Mwijage et al. J of Biol Res-Thessaloniki (2018) 25:13 https://doi.org/10.1186/s40709-018-0084-4 Journal of Biological Research-Thessaloniki RESEARCH Open Access Diferences in trophic resources and niches of two juvenile predatory species in three Pangani estuarine zones, Tanzania: stomach contents and stable isotope approaches Alistidia Paul Mwijage1,2*, Daniel Abel Shilla2 and John Ferdinand Machiwa2 Abstract Background: Estuaries are primary habitats that serve as feeding and nursery grounds for most juvenile marine fsh. However, estuaries have been used as fshing grounds by the artisanal fshers in Tanzania. The slow-growing preda- tory fsh at juvenile and sub-adult stages are among the most frequently caught species that functionally enhance multiple linkages of energy pathways within the food web. Stomach contents and stable isotopes (δ13C and δ15N) were used to describe the nutritional sources and trophic niches between the co-existing benthic, predatory species, Carangoides chrysophrys and Epinephelus malabaricus in the Pangani estuary, Tanzania. Results: The fndings indicated signifcant inter-specifc variations in dietary composition (PERMANOVA, p 0.001, pseudo-F 15.81). The prey-specifc index of relative importance (%PSIRI) indicated that juvenile shrimps = (%PSIRI =51.4) and Teleostei (%PSIRI 26.5) were the main diets of C. chrysophrys while brachyura (%PSIRI 38.8), juvenile= shrimps (%PSIRI 25.6) and Teleostei= (%PSIRI 23.3) were important diets of E. malabaricus. The isotope= mix- ing models indicated that= the predatory fsh species accumulate= nutrients derived from similar autotrophic sources, microphytobenthos, seagrass and macro-algae via consumption of small fsh, including clupeids and mugilids. Yet, they signifcantly showed diferent isotopic niche width with varying degree of niche overlap across the longitudinal estuary gradient. This situation was justifed by the presence of basal food sources among the estuarine zones that isotopically were diferent. Conclusion: The reliance of both predators on clupeids and mugilid preys that are trophically linked with estuarine and marine basal food sources, is an indication of low estuarine food webs’ connectivity to the fresh water related food web. This situation is most likely threatening the stability of the estuarine food web structure. Management strat- egies and plans in place should be cautiously implemented to ensure the balanced anthropogenic freshwater use in the catchment and fshing activities, for the maintenance of the Pangani estuarine ecosystem health. Keywords: Juvenile fsh, Predator, Trophic resources, Niches, Pangani estuary *Correspondence: [email protected]; [email protected] 1 Tanzania Fisheries Research Institute (TAFIRI)—Kyela Centre, P. O. Box 98, Mbeya, Tanzania Full list of author information is available at the end of the article © The Author(s) 2018. This article is distributed under the terms of the Creative Commons Attribution 4.0 International License (http://creat​iveco​mmons​.org/licen​ses/by/4.0/), which permits unrestricted use, distribution, and reproduction in any medium, provided you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons license, and indicate if changes were made. The Creative Commons Public Domain Dedication waiver (http://creat​iveco​mmons​.org/ publi​cdoma​in/zero/1.0/) applies to the data made available in this article, unless otherwise stated. Mwijage et al. J of Biol Res-Thessaloniki (2018) 25:13 Page 2 of 16 Background the assessment of what types of prey have been histori- Te stability of the food web structure is one of the main cally assimilated by the species which is revealed by sta- determinants of an ecosystem’s health [1]. Preservation of ble isotopes [16]. Carbon and nitrogen isotope ratios estuarine food web structure is thus vital for maintaining refect long-term diets [16] and quantify the primary the ecological and functional roles of an estuarine ecosys- food sources [17] assimilated by the species during the tem. Estuaries in the world are primary habitats for the time the organism is growing. Te use of stable isotopes growth and survival of most juvenile marine fsh [2, 3]. is important, especially when characterizing the diet and Tey serve as nursery grounds since they provide diverse feeding interaction of predatory fsh species like group- food and shelter for most marine fsh across all trophic ers that have high prey regurgitation and stomach vacu- levels along the estuarine food web [4, 5]. Such fsh com- ity index [18]. Due to the ability to integrate assimilated munity assemblages comprise predatory fsh which are dietary signatures on a long-term basis and diferentiate economically and ecologically important in the mangrove dietary sources of marine and fresh water origin, stable estuaries along the coast of Tanzania. In these habitats, isotope analyses can better infer the nutritional sources artisanal fshers tend to fsh unselectively by using inap- and feeding relationships of juvenile marine predatory propriate fshing gears [6]. Tis is a concern especially