Siné-Saloum Delta: an illustrated guide to Senegal’s remarkable mangrove estuary

Siné-Saloum Delta: An illustrated guide to Senegal’s remarkable mangrove estuary.

Jeffrey Chatellier & Zoë Shribman

i

Siné-Saloum Delta: An illustrated guide to Senegal’s remarkable mangrove estuary.

Table of Contents Copyright Dedication and Acknowledgements About the Authors

ii iii v

Foreword

vii

Abstract

9

General Overview

10

Geography of the Estuary

15

Soil Profiles

19

Estuary Formation

23

Food Web

26

Discussion of the Hypersaline Conditions in the Estuary

33

Major Changes in the Estuary

36

Footnotes

47

Copyright © 2018 Jeffrey Chatellier & Zoë Shribman. All Rights Reserved.

Written by Jeffrey Chatellier & Zoë Shribman.

Foreword by Dr. Marie-Christine Cormier-Salem.

Ebook design by Zoë Shribman.

Cover by Rizaldy Yudhista Nurzirwan.

Maps and Satellite Images created by Rizaldy Yudhista Nurzirwan, courtesy of Landsat Images, USGS: February 1986, April 2007, May 2018.

Photographs by Michael Jeddah, Kasia McCormick, Dominic Wodehouse, Dr. MarieChristine Cormier-Salem, and Mangrove Action Project.

Author photographs by Angga Pratama and Eleonora Fossile.

Publishing Service: StreetLib. Correspondence: Jeffrey Chatellier, j.chatellier@forestcarbon.com & Zoë Shribman, zoeshribman@gmail.com Suggested Citation: Chatellier, Jeffrey & Shribman, Zoë (2018). Siné-Saloum Delta: an illustrated guide to Senegal’s remarkable mangrove estuary. Streetlib Publishing. ii

To the people of the Niombaato, who welcomed me into their community.

- Jerey Chatellier

iii

Acknowledgements I would like to thank the following people and institutions for their support and inspiration to publish this book: •

Shimon Anisfeld,

•

Molly Williams,

•

•

Rob Bailis,

•

Gus Speth,

Margaret Chatellier, •

Peter Chatellier,

•

Kate Chatellier,

•

Mary Osmond,

•

Ibrahima Demba,

•

Doucoure Family,

•

Ibrahima Cisse,

•

Demba Sidibe,

•

Mamadou Diaw, •

•

Meghan Williams, •

John Leary,

•

Kasia Krynski,

•

Chris Thurlow,

•

Andrew Thurlow, •

•

Aziz Diatta,

Forest Carbon,

Mangrove Action Project,

•

Peace Corps Senegal Program,

•

Yale Agrarian Studies Program, •

• •

Luce Fellowship at Yale, The Montgomery Family, and

Yale Tropical Resource Institute.

- Jeffrey Chatellier

About the Authors Jeffrey Chatellier is a conservation scientist who learned about the Siné-Saloum Delta in Senegal after having spent three years working on community reforestation and conservation projects in the region as Peace Corps volunteer, working with the Senegalese Ministry of the Environment and Protection of Nature. He holds a bachelor’s degree in International Affairs and Economics from The George Washington University and a Masters’ Degree in Environmental Science from Yale University and was a Fulbright Research Scholar in Indonesia. Since 2010, he has worked as the Director of Forest Carbon, a Southeast Asia consultancy, where he has advised governments, companies, and international organizations on forest conservation, sustainability, and environmental policy. In 2017, he closed one of the largest conservation finance investments in Indonesia for a 22,000 ha peatland rainforest restoration project in South Sumatra that totaled 6.6 million USD.

v

Zoë Shribman is a marine biologist focused on studying mangrove and coastal ecosystem ecology and conservation. Shribman has been studying mangrove systems since 2014, and has experience working with mangroves in the United States, Indonesia, Thailand, and Australia. She strives to use her understanding of these complex coastal systems to integrate community outreach and education with advocacy work. Shribman has worked and collaborated with an array of international NGOs, including the Sierra Club, the New England Aquarium, Environment America, in the U.S., Mangrove Action Project in Thailand, and Blue Forests, Suara Pulau, and the Friends of the National Parks Foundation in Indonesia. She has also been actively involved in the People’s Climate Movement, the March for Science, and the Women’s March. Shribman wrote her undergraduate thesis about mangrove forest structure and biodiversity of natural systems in Indonesia, as part of a Henry Luce Foundation grant with Eckerd College. Shribman received her Bachelor of Science in Marine Science from Eckerd College in St. Petersburg, Florida. She loves studying languages, and is proficient in both Bahasa Indonesia and French.

Foreword “Eaters of shellfish.”

