Stable Oxygen and Carbon Isotopic Compositions of Lates stappersii Otoliths from Lake Tanganyika, East Africa

J. Great Lakes Res. 33:806–815 Internat. Assoc. Great Lakes Res., 2007

Stable Oxygen and Carbon Isotopic Compositions of Lates stappersii Otoliths from Lake Tanganyika, East Africa Aboubakar Sako1,*, Kenneth G. MacLeod2, and Catherine M. O’Reilly3 1Department

of Geology Geography and Environmental Sciences Stellenbosch University, Private Bag X1 Matieland 7602, South Africa 2Department

of Geological Sciences University of Missouri-Columbia Columbia, Missouri 65211-1380 3Department

of Biology Bard College Annandale-on-Hudson, New York 12504-5000 ABSTRACT. In this study, we measured growth trends in oxygen and carbon isotopic ratios in whole sagittal otoliths from three adult centropomid fish (Lates stappersii) from each of three sub-basins of Lake Tanganyika, East Africa. Sampling density was 20 to 50 samples per otolith. Both δ 18O and δ 13C values increase with age. The δ 18O data suggest that otoliths were precipitated near the expected equilibrium with the ambient environment (ca. +3.5‰) and support a migration pattern from surface waters during larval stages to deeper waters (40 to 80 m) for mature fish. Relatively high δ 18O values in the southern sub-basin are consistent with cooler temperatures in the region during seasonal upwelling. The δ 13C increase from otolith core to edge is large (up to 4‰) and is interpreted as due to ontogenetic changes in diet and contributions from a decrease in the proportion of respired CO2 incorporated into otolith carbonate as metabolic rates of the fish dropped with maturity. The data seem to successfully reveal life strategy and migration patterns of L. stappersii, document regional differences in lake conditions, and provide a record of temperature within the water column during which the fish lived. Higher resolution studies and analyses of historical samples could be used to constrain modern and past growth patterns, and to reconstruct past temperature gradients and productivity patterns in the lake. INDEX WORDS: ronment.

Ontogeny, δ 13C and δ 18O, temperature, growth profiles, sub-basins, ambient envi-

INTRODUCTION Lake Tanganyika has a potential yield of fish in several hundred thousand metric tons, and the fishery plays an important role in the livelihood of the people around the lake (Coulter 1977, Molsa et al. 1999). Three pelagic fish species, two clupeids, Stolothrissa tanganicae and Limnothrissa miodon, and one centropomid, Lates stappersii, support the bulk of artisanal and commercial fisheries in the lake (Coulter 1991). The stock levels and distribution of these fish have been subject to seasonal fluc*Corresponding

tuations in recent years (Ellis 1971, Pearce 1995, Plisnier 1997, Molsa et al. 1999). Although growth rate is the key variable that provides information on population structure of fish (Campana and Neilson 1985, Al-Hossaini et al. 1990), a reliable technique for assessing growth rates of the three species is lacking. Length-frequency analysis is the method used currently to assess growth rates of these fish. However, this method requires a large set of data with different age groups and does not provide direct information on individual fish. In the present study, we investigated the suitability of stable oxygen and carbon isotopic ratios of L. stappersii otoliths as a record of external conditions in which

author. E-mail: [email protected]

