The Diet of Limnothrissa miodon and Zooplankton Densities in Sanyati Basin, Lake

更新时间：2024-02-06 作者：用户投稿原创标记本站原创
Abstract: There has been a continuous decline in the catches of Limnothrissa miodon in Lake Kariba since 1994. Given the value and contribution of this sardine in the GDP (gross domestic product) of Zimbabwe, research on its ecology becomes vital. The diet composition of L. miodon in the Sanyati basin of Lake Kariba was explored using numerical and frequency of occurrence methods. Densities of zooplankton species in the riverine and pelagic zones of the basin were established. Zooplankton species were collected at each site using 60 μm mesh plankton, preserved and classified in the laboratory. Limnothrissa miodon samples (500 g) were collected from Kapenta rigs for gut analysis. Diet overlap and relative prey selection was analysed using Schoener index and Chesson’s index of selection respectively. ANOVA was done to establish the variations in the zooplankton densities in the riverine and pelagic sampling areas as well as proportional occurrences of prey item consumed. Significant differences on proportions of chironomids (F = 1.94, P < 0.05), nauplii (F = 10.24, P < 0.05) and Diaphanosoma (F = 20.98, P < 0.05) preyed by Kapenta size classes in riverine specimens were noted. Proportion of chironomids and Boina eaten by adult and sub-adult Kapenta in pelagic waters were significantly different (F = 2.55, P < 0.05) and (F = 2.21, P < 0.05). A hierarchical cluster analysis using species densities revealed no major divisions in faunal associations among cyclopoids. Findings of this study indicate that the densities of zooplankton species between pelagic and riverine sections of the Sanyati basin are different.
Key words: Limnothrissa miodon, zooplankton, Kapenta, pelagic, riverine, Lake Kariba.
1. Introduction
The Limnothrissa miodon (Kapenta) is a fresh water clupeid endemic to Lake Tanganyika which has been introduced to many African Lakes that include Kivu, Cabora Bassa and Kariba [1, 2]. The transplant of the sardine has led to a succesul colonization of underutilized pelagic zooplanktivorous community [3]. The sardine reaches sexual maturity at a all size (35 mm TL (total length)), it has a high fecundity (up to 55,000 ovules per female) and reproduces the whole year [4]. The sardine individuals observed in Lake Tanganyika adults are larger than in Lake Kariba due to the different nutrient status of the Lakes [5]. Lake Kariba is a river fed reservoir built along Zambezi River on the border between Zambia and Zimbabwe. The lake is divided into five hydrological basins namely: Mlibizi, Binga, Sengwa, Bumi and Sanyati[6].
After the Lake’s completion in the early 1960s artisanal (gill netting) fishery was the common fish industry dominated by indigenous riverine fish, which inhabited in littoral areas less than 20 m deep. Ten different individuals characterized the catches with the family cichlidae (38%) and characids (17%) dominating the catches [7]. Limnothrissa miodon was introduced into Lake Kariba in 1967 and 1969 . By 1970 the sardines had become established in the whole of Lake Kariba and its population was adequate to support fish industry. Commercial Kapenta fishery industry started in 1974 [8]. Fishing is done using rigs with a net of mesh size not less than 8 mm to catch the Kapenta [9]. Limnothrissa miodon maximum catches were recorded in 1990 estimated at 21,758 t fresh weight (FW) [9]. The catches vary according to river inflow and periods of drought are usually associated by low catches [3, 9].
The Kapenta catches declined by 24% (2,687t FW) and 11.1% (2,452t FW) in 1982 and 1991 respectively, a phenomenon that has been attributed to droughts [9, 10]. However, a trendy decline up to 50% of Kapenta catches has been noted in Lake Kariba as early as 1994[11]. Unfortunately, little research has been conducted to assess any further changes in the feeding patterns of this sardine [11]. Limnothrissa miodon forms an integral part of Lake Kariba’s pelagic zooplanktivorous community and as such contributes enormously to the stability and diversity of the lakes’ ecosystem. It is the dominant prey species (above 45%) of Hydrocynus vittatus [12]. Since climatic changes generate varying Kapenta catches and zooplankton composition [3, 13], investigating the food items selected by L. miodon in the lake provided an insight into the ecology and abundance of the sardine. In this study, the diet composition of Kapenta using Numerical and Frequency of Occurrence methods wasexplored[14, 15] as well as the densities of zooplankton species in the Sanyati basin found on the Zimbabwean shore.
