks and Benthic predatores (competition for prey) Small pelagics/Squid, Marine mammals/Tuna and Marine mammals/Pelagic sharks  form opposing groups in competition for main prey

ECOSIM: simulating the effects of fishing Possible scenario of fishing in Guinea Bissau during the decade of the 90´s. Combined gears Principal species affected (Biomass) - Benthic/fish feeders (+42%) - Cephalopods (+19%) - Sparids (+17%) - Mullets (-19%) - Rays (-21%) - Groupers/Snappers (-25%) - Pelagic sharks (-27%)

End of simulation (End of 1999) ECOSPACE: spatial representation Fishing effort by fleet Fishing rate=1.5

Final considerations:  Better estimates of landings and discards are needed in order to simulate the effect of fishing on the ecosystem.  The re-establisment of the statistical collection system in Guinea Bissau, which is in course, will lead to improved statistics.  Spatial dynamics are known to be important and it would be desirable to develop a spatial model considering the distribution of species by zone and depth.  Regular surveys, covering seasonal changes, would lead to a better understanding of variability in the system.