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Distribution and seasonal movement of pelagic fish in southern Lake Tanganyika
Fisheries Research 41 (1999) 63±71
Distribution and seasonal movement of pelagic ®sh in southern Lake Tanganyika Harris Phiria,*, Kunio Shirakiharab b
a Lake Tanganyika Research Unit, Department of Fisheries, PO Box 55, Mpulungu, Zambia Faculty of Bioresources, Laboratory of Fish Population Dynamics, Mie University, 1515 Kamihama, Tsu 514, Japan
Received 18 February 1997; accepted 16 November 1998
Abstract The distribution and seasonal movement of two planktivorous clupeids, Stolothrissa tanganicae and Limnothrissa miodon, and one predatory centropomid ®sh, Lates stappersi in the southern end (Zambian waters) of Lake Tanganyika were studied using purse seine ®shery statistics and species ratio data collected between 1984 and 1992. L. stappersi is the most abundant offshore among the three species and moves northwards into waters of other countries during the dry season. L. miodon occupies the inshore regions of the lake, and does not appear to move offshore beyond Zambian waters. S. tanganicae is more dominant in the offshore waters than L. miodon and there is a strong possibility that it moves out of the Zambian waters. The presence or absence of the predator, L. stappersi, seems to affect the distribution of the two prey clupeid species. Speciesbased management of either clupeid, which are captured together, may not be meaningful but an assumption of closeequivalent population in the wet season may be useful in implementing seasonal regulations. International management policies may have to be introduced for L. stappersi. # 1999 Elsevier Science B.V. All rights reserved. Keywords: Pelagic ®sh; Fishery management; Lake Tanganyika; Stolothrissa tanganicae; Limnothrissa miodon; Lates stappersi
1. Introduction Lake Tanganyika, the oldest lake in the African rift valley, contains many endemic ®sh species. Two endemic species of sardine-like planktivorous clupeids, Stolothrissa tanganicae Regan, 1917 and Limnothrissa miodon (Boulenger, 1906) and one centropomid species, Lates (Luciolates) stappersi (Boulenger, 1914) are distributed over the pelagic waters of the lake (Poll, 1953). These three species, captured together by purse seine and other ®shing *Corresponding author. Tel.: +81-59-231-9673; fax: +81-59231-9538; e-mail: [email protected]
nets, have been exploited as the most important ®sheries resources by all the riparian countries (Fig. 1(a)). Each species has close ecological interaction with the others (Coulter, 1991). S. tanganicae is preyed upon by L. stappersi and large L. miodon. S. tanganicae, L. miodon, and juvenile L. stappersi can utilize common food resources (Pearce, 1992). Commercial exploitation in southern Lake Tanganyika started in 1959 (Coulter, 1970), when the purse seine was ®rst used. The ®shery expanded signi®cantly in the 1980s and the number of purse seiners had increased from 11 at the end of 1984 to 19 in 1992. In the mid 1970s, the Zambian government introduced
0165-7836/99/$ ± see front matter # 1999 Elsevier Science B.V. All rights reserved. PII: S0165-7836(99)00008-9
64
H. Phiri, K. Shirakihara / Fisheries Research 41 (1999) 63±71
Fig. 1. (a) Map of Lake Tanganyika. The shaded area shows the Zambian part of the lake. (b) An enlargement of the Zambian part of the lake with isobaths in metres. The area used to calculate the effectiveness of effort is surrounded by the thin dotted line. (c) Rectangles used for specifying fishing area and their stratification based on distance from the shore shown by the bold lines.
a regulation of the seine net mesh size as a way to manage the ®shery. This strategy seems not to have been as effective as desired. After 1985, the catch of the clupeids had a decreasing trend while that of L. stappersi have more or less remained stable (Pearce, 1995). Although the population ecology of these pelagic ®shes has been studied (Mulimbwa and Shirakihara, 1994; Kimura, 1995; Tshibangu and Kinoshita, 1995), most of the studies have been conducted under the implicit assumption of closed populations in the study areas due to lack of proof of ®sh movement (Coulter, 1970; Roest, 1978; Pearce, 1985; Shirakihara et al., 1992; Chapman and van Well, 1978), in spite of there
being some signs of migration over the lake (Matthes, 1967). Although information on distribution and movement is essential for making rational ®shery management schemes, to date descriptions have been qualitative (Coulter, 1991; Pearce, 1992, 1995). One exception is the acoustic studies (e.g., Johannesson, 1974), but these did not give information by species. Another is catch statistic analysis. Shirakihara et al. (1992) estimated an index of population density from catch statistics at landing places, which are available in the northern part of the lake (Zaire and Burundi waters). Uncertainty appears in that relationship between landing places and actual ®shing areas is unclear.