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Otolith Isotopic Composition of Lates stappersii from Lake Tanganyika the fish live, as well as their growth patterns and ontogenetic diet shifts. The formation of an otolith begins before hatching and continues throughout life of the fish (Campana and Neilson 1985, Thresher 1999). Consequently, in a mature fish, the core region of an otolith (i.e., nucleus) records environmental conditions experienced by the fish at the larval stage, whereas the edge records conditions at the adult stage (Kalish 1989). Otoliths are composed of about 95% aragonite (CaCO3) with most of the remainder being organic matrix (Campana 1999, Thresher 1999). Stable isotopic compositions of oxygen and carbon of aragonite has been used to identify marine fish stocks (e.g., Gao et al. 2001, Ayvazian et al. 2004, Jamieson et al. 2004, Gao et al. 2004), to study life history of individual fish, to reconstruct past temperatures in which fish lived (e.g., Gao and Beamish 1999, Guerra 1999, Begg and Weidman 2001, Blamart et al. 2002, Gao 2002, Høie et al. 2004) and to estimate fish age (Guerra 1999, Weidman and Millner 2000). The usefulness of stable isotopes in these studies, however, requires that otolith aragonite was precipitated in equilibrium with the ambient water or that sources of any disequilibrium precipitation are known, and that their magnitude can be meaningfully estimated (Grossman and Ku 1986). Laboratory and field studies have demonstrated that otolith δ18O values are at or near isotopic equilibrium with ambient water conditions (δ18O waters: δ18Ow and temperature), and they are largely independent of biogenic influence (i.e., vital effects; Grossman and Ku 1986, Kalish 1991, Patterson et al. 1993, Thorrold et al. 1997). In contrast, otolith δ13C values are often far from equilibrium with dissolved inorganic carbon ratios (δ13CDIC) of the ambient water (e.g., Kalish 1991, Gauldie 1996, Thorrold et al. 1997) and are thought to be strongly influenced by vital effects. In addition to δ13CDIC, the other source for otolith carbon is the metabolically derived carbon from respired CO 2. Several studies have reported that otolith δ13C values are primarily derived from inorganic sources, and only a small fraction is metabolically-derived (e.g., Gauldie 1996, McConnaughey et al. 1997, Schwarcz et al. 1998, Weidman and Millner 2000). However, Kalish (1991) reported that more than 30% of the Australian salmon otolith δ 13C is of metabolic origin. The contribution of metabolic activities to otolith δ13C compositions may therefore be climate-and species-specific. Further, at larval and juvenile stages during which the fish is highly

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mobile and the metabolic rates are at maximum level, inclusion of a considerable amount of 13Cdepleted in the otolith matrix occurs. As the fish matures and becomes less mobile, the metabolic rates decrease, leading to a lower contribution from respired CO2 and 13C enrichment in otoliths (Edward et al. 1972, Schwarcz et al. 1998). Finally, an ontogenetic shift in diet such as feeding at higher trophic levels that affects the δ 13 C values of respired CO2 could be superimposed on trends in otolith δ13C attributable to metabolism alone (e.g., Fry 1988, Schwarcz et al. 1998). We measured δ13C and δ18O values in aragonite samples collected parallel to growth bands in 20 to 50 samples from the core to the edge of whole otoliths of L. stappersii, the top predator in the pelagic zone of Lake Tanganyika (Coulter 1991). This relatively coarse resolution is used as a first step to: (1) test the suitability of otolith δ18O values in assessing microhabitat changes of L. stappersii through lifetime, and (2) to examine whether otolith δ13C values suggest dietary as well as metabolic shifts in L. stappersii during ontogeny. These data will help guide future studies, and the method may be applicable to other large tropical lakes around the world. Limnology of Lake Tanganyika Lake Tanganyika is located south of the equator (3°20′ to 8°50′S), with maximum depth at 1,470 m. The bottom water of the lake (ca. 600 m) is divided into northern and southern basins by a major sill, whereas each of the two basins is divided into several sub-basins by minor sills. The lake is meromictic with a permanent anoxic hypolimnion. The depth of hypolimnion is about 150 m in the northern basin and up to 250 m in the southern basin (Tiercelin and Mondeguer 1991). The thermocline varies between 50 to 80 m in the northern basin and is up to 150 m below the surface in the southern basin (Coulter and Spigel 1991). Surface temperatures of Lake Tanganyika range from 23.4 to 28.2°C and are controlled by seasonal winds, which can cause upwelling of deeper waters (Plisnier et al. 1999, Coulter 1991). Due to stronger upwelling in the southern end of the lake during the windy periods, average surface water temperatures in the southern end of the lake are relatively lower (24.0 to 24.4°C) than those in the north (24.6 to 26.1°C) (Plisnier et al. 1999). Salinity is low (+0.58‰, Branchu and Bergonzi 2004) and fairly constant across the lake, suggesting that δ18Ow differences

