Temporal variability in the life history and reproductive biology of female dugongs in Torres Strait: The likely role of sea grass dieback

The extensive sea grass meadows in Torres Strait enable it to be a globally important habitat for the dugong, Dugong dugon, a marine mammal of cultural and dietary significance to Torres Strait Islanders and the basis for the substantial island-based fishery in the Torres Strait Protected Zone. Torres Strait sea grass communities are subjected to episodic diebacks which are now believed to be largely natural events. Information on dugong life history was obtained from specimens obtained from female dugongs as they were butchered for food by Indigenous hunters at two major dugong hunting communities in Torres Strait: Daru (9.04°S, 143.21°E) in 1978–1982 (a time of sea grass dieback and recovery) and Mabuiag Island (9.95°S, 142.15°E) in 1997–1999 (when sea grasses were abundant). Dugongs sampled in 1997–1999 had their first calf at younger ages (minimum of 6 cf. 10 years), and more frequently (interbirth interval based on all possible pregnancies 2.6±0.4 (S.E.) yr cf. 5.8±1.0 yr) than the dugongs sampled in 1978–1982. Pregnancy rates increased monotonically during 1978–1982, coincident with sea grass recovery. The age distribution of the female dugongs collected in 1997–1999 also suggested a low birth rate between 1973 and 1983 and/or or a high level of mortality for animals born during this period. These results add to the evidence from other regions that the life history and reproductive rate of female dugongs are adversely affected by sea grass loss, the effect of which cannot be separated from a possible density-dependent response to changes in dugong population size. Many green turtles in Torres Strait were also in poor body condition coincident with the 1970s sea grass dieback. The impacts of future sea grass diebacks need to be anticipated when management options for the traditional Torres Strait fisheries for dugongs and green turtles are evaluated.     

Sediment mobility due to currents and waves in the Torres Strait–Gulf of Papua region

The MECO hydrodynamic model (MECO Technical Report No. OMR-118/120, CSIRO Marine Research, 1998) was adapted for the Torres Strait–Gulf of Papua region at 0.05° resolution. Validation of the hydrodynamic model was carried out against observed current meter data and calculated tidal sea levels. Dispersal pathways of sediments derived from the Fly River, and from a resuspension event along the northern Great Barrier Reef were investigated using an Eulerian approach. Sediment input into Torres Strait is found to be greater during the Trade season by approximately 10%. Wave data were also obtained, and together with hydrodynamic model output, sediment mobility due to currents, waves and wave–current interactions was considered for both the Trade and Monsoon seasons. Sediment mobility in the Gulf of Papua is dominated by wave motion, whereas Torres Strait is a mixed environment of waves and tidal currents.     

Marine resources,biophysical processes,and environmental management of a tropical shelf seaway: Torres Strait,Australia–Introduction to the special issue

This special issue of Continental Shelf Research contains 20 papers giving research results produced as part of Australia's Torres Strait Co-operative Research Centre (CRC) Program, which was funded over a three-year period during 2003–2006. Marine biophysical, fisheries, socioeconomic-cultural and extension research in the Torres Strait region of northeastern Australia was carried out to meet three aims: 1) support the sustainable development of marine resources and minimize impacts of resource use in Torres Strait; 2) enhance the conservation of the marine environment and the social, cultural and economic well being of all stakeholders, particularly the Torres Strait peoples; and 3) contribute to effective policy formulation and management decision making. Subjects covered, including commercial and traditional fisheries management, impacts of anthropogenic sediment inputs on seagrass meadows and communication of science results to local communities, have broad applications to other similar environments.     

Seagrass-Watch: Engaging Torres Strait Islanders in marine habitat monitoring

Involvement in scientifically structured habitat monitoring is a relatively new concept to the peoples of Torres Strait. The approach we used was to focus on awareness, and to build the capacity of groups to participate using Seagrass-Watch as the vehicle to provide education and training in monitoring marine ecosystems. The project successfully delivered quality scientifically rigorous baseline information on the seasonality of seagrasses in the Torres Strait—a first for this region.     