H. Phiri, K. Shirakihara / Fisheries Research 41 (1999) 63±71
Using the data on Zambian purse seine ®shery, which include information on the ®shing area, the present study was conducted to clarify the distribution and seasonal movement of the three pelagic species in the southern part of the lake with special attention to their emigration. Climate can be a factor affecting ®sh distribution. The area surrounding the lake experiences two seasons; a dry season between May and September or October characterised by strong south winds between July and September that induce nutrient upwelling in the southern end of the lake, and a wet season for the rest of the year (Coulter and Spiegel, 1991). 2. Materials and methods Two types of data were used in this study. One was Zambia's catch statistics of the purse seine ®shery on a daily basis from 1984 to 1992 (see Table 1 for an example of a record of a ®shing boat); catch in weight by species category and ®shing area, and ®shing effort in number of hauls by ®shing area. The two clupeid species, L. miodon and S. tanganicae, were grouped under one category locally called ``kapenta'', and L. stappersi was recorded into another category called ``mvolo''. The ®shing area of a boat on one night was recorded by specifying a corresponding rectangle of 55 km square (Fig. 1(c)) in the southern part of the lake. The other types of data were species ratios in the clupeid catches for the same period. A sample was taken from a night's catch of a commercial purse seine Table 1 A shematic example of one purse seine boat's daily catch record in 6±15 May 1988 Date
6 7 8 9 10 11
Rectangle
3b 2f 2f 4b 4b 3e
Effort (hauls)
Catch (kg) Kapenta (Clupeids)
Mvolo (Lates stappersi)
2 2 2 2 2 3
0 0 0 0 0 16
125 975 924 141 551 3984
The rectangle is specified through the abscissa shown by the numerals and ordinate by alphabet (see Fig. 1(c)).
65
boat by staff of Lake Tanganyika Research Unit, Zambia. Catch in weight of each clupeid species was estimated and recorded together with sampling day and ®shing area on a rectangular basis. Every month 4±61 samples were taken. Catch per unit effort (CPUE) was used as an index of density. Using the ®rst data CPUE can be calculated on the daily and rectangle basis. To summarise information on distribution from so many data, it is necessary to use a method for averaging CPUE over a larger scale. The ratio method (Cochran, 1977) is applicable as CPUE is a ratio of catch to effort where both are variables with the correlation. Following the notation used by Cochran, a ratio estimate of Y/X from a sample (xi, yi), i1,2,. . .,n) is , X X yi xi : (1) i
i
Variance of the estimate is obtained on the basis that the ratio is nearly normally distributed if the sample is large enough. Thus con®dence limit of the estimate can be evaluated and the two estimates can be statistically compared. In application with special attention to seasonal change in distribution, ®rst the CPUE was calculated from the ®rst data on monthly and rectangle basis. C clupeidsrm ; frm
(2)
C L: stappersirm ; frm
(3)
CPUE clupeidsrm CPUE L: stappersirm
where r is rectangle, m is month, C is catch, and f is effort. C and f were summed over days belonging to m in all the years. This averaging may be sensitive to yearly change in effort. But both annual and monthly effort showed no high yearly variation with the maximum/minimum ratio from 1.90 to 2.80 and had remained almost stable since 1986 (Table 2). Secondly, the clupeid species ratio was estimated on a larger scale to overcome the problem of small sample size in the species ratio data. Considering past reports of inshore±offshore movements by clupeids (Matthes, 1967; Pearce, 1992; Coulter, 1991), Zambian ®shing areas (rectangles) were assigned to three strata, based on distance from the shore, as follows (Fig. 1(c)); 0±5 km, stratum 1 (inshore); 5±10 km, stratum 2 (mid-waters); and >10 km, stratum 3 (off-
66
H. Phiri, K. Shirakihara / Fisheries Research 41 (1999) 63±71
Table 2 Monthly effort in hauls from 1984 to 1992 Year
Month
Totals
J
F
M
A
M
J
J
A
S
O
N
D
1984 1985 1986 1987 1988 1989 1990 1991 1992
439 507 804 669 899 695 849 746 857
595 442 569 662 653 664 766 755 873
496 647 959 758 884 633 694 706 861
450 741 934 656 760 877 833 636 886
552 600 668 684 383 614 772 806 872
381 643 749 780 473 555 771 761 851
427 463 882 667 424 555 677 826 855
322 335 856 764 360 664 807 884 821
369 408 798 746 866 670 674 555 805
354 720 696 660 984 676 649 619 815
466 889 478 637 677 821 698 730 811
365 592 613 819 951 864 782 898 801
Mean
718.3
664.3
737.6
752.6
661.2
662.7
641.8
645.9
654.6
685.9
689.7
742.8
8257
SD
159.2
125.3
144.3
153
147.3
160.2
184.4
238.5
176.5
167.2
146.3
185.5
1408
shore). In consideration of the seasonal change in climate, months were classi®ed into four seasons as; May±July, early dry season; August±October, late dry season; November±January, early wet season; and February±April, late wet season. The species ratio (R) was estimated by the ratio method as , XX XX ^ js Srd Krd ; (4) R rj ds
C clupeidsjs : fjs
non-target species. To ascertain whether the three species are target species or not, the effectiveness of effort (Tanaka, 1960), was calculated, within the main ®shing area (Fig. 1(b)), for the two ®sh categories. !, , X X X ~ m f m =fm A Crm Ar Crm =frm frm ; r
r
r
(8)
rj ds
where S and K are the catch in weight of S. tanganicae and Kapenta (clupeids), respectively, d the day, s the season as de®ned above, and j is the stratum. Note that the summation of Srd, for example, was done over r belonging to j and d belonging to s. Thirdly, the CPUE of the clupeids by stratum and season was calculated. CPUE clupeidsjs
5216 6987 9006 8502 8314 8288 8972 8922 10108
(5)
Finally, the CPUE of each of the clupeid species is given by ^ js CPUE clupeids ; (6) CPUE S: tangancaejs R js ^ js CPUE clupeids : CPUE L: miodonjs 1 ÿ R js (7) Commercial ®shing boats tend to concentrate their effort on the waters where the target species are abundant. Therefore commercial catch statistics may fail to re¯ect the true distribution pattern of
where A is the area and Äf is the effective ®shing effort which is larger than f when effort concentrates on areas of higher ®sh density. Thus an index of more than unity for a species suggests that the species is targetted. Fig. 2 shows the distribution of ®shing effort in southern Lake Tanganyika. Monthly distribution pattern remained almost unchanged throughout the year. Although effort concentrated near the two ports of Mpulungu and Nsumbu (Fig. 1(b)), each rectangle was exploited with an average of 180 hauls for the study period except for the border areas. The number of hauls by season and stratum was more than 240. Sample sizes of the clupeid species ratios were equal to 48 or more in any season and any stratum. Thus the sample sizes used for each analysis on the basis of asymptotic normality were almost suf®cient. Monthly effectiveness of effort () ranged 1.8±4.2 for the clupeids and 1.8-4.6 for L. stappersi, suggesting that the three species were target species for the purse seine ®shery throughout the year.
