application of agrometeorology to aquaculture and fisheries

25 Pages·2010·1.2 MB·English

CHAPTER 13
APPLICATION OF AGROMETEOROLOGY
TO AQUACULTURE AND FISHERIES
13.1 INTRODUCTION and especially in aquaculture. It is hoped that this
discussion will encourage the application of existing
The products of feisheries have been an important agrometeorological information to aquatic animal
component of the world food supply for centuries. production and stimulate research on the topic.
The number of global capture fisheries has increased
in response to the demands of rising human popula-
tion. The Food and Agriculture Organization (FAO)
of the United Nations began keeping statistics on 13.2 CAPTURE FISHERIES
world fisheries production in the 1950s; since then,
the annual catch from capture fisheries has increased Most of the commercial catch of fisheries products is
from about 25 million tonnes to approximately 95 of marine origin; in 2004, 87.2 million tonnes were
million tonnes. Most authorities feel that capture taken from the oceans, compared with 8.7 million
fisheries around the world are exploited to or possi- tonnes from inland waters. The Asian region
bly beyond their sustainable limit. For its part, accounted for nearly half of the production. China
aquaculture has been growing in importance: in was the top fishing country, and four other Asian
2004 the supply from aquaculture reached 59 million nations were in the top ten. During the past 10 years,
tonnes, or 38.1 per cent of world fisheries produc- the world production of capture fisheries has fluctu-
tion. The world’s population will continue to grow ated between 88 million tonnes and 96 million
and demand for products from fisheries will also rise tonnes, with no upward or downward trend. Many
accordingly. Aquaculture must step in to meet this species have been overfished and pollution of the
increasing demand, for the catch by capture fisheries oceans and coastal waters has negatively affected
apparently cannot be increased. productivity. The production of wild fish popula-
tions is dependent upon optimum temperature and
Meteorology plays an important role in fisheries other favourable weather conditions. Meteorological
because solar radiation and air temperature influence data can be used in attempts to explain observed
water temperature, which in turn affects the natural changes in fisheries production. Forecasts of tempo-
productivity of inland and marine waters and the rary or long-term changes in climate can also be used
growth of fisheries species (Kapetsky, 2000). Weather to predict changes in fisheries populations that could
conditions also have a tremendous effect on the abil- influence the future catch.
ity of fishermen to capture fish and other aquatic
organisms, and on the safety of fishermen. Most fishing methods require operation of boats in
Nonetheless, fishing, like hunting and gathering, large bodies of water and are inherently dangerous
primarily involves the exploitation of living resources activities because of storms. Short-term weather
from natural populations; the management of these forecasts can be extremely useful for planning fish-
resources is limited largely to regulations on capture. ing activities. Moreover, information on the
intensity and tracks of storms is critical to the safety
Aquaculture will soon surpass fisheries as the major of fishermen. Historically, many fishermen have
source of aquatic protein, just as agriculture perished because of inadequate information about
surpassed hunting and gathering as a source of storms or because of a failure to heed warnings.
meat, grain and other foods. Agrometeorology has
become an important tool in agriculture and it can
be equally useful in aquaculture. Meteorological
data are already used in aquaculture (Szumiec, 1983; 13.3 AQUACULTURE
Boyd and Tucker, 1998; Kapetsky, 2000).
Nevertheless, there have been few attempts to Total world aquaculture production was
organize the effort so that the acquisition and 59.4 million tonnes in 2004. Of this, 32.2 million
application of meteorological data may also serve tonnes originated from freshwater aquaculture and
as a tool for practical aquaculturists and become an 27.2 million tonnes from marine aquaculture. The
important part of the training of aquacultural top five countries in the world in terms of aquacul-
scientists. The purpose of this report is to discuss ture production are in Asia, as are 11 of the top £
the application of meteorological data in fisheries 15 producers, and China accounts for over one half
13–2 GUIDE TO AGRICULTURAL METEOROLOGICAL PRACTICES
of the world aquaculture production. Although arid, plains regions (United States Soil Conservation
aquaculture is vital to the domestic food supply of Service, 1979). The fluctuation in water level in
many nations, aqua culture products also are impor- watershed ponds ranges from a few centimetres to
tant international commodities. For example, the more than a metre, with the greatest fluctuations
United States imports nearly 80 per cent of its occurring in arid climates, during droughts, and in
seafood, and many of these products are from aqua- ponds that seep excessively (Yoo and Boyd, 1994).
culture. Aquaculture production systems vary greatly Some ponds are never drained, while others may be
both among and within species. An overview of the drained annually for harvest. Sometimes, ponds
common production systems will be useful to read- may have multiple uses, and water may be with-
ers who may not be familiar with aquaculture. drawn for domestic use, livestock watering or
irrigation.
