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From a conservation standpoint, species that are managed without consideration of their population sizes and connectivity have the potential to be over‐exploited and/or incur population decline. The burrowing shrimp, Trypaea australiensis, is an important ecosystem engineer and fishery resource caught in large numbers for which population information is unknown for properly managing the species. Here, we determined the level of population structure of T. australiensis across three locations along the East Coast of New South Wales, Australia, using genome‐wide single nucleotide polymorphisms (SNPs) obtained through double digest Restriction‐site Associated DNA‐sequencing (ddRAD‐seq). Analysis of population structure, including pairwise Fst (−0.003 to −0.001), STRUCTURE (K = 2) and Discriminant Analysis of Principal Components (DAPC) showed no evidence of structure among locations. Our findings provide crucial preliminary population genetic data for a key cryptic species, that also suggests gene flow among sampling locations enables the management of fisheries throughout the study area as a single unit. [ABSTRACT FROM AUTHOR]

Establishing the activity, movement and habitat‐use patterns of endangered species is fundamental in designing effective conservation plans. In Australia, the critically endangered freshwater crayfish, Euastacus dharawalus, is facing considerable risk of extinction due to numerous stressors, including the proliferation of the invasive Cherax destructor throughout their range. However, little is known about the fine‐scale spatial ecology of E. dharawalus and if this overlaps with C. destructor. Here, we aimed to establish the activity, movement and habitat‐use patterns of E. dharawalus as well as conduct a preliminary assessment into the overlap of their spatial ecology with that of C. destructor. In 2020, 15 E. dharawalus and six C. destructor were radio tracked at a site along Wildes Meadow Creek in the Southern Highlands region of New South Wales, Australia. Additionally, in 2021, 12 more E. dharawalus were tracked at this site over the same period. Euastacus dharawalus exhibited high site fidelity, small home ranges and diurnal activity patterns. Both E. dharawalus and C. destructor indicated significant preference for bank habitat, indicating potential for individual competitive interactions. We also report a significant reduction in the movement and activity of E. dharawalus from 2020 to 2021, possibly in response to population control of C. destructor or environmental variation across the 2 years. Our results indicate that the preservation or restoration of bank habitat will be critical to the conservation of E. dharawalus. Further, due to the extent of habitat‐use overlap between E. dharawalus and C. destructor, we conclude that C. destructor represents a substantial competitive threat to E. dharawalus, and therefore, C. destructor population control efforts should continue within the range of E. dharawalus. [ABSTRACT FROM AUTHOR]

Simple Summary: Aggressive invasive species can outcompete native species in contests over resources, which can lead to the exclusion of a native species by the invader. Invasive freshwater crayfish are often more aggressive than their native counterparts, however contests between invasive and native crayfish are typically investigated under laboratory conditions and are rarely examined in a natural setting. We used a baited underwater camera in a stream located in New South Wales, Australia to examine behavioural interactions between an invasive crayfish and a critically endangered native crayfish to determine which species was dominant. We found the native species dominant over the invader when larger, however when the species were size-matched the dominance of the native was lost and neither species exhibited a significant pattern of dominance. This outcome indicates the invasive crayfish represents a threat to the native since it may be able to outcompete the native over resources. Further, this outcome contrasts to previous laboratory findings, highlighting the importance of field observations in establishing the extent of impact an invader may be having on a native species. Competitive behavioural interactions between invasive and native freshwater crayfish are recognised as a key underlying mechanism behind the displacement of natives by invaders. However, in situ investigations into behavioural interactions between invasive and native crayfish are scarce. In Australian freshwater systems, the invasive Cherax destructor has spread into the ranges of many native Euastacus species, including the critically endangered Euastacus dharawalus. Staged contests between the two species in a laboratory setting found E. dharawalus to be the dominant competitor, however, this has yet to be corroborated in situ. Here, we used baited remote underwater video (BRUV) to examine in situ intra- and inter-specific behavioural interactions between E. dharawalus and C. destructor. We sought to evaluate patterns of dominance and differential contest dynamics between the species to provide indications of competition between the two species. We found E. dharawalus to be dominant over C. destructor based on pooled interspecific interaction data and size-grouped interactions where C. destructor was the smaller opponent. Alarmingly, however, when C. destructor was within a 10% size difference the dominance of E. dharawalus was lost, contrasting with the outcomes of the laboratory-staged study. In addition, we report that small C. destructor initiated significantly more contests than larger conspecifics and larger E. dharawalus, a pattern that was not observed in smaller E. dharawalus. Further, intraspecific interactions between C. destructor were significantly longer in duration than intraspecific interactions between E. dharawalus, indicating a willingness to continue fighting. Concerningly, these outcomes point towards inherent and greater aggressiveness in C. destructor relative to E. dharawalus and that only larger E. dharawalus hold a competitive advantage over C. destructor. Therefore, we conclude that C. destructor represents a substantial threat to E. dharawalus through competitive behavioural interactions. Further, due to the disparity between our findings and those produced from laboratory-staged contests, we recommend the use of in situ studies when determining the behavioural impacts of invasive crayfish on natives. [ABSTRACT FROM AUTHOR]