H. Phiri, K. Shirakihara / Fisheries Research 41 (1999) 63±71
67
Fig. 2. Monthly distribution of fishing effort of the purse seine fishery in the Zambian waters averaged over the period of 1984±1992.
3. Results and discussion 3.1. Distribution and seasonal movement 3.1.1. Clupeids Monthly distribution of the clupeids is shown in Fig. 3. These species are widely distributed in Zambian waters. Some monthly changes in distribution pattern were observed in the eastern part: a high level of density near the shore in June, followed by an increase in density away from the shore in July±August and a subsequent decrease after September.
Detailed examination by species is done next. Species ratios were always higher than 0.5 (P<0.05) in the most offshore stratum 3 (Fig. 4(a)), indicating that S. tanganicae is the dominant of the two clupeid species in the offshore waters. The ratios in the inshore stratum 1 were lower than those in stratum 3 except for February±April, indicating that L. miodon is relatively abundant in the inshore waters especially from the late dry season of August±October. S. tanganicae is a coastal species, having higher density in the most inshore stratum (Fig. 4(b)). But the possibility of offshore movement in the dry season and subsequent inshore movement cannot be completely
68
H. Phiri, K. Shirakihara / Fisheries Research 41 (1999) 63±71
Fig. 3. Monthly distribution of the clupeids in the Zambian waters.
refuted because there is no signi®cant difference in CPUE among the three strata in late dry season and a decrease in CPUE of stratum 3 from this season to the next. L. miodon is also a coastal species, having higher density in the inshore stratum (Fig. 4(c)). This species shows an increase in CPUE of stratum 2 in the dry season and a decrease in the next season. In combination with an increase in the clupeids' density in July± August (Fig. 3), L. miodon is likely to move offshore in late dry season. Unlike S. tanganicae, however, the density of L. miodon in stratum 3 remained the lowest
(P<0.05) among the three strata throughout the four seasons (Fig. 4(c)). This suggests that the ®sh inhabiting the Zambian waters rarely move into the waters of other countries. 3.1.2. Lates stappersi Fig. 5 shows the monthly distribution of L. stappersi. This species is widely distributed in the Zambian waters but shows the most offshore distribution among the three species. In the western part, the density of this species is low even offshore. Since this region is mostly shallow (Fig. 1(a)), depth may be
H. Phiri, K. Shirakihara / Fisheries Research 41 (1999) 63±71
69
Fig. 4. (a) Seasonal change in species ratio, defined as the ratio of catch in weight of S. tanganicae to that of the two clupeid species. The symbol ``'' indicates species ratio with significant difference from 0.5 (P<0.05); (b) and (c) seasonal change in CPUE of the clupeids. The symbols ``*'' and ``O'' indicate the difference between stratum 1 and 3 (P<0.05) and between two consecutive seasons (P<0.05), respectively.
a limiting factor in the distribution pattern of this species. Monthly changes were observed in the central and eastern parts. From June±August, density was low in most of the regions. In September and October, density became higher away from the shore. From November±January, high-density areas extended inshore. In the remaining months, February±May, overall density decreased gradually. Since this species has a longevity of more than ®ve years (Pearce, 1985), mortality alone can hardly account for the low density observed between June and August. This phenomenon
must be a result of seasonal movement. L. stappersi moves into Zambian waters in late dry season, stays there until the end of the wet season and then gradually moves out of the Zambian waters. 3.2. Causes of seasonal movement In general, food availability, spawning and predator avoidance are plausible causes of seasonal movement. Spawning of S. tanganicae continues throughout the year with the main peak being in the ®rst half of the year (Coulter, 1970; Ellis, 1971; Pearce, 1985). As the
70
H. Phiri, K. Shirakihara / Fisheries Research 41 (1999) 63±71
Fig. 5. Monthly distribution of L. stappersi in the Zambian waters.