2. Materials and Methods
2.1 Study Area
The study was carried out in Sanyati basin, Lake Kariba, Zimbabwe. Lake Kariba is situated on the border between Zambia and Zimbabwe and stretches for 280 km in an east-west direction. It is located between 16?28′-18?06′ S, 26?40′-29?03′ E latitudinal positions [6]. The climate of Kariba can be divided into four distinguishable seasons based on rainfall and temperature: the cold dry season (June to August), hot dry season (September and November), hot wet season(November to March) and cool dry season (March to May) [16]. The erage mean temperature is 28 ?C and 17 ?C for the hot wet season and the cool dry season respectively. Annual precipitation over the lake is between 250-1,000 mm and falls between November and April. The mean annual potential evaporation from the lake is 500-3,600 mm [6].
2.2 Sampling Sites Selection
Using Lake Kariba maps, the Sanyati basin was divided into three distinct broad sampling areas namely littoral (0-20 m deep), pelagic (above 20 m deep) and riverine (river mouths). Of these three, the pelagic and riverine areas were sampled. Using computer generated random numbers, one sampling area from pelagic and riverine areas within the basin was selected. Three sampling points per area were first determined by an echo sounder that fishermen use to locate schools of Kapenta and the latitudinal coordinates of the points were noted on a GPS72 Garmin for replication of results.
2.3 Sampling
Samples of both L. miodon and zooplankton were collected concurrently from the two selected sampling areas during the month of March and June 2010. At each site, 60 μm mesh plankton net was used to collect zooplankton samples. Zooplankton samples were preserved in 95% alcohol for laboratory analysis [17]. Limnothrissa miodon samples were collected from the catches using the ordinary commercial Kapenta rig. After every lift before salting, 500 g of L. miodon was removed and a sub sample 300 g preserved in 70% alcohol. The remaining 200 g was chilled in a cooler box for refrigeration.
2.4 Laboratory Measurements
In the lab zooplankton samples were shaken to achieve uniform distribution of the organis and a subsample of 5 mL or 10 mL for dense and sparse samples respectively [18] was taken and placed in sedimentation chamber using an adjustable-volume pipette with a tip of 3 mm diameter. After 30 minutes resident time, subsamples were analyzed over an
inverted microscope at ×100 magnification. Species were identified and classified using 摘自：学士论文www.808so.com
guides from diagrams and descriptions in Refs. [19-23]. Limnothrissa miodon samples were measured to determine individual weight (g) and SL (tandard length)(mm) using an analytical balance and ruler respectively. Using length classes after Cochrane [5] species were grouped into two age classes namely sub adults(3.0-3.9 mm) and adults (≥ 4 mm) [24].
After measurements specimens were dissected over a dissecting microscope (magnification ? 40) to remove the stomachs, which were further weighed . Individual stomachs were dissected and the stomach contents emptied in a Petri dish containing a known volume of water (5-10 mL) [25]. Food items were identified with variable taxonomic accuracy where possible the species level was the primary target, followed by genus, family and order using literature and guides over an inverted microscope with magnification ? 100. The abundances of different prey items in the stomachs of L. moidon were recorded on counters and expressed as proportional percentages[25].
2.5 Data Analysis
Densities of zooplankton species, groups or developmental stages were calculated using the formula that was used by Masundire [18]:
Density of species (i) in lake (Ind.m3) = NiVL-1 (1)
Shannon-Weiner index of diversity (H?) was calculated using the densities of different zooplankton species in the samples [24]. Occurrence frequencies (Fi) of individual food items consumed by L. miodon of varying classes were calculated using the method described by Goitein et al. [15, 25]. Diet overlap was analysed using Schoener index [26]:?Sch = 1 – 0.5 (∑n?Pxi – Pyi?? (2) where, Pxi is the proportion of food category (i) in the diet of class (x); Pyi is a proportion of food category in the diet of class (y), and n is the number of food categories. Relative prey selection (S) was calculated using Chesson’s [27] index of selection: S = (rd-1)/ ∑(rd?1) (2) (3) where, r is the number of individuals located in the stomach for a specific prey category and d is the density (m3) of the prey category in the Lake. The values of the index range between 0 and 1, with the higher values indicating higher levels of selectivity.
All statistical analysis was done using MINITAB at 5% level of significance. All data were tested for normality and further arcsine tranormed to satiy the equality of variance and normality assumptions. ANOVA (one-way analysis of variance) was used to test for differences in zooplankton species densities between the riverine and pelagic sampling areas. Differences between the proportional occurrences of prey item consumed by L. miodon of varying length classes were also tested through ANOVA. A HCA(Hierarchical Cluster Analysis) was performed using Correlation Coefficient Distance and Single Linkage zooplankton density in MINITAB. Zooplankton densities were all standardized by subtracting the means and dividing by the standard deviation before the distance matrix was calculated to minimize the effect of scale differences.