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Sako et al. among lake basins should not be a significant factor in determining otolith δ18O patterns. However, increases in δ18Ow and decreases in temperature with depth (Fig. 1; Craig 1974, Plisnier et al. 1999) may be the dominant cause of ontogenetic δ18O trends in Lake Tanganyika. In addition, evaporation does strongly influence the δ18Ow of lake waters on a long-time scale. The δ18Ow compositions of the Ruzizi, Malagarasi, and Lufubu rivers, the three major rivers that contribute to the lake recharge, are isotopically lighter than those of the lake. Values for δ18Ow were –0.12‰, –1.42‰ and –3.72‰V-SMOW (V-SMOW = Vienna Standard Mean Ocean Water) for the Ruzizi, Malagarasi, and Lufubu rivers, respectively. In contrast, δ18Ow values of the lake near the surface (0 to 250 m) vary from +3.50 to +4.18‰ V-SMOW (Craig 1974). The strong enrichment in the lake’s δ18Ow is explained by the high relative loss of water from the lake due to evaporation (55.3 km3/year) compared to outflow (9.7 km 3 /year, Branchu and Bergonzini 2004). Because surface waters of the lake are well mixed and fractionation during evaporation overprints the δ18Ow of riverine sources, isotopic differences among rivers have little influence on surface δ18Ow values except near the input areas.

FIG. 1. A) Stable oxygen and carbon isotope profiles of the rainy season 1973 from Kigoma sub-basin (data from Craig 1974). B–C) Average temperature profiles of Lake Tanganyika from August 1993 to July 1994 (data from Plisnier et al. 1999).

Ecology of Lates stappersii Lates stappersii (Boulenger 1914) is the second largest fish species in Lake Tanganyika and the dominant predator in the pelagic zone (Coulter 1991). In contrast to other economically important fish species of the lake, S. tanganicae and L. miodon, L. stappersii is slow-growing and relatively long-lived (ca. 7 years, Roest 1988). Its mean length at maturity is thought to vary from one subbasin to another (237 to 278 mm; Pearce 1985, Mannini et al. 1996). Variations in growth of L. stappersii, which are most pronounced in juveniles, have been attributed to progressive ontogenetic changes in their diet rather than seasonal events (Moreau and Nyakagen 1992). Juvenile L. stappersii (total length < 70 mm) feed on zooplankton, whereas adults (ca. total length, TL = 130 mm, ca. 10 month old) preferentially feed on the dominant clupeid S. tanganicae (Ellis 1978, Coulter 1991). Because of declining metabolic and growth rates with maturity, enrichment in otolith δ13C values from the core to the edge is expected. Moreover, the ontogenetic changes in diet may allow the use of L. stappersii otolith δ13C in evaluating carbon transfer from the primary producers to

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the upper trophic level (i.e., plankton to L. stappersii via clupeids) and to assess the changes in growth patterns. MATERIALS AND METHODS Sampling and Otolith Preparation Nine adult specimens of L. stappersii were collected during the rainy season (February to March) of 2003, three specimens each from Bujumbura, Kigoma, and Mpulungu sub-basins. Bujumbura and Kigoma are within the northern basin whereas Mpulungu is in the southern basin (Fig. 2). Total lengths (TL in mm) and weight (W in g) of the fish were measured. Sagittal otoliths were extracted and stored in small zip lock bags for further processing. Photographs of a whole and thin section of an otolith were taken with a light microscope at Stellenbosch University, South Africa (Fig. 3). FIG. 2. Lake Tanganyika location in East Africa. The sampling locations are indicated by squares, along with the basin names.

Otolith Isotope Analysis Otoliths were cleaned and ultrasonicated in MilliQ water bath for 15 min and air-dried. Starting at the center of each otolith, concentric sampling paths parallel to growth lines were milled, using a mounted and variable speed Dremel rotary drill with a 0.3 mm diameter (tungsten carbide bur). Some time averaging occurred as radial growth of otoliths is faster than increase in thickness, but for the early formed portion of each otolith special care was taken to minimize contamination of late formed aragonite on the upper and lower otolith surface. For the nine otoliths, 20 to 50 subsamples were col-

FIG. 3. Photograph of whole (A, aragonite samples were collected vertically) and thin section (B, showing the core region) of L. stappersii otolith (scale bar = 1 mm).

