Three commercially important fish species, Lates niloticus (L.), Rastrineobola argentea (Pellegrin) and Oreochromis niloticus (L.) that are fished by artisanal fishermen of Lake Victoria, Tanzania part, were studied in Kagera, Mwanza and Mara beaches from October 1997 to July 1999. Catches, effort, exploitation and stock structure were investigated. Beaches for sampling were selected based on importance for landing the above named fish species. The number of boats found on beach that day, the number that lay idle and their means of propulsion were recorded. As many boats as possible were sampled for gear type and gear size. The catches were sorted into species and measured. Variation in the species and size composition of landings was observed between regions, between months and between gears used. The implications of the findings to management are discussed

Nsinda, P.E.

English

Aquatic Commons

Stock assessment of Lates niloticus (L,), Oreochromis niloticus (L.)
and Rastrineobola argentea (ie11egrin) using fisheries-dependent data
from Tanzania waters of LakeVictoria
P.E. NS1NDA Tanzania Fisheries Research Institute, .0. Box 475, Mwanza,
Tanzania
Abstract: Three commercially important fish species, Lates niloticus (L.)
Rastrineobola argentea (Pellegrin) and Oreochrornis niloticus (L.) that are fished by
artisanal fishermen of Lake Victoria, Tanzania part were studied in Kagera, Mwanza
and Mara beaches from October 1997 to July 1999. Catches, effort, exploitation and
stock structure were investigated. Beaches for sampling were selected based on
ìmportance for landing the above named fish species. The number of boats found on
beach that day, the number that lay idle and their means of propulsion were recorded.
As many boats as possible were sampled for gear type and gear size. The catches
were sorted into species and measured. Variation in the species and size composition
of landings was observed between regions, between months and between gears used.
The implications of the findings to management are discussed
Introduction
The best approach to managing inland fisheries depends on the availability of some
measure of stock abundance or associated population parameters on which to
formulate the decision-making process or evaluate the impact of a particular
management activity (Cowx 1996). Stock assessment is considered necessary to
gauge the effects of activities such as the impact of overfishing, a pollution event,
land drainage improvement works, river regulation, habitat restoration, stocking,
introduction of new species or to merely respond to requests for advice on the
management of waters.
Traditionally two methods of stock assessment are used, based on fishery dependent
and fishery independent data. Catch assessment surveys, based on the former, are
complimentary to experimental, fishery independent surveys as a method of collecting
data to assess the impact of fishing on the stock structure.
In Tanzania, fishery dependent data are collected by the Fisheries Department for
statistical reasons. However, the strategy lacks rigour and the data exhibit vagaries
which make their use questionable. Irrespective the use of fishery dependent methods
is extremely valuable for identifying exploitation patterns and catch characteristics
Consequently, a pilot study was set up to investigate the exploitation of the three main
commercial species in the Tanzanian sector of Lake Victoria (Nsinda & Mrosso
1999). This study was used to determine the variability in fishery effort and catch
composition between beaches. Four beaches were selected in the three main zones of
Lake Victoria, i.e. Mara, Mwanza and Kagera. These were indicative of the zone and
the main types of beaches in terms of species lauded. Two well defined
characteristics were elucidated from this study. The species and size of fish landed
usually reflects the location of the fishery and the gear used by boats operating out of
that beach. Also landings are usually dominated by one key species. It was also
possible to discriminate cohort growth patterns over time, especially for Lates
niloticus (L.) and Rastrineobola argentea (Pellegrin).
108
As a result of this pilot scale project, a more comprehensive catch assessment
programme was established for the Tanzanian sector of Lake Victoria. This was
