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Summary of fisheries and resources information for mainland Tanzania

by
L.B. Nhwani
Tanzania Fisheries Research Institute
P.O. Box 90, Kigoma, Tanzania


ABSTRACT
1. INTRODUCTION
2. THE ARTISANAL FISHERIES
3. THE LIGHT-ASSISTED FISHERY FOR SMALL PELAGIC SPECIES
4. THE SEMI-INDUSTRIAL SHRIMP TRAWL FISHERY
5. THE SEMI-INDUSTRIAL FISH TRAWL FISHERY
6. THE OTHER FISHERIES RESOURCES
7. BIBLIOGRAPHY


ABSTRACT

More than 95 percent of the total marine catch of 46,985 tonnes in 1986 was from the artisanal fishermen. The present productivity per unit area of these fishermen is estimated as 3.8 tonnes/km2. This is judged as being relatively high and indicative of the inshore grounds being at or near the level of full exploitation. Potential is believed to exist for additional catches of up to 500 tonnes per year of quality fish from rough grounds further offshore in the approaches to the Mafia Channel. This is likely to require semi-industrial fishing using short setlines.

Reference is made to light assisted purse seining for small pelagics adjacent to Dar es Salaam. This presently involves the use of motorised traditional boats, as well as two larger steel vessels. The catch from purse seining in 1986 is estimated at about 2,500 tonnes, of which some 70 percent is included with the above-mentioned catch by the artisanal fishermen. Reference is made to the total catch of sardines and small mackerels (from all fishing methods) being 7,090 tonnes. The preliminary estimate for the potential yield of small pelagics is 20,000 tonnes from all Tanzanian waters (including the waters around Zanzibar and Pemba Islands).

The other principle fisheries include both fish and shallow water shrimp trawling on grounds adjacent to the Rufiji Delta and Bagamoyo. An estimate of the landings of shrimp by trawlers in 1987 is 691 tonnes (whole weight). It is believed that an additional 500 to 700 tonnes is taken by the artisanal fishermen using seines, traps and scoop nets. The estimate given for the potential yield on the exploited grounds is 3,500 - 4,000 tonnes. Shrimp resources are known to exist outside the presently exploited areas.

The estimate given for the catch of trawl fish during 1986 is 875 tonnes, of which roughly half was by-catch from the shrimp trawlers. Various estimates are given for the potential yield based on previous trawl surveys.

1. INTRODUCTION

The coastline of mainland Tanzania extends for some 800 km (without including indentations due to river mouths and estuaries), from latitude 04°39'S at the northern border with Kenya to latitude 10°28'S at the southern border with Mozambique. The continental shelf is generally narrow with the 200 m depth contour being about 4 km offshore, except for the Zanzibar and Mafia Channels where the shelf extends for some 60 km. The seabed is mainly rough due to coral reefs and outcrops. Grounds suitable for trawling are found adjacent to the mouths of the five main rivers (Pangani, Wami, Ruvu, Rufiji and Ruvuma) and within the Zanzibar Channel; see Figure 1.

More than 95 percent of the total marine landings of 46,985 tonnes (1986) are from the artisanal fishermen (see Table 1.1). They operate from a large number of fishing villages, and use traditional craft (mostly non-motorised) and simple gears such as traps, handlines, gillnets and seines. The catches typically comprise a large number of species.

The situation for the main landing site at Dar es Salaam is somewhat atypical, as here many of the traditional mashuas have been motorised and are used for catching small pelagics (sardines and mackerels). Purse seine nets are used and the fishing is done at night with the assistance of lights (for attracting and concentrating the fish).

There are two larger non-traditional boats engaged in light assisted purse seining for small pelagics from Dar es Salaam. They are operated in the adjacent waters with crews recruited from amongst the artisanal fishermen. The catch from these boats during 1986 was 702 tonnes.

The other larger non-traditional boats are trawlers. Six of these were operated by the parastatal Tanzania Fishing Corporation (TAFICO) in 1986 for a catch of 146 tonnes (tail weight) of shrimps and 614 tonnes of fish. Only two of the six boats were used for catching shrimps. In the case of these boats the onboard procedures include sorting, grading, packaging and freezing shrimp for export. The commonly exploited grounds are off the mouths of the Rufiji, Wami and Ruvu rivers.

During 1986, foreign fishing licences were granted allowing four trawlers based from Kenya to be used off the southern coastline adjacent to the Rufiji Delta. The catch of shrimps from these boats during 1987 was reported as 274 tonnes (whole weight). The quantities of fish and other products caught are not yet available.

A few small trawlers owned by the Bagamoyo Fishermen's Cooperative are also engaged in catching shrimp adjacent to the mouth of the Ruvu River. The quantity of catch from these boats is negligible.

Figure 1. Coast of mainland Tanzania showing regions used for fisheries statistics purposes.

Table 1.1. Annual marine fish catches and other statistics 1977 - 1984.


1978

1979

1980

1981

1982

1983

1984

1986

Artisanal Fishery:

Catch Weights (tonnes)

46,708

33,104

29,593

32,716

23,669

33,371

39,810

45,501

No. of Fishermen

9,799

8,120

7,596

4,755

13,499

na.

13,783

12,619

No. of Boats

4,498

2,906

2,238

3.001

3,433

2,382

3,556

3,690

Catch/Boat (tonnes)

10.4

11.4

13.2

10.9

6.9

14.0

11.2

12.3

Industrial Fishery: (TAFICO and DARFISH)

Catch Weights (tonnes)

---

---

---

---

608

1,115

1,081

1,483

No. of Vessels

---

---

---

---

na.

10

10

10

Source: Statistics Section, Fisheries Division

Note: Values for 1982 extrapolated from previous years data. Values for 1985 not compiled at the time of writing.