13.3.1 Pond culture
Excavated ponds are made by digging a basin in
Aquatic animals are stocked in ponds, and fertilizer which to store water. Ponds may be filled by rain-
and feed are used to promote rapid growth. fall, runoff and infiltration of ground water. Such
Undesirable species can be excluded and water ponds cannot be drained, but sometimes water may
quality maintained within a desirable range. be removed with a pump. Small, excavated ponds
Production per unit area greatly exceeds that of of a few hundred square metres in area are widely
natural waters and culture animals can be harvested used for aquaculture in rural areas of India,
easily. Three basic types of ponds are used in aqua- Bangladesh and some other Asian countries. These
culture: watershed ponds, embankment ponds and ponds also may serve as sources of water for home
excavated ponds. The water budget for ponds may and farm uses. Where direct rainfall is the major
be expressed by the hydrologic equation: source of water for excavated ponds, ponds may dry
up or become very shallow during the dry season.
Inflow – outflow = ∆H (13.1)
Watershed and excavated ponds rarely receive
The hydrologic equation for ponds may be expanded inflow from wells, streams or other external bodies
as follows: of water. Fisheries production in such ponds often
is referred to as “rainfed” aquaculture. Rainfall,
(P + R + S + A) – overland flow, evaporation and seepage are critical
in
(E + So + O + C + Q) = ∆H (13.2) factors regulating the amount of water available for
rainfed aquaculture. Small, rainfed ponds are the
where P = precipitation; R = runoff; S = seepage in; most common aquaculture systems used by poor,
in
A = intentional additions from wells, streams, lakes rural people in tropical nations.
or other sources; E = evaporation; S = seepage out;
o
O = overflow; C = consumptive use for domestic Embankment ponds are formed by building an
use, irrigation, livestock watering or other purposes; embankment around the area for water storage.
Q = intentional discharge for water exchange or Surface areas of these ponds often are 0.2 to 2 ha
harvesting; and ∆H = change in storage. Depending and seldom over 10 ha. Watersheds consist of the
upon its design, construction, location and use, one above-water, inside slopes of embankments, and
or more of the terms listed above may not apply to little runoff enters ponds. Embankment ponds in
a specific pond. inland areas are supplied with water from wells,
streams or reservoirs. In coastal areas, embank-
Watershed ponds are made by building a dam ment ponds are filled with brackish water from
across a watercourse to impound surface runoff. estuaries or seawater. Drain structures consist of
They vary greatly in area, but most are greater than pipes with valves or gates with dam boards.
0.5 ha and less than 10 ha in area. These ponds also Embankment ponds are popular for commercial
have been called terrace ponds, and it is common aquaculture because water levels can be controlled
to construct a series of them on a watershed so that and ponds drained easily to facilitate harvest.
the overflow from one pond will be captured by These ponds usually are dedicated to aquaculture
another at a lower elevation. Watershed ponds fill use and are not sources of water for other
and often overflow during the rainy season, but the activities.
water level may decline drastically during dry
weather. The minimum ratio of watershed area to 13.3.2 Flow-through systems
pond volume necessary to maintain watershed
ponds varies from about 0.3 ha/1 000 m3 in moun- Flow-through systems for aquaculture include race-
tainous, humid areas to over 40 ha/1 000 m3 in ways, tanks and other culture units through which
CHAPTER 13. APPLICATION OF AGROMETEOROLOGY TO AQUACULTURE AND FISHERIES 13–3
water flows continuously. The culture species is Bivalve shellfish such as oysters, clams, mussels and
stocked at densities much greater than those used scallops can be laid on bottom plots in coastal waters
in ponds (Table 13.1). Water flow rates normally are for grow-out. It is more common, however, to place
two or three times the volume of the culture units young shellfish, called spat, on stakes, rocks and
per hour. Water sources are springs, streams and racks, or to attach them to ropes hanging from long
other bodies of surface water. Incoming water is the lines, rafts or other structures for grow-out by off-
main source of dissolved oxygen for fish and wastes bottom culture methods. These structures are placed
are flushed from culture units by the flowing water. in coastal waters in areas often used for navigation,
A constant supply of water is essential for fishing or other purposes. Seaweed is usually attached
flow-through systems. These systems are especially to ropes or netting for grow-out in coastal waters.
popular for the culture of trout in freshwater.
