Biological performance and genetics of restocked and wild bream in the Blackwood River Estuary

M.J. Gardner; A. Cottingham; N.M. Phillips; S.A. Hesp; J.A. Chaplin; G.I. Jenkins

In an attempt to replenish a heavily depleted population of Black Bream Acanthopagrus butcheri, in 2002-03, 220,000 4-8 month old juveniles of this species, cultured using broodstock from the Blackwood River Estuary, were released into this system. During subsequent monitoring by Potter et al. (2008) over the next 3.5 years, restocked fish were readily able to be distinguished from wild A. butcheri because their otoliths contained a purple stain (alizarin complexone) that had been used to mark all cultured fish before release. The results of that monitoring program showed that the restocking had led to a substantial increase in the abundance of A. butcheri in the estuary. Their biological comparisons, using data for restocked and wild fish collected in 2002-05 and also data for wild fish collected in the two years leading up to the restocking, appeared to indicate that cultured individuals did not grow as well, or mature as rapidly, as wild fish. As the monitoring of the study of Potter et al. (2008) ended in 2005 when the oldest of the restocked A. butcheri were still young (about 4 yrs), the extent to which restocked and wild fish would differ in their growth and reproductive performance as they became older could not be determined by that study. Furthermore, as the restocked fish were only beginning to become large enough to be legally caught and retained by fishers, it was also not known whether restocked fish would substantially contribute to catches of A. butcheri in the Blackwood River Estuary. Moreover, the genetic consequences of the restocking program, and their implications for the ongoing conservation of A. butcheri in the estuary, had not been investigated. In this study, additional samples of A. butcheri from the Blackwood River Estuary have been obtained from the catches of a commercial gillnet fisher in 2006, 2007 and 2009, resulting in a full data set for fish spanning 2000-09. Analyses of biological data revealed that, since the restocking, the growth of wild A. butcheri has changed, with the lengths at a given age of fish hatched in more recent years typically being least. A comparison of the data for the 2001 and 2002 cohorts of restocked fish with corresponding data for wild individuals hatched in 2001 and 2002 showed that the restocked fish grew more similarly to wild fish than previously thought, with the difference becoming less than 5% by the time restocked fish were 6 years old. Comparisons of the growth of the 2001 and 2002 cohorts of restocked fish, and their relative abundance in catches, revealed that, although their growth was virtually the same, survival of the latter cohort was far greater, which may be attributable to that latter cohort having spent less time in the hatchery (~7 vs ~4 months). The study also highlighted that restocked A. butcheri made a substantial contribution to commercial gillnet catches in 2006 (32%) and even more so in 2007 (66%) and 2009 (62%). The new reproductive data provided very strong evidence that all surviving restocked fish attained maturity by the time they were 300 mm in length. Age composition data for the A. butcheri caught by commercial gillnet fishing in 2007 and 2009 show that recruitment of fish from natural spawning between 1999 and 2004 has been poor. Observations of substantial numbers of small black bream in the Blackwood River Estuary by the commercial fisher who operates in this system raises the strong possibility that there has been one or more recent years of good recruitment, as could potentially be brought about by the restocking program, but this has yet to be confirmed by research data. The genetic component of this study focussed on comparing patterns of variation at seven microsatellite loci in 52 wild and 53 restocked A. butcheri collected in September 2009. The results indicate that the restocking is unlikely to have had a major effect on the genetic composition of the population in this system and provide no evidence of an increased incidence of inbreeding in the restocked individuals. However, the restocking has probably resulted in a slight reduction in genetic diversity in the population due to a loss of rare alleles in the restocked individuals. Although these alleles are still present in wild individuals, they are at a higher risk of being lost (via genetic drift) from the population in future generations because their frequency has declined due to the introduction of the restocked fish. Although the loss of these alleles could potentially limit the evolutionary potential of the population of A. butcheri in the Blackwood River Estuary, it should be borne in mind that, if the restocking had not taken place, rare alleles are likely to have been lost regardless, as the abundance of Black Bream in this system underwent further decline. This study has highlighted the value of long-term monitoring in fish restocking programs, particularly for species such as Black Bream with medium longevity, and of rigorously assessing the genetic implications of these programs. The results demonstrate that the restocking of A. butcheri in the Blackwood River Estuary has been very successful in most respects but also highlight the importance, when determining the numbers of fish to be released in any future restocking of Black Bream, of considering the effects of restocking density. The results also emphasise that, with appropriate broodstock selection and breeding and release protocols, a restocking program can have minimal genetic consequences. However, it should be borne in mind that, as the results of a restocking program can be highly species and situation specific, these programs need to be assessed on a case-by-case basis. In conclusion, this research has provided a detailed account of the outcomes of the restocking program, which is important for informing future restocking programs in Western Australia for Black Bream, in particular. Why the abundance of A. butcheri in the estuary declined is still poorly understood and research on the underlying causes of this decline is critical for the ongoing conservation of this population.

Biological performance and genetics of restocked and wild bream in the Blackwood River Estuary

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