Large Marine Embayment Research Articles

Environmental extremes drive population structure at the northern range limit of Atlantic salmon in North America.

Conservation of exploited species requires an understanding of both genetic diversity and the dominant structuring forces, particularly near range limits, where climatic variation can drive rapid expansions or contractions of geographic range. Here, we examine population structure and landscape associations in Atlantic salmon (Salmo salar) across a heterogeneous landscape near the northern range limit in Labrador, Canada. Analysis of two amplicon-based data sets containing 101 microsatellites and 376 single nucleotide polymorphisms (SNPs) from 35 locations revealed clear differentiation between populations spawning in rivers flowing into a large marine embayment (Lake Melville) compared to coastal populations. The mechanisms influencing the differentiation of embayment populations were investigated using both multivariate and machine-learning landscape genetic approaches. We identified temperature as the strongest correlate with genetic structure, particularly warm temperature extremes and wider annual temperature ranges. The genomic basis of this divergence was further explored using a subset of locations (n=17) and a 220K SNP array. SNPs associated with spatial structuring and temperature mapped to a diverse set of genes and molecular pathways, including regulation of gene expression, immune response, and cell development and differentiation. The results spanning molecular marker types and both novel and established methods clearly show climate-associated, fine-scale population structure across an environmental gradient in Atlantic salmon near its range limit in North America, highlighting valuable approaches for predicting population responses to climate change and managing species sustainability.

Molecular systematics and historical biogeography of the genus Gerrhonotus (Squamata: Anguidae)

AbstractAimMultiple geological and climatic events have created geographical or ecological barriers associated with speciation events, playing a role in biological diversification in Mexico. Here, we evaluate the influence of Neogene geological events and of Pleistocene climate change in the diversification of the genus Gerrhonotus using molecular dating and ancestral area reconstruction.LocationMexico and south‐central United States.MethodsA multilocus sequence dataset was generated for 86 individuals of Gerrhonotus from most Mexican biogeographical provinces and belonging to five of the seven currently recognized species, as well as two putative undescribed species. Phylogeographical structure was explored using Poisson‐Tree‐Processes molecular species delimitation. Divergence events were estimated based on the fossil record using a relaxed uncorrelated lognormal clock. Ancestral areas were estimated at divergence events across the tree using a probabilistic Bayesian approach.ResultsExtensive geographical structure was evident within three well‐supported clades. These clades probably diverged from each other in the early to mid‐Miocene, and their divergence was followed by six divergences in the late Miocene and eight divergences in the Pliocene. The ancestral origin of Gerrhonotus with keeled dorsal scales (keeled‐scale Gerrhonotus) was reconstructed to be across the Pacific Coast Province. Our phylogenetic analyses did not support the monophyly of Gerrhonotus.Main conclusionsMiocene and Pliocene geomorphology, perhaps in conjunction with climate change, appears to have induced allopatric divergence on a relatively small spatial scale in this genus. The late Miocene–Pliocene reduction in the highlands along the Tehuantepec fault probably created a large marine embayment that led to an early divergence in a clade of Gerrhonotus. Our analysis suggests uplifting of the Trans‐Mexican Volcanic Belt during this same time period resulted in additional diversification. This was followed by more recent, independent colonization events in the Pliocene from the Mexican Plateau to the Sierra Madre Oriental, Sierra Madre Occidental, Tamaulipas and Edwards Plateau provinces. A genus Gerrhonotus with the keeled‐scale species in addition to Coloptychon rhombifer (= G. rhombifer) is strongly supported. Inclusion of the smooth dorsal‐scale species in the genus is uncertain and maintained only tentatively.

Stepped Holocene sea-level rise and its influence on sedimentation in a large marine embayment: Maputo Bay, Mozambique

The sedimentary record in large marine embayments holds important information on Holocene environmental and sea-level changes. This paper investigates the evolution of a large subtropical marine embayment, Maputo Bay, in southern Mozambique. Using a combination of sediment cores, stable isotope data, seismic reflection profiling and 14C dating, we show complex back-barrier changes during the Holocene and link these to stepped rises in sea level. Our data reveal a sea-level “slowstand” followed by an abrupt rise that we correlate with the 8.2 ka global rise in sea level. Sedimentological, isotope and seismic evidence point to a subsequent slowstand interspersed by two punctuated rises in sea level that formed clear tidal ravinement surfaces. Sedimentary changes in the embayment are primarily forced by sea-level change, whereby tidal ravinement surfaces sandwich normal regressive packages formed via marginal progradation and shallowing of the system during “slow-” or still-stands. Large marine embayment systems hold great potential in unravelling relative sea-level change. Axiomatically, this reduces their utility as archives of climate-change.

