Structure of demersal fish Structure of demersal fish assemblages based on trawl surveys assemblages based on trawl surveys

in the continental shelf and upper in the continental shelf and upper slope off Guinea Bissauslope off Guinea Bissau

P. Amorim1, S. S. Mané2 and K. A. Stobberup1

1 IPIMAR: Instituto de Investigação das Pescas e do Mar (Portugal)

2 CIPA: Centro de Investigação Pesqueira e Aplicada (Guiné-Bissau)

Presented at the Symposium on “Marine fisheries, ecosystems, and societies in West Africa: half a century of change”, 24-28 June 2002, Dakar, Senegal.

Ecosystem characteristics

- an extensive continental shelf (about 150 km)

- considered an important nursery area in the region

- strong influence due to seasonal upwelling and alternation between dry and wet seasons

- located at the southern limit of the Canary Current System and the western limit of the Gulf of Guinea causing important shifts in oceanographic conditions.

These characteristics are expected to affect the composition and abundance of the resources

OBJECTIVE

Study demersal fish assemblages in Guinea Bissau considering the effects of DEPTH,

ZONE and TIME in abundance and species composition.

DATA SOURCES

- 5 surveys

- between 1988 and 1995

- CIPA-IPIMAR

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Survey 1988

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Survey 1989

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Survey 1990

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Survey 1991

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Survey 1995

CIPA - IPIMAR Surveys CIPA - IPIMAR Surveys

Survey Shallow Mid Deep Shallow Mid Deep Total1988 5 1 - 18 7 4 351989 36 27 - 14 6 5 881990 29 21 - 24 24 7 1051991 7 3 1 9 11 2 331995 48 18 - 11 - - 77

125 70 1 76 48 18 338

North South

Different areas covered in different years

Survey periods varied between the months of March and June (transition between dry and wet season)

Depth of trawl stations ranged from 10 to 500 meters

The northern area and the shallow waters were better sampled

Rocky bottoms were not covered due to gear limitations

Fishing stationsFishing stations

Zeexln3.shp

Province.shpCape VerdeGambiaGuineaGuinea BissauMaliMauritaniaSenegalSierra LeoneWestern Sahara

Interfundo_zeegbshp.shpRocky bancsSand with shellSand fine/ very fineSand medium/ coarse grained Sand medium/ coarse grained + rocky zones Sand silt/ Silt sandySiltSilt+ rocky zones

10°00' 10°00'

10°30' 10°30'

11°00' 11°00'

11°30' 11°30'

12°00' 12°00'

18°00'

18°00'

17°30'

17°30'

17°00'

17°00'

16°30'

16°30'

16°00'

16°00'

15°30'

15°30'

15°00'

15°00'

Zeexln3.shp

Province.shpCape VerdeGambiaGuineaGuinea BissauMaliMauritaniaSenegalSierra LeoneWestern Sahara

Interfundo_zeegbshp.shpRocky bancsSand with shellSand fine/ very fineSand medium/ coarse grained Sand medium/ coarse grained + rocky zones Sand silt/ Silt sandySiltSilt+ rocky zones

10°00' 10°00'

10°30' 10°30'

11°00' 11°00'

11°30' 11°30'

12°00' 12°00'

18°00'

18°00'

17°30'

17°30'

17°00'

17°00'

16°30'

16°30'

16°00'

16°00'

15°30'

15°30'

15°00'

15°00'

Sediment types

Source: SIAP SIG project

Zone North

Bottom mainly consist in silt

sediments

Zone South

Bottom mainly consist in sandy

sediments

Factors defined for statistical analysis:Factors defined for statistical analysis:

- Time (survey years)

- Zone (corresponding to different sediment types)

- Depth 1. Shallow (<50 m) above thermocline

2. Mid (50-200 m)

3. Deep (>200 m) Shelf-slope transition

Total data:Total data:

- 316 fish species

- 338 trawl stations

- Catch weight (kg) standardized by half-hour.

ANALYSIS OF COVARIANCEANALYSIS OF COVARIANCE

Results indicate significant effects of Depth and Zone in the total catch

Catch versus Depth

-1

0

1

2

3

4

5

0 100 200 300 400 500 600

Depth (m)

Lo

g (

kg/3

0 m

in)

Fish catch increased with depth

- due to the possible effects of seasonal upwelling

- incidental high catches of deepwater species (ex: Chlorophthalmus agassizi)

North South

The factor Zone was significant: lower catches in the north

Only the survey of 1990 presented significantly higher catches. The differences in survey area for the different years does not appear to have an effect on the results obtained.

