Restoring doomed fish

24 08 2012

I get called a doomsday merchant a lot, mainly because there’s not a lot of good news out there when it comes to biodiversity these days. However, now and again there is a success story worth shouting from the rooftops. This latest post comes from my PhD student, Jarod Lyon (also of the Arthur Rylah Institute in Victoria), who is working on restoring native freshwater fish in Australia’s largest river system – the Murray-Darling. The M-D also happens to be in a lot of trouble because of poor water management and years of neglect. However, some clever research and restoration proves that we can bring biodiversity back from the brink if done right. Jarod has posted here on Conservation Bytes before describing his work, and this latest post provides some detail on one species in particular.

Trout cod Maccullochella macquariensis were once considered to be widespread in the southern tributaries of the Murray-Darling Basin. However over the past fifty years, their distribution and abundance have declined dramatically, due to a number of disturbances including habitat loss, altered flow and temperature patterns, in-stream sedimentation, population fragmentation due to in-stream barriers and over fishing. Trout cod are listed nationally as endangered under the Environment Protection and Biodiversity Conservation Act (EPBC Act 1999) and listed under the Victorian Flora and Fauna Guarantee Act (FFG Act 1988). Trout cod are often accidentally caught when fishing for Murray cod. However, it is illegal to take a trout cod while angling.

Trout cod were historically abundant in the lower Ovens River system in South-Eastern Australia, however were locally extinct by the 1980s. In an attempt to re-introduce a viable population in the Ovens River, hatchery-reared juvenile trout cod were stocked in the Ovens River system for ten years starting in 1997. Our recent manuscript published in Marine and Freshwater Research assesses the success of this stocking regime (particularly in relation to recovery plan objectives) through a variety of techniques, including fish surveys and analysis of gonads, otoliths and genetic structure of the population.

We found that the Ovens River now holds a naturally self-sustaining population of trout cod – that is, the progeny of stocked fishes are now breeding.  Given that most threatened species re-introduction programs worldwide fail, this is somewhat of a good-news story for management of rare animals. In particular, we found that the length of the stocking program was a major factor in its success, as the long time period overcame the years where the survival of the stocked fingerlings was low. Interestingly, most fish to recruit to an adult size were stocked in 2003 or 2004 – meaning if this had been a five-year program, it would most likely have failed.

We discriminated the hatchery-reared fish from wild-born fish by analysis of otoliths (fish ear bones). Fish otoliths lay down annual growth rings, somewhat like the growth rings in trees. Before release, fish were marked with a fluorescent chemical dye, which made a permanent ‘mark’ on the otolith, and so when these otliths were removed for aging, we also examined the otoliths under a fluorescing microscope to look for the mark. This analysis revealed that while most trout cod in the system (at the time of sampling) were of hatchery origin, there were many unmarked (wild-born) fish in the population, and this number was increasing over time.

Genetic analysis of the population revealed that there was no difference in genetic diversity between the stocked population and the last remaining natural population, located in the Murray River downstream of Lake Mulwala. This is surprising, given that stocked fish populations are often thought to have lower genetic diversity due to the few adults contributing to the gene pool. In this case, the stocked population contained 55 alleles, while the natural population contained only 49. Possible reasons for this include that the hatchery protocols, which used many different combinations of breeding pairs, were robust, or that the population was too young to pick up any possible founder effects.

In summary, our research provides a unique case study and rare ‘good-news story’ for endangered species recovery. Re-introduction of populations is best done over a longer-than-typical period of time to account for unfavourable environmental conditions that reduce yearly survival of re-introduced individuals – a finding which might well have implications for other (even terrestrial) species-recovery efforts.


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