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Kimirei I.A.,Radboud University Nijmegen | Kimirei I.A.,Tanzania Fisheries Research Institute | Nagelkerken I.,Radboud University Nijmegen | Griffioen B.,Radboud University Nijmegen | And 2 more authors.
Estuarine, Coastal and Shelf Science

Tropical shallow-water habitats such as mangroves and seagrass beds are widely acknowledged as important juvenile habitats for various coral reef fish species, most of which are commercially important to fisheries. Spatio-temporal variability in ontogenetic habitat use by fish among these tropical coastal ecosystems has rarely been investigated, yet there are sufficient reasons to believe that this plays an important role. In the present study, we test the spatio-temporal variability in patterns of ontogenetic habitat use by some mangrove/seagrass-associated coral reef fishes (Lethrinus harak, Lethrinus lentjan, Lutjanus fulviflamma and Siganus sutor). Abundances of these four species were investigated during two years in Tanzanian coastal waters, using underwater visual census in mangrove, seagrass, shallow and deep mudflat, and shallow and deep coral reef habitats. The study covered four distinct seasons of the year and was done at two spatially separated (>40. km) locations. Averaged across locations, seasons and years, juveniles (≤10. cm length) of the four study species had significantly higher relative densities in shallow-water (mangroves and seagrass beds) than in deep-water habitats (deep mudflats or coral reefs), whereas the opposite pattern was found for the adults (>15. cm). These findings suggest a strong and general pattern of ontogenetic habitat shifts from shallow- to deep-water habitats. However, specific habitat-use patterns of juveniles as well as adults differed significantly in time and space. Various species showed subtle to considerable flexibility in juvenile as well as adult habitat use across seasons, years, or at different locations. Furthermore, for some species the data suggest presence of ontogenetic habitat shifts at one location but lack thereof at the other location. In summary, ontogenetic habitat use needs to be considered at various spatial and temporal scales for the interpretation of habitat utilization by fish during different life stages. This is important for conservation and management of these habitats, as essential habitats or seasons may be ignored or over-emphasized with respect to their importance for fish during different parts of their life cycle. © 2010 Elsevier Ltd. Source

Igulu M.M.,Radboud University Nijmegen | Igulu M.M.,Tanzania Fisheries Research Institute | Nagelkerken I.,Radboud University Nijmegen | Nagelkerken I.,University of Adelaide | And 3 more authors.

Coastal ecosystems are energetically connected through passive transport of nutrients but also by migrations of motile organisms. Mangroves are highly productive tropical ecosystems that replenish offshore populations of many species, but we know little about the degree to which this production is fuelled by prey from mangroves, especially in the cases in which mangroves are only accessible at high tide. Different results have been obtained on the importance of mangroves as feeding habitats, confounded by differences in species composition, seascape configuration, and methodology. In the present study, we took a more holistic approach by exploring reliance by fishes on mangroves as a feeding habitat at multiple ecological levels: from individuals to species to communities in mangrove ecosystems from across the globe, using a stable isotope approach. A two end-member mixing model showed a wide range (12-72%) in degree of reliance on mangrove food sources by fishes from different studies across the globe. However, analyzed at the levels of individual fish and species, reliance was low (for example, <25% for 55% of the species worldwide, or <50% for 85% of species, respectively) even though they were collected from sites that differed in geographical location, tidal regime, seascape structure, and species composition. The high fisheries productivity of mangroves appears to be energetically supported largely by food sources from adjacent habitats. In light of the ongoing rapid demise and fragmentation of mangrove and adjacent ecosystems, loss of ecosystem connectivity is likely to affect the productivity and functioning of tropical coastal ecosystems and the services they provide. © 2013 Springer Science+Business Media New York. Source