Relatively little of the total marine catch is exported, although this includes almost all the shrimp caught from trawlers (plus some of the shrimp caught by the artisanal fishermen). The quantities exported during 1986 comprised of 216 tonnes of shrimp (mainly tails), 157 tonnes of sea shells, 162 tonnes of beche-de-mer, 0.26 tonnes of turtle shells, 2.7 tonnes of shark fins and jaws, and negligible quantities (4,570 fish) of live aquarium fish.

2. THE ARTISANAL FISHERIES

Location: Except in the case of Dar es Salaam the fishing communities exist in many small villages scattered along the entire Tanzanian coastline. The fishing takes place almost entirely within the near shore waters to depths of 40 m, although sometimes there is handlining to 60 m depth on the upper edge of the continental slope.

Boats: Some 3,690 artisanal fishing boats were recorded for 1986 (not all were actually operated) of which 228 were equipped with outboard engines and 49 with inboard engines. The location of these boats according to the statistical regions depicted in Figure 1 are as shown in the following Table 2.1.

Table 2.1. Numbers of artisanal fishing boats by statistical region for 1986.

Item

Numbers of Boats and Engines by Region:

Tanga

Coast

Dar es Salaam

Lindi

Mtwara

Totals

Total Boats

841

807

656

683

703

3,690

With Outboards

76

17

118

13

4

228

With Inboards

2

14

24

7

2

49

Source: Statistics Section, Fisheries Division.

An independent determination of boat numbers is provided in Carrara (1986) based on data from an "aerial" survey done in November 1985 complemented by other data collected by the Statistics Section, Fisheries Division. The estimates of boat numbers are given below in Table 2.2.

Table 2.2. Numbers of artisanal fishing boats by type and region.

Boat Type

Numbers of Boats by following Regions:

Tanga

Coast

Dar es Salaam

Lindi

Mtwara

Totals

Mtumbwi

180

177

88

343

167

955

Ngalawa

384

390

363

172

13

1,322

Mashua

174

193

169

85

163

784

Boti

53

37

107

14

7

218

Totals

791

797

727

614

350

3,279

Source: Carrara (1986).

Note: Mtumbwi are dugout canoes of 2 - 4 m in length propelled by paddling, Nglawa are dugout canoes with sails and fitted with outriggers for stability, mashuas are planked boats of 5 - 10 m in length fitted with sails, and both are motorised boats.

Fishermen: The numbers of fishermen recorded for 1986 are given below in Table 2.3.

Table 2.3. Numbers of fishermen by statistical region in 1986.

Numbers of Fishermen in the following Regions:

Tanga

Coast

Dar es Salaam

Lindi

Mtwara

Total

3,311

2,594

3,421

1,475

1,818

12,619

Source: Statistics Section, Fisheries Division. Gear: The numbers of items of fishing gear in use in 1986 are as given in Table 2.4.

Table 2.4. Numbers of gears by type and by statistical region in 1986.

Gear Type

Numbers of Gears in the following Regions:

Tanga

Coast

Dar es Salaam

Lindi

Mtwara

Totals

Gill Nets

1,203

4,662

394

1,162

1,421

8,842

Shark Gill Nets

543

1,137

1,100

419

391

3,590

Beach Seines

35

292

491

113

82

1,013

Cast Nets

29

121

9

28

29

216

Hand Lines

1,206

2,456

4,134

4,896

786

13,478

Long Lines

107

---

---

---

14

121

Basket Traps

2,258

3,314

1,840

378

1,369

9,159

Fixed Traps

---

2,967

8

124

60

3,159

Unspecified Gears

459

---

---

---

---

459

Source: Statistics Section, Fisheries Division.

Catches: The catch by the artisanal fishermen during 1986 for each statistical region is shown below in Table 2.5.

Table 2.5. Catch weight by region in 1986.

Catch Weight (tonnes) for the following Regions:

Tanga

Coast

Dar es Salaam

Lindi

Mtwara

Total

4,402

10,745

10,656

14,051

5,327

45,181

Source: Statistics Section, Fisheries Division.

Note: Above values do not include sea shells (157 tonnes) and beche de mer (163 tonnes).

Catches per Unit Effort: These data are not being collected, however, a rough "order of magnitude" is provided by multiplying the numbers of boats (from Table 2.1) by an assumed 200 fishing days per year, and then dividing these into the catch weights. These estimates are shown in the following Table 2.6.

Table 2.6. Catch weights per unit effort by statistical region for 1986.

Catch Weights per unit Effort in the following Regions:

Tanga

Coast

Dar es Salaam

Lindi

Mtwara

Mean

(kg/boat day

26.2

66.6

81.2

102.9

37.9

61.2

Species Composition: The species composition of the catches as reported for 1986 are given below in Table 2.7.

Table 2.7. Catch weights by species group and statistical region for 1986.

Species Groups

Catch Weights (tonnes) in the following Regions:

Tanga

Coast

Dar es Salaam

Lindi

Mtwara

Totals

Demersal:

Emperors

580

1,952

2,938

2,886

604

8,960

Parrotfish

954

--

---

1,374

400

2,728

Rabbitfish

391

396

1,009

596

50

2,442

Silver Biddies

--

227

29

824

159

1,239

Sharks

233

255

51

354

234

1,127

Rays

315

252

64

222

182

1,035

Prawns/Lobster

60

561

31

83

1

736

Milkfish

--

--

--

42

549

591

Rock Cod

56

70

46

262

151

585

Queenfish

--

143

53

164

118

478

Catfish

32

319

13

32

76

472

Threadfin Breams

65

165

68

--

163

461

Octopus

34

53

76

7

196

366

Cobia

25

27

--

92

64

208

Mullets

46

50

21

--

--

117

Flatfish

--

60

10

--

11

81







21,626

Pelagic:

Sardines

205

1,174

1,350

1,293

773

4,795

Mackerels (small)

130

1,270

487

265

143

2,295

Halfbeaks

--

397

318

1,050

303

2,068

Jacks

144

168

268

523

229

1,332

Kingfish

75

238

202

168

120

803

Tunas

111

70

259

58

25

523

Billfish

71

--

--

81

46

198







12,014

Undetermined:

Others

877

2,899

3,363

3,674

729

11,542

Totals

4,402

10,745

10,656

14,051

5,327

45,181

Source: Statistics Section, Fisheries Division.

