Habitat utilisation by juveniles of commercially important fish species in a marine embayment in Zanzibar, Tanzania

Abstract

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 subadults. 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.

Seagrass structural complexity and landscape configuration as determinants of tropical fish assemblage composition

“…all fish identified to species level), 63 species were dominated by juvenile and/or subadult individuals, including 25 species that were almost exclusively restricted to the juvenile size class. This predominance of young fish in seagrass meadows has been found earlier in the studied embayment (Dorenbosch et al 2005a, Lugendo et al 2005 as well as elsewhere in the western Indian Ocean region (e.g. Gell & Whittington 2002, Gullström et al 2002, Dorenbosch et al 2005b).…”

“…Mixed plant assemblages (seagrass and macroalgae) are commonly scattered within the channel system. The southern bay shoreline is fringed by an extensive mangrove forest (~3000 ha) with 7 mangrove species identified and several tidally influenced creeks characterized by mangrove prop roots (Lugendo et al 2005). In the vicinity of the mangrove forest are widespread sand flats, partly covered by T. hemprichii and Cymodocea spp.…”

“…Even though multiple regression analyses did not show any significant relationships with fish density, biomass, or species richness, our study did show distinct multivariate patterns with a structural organization of fish assemblages, irrespective of size class, that was significantly correlated with the distance from seagrass meadows to neighboring mangrove and coral-reef habitats, suggesting that landscape configuration and connectedness among adjacent habitats are essential factors shaping fish assemblage composition. Lugendo et al (2005) examined habitat utilization of juvenile fish in Chwaka Bay and found a high number of species in seagrass channels. They suggested that this high diversity was due to the geographic location of seagrass habitats, in between an extensive mangrove forest and a deeper coral reef, and that the seagrass channels may operate as tide-related refuges (Dorenbosch et al 2004a) or as corridors for transient fish undertaking ontogenetic migration from mangrove to coral-reef habitat (Nagelkerken et al 2000).…”

Connectivity in reef fish assemblages between seagrass and coral reef habitats

“…non-nursery species) may migrate short distances away from the reef towards adjacent habitats for food (Tulevech & Recksiek 1994, Kaunda-Arara & Rose 2004 and increase densities of some non-nursery species in habitats adjacent to the reef. Seagrass beds within the Indo-Pacific may also function as corridors between mangroves and coral reefs for fish that undertake ontogenetic migrations, from shallow water habitats to the reef, once they mature (Lugendo et al 2005).…”

The mangrove forests as nursery habitats for the ichthyofauna of Mida Creek (Kenya, East Africa)

“…These isotopic signatures suggest that juvenile and/or adult fish probably choose more than one habitat to feed, find shelter or both (Lugendo et al, 2006(Lugendo et al, , 2007. Their movements may be triggered by several factors: tides, food availability, variation in salinity and/or ontogeny (Cocheret de la Morinière et al, 2003;Lugendo et al, 2005Lugendo et al, , 2006Nakamura et al, 2008;Kimirei et al, 2013). Mangroves may, therefore, be designated as potential nurseries for pelagic and demersal coral reef fish and crustaceans (Laegdsgaard & Johnson, 2001;Nagelkerken et al, 2002;Nagelkerken, 2009).…”