movements described above occur in the latter half of the year, spawning can hardly explain them. Its offshore and subsequent inshore movement is likely to be attributed to a decrease in the predator and food availability. This is because the seasons of the ®rst and second events coincide with the time of emigration of L. stappersi, and the time of the plankton bloom (Hecky, 1991), respectively. During the season of low density of L. stappersi, from June to August, the distribution of the clupeids and L. stappersi overlapped in the eastern part, whereas these species showed almost negative distributional association in
the remaining seasons (Figs. 3 and 5). Since the predator showed the highest abundance in most seasons, it is probable that the presence or absence of the predator affects the distribution of the prey clupeids. 3.3. Management implications Our attention is focussed on whether populations of the three pelagic species can be considered to be closed in Zambian waters for management purposes. The assumption of closed population is applicable to L. miodon, so this coastal species could be managed as
H. Phiri, K. Shirakihara / Fisheries Research 41 (1999) 63±71
one population within Zambian waters. But such a view may not be appropriate for S. tanganicae, which is a more offshore species and may emigrate in the late dry season. Practically, single-species management is not possible for the two clupeid species, which are captured together and treated as one commercial category. However, the assumption of the closeequivalent population can be accepted in the wet season when the density of S. tanganicae is low in the offshore waters. This assumption will be useful in evaluating the effectiveness of future domestic ®shery regulations, and estimating population parameters using catch statistics in this season. On the other hand, L. stappersi moves extensively in and out of the Zambian waters. Therefore, international management schemes will have to be implemented. While this paper makes a ®rst step towards appropriate de®nition of stocks for these species, more study will be required to verify the conclusions. These would involve changes in distribution areas with body size and spawning migrations. Acknowledgements We are grateful to the late Dr. R. Mubamba, Department of Fisheries, Zambia for his permission to use the Zambian ®sheries statistics. We thank Drs. Y. Natsukari, M. Nagoshi and M. Kawanabe for their encouragement to our work. Mr. L. Mwape and staff of Lake Tanganyika Research Unit, Zambia made every effort to support our work. This work was ®nancially supported by the Ministry of Education, Science and Culture, Japan (no. 02041058). References Boulenger, G.A., 1906. Fourth contribution to the ichthyology of Lake Tanganyika. Report on the collection of fishes by Dr. W.A. Cunnington during the Third Tanganyika Expedition, 1904±1905. Transactions of the Zoological Society of London, 17(6): 537±576. Boulenger, G.A., 1914. Mission stappers au Tanganika-Moeros. Diagnoses de poissons nouveaux: I Acanthopterygiens, Opisthomes, Cyprinodontes. Revue de zoologie et le botanique africaines, 3: 442±447. Chapman, D.W., van Well, P., 1978. Growth and mortality of Stolothrissa tanganicae. Trans. Am. Fisheries Soc. 107, 26±35. Cochran, W.G., 1977. Sampling Techniques. Wiley, New York, 428 pp.
71
Coulter, G.W., 1970. Population changes within a group of fish species in Lake Tanganyika following their exploitation. J. Fish Biol. 2, 329±353. Coulter, G.W., 1991. Pelagic fish. In: Coulter, G.W. (Ed.), Lake Tanganyika and its Life. Oxford University Press, New York, pp. 23±38. Coulter, G.W., Spiegel, R.H., 1991. Hydrodynamics. In: Coulter, G.W. (Ed.), Lake Tanganyika and its Life. Oxford University Press, New York, pp. 49±75. Ellis, C.M.A., 1971. The size at maturity and breeding seasons of sardines in southern Lake Tanganyika. Afr. J. Tropical Hydrobiol. Fisheries 107(4), 59±66. Hecky, R.E., 1991. Pelagic ecosystem. In: Coulter, G.W. (Ed.), Lake Tanganyika and its Life. Oxford University Press, New York, pp. 90±110. Johannesson, K.A., 1974. Preliminary quantitative estimates of pelagic fish stocks in Lake Tanganyika by use of echointegration techniques. FAO Document EIFAC/74/1/Symp., vol. 54, Rome. Kimura, S., 1995. Growth of the clupeid fishes, Stolothrissa tanganicae and Limnothrissa miodon, in the Zambian waters of Lake Tanganyika. J. Fish Biol. 47, 569±575. Matthes, H., 1967. Preliminary investigations into the biology of the Lake Tanganyika Clupeids. Fisheries Res. Bull. Zambia 4, 39±45. Mulimbwa, N., Shirakihara, K., 1994. Growth, recruitment and reproduction of sardines (Stolothrissa tanganicae and Limnothrissa miodon) in northern Lake Tanganyika. Tropics 4(1), 57±67. Pearce, J.M., 1985. A description and stock assessment of the pelagic fishery in the southeast arm of the Zambian waters of Lake Tanganyika. Report of the Department of Fisheries. Zambia, 59 pp. Pearce, J.M., 1992. The development of the pelagic fishery since 1980 and its effects on the pelagic fish stocks in southern Lake Tanganyika. Report of the Department of Fisheries. Zambia, 102 pp. Pearce, J.M., 1995. Effects of exploitation on the pelagic fish community in the south of Lake Tanganyika. In: Pitcher, T.J., Hart, J.B. (Eds.), The Impact of Species Changes in African Lakes. Chapman and Hall, London, pp. 425±441. Poll, M., 1953. Poisson non-Cichlidae. Resultats scientifiques de l'exploration hydrobiologique du Lac Tanganyika (1946±1947). Institut Royal des sciences naturelles de Belgique 3(5A), 1±251. Roest, F.C., 1978. Stolothrissa tanganicae: population dynamics, biomass evolution and life history in the Burundi waters of Lake Tanganyika. CIFA Tech. Pap., CPCA 5, 42±62. Shirakihara, K., Use, K., Kamikawa, S., Mambona, W.B., 1992. Population changes of sardines in northern Lake Tanganyika. Afr. Study Monographs 13(1), 57±67. Tanaka, M., 1960. Studies on the dynamics and management of fish populations. Bull. Tokai Reg. Fish. Res. Lab. 28, 1±200. Tshibangu, K.K., Kinoshita, I., 1995. Early life history of two clupeids, Limnothrissa miodon and Stolothrissa tanganicae, from Lake Tanganyika. Jpn. J. Icthyol. 42(1), 81±87.