Areas of Fishing Grounds: The area along the mainland coast available to the artisanal fishery was estimated at over 12,000 km2 by Wijkstrom (1974); regionally divided into Tanga (2,200 km2), Coast including Dar es Salaam (8,100 km2), Lindi (1,550 km2) and Mtwara (310 km2). (A more comprehensive breakdown of the areas is given in FAO/IOP (1979), however, they are not reported here as they also include the grounds adjacent to Zanzibar and Pemba Islands).

Productivities per Unit Area: The areas quoted above were divided into the catch weights for 1986 presented earlier to provide the annual productivities per unit area Shown in Table 2.8.

Table 2.8. Productivities per unit area in 1986.

Productivities per Unit Area in the following Regions:

Tanga

Coast and Dar es Salaam

Lindi

Mtwara

Total

(tonnes/km2)

2.0

2.6

9.1

17.2

3.8

Note: The basis for the extremely high value for the Mtwara Region is presently unknown, and as such the value should be treated with suspicion.

Resource Assessments: 1. At the FAO/IOP Workshop held in the Seychelles in 1978 (see FAO/IOP, 1979, and Venema, 1984) it was determined that 5 tonnes/km2 was a useful value for the potential productivity of reef areas. This was used in obtaining the preliminary estimates of potential yields shown in Table 2.9. The contents of the table do not provide a full coverage for the mainland grounds, and includes grounds adjacent to Pemba.

Table 2.9. Demersal biomass and potential yield estimates.

Locations

Area
(km2)

Whether Trawlable

Density
(t/km2)

Biomass
(tonnes)

Potential Yield
(tonnes)

Pemba reef

888

No

?

?

(4,440)

Mafia mangrove

320

No

?

?

?

Mtwara-Lindi reef

1,295

No

?

?

6,500

Totals

2,503




(10.940)

Source: FAO/IOP (1979), Venema (1984).

Note: Potential yields for Pemba and Mtwara-Lindi reefs assumes potential productivity of 5 tonnes/km2.

2. The East African Marine Fisheries Organisation (EAMFRO) conducted exploratory and experimental fishing exercises with handlines, droplines and longlines during the period 1969 - 1976, mostly on the deep reefs at the entrances of the Mafia and Zanzibar Channels. Droplines were unable to catch fish at commercial rates because of the limited number of hooks (Morris, 1972), whilst longlines of traditional length were found to be difficult to set and retrieve in the strong currents and on the rougher substrates where catch rates were high. A short setline was evolved enabling a daily fishing effort from a suitably equipped boat of over 2,000 hooks (Tarbit, 1975 and 1976).

The species composition and catch rates obtained varied with depth and substrate as shown in the following Table 2.10. Generally the catch rates declined with depth, and varied from rates up to 50 kg/100 hooks on rough substrates, to less than 35 kg/100 hooks on coral rubble. The areas of rough substrate were found to be extensive only at the entrances of the Mafia Channel.

Table 2.10. Catch rates and species compositions from short setline sets off the Mafia shelf.

Depth Range (m)

45-60

60-75

75-120

120-250

Mean Catch Rates (kg/100 hooks)

33.2

20.9

19.5

15.9

Species Composition:


Depth 45-120 m


Preferred Substrate


Lethrinella wagiensis

(19.4)

Coral Rubble


Sharks

(17.3)

All Substrates


Lutjanus sanguineus

(15.9)

Rough Substrate


Lutjanus bohar

(9.7)

Rough Substrate


Epinephelus flavocaeruleus

(5.2)

Rough Substrate


Lutjanus rivulatus

(2.7)

Rough Substrate


Lethrinus nebulosus

(2.5)

Coral Rubble


Lutjanus sebae

(1.7)

Coral Rubble


Others

(25.5)

All Substrates


Depth 120-250 m




Pristipomoides spp

(31.9)

All Substrates


Sharks

(29.8)

All Substrates


Etelis spp

(10.6)

All Substrates


Epinephelus spp

(7.6)

All Substrates


Polysteganus spp

(6.9)

All Substrates


Others

(13.2)

All Substrates

Source: Tarbit (1984)

Note: Percentages by weight are given in brackets.

The estimated potential daily catch rates with an assumed effort of 2,000 hooks/day are shown in Table 2.11. The values were taken as indicating some potential for semi-industrial fishing at the northern and southern entrances to the Mafia Channel, where the initial catch rates may exceed 1 tonne per day.

An attempt to estimate the density of fish off the Mafia shelf, based on the probable range of attraction of baited hooks during a soak time of one hour, produced values of 15 tonnes/km2 for rough ground and 3.8 tonnes/km2 for substrates with less profile. This enabled an estimate of 3,036 tonnes for the biomass of species available to a line fishery in depths from 45 m to 100 m; composed of 1,520 tonnes from 100 km2 of rough ground and 1,516 tonnes from 400 km2 of smoother ground (Tarbit, 1975 and 1976). Having in mind that the catches would comprise mainly of medium to long-lived species, the (long term) potential yield was taken to represent some 10 to 15 percent of the biomass, or 300 to 450 tonnes annually.

Table 2.11. Catch weights, efforts and projected catch rates from short setline sets off the Tanzanian coast.