Ngatunga B.P.,Tanzania Fisheries Research Institute
Freshwater Biology

Genetic differences among freshwater fish populations are dependent on life-history characteristics of the species, including the range of adult dispersal and the extent of homing to natal breeding grounds. However, the effects of variation in such characteristics on population genetic connectivity are rarely studied comparatively among closely related species. We studied population genetic structure within three congeneric cyprinid species from the Lake Malawi catchment that differ substantially in life-history traits and conservation status, using a combination of microsatellite and mitochondrial DNA markers. Mpasa (Opsaridium microlepis) is a large (70 cm total length) migratory species that spawns in rivers, but as an adult is exclusively known from the main lake body. Sanjika (Opsaridium microcephalum), is a medium size (30 cm total length) species that exists in lake breeding, river-lake migratory and apparently landlocked populations. Dwarf sanjika (Opsaridium tweddleorum) is a small non-migratory species (15 cm total length) that persists in small tributaries surrounding the main lake and adjoining rivers. The results revealed striking differences among the three species in spatial genetic structuring. The river-lake migratory mpasa showed only weak yet significant population genetic structure within the main Lake Malawi catchment, suggesting that there is no strong natal homing. The habitat-generalist sanjika showed only weak spatial genetic differentiation at microsatellite loci within the Lake Malawi catchment, but moderate structure in mitochondrial DNA, potentially reflecting male-biased dispersal. The river-restricted dwarf sanjika showed strong genetic structure in both microsatellite and mitochondrial DNA, suggesting strictly limited dispersal at both adult and juvenile stages. We conclude that contrasting migration life histories have resulted in dramatically different patterns of population genetic structure among these congeneric species. The observed patterns demonstrate how divergent life-history evolution may strongly influence broader patterns of population genetic connectivity in freshwater fish, with consequences for management and conservation. Specifically the results suggesting gene flow among Lake Malawi populations of mpasa, an IUCN red-listed 'Endangered' species endemic to the lake catchment, imply that conservation initiatives operating at both local and catchment scales are needed to reverse local population decline. Copyright © 2016 John Wiley & Sons Ltd. Source

Everson I.,Anglia Ruskin University | Taabu-Munyaho A.,National Fisheries Resources Research Institute NaFIRRI | Kayanda R.,Tanzania Fisheries Research Institute
Fisheries Research

Data from the first series of lakewide fisheries acoustic surveys of Lake Victoria, East Africa, have been re-analysed according to current protocols. Surveys took place in February and August each year between 1999 and 2002. The primary aim has been to estimate the standing stock of Nile perch and dagaa, the main species taken in commercial fisheries on the lake. The results show that over the period of the surveys from 1999 to 2002 there was no significant trend in the standing stock of either species with time although there was a significant seasonal effect higher values in February as compared to August. Information from bottom trawls during the surveys supports these conclusions. The results have been considered in the context of a food web from which it is concluded that in order for sufficient food to be present for the Nile perch there must be a significant proportion of the decapod crustacean Caridina present. It is noted that this species can be estimated acoustically using multifrequency echosounders but is very difficult with the single frequency system used on the 1999-2002 surveys. Refinements in the methodology will permit the simultaneous assessment of several key components in the food web and open the way to effective ecosystem-based fisheries management. © 2012. Source

Kimirei I.A.,Radboud University Nijmegen | Kimirei I.A.,Tanzania Fisheries Research Institute | Nagelkerken I.,Radboud University Nijmegen | Nagelkerken I.,University of Adelaide | And 3 more authors.

Mangroves and seagrass beds have long been perceived as important nurseries for many fish species. While there is growing evidence from the Western Atlantic that mangrove habitats are intricately connected to coral reefs through ontogenetic fish migrations, there is an ongoing debate of the value of these coastal ecosystems in the Indo-Pacific. The present study used natural tags, viz. otolith stable carbon and oxygen isotopes, to investigate for the first time the degree to which multiple tropical juvenile habitats subsidize coral reef fish populations in the Indo Pacific (Tanzania). Otoliths of three reef fish species (Lethrinus harak, L. lentjan and Lutjanus fulviflamma) were collected in mangrove, seagrass and coral reef habitats and analyzed for stable isotope ratios in the juvenile and adult otolith zones. δ13C signatures were significantly depleted in the juvenile compared to the adult zones, indicative of different habitat use through ontogeny. Maximum likelihood analysis identified that 82% of adult reef L. harak had resided in either mangrove (29%) or seagrass (53%) or reef (18%) habitats as juveniles. Of adult L. fulviflamma caught from offshore reefs, 99% had passed through mangroves habitats as juveniles. In contrast, L. lentjan adults originated predominantly from coral reefs (65-72%) as opposed to inshore vegetated habitats (28-35%). This study presents conclusive evidence for a nursery role of Indo-Pacific mangrove habitats for reef fish populations. It shows that intertidal habitats that are only temporarily available can form an important juvenile habitat for some species, and that reef fish populations are often replenished by multiple coastal habitats. Maintaining connectivity between inshore vegetated habitats and coral reefs, and conserving habitat mosaics rather than single nursery habitats, is a major priority for the sustainability of various Indo Pacific fish populations. © 2013 Kimirei et al. Source

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