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TABLE 1. Summary of otolith oxygen isotopic ratios (expressed in per mil) of L. stappersii, collected from three Sub-basins of Lake Tanganyika. Range 1 = Maximum δ 18O - Minimum δ 18O; Range 2 = Edge δ 18O - Core δ 18O. Sub-basin ID Bujumbura BU22 Bujumbura BU24 Bujumbura BU42 Kigoma KG4 Kigoma KG8 Kigoma KG12 Mpulungu MP77 Mpulungu MP81 Mpulungu MP97 1 SD = standard deviation

TL (mm) 230 360 340 305 307 296 199 240 226

W (g) 234 246 291 175 191 177 53 86 85

Mean δ18O (SD1) 1.59(0.18) 1.51(0.18) 1.34(0.16) 1.53(0.20) 1.45(0.18) 1.25(0.16) 1.56(0.15) 1.47(0.27) 1.71(0.23)

N 50 19 19 24 36 33 33 34 34

Minimum δ18O 0.98 0.95 0.84 1.06 0.85 1.01 1.11 0.60 1.18

Maximum δ18O 1.84 1.75 1.56 1.79 1.68 1.53 1.73 1.88 2.00

Range 1 0.86 0.80 0.72 0.73 0.83 0.52 0.62 1.28 0.82

Range 2 0.42 0.70 0.72 0.57 0.80 0.38 0.32 1.28 0.34

TABLE 2. Summary of otolith carbon isotopic ratios (expressed in per mil) of L. stappersii, collected from three Sub-basins of Lake Tanganyika. Sub-basin Bujumbura Bujumbura Bujumbura Kigoma Kigoma Kigoma Mpulungu Mpulungu Mpulungu

ID BU22 BU24 BU42 KG4 KG8 KG12 MP77 MP81 MP97

TL (mm) 230 360 340 305 307 296 199 240 226

W (g) 234 246 291 175 191 177 53 86 85

N 50 19 19 24 36 33 33 34 34

Mean δ13O (SD) –9.63(0.99) –10.20(0.54) –9.76(0.50) –9.89(0.74) –8.73(0.58) –9.09(0.84) –10.25(0.50) –9.78(0.65) –9.52(0.67)

lected with an average spacing of < 200 µm. Powdered sample from each sampling pass (~50 µg) was transferred to an individual reaction vial and the otolith was cleaned (compressed air) before the subsequent sample was milled. Samples were reacted in 100% H3PO4 at 70°C on a Kiel III carbonate device and the CO2 generated was cryogenically distilled before online measurement of carbon and oxygen isotopic ratios on Finnigan MAT DeltaPlus gas ratio mass spectrometer. Analyses were performed in the Biogeochemistry Lab at the University of Missouri, Columbia, and results are reported in the standard delta notation, relative to the Vienna PDB (V-PDB) standard. Instrument calibration within each run was based on multiple analyses of the NBS-19 carbonate standard using nominal values of +1.95‰V-PDB for δ13C and –2.20‰V-PDB for δ18O. External precision, estimated from uncorrected results for NBS-19, was better than ±0.03‰ for δ13C and ±0.05‰V-PDB for δ18O (1σ standard deviation).

