Effects of High pH on

Macrobrachium rosenbergii Postlarvae

 Charles G. Hummel and Dallas E. Alston
 Department of Marine Sciences
 University of Puerto Rico -Mayaguez
 Mayaguez, PU 00708 USA


 By adjusting pH through additions of NaOH solution
(r,8N), the response of recently metamorphosed Macrobrach-
ium rosenbergii postlarvae to pH values ranging from 7.5 to
12.0 was determined. At pH 9.0 weak animals and subse-
quent mortalities were observed. At pH 9.5 or greater, mass
mortalities occurred. The effect of high pH resulting from


algal photosynthesis in water with a heavy phytoplankton
bloom was also evaluated. High mortality rates were recorded
in water with a heavy bloom and high pH (10.0-10.5) com-
pared with control water without a bloom and with a lower pH
(8.5-9.0).


 Photosynthetic activity of unicellular algae is primarily respon-
 sible for pH variations in aquacultural ponds. It is widely
 suspected that high afternoon pH in ponds with heavy plankton
 blooms may kill fish fry (Boyd, 1979). Mortalities of adult
 freshwater prawns, Macrobrachium rosenbergii, have been at-
 tributed to higher pH values resulting from CO2 uptake by
 photosynthesizing algae (AQUACOP, 1979).
 Fujimura (1974) suggests that postlarval M. rosenbergii be
 cultured in water with an already established algal population
 (green water). The present experiments were therefore designed
 to test the response of M. rosenbergii postlarvae to high pH levels
 which may occur in culture systems with heavy phytoplankton
 blooms.

 MATERIALS AND METHODS
 Chemically Adjusted pH
 This experiment tested the shock effect of abrupt pH increases
 on the mortality of M. rosenbergii postlarvae. Experimental units
 were twenty 37-liter aerated aquaria filled with 20 liters of tap
 water. The pH was adjusted in each aquarium by adding quantities
 of concentrated NaOH solution (r8N). The pH values ranged
 from pH 7.5 to 12.0 with 0.5 pH intervals. The ten pH treatment
 levels were replicated twice in a completely randomized design.
 Thirty recently metamorphosed postlarvae were acclimatized to
 freshwater of pH 8.2 and placed in each aquarium. The pH was
 monitored with a Corning 3D pH meter. Temperature ranged
 from 28 to 29*C. Mortalities were recorded at 0.5, 1, 2, 4, 6, 8, 24,
 48, 72, and 96-hour intervals. All dead animals were removed from
 the aquaria.

 Photosynthetically Altered pH
 By comparing two water treatments, green and clear, this experi-
ment tested the effect of high pH resulting from photosynthetic ac-
tivity of phytoplankton on the survival of M. rosenbergii postlar-
vae. Experimental units were eight fiberglass pools (4.12 m2). Four
pools were filled to 30 cm with clear water containing no algae and
four pools were filled to the same level with green water containing
a heavy bloom of the blue-green algae Anacystis sp. Each pool was
provided with 3 lengths (40 cm each) of 3.75 cm diameter PVC
pipe as shelter. The pools were outdoors and exposed to full
sunlight.


VOL. XX-PROCEEDINGS of the CARIBBEAN FOOD CROPS SOCIETY


 TABLE 1. Percent mortality of postlarvae exposed to various pH levels.


 Exposure Time (hours)
 -------------------------------------
 pH 0.5 1 2 4 6 8
 ----------------------------------------------
 7.5 0 0 0 0 0 0
 8.0 0 0 0 0 0 0
 8.5 0 0 0 0 0 1
 9.0 0 0 13 23 36 40
 9.5 0 3 50 100 100 100
 10.0 0 3 73 100 100 100
 10.5 0 26 100 100 100 100
 11.0 0 30 100 100 100 100
 11.5 1 48 100 100 100 100
 12.0 100 100 100 100 100 100



 Each pool was stocked with 140 recently metamorphosed post-
 larvae (r30 mg). The animals were stocked at 0700 hours and were
 harvested 96 hours later.

 RESULTS
 Chemically Adjusted pH
 No significant mortalities were recorded for animals exposed to
 pH 7.5-8.5 after 96 hours of exposure. At pH 9.0 dead animals
 were observed after 2 hours of exposure. Increasing pH above 9.0
 resulted in higher mortalities in less time (Table 1). Beyond 8 hours
 exposure no further mortality was recorded at any pH level.

 Photosynthetically Altered pH
 Algal growth and photosynthetic activity were heavy in the green
water treatment (average afternoon maximum pH 10.5 and 91
mg/l COD) and light in the clear water (average afternoon max-
imum pH 9.0 and 12mg/l COD). Average total hardness was 78
mg CaCO3 for the green water treatment and 126 mg CaCO /1 for
the clear. Dissolved oxygen never fell below 7.3 mg/l, nor did am-
monia concentration rise to more than 0.01 mg in any of the eight
pools. Temperature fluctuated equally in all pools from 28C in
the morning to 360C in the afternoon.
 A significantly (p>0.01) higher postlarval mortality was recorded
for the green water treatment. An average of 78.5 animals were


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