This is the name given to the inhabitants of the Saloum Islands, the Niominka, who are highly dependent on mangrove resources for their livelihood. The artificial shellmiddens, remarkable cultural landscape listed collectively as a UNESCO World Heritage Site in 2011, are evidence of the shellfish harvesting and fishing in the area for over 2000 years.

In fact, the Saloum Islands in Senegal are home to one of West Africa's most incredible mangrove areas. These socio-ecosystems, between land and sea, encompass a complex network of creeks, rivers and small tidal channels called bolon. They are bordered by mangrove trees, leaving to discover, at low tide, vast mudflats and hypersalinated flats behind the mangroves, called tann. Mangrove forest and mudflats are habitat to an amazing wealth of birds and aquatic species, of which this book gives a very good illustration.

The climatic deterioration of the years 1960-70, the sea level rise and coastal erosion, obvious at the Sangomar breach, resulted in a severe salinization of the waters and the soils. Besides the retreat of the mangrove forest and the decline of a few fish species like Pristis pristis (represented in West African currency on FCFA coins, famous for his saw or rostrum), those climatic changes signed the quasi disappearance of the cultivation of millet and rice. Nevertheless, the population has managed to adapt to this salty environment.

Excellent sailors, men perform long-distance sea fishing migration. Women harvest shellfish and transform them into products (pane, tuffë, yeet) sought by both local and urban convii

sumers. They exploit salt and honey, and are more and more involved in arboriculture and the confection of ditakh (fruit of Detarium senegalense) juice, the best of Senegal according to connoisseurs.

This concise book is a lovely introduction of the fascinating nature of the Delta. The two authors, Jeff Chatellier and Zoë Shribman, experts in Environmental Science, respectively, in Agroforestry and Marine Science, provide a full-of-information book that captures the essence of the delta and of its moving landscapes, according to the tide, the season and in the face of globalization (either climatic, demographic, political or economic). This book will serve as a helpful resource to those looking to learn more about the singularity of the Saloum Delta, and the significance of the area’s valuable mangrove ecosystems.

From my personal experience enjoying the wonders of this complex mangrove socioecosystem, I am delighted to support this publication as a new means of discovering the unstable, but not less sustainable balance between people and nature that is the Saloum Delta.

Dr. Marie-Christine Cormier-Salem Directrice de Recherche, IRD Unité Patrimoines Locaux et Gouvernance, PALOC, UMR 208 Sorbonne Universités, IRD, MNHN, Département HNS

viii

Abstract The Saloum estuary of Senegal is a valuable natural resource for local communities.

This commercially-important fish and shellfish habitat provides the community with a protein-rich food supply, as well as employment opportunities in the fishing industry. This delta also supplies a source of cooking fuel and building materials for local people. Decreased fish catches in the region have focused attention on the role mangroves play as a fish nursery, prompting the government to prohibit the cutting of live mangroves. With 12% of Senegal’s population living in the estuary, development and environmental organizations are creating projects to protect the biodiversity of the estuary and maintain its function as a rich source of natural resources. A strong scientific understanding of the estuary is critical for developing projects to maintain and improve the health of this vast ecosystem.

Mangrove seedlings in the SinĂŠ-Saloum Delta. Photo by Michael Jeddah.

9

General Overview

Mangroves along a tidal creek in the Siné-Saloum Delta. Photo by Michael Jeddah.

The Siné-Saloum Delta covers an area

trees and shrubs that live along protected

of 2,250 km2 in the mid-western part of

coastlines, characterized by their ability to

Senegal, south of Dakar located between

withstand flooded and salty water, are a dis-

13°35’ and 14°10’ latitude North and

tinguishing component of this estuary.5

16°03’ and 16°50’ longitude West.1 This es-

The Saloum Delta is an important part of a

tuary is classified as a Marine Protected

larger stretch of mangroves that extends

Area, known as the Delta du Saloum

across the coast of Western Africa known

UNESCO-MAB Biosphere Reserve, with

as “les Rivières du Sud”.6 This is the larg-

many uses and governing authorities.2,3

est contiguous mangrove forest in the

The Saloum Delta was also named a

world at 7,887 km2, comprising approxi-

Ramsar site in 1984.4 Mangroves, tropical

mately 5% of global mangrove area.5

10

Map of the Siné-Saloum Delta within the context of Senegal, in Africa. Image courtesy of Landsat 2018 and Rizaldy Yudhista Nurzirwan.

The Saloum estuary can be characterized as an inverse estuary,17 with the lowest salinity levels at the Atlantic Ocean, increasing along a coastal to inland gradient. Fresh water flow in the Saloum River is nil and as a result never exports freshwater to the ocean.7 Seawater, on the other hand, is imported daily into the system. This reverse estuary function is caused by the low levels of rainfall in the river basin and high levels of evaporation. The regional annual precipitation increases southward and ranges from

600 to 800 mm.8 The entire river basin of 29,720 km2 has minimal slope with the highest elevation reaching only on average 40 m.1,8 This small elevation gradient limits the amount of runoff that enters the Saloum River as evaporation rates are very high.