Submarine sediment and landform record of a palaeo‐ice stream within the British−Irish Ice Sheet

This paper examines marine geophysical and geological data, and new multibeam bathymetry data to describe the Pleistocene sediment and landform record of a large ice‐stream system that drained ∼3% of the entire British−Irish Ice Sheet at its maximum extent. Starting on the outer continental shelf NW of Scotland we describe: the ice‐stream terminus environment and depocentre on the outer shelf and continental slope; sediment architecture and subglacial landforms on the mid‐shelf and in a large marine embayment (the Minch); moraines and grounding line features on the inner shelf and in the fjordic zone. We identify new soft‐bed (sediment) and hard‐bed (bedrock) subglacial landform assemblages in the central and inner parts of the Minch that confirm the spatial distribution, coherence and trajectory of a grounded fast‐flowing ice‐sheet corridor. These include strongly streamlined bedrock forms and megagrooves indicating a high degree of ice‐bed coupling in a zone of flow convergence associated with ice‐stream onset; and a downstream bedform evolution (short drumlins to km‐scale glacial lineations) suggesting an ice‐flow velocity transition associated with a bed substrate and roughness change in the ice‐stream trunk. Chronology is still lacking for the timing of ice‐stream demise; however, the seismic stratigraphy, absence of moraines or grounding‐line features, and presence of well‐preserved subglacial bedforms and iceberg scours, combined with the landward deepening bathymetry, all suggest that frontal retreat in the Minch was probably rapid, via widespread calving, before stabilization in the nearshore zone. Large moraine complexes recording a coherent, apparently long‐lived, ice‐sheet margin position only 5–15 km offshore strongly support this model. Reconstructed ice‐discharge values for the Minch ice stream (12–20 Gt a−1) are comparable to high mass‐flux ice streams today, underlining it as an excellent palaeo‐analogue for recent rapid change at the margins of the Greenland and West Antarctic Ice Sheets.

Integrated bio- and lithofacies analysis of coarse-grained, tide-dominated deltaic environments across the Cretaceous/Paleogene boundary in Patagonia, Argentina

Four facies associations representing, from sea to land, namely (1) tidal bars in distal delta front, (2) proximal tidal bar complexes, (3) tidal channel and tidal flat deposits, and (4) multi-episodic, tide-influenced channels, resulted from the bio- and lithofacies analysis of the siliciclastic Cretaceous/Paleogene Lefipán Formation (Patagonia, Argentina). Lithofacies analysis indicates the existence of coarse-grained, tide-dominated deltas fed with sediments of braided rivers, a rare type of delta that was developed at the western margin of a large marine embayment during the Cretaceous/Paleogene (K/Pg) boundary interval. Differences in taxonomic composition and ecological structure of the biofacies revealed variations in environmental factors, with salinity exerting the principal influence on biofacies. Salinity varied from oligohaline and mesohaline in the deltaic, high-stress environments, as typified by monospecific bivalve assemblages, to diverse and trophically more complex associations, indicating more normal marine and stable conditions. Thus, the biofacies provided an independent tool to check the results of the lithofacies analysis in complex transitional environments. A comparison of Maastrichtian and Danian faunas suggests that the K/Pg boundary event disrupted the stable Maastrichtian communities via the extinction of dominating species, leading to less diverse but more varied Danian assemblages. A shift in trophic structure to increased percentages of deposit-feeders in the Danian was not a direct consequence of impact-related environmental perturbations at the boundary in the studied onshore environment. This contrasts with previous studies of siliciclastic offshore settings, where such a shift in trophic composition was interpreted as the consequence of a productivity crisis at the boundary.

