Phase III – Larval Settlement Rates and Reef Fish Recruitment Dynamics in Coastal Kenya: Implications for Fisheries Management and Conservation in Eastern Africa

Fish recruitment variability and larval supply within marine habitats have an effect on community structure of adults, yield of stocks and on patchiness of populations in space and time. However, despite this effect, few studies have examined this variability in in coastal East Africa and the Western Indian Ocean (WIO) in general, especially in relation to marine park boundaries. In particular, there is a general lack of knowledge about the sources of larval and juvenile fishes found on reefs. Also, little is known about the rates at which fish settle from the plankton and hence the extent to which fish stocks may be limited by larval settlement. The overall goal of this research project was to provide useful data in designing additional marine protected areas in coastal Eastern Africa and for managing exploited reef fish stocks. The specific objectives therefore were: (1) To evaluate seasonal settlement rates of pre-settlement larval stages of coral reef fish species. (2) To assess the abundance of pre-settlement larval stages and juvenile and adult fish within protected and unprotected reefs. (3) To compare the influence of different habitat types (reef vs. seagrass sites) on larval settlement and fish recruitment between protected and unprotected reefs. (4) To estimate larval pelagic duration and relate it with the composition and abundance of settling fish species within and outside the protected reefs. In addition, the project aimed at establishing resources (e.g. reference materials and manpower) necessary for capacity building for institutions in Eastern Africa in recruitment studies and otolith aging techniques, useful in most population dynamic studies, as well as to increase the regional capacity for species identification of juvenile and larval fish.

The study was conducted at protected reefs in the Kenyan Marine National Parks, Malindi and Watamu, and the adjacent fished sites. The target beneficiaries of the project included the scientific community, MPAs managers, decision and policy makers, and other relevant stakeholders in the WIO region.

Larval supply, juvenile recruitment rates and adult fish abundance were sampled using a combination of oblique plankton tows, light-traps, fish-traps and visual counts. Additionally, pelagic larval duration was discerned from otolith microstructure. The data was analyzed for seasonality of recruitment and the extent to which reef fish abundance and diversity may be limited by larval settlement rates. The project, the first of its kind in East Africa, provided a deeper understanding of the importance of reefs and surrounding seagrass areas for fish population dynamics and the importance of recruitment dynamics on fishery management and conservation.

Findings revealed that over 80% of the total larvae in the parks are comprised of preflexion larvae with relatively smaller proportions of the flexion and postflexion. Gobiidae, Blenniidae, Engraulidae and Apogonidae dominated the preflexion larvae component. There also existed a wide size range of larvae ((preflexion, flexion and postflexion) for some groups like Blenniidae, Gobiidae, Engraulidae, Labridae and Scaridae an indication of the possibility of some larvae completing their pelagic phase within the parks. Larval densities differed significantly between seasons in both parks, with higher densities during the calm northeast monsoon (NEM) season as compared to the rough SEM season, possibly due to optimal conditions for food production and warmer temperatures during this season. Larval abundance in Malindi Park was significantly higher during spring tides than neap tides. Moreover, nocturnal larval abundance in this park was 13-fold greater than daytime concentrations, regardless of tidal regime. Spectral time-series analysis revealed that larval fish supply in Malindi Park occurred in a 30-day cyclical pattern associated with the new moon and was synchronised in the long-term with the northeast monsoon season.

Findings further indicate a significant positive influence of zooplankton density on fish larval abundance in Malindi Park, and a positive and significant influence of temperature in Watamu Park. Distinct assemblages of larvae occurred within the parks, which could be an indication of within park patchiness in larval distribution and, perhaps existence of retention features. Notably, Watamu Park, which is more enclosed with restricted connection to open waters, was found to be dominated by fish larvae from demersal spawners; while the more open Malindi Park was, dominated by the larval pool from pelagic spawners.

The principal output of this research project is notably, a guide that provides a laboratory and field identification tool for a total of 37 fish families, commonly occurring along the coastal Kenya. This guide, the first descriptive work for the early life history stages of fishes in the wider WIO region contains photographs of larvae from archived materials as well as from fresh samples collected along the Kenyan coast.

Since seascape and oceanographic features appear to influence larval pools on these parks, findings from this research project may form important consideration in locating marine parks. These results also provide useful baseline data on larval distribution in the WIO, and will be useful in understanding population dynamics of adult populations when complimented with additional oceanographic data. The guide will be a very useful identification tool for larval stages of fish not only for fishes occurring along the Kenyan coast but within the whole WIO region. Future studies can focus on collection of more robust data so as to determine the existence and scale of larval retention within these parks.