Oceanographic Features Along the Kenyan Coast : Implications for Fisheries Management and Development

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Changing storminess and global capture fisheries

This is the author accepted manuscript. The final version is available from Nature Publishing Group via the DOI in this record.Climate change-driven alterations in storminess pose a signifcant threat to global capture fsheries. Understanding how storms interact with fshery social-ecological systems can inform adaptive action and help to reduce the vulnerability of those dependent on fisheries for life and livelihood.N.C.S. acknowledges the financial support of the UK Natural Environment Research Council (NERC; GW4+ studentship NE/L002434/1), Centre for Environment, Fisheries and Aquaculture Science and Willis Research Network

Multiple stressors cause rapid ecosystem change in Lake Victoria

1. Lake Victoria endured multiple stresses over the past century including population growth, increased cultivation of land, meteorological variability, resource extraction, intensive fishing, introduction of exotic species and more recently climate warming. These stressors became manifest through a fundamental and rapid change in the fish community and fishery in the early 1980s and visible eutrophication. However, the relation of these two phenomena and the possible interaction of the multiple stressors have been difficult to establish because of the temporally fragmented nature of the environmental data.2. Comprehensive limnological observations from the 1960s were repeated in the 1990s and established the eutrophication of the lake, but these do not provide insight to the time course of when changes in trophic state occurred. Comprehensive fishery catch data from 1965 to the present provide a time course of the change in community composition and yield but cannot be correlated in time with discontinuous and sparse limnological data to determine possible cause–effect relationships.3. Palaeolimnologic studies were conducted on three cores, two offshore and one nearshore, to establish a time course for the eutrophication of the lake that can be related to time-based data on the fishery. In the 1920s, the cores recorded an increase in nitrogen content of the sediments, but there was no significant response in the paleo-productivity indicators of biogenic Si deposition and change δ 13 C of deposited organic matter. Phosphorus deposition began to increase in the 1940s in all three cores after which biogenic Si deposition increased steadily over time. Responses in δ 13 C of organic matter begin in the 1960s at the coring sites. In the 1970s, the δ 13 C of organic matter at the nearshore site increased nearly 3‰ in a 10-year period likely as a response to a dramatic increase in internal P loading caused by spreading anoxia.4. Nile perch, the large predatory fish introduced in 1954, had become established through much of the lake at low abundances by the 1970s. In 1980, the catch of this fish began to increase, and by the end of the decade, the Lake Victoria fishery was the largest lake fishery in the world; and Nile perch dominated the catch. While catches of some other fishes also increased, the endemic haplochromines suffered a catastrophic decline in abundance and loss of biodiversity.5. The detailed chronostratigraphies for these sediment cores established that the major changes in the trophic condition of the lake were accomplished prior to the change in the fish community and that the increased primary productivity of the lake likely contributed to the increased fish catches after 1980. The increased algal abundance also would have greatly reduced visibility and facilitated the emergence of Nile perch as the dominant top predator.6. Thematic implications : multiple stresses were present in Lake Victoria over several decades, but transition to a new ecosystem state with a transformed food web and highly productive algal community may have been triggered by a period of low wind stress and then generally warming climate since the 1970s. Unless phosphorus loading is stabilised or reduced, the ecosystem’s diversity and balanced productivity will not recover, and other state transitions may occur to the detriment of the lake and its riparian populations.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/78648/1/j.1365-2427.2009.02374.x.pd

Basin-Scale Control on the Phytoplankton Biomass in Lake Victoria, Africa

The relative bio-optical variability within Lake Victoria was analyzed through the spatio-temporal decomposition of a 1997–2004 dataset of remotely-sensed reflectance ratios in the visible spectral range. Results show a regular seasonal pattern with a phase shift (around 2 months) between the south and north parts of the lake. Interannual trends suggested a teleconnection between the lake dynamics and El-Niño phenomena. Both seasonal and interannual patterns were associated to conditions of light limitation for phytoplankton growth and basin-scale hydrodynamics on phytoplankton access to light. Phytoplankton blooms developed during the periods of lake surface warming and water column stability. The temporal shift apparent in the bio-optical seasonal cycles was related to the differential cooling of the lake surface by southeastern monsoon winds. North-south differences in the exposure to trade winds are supported by the orography of the Eastern Great Rift Valley. The result is that surface layer warming begins in the northern part of the lake while the formation of cool and dense water continues in the southern part. The resulting buoyancy field is sufficient to induce a lake-wide convective circulation and the tilting of the isotherms along the north-south axis. Once surface warming spreads over the whole lake, the phytoplankton bloom dynamics are subjected to the internal seiche derived from the relaxation of thermocline tilting. In 1997–98, El-Niño phenomenon weakened the monsoon wind flow which led to an increase in water column stability and a higher phytoplankton optical signal throughout the lake. This suggests that phytoplankton response to expected climate scenarios will be opposite to that proposed for nutrient-limited great lakes. The present analysis of remotely-sensed bio-optical properties in combination with environmental data provides a novel basin-scale framework for research and management strategies in Lake Victoria

The Distribution of Skates and Rays along the Kenyan Coast.

The distribution, abundance, reproductive biology and the economic importance of skates and rays along the Kenyan Coast was studied between January 1980 and December 1981. The common species described in this paper are Raja miraletus, Taeniura lymna. Myliobatis aquila, Dasyatis thetidis. D. uarnak and D. sephen. These fish were present throughout the year with increased catches being realised in August, December and March. The size distribution of each fish is described. Linear regression analysis of the length/weight relationship for all the species indicate allometric growth. Raja miraletus. D. thetidis and D. uarnak exhibit sexual dimorphism. All fish, except Manta birostris, are carnivorous, feeding on crustaceans, molluscs and fishes. The distribution of skates and rays is discussed with reference to depth, temperature, salinity and the monsoonal phenomena. Temperature changes and low salinity water during the rainy season may act as a trigger mechanism for spawning.Publishe

Oceanographic Features Along the Kenyan Coast: implications for Fisheries Management and Development. Special Project Report.

This Special Project Report has the following major headings: 1. Introduction (a) Position; (b) Nature of Coastline; (c) Fisheries Status. 2. Oceanography (a) Currents and Water Masses; (b) Local Upwellings and Vertical Mixing Between the Water Masses; (c) Primary Productivity and the General. 3. Fish stocks and fisheries; (a) Crustaceans: (b) Spawning, Breeding and Recruitment Activity; 4. Stock assessment: Maximum Sustainable Yield (MSY) Calculations; Along the Kenyan Coast from the Surplus; Production Model 5. Management: (a) Possible Management Strategies; (b) Issues for International Fishery Management. 6. Developments: Industrial and Foreign Fishing. 7. Recommendations. 8. Literature cited. 9. Appendix figures 1-20 and Tables 1-3.Master