The Biology and Management of Size Variation

The wide range of sizes that develops in prawn populations and particularly in the males is a very typical characteristic of Macrobrachium rosenbergii culture and a major obstacle to increased profitability. Prawn prices are size dependent and those below a minimal size are often used as bait for fish or are discarded by the growers. Male populations are bi- or multi-modal and positively skewed, comprising a large fraction of small, unmarketable individuals and a fraction of large prawns. Early workers recognised obvious but only superficial parallels between the prawn growth pattern and that of the common carp, with its rapidly growing shoot carp or tobi-koi (Nakamura & Kasahara 1957). Removal of the shoot carp and of the large prawns leads to a rapid compensatory growth of the smaller animals. The prawn growth pattern is, however, much more complicated than that of the common carp. Prawn size variation actually reflects a complex population structure, composed of three sexually mature male morphotypes (small male, orange claw male and blue claw male), which differ in their morphology, physiology and behaviour, and transform from one morphotype into another. Recent research has revealed that prawn growth is affected by a very early determination of the male developmental pathway, based on the juvenile relative size ranking. Prawn growth regulation - suppression as well as enhancement - is achieved mainly by means of social interactions among individuals. In the tropics, with the traditional all-year culture system, size variation is only partly and inefficiently managed by the selective removal of large individuals, and the industry does not realise its full potential. In the temperate zone, prawn culture has not yet achieved true economic viability (see Chapters 11 and 20). It became obvious that a major limiting factor for prawn culture was the prawn growth pattern. Many studies, including numerous MSc and PhD dissertations related directly and indirectly to prawn size variation, have been published. Research areas were extremely diverse (morphology, anatomy, physiology, biochemistry, ecology and social behaviour). In this review we have assembled pieces of information related to size variation, in order to try to solve the puzzle of the prawn's complex growth pattern and to evaluate the strategies for its efficient management.