Minimum δ13C –12.74 –11.90 –11.52 –11.95 –11.13 –10.94 –11.79 –12.34 –11.21

Maximum δ13C –8.56 –9.46 –9.18 –8.93 –7.96 –8.20 –9.58 –9.19 –8.61

Range 1 4.18 2.44 2.34 3.02 3.17 2.74 2.21 3.15 2.60

Range 2 4.11 2.31 2.35 2.68 1.63 2.52 1.07 2.93 2.25

RESULTS Otolith values (Table 1) were close to the expected equilibrium value assuming temperatures of 23.4 to 28.2°C and δ 18 O w of +3 to 4‰ V-SMOW , whereas otolith δ13C values deviated considerably from the equilibrium with DIC (+1.5 to +0.36‰V-PDB in the lake’s surface waters (Fig. 4, Table 2). Oxygen isotopic compositions were slightly higher around otolith edges of the fish collected from the Mpulungu Sub-basin (+1.62 to +1.88‰V-PDB) compared to those from the northern basin (+1.40 to +1.65‰ V-PDB). In all nine samples, otolith δ 13C and δ 18O values increased from the cores to the edges with larger fluctuations in δ13C values compared to those of δ18O (Standard deviations, Tables 1–2). Additionally, the isotopic compositions along the growth profiles exhibited the same trend in the three sub-basins, which was characterized by a steep increase in both δ18O and δ13C values (Fig. 5). Thus, the increase in otolith δ13C values was greater between larval and the beginning of piscivoδ18O

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FIG. 4. Otolith δ 13C and δ 18O growth series (in ‰, relative to the Vienna Peedee Belemnite, VPDB) of L. stappersii collected from the Bujumbura (BU), Kigoma (KG), and Mpulungu sub-basins (MP). Both isotope signatures increase with age from the larval stage (cores) to time of fish captures (edges). rous stage (+0.80 to +2.90‰ V-PDB) compared to the adulthood (+0.3 to +1.20‰ V-PDB, Fig. 5). The total increase in otolith δ13C values from the larval to the adult stage varied between +1.07 to +4.11‰ V-PDB (Table 2). DISCUSSION Otolith δ18O as an Indicator of Microhabitat Changes Assuming approximate oxygen isotopic equilibrium between otoliths and the ambient water, increase in δ18O from the core to the edge of each otolith suggests some combination of decreasing temperatures and increasing δ18Ow are experienced by L. stappersii throughout their life cycle. This pattern is particularly consistent with the combined effects of δ18Ow and temperature, as several studies have established a direct relationship between

otolith δ18O, δ18Ow and the ambient temperature (e.g., Kalish 1991, Thorrold et al. 1997, Gao 2002, Høie et al. 2004). That is, otolith δ18O values suggest that larval L. stappersii occupy warm and relatively δ18O-depleted surface waters and migrate to deeper, cooler and δ18O-rich waters as they mature. The narrow range of otolith δ18O values at maturity indicates that adult L. stappersii preferentially live in a depth with small variability in δ18Ow and temperature (Fig. 5). High otolith δ18O values of specimens from the south compared to those from the northern sub-basins is consistent with higher δ18Ow values and lower temperature in the southern basin due to the pattern of upwelling. The relationship between otolith δ18O values and the lake’s temperature was evaluated using the empirical equation developed by Gao (2002) for wild-caught cod otoliths.

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FIG. 5. Otolith δ 18O profiles of L. stappersii from the three sub-basins and estimated otolith temperatures (°C). Otolith temperatures were calculated, assuming the average ambient δ 18Ow of the lake to be +3.50‰. T (°C) = 17.6 – 4.2 (δc – δw)

(1)

Where T = the ambient lake water temperature, δc = otolith δ18O in V-PDB, and δw = the average water δ18O of the lake in V-SMOW, assumed to be +3.5‰ (Craig 1974). The calculated otolith temperatures ranged from 23.6 to 28.7°C (Fig. 5, Table 3). These values match well with the thermal structure of Lake Tanganyika. For comparison with reported ambient δ18Ow values (Fig. 1), δ18Ow values were also calculated using Eq. 1. The average ambient temperatures of the lake at 0 m were estimated from Figure 1 to be 26.2°C, 26.44°C, and 25.85°C for Bujumbura, Kigoma, and Mpulungu sub-basins, respectively. The average calculated δ18Ow values at core regions were +3.00 ± 0.06‰, +3.08 ± 0.12‰ and + 3.09 ± 0.48‰ for specimens caught in Bujumbura, Kigoma, and Mpulungu sub-basins, respectively. Further, the difference between measured (+3.50‰ V-SMOW) and estimated δ 18O w