justified on the quality of data provided at a relatively cheap cost, and to establish a
long term data collection programme which will provide valuable insight into the
exploitation trends in the fishery. The paper provides the output for the first few
months of the revised programme and is the forerunner to annual reports on the
fisheiy.
Materials and methods
In the pilot survey, three beaches, Kayenze, Choie, and Igombe, were selected for
sampling on the basis of accessibility and their importance for L. niloticus,
Oreochrornis niloticus (L.) and R, argentea fisheries (Fig. 1).
Four trips were made to each beach in October 1997, and February, June and
September 1998. The number of boats on beach that day, the number that lay idle and
the number that went out fishing were recorded. For each beach as many active boats
as possible were sampled to determine the type of gear, means of propulsion, number
and mesh size of nets. The catch of each boat was sorted into species and recorded,
and correlated against the principal gear used. Length (nearest cm below) and weight
(nearest g) measurements of the entire sample or sub sample of L. niloticus and O.
niloticus were taken from each sampled boat. For R. argentea, a sample of 0.125 kg
was taken and preserved in 5% formalin for analysis in the laboratory.
The revised catch assessment surveys were conducted in three areas (Fig. 1): A -
Mwanza to Bunda; B - Mara and C - Kagera. between April and July 1999. Five
beaches in the Kagera zone , and seven each from the Mwanza and Mara zones were
selected on the basis of their importance either for L. niloticus, O. niloticus or R.
argentea fisheries. Due to resource limitations, the beaches were surveyed on a rota
basis, i.e. all the beaches in a particular area were sampled on a three-monthly
rotation. Sampling on the beaches used the same methods as in the pilot survey.
Results
Catch by region
Catch rates varied considerably between months, and hence regions. The poorest
catches were in May 1997 in the Mwanza region, whilst consistently higher catches
were recorded for the other two regions. (Table 1). Once a full year
collected these will be used to estimated the total catch from the fishery based on
raising factors collected from the recently completed frame survey.
The catch rates for the three species also varied between regions and months (Table
1), possibly reflecting seasonal/regional changes in the fishery. This will be examined
more fully in the next reporting period.
The modal length for O. niloticus caught by 1-9' mesh sized gilinets and handlines
and longlines with hook size 7 to 12, varied between months, but during the latter part
of the programme, when more consistent data were collected, there was a modal
progression from 29-30 cm in April to 36-37,5 cm TL in July (Fig. 2). A considerably
109
greaternumber of smaller fish were caught in June 1998 and July 1999, presumably
because juveniles were recruiting into the fishery. This will again be assessed during
the later phases of the project. This influx of smaller fish was not, however, due to
variation in the gears used between surveys (see later).
Lates niloticus, caught by gilinet with mesh sizes ranging from 4.5' to 7 and longline
hook size numbers 7 to 13, showed evidence of recruitment, i.e. a decline in the
median size of fish landed, in both June 1998 and June 1999, which is consistent with
the growth of the smaller fish caught during the February survey (Fig. 3).
Rastrineobola argentea caught by seine and lift nets exhibited a modal progression in
size with recruitment to the fishery possibly around the July to September period each
year (Fig. 4), when a greater proportion of smaller fish and a bimodal distribution of
the catch was observed. However, the latter could be due to two mesh sizes of lift net
(5 and 10 mm) being operated in the fishery at this time.
Other species encountered in catches on the surveyed beaches included Bagrus
docrnak , Glanas ganiepinus (Burchell), Protopterus aethiopicus Heckel,
Labeo victorianus Boulenger, Synodontis afrofischeri Hilgendorf, Synodontis
victoriae Boulenger, Schuhe intermedias , Brycinus jacksonii (Boulenger),
Mormyrus kanuwne and hapiochromine cichlids. The greatest fish species
diversity was recorded in the Mwanza and Mara regions (all species), whilst only the
