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6 R. FRANKHAM Meltzer 1993; Wilson et al. 1994; Snyder et al. 1996). Con- sequently, there are advantages in terms of reproductive fitness, genetic diversity, minimizing genetic adaptation to captivity, cost and safety to maintaining a species as isolated populations with occasional translocations. A similar management regime was recommended by Ballou (1993), based on computer simulations of the risk of disease epidemics and the impacts of inbreeding depression. The fragmentation regime results in higher levels of inbreeding, so it must be balanced with occasional out- crossing to maintain inbreeding at acceptable levels (Frankham et ai. 2002). While this fragmentation regime has much to recommend it, it is not currently the recom- mended genetic management for threatened species. If genetic adaptation to captivity is due to rare alleles that are partially recessive and deleterious in the wild, then different captive populations will utilize at least partially different alleles. Thus, crosses between them should exhibit partial recovery of fitness in the wild. Crosses among rep- licate populations within sizes of Ne = 500 and Ne = 250 after 45 generations resulted in partial recovery of fitness under simulated wild conditions (Fig. 2; Woodworth et ai. 2002). These populations had inbreeding coefficients of approximately 5% and 10% so the recovery is far too great to be solely attributable to reversal of inbreeding depression. Immigration from wild into captive populations Immigration from the wild is expected to reduce the rate of genetic adaptation to captivity, with the reduction being greater for immigration in later, rather than in earlier 1.5 1.0 rn rn Ql .5 u::: 0.5 0.0 ~ ~ Ql 0 0 0 0 m ~ ~ ~ ~ g ~ 0 m N 8 .5 ~ m ~ ~ ~ ~ 000 m mom N N ~ Fig. 2 Effects of crossing replicate captive populations upon fitness when they are returned to the wild, Reproductive fitness of the Ne populations (500, 250, 100, 50 and 25) and Pooled populations formed from crossing replicates within treatments at generation 45 (500P, 250P, lOOP, SOP and 25P), when all were transferred to simulated 'wild' conditions at generation 50. Wild, base population and inbred control populations are shown for comparisons. generations, and greater for higher than for lower immi- gration rates (Frankham & Loebel1992). For most threatened species, immigration from the wild is not possible, as the wild population is too small to justify depleting it. Immigra- tion is mainly being used in fish, and the primary purpose is to supply more individuals for harvest rather than for conservation purposes. Araki et ai. (2007) have shown that fitness in the wild is indistinguishable from wild steelhead trout (Onchorynchus mykiss) for supplemental hatchery stock (only one generation in captivity), while hatchery stock that had been in captivity for multiple generations had relative reprod uctive successes of 6-11 %, 31-45% and 30- 44% compared to that of wild stock in different years. What if the wild environment has changed since foundation of captive population? For many species, especially those that must spend 100-200 years in captivity, the wild environment may have changed by the time they can be reintroduced into the wild. Under these circumstances, I would recommend crossing all captive populations prior to reintroduction to begin the new wild population with maximum heterozygosity and maximum genetic diversity. In this way, it may have sufficient fitness to establish in the wild and genetic diversity to evolve to cope with both its new environment and future environmental change. I know of no empirical studies that address this issue. What will be the impact on native diversity of releasing captive species after 100-200 years? Will the released spe- cies become invasive? It is unlikely that released captive species will become invasive, as repeated releases of domesticated rabbits failed to establish in Australia before the 'successful' introduction of wild rabbits resulted in an invasive species (Fenner & Ratcliffe 1965). Related data also exists for birds (Fyfe 1978). Recovery of wild fitness after reintroduction Populations can recover their loss of wild fitness through natural selection if they have genetic diversity and sufficient fitness to establish in the wild. For example, our Ne = 500 treatment after 50 generations in captivity had a relative fitness in the wild environment of only 14% that of the wild population (Woodworth et ai. 2002). However, it recovered to 70% of the fitness of the wild population after only 12 generations in the wild environment (Margan et ai. 1998). Such recovery will be most rapid in large reintroduced populations with ample genetic diversity and less in small populations with limited genetic diversity. Conclusions Genetic adaptation to captivity increases with selection differential, genetic diversity, effective population size and @ 2007 The Author Journal compilation @2007 Blackwell Publishing Ltd