This is combined with the fact that the surface area of the drainage basin is made up of permeable sands which further limit runoff into the estuary. High average evaporation rates for the area, ranging from 1,900 to 2,400 mm also limit freshwater runoff to the system. With freshwater inputs at practically zero for most of the year, tides are the main causal agents of water flow in the estuary.

11

Boys playing in the Saloum Delta.

A man driving a boat in the Saloum

Photo by Kasia McCormick.

Delta. Photo by Kasia McCormick.

12

This map illustrates the various regions of the SinĂŠSaloum Delta, Senegal.

Image courtesy of Landsat, 2018 and Rizaldy Yudhista Nurzirwan.

The tidal regime is semi-diurnal7 and

the outward flow of water. This combined

microtidal, with an average range of 0.8

with high levels of evaporation reduces the

m.9 The vast areas of mudflats, composed

amount of water leaving the system which

of sandy silt and known as tannes,1 experi-

also ultimately reduces the ebb velocity as

ence inundation only during spring tides.

less water must leave the system. This

This tidal regime is asymmetric with the

type of hydrology limits the export of or-

flood lasting approximately 7 hours, and

ganic matter which means that the estuary

the shorter ebb lasting 5 hours and 25

is more likely a system with net import

minutes.9,10 As a result, the high-water pe-

than net export.

riod lasts longer than the low-water, which means the estuary is inundated more often than it is dry. The velocities of the current are much faster during the flood stage than the ebb, with velocities reaching up to 100 cm/s while the ebb maxes out close to 65 cm/s.9 The amount of seawater that enters the estuary is more than the 9

amount that flows back to the ocean. The mangrove-colonized delta comprised of complex systems of tidal channels reduces

The tidal floods carry organic matter produced in the mangrove forests to the upper parts of the estuary that are hypersaline and void of mangrove vegetation.9 The flow of seawater reaches distances that are up to 100 km upstream and has contaminated surface water and continental terminal groundwater sources.8 At the mouth of the estuary, salinities have been measured at 36.7 ppt and up to 130

13

ppt.8,9,23 The presence of hypersaline wa-

deep ocean waters, to the warmer, sunlit

ters inland has led to aquifer contamina-

zone at the surface. The prevailing trade

tion in the northern part of the estuary.8

winds drive the surface water away from

The climate of the estuary consists of a dry

shore, drawing deeper water upward to

season between November and June and a

take its place. Coastal areas that experi-

rainy season between July and September.

ence upwellings receive an abundance of

The mean temperature is 28.6°C with a hu-

krill and other nutrients and become feed-

midity of 55.4%.9 Rainfall during the rainy

ing grounds for many marine and avian

season has decreased since prolonged

species.11 Despite this, some research indi-

drought that began in the 1960s and

cates that upwellings in the coastal waters

1970s.9 Starting at the end of November,

of Senegal only have a slight modification

the waters of the estuary become very cold

on the trophic organization of the

as northeast trade winds induce oceanic

estuary.12 The estuary’s water becomes

upwellings which reach a peak near the

relatively warmer as the rains emerge and

end of the dry season in April.9 Upwellings

the upwellings cease.

are the rising of cold and nutrient-rich Locusts, one of the harsh living conditions in a village in the Saloum Delta.

Photo by Kasia McCormick.

14

Geography of the Estuary

The Saloum River. Photo by Michael Jeddah.

There are three major channels in the

input becomes zero. The Diomboss, lo-

delta named Saloum, Diomboss, and Bandi-

cated between the other two main chan-

ala. The Saloum, located in the northern

nels, is a deep ria (submerged river valley)

part of the delta, has widths up to 1-2 km

that has a width of 4 km at the mouth and

and depths reaching 13-25 m between the

depths ranging from 10-25 m.9 The Bandi-

mouth of the river and the inland city of

ala, the smallest of the three, has a width

Kaolack where it narrows to less than 500

that is less than 500 m wide with depths

m with depths less than 8 m. Of all the riv-

less than 10 m.9 This river has a lower sa-

ers in the delta, the Saloum singularly has

linity level due to the presence of exfiltra-

significant fluvial input from runo, which

tion of fresh water from the Nema River

only occurs during the three-month rainy

which is located a few kilometers north of

season.1 During the dry season, this fluvial

Missirah.