The potential impact of bedform migration on seagrass communities in Torres Strait,northern Australia

Seagrass communities in the northwest of Torres Strait are known to disappear episodically over broad areas. Sediment mobility surveys were undertaken within two study areas during the monsoon and trade wind seasons, in the vicinity of Turnagain Island, to find out if the migration of bedforms could explain this disappearance. The two study areas covered sand bank and sand dune environments to compare and contrast their migration characteristics. Repeat multibeam sonar surveys were used to measure dune-crest migration during each season.     

Estimating the sustainable lobster (Panulirus ornatus) catch in Torres Strait,Australia, using an age-structured stock assessment model

The ornate rock lobster (Panulirus ornatus) is distributed across Torres Strait between Australia and Papua New Guinea and supports the most important commercial fishery to Islanders in the region. The long-term sustainability of the fishery is of importance not only to local island communities but also to the bilateral relationship between the two countries. In this paper, we developed an age-structured fishery model to assess the current stock status and estimate the long-term sustainable catch. The model was fitted to catch statistics and survey-based population estimates through a weighted maximum likelihood approach. Natural mortality was estimated at 0.732 yr⁻¹, and fishing mortality ranged from 0.19 to 0.65 yr⁻¹ from 1989 to 2005. The maximum sustainable yield was estimated at about 640 tonnes whole weight with a corresponding fishing mortality of 0.5 yr⁻¹. A stock–recruitment model was established based on outputs of the stock assessment model. To obtain the maximum sustainable yield, spawning stock should be maintained at 120×10⁴ lobsters. The lobster spawning stock has been fished down below this level twice since 1989, in 1999 and 2002, and was higher than this level in 2004 and 2005. These results suggest that the Torres Strait lobster stock is not over-fished and that the current harvest strategy is likely to maintain the stock at or above the level that would support the maximum sustainable yield if the current cap on fishing effort remains in place.     

Spatial patterns of sub-tidal seagrasses and their tissue nutrients in the Torres Strait,northern Australia: Implications for management

The distribution and nutritional profiles of sub-tidal seagrasses from the Torres Strait were surveyed and mapped across an area of 31,000 km². Benthic sediment composition, water depth, seagrass species type and nutrients were sampled at 168 points selected in a stratified representative pattern. Eleven species of seagrass were present at 56 (33.3%) of the sample points. Halophila spinulosa, Halophila ovalis, Cymodocea serrulata and Syringodium isoetifolium were the most common species and these were nutrient profiled. Sub-tidal seagrass distribution (and associated seagrass nutrient concentrations) was generally confined to northern-central and south-western regions of the survey area (<longitude 142.60), where mean water depth was relatively shallow (approximately 13 m below MSL) and where sediments were comprised primarily muddy sand to gravelly sand. Seagrass nitrogen and starch content, the most important nutrients for marine herbivores, were significantly correlated with species and with the plant component (above or below ground). For all seagrass species, the above-ground component (shoots and leaves) possessed greater nitrogen concentrations than the below-ground component (roots and rhizomes), which possessed greater starch concentrations. S. isoetifolium had the highest total nitrogen concentrations (1.40±0.05% DW). However, it also had higher fibre concentrations (38.2±0.68% DW) relative to the other four species. H. ovalis possessed the highest starch concentrations (2.76±0.12% DW) and highest digestibility (83.24±0.66% DW) as well as the lowest fibre (27.2±0.66% DW). The high relative abundance (found at 55% of the sites that had seagrass) and nutrient quality characteristics of H. ovalis make it an important source of energy to marine herbivores that forage sub-tidally in the Torres Strait. There were two regions in Torres Strait (north-central and south-western) where sub-tidal seagrass meadows were prevalent and of relatively higher nutritional value. This spatial and nutritional information can be used by local agencies to manage and to protect the ecological, economic and cultural values of the sub-tidal seagrass ecosystems and associated fisheries of the Torres Strait.     