Latitude
(°S)

Effort

Catch

Catch Rates

Effort

Catch

Catch Rates

Effort

Catch

Catch Rates

(Depth 50-100m)

(Depth 100-200m)

(Depth 200-400m)

4

1,026

135

263

605

214

707

416

27

130

5

3,275

382

233

952

122

256

616

199

646

6

1,399

187

267

463

133

574

621

123

396

7

4,039

2,030

1,005

260

52

400

---

---

---

8

1,171

293

500

638

66

207

130

5

77

9

360

54

300

130

20

307

---

---

---

10

182

27

296

159

0

---

---

---

---

Source: Tarbit (1984)

Note: The effort unit is No. of hooks, the catches are in kg, and the projected catch rates are as kg/2,000 hooks/day.

3. THE LIGHT-ASSISTED FISHERY FOR SMALL PELAGIC SPECIES

Location: This type of fishing is presently undertaken in the nearshore waters immediately adjacent to Dar es Salaam (and Zanzibar town). This is in part the consequence of the large market for fresh fish at these places, and the support needed to ensure adequate supplies and maintenance of the boats. Previously in 1980 and 1981 a single purse seiner was successfully deployed in the waters adjacent to Tanga, see Nhwani (1981).

Boats: The number of traditional boats (mashuas) engaged in the fishery is unknown. In addition there are two larger steel boats (20 m and 12 m in length) of modern design, based in Dar es Salaam and operated by DARFISH. (Another six modern aluminium vessels of 9 m length are operated by the Zanzibar Fishing Corporation adjacent to Zanzibar town).

Gear: The fishing is undertaken at night, on fish concentrated around lighted dinghies, using purse seines and to a lesser extent scoop nets. The dimensions of the purse seines used from the traditional boats are 100 m to 150 m in length and about 15 m deep. The purse seine operated from the larger steel boat is 400 m long x 64 m deep, while that used from the smaller steel boat is 170 m long x 30 m deep. These latter boats are each supported by a net-boat (10 and 12 m length) and four 4 m lamp dinghies. In the case of all the boats, the hauling of the purse seine nets is done by hand.

Catches: Those from the traditional boats are recorded within the official statistics as part of the artisanal catches; see previous Section 2. In the absence of specific data, it seems probable that much of the 1,350 tonnes of sardines and 487 tonnes of (small) mackerels recorded for Dar es Salaam in 1986 (see Table 2.7) were landed from these boats. During that year an additional 702 tonnes of small pelagics were landed from the two DARFISH boats (Source: Statistics Section, Fisheries Division).

Catches per Unit Effort: Mean catches per unit effort achieved with the traditional boats operated from Dar es Salaam during 1986 are given in Table 3.1. Similar data are presented in respect to the large steel purse seiner (net 400 m x 64 m) in Table 3.2.

Species: The main species groups recorded during a study commenced in 1986 (in respect to the traditional boats operated from Dar es Salaam) are given in Table 3.3. Similar data for the large steel purse seiner are given in Table 3.4.

Areas of Fishing Grounds: The area of the fishing ground is not known. Mostly the fishing is done within 1 - 2 hours travel from Dar es Salaam.

Productivities per Unit Area: This is not known.

Resource Assessments: At the FAO/IOP Workshop held in the Seychelles in November 1978 (FAO/IOP, 1979) it was decided to adopt the very preliminary value of 20,000 tonnes/year as the potential yield of small pelagics (for all marine waters including those around Zanzibar and Pemba Islands), based on a biomass estimate of 43,000 tonnes (Venema, 1984). These estimates are based largely on the results of acoustic surveys done in 1976 and 1977 from the R/V Prof. Mesyatsev.

Subsequently in 1982-1983 three surveys were undertaken with the R/V Dr Fridtjof Nansen (Iversen et al, 1984), from which it was reported that the registrations (by the acoustic gear) were few and far apart. The estimates of biomass obtained for the investigated area were about 170,000 tonnes for the first survey and about 90-100,000 tonnes for the remaining two surveys. The inshore grounds were not surveyed, but from extrapolation it was suggested that the biomass there might be between about 15-40,000 tonnes. No estimates for the potential yield of the small pelagics was given.

(Note: The sum of the catches of sardines and (small) mackerels by the artisanal fishermen during 1986 is 7,090 tonnes (see Table 2.7), and when combined with the catches from the larger steel purse seine boats (702 tonnes in 1986) gives a total mainland catch of 7,792 tonnes of small pelagics in that year.)

Table 3.1. Catches per unit effort by traditional purse seiners during 1986.

Month

Catches per Unit Effort
(kg/night/boat)

Jan.

165

Feb.

215

Mar.

126

Apr.

99

Jun.

176

Oct.

81

Nov.

289

Dec.

240

Source: Nhwani and Makwaia (1988)

Table 3.2. Catches per unit effort by the large steel purse seiner in 1980, 1981 and 1985.

Month

Catches per Unit Effort
(kg/night/vessel)

Oct. 1980

891

Nov. 1980

944

Dec. 1980

412

Jan. 1981

447

Feb. 1981

238

Mar. 1981

356

Jul. 1985

456

Aug. 1985

633

Sep. 1985

859

Oct. 1985

1,266

Nov. 1985

977

Dec. 1985

1,105

Sources: Nhwani and Makwaia (1988), and Nhwani (1981b).

Note: During 1980 and 1981 the vessel was operated adjacent to Tanga, while in 1985 it was operated adjacent to Dar es Salaam.

Table 3.3. Main species groups in catches from traditional purse seine boats during 1986.

Family/Species

Percentage by Weight in Catches

Weighted Average

Jan.

Feb.

Mar.

Apr.

Jun.

Oct.

Nov.

Dec.

Clupeidae:

Sardinella gibbosa

22

23

87

55

--

26

81

7

41

S. albella

--

--

--

3

--

2

--

10

1

S. sirm

--

22

1

-

96

37

--

--

24

Herklotsichthys punctatus

38

1

2

7

--

--

--

--

5

Engraulidae:

Stolephorus indicus

--

--

1

--

--

--

--

--

--

S. heterolobus

1

42

--

22

--

--

--

--

--

Leiognathidae:

Gaza minuta

8

3

--

1

--

--

--

--

1

Leiognathus leuciscus

28

--

--

5

--

--

--

2

6

Scombridae:

Rastrelliger kanagurta

1

4

3

6

--

1

2

50

7

Carangidae:

Decapterus russelli

--

1

3

--

2

35

17

8

7

Others

2

3

4

--

--

--

--

--

1

Source: Nhwani and Makwaia (1986).