15

There are two major groups of islands

River and extend to the mouth of the

in the estuary that are separated from each

Diomboss. The northern sand barrier con-

other by the Diomboss channel with the

sists of the 19 km long Sangomar spit.9 It

Saloum and Bandiala separating them from

is a southward stretching spit that is fed

the mainland. The Gandoul islands are in

by the north south littoral drift. The spit

the north and the Betenti and Fathala Is-

has undergone major changes in recent his-

lands are in the south. The islands cover

tory and was breached in 1987.9 The open-

an area of approximately 800 km2.9

ing of the breach has created two mouths

The

three main channels are interconnected by

for the

a large network of small tidal channels cov-

larger of the two, is located between the

ering an area close to 800 km2 whose

spit and the newly created island, with a

coasts are colonized by mangroves.9

width of 4 km. The Sangomar mouth is lo-

The estuary is provided protection from the Atlantic Ocean by sand barriers.

These sand barriers, representing 15% of the exposed portion of the delta, were cre1

ated by littoral drift. The southern sand barriers start at the mouth of the Gambia

Saloum. The Lagoba mouth, the

cated to the south of the newly formed Sangomar Island and is 1.8 km wide. The island continues to extend southwards at a rate of 192.5 m/year.9 The part of the spit connected to the mainland has eroded northwards.

In this time series, the Sangomar spit is connected to the mainland in 1986, but in the more recent images in 2007 and 2018, Sangomar Island has been formed. Image courtesy of Landsat, 1986, 2007, 2018 and Rizaldy Yudhista Nurzirwan.

16

Detailed map of the SinĂŠ-Saloum Delta region. Image courtesy of Landsat, 2018 and Rizaldy Yudhista Nurzirwan.

17

Sunsets in the delta. Photos by Michael Jeddah.

18

Soil Profiles

A river bank within the Siné-Saloum Delta. Photo by Michael Jeddah.

The bulk of the soil material in the

Chemically, the sediment in the Sa-

mangrove forests of the estuary is made up

loum estuary is characterized by its high

of heavy clay with many inclusions of fi-

levels of acidity and salinity.13 The acidity

brous Rhizophora roots. The macroporosity

is related to sulfur and sulfuric compounds

of the soil allows water to penetrate areas

while the salinity is controlled by the in-

away from the channel banks.13 This ulti-

fluxes of saline seawater.13 The acidity of

mately increases the amount of water in

the soil is different under the different spe-

the system which causes most of the soil

cies of mangroves. The acidity of soils un-

in the estuary to be unripe (liquid mud

der Rhizophora mangle is around 6-6.5 in

and un-kneadable) and half ripe (fairly

pH, while the acidity of soil under Avicen-

soft, sticks to hands, and can be squeezed

nia germinans is closer to 7 in pH because

through fingers).13

the sulfur content under these tree is 19

Young Rhizophora sp. mangroves growing with the support of their aerial prop roots. Photo by Dr. Marie Christine Cormier-Salem.

Abundant crabs can be seen inhabiting a mudflat in the estuary at low tide. Photo by Dr. Marie Christine Cormier-Salem.

20

low.13 The tannes have a very high level of

vertically and seems to be independent of

acid as the pyrites in the soil of the tannes

the mangrove water table.13 The tanne’s sa-

are oxidized, producing sulfates which lead

linity level has little seasonal variation

to increased acidity levels. High salinity is

while the tidal channel bank’s salinity

one of the most defining characteristics of

changes seasonally due to increased fresh-

the Saloum estuary with levels reaching 55

water input from rain and runoff. The

ppt in the middle of the estuary and up to

tanne’s high salinity is accompanied by

90 ppt in the upper portion of the Saloum

high acidity, which suppresses plant life

river in Kaolack emphasizing that the en-

creating the bare mudflat. Salt accumula-

tire estuary has salinity levels higher than

tion is also encouraged due to the lack of

the ocean.8 The tidal inundations of the es-

slope in these inland areas preventing accu-

tuary bring a daily supply of saline water.

mulated salt from running off.

The groundwater table has a lateral evolution from the channel bank until it reaches the tanne. The water table of the tanne operates differently as its movement occurs

A tanne, or mudflat composed of sandy silt, with Baobab trees growing in the Siné-Saloum Delta. Photo by Dr. Marie Christine Cormier-Salem.

21

Aerial roots of Laguncularia racemosa.

Rhizophora roots along a small tidal creek at low tide. Photo by Mangrove Action Project.

Photo by Mangrove Action Project.

Estuary Formation

A shell midden in the Siné-Saloum Delta. Photo by Dr. Marie Christine Cormier-Salem.