Photosynthetic responses of subtidal seagrasses to a daily light cycle in Torres Strait: A comparative study

In this study, we examined the photosynthetic responses of five common seagrass species from a typical mixed meadow in Torres Strait at a depth of 5–7 m using pulse amplitude modulated (PAM) fluorometry. The photosynthetic response of each species was measured every 2 h throughout a single daily light cycle from dawn (6 am) to dusk (6 pm). PAM fluorometry was used to generate rapid light curves from which measures of electron transport rate (ETR_max), photosynthetic efficiency (α), saturating irradiance (E_k) and light-adapted quantum yield (ΔF/F′_m) were derived for each species. The amount of light absorbed by leaves (absorption factor) was also determined for each species. Similar diurnal patterns were recorded among species with 3–4 fold increases in maximal electron rate from dawn to midday and a maintenance of ETR_max in the afternoon that would allow an optimal use of low light by all species. Differences in photosynthetic responses to changes in the daily light regime were also evident with Syringodium isoetifolium showing the highest photosynthetic rates and saturating irradiances suggesting a competitive advantage over other species under conditions of high light. In contrast Halophila ovalis, Halophila decipiens and Halophila spinulosa were characterised by comparatively low photosynthetic rates and minimum light requirements (i.e. low E_k) typical of shade adaptation. The structural makeup of each species may explain the observed differences with large, structurally complex species such as Syringodium isoetifolium and Cymodocea serrulata showing high photosynthetic effciciencies (α) and therefore high-light-adapted traits (e.g. high ETR_max and E_k) compared with the smaller Halophila species positioned lower in the canopy. For the smaller Halophila species these shade-adapted traits are features that optimise their survival during low-light conditions. Knowledge of these characteristics and responses improves our understanding of the underlying causes of changes in seagrass biomass, growth and survival that occur when modifications in light quantity and quality arise from anthropogenic and climatic disturbances that commonly occur in Torres Strait.     

Harvest patterns and effort dynamics of indigenous and non-indigenous commercial sectors of the eastern Torres Strait reef line fishery

The reef line fishery (RLF) in eastern Torres Strait (ETS) is unique in that it has both a commercial indigenous sector and a commercial non-indigenous sector. Recently, concerns have been expressed by all stakeholders about the long-term sustainability of the fishery. These concerns have been exacerbated by the lack of detailed catch and effort information from both sectors, which has precluded any formal assessment of the fishery. In this paper, we characterise the harvest patterns and effort dynamics of the indigenous and non-indigenous commercial sectors of the ETS RLF using a range of data sources including commercial logbooks, community freezer records, voluntary logbooks and observer surveys. We demonstrate that bycatch is a significant component of the catch for both sectors and identify substantial differences in harvest patterns and effort dynamics between the sectors. Differences between sectors were observed in species composition and spatial and temporal patterns in catch, effort and catch per unit effort. These results highlight the inherent variation in catch and effort dynamics between the two commercial sectors of the ETS RLF and provide valuable information for the development of future assessments and appropriate management strategies for the fishery. The more reliable estimates of harvest patterns and effort dynamics for both sectors obtained from observer surveys will also assist in resolving issues relating to allocation of reef fish resources in Torres Strait.     

Spatial variability in community structure of Dictyoceratida sponges across Torres Strait,Australia

Spatial variability in community structure of dictyoceratid sponges (class Demospongiae; order Dictyoceratida) was examined on coral reefs in Torres Strait, an archipelago of islands and reefs between northern Queensland, Australia, and Papua New Guinea. Dictyoceratid sponge abundances and environmental factors were recorded at four locations, separated by 50–220 km. Each location was subdivided into 5–7 sites, each ?2 km apart. At each site, four 50×2 m belt transects were quantitatively surveyed, recording the dictyoceratid numbers, substrate type (rock, rubble and sand), water clarity, degree of reef slope and depth. Dictyoceratid abundance was similar among locations over large spatial scales, averaging 15.5 individuals per 100 m², but varied significantly among sites within particular locations (i.e. small scale heterogeneity). Twenty-three dictyoceratid species were recorded in Torres Strait, with approximately half (12/23) found in only one location. The canonical correspondence analysis determined that the measured environmental factors explained only 26% of the spatial variation. Cluster analysis revealed a complex dictyoceratid community structure with similarities among neighbouring sites and among sites separated by hundreds of kilometres. Conversely the abundance and composition of dictyoceratids could vary greatly between neighbouring sites 2 km apart and on the same reef complex. The results of this study suggest that spatial variability of dictyoceratid sponges in Torres Strait is influenced by a combination of environmental, biological and stochastic processes.     