The northern sector of the last British Ice Sheet: Maximum extent and demise

Strongly divided opinion has led to competing, apparently contradictory, views on the timing, extent, flow configuration and decay mechanism of the last British Ice Sheet. We review the existing literature and reconcile some of these differences using remarkable new sea-bed imagery. This bathymetric data provides unprecedented empirical evidence of confluence and subsequent separation of the last British and Fennoscandian Ice Sheets. Critically, it also allows a viable pattern of ice-sheet disintegration to be proposed for the first time. Covering the continental shelf around the northern United Kingdom, extensive echosounder data reveals striking geomorphic evidence – in the form of tunnel valleys and moraines – relating to the former British and Fennoscandian Ice Sheets. The pattern of tunnel valleys in the northern North Sea Basin and the presence of large moraines on the West Shetland Shelf, coupled with stratigraphic evidence from the Witch Ground Basin, all suggest that at its maximum extent a grounded ice sheet flowed from SE to NW across the northern North Sea Basin, terminating at the continental-shelf edge. The zone of confluence between the British and much larger Fennoscandian Ice Sheets was probably across the northern Orkney Islands, with fast-flowing ice in the Fair Isle Channel focusing sediment delivery to the Rona and Foula Wedges. This period of maximum confluent glaciation (c. 30–25 ka BP) was followed by a remarkable period of large-scale ice-sheet re-organisation. We present evidence suggesting that as sea level rose, a large marine embayment opened in the northern North Sea Basin, as far south as the Witch Ground Basin, forcing the two ice sheets to decouple rapidly along a north–south axis east of Shetland. As a result, both ice sheets rapidly adjusted to new quasi-stable margin positions forming a second distinct set of moraines (c. 24–18 ka BP). The lobate overprinted morphology of these moraines on the mid-shelf west of Orkney and Shetland indicates that the re-organisation of the British Ice Sheet was extremely dynamic — probably dominated by a series of internally forced readvances. Critically, much of the ice in the low-lying North Sea Basin may have disintegrated catastrophically as decoupling progressed in response to rising sea levels. Final-stage deglaciation was marked by near-shore ice streaming and increasing topographic control on ice-flow direction. Punctuated retreat of the British Ice Sheet continued until c. 16 ka BP when, following the North Atlantic iceberg-discharge event (Heinrich-1), ice was situated at the present-day coastline in NW Scotland.

Foraminiferal reconstruction of mid- to late-Holocene ocean circulation and climate variability in Disko Bugt, West Greenland

Benthic foraminifera from two cores collected from Disko Bugt, a large marine embayment in West Greenland, are used to investigate hydrographic changes during the mid to late Holocene. The records document fluctuations in the West Greenland Current (WGC) linked to large-scale changes in North Atlantic circulation and, for one core, impacts of local climate changes. In particular, changes in the cold East Greenland Current (EGC) and the warm, more saline Irminger Current (IC) influence the WGC and propagate into Disko Bugt. The cores record subsurface warming associated with the `Holocene Thermal Maximum' (HTM) from c. 6 to 5 ka cal. BP. Gradual cooling of WGC from 5 ka cal. BP was interrupted by an abrupt warm interval from 3.9 to 3.5 ka cal. BP, before cooling became more prominent from 3.5 to 2 ka cal. BP. This cold period corresponds to the well-documented Northern Hemisphere cooling of the neoglacial and we interpret it as reflecting enhanced flux from the EGC and reduced IC flux to the WGC in Disko Bugt. Colder subsurface conditions from 3.5 ka cal. BP correspond to major hydrographic and climate changes elsewhere in West Greenland, reflecting the final transition from HTM to the neoglacial in this region. Relatively warm conditions identified from c. 2.0 to 1.4 ka cal. BP are followed by gradual cooling to 0.7 ka cal. BP, interrupted by a brief warm episode around 1 ka cal. BP. The late-Holocene fluctuations in WGC recorded in Disko Bugt can be linked to North Atlantic oceanographic changes identified on the East Greenland and North Icelandic shelf.

