GIANT FRESH WATER PRAWN Size management of giant freshwater prawns farmingDr. Mhd. Ikhwanuddin,Institute of Tropical Aquaculture,Universiti Malaysia Terengganu.
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.
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.
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’.
• 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.
• 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.
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.
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.
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.
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.
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.
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.
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.
(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.