Nearshore wave environments around a sandy cay on a platform reef,Torres Strait,Australia

Waves are the primary factor affecting reef-island morphology. This study examines spatial and temporal variations of wave characteristics in the nearshore around Warraber Island, a sandy cay on a platform reef in Torres Strait Australia, based on field measurements during the predominant southeasterly wind season. Water pressure was recorded simultaneously, and transformed to water surface wave spectra, at a location close to the reef edge and across the nearshore at different locations around the island. Wave environments off the reef were estimated based on wave characteristics measured at the reef-edge location and found to be primarily dominated by sea. Low and high wave-energy events were identified, based on wave energy level at the reef-edge location.     

Population biology of coral trout species in eastern Torres Strait: Implications for fishery management

Coral trout (Plectropomus spp.) are the main target species for commercial fishers in the eastern Torres Strait Reef Line Fishery (ETS RLF). The four species of coral trout known to occur in Torres Strait: Plectropomus leopardus, Plectropomus maculatus, Plectropomus areolatus and Plectropomus laevis are currently managed as a single species in Torres Strait, as there is no species-specific biological information available for the region which could be used to assess whether species differ in their response to fishing pressure. The aim of our study was to determine whether it is appropriate (biologically) to manage coral trout in the ETS RLF as a single species group or whether different management arrangements are required for some species. We used catch data and biological data from samples collected by commercial fishers to examine the distribution within Torres Strait and estimate a range of biological parameters for P. leopardus, P. maculatus and P. areolatus. Insufficient P. laevis samples were collected to reliably examine this species. Results indicated that the population biology, particularly the reproductive biology, of P. areolatus was substantially different to both P. leopardus and P. maculatus. Although it is difficult to predict the response to fishing, P. areolatus may be more vulnerable to fishing than P. leopardus and P. maculatus, due to the larger size at sex change observed for this species and the very low proportion of males protected by the current minimum size limit. Therefore, while the common management arrangements for P. leopardus and P. maculatus appear to be adequate for these species, separate management arrangements are needed for the sustainable harvest of P. areolatus populations in the ETS. Specifically, we recommend the introduction of a maximum size limit for P. areolatus, in addition to the current minimum size limit, which may allow a proportion of males some protection from fishing.     

Multiple time-scale modelling of the circulation in Torres Strait—Australia

Ocean circulation influences nearly all aspects of the marine ecosystem. This study describes the water circulation patterns on time scales from hours to years across Torres Strait and adjacent gulfs and seas, including the north of the Great Barrier Reef. The tridimensional circulation model incorporated realistic atmospheric and oceanographic forcing, including winds, waves, tides, and large-scale regional circulation taken from global model outputs. Simulations covered a hindcast period of 8 years (i.e. 01/03/1997–31/12/2004), allowing the tidal, seasonal, and interannual flow characteristics to be investigated. Results indicated that the most energetic current patterns in Torres Strait were strongly dominated by the barotropic tide and its spring-neap cycle. However, longer-term flow through the strait was mainly controlled by prevailing winds. A dominant westward drift developed in summer over the southeasterly trade winds season, which then weakened and reversed in winter over the northwesterly monsoon winds season. The seasonal flow through Torres Strait was strongly connected to the circulation in the north of the Great Barrier Reef, but showed little connectivity with the coastal circulation in the Gulf of Papua. Interannual variability in Torres Strait was highest during the monsoon period, reflecting variability in wind forcing including the timing of the monsoon. The characteristics of the circulation were also discussed in relation to fine sediment transport. Turbidity level in Torres Strait is expected to peak at the end of the monsoon, while it is likely to be at a low at the end of the trade season, eventually leading to a critically low bottom light level which constitutes a severe risk of seagrass dieback.     