Habitat partitioning by five congeneric and abundant Choerodon species (Labridae) in a large subtropical marine embayment

The habitats occupied by the juveniles and adults of five morphologically similar, diurnally active and abundant Choerodon species in the large subtropical environment of Shark Bay, a “World Heritage Property” on the west coast of Australia, have been determined. The densities of the two life cycle stages of each Choerodon species in those habitats were used in various analyses to test the hypotheses that: (1) habitats are partitioned among these species and between their juveniles and adults; (2) such habitat partitioning is greatest in the case of the two Western Australian endemic species, i.e. Choerodon rubescens and Choerodon cauteroma; and (3) the extent of habitat partitioning between both of these two species and the only species that is widely distributed in the Indo-West Pacific, i.e. Choerodon schoenleinii, will be less pronounced. Initially, catches of each of the five congeneric species, obtained during other studies in Shark Bay by angling, spearfishing and otter trawling, were collated to elucidate the broad distribution of these species in that embayment. Underwater visual census was then used to determine the densities of the juveniles and adults of each Choerodon species at sites representing the four habitat types in which one or more of these species had been caught, i.e. reefs in marine waters at the western boundary of the bay and seagrass, reefs and rocky shorelines in the two inner gulfs. The compositions of the Choerodon species over marine (entrance channel) reefs and in seagrass were significantly different and each differed significantly from those in both inner gulf reefs and rocky shorelines, which were, however, not significantly different. Choerodon rubescens was restricted to exposed marine reefs, and thus occupied a different habitat and location of the bay than C. cauteroma, the other endemic species, which was almost exclusively confined to habitats found in the inner gulfs. Choerodon cauteroma differed from other Choerodon species in being relatively abundant in each inner gulf habitat and undergoing an ontogenetic change in habitat. Thus, while its juveniles lived predominantly in seagrass, its adults occupied mostly reefs and rocky shorelines. Choerodon cephalotes occurred almost exclusively in seagrass, while C. cyanodus was relatively most abundant along rocky shorelines. Choerodon schoenleinii, the only one of the five Choerodon species to be widely distributed outside Australia, occupied reefs and, to a lesser extent, rocky shorelines in the inner gulfs. The above differences in habitat among Choerodon species would reduce the potential for competition for spatial resources among these abundant and similar species and between the juveniles and adults of one of the species.

Comparisons between the influences of habitat, body size and season on the dietary composition of the sparid Acanthopagrus latus in a large marine embayment

Seasonal samples from Shark Bay on the west coast of Australia were used to determine (1) the habitats occupied by the juveniles and adults of Acanthopagrus latus in this large subtropical marine embayment and (2) the extent to which the dietary composition of this sparid is influenced by habitat type, body length and season. Sampling was undertaken in two habitat types in which A. latus was known to be abundant, namely mangrove ( Avicennia marina) creeks and nearby rocky areas, the latter comprising sandstone boulders and/or limestone reefs. The mean total length ±95% CLs of A. latus was far lower in mangrove creeks, 126 ± 6.1 mm, than in rocky areas, 313 ± 4.7 mm. As A. latus attains maturity at ca. 245 mm, the juveniles of this species typically occupy mangrove areas and then, with increasing body size, move to nearshore rocky areas, where they become adults. The species composition of the food ingested by juvenile A. latus in mangrove creeks differed markedly from that of large juveniles and adults in rocky areas. Based on analyses of data for both habitat types combined, this difference was far greater than that between size classes and season, which was negligible. There were indications, however, that, overall within each habitat, the dietary composition did change seasonally, although not with body size. Acanthopagrus latus fed predominantly on mangrove material, sesarmid crabs and small gastropods in mangrove habitats, and mainly on Brachidontes ustulatus in rocky areas, where this mytilid bivalve is very abundant. The mangrove material, which contributed nearly 40% of its overall dietary volume in mangrove creeks, consisted mainly of lateral root primordia. This apparently unique food source for a teleost is presumably ingested through subsurface nipping, which would be facilitated by the mouth and dentitional characteristics of sparids. The almost total lack of correspondence in the dietary compositions of fish in the length class that was well represented in both mangrove and rocky areas illustrates the extent to which this sparid is capable of opportunistic feeding behaviour.