Table 3.4. Main families in catches from large steel purse seiner.

Family

Percentage by Weight in Catches

Tanga (1980/81)

Dar es Salaam (1985)

Clupeidae (Sardinellas)

73

25

Engraulidae (Anchovies)

1

46

Leiognathidae (Ponyfish)

3

7

Scombridae (Indian mackerel)

5

8

Carangidae (Scads & trevally)

13

5

Caesionidae (Fusiliers)

-

7

Others

5

2

Sources: Nhwani and Makwaia (1988), Nhwani (1981b).

4. THE SEMI-INDUSTRIAL SHRIMP TRAWL FISHERY

Location: Shallow water shrimp are exploited in depths to 20 m mainly adjacent to the estuaries of the Wami, Ruvu and Rufiji rivers. It is possible that exploitable concentrations also exist in the Kilwa/Lindi area and adjacent to the Ruvuma river. The resources on these latter grounds have not been surveyed.

Boats: During 1987 ten shrimp trawlers were licensed for this fishery. These included the two larger boats (32 m, 600 HP) owned by the Tanzania Fisheries Corporation (TAFICO), four licensed foreign boats (25 m, 380 HP) operated by the Kenya based Alpha Fishing Company, and four leased boats operated by local entrepreneurs.

Gear: The boats operated by TAFICO and the Alpha Fishing Company are twin-rigged. It is not known whether this was so for the other four boats. All trawling is confined to the daylight hours.

Catches: In 1987 the catch of shrimps from the TAFICO boats was 280 tonnes (est. whole weight equivalent to 149 tonnes of tails). An additional 274 tonnes (whole weight) is claimed to have been caught from the four licensed foreign trawlers. In the absence of data for the other four boats, it is assumed that two had a catch rate equivalent to the licensed foreign boats and the remaining two had no catch. On this basis the combined catch from the ten boats would be 691 tonnes. (Sources: TAFICO and the Fisheries Division).

A more detailed description of the catches landed (including fish by-catch) from the TAFICO boats is given in the following Table 4.1. In respect to these data the ratio of fish: shrimp (tail weight) is 3.2: 1, and hence equivalent to a ratio of fish: shrimp (whole weight) of about 1.93: 1. Assuming this latter ratio applied in respect of all the ten boats, then an estimate of the total fish by-catch is 1,334 (=691 x 1.93) tonnes. This would be an under-estimate as not all the by-catch is landed. The proportion discarded by the two Tafico boats is unknown.

Catches per Unit Effort: These data are not available. It is possible, however, to determine the "order of magnitude" with reference to the data given in Table 4.1. If the number of fishing days per month is assumed as 20 days, during each of which there is 10 hours trawling, then the estimate obtained for the average catch per unit effort is about 350 kg/day or 35 kg/hour (both values as tail weight). In respect to the months of highest productivity, the estimates are respectively about 500 kg/day and 50 kg/hour. These values relate to the use of the larger TAFICO boats twin rigged with trawls of about 25 m headline length, and are not applicable to the trawl fleet generally.

Table 4.1. Monthly landings for TAFICO boats during 1986.

Month

MAMA TAFICO

SADAANI

Fish
(kg)

Shrimp
(kg)

Fish
(kg)

Shrimp
(kg)

January

8,637

7,020

----

----

February

25,280

9,564

----

----

March

14,351

4,428

7,440

640

April

27.206

6,108

39,944

7.648

May

22,071

7,608

14,785

8,226

June

26,090

10,168

28,095

10,500

July

12,390

4,956

24,281

9,466

August

24,850

9,682

29,836

6,876

September

24,263

7,122

25,583

6,976

October

31,709

6.608

22,318

6,996

November

13,462

5,800

25,177

2.196

December

13,602

5,254

----

----

Totals

244,010

84,318

217,459

59,533

Means

20,334

7.026

24.162

6,615

Source: TAFICO

Note: The values given for shrimp are as tails.

Species Composition: The. species composition in the trawl catches from the TAFICO boats is given in the following Table 4.2.

Table 4.2. Species composition in the trawl catches.

Species

Percentage by Weight in the Catches

Penaeus indicus

60

Metapenaeus monoceros

20

Penaeus monodon

15

Penaeus semisulcatus

4

Penaeus japonicus

1

Source: TAFICO

Areas of Fishing Grounds: Estimates of the areas of the shrimp trawl grounds adjacent to the Rufiji delta, and the Wami and Ruvu rivers (near Bagamoyo) are given in FAO/IOP, 1979 (Appendix 7) as 2,880 km2 and 950 km2 respectively.

Productivities per Unit Area: Dividing the sum of the above areas into the estimate for the catch of shrimp from trawlers during 1987 (691 tonnes whole weight) gives a productivity of 180 kg/km2. Similarly an estimate of the productivity for the fish by-catch of 348 kg/km2 is obtained by dividing the sum of the areas into the estimated quantity of by-catch (1,334 tonnes).

Resource Assessment: Commercial trawling commenced from 1969, immediately following surveys during June through December 1968 using the R/V Sagama Maru of the Kanagawa Prefectural Government of Japan; for the results refer to Kanagawa Prefectural Government (1969). The catch and effort data for the commercial boats from 1969 through 1971 were subsequently used to assess the shrimp biomass and potential yields for the Rufiji and Bagamoyo grounds, as described briefly below from FAO/IOP, 1979 (Appendix 7).