13.3.4 Water-reuse systems
13.3.3 Open-water culture methods
There are two basic types of water recirculation
Aquatic organisms also are cultured in open waters systems. One type is built outdoors and consists of
of oceans, estuaries, lakes and streams by confining culture units from which water passes through a
them at high density (Table 13.1) in enclosures or sedimentation basin and then into a larger, earthen
by placing sessile organisms on bottom plots or pond for treatment by natural biological processes
attaching them to a structural framework. Cages before being returned to the culture units for reuse.
and net pens are constructed of netting secured to a Mechanical aeration sometimes is applied in the
supporting framework. Cages vary in size from 1 m3 treatment pond to enhance dissolved oxygen
to more than 2 000 m3, and they float on or near concentration and promote microbial activity. The
the water surface. Fish in cages are supplied with other type of water-reuse system usually is placed in
manufactured feed daily. Pens are made by install- a greenhouse or other structure: water from culture
ing vertical poles and attaching netting to form an units passes through mechanical and biological
enclosure in which to culture fish. Pens are larger filters and is aerated before being reused in culture
than cages and stocked at lower densities than cages units. Effluents may overflow from outdoor systems
(FAO, 1984). The fish in pens usually are fed, and during rainy weather, and water must occasionally
they have free access to natural food organisms in be discharged from indoor systems when new water
the water and sediment of the enclosed area. is applied to lower salinity or filters are cleaned.
Table 13.1. Typical standing biomass ranges for different kinds of aquaculture
Culture method Standing biomass
Ponds (fish and shrimp)
Extensive 0.025 to 0.05 kg m–3
Semi-intensive 0.05 to 0.5 kg m–3
Intensive 0.5 to 5 kg m–3
Flow-through systems
Trout 160 to 240 kg m–3
Channel catfish 75 to 150 kg m–3
Carp 200 to 300 kg m–3
Cages
Trout 20 to 40 kg m–3
Tilapia 150 to 250 kg m–3
Channel catfish 100 to 200 kg m–3
Water-recirculating systems (finfish) 100 to 200 kg m–3
Shellfish plots 0.5 to 150 kg m–2
13–4 GUIDE TO AGRICULTURAL METEOROLOGICAL PRACTICES
13.4 CLIMATE, WEATHER AND The drought of 2000 in the south-eastern United
HYDROLOGY States was especially severe, and in eastern
Mississippi a 38 to 40 cm rainfall deficit and above-
Aquaculture depends upon a constant supply of average evaporation because of summer
water and the total volume of water used is great temperatures that were warmer than usual caused
compared with traditional agricultural crops. many ponds to shrink to about 40 per cent of
Consumptive water use in aquaculture is much less normal volume. There was no means of replacing
than total water use and consists of water removed this water. It was estimated that the economic loss
in animals at harvest, about 0.75 m3 per tonne of to catfish farmers resulting from the drought was
production, and water lost in seepage and evapora- US$ 11.3 million (Hanson and Hogue, 2001). In
tion. Most water is discharged from culture units the western Delta region of Mississippi, water
and passes downstream (Boyd, 2005). The value of levels in ponds could be maintained during the
aquaculture products per unit of consumptive water 2000 drought by additions from wells. Higher
use is greater than for traditional agricultural crops pumping costs were incurred, however, because
(Boyd, 2005). Nevertheless, aquaculture facilities more water than normal was pumped to offset the
should be designed for efficient water use and aqua- rainfall deficit, which was 25 cm above the normal
culturists should be knowledgeable about local figure for the period from June through October
hydrologic conditions. (Hanson and Hogue, 2001).
13.4.1 Precipitation 13.4.2 Evaporation
All water sources for inland fisheries and aqua- Lake evaporation is often estimated by multiply-
culture are derived from precipitation, and the ing 0.7 by Class A pan evaporation, and several
amount and annual distribution of precipitation is techniques have been used to adjust the pan coef-
a critical factor (Kapetsky, 2000). Ponds and other ficient for local conditions (WMO, 1973). Boyd
production systems should be managed in (1985) measured daily evaporation from a plastic-
harmony with rainfall so that adequate water lined, 0.04 ha pond at Auburn, Alabama, and
levels can be maintained. The depth of precipitation compared the values with daily evaporation from
measured in a raingauge is the same as the depth an adjacent Class A evaporation pan. The correla-
of water falling directly into a water body near the tion (R2) between pan and pond evaporation
gauging station. It is well known that precipitation increased with the length of the period of meas-
varies greatly among locations and temporally at a urement: 0.668 for daily measurements, 0.902 for
given place. In fisheries and aqua culture, the weekly totals and 0.995 for monthly values. The
precipitation excess or deficit is a more important pan coefficient ranged from 0.72 in March to 0.90
variable than precipitation alone (Boyd, 1986; Yoo in September, with an annual average of 0.81. The
and Boyd, 1994). This variable is the difference in pan coefficient for estimating evaporation for
precipitation and pond evaporation measured on small ponds is larger than the pan coefficient for
a monthly or annual basis. At most sites, there will estimating lake evaporation because the physical
be periods with a precipitation excess, and other conditions of a small pond more nearly reflect
times, there will be a precipitation deficit (Figure those of the evaporation pan than do those of a
13.1). Annual precipitation excess usually occurs larger body of water.