Distribution and seasonal movement of pelagic ®sh in southern Lake Tanganyika Harris Phiria,*, Kunio Shirakiharab b
a Lake Tanganyika Research Unit, Department of Fisheries, PO Box 55, Mpulungu, Zambia Faculty of Bioresources, Laboratory of Fish Population Dynamics, Mie University, 1515 Kamihama, Tsu 514, Japan
Received 18 February 1997; accepted 16 November 1998
Abstract The distribution and seasonal movement of two planktivorous clupeids, Stolothrissa tanganicae and Limnothrissa miodon, and one predatory centropomid ®sh, Lates stappersi in the southern end (Zambian waters) of Lake Tanganyika were studied using purse seine ®shery statistics and species ratio data collected between 1984 and 1992. L. stappersi is the most abundant offshore among the three species and moves northwards into waters of other countries during the dry season. L. miodon occupies the inshore regions of the lake, and does not appear to move offshore beyond Zambian waters. S. tanganicae is more dominant in the offshore waters than L. miodon and there is a strong possibility that it moves out of the Zambian waters. The presence or absence of the predator, L. stappersi, seems to affect the distribution of the two prey clupeid species. Speciesbased management of either clupeid, which are captured together, may not be meaningful but an assumption of closeequivalent population in the wet season may be useful in implementing seasonal regulations. International management policies may have to be introduced for L. stappersi. # 1999 Elsevier Science B.V. All rights reserved. Keywords: Pelagic ®sh; Fishery management; Lake Tanganyika; Stolothrissa tanganicae; Limnothrissa miodon; Lates stappersi
1. Introduction Lake Tanganyika, the oldest lake in the African rift valley, contains many endemic ®sh species. Two endemic species of sardine-like planktivorous clupeids, Stolothrissa tanganicae Regan, 1917 and Limnothrissa miodon (Boulenger, 1906) and one centropomid species, Lates (Luciolates) stappersi (Boulenger, 1914) are distributed over the pelagic waters of the lake (Poll, 1953). These three species, captured together by purse seine and other ®shing *Corresponding author. Tel.: +81-59-231-9673; fax: +81-59231-9538; e-mail: [email protected]
nets, have been exploited as the most important ®sheries resources by all the riparian countries (Fig. 1(a)). Each species has close ecological interaction with the others (Coulter, 1991). S. tanganicae is preyed upon by L. stappersi and large L. miodon. S. tanganicae, L. miodon, and juvenile L. stappersi can utilize common food resources (Pearce, 1992). Commercial exploitation in southern Lake Tanganyika started in 1959 (Coulter, 1970), when the purse seine was ®rst used. The ®shery expanded signi®cantly in the 1980s and the number of purse seiners had increased from 11 at the end of 1984 to 19 in 1992. In the mid 1970s, the Zambian government introduced
0165-7836/99/$ ± see front matter # 1999 Elsevier Science B.V. All rights reserved. PII: S0165-7836(99)00008-9
64
H. Phiri, K. Shirakihara / Fisheries Research 41 (1999) 63±71
Fig. 1. (a) Map of Lake Tanganyika. The shaded area shows the Zambian part of the lake. (b) An enlargement of the Zambian part of the lake with isobaths in metres. The area used to calculate the effectiveness of effort is surrounded by the thin dotted line. (c) Rectangles used for specifying fishing area and their stratification based on distance from the shore shown by the bold lines.
a regulation of the seine net mesh size as a way to manage the ®shery. This strategy seems not to have been as effective as desired. After 1985, the catch of the clupeids had a decreasing trend while that of L. stappersi have more or less remained stable (Pearce, 1995). Although the population ecology of these pelagic ®shes has been studied (Mulimbwa and Shirakihara, 1994; Kimura, 1995; Tshibangu and Kinoshita, 1995), most of the studies have been conducted under the implicit assumption of closed populations in the study areas due to lack of proof of ®sh movement (Coulter, 1970; Roest, 1978; Pearce, 1985; Shirakihara et al., 1992; Chapman and van Well, 1978), in spite of there
being some signs of migration over the lake (Matthes, 1967). Although information on distribution and movement is essential for making rational ®shery management schemes, to date descriptions have been qualitative (Coulter, 1991; Pearce, 1992, 1995). One exception is the acoustic studies (e.g., Johannesson, 1974), but these did not give information by species. Another is catch statistic analysis. Shirakihara et al. (1992) estimated an index of population density from catch statistics at landing places, which are available in the northern part of the lake (Zaire and Burundi waters). Uncertainty appears in that relationship between landing places and actual ®shing areas is unclear.