The observed mean annual catch rates (relative to the F/V Taiku Maru) using twin-rigged 19 m (headline length) nets and trawling at a speed of 2 knots varied between 22.3 to 26.4 kg/hour on the Bagamoyo grounds and from 25.4 to 31.8 on the Rufiji grounds. (It is not clear in the original report whether these values refer to whole weight or tail weight, however, on the basis of a comparison with the catches per unit effort of the TAFICO boats given above, it is presumed here that the values refer to tail weight.)

The area "swept" by the trawl nets during an hour was estimated as 0.0851 km2 (assuming that the trawl nets were 100 percent efficient, and that the horizontal "width" of the trawls was 60 percent of the headline length). The areas of the two grounds were taken as 950 km2 and 2,880 km2 respectively.

Using the above values, the mean biomass were determined as 280 (= 25 kg/hour x 950 km2/0.0851 km2) tonnes for the Bagamoyo grounds and 940 for the Rufiji grounds. It was further assumed that the natural mortality coefficient was M=3 (annual), and hence that the potential yields were 1.5 times the biomass. On this basis the potential yields were estimated as 420 tonnes and 1,410 tonnes respectively; ie. 1,830 tonnes (tail weight) or roughly 3,050 tonnes (whole weight) in total.

(Note: 1. These potential yields refer only to that part of the stock being exploited from trawlers. In order to have some rough estimate of the potential yield to both the artisanal and trawler fishermen, it would be necessary to add the mean annual catch (for the period 1969 through 1971) by the artisanal fishermen. The artisanal catch of shrimp during these years ranged from 264 to 509 tonnes.

2. Additional but unknown quantities of shrimps exist outside the area surveyed; eg. adjacent to the mouths of the Pangani and Ruvuma Rivers.)

5. THE SEMI-INDUSTRIAL FISH TRAWL FISHERY

Location: The presently exploited grounds are within and adjacent to the Mafia Channel between about 20 - 70 m depth, and within the Zanzibar Channel adjacent to the mainland in about 10 - 30 m depth

Boats: During 1986 there were five trawlers engaged exclusively in the capture of fish. These included four owned by the Tanzania Fishing Corporation (TAFICO) and the M/V Mafunzo owned by the Mbegani Fisheries Development Centre. These boats range in length from about 20 - 25 m.

Gear: All the boats were rigged as stern trawlers. The dimensions of the trawl nets used are not known, except in the case of the M/V Mafunzo which was equipped with a Calypso trawl net having an effective horizontal "width" estimated as 25 m (from van Nierop, 1987a).

Catches: The official statistics give the fish catch from the six trawlers operated by TAFICO during 1986 as 613 tonnes. This includes 461 tonnes of by-catch from the two shrimp trawlers (see Table 4.1), in which case the landings from the other four boats totalled 152 tonnes. The catch during 1986 from the M/V Mafunzo is reported in van Nierop (1987b) as 262 tonnes. The combined catch from the trawlers used exclusively for catching fish was therefore 414 tonnes.

Catches per Unit Effort: These data are available only in respect to the use of the M/V Mafunzo. During 1986 the mean catch rates achieved on the grounds west of Mafia Island, and on the southern and northern halves of the grounds in the Zanzibar Channel were 814 kg/hour, 260 kg/hour and 459 kg/hour respectively; see van Nierop (1987b).

Species Composition: The main species groups contributing to the catches from the M/V Mafunzo during 1986 are given in the following Table 5.1.

Table 5.1. Percentages by weight of main species groups during 1986.

Species Groups

Percentage in the Catch by Weight

West of Mafia Island

Southern Zanzibar Channel

Northern Zanzibar Channel

Leiognathidae

35

21

30

Mullidae

15

20

28

Gerridae

7

15

6

Nemipteridae

3

7

2

Sphyraenidae

1

3

4

Carangidae

9

5

6

Shark sand Rays

13

5

2

Large Bony Fish

5

2

3

Others

12

22

19

Source: van Nierop (1987b).

Areas of Fishing Grounds: The areas of the fishing grounds exploited from the M/V Mafunzo have been determined as 237 km2 for west of Mafia Island, and 172 km2 and 305 km2 respectively for the southern and northern parts of the grounds in the Zanzibar Channel; see van Nierop (1987a & b). The areas of the grounds exploited from the TAFICO boats is not known.

Productivity per Unit Area: Dividing the catches from the M/V Mafunzo during 1986 by the above-mentioned areas provides estimates of the present productivities of 135 kg/km2 for west of Mafia Island, and 453 kg/km2 and 498 kg/km2 for the southern and northern Zanzibar Channel grounds respectively. It is likely that the boats owned by TAFICO are also operated on the grounds west of Mafia Island, but the quantities landed specifically from these grounds are not known.

Resource Assessments: 1. The results of three surveys carried out in Tanzanian waters from the R/V Prof. Mesyatsev are given in Birkett (1978), VNIRO (1978) and Burczynski (1976). Apart from a limited amount of recording with acoustic equipment, there were 77 trawl hauls for a catch of 56 tonnes of fish and Crustacea. The areas surveyed were north of Mafia Island to north of Pemba Island (including adjacent to both Pemba and Zanzibar Islands). The data from these surveys, along with other information, were used at the FAO/IOP Workshop held in the Seychelles to estimate biomass and potential yield values. The values for those resources on grounds believed suitable for trawling are shown in Table 5.2 (from Venema, 1984). The proportion of the potential yield accessible to fishermen based from the mainland is not known.

Table 5.2. Biomass and potential yield estimates.

Location

Area
(km2)

Whether Trawlable

Density
(t/km2)

Biomass
(tonnes)

Potential Yield
(tonnes)

Pemba other

1,312

Yes

1.65

2,165

540

Zanzibar <200 m

6,192

Yes

1.65

10,220

2,500

Zanzibar >200 m

411

Yes

2.7

1,110

280

Mafia other

7,936

Yes

(0.45)

3,532

900

Mtwara-Lindi other

565

?

?

?

