Population fragmentation in the Murray Hardyhead Craterocephalus fluviatilis McCulloch, 1912 (Teleostei: Atherinidae) : ecology, genetics and osmoregulation.

Population fragmentation is a common symptom of the decline of species, including freshwater fishes. It occurs naturally, but has also proliferated in response to human interventions that increase the prevalence and intensity of isolating barriers and events. In regulated rivers, for example, fish are affected by the loss of connectivity between habitats that is associated with hydrological changes. The process has evolutionary consequences by limiting gene flow, reducing genetic diversity and rendering the isolates vulnerable to local environmental changes. Comparative studies of related species may help to elucidate the causes and consequences of fragmentation. For example, they may identify habitat features that influence the spatial separation of congeneric species. An opportunity for such a study arises with small fishes (Atherinidae) in the intensively-regulated River Murray, southeastern Australia. Whereas the unspecked hardyhead Craterocephalus stercusmuscarum fulvus is widespread and abundant, the Murray hardyhead C. fluviatilis has a patchy distribution and is listed as 'endangered‘ by the International Union for Conservation of Nature and 'vulnerable‘ under the Australian Environment Protection and Biodiversity Conservation Act 1999. These two species rarely cohabit, implying that they could be separated by particular habitat characteristics. In the past, several species of Craterocephalus, including C. fluviatilis and the Darling River hardyhead C. amniculus, have been regarded as C. eyresii sensu lato. The taxonomic separation of C. s. fulvus has been confirmed, but some doubt remains about the relationship of C. fluviatilis and C. amniculus. This issue needs resolution to ensure that appropriate targets are set for conservation. This study is a comparative investigation of the aforementioned species. It was designed (1) to identify the habitat characteristics that influence the distribution and abundance of C. fluviatilis and, given that salinity emerged as a key factor, (2) to explore the biological implications of salinity through a comparative study of osmoregulation in C. fluviatilis and C. s. fulvus, (3) to determine whether the osmoregulatory responses of population isolates of C. fluviatilis differ at varying salinities, and (4) to evaluate the genetic population structure of C. fluviatilis, confirm its taxonomic separation from C. amniculus and identify genetic 'management units‘ for conservation. Field sampling showed that C. fluviatilis is confined mainly to saline waters (0.4-20‰), whereas C. s. fulvus is absent from salinities >7‰. Comparisons were made of osmoregulation in these two taxa over a salinity range of 0.03-85‰, with additional reference to the small-mouth hardyhead Atherinosoma microstoma, a related estuarine species that tolerates salinities >94‰. The three species all are euryhaline, although the osmoregulatory ability of C. s. fulvus falters above about 35‰ salinity. C. fluviatilis is a better osmoregulator than A. microstoma at salinities <1‰, but both species tolerate hypersaline conditions (85‰). Osmoregulation was compared in C. fluviatilis from two isolated populations in different salinity regimes (Wyngate: 0.4-1.5‰, Disher Creek: c. 1.0-45‰) to determine whether they show related phenotypic differences. Fish from both populations remained healthy at salinities from 5-65‰. The Disher Creek population maintained a significantly lower blood osmotic concentration than the Wyngate population at salinities ≤1‰, suggesting that there is a physiological difference between them. The genetic population structure of C. fluviatilis and its taxonomic distinction from C. amniculus were investigated using complementary allozyme and mtDNA markers. This confirmed that C. fluviatilis is genetically distinct from its sister taxon, C. amniculus. It also identified several genetically-defined 'management units‘ as a framework for future conservation. Further, it revealed that C. fluviatilis in habitats downstream of Lock 1 on the Murray (274 km from the river mouth) displays a genetic signature indicating introgression with C. amniculus. Clearly, these findings have implications for the conservation of C. fluviatilis. For example, isolates can be prioritised for protection, and re-introduction programs can be modified accordingly. The findings may be applied to other freshwater fish, especially populations of closely-related species subject to salinisation or other stressors, and they may also contribute toward understanding of the factors and processes underlying rarity and fragmentation. It is clear that salinity can be a significant factor in population fragmentation, and that closelyrelated species with similar ranges may be segregated by differences in osmoregulatory ability.Thesis (Ph.D.) -- University of Adelaide, School of Earth and Environmental Sciences, 200