1 2 3 4 5

01

23

4

Trawl Catch by Survey

Log

(kg/

half

hour

)

Norte Sul

01

23

4Trawl Catch by Zone

Log

(kg/

half

hour

)

MULTIVARIATE ANALYSISMULTIVARIATE ANALYSIS

Study of demersal fish assemblages and the effects of Time, Depth and

Area

a. Non-parametric Multi-Dimensional Scaling (MDS)

Ordination method based on a matrix of similarities: constructs a map or configuration of samples, which attempts to satisfy the similarity between samples by their location in the map. The effect of factors can be studied by observing the pattern formed, when grouping the samples by factors.

Fourth-root transformation applied to data:

- gives lesser weight to abundant species

- considers the ocurrence of rare species

Similarities between stations calculated using the Bray-Curtis similarity measure:

- not sensitive to double zeros in the matrix data

Shallow

Mid

Deep

Stress: 0.15

North

South

Stress: 0.15

MDS plot for Zone and Depth effects

Depth

Zone

Clear groups are formed when

presenting the stations by Depth

and Zone

Low value of stress

Station relationships well represented, considering

the high number of stations

1988

1989

1990

1991

1995

Stress: 0.15

Time

MDS plot for Time effects

No pattern was found when stations were grouped by Time

South - Shallow

South - Mid

South - Deep

North - Shallow

North - Mid

North - Deep

Stress: 0.15

Zone + Depth

MDS plot for combined effects

A grouping of stations considering the combined effects of Zone and Depth show a

clear pattern of aggregation.

b. Determine whether the effects of Depth and Zone are significant (using the ANOSIM procedure)

ANOSIM is an non-parametric extension of ANOVA to test for the significant effects

A 2-way crossed test based on matrix of similarities was undertaken to test for the significant effects of Depth and Zone

Time factor was not considered based on the MDS results, which did not show any clear pattern.

The results obtained confirme that Depth and Zone produce

significant effects in demersal fish assemblages.

c. Simper

PRIMER procedure for the grouping of typical and discriminant species.

Identify the average contribution of each specie to the similarity - Typical species - and dissimilarity - Discriminants species - between groups of samples.

Typical species contribute to the similarity within a group

Discriminants species contribute to the dissimilarity between groups

This analyse was perfomed considering only Depth and Zone.

10°30' 10°30'

11°00' 11°00'

11°30' 11°30'

12°00' 12°00'

18°00'

18°00'

17°30'

17°30'

17°00'

17°00'

16°30'

16°30'

16°00'

16°00'

15°30'

15°30'

Syacium guineensisBothus podsTrachinocephalus myopsRaja miraletusChelidonichthys gabonensisPseudupeneus prayensisSolitas gruveliPagellus bellottii bellottiiDicologlossa hexophthalmaChilomycterus spinosus mauretanicusParaconger notialis

Chlorophthalmus agassiziSynagrops microlepisPterothrissus bellociPeristedion cataphractumAriomma melanumChascanopsetta lugubrisAriomma bondiSynagrops bellusBembrops heterurusMerluccius polli

Cynoglossus canariensisBrachydeuterus auritusGaleoides decadactylusPteroscion peliIlisha africanaPseudotolithus senegalensisTrichiurus lepturusCitharichthys stampfliiPseudotolithus typusCynoponticus feroxVanstraelenia chirophthalmus

Less than 2 samples in this group: it was

not possible determined the typical species

Typical speciesBrotula barbata Vanstraelenia chirophthalmus Citharus linguatula Lepidotrigla cadmani Trachurus trecae Brachydeuterus auritus Dentex angolensis Zeus faber Saurida brasiliensis Umbrina canariensis Dentex maroccanus Microchirus frechkopi Raja miraletus Leptocharias smithii

Raja miraletus Trachurus trecae Ariomma melanum Heteropriacanthus cruentatus Dentex congoensis Lepidotrigla cadmani Dentex angolensis Antigonia capros Pagellus bellottii bellottii Spicara alta Saurida brasiliensis Lepidotrigla carolae

Z eus faber Chelidonichthys gabonensis Citharus linguatula Solitas gruveli

Temperature Dissolved Oxygen Chlorophyll

decrease with depth

Similar values in both areas

Shallow water is characterised by higher values of temperature, chlorophyll and oxygen in contrast with Deeper

waters

Salinity little variation (surveys undertaken in a limited period of the year)

Depth factor Enviromental parameters

Oceanographic stations indicated:

Zone:

Clear separation of bottom type between the 2 zones

Different types of sediment

Different benthos communities

Different demersal fish assemblages

Diet preferences

Prey-predator relationships

CONCLUSIONS Results indicate important spatial structure (by depth and zone) in the Guinea-Bissau ecosystem, both in terms of total catches and fish assemblages.

There were no detectable time effects, but the period considered (1988-1995) is limited. Future studies should attempt to lengthen the time period in order to study temporal dynamics.

The various methods applied appear to be useful and complementary in studying the structure of demersal fish communities, considering spatial and temporal dynamics.