The development of the delta has

likely explained by harsh environmental

taken place within the “Continental Termi-

conditions. The older shell middens lie

nal” which acts as the boundaries of the es-

closer to the continental terminal and have

tuary. The presence of shell middens offers

been eroded by meandering channels lead-

valuable information on the formation of

ing to detritus build up creating a re-

the delta. Their age indicates a date at

worked layer that consists of shells, mud,

which the delta was producing mollusks

and plant debris.1

on banks and had higher topographical areas for shucking. Carbon dating shows that shell midden construction took place over two separated periods: 1940-1140 and 600-0 BP.1 The break in time is most

The post glacial Holocene period is marked by significant climatic variations which prompted the formation of the delta. Between 12,000 and 4000 BP, the region experienced wet conditions with a pe-

23

riod of aridity lasting from 7500 to 6500

nel, explaining its relative greater depth

BP.1 There was then a transition period

compared to the other channels at the pre-

that lead to an arid period starting in 3300

sent time. The third phase consisted of an

BP that was interrupted by short wet peri-

estuary that was almost completely pro-

ods from 2900-2500 BP and 1200-750.1 for-

tected from barrier beaches allowing for

mation of the delta can be described as a

sedimentation deposit. These barriers

post glacial Holocene sedimentary se-

made up the inner barrier islands that ex-

quence that was deposited during five evo-

ist in the present-day estuary. The fourth

lutionary stages.1

phase began in 1990 BP and lasted until

These stages can be described as: submerged postglacial ria, funnel-like estuary, barred estuary, cuspate delta, and reverse 1

estuary. In the first stage, the Nouakchottian sea submerged the deeply excavated mouth of the Saloum river in the north of the estuary and Nema river in the south around 5500 BP and lasted until about 3000 BP.

1

Macrotidal conditions most

likely prevailed creating a high-energy system that left sediment deposits of variable thickness. The second phase

600 BP and can be characterized as a depositional period for the estuary.1 Barrier islands, comprised of wind transported sediments from the mainland, support the notion that this phase was marked by arid conditions.1 Despite this, the mangroves began to colonize protected channels. The final reverse estuary stage of the delta’s formation was similar to conditions currently found in the estuary.

A view of a shell midden along a riverbank.

Photo by Dominic Wodehouse.

consisted of the formation of barrier beaches which created a funnel like estuary with only one major opening to the sea. The barrier formation in the north and the south forced most the flow through the Diomboss chan-

24

Shell midden with baobab trees. Photo by Dr. Marie Christine Cormier-Salem.

Views from a shell midden in Ndiouta in the SinĂŠSaloum Delta. Photos by Dr. Marie Christine CormierSalem.

25

Food Web One of the many species of mangroves living in the delta, this Rhizophora tree is in flower.

Photo by Mangrove Action Project.

The mangrove vegetation of estuary is

69-73 species found globally.5,15 Senegal’s

characterized by the dominant Rhizophora

total mangrove forest area comprises

species (including Rhizophora racemosa, R.

1,279.45 km2 of the 181,077 km2 of man-

mangle, and the hybrid species R. x harri-

grove area across the globe.5,16

sonii) and the Avicennia species, Avicennia germinans.5 The estuary is also home to some Laguncularia racemosa stands, as well as the presence of the mangrove fern Acrostichum aureum and Conocarpus erectus which often lives at higher elevations.5 There are these seven species of mangroves in Senegal, compared to 12 species across the entire Atlantic East Pacific region (the Americas and West and Central Africa) and the

The Rhizophora mangroves are commonly referred to as red mangroves, Avicennia mangroves as black mangroves, and Laguncularia mangroves as white mangroves.

Within these mangrove vegetative areas, the mangrove species composition follows a predictable pattern, known as zonation.

The Rhizophora species are most often found on the edges of tidal channels creating a Rhizophora fringe eect. The Avicennia

26

Submerged Rhizophora. Photo by Dr. Marie Christine Cormier-Salem.

Juvenile Avicennia and Rhizophora trees. Photo by Dominic Wodehouse.

27

and Laguncularia species in this estuary are

the Saloum estuary was calculated at 260

frequently found on the more elevated por-

g/m2/year.7 Another study indicates that

tions of the mudflats. The variation in sa-

the average biomass production of the man-

linity may also impact the composition of

groves in the estuary is 180-210

mangrove stands. R. mangle and R. racemosa

g/m2/year.9 The estuary is comprised of

tend to dominate in areas where there is a

800 km2 of mangroves which means that

low variation in salinity levels, while A. ger-

the annual primary production of fresh

minans, and L. racemosa are documented to

matter is 186,000 t/year using the 210 g/

dominate in areas of high salinity

m2/year.

variation.17 Production of leaf litter from these mangrove species is an important source of organic matter and nutrients into the system. Net primary production of fresh matter of Rhizophora mangroves in

The Saloum estuary has a diverse population of terrestrial animals including antelopes, hyenas, teals, and monkeys. Additionally, many kinds of marine and

A goliath heron, Ardea goliath, takes o from its perch in the mangroves. Photo by Michael Jeddah.