in humid climates and an annual precipitation
deficit occurs in arid ones. For example, the annual Monthly mean air temperature and average monthly
precipitation excess averages 16.5 cm in humid solar radiation also were correlated with pond evapo-
central Alabama (United States). The precipitation ration (Boyd, 1985). The regression equations were:
deficit is 54.6 cm in semi-arid southern Kansas and
158.8 cm in the desert region of Southern E = –2.15 + 0.268 Rad R2 = 0.642 (13.3)
p
California. There are few places in the world where
direct rainfall will sustain a pond. There usually E = –4.406 + 5.753 T R2 = 0.862 (13.4)
p
must be one or more external water sources, such
as runoff from a watershed, inflow from seepage or where E represents monthly pond evaporation
p
additions from wells or other water bodies. (mm month–1), Rad is mean monthly solar radiation
(g·cal cm–2 day–1) and T is air temperature (°C).
Drought can be particularly devastating in
watershed ponds. Where groundwater is not The normal way of reducing evaporative loss of
available for refilling ponds, water levels may water stored in ponds for irrigation and other uses
decrease drastically, causing overcrowding of fish. is to make them deeper. Ponds that are 9 m and 4 m
CHAPTER 13. APPLICATION OF AGROMETEOROLOGY TO AQUACULTURE AND FISHERIES 13–5
deep and are exposed to the same conditions will evapotranspiration require equipment or data
have the same amount of evaporation from their seldom available to aquaculturists.
surfaces. Nevertheless, the evaporation loss per
cubic metre of water storage will be more from the
13.4.3 Overland flow and runoff
shallower pond by a factor of 9/4. In aquaculture, it
usually is not possible to use this approach to reduce The amount of overland flow entering ponds
water loss by evaporation because deep ponds strat- depends upon the watershed area, amount of
ify thermally, making management more difficult. precipitation, infiltration, evapotranspiration and
the runoff-producing characteristics of watersheds.
Evapotranspiration usually is not a major factor in Individual watersheds vary greatly with respect to
aquaculture ponds because vascular aquatic plants the percentage of precipitation that is transformed
are discouraged by deepening pond edges, by to overland flow. Steep, impervious watersheds may
turbidity resulting from plankton and by application yield 75 per cent overland flow, while flat water-
of aquatic weed control techniques (Boyd and sheds with sandy soils may yield less than 10 per
Tucker, 1998). Nevertheless, in hydrologic cent overland flow. Estimates of overland flow can
assessment of aquaculture projects, be made using the curve number method (United
evapotranspiration on watersheds is an issue. Yoo States Soil Conservation Service, 1972). In this
and Boyd (1994) recommend the Thornthwaite method, the depth of overland flow is estimated
method to estimate evapotranspiration for from the depth of rainfall produced by a given
aquaculture and fisheries purposes because it storm, antecedent soil moisture conditions, hydro-
requires only information on mean monthly air logic soil group, land use and hydrologic condition
temperature. Other methods of measuring on a watershed.
Preciptation and Preciptation and
pond evaporation, mm pond evaporation, mm
Dallas, Fresno,
Texas California
Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec
Auburn, Bangkok,
Alabama Thailand
Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec
Figure 13.1. Pond evaporation (solid line) and precipitation (dotted line) at four sites. Dark shading indi-
cates an evaporation excess, while light stippling indicates a precipitation excess. (Data from Wallis, 1977;
Farnsworth and Thompson, 1982; Boyd, 1985; and Meteorological Department of Thailand, 1981)
13–6 GUIDE TO AGRICULTURAL METEOROLOGICAL PRACTICES
Estimation of peak discharge is important in pond drained near the end of the dry season so that they
design and construction to prevent damage or will refill during the rainy season when water is abun-
destruction of dams and pond banks by erosive dant. In the south-eastern United States, the period
water overflows during intense storm events. In between December and March has the most rainfall
particular, spillways must be adequate to bypass and least evapotranspiration, and the majority of
excessive runoff and prevent dam failure. The selec- overland flow occurs during this time. Ponds in this
tion of the rainfall return period for use in spillway region typically are drained for harvest in the fall to
design should be based on the human and environ- ensure that they will refill in winter and spring.
mental consequences and expense of dam failure.