H. Phiri, K. Shirakihara / Fisheries Research 41 (1999) 63±71
Using the data on Zambian purse seine ®shery, which include information on the ®shing area, the present study was conducted to clarify the distribution and seasonal movement of the three pelagic species in the southern part of the lake with special attention to their emigration. Climate can be a factor affecting ®sh distribution. The area surrounding the lake experiences two seasons; a dry season between May and September or October characterised by strong south winds between July and September that induce nutrient upwelling in the southern end of the lake, and a wet season for the rest of the year (Coulter and Spiegel, 1991). 2. Materials and methods Two types of data were used in this study. One was Zambia's catch statistics of the purse seine ®shery on a daily basis from 1984 to 1992 (see Table 1 for an example of a record of a ®shing boat); catch in weight by species category and ®shing area, and ®shing effort in number of hauls by ®shing area. The two clupeid species, L. miodon and S. tanganicae, were grouped under one category locally called ``kapenta'', and L. stappersi was recorded into another category called ``mvolo''. The ®shing area of a boat on one night was recorded by specifying a corresponding rectangle of 55 km square (Fig. 1(c)) in the southern part of the lake. The other types of data were species ratios in the clupeid catches for the same period. A sample was taken from a night's catch of a commercial purse seine Table 1 A shematic example of one purse seine boat's daily catch record in 6±15 May 1988 Date
6 7 8 9 10 11
Rectangle
3b 2f 2f 4b 4b 3e
Effort (hauls)
Catch (kg) Kapenta (Clupeids)
Mvolo (Lates stappersi)
2 2 2 2 2 3
0 0 0 0 0 16
125 975 924 141 551 3984
The rectangle is specified through the abscissa shown by the numerals and ordinate by alphabet (see Fig. 1(c)).
65
boat by staff of Lake Tanganyika Research Unit, Zambia. Catch in weight of each clupeid species was estimated and recorded together with sampling day and ®shing area on a rectangular basis. Every month 4±61 samples were taken. Catch per unit effort (CPUE) was used as an index of density. Using the ®rst data CPUE can be calculated on the daily and rectangle basis. To summarise information on distribution from so many data, it is necessary to use a method for averaging CPUE over a larger scale. The ratio method (Cochran, 1977) is applicable as CPUE is a ratio of catch to effort where both are variables with the correlation. Following the notation used by Cochran, a ratio estimate of Y/X from a sample (xi, yi), i1,2,. . .,n) is , X X yi xi : (1) i
i
Variance of the estimate is obtained on the basis that the ratio is nearly normally distributed if the sample is large enough. Thus con®dence limit of the estimate can be evaluated and the two estimates can be statistically compared. In application with special attention to seasonal change in distribution, ®rst the CPUE was calculated from the ®rst data on monthly and rectangle basis. C clupeidsrm ; frm
(2)
C L: stappersirm ; frm
(3)
CPUE clupeidsrm CPUE L: stappersirm
where r is rectangle, m is month, C is catch, and f is effort. C and f were summed over days belonging to m in all the years. This averaging may be sensitive to yearly change in effort. But both annual and monthly effort showed no high yearly variation with the maximum/minimum ratio from 1.90 to 2.80 and had remained almost stable since 1986 (Table 2). Secondly, the clupeid species ratio was estimated on a larger scale to overcome the problem of small sample size in the species ratio data. Considering past reports of inshore±offshore movements by clupeids (Matthes, 1967; Pearce, 1992; Coulter, 1991), Zambian ®shing areas (rectangles) were assigned to three strata, based on distance from the shore, as follows (Fig. 1(c)); 0±5 km, stratum 1 (inshore); 5±10 km, stratum 2 (mid-waters); and >10 km, stratum 3 (off-
66
H. Phiri, K. Shirakihara / Fisheries Research 41 (1999) 63±71
Table 2 Monthly effort in hauls from 1984 to 1992 Year
Month
Totals
J
F
M
A
M
J
J
A
S
O
N
D
1984 1985 1986 1987 1988 1989 1990 1991 1992
439 507 804 669 899 695 849 746 857
595 442 569 662 653 664 766 755 873
496 647 959 758 884 633 694 706 861
450 741 934 656 760 877 833 636 886
552 600 668 684 383 614 772 806 872
381 643 749 780 473 555 771 761 851
427 463 882 667 424 555 677 826 855
322 335 856 764 360 664 807 884 821
369 408 798 746 866 670 674 555 805
354 720 696 660 984 676 649 619 815
466 889 478 637 677 821 698 730 811
365 592 613 819 951 864 782 898 801
Mean
718.3
664.3
737.6
752.6
661.2
662.7
641.8
645.9
654.6
685.9
689.7
742.8
8257
SD
159.2
125.3
144.3
153
147.3
160.2
184.4
238.5
176.5
167.2
146.3
185.5
1408
shore). In consideration of the seasonal change in climate, months were classi®ed into four seasons as; May±July, early dry season; August±October, late dry season; November±January, early wet season; and February±April, late wet season. The species ratio (R) was estimated by the ratio method as , XX XX ^ js Srd Krd ; (4) R rj ds
C clupeidsjs : fjs
non-target species. To ascertain whether the three species are target species or not, the effectiveness of effort (Tanaka, 1960), was calculated, within the main ®shing area (Fig. 1(b)), for the two ®sh categories. !, , X X X ~ m f m =fm A Crm Ar Crm =frm frm ; r
r
r
(8)
rj ds
where S and K are the catch in weight of S. tanganicae and Kapenta (clupeids), respectively, d the day, s the season as de®ned above, and j is the stratum. Note that the summation of Srd, for example, was done over r belonging to j and d belonging to s. Thirdly, the CPUE of the clupeids by stratum and season was calculated. CPUE clupeidsjs
5216 6987 9006 8502 8314 8288 8972 8922 10108
(5)
Finally, the CPUE of each of the clupeid species is given by ^ js CPUE clupeids ; (6) CPUE S: tangancaejs R js ^ js CPUE clupeids : CPUE L: miodonjs 1 ÿ R js (7) Commercial ®shing boats tend to concentrate their effort on the waters where the target species are abundant. Therefore commercial catch statistics may fail to re¯ect the true distribution pattern of
where A is the area and Äf is the effective ®shing effort which is larger than f when effort concentrates on areas of higher ®sh density. Thus an index of more than unity for a species suggests that the species is targetted. Fig. 2 shows the distribution of ®shing effort in southern Lake Tanganyika. Monthly distribution pattern remained almost unchanged throughout the year. Although effort concentrated near the two ports of Mpulungu and Nsumbu (Fig. 1(b)), each rectangle was exploited with an average of 180 hauls for the study period except for the border areas. The number of hauls by season and stratum was more than 240. Sample sizes of the clupeid species ratios were equal to 48 or more in any season and any stratum. Thus the sample sizes used for each analysis on the basis of asymptotic normality were almost suf®cient. Monthly effectiveness of effort () ranged 1.8±4.2 for the clupeids and 1.8-4.6 for L. stappersi, suggesting that the three species were target species for the purse seine ®shery throughout the year.
