Use of replanted mangroves as nursery grounds by shrimp communities in Gazi Bay, Kenya

Study Number:

47

Author:

B.I. Crona & P. Ronnback

Abstract:

This study quantitatively assessed the distribution of postlarval and juvenile shrimps in natural, degraded and replanted stands of Sonneratia alba mangroves in Gazi Bay, Kenya in 2002e2003. Two plantations (matrix and integrated) differing in historical status and planting strategy were studied. Sampling was conducted using stake nets (2 mm mesh), each net enclosing 9 m2 of intertidal microhabitat. A total of 615 shrimps from 19 species/taxa were caught, including several penaeid species of major commercial importance. Penaeids dominated the catch (66%) followed by Macrobrachium spp. (16%) and Acetes sp. (6%). Shrimp abundance ranged from 0.42 to 10.0 ind. Per net (9 m2) for individual sites across spring tides and significant differences were detected between sites and over time (p<0.001). Results showed no significant difference in diversity of species/taxa between sites. However, multivariate analysis revealed significant differences in community assemblages between sites, except for the natural stand and integrated plantation. These two sites harbored higher abundances of the majority of all taxa caught. The observed distribution patterns are discussed with regard to measured environmental parameters such as elevation, structural complexity and sediment characteristics. 

Main Results and Conclusions:

• Post larval and juvenile shrimp assemblages (primarily penaeid shrimp, other species discussed) were studied at two different replanted mangrove stands (mangrove species used: Sonneratia alba), and then compared to shrimp assemblages at a natural stand and a deforested stand.  
• The characteristics of the replanted mangrove stands are as follows: “Site MP (matrix plantation) was planted on a clear-cut strip of fringing mangrove in a 1 X 1 m matrix, which has resulted in a dense monostand of S. alba with a relatively high degree of canopy cover and a more homogenous root complexity throughout the site than plantation IP. The latter site (integrated plantation (IP)) was planted in a degraded, but partly forested area adjacent to a natural stand of S. alb. Consequently, this site had a more heterogeneous appearance with natural canopy gaps and a higher variability in root complexity due to the diversity in age of the existing trees”(536).
• Differences in sedimentation between sites was observed: “The clear-cut area was characterized by coarser sediments, predominantly sand, while the forested sites had a higher percentage of the fine sediment fraction (< 0.075 mm) with highest values found in the matrix plantation (Table 1)”(538).
• The results from studying species richness of shrimp caught in each mangrove plot are as follows:
  • “No pronounced differences in species richness among sites were found. Instead diversity measures seemingly differed more between years (Table 2)”(539).
  • “The total number of shrimp species/taxa encountered at each site was also lower in the matrix plantation (eight taxa) compared to the other sites (10e 13 taxa)”(539).
  • “…the natural stand and integrated plantation had a more even relative distribution by species/taxa to total abundance (Fig. 2)”(539).
• The variability of shrimp abundance was high at each stand, and the amount of shrimp caught over the course of the study was inconsistent (541). However, results from certain shrimp assemblages show preference to natural and integrated mangrove replanting techniques:
  • “Penaeus semisulcatus, Acetes sp. and Macrobrachium spp. were found to be strongly associated with the integrated plantation and the natural stand (Fig 2)…”(541).  A possible explanation for this is the close proximity of seagrass beds in Gazi Bay—juveniles of these shrimp species occur in seagrass beds (de Freitas, 1986; Loneragan et al., 1994; Macia, 2005)”(541).
  • “Most individuals of Penaeus indicus were caught in the matrix plantation”(542). This may be explained by this species preference of mangroves over unvegetated habitat (Ronnback et al. 2001).
  • “The present study found M. monoceros to be moderately represented in all sites, although with a preference for forested areas”(543). This may be explained by this species preference of fine sediments and organic content found in higher concentrations in forested areas.
• Conclusions:
  • “Slightly higher abundances of the majority of shrimp species/taxa were caught in the integrated and natural stand of Sonneratia alba studied”(543).
  • These two sites had more even distributions of species found in the catches used for the study (543).
  • These two sites also “harbored a higher diversity of penaeid species likely due to greater heterogeneity in terms of structural complexity as well as longer inundation time”(543).

Works Cited:

de Freitas, A.J., 1986. Selection of nursery areas by six Southeast African Penaeidae. Estuarine, Coastal and Shelf Science 23, 901–908.

Loneragan, N.R., Kenyon, R.A., Haywood, M.D.A., Staples, D.J., 1994. Population dynamics of juvenile tiger prawns (Penaeus esculentus and P. semisulcatus) in seagrass habitats of the western Gulf of Carpentaria, Australia. Marine Biology 119, 133–143.

Macia, A., 2005. Juvenile penaeid shrimp density, spatial distribution and size composition in four adjacent habitats within a mangrove fringed bay on Inhaca Island, Mozambique. Western Indian Ocean Journal of Marine Science, in press.

Ronnback, P., Macia, A., Almqvist, G., Schultz, L., 2001. Do penaeid shrimps have a preference for mangrove habitats? Distribution pattern analysis on Inhaca Island, Mozambique. Estuarine, Coastal and Shelf Science 55, 427–436.