Fish Recruitment Dynamics in Seagrass Habitats of Varying Coverage in Watamu, Northern Coastal Kenya

Abstract

Seagrass habitats are important nursery and recruitment areas for marine fishes; hence their conservation is an important aspect in fishery management. This study examined patterns in fish recruitment in seagrass habitats of varying seagrass cover; specifically, larvae and juvenile composition and abundance in seagrass habitats of Watamu. The study tested the hypotheses that sites, seasonality and hydrographic parameters influence larvae and juvenile composition and abundance. Two sites of varying seagrass cover were identified, Watamu Blue Lagoon (Site 1) and Watamu Beach (Site 2). Monthly samples of fish larvae and juveniles were collected during the Southeast monsoon (June-August) and Northeast monsoon (November-January) from 2019 to 2021. Environmental variables (water temperature, salinity and chlorophyll-a) were measured at each sampling occasion. Fish larvae were sampled by towing a 500 µm net behind a boat at a speed of 1 knot (meter per second) for 20 minutes, while juvenile fish were collected using a seine net of stretch mesh size 1.5 cm. Larvae were sorted on a dissecting microscope, identified to the lowest taxa possible. Developmental stage of larvae was determined as either preflexion, flexion or postflexion. Larvae belonged to a total of 35 families while juvenile belonged to 28 families. Majority of larvae (60%) were collected during the Northeast Monsoon (NEM). Larvae at preflexion stage dominated the catch during the NEM season while Southeast Monsoon (SEM) samples were dominated by larvae at postflexion stage. For juveniles, the dominant species were Lutjanus fulviflamma (family Lutjanidae), Lethrinus harak and L. mahsena (family Lethrinidae) and Siganus sutor (family Siganidae). Mean larval abundance was highest in November 2019 with a peak of 8.09 in site 1 and 12.14 larvae/100m3 in site 2. Mean juvenile abundance was also highest in year 1, December with 22 fish for site 1 and 50 fish in site 2. Differences in larval and juvenile abundance between the two sites were not significant (larvae p = 0.94, juvenile p=0.32), seasonal variation in larval abundance was significant for larval abundance (larvae p = 0.04, juvenile = 0.85). A few fish families such as Lutjanidae and Siganidae, were sampled both as larvae and as juveniles. A seasonal pattern was observed in larval and juvenile abundance, with an abundant and diverse larval and juvenile assemblage sampled in NEM compared to SEM. A generalized linear model showed that Chlorophyl-a, temperature, and zooplankton abundance positively correlated with larval abundance. Chlorophyll-a (positively) and salinity (negatively) correlated with juvenile abundance. The findings of the study demonstrate the importance of seagrass habitat to larval and juvenile developmental stages of marine fishes.