H. Phiri, K. Shirakihara / Fisheries Research 41 (1999) 63±71
67
Fig. 2. Monthly distribution of fishing effort of the purse seine fishery in the Zambian waters averaged over the period of 1984±1992.
3. Results and discussion 3.1. Distribution and seasonal movement 3.1.1. Clupeids Monthly distribution of the clupeids is shown in Fig. 3. These species are widely distributed in Zambian waters. Some monthly changes in distribution pattern were observed in the eastern part: a high level of density near the shore in June, followed by an increase in density away from the shore in July±August and a subsequent decrease after September.
Detailed examination by species is done next. Species ratios were always higher than 0.5 (P<0.05) in the most offshore stratum 3 (Fig. 4(a)), indicating that S. tanganicae is the dominant of the two clupeid species in the offshore waters. The ratios in the inshore stratum 1 were lower than those in stratum 3 except for February±April, indicating that L. miodon is relatively abundant in the inshore waters especially from the late dry season of August±October. S. tanganicae is a coastal species, having higher density in the most inshore stratum (Fig. 4(b)). But the possibility of offshore movement in the dry season and subsequent inshore movement cannot be completely
68
H. Phiri, K. Shirakihara / Fisheries Research 41 (1999) 63±71
Fig. 3. Monthly distribution of the clupeids in the Zambian waters.
refuted because there is no signi®cant difference in CPUE among the three strata in late dry season and a decrease in CPUE of stratum 3 from this season to the next. L. miodon is also a coastal species, having higher density in the inshore stratum (Fig. 4(c)). This species shows an increase in CPUE of stratum 2 in the dry season and a decrease in the next season. In combination with an increase in the clupeids' density in July± August (Fig. 3), L. miodon is likely to move offshore in late dry season. Unlike S. tanganicae, however, the density of L. miodon in stratum 3 remained the lowest
(P<0.05) among the three strata throughout the four seasons (Fig. 4(c)). This suggests that the ®sh inhabiting the Zambian waters rarely move into the waters of other countries. 3.1.2. Lates stappersi Fig. 5 shows the monthly distribution of L. stappersi. This species is widely distributed in the Zambian waters but shows the most offshore distribution among the three species. In the western part, the density of this species is low even offshore. Since this region is mostly shallow (Fig. 1(a)), depth may be
H. Phiri, K. Shirakihara / Fisheries Research 41 (1999) 63±71
69
Fig. 4. (a) Seasonal change in species ratio, defined as the ratio of catch in weight of S. tanganicae to that of the two clupeid species. The symbol ``'' indicates species ratio with significant difference from 0.5 (P<0.05); (b) and (c) seasonal change in CPUE of the clupeids. The symbols ``*'' and ``O'' indicate the difference between stratum 1 and 3 (P<0.05) and between two consecutive seasons (P<0.05), respectively.
a limiting factor in the distribution pattern of this species. Monthly changes were observed in the central and eastern parts. From June±August, density was low in most of the regions. In September and October, density became higher away from the shore. From November±January, high-density areas extended inshore. In the remaining months, February±May, overall density decreased gradually. Since this species has a longevity of more than ®ve years (Pearce, 1985), mortality alone can hardly account for the low density observed between June and August. This phenomenon
must be a result of seasonal movement. L. stappersi moves into Zambian waters in late dry season, stays there until the end of the wet season and then gradually moves out of the Zambian waters. 3.2. Causes of seasonal movement In general, food availability, spawning and predator avoidance are plausible causes of seasonal movement. Spawning of S. tanganicae continues throughout the year with the main peak being in the ®rst half of the year (Coulter, 1970; Ellis, 1971; Pearce, 1985). As the
70
H. Phiri, K. Shirakihara / Fisheries Research 41 (1999) 63±71
Fig. 5. Monthly distribution of L. stappersi in the Zambian waters.
