PATTERNS OF JUVENILE REEF FISH DIVERSITY ACROSS A MANGROVE-SEAGRASS-CORAL REEF CONTINUUM IN SOUTH COAST KENYA

Abstract

Studying assemblage patterns of juvenile fishes across ecologically connected habitats is necessary for understanding connectivity. Mangroves, seagrass beds, and coral habitats are ecologically essential for the survival of juvenile fishes. These habitats are connected through various mechanisms related to larval supply, ontogenic feeding, juvenile migration, and opportunistic adult phases. A mangrove-seagrass-coral reef continuum on Kenya's south coast was investigated to assess connectivity using juvenile fish assemblages. Between August 2021 and August 2022, a beach seine was used to collect samples monthly for two days. Fish species diversity across habitats was determined using Shannon-Weiner diversity index (H'). Detrended Correspondence Analysis (DCA) was applied to investigate species associations with the three habitats. The trophic level of each fish species was obtained from FishBase. Sea surface temperature and salinity were measured monthly at each site using a YSI multi-parameter probe. Species associations with temperature and salinity were assessed using Canonical Correspondence Analysis (CCA). A total of 8,575 fish specimens representing 228 species from 52 families were recorded. The coral reef habitat had the most individuals (n = 3,186) and was the most species-rich (168 species), followed by seagrass (2,420 individuals, 132 species) and mangrove (1,783 individuals, 119 species). Significant differences in species diversity were observed across the habitats during the northeast monsoon (NEM) and the southeast monsoon (SEM) (p = 0.026). The Kruskall -Wallis test showed no significant differences in the mean number of individuals per haul across habitats H (2, N = 60) = 4.81, p = 0.090). It also showed no significant difference in the mean number of individuals per haul between seasons H(1, N=60) = 2.10, p = 0.150). A two-way ANOVA revealed significant differences in fish diversity across the habitats (df = 2; f = 5.677; p = 0.006). Pair-wise comparison using Tukey HSD test revealed differences in species diversity between the coral and mangrove habitats (p = 0.044), but no significant differences in species diversity between seasons (df = 1; f = 0.818; p = 0.097). Overall, 69 fish species were common across the three habitats. DCA results revealed a strong association of species Pempheris mangula, Yarica hyalosoma, Pristotis obtusirostris, Taeniamia fucata, and Apogon fragilis to mangrove habitat. CCA results showed a positive relationship with increased temperature in the mangrove habitat represented by Apogon fragilis and Taeniamia fucata, while species, such as Siganus luridus and Sphyraena barracuda, showed a positive relationship with increased salinity in seagrass habitat. The Kolmogorov-Smirnov (K-S) tests revealed no significant difference in trophic signatures between the mangrove and seagrass habitat (D = 0.2308; p = 0.4402), but a significant difference between the mangrove and coral habitat (D = 0.5385; p = 0.00055) as well as between the seagrass and coral habitat (D = 0.4615; p = 0.004906). This study demonstrates the existence of connectivity across a mangrove-coral seagrass continuum in Shimoni and recommends long-term studies on nearshore habitat connectivity for better conservation and management strategies, including ecosystem-based fisheries management.