The rational method (also known as the Lloyd– Storage of runoff in ponds lessens stream flow, but
Davies formula or the design peak runoff method) once ponds are full, water entering ponds flows
developed to design storm drainage systems is through them and into streams. Overflow struc-
widely used to design overflow structures and spill- tures usually release water slowly, and water is
ways in watershed ponds. The equation for the detained in ponds for a few hours to a few days.
rational method is: Ponds on the catchment of a stream usually do not
reduce annual stream flow appreciably (Silapajarn
Q = CiA (13.5) and Boyd, 2005), but they will tend to lengthen the
time that runoff enters streams (Schoof and Gander,
where Q = peak runoff discharge, C = runoff coeffi- 1982). This flattens the stream hydrograph and can
cient, i = maximum rainfall intensity for the reduce flood levels.
concentration time of the watershed and the
selected return period, and A = watershed area. Embankment ponds often are constructed on flood
Runoff coefficients and the equation for estimating plains. If a large proportion of the area of a flood
the variable i can be found in most hydrology texts. plain is occupied by ponds, the cross-sectional area
An intensity–frequency–duration plot for area rain- for flood flow will be reduced and flood levels will
fall is also required for solving the rational method increase. Some countries restrict the extent to which
equation. flood plains can be obstructed. In the United States,
the Natural Resource Conservation Service has a
Runoff consists of overland flow plus groundwater rule that no more than 40 per cent of flood plains
discharge, and the two sources make up stream can be blocked. Embankments of ponds on flood
flow. Thus, stream gauging provides the most relia- plains should be high enough to prevent floods
ble estimates of runoff. Runoff usually is between from overtopping them.
15 and 40 per cent of annual rainfall for catchments
large enough to support permanent streams. Rough
13.4.4 Hydroclimate
estimates of annual stream flow can be obtained by
subtracting evapotranspiration from precipitation. The study of hydroclimate embraces the influences
Overland flow is only a fraction of runoff and one of climate on water availability (Langbein, 1967).
or two of the largest rainfall events during a year In some places, more rain falls each month than is
may contribute most of the overland flow. In lost by evapotranspiration. The excess water either
Alabama, the average annual runoff for watersheds infiltrates the land surface or becomes stream flow.
of the Piedmont Plateau region is 52 cm year–1, and In other places, monthly rainfall never meets the
average overland flow for typical watersheds within demands of evapotranspiration. Such regions have
this region is 22 cm year–1 (Boyd and Shelton, no permanent streams, and runoff is limited to
1984). unusually heavy rains. Most places have a hydro-
climate between these two extremes, in which some
Water levels in aquaculture ponds should be main- seasons have excess rainfall and others have a
tained 10 to 15 cm below overflow structures so precipitation deficit.
that most rainfall and runoff may be conserved. In
arid climates, the savings of water may be small, A common way of describing the hydroclimate of
but in humid climates, rain falling directly into an area is to plot monthly rainfall totals and
ponds often is almost enough to replace losses to monthly potential evapotranspiration estimates
seepage and evaporation (Boyd, 1982). over an entire year. A net gain in soil moisture
occurs in any month in which precipitation exceeds
Rainfed aquaculture ponds usually are drained at potential evapotranspiration. When the soil is at
intervals of one to several years for harvest. At most field capacity, some rainwater infiltrates deeper to
sites, the year can be divided into periods on the basis become groundwater, while the remainder becomes
of the amount of precipitation. Ponds should be overland flow. The proportion of rainwater that
CHAPTER 13. APPLICATION OF AGROMETEOROLOGY TO AQUACULTURE AND FISHERIES 13–7
infiltrates more deeply depends upon the rate that used to estimate the water budget for a project.
water moves downward through the soil and under- Suppose that a fish farm with 20 embankment
lying geological material. Net loss of soil moisture ponds, each with a water surface of 5 ha and an
occurs when rainfall is less than potential evapotran- average depth of 1.5 m, is to be built on loamy clay
spiration. If the period of net water loss continues, soil where annual rainfall and Class A pan evapora-
soil moisture depletion may limit plant growth. tion are 120 cm and 100 cm, respectively. It is
Soil moisture recharge occurs when precipitation assumed that seepage will be 0.25 cm day–1 (Yoo
exceeds evapotranspiration and the soil moisture and Boyd, 1994), because properly constructed
content is below field capacity. The annual hydro- ponds on loamy clay soil should not seep much.
climate of a locality in Alabama is given in Figure Pond evaporation will be taken as 0.8 times pan
13.2. The figure illustrates periods of water surplus, evaporation. Ponds will be drained once per year
soil moisture utilization, water deficiency and soil for harvest, water exchange will not be used and
moisture recharge. storage volume will be sufficient to prevent over-
flow after rains. Runoff from the embankments can
For the hydroclimate illustrated in Figure 13.2, it is be neglected. The storage change (∆H) will be 1.5 m
clear that most overland flow and aquifer recharge because ponds will be filled and drained once each
occur between mid-December and May. The peak year. The water source will be groundwater from
discharges of streams also occur between December wells. The total amount of well water needed in an
and May. From early May until mid-December, average year can be calculated as:
streams are sustained primarily by base flow. Many
small streams cease to flow during this period. Inflow – Outflow = ∆H
Unusually heavy rains during the summer and fall
generate some overland flow and may recharge (A + P) – (E + So) = ∆H
aquifers slightly.