first three species, plus hapiochromines, were recorded ìn the Kagera zone.
Catch by gears
The catch distribution of the various fish species by different gears exhibits some
marked difference. Gilinetting, the predominant gear used for O. niloticus, exhibited
two modal distributions at around 19 cm and 32 cm TL, respectively. This almost
certainly reflects the use of nets from 1.
of nets employed were >5
which were >30 cm TL. The encircling gilinet catch distribution was distributed
around a mode of 37 cm TL with few fish caught <29 cm TL. This was because the
encircling nets used were 5-6
By contrast, the majority of O. niioticus landing from long Jining and hand lining
were <130 cm and as small as 12 cm TL. The modal length was around 21 cm,
considerably lower than fish caught by netting. This is probably because small hook
sizes (longline 7 and 9; hand line 10-12) are used in the fishery.
The size composition of Nile perch catches by gillnetting and long lining similarly
exhibited marked differences. The nican length of capture by gilinetting was 46 cm
TL but 55 cm by long lining. Few fish <30 cm were landed, but long lining caught a
considerably larger proportion of fish greater than 80 cm TL.
Landing from the two main gears used for capture of Rastrineobola showed marked
differences. A bimodal distribution (modes at 45 and 60 mm TL) was found for the
lift net catches hut a negatively skewed distribution with a mode around 55 mm TL
was found for seine catches. The Rastrineohola data in July 1999 exhibited a
measuring bias by the recorders, which has since been addressed. The recorders had a
110
tendency to measure to the nearest 5 mía, hence the peaks at five mm intervals.
However, the underlying trend in size at landing is still evident.
s cuss io n
The preliminary assessment of the commercial landings from the Tanzanian fishery
exhibit sorne prominent characteristics which will have a bearing on the management
of the fishery in the future.
Active fishing for O. niloticus using gilinets is illegal in the lake but is commonly
practiced. However, size distribution of the catch from the netting operation was of
larger fish, thus the damage done by this method is largely one of increasing capture
efficiency and not necessarily exploiting smaller individLials. It must be borne in
mind this scenario may change if catches decline and the fishermen reduce the mesh
sizes employed.
Perhaps of more concern is the high proportion of smaller fish caught by the hook and
line methods, especially hand lining. The rnaority of these fish are below the age at
first maturity thi Nile tilapia which is >30 cm TL (Ojuok 1999). Managing the hook
and line fishery is considered to be difficult, mainly because it is widely practised on a
subsistence basis. However, effort must be made to prevent expansion of the fishery
and ban hook and line fishing in known nursery areas. Importantly, the majority of
Nile tilapia landed by gill netting is >30 cm, and efforts must be made to ensure the
mesh sizes do not reduce if catches decline.
Nile perch catch statistics again exhibited variation in size distribution of landed fish
between gears used; long lining tended to catch more larger fish. The major cause of
concern is that the majority of fish caught by gilinetting was below 50 ein TL. The
majority of these fish are below the size of maturity (55 cm in Tanzanian waters,
Mkumbo & Ezekeil 1999), suggesting that overfishing is prevalent in the fishery and
action must he taken to reduce the problem.
The size distribution of the exploited stock of Rastrineobola varied between gears,
probably as a result of seasonal variability in the monitoring. It is possible that the
records covered recruitment of cohorts of juveniles into the fishery, especially in
October 1997 and July 1999 sampling which was more prevalent in the lift net
catches, or differences in mesh size of nets used (i.e. 5 and 10 mm mesh) may
influence the size distribution. Irrespective, the size of fish exploited by the fisheries
is mainly above 45 cm, the size at maturity of Rastrineobola in Uganclan waters in
Lake Victoria (Wandera 1999).
The output of the catch assessment studies on Tanzanian waters of Lake Victoria
shows the importance of integrating fishery dependent data with biological
information. It also justifies the need to expand on this aspect of stock assessment in