species throughout the period they spent in the estuarine when the species caught include slow-growing fsh that nursery ground. attain large body size before reaching maturity [7, 8]. Juvenile marine fsh are currently dominating fsh Te dominance of many marine juvenile fsh in the assemblages in the Pangani estuarine system. Tis has Pangani mangrove estuary along with intense fshing resulted by the increasingly inland intrusion of sea water practices raises concern about the abundance of resource through tidal actions as a consequence of reduction in base and how do predatory fsh species with similar feed- fresh water infow into the estuary [19–21]. Te preda- ing habits, partition dietary resources and thus, manage tory marine fsh species present in the estuary among to co-exist. Te presence of species that occupy the same others, include lutjanidae (e.g. Lutjanus argentimacula- trophic level and demonstrate similar trophic niches tus), sphyraenidae (e.g. Sphyraena jello), hemiramphidae in such a highly disturbed estuary can be an indication (e.g. Hyporhamphus dussumieri), gobiidae, serranidae of the stability of the food web structure [6]. Tis is fre- (e.g. Epinephelus species), carangidae (e.g. Carangoides quently related to the occurrence of predatory fsh with species) and others [22]. Tese species difer in salin- trophic linkages originating from diverse trophic base ity tolerance along the salinity gradient, as well as feed- resources [9, 10]. Under such circumstances, loss of con- ing habitats and area within the water column [23]. Te nectivity within and across trophic levels is unlikely to feeding of some fsh species is directly or indirectly con- occur due to energy fow higher up the food web through nected with the mangrove habitats. Few marine preda- multiple channels [6, 10]. Pelagic, benthopelagic and tory species are abundant in the upper Pangani estuarine even benthic predatory fsh are important to maintain zone where the banks of the estuary are fringed with the food web stability since they link pelagic and benthic coconut palm trees. Lower and middle estuarine zones food chains in aquatic ecosystems through feeding rela- are dominated by mangrove forest [24]. Greasy grouper tionships [10, 11]. In addition, knowledge on the trophic or black-spot estuary cod, Epinephelus malabaricus and interaction among autotrophs, primary consumers and longnose trevally, Carangoides chrysophrys, are abundant predatory fsh can help describe how these predators predator species in estuaries at juvenile and sub-adult afect and depress the biomass of organisms down the stages [7, 23, 25, 26]. In the Pangani estuary, they are food web. Studies on the trophic relationships of marine abundant in the lower, middle and upper or innermost juvenile predators that link top predators and primary zone from the estuary mouth. Tey are among the mul- consumers are limited in estuarine systems [10, 12] par- tiple fsh caught by local fshermen throughout the Pan- ticularly in Tanzania where estuaries are highly disturbed gani estuarine zone. Unfortunately, no documented data due to fshing. are available for the movement, feeding pattern and other Stable isotopes and stomach content analyses are meth- ecological features of these species in Tanzanian estuar- ods that complement each other to gain more insight into ies [8]. A study conducted by Bhat et al. [23] in the Agha- the feeding relationships of fsh species and the degree nashini estuary, in India indicated that C. chrysophrys of dietary resources sharing in diferent aquatic systems was abundant throughout the year and across the estua- [2, 6, 13, 14]. Integrating these two methods is important rine reaches with average salinity ranging from 6.5 ppt for the interpretation of feeding results. Trough stom- (inner estuary) to 22.5 ppt (near estuary mouth). ach content analysis, it is easier to determine what types Like other groupers which are slow growing, juve- of prey (at high taxonomic level) are instantly ingested nile E. malabaricus use estuaries and other shallow by predators [15]. However, the method does not allow habitats over a year and after reaching sexual maturity Mwijage et al. J of Biol Res-Thessaloniki (2018) 25:13 Page 3 of 16 they migrate to deeper waters for spawning [7]. This Methods grouper is a protogynous hermaphrodite [8]. There- Study area fore, uncontrolled fishing of pre-mature individuals in Te study was carried out in the Pangani estuary the Pangani estuary can interfere with the reproduc- located along the west Indian Ocean, in northern Tan- tion cycle of the species. This grouper is also a vora- zania coastal areas at 38° 50′E, 5° 20′S and 39°E, 5°26′S cious predator, feeding on fish, macro-crustaceans and [32] (Fig. 1). Te estuary is permanently open, funnel- in some cases even on octopuses. The species uses a shaped and part of the Pangani River Basin of about sit and wait, ambush hunting style foraging behavior, 43,650 km2 [19, 33]. Tis basin is the third largest in and it is tolerant to low salinity and turbid environ- Tanzania.