values is about +0.5‰ V-SMOW , suggesting that L. stappersii otolith δ18O should be a suitable tool for reconstructing the environmental temperatures with errors less than +1‰. However, to be able to predict the exact seasonal or annual changes in water temperatures, using otolith δ18O values, more data on the ambient δ18Ow values of the lake are needed. An accurate estimation of L. stappersii age through otolith microstructure may also allow microsampling of aragonite materials at seasonal and annual resolutions, and that could provide further insights into the ecology of L. stappersii and their stock structure overtime. Otolith δ13C Compositions and Growth Patterns 13 Otolith δ C is much lower than δ13CDIC, suggesting a significant incorporation of respired CO2 with low δ13C values during otolith formation. In

Otolith Isotopic Composition of Lates stappersii from Lake Tanganyika TABLE 3. Predicted lake temperatures (T; °C), occupied by L. stappersii calculated from otolith δ18O and ambient δ18Ow. Maximum (0 m depth) and minimum (80 to100 m) temperatures are similar to Lake Tanganyika surface and deep-water temperatures, respectively. Mean T (SD1)

Min T

25.6 (0.4) 26.0 (0.8) 26.7 (0.7)

24.6 25.0 25.0

28.2 28.3 28.3

3.6 3.3 3.3

25.9 (0.8) 26.2 (0.7) 27.0 (0.7)

24.8 25.2 25.9

27.8 28.7 28.0

3.1 3.5 2.2

Mpulungu MP77 25.3 (0.6) MP81 26.1 (1.1) MP97 24.7 (0.9) 1SD = standard deviation

24.7 24.4 23.6

27.2 29.8 26.9

2.5 5.4 3.3

Sample Bujumbura BU22 BU24 BU42 Kigoma KG4 KG8 KG12

Max T Range

addition, all specimens exhibit an increase of δ13C from 1‰ to 3‰ through ontogeny which could be explained as decreasing metabolic contribution (consistent with slowing growth rates with age; Schwarcz et al. 1998, Begg and Weidman 2001) combined with a change in the isotopic composition of respired CO 2 (consistent with changes in diet through the life cycle). To estimate the relative contribution of respired CO 2 through ontogeny, we used an equilibrium model between otolith δ 13C values and the possible sources of carbon incorporation into otolith matrices (Schwarcz et al. 1998, Jamieson et al. 2004). The model is based on the assumption that there is equilibrium between the otolith δ 13 C and the δ 13 C DIC of the endolymph (fluid medium), and the endolymph δ13CDIC composition is similar to the fish blood or flesh δ13C. The latter will vary with fish diet and external DIC. The model also includes variations in the isotopic fractionation between aragonite and endolymphatic bicarbonate (∆arag-HCO3, ca. 2.7‰; Romaneck et al. 1992). Consequently, otolith δ13C value at any point is the weighted sum of the fraction (M) of carbon derived from metabolic activity, which has approximately the same δ13C values as the prey (δ13Cd), and fraction derived from external DIC with a composition δ13CDIC (Eq. 2).

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δ13Cc = Mδ13Cd + (1-M)δ13CDIC + ∆arag-HCO3 (2)