100

Totals

16,416




(4,320)

Source: FAO/IOP (1979), Venema (1984).

Note: Densities on trawlable ground based on Prof. Mesyatsev survey results (except for Mafia other). Potential yield for Mtwara-Lindi other assumes potential productivity of 0.5 tonnes/km2.

2. During 1982-1983 three surveys were undertaken with the R/V Dr Fridtjof Nansen (Iversen et al, 1984). The distribution and abundance of the fish in depths from 10 m to about 500 m were investigated acoustically and by trawling. The biomass from the trawl results in the investigated area north of 9°S was estimated as about 50,000 tonnes (includes areas adjacent to Zanzibar and Pemba Islands). The catch rates were observed to be greatest in depths less than 50 m. The highest mean was 959 kg/hr in 20 - 50 m depth near Mafia during the first survey. Otherwise in this depth range the means were between about 150 and 450 kg/hr.

The area of seabed covered during an hour of trawling was about 0.1 km2 (when assuming a speed of 5 km/hr), in which case when assuming the trawl net to be 100 percent efficient, provides 1.5 to 4.5 tonnes/km2 as an indication of the density range in these depths. Applying an overall mean of 3 tonnes/km2 to the about 8,350 km2 given in Iversen et al (1984) for the area between < 20 m and 50 m depth provides an estimate for the biomass of 25,000 tonnes.

On the basis of the results from the trawling survey, a rather large portion of the unrealised potential would be ponyfish (Fam. Leiognathidae), which have a low market value. These accounted for some 20 - 40 percent of the catches (from waters less than 50 m). The predominant pelagic species were Sardinella sirm, S. gibbosa, S. albella, Decapterus russelli, and D. macrosoma. These were judged as not particularly abundant as the acoustic registrations were few and far apart. The catch rates of shrimp were rather poor; the biggest catch (130 kg/hr) was made east of Dar es Salaam during a haul at 325 m depth. The deep sea lobster and squid catches were also very small.

3. The catch and effort data in respect to the commercial cruises with the M/V Mafunzo during each of 1985 and 1986 were used for the estimation of biomass and potential yields; see van Nierop, 1987a & b. The trawling was undertaken in the Zanzibar Channel adjacent to the mainland in the depth range of 5 to 15 m, and in the Mafia Channel at depths between 30 and 40 m.

In respect of each haul, the area of seabed travelled over was determined as the product of the distance travelled by the "horizontal" width of the trawl net. These were then summed for each month and divided into the sum of the catches for the month. On the assumption that the trawl net was 100 percent efficient (ie. all the fish in the path of the net were caught) the catches per unit area travelled were taken as estimates of the mean fish biomass per unit area (of the fishing grounds). The products of these latter values and the respective areas of the fishing grounds provided the estimates of biomass shown in Table 5.3.

In estimating the potential yields from the biomass values, the natural mortality coefficient (annual) was taken as M = 0.7 in the equation Y = M x B' x exp[(C/ (M x B')) - 1] where B' is the estimated biomass and C is the recorded catch; from Garcia et al (1987). The estimates so obtained are given in the following Table 5.4. The associated estimates for the potential yield per unit area are about 1.1 tonnes/km2 for west of Mafia Island and about 0.8 tonnes/km2 for the Zanzibar Channel.

Table 5.3. Estimates of biomass in each month for the grounds exploited with the M/V Mafunzo.

Month

Estimates of Biomass (tonnes)

West of Mafia Island 1985

Zanzibar Channel

West of Mafia Island 1986

Zanzibar Channel

January

---

1,223

---

672

February

---

---

653

300

March

---

1.486

---

536

April

---

1,348

---

2,631

May

---

---

---

2,184

June

1,075

883

---

1,844

July

---

---

---

910

August

---

---

---

1.167

September

---

1,131

---

1,464

October

775

606

---

1,626

November

930

1,460

---

1,398

December

---

---

1,341

1,085

Mean

894

1,164

933

1,253

Source: van Nierop, 1987a & b.

Table 5.4. Estimates of potential yield for the grounds exploited from the M/V Mafunzo.

Item

West of Mafia Island

Zanzibar Channel 1985

West of Mafia Island

Zanzibar Channel 1986

Catch (tonnes)

74

158

32

230

Potential Yield (tonnes)

259

364

252

419

Source: van Nierop, 1987a & b.

6. THE OTHER FISHERIES RESOURCES

Shallow-water Lobster

Bwathondi (1973) has identified Panulirus ornatus, P. longipes, P. versicolor, P. homarus and P. penicillatus as occurring in Tanzanian waters. The first two are the most abundant, contributing more than 80 percent to the landings (Bwathondi, 1980). The most common fishing method involves divers with a hand-held net and an octopus; the latter is used to flush the lobster so that they can be scooped up by the net. Dealing with the period 1966 through 1975, Bwathondi and Mwaya (1984) refer to annual catches reaching 80 tonnes. More recent data are not available as the catches of lobster are not identified separately within the official statistics. No assessments of the lobster biomass or potential yields have been undertaken.

Deep-water Shrimp

Deep-water shrimp (possibly Heterocarpus sp.) were caught during surveys with the R/V Dr Fridtjof Nansen and the R/V Prof. Mesyatsev; see Iversen et al (1984), Birkett (1978) and VNIRO (1978). The catch rates achieved were generally low, although a haul in 325 m depth east of Dar es Salaam with the first mentioned vessel gave a catch rate of 130 kg/hour. Insufficient data have been collected to allow any assessment of potential yields. There has been no commercial fishery on this resource.

Deep-water Lobster

Lobsters identified as Linuparus somniosus and Metanephrops andamanicus were regularly caught in depths of 250 to 320 m at the southern end of the Zanzibar Channel during exploratory surveys with the R/V Prof. Mesyatsev; see Birkett (1978), VNIRO (1978) and Burczynski (1976). The catch rates ranged up to 50 kg/hour. Insufficient data were collected to allow an assessment of the potential yields. These lobsters have not been subject to exploitation.