28

Two pied kingfishers, Ceryle rudis, in the delta. Photo by Michael Jeddah.

aquatic animals live in these areas, includ-

birds come to the delta each year to nest

ing Common Bottlenose Dolphins (Tursiops

and spend the winter.4 Studies have shown

truncatus), Nile Crocodiles (Crocodylus niloti-

that large bird populations can increase

cus), and sea turtles.4 Many species that

the growth of mangrove tree species be-

rely on this mangrove estuary have popula-

cause guano adds increased nitrogen to the

tions at risk, including the Atlantic Hump-

soil. Additionally, the migratory birds in

backed Dolphin (Sousa teuszii), classified as

the region rely on the wetlands of both the

critically endangered by the IUCN Red List

mangrove forests and the nearby rice culti-

of Threatened Species, the West African

vation fields, indicating the important con-

Manatee (Trichechus senegalensis), classified

nections between local ecosystems.6

as vulnerable, and the red colobus monkey (Piliocolobus badius), which is classified as endangered.3,4,18,19,20,21 Migratory birds are numerous in the park with over 1,000 species, and astonishingly about 120,000

The estuary has a high level of fish species richness with 114 species recorded in 1996.22 However, during a more recent survey, only 73 species were caught and documented.23 Most of the fish population

29

is represented by 34 species.24 Mangrove

many species of birds, fish, and monkeys.

oysters (Crassostrea cuculata) are found

Their presence increases the rate of break-

throughout the park attached to the sub-

down of organic matter through digestive

merged stilt roots and lower branches of

processing. They also aect the mangrove

Rhizophora mangrove trees, as well as

species composition because of their preda-

clams (Anadara senilis). Shrimp also use the

tion of propagules and seedlings in areas

mangroves as a nursery and have large

well suited for their habitat. In addition to

populations. Fiddler crabs (Uca sp.) are

the mangroves, scientists have recorded

very numerous along the mudflats where

188 other plant species throughout the Sa-

they live in burrows and forage for leaf lit-

loum delta.4

ter and mangrove propagules. These crabs play an important role in the ecosystem since they are a source of food for the

One study classified the benthic marine species of the estuary into 37 functional groups consisting of 29 fish groups,

Blue-cheeked bee-eaters, Merops persicus, sit in a grove of Rhizophora trees. Photo

by Michael Jeddah.

30

5 invertebrate groups, 2 primary producer

Mangroves around the world have

groups, and a detritus group. The study cre-

been seen to support fish populations as

ated a model to show trophic interactions

they help with nutrient turnover and pro-

throughout the estuary. The biomass levels

vide support for epiphytic plant growth

of each group were calculated with fish bio-

that increases the food supply.24 The

mass calculated at 9.1 tons/km2, shrimp and crabs calculated at 4.7 tons/km2, and primary producers calculated at 114 tons/km2.25 The trophic level of the groups ranged from 2-3.8 suggesting a small trophic food web with most consumer populations comprised of juveniles.25 Most of the biomass produced in the estuary is lower in the pyramid making it not only short but wide. This trophic pyramid shape suggests a stable food web.

25

This estuary is a

bottom up driven food chain, as the high productivity of the mangroves provides large quantities of organic matter which are then consumed by detritus feeders at the base of this short and wide pyramid.

A firefinch, Lagonosticta sp., perches on a branch. Photo by Michael Jeddah.

assemblages of fish are influenced by salinity and habitat availability. In the upper parts of the estuary where salinity borders on hypersaline levels, there is the lowest species richness count, while in the lower part of the estuary, species richness is much higher.

The presence of mangroves provides habitat and a food source for bottom feeders and their subsequent predators increasing overall species richness. The abundance of fish in the estuary changes on a temporal scale with levels dropping before and during the rains. This may be a result

A red-billed hornbill, Tockus erythrorhynchus.

of adult and sub-adult fish leaving the estu-

Photo by Dominic Wodehouse.

31

ary to spawn in the sea.23 At the end of the

flood dominated which slows the expul-

wet season, it has been recorded that juve-

sion of organic matter produced by man-

niles born in the sea enter the estuary

groves. This low export of nutrients in-

where they mature. Juveniles of fish spe-

creases the trophic richness of the

cies that live on the continental shelf as

estuary.22 Relatively stable environmental

adults make up the majority of the fish in

conditions free of major flooding also al-

the estuary.

low fish assemblages to stay intact through-

This high species richness occurs despite the Saloum being an inverse estuary with high salinity levels that could be stressful to fish development, adaptation, and survival. However, there are many explanations as to why the Saloum estuary has such a high level of species richness.

The circulation of water in the estuary is

out the year. Despite the high salinity, most of the main functions of the estuary are preserved; in particular, nursery function is especially well-developed for many species.23

An osprey, Pandeon haliaetus, flies over the mangroves with a freshly caught fish. Photo by Michael Jeddah.

32

Discussion of the Hypersaline Conditions in the Estuary

The Siné-Saloum Delta. Photo by Michael Jeddah.

