Surveys of intertidal and subtidal biota of the Derwent Estuary - 2010

Summary This study examines patterns of diversity and abundance of fish, invertebrates and algae on intertidal to subtidal rocky reefs and their adjacent sediments within the Derwent Estuary. It is based on two quantitative survey methods that formed the basis of a detailed survey of the estuary, primarily undertaken between February-April 2010. The survey area ranged from Claremont Point to Tinderbox and encompassed up to 24 sites along this estuarine gradient, surveying the typical range of depths that rocky reefs and adjacent sediments occur in along that gradient. The primary aims of the study were: (1) to provide a substantial quantitative baseline of current biological assemblages in these habitats as a snapshot by which future change could be measured, spatial patterns could be recognised, and management issues identified; (2) to undertake a detailed search for the threatened seastar Marginaster littoralis, and other rare seastar species that could be resident in the Derwent estuary to better understand their conservation status; (3) to quantitatively describe the current distribution of introduced species within the estuary and better understand the threats they might provide to native species. The two survey methods utilised included (1) a standard belt transect methodology widely used by TAFI for biodiversity surveys in temperate Australia; (2) a 20 minute timed swim (and intertidal search) with multiple replicates at multiple depth bands, recording fishes, readily identifiable fishes, invertebrates and brown and green macroalgae. For method 1, fifty four species of fish, thirty five species of mobile macro-invertebrates, twenty one species of sessile invertebrates and seventy two species of brown, green and red algae were recorded along standard subtidal rocky reef surveys. Fish, invertebrate and algal diversity generally increased from northern to southern-most sites within the estuary, in a pattern typical of estuarine diversity gradients. Fish species such as Trachinops caudimaculatus (Hulafish), Latridopsis forsteri (Bastard trumpeter), Dinolestes lewini (Long-fin pike), Notolabrus tetricus (Blue-throat wrasse) and Acanthaluteres vittiger (Toothbrush leatherjacket) were the most abundant. The abundance of these species increased towards the southern sites of the estuary with the exception of Hulafish which were more abundant towards the northern parts of the estuary. The most abundant invertebrates were the echinoderms Patiriella regularis (introduced regular seastar), Heliocidaris erythrogramma (Purple urchin), Meridiastra calcar (Eight-armed seastar), Amblypneustes ovum (Short-spined urchin) and the Triton shell Cabestana spengleri. Introduced species of echinoderms and crustaceans were more abundant in the northern sites of the estuary whereas native invertebrates were more abundant in the southern part of the estuary. Overall, introduced species numerically dominated the mobile macro-invertebrates within the estuary from Claremont Point to Bellerive Bluff. Dominant algae included species of encrusting Peyssonnelia (red algae) and the brown kelp Ecklonia radiata, Lessonia corrugata and Carpoglossum confluens, but the distribution of most foliose algae was restricted to sites seaward of Rosny Point, with filamentous red algae in particular dominating the northern section of the estuary. Bellerive Bluff to Rosny Point marked the most significant transition in algal assemblages and corresponded with a change from silty tube-worm matting dominated reef systems to reefs with an increasing cover of encrusting corraline algae and encrusting sponges. Notable features of the algal surveys included the abundance of the red algae Aodes nitidissima in the Rosny Point region where other foliose algae are rare, and the presence of the North Pacific kelp Undaria pinnatifida between the Grange and Alum Cliffs. Undaria is an introduced species of brown algae that is seasonally abundant with peaks in early summer. We suspect that Aodes is also an introduced species given its restricted range in this survey and the distribution of past specimens collected in Tasmania that are also restricted to the estuary. For method 2, seventy four species of fish, one hundred and forty seven species of macro-invertebrates and forty six species of brown and green algae were recorded throughout the timed intertidal and subtidal surveys. As with results from standard surveys, diversity of fish, invertebrate and algae increased from northern to southern-most sites within the estuary. Fish species such as T. caudimaculatus, N. tetricus, Fosterygion varium (introduced Many-rayed threefin), Grahamina gymnota (introduced Estuarine threefin) and Neoodax balteatus (Little rock whiting) had the largest rank abundances. Spatial distribution patterns were clearest for the two introduced threefin species which were most strongly associated with the mid-upper estuarine sites. The upstream assemblage was characterised by high abundance of gobies, blennies and threefins. A similar, but reversed pattern was evident with many marine species, particularly the numerically abundant N. tetricus, a species essentially absent upstream from Rosny Point. The most abundant invertebrates were Mytilus galloprovincialis (Blue mussel), Petrolisthes elongatus (introduced New Zealand porcelain crab), the introduced echinoderms Asterias amurensis (Northern Pacific seastar), P. regularis and the introduced fan worm Myxicola infundibulum. Some clear patterns of species distribution occurred along the estuary, including an increased abundance of P. regularis at sites