The Lower Lakes fish inventory: distribution and conservation of freshwater fishes of the Ramsar Convention wetland at the terminus of the Murray Darling Basin, South Australia

Scotte Wedderburn and Michael Hamme

Range-wide population genetics study informs on conservation translocations and reintroductions for the endangered Murray hardyhead (Craterocephalus fluviatilis)

1. Freshwater ecosystems worldwide harbour disproportionately high numbers of endemic species under threat from human activity, particularly accelerated habitat fragmentation. The Murray–Darling Basin in south‐eastern Australia, one of the country's largest and arguably its most vulnerable freshwater ecosystem, is inhabited by a number of small‐bodied fishes that are threatened with imminent extinction. 2. Here an extensive microsatellite dataset was used, supplemented by additional allozyme and mitochondrial DNA analyses, to assess the genetic diversity, population structure and contemporary migration patterns in the Murray hardyhead Craterocephalus fluviatilis, one of Australia's most threatened fishes. 3. Genetic diversity estimates, primarily based on 413 fish collected during the latter period of intense drought (1997–2010) from 23 sites and genotyped at 14 microsatellite loci, were higher than those previously detected for other regionally co‐occurring small‐bodied freshwater fishes. 4. Population structure analyses identified a subtle primary split between ‘lower Murray’ (lower river reaches) versus ‘upstream Murray’ (upper river reaches) and a total of nine genetically similar sub‐populations. This includes unexpected sub‐population differentiation in the Lower Lakes, a region at the terminus of the Murray–Darling Basin that most often has inter‐connected habitat. 5. Very low levels of contemporary migration were detected between most inferred populations (<2%) during the drought, with all exceptions involving moderate levels of migration from an upstream sub‐population into an adjacent downstream sub‐population. 6. This article describes how these genetic data have guided translocation and reintroduction efforts in recent years. We advocate the use of assisted gene flow as a central component of continuing efforts to rescue this species from imminent extinction.Samantha Thiele, Mark Adams, Michael Hammer, Scotte Wedderburn, Nick S. Whiterod, Peter J. Unmack, Minami Sasaki, Luciano B. Beheregara

Zooplankton response to flooding of a drought refuge and implications for the endangered fish species Craterocephalus fluviatilis cohabiting with alien Gambusia holbrooki

Disruption to a river's natural flow regime changes its ecological character, which becomes unfavourable for previously adapted biota. The zooplankton particularly are affected, and survival of larval and juvenile fish is largely determined by their availability. Alien fishes can also impact on recruitment in native fishes, sometimes through competition. In this regard, the invasive eastern Gambusia Gambusia holbrooki is linked to the decline of several fish species. It can have a substantial influence in shaping plankton communities, which implies that it competes with native fish that rely on the microfauna. The effects of river regulation and over abstraction of water in the Murray-Darling Basin, south-eastern Australia, were exacerbated by drought from 1997 to 2010. Consequently, the endangered Murray hardyhead Craterocephalus fluviatilis underwent substantial population decline and extirpations. The purpose of this study is to determine if a link exists between zooplankton response to flooding of a drought refuge and the recruitment success of C. fluviatilis in the presence of G. holbrooki. Flooding triggered sharp and substantial increases in the zooplankton and their eggs, which was the sole food of C. fluviatilis. This apparently benefitted the recruitment of C. fluviatilis, and sometimes alleviated diet overlap with G. holbrooki. Conversely, the zooplankton in a nearby non-flooded refuge was low in abundance and diversity, and all fish species were extirpated. The findings indicate that the flooding of drought refugia with relatively small volumes of water can be timed with ecological cues that would otherwise be desynchronized in highly regulated rivers, particularly during drought. © 2013 Springer Science+Business Media Dordrecht.Scotte D. Wedderburn, Karl A. Hillyard, Russell J. Shie