A = (∆H + E + So) – P
13.4.5 Water budgets
A = 150 cm + (100 cm × 0.8) + (0.25 cm day–1 ×
The hydrologic equation and local climatic data 365 days) – 120 cm = 201.25 cm. (13.6)
can be valuable in planning hydrologically respon-
sible aquaculture projects. The size of an aquaculture For the 100 ha farm, 2 012 500 m3 of water must be
project should not exceed the availability of water, supplied by the well. Suppose the plan also requires
for if water shortages occur, aquaculture crops may a capacity to fill all ponds within a 60-day period.
be damaged or lost (Boyd and Gross, 2000). Water Ponds are 1.5 m deep, and 1 500 000 m3 would be
budgets should be estimated for planning and required to fill 100 ha of ponds over 60 days. This is
designing new projects. The following example a continuous pumping rate of 17.4 m–3 minute–1
illustrates how the hydrological equation may be from the wells.
The above example was based on average condi-
200 tions. Reference to the historical rainfall and
180 evaporation data for the site could suggest the driest
conditions normally expected. This would allow for
160
the design of extra well capacity so that plenty of
140
water will be available during dry years. If such data
120
es are not available, a safety factor of 1.5 is
metr 100 recommended.
Milli 80 Rainfall
Many governments are developing water quality
60
Potential regulations for aquaculture effluents, and the
evapotranspiration
40
volume of water discharged by ponds has become
20 an important variable. Suppose that a 1 ha pond of
0 1.5 m average depth has a 20 ha watershed. Rainfall
in the area is 1 100 mm yr–1, Class A pan evapora-
Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec tion is 980 mm yr–1 and watersheds typically yield
18 per cent of annual rainfall as overland flow. The
Figure 13.2. Hydroclimate at a site in Alabama. Soil
moisture status: (A) surplus, (B) utilization, pond is constructed on tight clay soil, and the seep-
(C) deficiency and (D) recharge. Runoff occurs age rate will be taken as 0.1 cm day–1. The pond is
primarily when there is a soil moisture surplus. drained only at intervals of several years. Assuming
13–8 GUIDE TO AGRICULTURAL METEOROLOGICAL PRACTICES
that the pond typically is full but not overflowing Organisms use oxygen in respiration in order to
on 1 January of each year, that is, ∆H = 0 cm, the oxidize organic nutrients and release biologically
overflow for a year will be estimated. The appropri- useful energy. Ecologically, respiration is basically
ate form of the hydrologic equation and its solution the opposite of photosynthesis. During daylight,
are: photosynthesis usually produces oxygen faster than
oxygen is consumed in respiration, and dissolved
(P + R) – (E + So + O) = ∆H oxygen concentration increases from morning to
afternoon (Figure 13.3). Photosynthesis stops at
O = ∆H – (P + R) + (E + So) night, but respiration continues to cause dissolved
oxygen concentration to decline at night
O = 0.0 cm – (1.1 m × 1 ha) – (1.1 m × 0.18 × (Figure 13.3). Differences in dissolved oxygen
20 ha) + (0.98 m × 0.8) + (0.001 m day–1 × concentration between day and night become more
365 days) × 104 = 39 100 m3 year–1 extreme as phytoplankton abundance increases
(Figure 13.3). Aquaculture ponds typically have
O = –39 110 m3 year–1 (13.7) dense plankton blooms and wide daily fluctuations
in dissolved oxygen concentration.
The overflow has a negative sign because it repre-
sents water lost from the pond. This volume of Organic matter from photosynthesis is the source
overflow is more than twice the pond volume. of organic compounds used by phytoplankton and
other plants to elaborate their biomass. The oxygen
The overflow estimated in the example above is for released by the process is a major source of dissolved
an entire year. The spillway design, however, should oxygen needed in respiration by aquatic animals,
be based on the most runoff expected following the bacteria and aquatic plants living in water bodies.
largest single daily rainfall event for a selected In aquaculture, fertilizers may be applied to ponds
return period, namely, the 25-year, 50-year or to increase phytoplankton productivity and permit
100-year rainfall event (Yoo and Boyd, 1994). greater aquatic animal production, but manufac-
tured feeds may be offered to culture animals to
increase production beyond that achievable from
natural food.