3. Results
3.1 Gut Composition of L. miodon
Eight prey items were observed in Kapenta stomachs from pelagic and riverine sampling stations with Cyclopoid adults, Chironomids and Chaoborus being the most frequent prey items. However, Chaoborus and Calanoids were only found in riverine specimens (Fig. 1). Consumption of chironomids and chaoborus by Kapenta increased with age class while preying on cyclopoids (adults and copepodites) decreased with age (Figs. 2a and 2b). Significant differences on proportions of chironomids (F = 1.94, P < 0.05), nauplii (F = 10.24, P < 0.05) and Diaphanosoma(F = 20.98, P < 0.05) preyed by Kapenta size classes in riverine specimens were noted. The proportion of chironomids and Boina eaten by adult and sub-adult KapentainpelagicwatersofSanyatiBasinwere
significantly different (F = 2.55, P < 0.05) and (F = 2.21, P < 0.05) respectively. 3.2 Zooplankton Densities
Four zooplankton species and a single dipteran(Chaoborus edulis) were observed from the sampled sites of Sanyati Basin. A hierarchical cluster analysis using species densities revealed no major divisions in faunal associations among cyclopoids. Cyclopoids(adults, nauplii and copepodites) were the dominating species in both pelagic and riverine and seemed to be associated with every other species regardless of the site. Diaphanosoma were closely linked with the cyclopoids in terms of densities thus it cascaded in the same cluster with cyclopoids. Boina had the furthest linkage distance from all species forming a cluster while Chaoborus and calanoids formed another cluster which is well distinguished from others (Fig. 3).
A cluster analysis of zooplankton densities indicated that riverine and pelagic samples accounted above 61% similarity. Nevertheless, pelagic samples displayed a high degree of similarityabove 85%. There was a high degree of individuality between riverine and pelagic sampling sessions that were conducted in March (R), June (R2), March (P), June (P2) as displayed in Fig. 4.
Cyclopoid species (cyclopoid adults, nauplii and copepodites) constituted a relatively high density of the zooplankton community in the basin, with the highest densities recorded in June both riverine and pelagic sites. Cladocerans (Diaphanosoma and Boina) had the lowest densities whilst calanoids were only encountered in riverine sampling sessions as shown in Figs. 5a and 5b.
The zooplankton species density between the riverine and pelagic samples were significantly different (P0.05) (Table 1). Variations among samples within the same site were not significant (P > 0.05).
3.3 Zooplankton Diversity
The established mean species diversity of zooplankton in pelagic and riverine sites was low (< 0.3). Nevertheless, the riverine site had higher diversity in terms of species richness compared to the pelagic zone of the basin (Table 2). Cyclopoid adults, nauplii
and copepodites had the highest species diversity with
Calanoids being the least diverse in both sites.
3.4 Kapenta Prey Selectivity
Riverine sub-adults and adults specimens showed a high preference of Chaoborus showing an erage above 0.8. Pelagic Kapenta classes selected copepodite, cyclopoid adults and Diaphanosoma. However, selection of boina, calanoids and nauplii was low for both sites, but a slight increase in selectivity was noticed pelagic waters (Fig. 6). 3.5 Diet Overlaps
There was a high dietary overlap (αShc ~ 1) between the different sizes of Kapenta from pelagic and riverine as shown in Table 3.
3.6 Size Class Distribution of Kapenta
Juveniles (X ≤ 29 mm) and sub-adults (30-39 mm) of Kapenta constituted above 50% of catches from
pelagic and riverine respectively. Adults greater than 50 mm could not exceed 6% of the catches on both sites (Fig. 7). The mean (SL) of Kapenta classes for the two sites were above 30 mm whilst juveniles were only observed in riverine specimens.
4. Discussion
4.1 Gut Content of L. miodon
The presence of chironomids in the diet of different age classes of Kapenta in the Sanyati basin concurs with findings of other researchers [2, 28] who studied the feeding ecology of Kapenta in Lake Kivu. Though some authors [23, 29] argue that Kapenta in pelagic waters are exclusively zooplanktivore, the findings from this study offer a sharp contrast to this notion given that chironomids were noticed in stomachs of pelagic specimens. However this may be attributed to the distance of the sampling site from the island. Presence of all cladocerans (Boina longirostris, Boinopsis deitersi and Diaphanosoma excisum) was also observed in the stomachs of Kapenta by Goswami et al. [23]. Although Masundire et al. [18, 23] encountered rare large cladocerans like Daphinia lumholtz, few individuals of this nature were encountered in the current study. This contrast could be a reflection of a shift in prey size as a result of increased predation which is a known phenomenon in classical feeding ecology [29].
The prey size occurrence in stomachs of different Kapenta sizes could be linked with the length size of Kapenta (Figs. 2 and 3). Similarly, Isumbiso et al. [28]
established that prey size is relatively proportional to fish size. This interaction is similar to interfaces in terrestrial ecosystems where large carnivores such as Panthera leo predates on larger herbivores like Tragelaphus strepsiceros, Connochaetes taurinus while all predator like Lycaon pictus resort to all antelopes like Aepyceras melampus [30]. It is argued that this feeding pattern reduces competition between sub-adult and adult classes of Kapenta considering the predation of chironomids, chaoborus and cyclopoids.
