Browsing by Author "Van der Velde, Gerard"

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    Different Fish Composition in Seagrass Beds Adjacent to Extensive Mangrove Areas as Opposed to Coral Reefs
    (2007) Dorenbosch, Martijn; Grol, Monique G. G.; Nagelkerken, Ivan; Lugendo, Blandina R.; Van der Velde, Gerard
    Little is known about fish assemblages on seagrass beds located adjacent to different habitats. Visual census surveys were used to study the fish composition of two types of seagrass habitats in Zanzibar (Tanzania): seagrass beds adjacent to extensive mangrove areas in an embayment (bay seagrasses) and seagrass beds situated on the continental shelf adjacent to coral reefs (reef seagrasses). At species level, 39 fish species were common in the seagrass habitats, of which nine showed significantly higher densities in bay seagrasses, and four species were exclusively observed in bay seagrasses. Seine net data supported these data a the connectivity with mangroves.
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    Fish Community Composition of a Tropical Nonestuarine Embayment in Zanzibar, Tanzania
    (Wiley, 2007) Lugendo, Blandina R.; Nagelkerken, Ivan; Jiddawi, Narriman S.; Mgaya, Yunus D.; Van der Velde, Gerard
    By using a seine net, fish samples were taken from the nonestuarine Chwaka Bay (Zanzibar, Tanzania) from the mangroves, mud/sand flats and seagrass beds. Sampling was done twice per month between November 2001 and October 2002. In total, 150 fish species belonging to 55 families were identified. Diversity (H′) ranged from 1.9 in mud/sand flats to 3.4 within the Chwaka seagrass beds. Mean density of fishes was significantly higher in the mangrove creeks than in any other habitat (mean = 238.7 ind./1000 m2). Highest, but non-significantly different mean biomasses were recorded in the mangrove creeks (1.7 kg/1000 m2) and in the Marumbi seagrass beds (1.6 kg/1000 m2). The mangrove channel had the lowest biomass (0.6 kg/1000 m2). A high overlap in species composition (as high as 93.4% similarity) was found for adjoining habitats (i.e. mangrove creeks and mangrove channel), while habitats that were far apart showed low overlap (6.6% similarity for the Marumbi seagrass beds and mangrove creeks). On average, 58.4 and 63.2% in terms of abundance and biomass, respectively, of the fish assemblage of Chwaka Bay were of commercial fishery importance. Thus, Chwaka Bay appears to be an important juvenile habitat for various commercially important fish species.
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    Habitat Utilization by Juvenile of Commercially Important Fish Species in a Marine Embayment in Zanzibar, Tanzania
    (2005) Lugendo, Blandina R.; Pronker, Anna E.; Cornelissen, Ilse; De Groene, Arjan; Nagelkerken, Ivan; Dorenbosch, Martijn; Van der Velde, Gerard; Mgaya, Yunus D.
    Habitat utilisation by juveniles of 13 commercially important fish species was studied in five habitats located in Chwaka Bay, Zanzibar: mangrove creeks, mangrove channel, sand/mud flats, a seagrass area close to mangroves, and a seagrass area far from mangroves. Fish samples were collected from each habitat using a seine net, and fish abundance and size were measured to determine habitat utilisation. The seagrass beds near to mangroves showed the most diverse fish assemblage of all habitats, possibly because it functions as a corridor between the mangroves and deeper parts of the embayment. Juveniles of Cheilio inermis, Hipposcarus harid, Leptoscarus vaigiensis, and Scolopsis ghanam inhabited seagrass beds only. Juveniles of Gerres filamentosus and Monodactylus argenteus were mainly found in the mangrove habitats. Lethrinus variegatus, Pelates quadrilineatus and Siganus sutor were found in more than two habitats, with highest abundances in seagrass beds. Juveniles of Gerres oyena, Lethrinus lentjan, Lutjanus fulviflamma and Sphyraena barracuda were the most generalist species and were found in all studied embayment habitats. Visual census surveys supported the seine net data showing that most fishes in the embayment habitats were juveniles or sub-adults. In terms of habitat utilisation by different size classes, five of the 13 species (Lethrinus lentjan, L. variegatus, P. quadrilineatus, Siganus sutor and Sphyraena barracuda) were found as small-sized individuals in shallow and turbid mangrove areas, whereas large-sized individuals were observed in deeper and less turbid seagrass beds. A possible explanation for this pattern could be an ontogenetic shift in habitat utilisation, although this could not be proven. The patterns observed in the present study show a high similarity to those observed in marine embayments in the Caribbean, indicating that similar mechanisms are at work which make these systems attractive juvenile habitats.
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    The Importance of Mangroves, Mud and Sand Flats, and Seagrass Beds as Feeding Areas for Juvenile Fishes in Chwaka Bay, Zanzibar: Gut Content and Stable Isotope Analyses
    (Wiley, 2006) Lugendo, Blandina R.; Nagelkerken, Ivan; Van der Velde, Gerard; Mgaya, Yunus D.
