Steeg D. Hoeksema

Massive mortalities of the black bream Acanthopagrus butcheri (Sparidae) in two normally-closed estuaries, following extreme increases in salinity

Salinities in some normally-closed estuaries in the central south coast of Western Australia are now frequently becoming highly elevated. This is due to: (1) high evaporation rates in water volumes that, by summer, are already low as a result of atypically dry winters; and (2) increased salt run-off following vegetation clearing in the catchments. A few black bream (Acanthopagrus butcheri) died in the basin and lower reaches of the main tributary of Culham inlet when salinities reached [similar] 77 and 67, respectively, and an estimated 1.3 million black bream died in the tributary during the next two months when salinities continued to increase. All black bream in the basin and the lower reaches of the tributary of another estuary were apparently killed when salinities reached [similar] 83 1385. It is proposed that A. butcheri becomes stressed at salinities of 60 and typically die before they reach [similar] 85. In both estuaries, a rock bar in the tributary prevented black bream from moving to refugia in upstream areas where salinities were lower.

Effects of a harmful algal bloom on the community ecology, movements and spatial distributions of fishes in a microtidal estuary

Harmful algal blooms can adversely affect fish communities, though their impacts are highly context-dependent and typically differ between fish species. Various approaches, comprising univariate and multivariate analyses and multimetric Fish Community Indices (FCI), were employed to characterise the perceived impacts of a Karlodinium veneficum bloom on the fish communities and ecological condition of the Swan Canning Estuary, Western Australia. The combined evidence suggests that a large proportion of the more mobile fish species in the offshore waters of the bloom-affected area relocated to other regions during the bloom. This was indicated by marked declines in mean species richness, catch rates and FCI scores in the bloom region but concomitant increases in these characteristics in more distal regions, and by pronounced and atypical shifts in the pattern of inter-regional similarities in fish community composition during the bloom. The lack of any significant changes among the nearshore fish communities revealed that bloom impacts were less severe there than in deeper, offshore waters. Nearshore habitats, which generally are in better ecological condition than adjacent offshore waters in this system, may provide refuges for fish during algal blooms and other perturbations, mirroring similar observations of fish avoidance responses to such stressors in estuaries worldwide.

Complexity of spatial and temporal trends in metal concentrations in macroinvertebrate biomonitor species in the Severn Estuary and Bristol Channel

Cadmium, chromium, copper, nickel, lead and zinc concentrations in the bivalve mollusc Macoma balthica and the polychaete annelids Hediste diversicolor and Arenicola marina were measured, during winter and summer, at sites throughout the Severn Estuary and Bristol Channel. The mean concentration of each metal in A. marina was greater in the lower Severn Estuary than in the far less contaminated outer Bristol Channel and the concentration of a given metal, e.g. Cr, in a species occasionally peaked at a site, reflecting local metal contamination. The concentrations of each metal in each of these biomonitor species almost invariably differed significantly among sites and often seasons and there were sometimes interactions between site and season. This indicates that the various factors that determine the concentration of a metal in a species operate in a complex manner and that their individual effects can vary among sites and/or seasons. The rank order of each metal concentration in each species at a site within the estuary frequently did not match the sequence for the concentration of that metal measured in the sediment at that site at the same time. This lack of correspondence is likely to be due, at least in part, to one or both of the following: (1) variations in the bioavailability of certain metals among sites due to differences in such features as the metal-binding properties of the sediments; (2) the effects of the constant transport and redistribution of the sediments and thus also of their associated trace metals by the very strong tidal action that characterizes the Severn Estuary. This would mean that single time measurements do not accurately reflect the overall trace metal environment to which the biomonitor organism had been exposed in the weeks/months prior to sampling. Marked differences in the concentrations of certain metals, e.g. Cu and Zn, in co-occurring biomarker species could frequently be related to differences between the ability of these species to regulate certain metals. Non-metric multi-dimensional scaling ordination and associated tests emphasize that the relationships between the concentrations of the various metals differed markedly among species and between sites and seasons in individual species and elucidated which metals contributed most to those differences. If the proposed scheme for harnessing tidal power in the Severn Estuary proceeds, the data in this paper provide a baseline for assessing the impact of such major changes on the bioavailability of trace metals in this estuary. This information will also be invaluable for predicting the changes likely to occur in other estuaries that become subjected to major structural changes.

Implications of the divergent use of a suite of estuaries by two exploited marine fish species

Biological characteristics of the marine species King George whiting Sillaginodes punctatus and Australian herring Arripis georgianus in three seasonally open estuaries (Broke, Irwin and Wilson Inlets), one permanently open estuary (Oyster Harbour) and one normally closed estuary (Wellstead Estuary) on the south coast of Western Australia have been determined and compared. Sillaginodes punctatus enters the seasonally and permanently open estuaries early in life and reaches total lengths (L(T)) >280 mm at which it can be legally retained and thus contributes to commercial and recreational fisheries in these systems. This sillaginid almost invariably emigrates from these estuaries before reaching its typical size at maturity (L(T50)) and does not return after spawning in marine waters. In contrast, virtually all female A. georgianus (≥ 98%) in the three seasonally open estuaries and the majority in the normally closed (89·5%) and permanently open estuaries (83%) exceeded the L(T50) of this species at maturity, reflecting the fact that the nursery areas of this species are predominantly located much further to the east. Although adult females of A. georgianus in seasonally open and normally closed estuaries had developed mature ovaries by autumn, at which time they were prevented from migrating to the sea by closure of the estuary mouths, this species did not spawn in those estuaries. The oocytes in their ovaries were undergoing extensive atresia, a process that had been incipient prior to oocyte maturation. As the adult females of A. georgianus in the permanently open Oyster Harbour at this time all possessed resting gonads, i.e. their oocytes were all previtellogenic, the adults that were present in that estuary earlier and were destined to spawn in autumn must have emigrated from that permanently open estuary to their marine spawning areas prior to the onset of gonadal recrudescence. The body masses at length of A. georgianus, which were almost invariably higher in summer and autumn than in winter and spring, were greater in the very productive environments of the seasonally open and normally closed estuaries than in the less productive and essentially marine environment of Oyster Harbour and coastal marine waters. In general, the same pattern of differences between water bodies was exhibited by the growth of A. georgianus and by the more restricted data for body mass at L(T) and growth of S. punctatus. Despite an increase in anthropogenic activities in Wilson Inlet over the last two decades, the growth of both species was very similar to that recorded 20 years earlier. The fisheries implications of the results for the two species are discussed.