Most research focuses on the hyper-

Nema, one of the original submerged ria’s

saline environments of the upper Saloum

during the estuary’s early formation. The

but the salinity levels throughout the estu-

Nema is the only constant source of fresh-

ary are not uniform. The Diomboss and

water for the Bandiala and is fed by ground-

Bandiala channels have considerably lower

water exfiltration with baseflows of 1,054

salinities than the Saloum, with approxi-

m3/day during the dry season and 2,808

mately 38 ppt in the Diomboss, and 42 ppt

m3/day during the rainy season.8 Salinity

in the Bandiala.8 The majority of the estu-

tests demonstrate that Dioto, the point at

ary is closer to salinity levels of the ocean

which the Nema meets the estuary, has the

during the rainy season and salinity only

lowest salinity levels for the entire

increases to a maximum of 50 ppt during

estuary.24 The Bandiala, during the rainy

the dry season.24 The Bandiala is fed by the

season, is the only channel of the estuary 33

that exhibits a decreasing salinity gradient

ern part which is colonized by mangrove

moving inland.23

forests and has greater fish species rich-

As mentioned before there is considerable variation in the rainfall throughout the estuary with the north receiving much less rain than in the south. This is evident by the lower salinity levels in the southern lying Diomboss and Bandiala channels, as well as by the presence of more mangrove forests that cannot survive with the further inland hypersaline waters of the Saloum. Also, the shorter Diomboss and Bandiala channels are flushed more frequently

ness, and the upper Saloum River portion which has little vegetation and species richness along its banks due to hypersaline conditions. The Diomboss and Bandiala channels, while slightly more saline than the ocean, are highly productive ecosystems as they support mangroves, which lead to increased organic matter and fish production.

Salt accumulating on Avicennia leaves as an adaptation to the hyper-

and experience less evaporation than the

saline conditions in the estuary.

Saloum. The resulting eect is an estuary

Photo by Dr. Marie Christine

comprised of two distinct zones, the south-

Cormier-Salem.

34

The estuary’s climate is harsh, dry, and arid. Photo by Dominic Wodehouse.

The arid environment leaves many mangrove leaves covered in a thick layer of dust. Photo

by Dominic Wodehouse.

The soils are so oversaturated that salt appears on the surface.

Photo by Dominic Wodehouse.

35

Major Changes in the Estuary

A fishing community in the SinĂŠ-Saloum Delta. Photo by Dominic Wodehouse.

Aerial photos have shown that there

community.27 Some authors have pointed

has been a major retreat in mangrove vege-

to mangrove deforestation from fuelwood

tation increasing the area covered by bar-

harvesting as one of the main causes for

ren salt flats. This globally threatened eco-

the reduction in fish populations and have

system suered a loss of 34.8% of the man-

emphasized the need to restore

grove area throughout the Saloum Delta

mangroves.26

between 1972 and 1986.2 A reduction in the fish population has occurred, causing the catches of local fishermen to decrease by more than 5 times over the past 30 years.26 (Diouf et al. 1999). As fishing is the main economic activity in the region, the decrease in the fish stock has had a serious impact on the well-being of the

Mangroves enhance an estuary’s capacity as a fishery due to the high levels of organic matter that create a large food base, as well as by providing habitat. However the Saloum estuary is not uniform, with mangrove vegetation and ecology varying greatly with thick and healthy man36

A harvest of Arca and Murex Both men and women participate in fishing activities in the delta. Photo by Dr.

shellfish. Photo by Dr. Marie Christine Cormier-Salem.

Marie Christine Cormier-Salem.

37

grove cover in the southern part of the es-

note that these observations may be due to

tuary, and little cover at all in the northern

lower salinity levels that may allow for

part. This northern region has more urban

greater species diversity as many species

areas where fuelwood needs have put pres-

cannot tolerate the high salinities in the

sure on the existence of mangrove forests.

north.24

However, it is more likely that climate change in the form of reduced rainfall and arid conditions are the major agents of mangrove retreat in the delta.7 In any case, research needs to specify which portion of the delta it is referring to due to varying dierences in ecology between the north and south. Mangroves have a positive impact on the estuary’s fishery function as species diversity and total biomass from catches is higher in the southern part of the estuary.24 However, it is important to

The presence of mangroves may also be a function of the salinity, with forest cover being described as luxuriant in the less saline parts of the estuary.23 These findings have significant policy implications for development projects that seek to reforest degraded mangrove ecosystems. If salinities continue to rise in the north of the estuary then mangrove retreat will continue to occur there. The shorter Diomboss and Bandiala channels have closer

A fishing boat rides along next to the mangroves. Photo by Michael Jeddah.

38

The day’s catch is laid out to dry after a successful day of fishing. Photo by Dr. Marie Christine Cormier-Salem.

Women bring in the Murex shellfish they have collected in Niodior. Photo by Dr. Marie Christine Cormier-Salem.