Since L. stappersii change diet and habitat through ontogeny, we considered end-member scenarios for larva assuming life at the surface (δ13CDIC = 1.4‰) and diet of plankton (δ13Cd = –24.5‰V-PDB), and adults assuming life at 100 m (δ13CDIC = 0.4‰) and a diet of S. tanganicae (δ 13 C d = –21‰ V-PDB ; O’Reilly et al. 2002). Solving for metabolic contribution with these numbers yielded ranges in metabolic fraction differences between otolith cores and edges of 0.70 to 4.23% (with an exceptional value of 10.19% for BU22). Further, the metabolic contributions to L. stappersii otolith δ13C compositions at individual sampling points were greater (53 to 65%) than those reported for fish from temperate zones (20 to 30%; Gauldie 1996, McConnaughey et al. 1997, Schwarcz et al. 1998, Weidman and Millner 2000). Metabolic activities and changes in feeding habits through ontogeny are the major explanatory factors of the sequential enrichment in L. stappersii otolith δ13C values. The steep increase in otolith δ13C values at the early stage of L. stappersii life relative to the adult, characterized by high growth rates, was clear evidence that spatial and seasonal changes in δ13CDIC (i.e., upwelling and latitudinal variations) may have less impacts on the early fish growth compared to ontogenetic changes in diet. Despite the general trend of enrichment in otolith δ13C from larval to adulthood, the data showed that the maximum carbon isotopic values were not recorded at otolith edges (Range 1 > Range 2). The highest carbon isotopic values may be attained at the onset of sexual maturation of L. stappersii. During this period, most of metabolic carbon is internally diverted for reproductive processes, leading to high otolith δ13C values (Schwarcz et al. 1998, Weidman and Millner 2000, Begg and Weidman 2001, Gao et al. 2001). Consequently, the reproductive stage is the final important factor affecting enrichment of δ13C composition in L. stappersii otoliths. Our results suggested that otolith δ13C profiles could be used to assess growth rates and life history of individual fish. However, seasonal migration of L. stappersii may obscure the interpretation of δ13C at otolith edges for fish from different sub-basins. Further studies that include more individuals, micromilling at seasonal and annual resolution as well as actual values of DIC across the lake will provide a better understanding of the impacts of seasonal changes of temperature and metabolic rates on the lake’s productivity. These new data highlighted the

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ecological and environmental importance of using otolith δ13C and δ18O in large tropical lakes such as Lake Tanganyika. Owing to its habitat preference at adulthood (i.e., deep pelagic waters), its relative longevity and the high seasonal fluctuations in surface water temperatures, L. stappersii otolith δ18O can be used to estimate temperature. The study also has demonstrated that otolith δ13C could be used to assess the growth and metabolic rates of L. stappersii and, thus, evaluate changes in the lake’s productivity. Examination of L. stappersii otoliths has been shown to be a promising tool for investigating their life history and interactions with the surrounding environment. ACKNOWLEDGMENTS We thank Willy Mbemba, Ismael Kimirei, Mukuli Mupape, and Lawrence Makasa for their invaluable assistance in sample collections. Logistic assistance from Tanzania Fisheries Research Institute and Zambian Fisheries Unit is greatly appreciated. Eric Livingston and Damon Bassett from the Biogeochemistry Lab, University of Missouri, Columbia, assisted with isotopic analyses. REFERENCES Al-Hossaini, M., Liu, Q., and Pitcher, T.J. 1990. Otolith microstructure indicating growth and mortality among plaice, Pleuronectes platessa L., post-larval subcohorts. J. Fish. Biol. 35A:81–90. Ayvazian, S.G., Bastow, J.P., Edmonds, J.S., How, J., and Nowara, G.B. 2004. Stock structure of Australian herring (Arripis georgiana) in southwestern Australia. Fish. Res. 67:39–53. Begg, G., and Weidman, C. 2001. Stable δ13O and δ18O isotopes in otoliths of haddock Melanogrammus aeglefinus from the northwest Atlantic Ocean. Mar. Ecol. Prog. Ser. 216:223–233. Blamart, D., Escoubeyrou, K., Jeuillet-Leclerc, A., Ouahdi, R., and Lecomte-Finiger, R. 2002. Composition isotopique δ18O–δ13C des otoliths des populations de poisons récifaux de Tairo (Tumotu, Polynésie française): implications isotopiques et biologiques. C.R. Biol. 325:99–106. Boulenger, G.A. 1914. Mission Stappers au TanganikaMoero. Diagnoses de poissons nouveaux. I. Acanthoptérygiens, Opisthomes, Cyprinodontes. Rev. Zool. Bot. Afr. 3:442–447. Branchu, P., and Bergonzini, L. 2004. Chloride concentrations in Lake Tanganyika: an indicator of the hydrological budget? Hydrol. Earth System Sci. 8(5):256–265.

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