Form drag is a major component of bed shear stress associated with tidal flow in the vicinity of an isolated sand bank,Torres Strait,northern Australia

Tidal current and elevation data were collected from five oceanographic moorings during October 2004 in Torres Strait, northern Australia, to assess the effects of large bedforms (i.e., sand banks) on the drag coefficient (C_D) used for estimating bed shear stress in complex shallow shelf environments. Ten minute averages of tidal current speed and elevation data were collected for 18 days at an on-bank site (<7 m water depth) and an off-bank site (<10 m). These data were compared to data collected simultaneously from two shelf locations (<11 m) occupied to measure regional tidal behaviour. Overall C_D estimates at the on- and off-bank sites attained 7.0±0.1×10⁻³ and 6.6±0.1×10⁻³, respectively. On-bank C_D estimates also differed between the predominant east–west tidal streams, with easterly directed flows experiencing C_D=7.8±0.18×10⁻³ and westerly directed flows C_D=6.4±0.12×10⁻³. Statistically significant differences between the off-bank and on-bank sites are attributed to the large form drag exerted by the sand banks on the regional tidal currents, and statistically significant differences between the westward and eastward flows is ascribed to bedform asymmetry. Form drag from the large bedforms in Torres Strait comprises up to 65% of the total drag coefficient. When constructing sediment transport models, different C_D estimates must therefore be applied to shelf regions containing steep bedforms compared to regions that do not. Our results extend the limited inventory of seabed drag coefficients for shallow shelf environments, and can be used to improve existing regional seabed mobilisation models, which have direct application to environmental management in Torres Strait.     

Numerical modelling of the suspended sediment transport in Torres Strait

One-dimensional vertical and three-dimensional fine-resolution numerical models of sediment transport have been developed and applied to the Torres Strait region of northern Australia. The one-dimensional model, driven by measured waves and currents, was calibrated against measured suspended sediment concentrations using a sequential data assimilation algorithm. The algorithm produced a good match between model and data, but this was achieved only by allowing some temporal variability in parameter values, suggesting that there were underlying uncertainties in the model structure and forcing data. Implications of the assimilation results to the accuracy of the numerical modelling are discussed and the need for observational programmes having an extensive spatial and temporal coverage is highlighted. The three-dimensional sediment model, driven by modelled waves and currents, simulates sediment transport over the shelf during the monsoon and trade-wind seasons covering 1997–2000. The model predicts strong seasonal variability of the sediment transport on the shelf attributed to seasonally varying hydrodynamics, and illustrates significant inter-annual variability of the sediment fluxes driven by extreme events. The developed model provides a platform for testing scientific hypothesis. With additional calibration, including uncertainty analysis, it can also be used in a management context.     

Evaluating ecosystem-based management options: Effects of trawling in Torres Strait,Australia

A suite of management options for a prawn trawl fishery in Torres Strait, Australia was assessed for impacts on the benthic fauna using a dynamic management strategy evaluation approach. The specification of the management options was gained through consultation with stakeholders. Data for the model was drawn from several sources: the fleet data from fishery logbooks and satellite vessel monitoring systems, benthic depletion rates from trawl-down experiments, benthic recovery rates from post-experiment recovery monitoring studies, and benthic distribution from large-scale benthic surveys. Although there were large uncertainties in the resulting indicators, robust measures relevant to management were obtained by taking ratios relative to the status quo. The management control with the biggest effect was total effort; reducing trawl effort always led to increases in benthic faunal density of up to 10%. Spatial closures had a smaller benefit of up to 2%. The effect of closing a set of buffer zones around reefs to trawling was indistinguishable from the status quo option. Closing a larger area, however, was largely beneficial especially for sea cucumbers. When the spatial distributions of fauna prior to fishing were accounted for, fauna with distributions positively correlated with effort improved relative to those negatively correlated. The reduction in prawn catch under effort reduction scenarios could be ameliorated by introducing temporal closures over the full-moon period.     