movements described above occur in the latter half of the year, spawning can hardly explain them. Its offshore and subsequent inshore movement is likely to be attributed to a decrease in the predator and food availability. This is because the seasons of the ®rst and second events coincide with the time of emigration of L. stappersi, and the time of the plankton bloom (Hecky, 1991), respectively. During the season of low density of L. stappersi, from June to August, the distribution of the clupeids and L. stappersi overlapped in the eastern part, whereas these species showed almost negative distributional association in
the remaining seasons (Figs. 3 and 5). Since the predator showed the highest abundance in most seasons, it is probable that the presence or absence of the predator affects the distribution of the prey clupeids. 3.3. Management implications Our attention is focussed on whether populations of the three pelagic species can be considered to be closed in Zambian waters for management purposes. The assumption of closed population is applicable to L. miodon, so this coastal species could be managed as
H. Phiri, K. Shirakihara / Fisheries Research 41 (1999) 63±71
one population within Zambian waters. But such a view may not be appropriate for S. tanganicae, which is a more offshore species and may emigrate in the late dry season. Practically, single-species management is not possible for the two clupeid species, which are captured together and treated as one commercial category. However, the assumption of the closeequivalent population can be accepted in the wet season when the density of S. tanganicae is low in the offshore waters. This assumption will be useful in evaluating the effectiveness of future domestic ®shery regulations, and estimating population parameters using catch statistics in this season. On the other hand, L. stappersi moves extensively in and out of the Zambian waters. Therefore, international management schemes will have to be implemented. While this paper makes a ®rst step towards appropriate de®nition of stocks for these species, more study will be required to verify the conclusions. These would involve changes in distribution areas with body size and spawning migrations. Acknowledgements We are grateful to the late Dr. R. Mubamba, Department of Fisheries, Zambia for his permission to use the Zambian ®sheries statistics. We thank Drs. Y. Natsukari, M. Nagoshi and M. Kawanabe for their encouragement to our work. Mr. L. Mwape and staff of Lake Tanganyika Research Unit, Zambia made every effort to support our work. This work was ®nancially supported by the Ministry of Education, Science and Culture, Japan (no. 02041058). References Boulenger, G.A., 1906. Fourth contribution to the ichthyology of Lake Tanganyika. Report on the collection of fishes by Dr. W.A. Cunnington during the Third Tanganyika Expedition, 1904±1905. Transactions of the Zoological Society of London, 17(6): 537±576. Boulenger, G.A., 1914. Mission stappers au Tanganika-Moeros. Diagnoses de poissons nouveaux: I Acanthopterygiens, Opisthomes, Cyprinodontes. Revue de zoologie et le botanique africaines, 3: 442±447. Chapman, D.W., van Well, P., 1978. Growth and mortality of Stolothrissa tanganicae. Trans. Am. Fisheries Soc. 107, 26±35. Cochran, W.G., 1977. Sampling Techniques. Wiley, New York, 428 pp.
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Coulter, G.W., 1970. Population changes within a group of fish species in Lake Tanganyika following their exploitation. J. Fish Biol. 2, 329±353. Coulter, G.W., 1991. Pelagic fish. In: Coulter, G.W. (Ed.), Lake Tanganyika and its Life. Oxford University Press, New York, pp. 23±38. Coulter, G.W., Spiegel, R.H., 1991. Hydrodynamics. In: Coulter, G.W. (Ed.), Lake Tanganyika and its Life. Oxford University Press, New York, pp. 49±75. Ellis, C.M.A., 1971. The size at maturity and breeding seasons of sardines in southern Lake Tanganyika. Afr. J. Tropical Hydrobiol. Fisheries 107(4), 59±66. Hecky, R.E., 1991. Pelagic ecosystem. In: Coulter, G.W. (Ed.), Lake Tanganyika and its Life. Oxford University Press, New York, pp. 90±110. Johannesson, K.A., 1974. Preliminary quantitative estimates of pelagic fish stocks in Lake Tanganyika by use of echointegration techniques. FAO Document EIFAC/74/1/Symp., vol. 54, Rome. Kimura, S., 1995. Growth of the clupeid fishes, Stolothrissa tanganicae and Limnothrissa miodon, in the Zambian waters of Lake Tanganyika. J. Fish Biol. 47, 569±575. Matthes, H., 1967. Preliminary investigations into the biology of the Lake Tanganyika Clupeids. Fisheries Res. Bull. Zambia 4, 39±45. Mulimbwa, N., Shirakihara, K., 1994. Growth, recruitment and reproduction of sardines (Stolothrissa tanganicae and Limnothrissa miodon) in northern Lake Tanganyika. Tropics 4(1), 57±67. Pearce, J.M., 1985. A description and stock assessment of the pelagic fishery in the southeast arm of the Zambian waters of Lake Tanganyika. Report of the Department of Fisheries. Zambia, 59 pp. Pearce, J.M., 1992. The development of the pelagic fishery since 1980 and its effects on the pelagic fish stocks in southern Lake Tanganyika. Report of the Department of Fisheries. Zambia, 102 pp. Pearce, J.M., 1995. Effects of exploitation on the pelagic fish community in the south of Lake Tanganyika. In: Pitcher, T.J., Hart, J.B. (Eds.), The Impact of Species Changes in African Lakes. Chapman and Hall, London, pp. 425±441. Poll, M., 1953. Poisson non-Cichlidae. Resultats scientifiques de l'exploration hydrobiologique du Lac Tanganyika (1946±1947). Institut Royal des sciences naturelles de Belgique 3(5A), 1±251. Roest, F.C., 1978. Stolothrissa tanganicae: population dynamics, biomass evolution and life history in the Burundi waters of Lake Tanganyika. CIFA Tech. Pap., CPCA 5, 42±62. Shirakihara, K., Use, K., Kamikawa, S., Mambona, W.B., 1992. Population changes of sardines in northern Lake Tanganyika. Afr. Study Monographs 13(1), 57±67. Tanaka, M., 1960. Studies on the dynamics and management of fish populations. Bull. Tokai Reg. Fish. Res. Lab. 28, 1±200. Tshibangu, K.K., Kinoshita, I., 1995. Early life history of two clupeids, Limnothrissa miodon and Stolothrissa tanganicae, from Lake Tanganyika. Jpn. J. Icthyol. 42(1), 81±87.