    The relative importance of bay habitats, consisting of mangrove creeks and channel, seagrass beds, and mud and sand flats, as feeding grounds for a number of fish species was studied in Chwaka Bay, Zanzibar, Tanzania, using gut content analysis and stable isotope analysis of carbon and nitrogen. Gut content analysis revealed that within fish species almost the same food items were consumed regardless of the different habitats in which they were caught. Crustaceans (mainly copepods, crabs and shrimps) were the preferred food for most zoobenthivores and omnivores, while fishes and algae were the preferred food for piscivores and herbivores, respectively. The mean d13C values of fishes and food items from the mangrove habitats were significantly depleted to those from the seagrass habitats by 6 9 and 9 7% for fishes and food items, respectively, and to those from the mud and sand flats by 3 5 and 5 8%, respectively. Fishes and food items from the mud and sand flats were significantly depleted as compared to those of the seagrass habitats by 3 4 and 3 9%, for fishes and food, respectively. Similar to other studies done in different geographical locations, the importance of mangrove and seagrass themselves as a primary source of carbon to higher trophic levels is limited. The different bay habitats were all used as feeding grounds by different fish species. Individuals of the species Gerres filamentosus, Gerres oyena, Lethrinus lentjan, Lutjanus fulviflamma, Pelates quadrilineatus and Siganus sutor appeared to show a connectivity with respect to feeding between different habitats by having d13C values which were in-between those of food items from two neighbouring habitats. This connectivity could be a result of either daily tidal migrations or recent ontogenetic migration. # 2006 The Authors Journal compilation # 2006 The Fisheries Society of the British Isles
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    Mangrove Fish Production is Largely Fuelled by External Food Sources: A Stable Isotope Analysis of Fishes at the Individual, Species, and Community Levels from Across the Globe
    (2013) Mgaya, Yunus D.; Van der Velde, Gerard; Nagelkerken, Ivan; Igulu, Mathias
    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 pro
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    Relative Importance of Mangroves as Feeding Habitat for Juvenile Fish: A Comparative Study on Mangrove Habitats with Different Settings
    (2007-01) Lugendo, Blandina R.; Nagelkerken, Ivan; Kruitwagen, Guus; Van der Velde, Gerard; Mgaya, Yunus D.
    The importance of mangroves as feeding grounds for fish and other macrozoob-enthos in the Indian Ocean and elsewhere has been a subject of debate. This could partly be due to the fact that studies describing this role have been conducted in mangrove systems that differed in their settings. By using stable isotope analysis of carbon and nitrogen, we investigated two different settings of mangroves along the Tanzanian coast, to establish if mangrove setting influences the extent to which this habitat is utilized as a potential feeding ground by fish. The two mangrove settings were: mangrove-lined creeks which retain water during low tides and fringing mangroves that drain completely during low tides. The δ13C signatures of most fishes from the mangrove-lined creeks were similar to those of food items from the mangrove habitat, which suggests that these fishes feed from the mangrove habitats. In contrast, the overlap in δ13C of some food items from the fringing mangroves with those from adjacent habitats, and the more enriched δ13C signatures of fishes from the fringing mangroves with respect to most typical food items from the mangrove habitat could be an indication that these fishes feed from both habitats but to a lower extent from the fringing mangroves. The results suggest that fishes feed more from the mangrove-lined creeks as compared to fringing mangroves which is probably related to differences in the degree of mangrove inundation. The more or less continuous access provided more time for fishes to stay and feed in the mangrove-lined creeks compared to fishes from the fringing mangroves, which have access to these mangroves only during high tide and have to migrate to adjacent habitats with the ebbing tide.
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    Relative Importance of Mangroves as Feeding Habitats for Fishes: A Comparison between Mangrove Habitats with Different Settings
    (2007) Lugendo, Blandina R.; Nagelkerken, Ivan; Kruitwagen, Guus; Van der Velde, Gerard
    The importance of mangroves as feeding grounds for fish and other macrozoob-enthos in the Indian Ocean and elsewhere has been a subject of debate. This could partly be due to the fact that studies describing this role have been conducted in mangrove systems that differed in their settings. By using stable isotope analysis of carbon and nitrogen, we investigated two different settings of mangroves along the Tanzanian coast, to establish if mangrove setting influences the extent to which this habitat is utilized as a potential feeding ground by fish. The two mangrove settings were: mangrove-lined creeks which retain water during low tides and fringing mangroves that drain completely during low tides. The δ13C signatures of most fishes from the mangrove-lined creeks were similar to those of food items from the mangrove habitat, which suggests that these fishes feed from the mangrove habitats. In contrast, the overlap in δ13C of some food items from the fringing mangroves with those from adjacent habitats, and the more enriched δ13C signatures of fishes from the fringing mangroves with respect to most typical food items from the mangrove habitat could be an indication that these fishes feed from both habitats but to a lower extent from the fringing mangroves. The results suggest that fishes feed more from the mangrove-lined creeks as compared to fringing mangroves which is probably related to differences in the degree of mangrove inundation. The more or less continuous access provided more time for fishes to stay and feed in the mangrove-lined creeks compared to fishes from the fringing mangroves, which have access to these mangroves only during high tide and have to migrate to adjacent habitats with the ebbing tide.