The cannibalistic nature of L. miodon specimen stomachs in the current study corresponds with the studies carried in East African Lakes [4]. Previously in Lake Kariba, Pole et al. [31] reported that larvae and
juveniles of Kapenta were frequently the only gutcontent of adults as all as 60 mm in length. Cannibali is advantageous for adults because they would acquire more energy faster than chasing the microscopic zooplankton and also the search and detection of prey is easier. Kapenta specimens as all as 45 mm SL were observed to be cannibalistic. Nevertheless, it cannot be diissed that sub-adult individuals can exert cannibali on juveniles given that Mandima [32] acknowledged that fishes can cannibalize a prey up to about 35% of their body length. Nevertheless there is no substantial support to this since no larvae were found in the gutcontents of these sub-adults. Noticeable selectivity for Chaoborus by different classes of Kapentainriverine was highly significant from our observations. The distinct size of Chaoborus makes it easier to be spotted and hence makes a species of metabolicimportancein terms of energy content compared with all zooplanktons. However chaoborus species are dipterans which are composed of a greater proportion of chitin material that is indigestible so it could not be the case of energy. 4.2 Zooplankton
Zooplankton densities dominated by copepods and a few cladocerans is similar to those observed in Lake Kivu [28]. However, the densities were slightly higher than of this study which could be attributed to differences in nutrient status of the lakes [28]. Densities of cyclopoids are akin to the findings of Masundire [18] in same basin in June 1986 and March 1987 that coincide with the sampling months of this study. This stability could suggest some form of dynamic equilibrium might he been reached in the ecosystem since copepods potray a strong resilience upon predation by L. miodon when compared to cladocerans [28].
Observed densities of Boina, Diaphanosoma and Calanoids were lower than 1,000 individuals per cubic meter for each species as compared to other studies [18, 29, 33]. It has been observed that Kapenta has high preference for larger organis such as calanoids than aller onesand could possibly be the reason for the variation. The observed variability of Diaphanosoma abundances as compared to copepods in our findings could be as a result of predation [28]. This can also be related to the poor resilience of cladocerans after predation. Although the riverine site(Nyaodza River mouth) has a aller catchment area, the activities therein particularly farming and mining like the Sanyati River could influence zooplankton densities of the riverine environment to be different from the pelagic site [33]. This corresponds well with the variations in the nutrient status established through MANOVA. The notable difference was that of chaoborus and calanoids that were found in riverine sites only. Masundire [18] noted that such species are associated with microhabitats such as river mouth that tend to be richer in nutrients such as nitrates and phosphates than pelagic areas. Water transparency tends to influence habitat preference and utilization by Calanoids and Chaoborus hence the abundance of L. miodon which is a visual feeder [29, 31, 34]. Thus predation on large form of zooplankton (Chaoborus and Calanoids) is expected to be reduced under condition of low water transparency [35]. 4.3 Size Class Distribution
Few juveniles were encountered in the pelagic waters compared to riverine environments. The nature of the riverine environment offers cover that reduces predation threat upon juveniles by large fish. High frequencies of juveniles support that riverine sites are Kapenta spawning sites [12, 28, 36]. This is facilitated by aquatic weeds and low water transparency. Fishing in shallow or riverine areas where juveniles occur could result in high juvenile mortality leading to recruitment overfishing. In Lake Kariba harvesting of fish in riverine sites is illegal because the fishing gear has a narrow selection (lift net mesh size of 8 mm) which could reduce the juveniles significantly as noted by
5. Conclusions
The densities of zooplankton species between pelagic and riverine sections of the Sanyati basin were observed to be different. The characteristics of the two sampled areas of Sanyati Basin are unique from each other and support certain zooplankton species accordingly. Densities of cyclopoid species he not changed over the years compared to cladocerans whose density and diversity is on a downward trend. Preying on chironomids by L .miodon classes is more defined in adults compared to sub-adult individuals. Sub-adult L. miodon prey on nauplii species than any other as compared to the adult class. Predation of Cladocerans, cyclopoid adults and copepodites by different Kapenta classes is not different between adults and sub-adults of Kapenta. It is therefore recommended that more substantial data be gathered on the dynamics and feeding ecology of L. miodon, density and abundance of zooplankton species in the Sanyati Basin in this era of climate change and continued environmental woes.
Acknowledgments
The authors are grateful to the Kapenta Company & Zambezi Protein for the assistance and support in Kapenta and zooplankton sampling. The contribution of the Parks and Wildlife Management Authority of Zimbabwe Fisheries Research Section is greatly acknowledged. The immense support and guidance by the University of Zimbabwe Lake Kariba Research Station (ULKR) staff made this study pleasant.
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