39

proximity to the ocean and are flushed

material influx, and overfishing. Local com-

more frequently than the longer Saloum, al-

munities attribute overfishing as the main

lowing them to avoid hypersaline condi-

cause of this problem. Fishers in the estu-

tions. This would suggest that reforestation

ary use gill nets which kill small juvenile

programs should focus on the southern

fish, greatly reducing the overall popula-

part of the estuary where the probability

tion. In addition, the sheer number of fish-

for survival is higher. If conditions for man-

ing vessels throughout the estuary is in the

grove colonization existed in the northern

thousands.9 Although local people have for

part of the estuary, trees would recruit natu-

a long time maintained territorial use

rally as mangrove propagules have been ob-

rights for fish, shellfish and other mangrove

served in the upper portions of the Saloum.

resources, before 2004 there were no stateimposed policies to limit the amount of fishing in the estuary. The first marine protected area in the estuary was established in the Bandiala River and has already seen success in creating a spawning ground for once rare fish species.29 A series of marine protected areas could be a potential answer

The rivers of the delta are integral to life in this harsh environment. Photo by Michael

as it would ensure greater diversity in the fish population. However it is clear that the

Additionally, droughts cause mangrove

local community is fishing beyond the carry-

loss, but higher rainfall lets the mangroves

ing capacity of the estuary. Due to the

recover.6 Although Avicennia species can withstand salinities over 50 ppt, Rhizophora species are limited by salinities of 20-30

Fishermen rely on the success of local fish populations every day. Photo by Dr. Marie Christine Cormier-Salem.

ppt which can lead to black mangroves replacing their red counterparts.28 The reduced fish population numbers in the estuary are most likely due to an array of dierent factors including increased salinities, reduction in habitat and organic 40

There is an abundance of salt in this pond in BĂŠtanti. Photo by Dr. Marie Women harvest salt from a pond in Niodior.

Christine Cormier-Salem.

Photo by Dr. Marie Christine Cormier-Salem.

41

decentralized nature of the industry, bu-

aridity will continue to have the greatest

reaucratic quota systems might be difficult

impact on the estuary, influencing zonation

to implement. However, in the Bandiala is-

and colonization of mangroves, as well as

lands, shrimp fishing has been regulated

fish population and species diversity. Addi-

with four days of fishing cycled with four

tionally, anthropogenic activities such as fu-

days of resting. While this biological rest

elwood harvesting and intensive fishing

policy may not be the ultimate solution, it

may exacerbate these issues as the estu-

is easily enforced as it is clear that no one

ary’s high salinities and lack of freshwater

should be out in the water during the rest

leave the area vulnerable in the face of

days and it emphasizes to local populations

these stressors. Local agriculture expansion

that managing the fishery is required as a

also poses a serious threat to this estuary.5

mechanism to preserve the vital fishery resource. This type of management could be introduced further in the Siné-Saloum estuary as a first step to increase the fish population in the estuary.

Mangrove restoration efforts should focus on techniques using natural regeneration. Creating single species plantations is not proven to be an effective way to bring successful habitat back to disturbed mangrove ecosystems.2,

32

This type of restora-

tion combined with the critical aspect of working closely with local people is known as Community-Based Ecological Mangrove Restoration, or CBEMR.31 CBEMR advocates a science-based method for restoring Oysters harvested in the mangrove trees.

degraded mangrove habitat, by adjusting

Photo by Dominic Wodehouse.

site hydrology and consequently allowing

These mangrove systems are critical to local communities as fishing, oyster and shellfish harvesting, honey production, rice cultivation, and coastal tourism centered around birding in the mangrove forests are vital components of the local economy.5,30 Climatic variation in the form of increased

mangroves to return naturally to the area.31,

32

The NGOs Wetlands Interna-

tional Africa and Mangrove Action Project are collaborating in Senegal to empower local communities to successfully bring back mangroves while establishing sustainable livelihoods and local community action.32 42

Fishermen prepare their nets for a trip out into the delta. Photo by Dr. Marie Christine Cormier-Salem.

Many fishing boats gather along the shore. Photo by Kasia McCormick.

43

Fish smoke stoves are used to preserve Fish are dried and salted in Toubakouta Village. Photo by Mangrove Action Project.

the day’s catch in Toubakouta Village.

Photo by Dominic Wodehouse.

44

A bee hive is used as sustainable livelihood in a mangrove tree in Djinak. Photo by Dr. Marie Christine Cormier-Salem.

Oysters are cooked over a fire.

Photo by Dominic Wodehouse.

A woman dries stingray caught in the delta. Photo by Dominic Wodehouse.

45

Two fishing boats rest on still water.

A community living directly on the

Photo by Mangrove Action Project.

delta. Photo by Dominic Wodehouse.

46

Footnotes

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