Recent trends in distribution and abundance of commercial fish stocks in Lake Victoria (East Africa) based on GIS

Transcription

1 Recent trends in distribution and abundance of commercial fish stocks in Lake Victoria (East Africa) based on GIS representation from Acoustic surveys 1 Taabu A. M, 2 R. Tumwebaze and 3 R. Kayanda 1 National Fisheries Resources Research Institute (NaFIRRI), P.O. Box 343, Jinja, Uganda 2 Department of Fisheries Resources (DFR), Entebbe Uganda 3 Tanzania Fisheries Research Institute (TAFIRI), Mwanza Tanzania

2 Why GIS Content Introduction (Background) Materials and methods Trends of biomass estimates Spatialp distribution patterns Size structure Key observations/conclusions Suggestions for management

3 Use of GIS The situation presented here is not pleasing But perhaps a classical example how GIS can convey the message We kept communicating i through h tables, figs etc but.. The Time GIS was used

4 Use of GIS 6 out of 25 fish processing factories have closed their operations Saturday Vision 22nd Sept 2007

5 Introduction The first stage of management of any resource is to evaluate the status of the resource envelop a) How much? -Biomass b) What sizes? -Size structure; c) where are they found? Distribution patterns d) what changes? Trends in fish stocks in time and space e) What direction to take? Appropriate advice to management

6 Background information Prior to 1950,the lake was fished down 5 exotic fish species were introduced Predatory Nile perch flourished Stocks of native cichlids crushed (Google) Currently, 4 Major commercial species Nile perch (basis of the export fishery) Nile tilapia Rastrineobola argentea (Dagaa) Haplochromine cichlids b) over capacity is leading to a 2 nd collapse

7 Materials and methods Six Acoustic surveys (Aug 2005 to Feb 2008) The Simrad EY 500 and EK 60 echosounders Stratification by depth Collection of Goe-referenced Acoustic data Analysis -Echoview 4.0 software Nile perch - single target detections Dagaa and haplochromines - echo integration Spatial distribution - Arcview 3.2 and ArcGis 9.2 Correlation of results with historical data

8 Total Biomass estimates 3,000,000 2,500,000 R2 = ,000,000 1,500,000 1,000, ,000 - AUG 1999 AUG 2000 AUG AUG AUG AUG AUG AUG AUG Biomass (tonnes)

9 Biomass by different fish types Bioma ass (t x ) Other species Rastrineobola Nile perch Proportions of the major species have changed Nile perch reduced while Dagaa and Haplos have increased o.7m 1.4 m m

10 Seasonal abundance 1,500,000 Ln Dagaa Haps 1,200,000 Biomass s 900, , ,000 0 Aug 05 Feb 06 Aug 06 Feb 07 Aug 07 Survey yperiod

11 Spatial distribution of Nile perch Aug 2005 Higher densities NE, NW and SE Deep stratum not sufficiently covered

12 Spatial distribution of Nile perch Feb 2006 Higher densities NE, NW and SE Deep stratum not sufficiently covered

13 Spatial distribution of Nile perch Aug 2006 Higher densities NE, NW, SE Clusters in the SW

14 Spatial distribution of Nile perch Feb 2007 Higher densities in the northern quadrants More uniform in the south and deep transects

15 Spatial distribution of Nile perch Aug 2007 Higher densities northern quadrants Very low densities in the southern except SE coast

16 Spatial distribution of Nile perch Feb 2008 Clusters around island zones on the north Very low densities in the southern except SE coast

17 Spatial distribution of Dagaa Aug 2007 Higher densities in the northern quadrants including deep stratum Lower densities in the sw

18 Spatial distribution of Dagaa Feb 2008 A more uniform distribution across the entire lake

19 Spatial distribution of other fish types Aug 2007 Higher densities in inshore areas Major components Haplochromines and tilapia

20 Spatial distribution of other fish types Feb 2008 Clusters along the inshore areas More evenly distributed

21 Sizes that are now rare

22 Size distribution of Nile perch - Acoustics Max target t cm, TL < 1% of the targets were > 50 cm, TL Nile perch targe ts N o. of Tar rg ets Total Length (cm)

23 The relationship btn biomass (grey shading) and mean maximum ( ) and average ( ) lengths of Nile perch in Lake Victoria Bioma ass (t x ) There is a 50 strong 40 correlation r=0.99, 30 p< ) l length (cm Total Aug 05 Feb 06 Aug 06 Feb 07 Aug 07 Feb 08

24 Key observations Biomass = 2.17 ± 0.13 m t, = 2.05 ± 018mt 0.18 There are changes in species composition various effort levels The Nile perch fishery is getting dominated by smaller sized fish Localized abundance distribution mainly in the north and SE of the lake Dagaa biomass has continued in increase

25 Suggestions Observance of minimum slot size (50 85 cm TL)couldleadtoincreaseinnosoflarge fish Completion and implementation of the Regional Plan of Action (RPOA) on fishing capacity Need to develop a strategy toharvestdagaa Need a better method to estimate tilapia stocks More efforts to map and protect fish breeding areas Model environmental and effort data to isolate the major driving force

26 Appeal I invite collaborative research proposals to address the issues Lake Victoria is a BIG BIG Laboratory whose proposals are easy to market

27 I thank you for listening