upstream from Bellerive Bluff and high abundance of M. calcar at sites seaward of that location. For algae, the most abundant brown and green taxa were Ulva spp., E. radiata, Codium fragile, L. corrugata and Dictyopteris muelleri. These algae generally increased towards the southern part of the estuary with the exception of Ulva spp., which was consistently present in most sites. Most foliose algae were not present at sites upstream of Bellerive Bluff. Ecklonia radiata, present at Bellerive Bluff in depths to below 5 m, was not present at Rosny Point at any depth, despite the close proximity, indicating a very strong environmental gradient between these sites that was also reflected in the distribution of many other species, and the extent of cover of silt laden tube-worm matting that was prevalent across depths at the more upstream sites. Overall, this survey has successfully provided a comprehensive quantitative snapshot of the current distribution of much of the epifaunal biodiversity associated with reef systems in the Derwent estuary. It revealed many strong spatial patterns in species distributions and should provide a robust baseline from which to measure and assess future change. The clear break in biological assemblages between Rosny Point and Bellerive Bluff deserves further investigation to assess causes underlying this, including the extent that the tube-worm matting dominating the reef at Rosny Point and upstream sites plays. This matting may represent an alternative stable state to the algal dominated reef historically described from Rosny Point, the restoration of which may be seen to be an ultimate management goal with respect to amelioration of human impacts within the estuary. Despite thorough searching for the presence of the threatened Derwent river seastar Marginaster littoralis, none were found. This is despite extensive searching both intertidally and subtidally within the core habitat of this species. The co-occurrence and super-abundance of P. elongatus and P. regularis in these habitats suggests that if any individuals of M. littoralis are left they would be subject to severe competition and predation by these species. Consideration of the difficulty of exploiting an intertidal habitat within the core range of M. littoralis suggests that winter low salinity coupled with summer desiccation during spring tides and high temperatures would make an obligate mid-estuarine intertidal niche’ impossible. If the species is valid and continues to persist, in addition to the intertidal zone it is described from, it must also occupy subtidal habitats below salinity lows during winter, or additional intertidal habitats away from the influence of physical extremes. Our searches included many of these habitats but failed to detect any specimens. As P. regularis displayed great morphological variability within the central area of occupancy of M. littoralis, some specimens of which displayed similar features (such as an off-white marginal fringe), we suggest a revision of the taxonomy of this species be undertaken with regard to the variability of M. calcar characters, and a molecular genetic comparison to be made between these species once molecular techniques evolve to cope with the formalin preserved type specimens. Despite the rare species focus of our surveys, and the possibility of encountering several species of rare seastars known from adjacent coastal areas, no such species were found. If they are present, they would presumably be in low numbers and in isolated populations. Clearly, such species are difficult to detect, and require greater intensity of search effort to delineate their range than was possible in this survey. This was illustrated from our soft sediment searches undertaken adjacent to the reef margins, as these searches detected only one Spotted handfish Brachionichthys hirsutus despite many of our survey sites falling within the range and core habitat of this species. Introduced species numerically dominate the mobile invertebrate fauna at most of the sites we surveyed upstream from Bellerive Bluff. At many sites they contributed well over 80% of all invertebrate individuals counted along transects. In addition to the species described previously, they include the Piecrust crab (Metacarcinus novaezelandiae) and the green shore crab (Carcinus maenus), a species recently established in the estuary. A similar pattern is evident with sedentary invertebrates, with the Pacific oyster (Crassostrea gigas) distributed throughout the estuary and being particularly abundant in the upper sites examined. The fanworm Myxicola infundulibulum was abundant throughout soft silty sediments. Two species of introduced threefin Fosterygion varium and Grahamina gymnota were abundant in mid estuarine sites (Bedlam Walls to Bellerive Bluff) and several introduced species of algae were found at the more seaward sites. These were the large brown algae Undaria pinnatifida and the recently arrived red algae Grateoloupia turuturu. A further species of red algae, Aodes nitidissima is also suspected to be introduced, due to its restricted range within mid-estuarine sites. Many of these introduced species are in pest abundances, and the super-abundance of P. elongatus and P. regularis at many sites clearly threaten a range of native species at these sites and suggests that they may pose a biodiversity threat more widely, particularly to rare species such as Parvulastra vivipara and Smilasterias tasmaniae, seastars that occupy similar sheltered rocky shoreline habitats in nearby environments. Further investigation is urgently needed to quantify the threats that abundant introduced taxa pose