all three countries. This is being developed in Uganda (PhD studies of L. Muhoozi),
and existing assessment in Tanzania and Kenya will be intensified to monitor the
exploitation patterns of the commercial fisheries. In addition, once more data become
available, further analysis will be carried out to determine the exploitation
characteristics of the three major fisheries using stock assessment tools. Much of this
analysis will be carried out in time for the next FIDAWOG workshop.
Acknowledgements
I thank the Lake Victoria Fisheries Research Project (Ref: ACP-RPR 227) for
providing the funds for the surveys. I would also like to thank TAFIRI for the chance
they have given me in participating on the project.
Reference
Cowx I.G. (1996) The integration of fish stock assessment into fisheries
management. In: 1G. Cowx (ed.) Stock Assessment in Inland Fisheries.
Oxford: Fishing News Books, Blackwell Science, pp. 495-507.
Marten G.G. (1979) Impact of fishing on the inshore fishery of Lake Victoria (East
Africa). Journal of the Fisheries Research Board of Canada 36, 891-900.
Mkurnbo O.C. & Ezekiel (1999) Distribution and abundance of fish stocks in Lake
Victoria, Tanzania. This volume.
Nsinda PE & Mrosso H.D.J. (1999) Stock assessment of Lates niloticus (L.),
Oreochromis niloticus (L.) and Rastrineobola argentea (Pellegrin) using
fisheries-dependent data from Tanzania waters of Lake Victoria. In: D.
Tweddle & 1.0. Cowx (eds) Proceedings of the Third FIDAWOG Workshop
of the Lake Victoria Fisheries Research Project, Jinja, February 1999.
Ojuok J.E. (1999) Reproductive biology of Oreochromis niloticus (L.) in the Nyanza
Gulf of Lake Victoria. This volume
Wandern S.B. (1999) Reproductive biology of Rastrineobola argentea (Pellegrin) in
the northern waters of Lake Victoria. This volume
112
l'abée J. Estimated daily landings ftr (kg c1a») i/ic three nia jor comnniercia! f/v/i
species at monitored landing beaches be!u'een October 1997 and Jim/v 1999
Effort
(no of boats)
No. of boats CPUE
sampled (kg.boaf')
Estimated catch
(kg day')
Oreoch ramis niloticus
October1997 24 5,7 143.3
February 1998 9 2.5 16.3
June 1998 22 3.1 40.3
September 1998 18 2.7 70.4
April 1999 30 13 9.5 282.9
May 1999 10 2 20.0 193.0
June 1999 16 12 4.0 59.0
July 1999 16 8 37.0 1248.0
Rastrineobola aigeíitea
October 1997 107 125.0 10500.0
February 1998 76 160.8 12584.5
June 1998 36 20.3 1458.0
September 1998 34 141.1 9593.1
April 1999 34 10 89.1 3029.9
May 1999 5 5 33.0 166.0
June 1999 14 8 172.0 2541.0
July 1999 12 7 187.0 2242.0
Lates niloticus
October 1997 96 41.5 6825.3
February 1998 42 101.2 3333.7
June 1998 46 55.6 2559.3
September 1998 83 64.5 5671.3
April 1999 41 10 40.7 1668.7
May 1999 89 9 134.0 187.0
June 1999 26 7 98.0 2479.0
July 1999 24 10 56.0 1278.0
- 2°S
-
305
Bukoba
32 E
a,
InternaUonal bouncla,y
Ftg. L Map of'thc Tanzanian waters of Lake Victoria.
UGANDA
IMoe9a Bay
TANZANIA
C
lo
Ukerewe i://
i51 Korm I.
Ernie Pasha Gull Mwanza
Mwarrza Gulf
114
330E
B
a
Speke Gulf
34°E
R
L AIbO't
Musoma
D.
o
kilometres
9
2,0 49 6,0 82 100
10 -
5-
o
15
10
5-
o
15
10 -
5-
o
20
15
10
5
o
10
5
o
October 1997
June 1998
September 1998
Aprii 1999
o
115
length class (cm)
Fig. 2. Length distribution of Oreochromis niloticus caught by commercial
fisheries in Tanzania.
15 - February 1998
8-
6-
4-
2-
o
8
6
4-
2-
8-
5-
4-
3-
2-
1-
o
116
February 1998
September 1998
, -r - !i'IY r. r
Mey 1999
T'
10 20 30 40 50 60 70 80 90 100 110 120
Fig. 3. Length distribution of Lates niloticus caught by commercial fisheries in
Tanzania.
15
1
October 1997
u
6-
4-
2-
April 1999
15 -
io
5-
o
20 -
15 -
10
5-
0
5
O
15
10 -
5-
O
15
io -
4
2
o
10 20
July 1999
October 1997
February 1999
June 1998
10 September 1 998
April 1999
May 1999
30 40 50 50 70 80
Fig. 4. Length distribution of Rasirineobola argentea caught by commercial
fisheries in Tanzania.
117


Stock assessment of Lates niloticus (L.), Oreochromis niloticus (L.) and Rastrineobola argentea (Pellegrin) using fisheries-dependent data from Tanzania waters of Lake Victoria

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Stock assessment of Lates niloticus (L.), Oreochromis niloticus (L.) and Rastrineobola argentea (Pellegrin) using fisheries-dependent data from Tanzania waters of Lake Victoria

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