7. BIBLIOGRAPHY

Birkett, L. (1978) - Western Indian Ocean Fishery Resources Survey. Report on the cruises of R/V Professor Mesyatsev, December 1975 - June 1976/July 1977. Tech. Rep. Indian Ocean Programme., (21): 97 p.

Burczynski, J. (1976) - Echo survey along the East Africa coast from Mombasa to Laurenzo Marques by R/V Professor Mesyatsev in January/February 1976. FAO Fisheries Travel Report and Aide Memoire, (1162) Suppl. 1: 28 p.

Bwathondi, P.O.J. (1973) - The biology and fishery potential of palinurid lobsters in Tanzania. M.Sc. Thesis University of Dar es Salaam. 139 p.

Bwathondi, P.O.J. (1980) - The spiny lobster fishery in Tanzania. Proc. Symp. on the coastal and marine environment of the Red Sea, Gulf of Aden and Tropical Western Indian Ocean. Khartoum 9-14 Jan. 1980. Vol. II: 281-292.

Bwathondi, P.O.J. and G. Mwaya, (1984) - The fishery of Crustacea and molluscs in Tanzania. The Proceedings of the Norad-Tanzania Seminar to Review the Marine Fish Stocks in Tanzania (Mbegani, Tanzania, 6-8 March 1984). Tanzania Fisheries Research Institute, Dar es Salaam; and Norwegian Agency for International Development, Bergen: 19-28.

Carrara, G. (1986) - Artisanal Marine Fishery Statistics Frame Survey Tanzania Mainland RAF/79/065/WP/38/86/E: 38 p.

Dorsey, K.T. (1969) - Report on the prawn fisheries of the Rufiji delta with particular reference to possible changes resulting from modification to the environment by the proposed dam at Stiegler's Gorge. FAO/TCP/URT/8806(i), Project Technical Paper No. 2, FAO, Rome: 53 p.

FAO/IOP (1979) - Report of the Workshop on the fishery resources of the Western Indian Ocean South of the Equator. Mahe, Seychelles, 23 October - 4 November 1978. Dev. Rep. Indian ocean programme., (45): 102 p.

Garcia, S., P. Sparre and J. Csirke. (1987) - A note on rough estimators of fishery resources potential. FISHBYTE, Vol. 5 (2): 11-16.

Iversen, S.A., S. Myklevoll, K. Lwiza and J. Yonazi, (1984) - Tanzanian marine fish resources in the depth region 10-500 m investigated by R/V "Dr Fridtjof Nansen". The Proceedings of the Norad-Tanzania Seminar to Review the Marine Fish Stocks in Tanzania (Mbegani, Tanzania, 6-8 March 1984). Tanzania Fisheries Research Institute, Dar es Salaam; and Norwegian Agency for International Development, Bergen: 45-83.

Kanagawa Prefectural Government (1969) - The report of survey on the prawn fishing grounds along the coast of Tanzania, Japan: 120 p.

Morris, R.E., (1972) - Scientific Investigations - Demersal Fish. EAMFRO Annual Report 1972: 17-19.

Nhwani, L.B., (1981) - Purse seine fishery of Zanzibar. Tanzania National Scientific Research Council; Dar es Salaam. Bulletin of Research and Information. 1 (2): 33-39.

Nhwani, L.B., (1981b) - The dagaa fishery of Tanga. The Tanzania Society, Dar es Salaam. Tanzania Notes and Records No. 86/87: 29: 33.

Nhwani, L.B. and E.D. Makwaia, (1986) - The biology and fishery of small pelagic fishes in the coastal waters of Tanzania. Research Project Progress Report. Tanzania Fisheries Research Institute, Dar es Salaam: 6 p.

Nhwani, L.B. and E.D. Makwaia, (1988) - Aspects of the fishery and biology of small pelagic fishes of Dar es Salaam, Tanzania. FISHBYTE (In press).

Tarbit, J. (1975) - Some aspects of the demersal fisheries of the CINCWIO region. IOC/UNESCO/EAC Seminar Co-operative Investigations in the North and Central Western Indian Ocean. Nairobi 1976: 15 p.

Tarbit, J. (1974-76) - Demersal Fisheries Research: Terminal Report 1974-76. EAMFRO Annual Report 1976: 28 p.

Tarbit, J. (1984) - Inshore Fisheries of the Tanzanian Coast. The Proceedings of the Norad-Tanzania Seminar to Review the Marine Fish Stocks in Tanzania (Mbegani, Tanzania, 6-8 March 1984). Tanzania Fisheries Research Institute, Dar es Salaam; and Norwegian Agency for International Development, Bergen: 29-44.

Van Nierop, M., (1987a) - Analysis of trawl data of the training vessel "Mafunzo" for 1985. UNDP/FAO RAF/79/065/WP/33/87/E: 33 p.

Van Nierop, M., (1987b) - Analysis of trawl data of the training vessel "Mafunzo" for 1986. UNDP/FAO RAF/79/065/WP/36/87/E: 36 p.

Venema, S.C. (1984) - Resources surveys other than those by Dr. Fridtjof Nansen. The Proceedings of the Norad-Tanzania Seminar to Review the Marine Fish Stocks in Tanzania (Mbegani, Tanzania, 6-8 March 1984). Tanzania Fisheries Research Institute, Dar es Salaam; and Norwegian Agency for International Development, Bergen: 85-96.

VNIRO, (1978) - Western Indian Ocean Fisheries Resources Survey. FAO/UNDP/USSR Cooperative project Dev. Rep. Indian Ocean Programme, (46): 130 p. (restricted distribution).

Wijkstrom, U., (1974) - Processing and Marketing Marine Fish - Possible guidelines for the 1975-79 period. Proc. International Conference on Marine Resources Development in East Africa. Univ. Dar es Salaam. 55-67.


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