Introduction to the model of the Guinea-Bissau Continental Shelf Ecosystem and an exercise on simulating the effects of

1. Presented at the Symposium on “Marine fisheries, ecosystems, and societies in West Africa: half a century of change”, 24-28 June 2002, Dakar, Senegal. Introduction to the model of the Guinea-Bissau Continental Shelf Ecosystem and an exercise on simulating the effects of fishing Patrícia Amorim1, Gregório Duarte2, Miriam Guerra1, Telmo Morato3 and Kim A. Stobberup1 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)

2. Model construction • Parameter estimation 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:Published data and Fishbase information • Non Fish Groups:Other models with a similar division of functional groups (Opitz, 1993 and Arreguín-Sanchez et al, 1993)

3. Improvements on the “first” preliminary model: • incorporation of more reliable estimates on artisanal catches (D. Thiam, 1993) - artisanal catches were increased from 17000 to 46000 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.

4. Guinea Bissau Ecopath Model 31 ecological groups: - Marine mammals - Turtles - Seabirds - Fish groups (15) - Squid - Benthos groups (7) - Zooplankton - Primary producers (2) - Detritus (2) • 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 40816 km2. • 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.

5. CIPA-IPIMAR surveys: trawl stations • Surveys covered the same area • 138 stations

6. 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 • Fish groups(including 166 species) • Biomass estimates • CIPA-IPIMAR trawl surveys average of 1990 and 1991 Estimates of total biomass Survey 1990  63 000 Ton Survey 1991  37 000 Ton • Great inter-annual variability of the total catch * Biomass estimated by model

7. Fish Groups Problems found: 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 Groups poorly sampled by trawl surveys Published data

8. CIPA-IPIMAR surveys: Benthic grab samples • 91 stations

9. Benthos groups Cephalopods* Gastropods/Bivalves Shrimps* Crabs Small crustaceans Annelids Equinoderms - Biomass estimates Survey 1990  40 t/km2 Survey 1991  59 t/km2 * Biomass estimated by model • Dominant groups: • Annelids: B=21.7 t/km2 • Gastropods/Bivalves: B=16.1 t/km2 • Crabs: B= 9.1 t/km2 • These groups constitute an important part of the diet of many fish groups

10. 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

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

12. Catches by Fleet • Artisanal fishing dominated in terms of total catches • Total catch estimate  46000 ton. (D. Thiam 1993 ) • Industrial fishing: Total catch estimate  45000 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

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

14. Results Box Diagram

15. 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

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

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

18. 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.