The use of monosex fish is intrinsically desirable in a variety of fish species in a range of aquaculture production systems. The potential advantages sought from their use may include one or more of the following features: achievement of higher average growth rate, elimination of reproduction, reduction of sexual/territorial behaviour, reduction of variation in harvest size, and reduction of risk of environmental impact resulting from escapes of exotic species.Fish as a group have systems of sex determination which are of considerable biological interest and significance for studies in evolutionary biology. However, they are very variable, relatively poorly understood and give rise to much variation in sex ratio between, and within, species. Enough is known, however, to enable us to say that these systems are often employed in ways which sharply distinguish the fishes from groups such as mammals, birds and reptiles. As a consequence, manipulations of sexual phenotype designed to produce monosex populations are not straightforward and the results are not necessarily predictable. This paper reviews the techniques for production of monosex males, and considers in detail the case of the Y Y / G M T technology in the Nile tilapia, which is the only example of a genetic technology for the production of monosex males so far widely adopted by the aquaculture industry. The considerable benefits accruing from the use of GMT are described. An attempt at projecting future developments in this area of aquaculture is made. 0 0 4 4 -8 4 8 6 / 0 1 / $ -see front matter ©2001 Elsevier Science B.V. All rights reserved. PII: S 0 0 4 4 -8 4 8 6 ( 0 1 ) 0 0 5 9 0 -7 284 JA. Beardmore et al / Aquaculture 197 (2001) 283-301

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Two molecular techniques which reveal highly variable DNA polymorphisms, RAPD and multilocus DNA fingerprinting, were used to evaluate genetic diversity between six aquacultural strains of Oreochromis niloticus (tilapia) from the Philippines. The results using both techniques were in close agreement. Within-strain heterozygosity values were similar and were correlated between the two data sets, but statistical errors associated with the RAPD data set were lower. Although genetic distances between strains were greater using DNA fingerprinting, the distances measured using both methods were significantly correlated. Both methods were useful in estimating variation between strains, but they offered different advantages. RAPD was technically easier to perform and produced results with low statistical error, whereas DNA fingerprinting detected greater genetic differentiation between strains. The theoretical basis for using RAPD and multilocus minisatellite markers for population studies is discussed.

This paper first considers the perception and dimensions of biodiversity. We suggest that biodiversity can be viewed as a biospatial, hierarchically distributed structure of variability among living organisms with five levels of complexity (and including agricultural systems). Loss of biodiversity, value of biodiversity and knowledge of aquatic biodiversity are reviewed briefly. Methods for measuring biodiversity, especially at the genetic level, are discussed and considerable emphasis is laid on effective population size as a controlling factor in biodiversity.Finally, a range of impacts of aquaculture upon biodiversity are considered, including competitive and introgressive effects from farmed stocks. The development and use of reversibly sterile strains is seen as highly beneficial to both aquaculture and the maintenance of biodiversity.

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