Composition and distribution of seabed and suspended sediments in north and central Torres Strait,Australia

Widespread seagrass dieback in central Torres Strait, Australia has been anecdotally linked to the delivery of vast quantities of terrigenous sediments from New Guinea. The composition and distribution, and sedimentological and geochemical properties, of seabed and suspended sediments in north and central Torres Strait have been determined to investigate this issue. In northern Torres Strait, next to Saibai Island, seabed sediments comprise poorly sorted, muddy, mixed calcareous–siliciclastic sand. Seabed sediments in this region are dominated by aluminosilicate (terrigenous) phases. In central Torres Strait, next to Turnagain Island, seabed and suspended sediments comprise moderately sorted coarse to medium carbonate sand. Seabed sediments in this region are dominated by carbonate and magnesium (marine) phases. Mean Cu/Al ratios for seabed sediments next to Saibai Island are 0.01, and are similar to those found in New Guinea south coastal sediments by previous workers. Mean Cu/Al ratios for seabed sediments next to Turnagain Island are 0.02, indicating an enrichment of Cu in central Torres Strait. This enrichment comes from an exogenous biogenic source, principally from foraminifers and molluscs. We could not uniquely trace terrigenous sediments from New Guinea to Turnagain Island in central Torres Strait. If sediments are a factor in the widespread seagrass dieback in central Torres Strait, then our data suggest these are marine-derived sediments sourced from resuspension and advection from the immediate shelf areas and not terrigenous sediments dispersed from New Guinea rivers. This finding is consistent with outputs from recently developed regional hydrodynamic and sediment transport models.     

1994年台湾海峡7.3级地震发震断层的地震学证据

通过地震学观测资料的分析和研究,并综合其他作者的研究成果,论证了台湾海峡１９９４年７．３级地震发震断层的主破裂面走向应为北西向,地震的发生是由分布于该区域的巴士系构造活动所致。     

台湾海峡架桥铺路的层析技术问题的初步分析

关于在台湾海峡架桥和修筑隧道的问题从1987年至2002年在福省召开过三次"台湾海峡两岸地质地震和架桥与修筑隧道的研讨会",以后陆续出版了相应的论文和论文集,提出了"台湾海峡架桥和修筑隧道的可行性研究",和"台湾海峡隧道工程设计总建议"等[1-4],论述了修建该工程的可行性等问题.提出平潭岛可作为海峡北线的端点,探讨了如何具体建桥和相关工程的问题.近年来地球层析技术的发展,可对台湾海峡隧道工程的工程地质问题,海港的开发和沿海陆地的充分利用等,做出探测台湾海峡地下地层三维成像细结构和地球物理场的空间分布,作为台湾海峡架桥铺路工程基本设计的必要依据.为了让大中小轮船在台湾海峡顺利通航,利用现代的科技研讨台湾海峡两岸连接的最佳方案.为了架成大型的高桥和修建堤坝式的地基,必须用地震层析和应力波等方法探测桥梁和堤坝地基的结构和地层的坚实程度,以便决定桥墩深入海底的深度,和设计坚固而又经济的桥墩与堤坝的基础.参考开采海底石油平台施工和荷兰填海造田的工程经验,以双边海堤与大型高桥及活动的船闸相结合,从平潭岛东至台湾新竹西海岸,在124千米距离上架设4-6段桥梁和5-7段堤坝型的路基,既让大中小船舶照常航行,海洋生物照常游动,又可在其上通车,以缩短两岸距离,适应中国大陆和台湾经济快速发展的需要,争取和平沟通的愿望.由于架桥铺路的工程费用较修筑隧道的投资低得多,在福建近海地带局部填海造地,充分利用海洋资源和建筑深水码头,可得到较大的经济补偿,在现阶段较易实施.本文初步分析了有关的地球层析和工业CT技术的作用问题.