This document discusses size management strategies for farming giant freshwater prawns. It describes the different male morphotypes (blue claw, orange claw, small claw) and their behaviors, with blue claw males being the most dominant and aggressive. Population density, sex ratio, and heterogeneous individual growth rates affect the proportions of male morphotypes. Orange claw males exhibit a "leapfrog" growth pattern where they surpass blue claw males in size before transforming. The presence of blue claw males suppresses the growth of small males. The document recommends grading, cull harvesting, and using substrates to improve farm productivity by reducing dominance hierarchies.

1. GIANT FRESH WATER PRAWN
Size management of giant freshwater prawns
farming
Dr. Mhd. Ikhwanuddin,
Institute of Tropical Aquaculture,
Universiti Malaysia Terengganu.

2. Major male morphological
characteristics
• 3 major morphotypes have been described for
sexually mature male M. rosenbergii.
• The most immediately distinctive feature is the size
and colour of the claws, and the robustness of their
spines:
• Blue claw males (BC) have extremely long blue claws
(second pereiopods) with longer and stronger spines
than OC males;
• Orange claw males (OC) have golden coloured claws
that are generally shorter and have shorter and less
strong spines than those of BC males; and
• Small males (SM) have small, slim, almost
translucent claws.

3. Major male morphological
characteristics
• There are also a number of intermediary forms
between these major morphotypes.
• The transition from the small male (SM) to the orange
claw (OC) morphotype is gradual.
• The OC is therefore sometimes referred to as the
strong orange claw (SOC), and an intermediate stage
between these two forms, the weak orange claw male
(WOC), has been recognized in research work.
• Another intermediate form, this time between the
orange claw (OC) and the blue claw (BC) is known as
the transforming orange claw (TOC); this is the last
stage of the SOC male before it transforms into the
BC male.

5. Behaviour
• The behavioural characteristics of the
morphotypes - are of essential importance in
the management of freshwater prawn grow-
out facilities.
• BC males are aggressive, dominant and
‘territorial’.
• OC males are aggressive, subdominant and
‘non-territorial’.
• SM males are submissive and ‘non-
territorial’.

6. • Less physical contact and fewer displays of claw
position and movement occur in SM than in OC and
BC.
• BC males are dominant over OC males which, in turn,
are dominant over SM.
• Interactions between BC males are often only for
show, with little physical contact.
• BC and OC males with equal claw size are evenly
matched but a BC with larger claws than an OC.
• Dominance of BC over OC seems to confer priority of
access to preferred areas (e.g. shaded protected
crevices).
• Laboratory studies have shown that competitors are
evicted from the vicinity of a limiting resource, such as
shelter, food, and receptive females.

7. • Females approach males 2 to 3 days before their pre-mating
moult.
• At first the female is chased away but later, after several hours of
persistence, is allowed to remain near the male.
• About a day before the pre-mating moult the female is already
totally accepted by the male, positioned below it or between its
long second pair of claws.
• As a result of this early pair bonding, fertilisation can occur from
several minutes to half an hour after moulting.
• All three male morphotypes have similarly high rates of fertilising
receptive females.
• Males do not attack or injure the females that they have just
fertilized.
• BC tend to guard the female for two or three days following
mating, by which time the female’s exoskeleton is hard enough to
withstand attacks by other prawns.
• However, OC do not appear to groom or protect the females.
• There are reports of injuries inflicted by OC on females during this
period.
• SM mating can be achieved when there are three or more SM
present; while the BC is chasing away some of the runts, the
female remains unprotected.

8. The effect of the sex ratio
• The proportion of females under grow-out conditions tends to be
greater than males, possibly for the following reasons:
1. females may already outnumber males at stocking; and
2. selective male mortality may occur in crowded pond populations.
• Since the highest prices are generally obtainable for the largest
animals - females in a population seem to be disadvantage at first
glance.
• It would appear to indicate that there would be a strong incentive to
rear all-male populations of prawns.
• However, the effect of density on average weight is more extremely
pronounced in all-male, compared to all-female populations.
• The use of all-male populations would therefore not remove the need
to manage size variation and harvesting procedures very carefully.
• If maximizing the total weight of prawns produced per hectare is the
main goal, the rearing of all-female populations at very high densities
would be sensible.
• However, if maximizing the income from the pond is the main goal,
proper management of mixed-sex or all-male populations would be
best, since the larger-sized prawns normally have the greatest unit
value.