13.5 CLIMATE, WEATHER AND WATER
QUALITY Although it is well known that short-term varia-
tions in the photosynthesis rate result from the
Solar radiation is necessary for aquatic plant growth effect of cloud cover on the amount of incoming
and is a major factor regulating water temperature. radiation, it has not been demonstrated that the
Wind mixing has a strong influence on the thermal growth rate in aquatic animals is affected by this
and chemical dynamics of water bodies. Finally, variation. For most fisheries species, there is a time
extreme weather events such as floods, droughts, lag between primary production and its use in fish
hurricanes and unseasonable temperatures can
adversely influence water quality and have negative
impacts on fisheries and aquaculture.
13.5.1 Solar radiation L)
g/
m
Phytoplankton are the base of the food chain that n (
culminates in fisheries production in natural ge
y
systems. Planktonic algae require solar radiation, ox
d
water and inorganic nutrients to conduct ve
ol
photosynthesis, a process by which they use Diss
chlorophyll and other pigments to capture photons
Heavy plankton bloom
of light and transfer the energy to organic matter.
Moderate plankton bloom
The photosynthesis reaction is provided below in Light plankton bloom
its simplest form:
6 a.m. Noon 6 p.m. Midnight 6 a.m.
Time of day
6CO + 6HO CH O + 6HO (13.8)
2 2 6 12 6 2
Inorganic nutrients Figure 13.3. Effect of time of day and phyto-
plankton density on concentrations of dissolved
oxygen in surface water
CHAPTER 13. APPLICATION OF AGROMETEOROLOGY TO AQUACULTURE AND FISHERIES 13–9
production (McConnell, 1963), and short-term the number of consecutive days necessary for
fluctuations in solar radiation are not reflected in dissolved oxygen concentrations to decline to
fluctuations in the growth of fish, shrimp and other 2.0 mg l–1 and 0.0 mg l–1 in ponds with different
fisheries species. Fisheries production integrates Secchi disk visibilities and solar radiation inputs
short-term fluctuations in solar radiation and (Table 13.2). Romaire and Boyd (1979) also used
photosynthesis, but differences may be obvious on local solar radiation records to calculate the
an annual basis. Doyle and Boyd (1984) studied probabilities of the number of days with low solar
sunfish production in three ponds at Auburn, radiation, as illustrated in Table 13.3 with data
Alabama, each stocked, fertilized and managed the for Auburn, Alabama.
same way for eight consecutive years. Fish produc-
tion averaged 362 kg ha–1 and varied from 270 to The Secchi disk mentioned above is a disk 20 cm in
418 kg ha–1 over the period, with a coefficient of diameter that is painted on its upper surface with
variation of 27 per cent. Several factors, including alternate black and white quadrants, weighted on
dates of stocking and harvest, water temperature, its underside, and attached to a calibrated line. It is
fish size at stocking and amounts of aquatic weeds lowered into the water until it just disappears from
in ponds, varied among years and no doubt influ- view and raised until it just reappears. The average
enced production. Average daily solar radiation of the depths of disappearance and reappearance is
data for the period April through September at a the Secchi disk visibility. In most aquaculture
nearby site, however, exhibited a positive correla- systems, plankton is the major source of turbidity,
tion (R2 = 0.51) with sunfish production. so Secchi disk visibility provides an indirect meas-
ure of plankton abundance (Almazan and Boyd,
Many types of aquaculture are based on feed 1978). The transparency of lakes and other water
inputs. Annual variation in production among bodies is often compared by the extinction coeffi-
channel catfish ponds to which feed was applied cient (K) calculated from light intensity at the
was much less (coefficient of variation = 8 per cent) surface (I ) and at another depth (I ) in metres
o Z
than in fertilized ponds where fish growth (Wetzel, 2001) as follows:
depended upon primary productivity (Doyle and
Boyd, 1984). Production of channel catfish in lnI lnI
K= O Z (13.9)
ponds with feeding was not correlated with solar Z
radiation.
It is well known that prolonged cloud cover lessens The Secchi disk visibility provides a simple means
the rate of photosynthesis and dissolved oxygen of estimating the extinction coefficient because
production. Several days with overcast skies can Idso and Gilbert (1974) found that the following
result in dissolved oxygen depletion in ponds. relationship existed between the two variables:
Mechanical aeration often is used to enhance the
dissolved oxygen supply and prevent dissolved ∑
x (13.10)
oxygen stress to cultured species. Historical data on X= i=69 449 / 50 = 1 388.9
n
the amount of solar radiation, the frequency of
overcast skies or the duration of sunshine per day at
a particular site can suggest if cloudy weather is where Z = Secchi disk visibility (m).