9. The effect of density
• The proportions of the various male morphotypes
change significantly with density.
• High density results in a larger proportion of SM.
• The frequency of the large BC males is highest at
low densities.
• At high densities many prawns are in close contact
with BC males, which inhibit their growth.

10. The effect of uneven male growth rate
• Newly metamorphosed PL relatively even in size but
size variation soon becomes noticeable.
• Individual prawns grow at different rates =
heterogeneous individual growth (HIG).
• Fast-growing individuals = ‘jumpers’ - 15 times larger
than the population mode within 60 days after
metamorphosis.
• Jumpers became obvious within two weeks after
metamorphosis.
• Slow-growing prawns = ‘laggards’ - 5 weeks following
metamorphosis.
• Growth suppression in laggards depends upon the
presence of the larger jumpers.
• Male jumpers develop mainly into BC and OC males.
• Laggards develop mainly into small males.

11. The social control of growth
• Social interactions between freshwater
prawns are extremely important in
regulating growth.
• In freshwater prawns the most important
social interactions are the growth
enhancement of OC males (what is known
as the ‘leapfrog’ growth pattern) and the
growth suppression of SM by BC males.

12. Growth enhancement of orange claw males
• The change of OC males into BC males is sometimes called a
metamorphosis because the differences between these morphotypes are so
dramatic.
• An OC metamorphoses into a BC after it becomes larger than the largest BC
in its vicinity.
• As a new BC male it then delays the transition of the next OC to the BC
morphotype, causing it in turn to attain a larger size following its
metamorphosis.
• The newly transformed BC is larger (sometimes much larger) than the
largest BC previously present.
• This is known as the ‘leapfrog’ growth pattern, because the weight of one
type of animal leaps over another.
• BC males dominate OC males, regardless of their size, probably because of
their larger claws.
• A prawn that has metamorphosed into a BC male and is larger than any
other BC in its vicinity (following the ‘leapfrog’ growth pattern) becomes the
most dominant prawn in the vicinity until it is overtaken by another prawn
metamorphosing from OC to BC.
• The ‘leapfrog’ growth pattern results in the gradual descent in the social rank
of existing BC males.
• When a new and larger BC appears on the scene, the ‘social ranking’ of all
BC males present before that event fall.

13. Growth suppression of small males
• The growth of runts (SM) is stunted by the presence of BC males.
• Food conversion efficiency seems to be the major mechanism
controlling this growth suppression in runts.
• Runts have poorer (higher FCR) feed efficiency when BC males are
present.
• This seems to be governed by physical proximity; the phenomenon
has not been demonstrated when these two types of prawns are
separated, even when they are in the same water system and can
see each other (i.e. chemoreception and sight are not factors).
• SM are sexually active.
• While they stay small they attract less aggression from dominant BC
males (which are busier interacting with OC males) and are probably
less vulnerable to cannibalism since they can shelter in small
crevices.
• Being small and highly mobile, runts can find food on the bottom
before being chased away by larger prawns, whether they be males
or females.
• If BC males are removed from the population, some runts will
increase their growth rate, and transform into OC males and, finally
into BC males, following the normal ‘leapfrog’ growth pattern.
• This highlights the importance of regular cull-harvesting.

15. Managing grow-out in the light of
heterogeneous individual growth (HIG)
• In grow-out management:
• there is an opportunity to improve the final harvest, both in terms of
average market size and total production, by grading, because the
prawns that are going to grow fastest become identifiable within 2-5
weeks after the PL metamorphose from larvae.
• there is an opportunity to increase the final harvest, both in terms of
average market size and total production, by cull-harvesting. This
removes the BC males, many of the OC males, and the larger
females, thus encouraging small males to grow faster in the absence
of dominant males (sometimes known as compensatory growth).
• there are opportunities for increasing productivity through the use of
substrates. These provide refuges for newly moulted prawns (which
increases survival) and decrease the frequency of fighting (which
reduces growth suppression). This results in fewer SM, more OC and
BC males, and higher average harvest weights.
• the potential advantages of monosex culture, not only because of
the differential growth rates of males and females but also because
there is less HIG in females than in males, may eventually become
exploitable.

16. (A) A cross section of thoracic spine (TS) between the fifth walking legs.
(B) A gonopore complex (GP) of a prawn at 45days old.
(C) Cross section of a prawn showing the vas deferens (VD) and a gonopore complex.
(D) An appendix masculina (AM) located at the inner part of the second swimmeret.