SD
likely to be a common problem in a particular
region. Forecasts of periods with heavy cloud cover Light penetration to the bottom of ponds can result
could be useful in alerting aquaculturists to the in growth of undesirable aquatic weeds. The most
likelihood of dissolved oxygen depletion and the common procedure for preventing weed growth is to
need to prepare for the events. encourage phytoplankton turbidity, which restricts
light penetration. The depth limit of aquatic weed
Boyd et al. (1978b) provided an equation for growth usually is about twice the Secchi disk visibility
estimating the decline in dissolved oxygen (Hutchinson, 1975). The target Secchi disk visibility
concentration in ponds at night. Romaire and in ponds varies among culture species and methods,
Boyd (1979) developed an equation to predict the background turbidity of waters and preference of
daytime increase in dissolved oxygen in ponds managers, but 30 to 45 cm usually is considered
based on solar radiation, chlorophyll-a optimum. Plankton abundance in aquaculture ponds
concentration, Secchi disk visibility and usually will restrict underwater weed growth where
percentage saturation with dissolved oxygen at water is 90 cm or more in depth. Unless pond edges
dawn. The two equations were used to produce a are deepened, shallow water areas in ponds may
model (Romaire and Boyd, 1979) for predicting become weed infested in spite of turbidity. Aquatic
13–10 GUIDE TO AGRICULTURAL METEOROLOGICAL PRACTICES
Table 13.2. Number of consecutive days (simulated) necessary for dissolved oxygen (DO) concentrations
to decline to 2.0 and 0.0 mg l–1 in ponds given varying amounts of solar radiation and
Secchi disk visibilitiesa (from Romaire and Boyd, 1979)
Solar radiation (langleys/day)
Secchi disk
visibility (cm) 50 100 150 200 250 300 400
DO = 0.0 mg l–1
10 1 1 1 1 1 2 >8
20 1 1 1 1 1 5 >8
30 1 1 1 2 2 6 >8
40 1 1 1 2 3 6 >8
50 1 1 2 2 3 6 >8
60 1 2 2 3 4 6 >8
70 2 2 2 3 4 6 >8
80 2 2 3 3 4 7 >8
DO = 2.0 mg l–1
<30 0 0 0 0 0 0 0
40 1 1 1 1 1 1 1
50 1 1 1 1 1 1 3
60 1 1 1 1 1 2 4
70 1 1 1 1 2 2 5
80 1 1 1 2 2 3 6
a Assumptions are: (1) the initial average DO concentration at dusk is 10.0 mg l–1; (2) pond is 1 m in depth and contains 2 240 kg/ha of
catfish; (3) water temperatures are 30°C at dusk and 29°C at dawn.
weeds, such as Eichhornia crassipes (water hyacinth) accumulates at the surface removes carbon dioxide,
and Lemna spp. (duckweed), that float on the water causing pH to rise and dissolved oxygen
surface are especially troublesome in aquaculture concentrations to increase (Boyd and Tucker, 1998).
ponds because they cannot be controlled by Abeliovich and Shilo (1972) and Abeliovich et al.
manipulating turbidity. (1974) demonstrated that the combination of
intense light, low carbon dioxide concentration,
Some species of planktonic algae respond to changes high pH, and high dissolved oxygen concentration
in light intensity by altering their position in the caused death in blue-green algae through disruption
water column. The effects of light on some species of enzyme systems and photo-oxidation. Boyd et
of blue-green algae, such as Anabaena spp., can lead al. (1975) demonstrated that sudden die-offs of
to adverse impacts in aquaculture systems. Gas blue-green algae in channel catfish ponds occurred
vacuoles filled mainly with carbon dioxide form in during periods of calm, clear weather when scums
these algae in response to low light intensity. The of blue-green algae accumulated on pond surfaces.
increased buoyancy causes the algae to rise until In some cases, almost all blue-green algae suddenly
higher light intensities increase the rate of die, or in other situations, only a small percentage
photosynthesis, thus removing carbon dioxide and of the algae die (Boyd et al., 1978a). The dead algae
collapsing gas vacuoles (Fogg and Walsby, 1971). will be decomposed rapidly by bacteria, and in
Lack of turbulence during calm weather in shallow many cases, bacterial respiration will cause dissolved
ponds allows algae to rise more rapidly than oxygen concentration to decline to a critical level.
photosynthesis can cause gas vacuoles to collapse
and lessen buoyancy. The algae float to the surface Benthic algae often form mats on the bottoms of
where they encounter excessive light intensity. clear water bodies. These mats of algae are called
Rapid photosynthesis by phytoplankton that “lab-lab”, and they can be an important source of