Catchability and selectivity of juvenile snapper ( Pagrus auratus, Sparidae) and western butterfish ( Pentapodus vitta, Nemipteridae) from prawn trawling in a large marine embayment in Western Australia

Allegations of growth overfishing of the already depleted snapper ( Pagrus auratus) stock in Denham Sound, Shark Bay, Western Australia, due to prawn trawling, and concerns for by-catch species generally, led to the investigation of size-related catchability and cod-end selectivity of juvenile snapper and western butterfish ( Pentapodus vitta). The cod-end selectivity of prawn trawling was investigated by fitting a fine-mesh cod-end cover to commercial trawl nets to measure the size of fish passing through the cod-end. A Leslie–Delury depletion experiment involving trawling the same area over four consecutive nights was used to determine the catchability coefficients of snapper and butterfish. The size of snapper and butterfish caught on the trawlable grounds ranged from 3 to 18 and 5 to 20 cm fork length (FL), respectively. Ontogenetic changes in habitat preference meant that snapper were susceptible to commercial trawling only between approximately 9–17 months of age. Retention of snapper and butterfish in the trawl nets increased with increasing length, with the deeper-bodied snapper retained at smaller lengths. Catchability coefficients also increased with fish length. A single trawl pass was estimated to remove up to 57% and 52% of juvenile snapper and butterfish, respectively. The rate of natural mortality ( M) for juvenile snapper was estimated to be high; mean M = 2.58 (92% year −1). Catchability of butterfish was low prior to their sexual maturation and trawling is assessed not to be a threat to this species. To reduce the impact of prawn trawling on snapper recruitment in Denham Sound, restricting trawling from areas where juvenile snapper are in high abundance, combined with fish escape panels, would be preferable to increasing mesh size in commercial trawl gear.

MODERN DISTRIBUTION OF BENTHIC FORAMINIFERA FROM DISKO BUGT, WEST GREENLAND

Benthic foraminifera were investigated from 20 grab samples collected from a large marine embayment, Disko Bugt, along the west Greenland margin. Agglutinated and calcareous foraminifera were found throughout Disko Bugt, with agglutinated species dominating 19 out of the 20 samples. The most common species are Adercotryma glomerata, Spiroplectammina biformis, Textularia earlandi, Cribrosto- moides crassimargo and Reophax fusiformis. Six faunal assemblage zones were identified from cluster analysis and detrended correspondence analysis. The relationship between the foraminifera and environmental variables was investigated using canonical correspondence analysis. From this analysis, bottom water salinity and water depth were identified as the most important environmental variables explaining the fora- miniferal assemblages. This result suggests water mass characteristics in Disko Bugt are an important control on the benthic foraminiferal assemblage zones. Where the warm and saline West Greenland Current water mass impinges on the seafloor, the assemblage is dominated by A. glomerata, T. earlandi, R. fusiformis and Reophax pilulifer. Sites with the cold, lower salinity Polar Water mass at the seafloor are dominated by S. biformis and A. glomerata, with common Cuneata arctica and C. crassimargo. In shallow locations with a relatively warm and low salinity surface water mass and coarse sediments the fauna is dominated by Cibicides lobatulus. In similar hydrographic locations with fine-grained sediments a mixed faunal assemblage dominated by Ammos- calaria pseudospiralis and Eggerella advena is found.

Comparisons between the biology of two co-occurring species of whiting (Sillaginidae) in a large marine embayment

We compare the biology of the tropical species Sillago analis and the temperate species Sillago schomburgkii in Shark Bay, a large subtropical marine embayment on the west coast of Australia. This environment constitutes approximately the southernmost and northernmost limits of the distributions of these two species, respectively. The annuli visible in sectioned otoliths of S. analis and S. schomburgkii form annually. Their numbers were thus used to age the individuals of these two species, which are morphologically very similar and live in the same habitats. Although the growth rates of S. analis and S. schomburgkii are very similar until maturity is attained, they subsequently diverge, with S. schomburgkii investing relatively more energy into somatic growth. The maximum total lengths and ages of both the females (320 mm, 6 years) and males (283 mm, 8 years) of S. analis were not as great as those of the females (383 mm, 9 years) and males (302 mm, 9 years) of S. schomburgkii. In Shark Bay, S. schomburgkii spawns earlier and longer than S. analis, i.e. August–December vs. January–March, which would result in the juveniles of these two species recruiting into nursery areas at different times. In addition, S. schomburgkii spawns earlier and for longer in Shark Bay than in temperate marine waters 800 km further south, presumably reflecting the fact that, in that subtropical embayment, water temperatures over which this species typically spawns are attained earlier and last for longer. However, although environmental conditions in Shark Bay and those temperate marine waters differ markedly, the growth of the corresponding sexes of S. schomburgkii in these two water bodies is similar.

Early Holocene palaeoceanography and deglacial chronology of Disko Bugt, West Greenland

Two new cores from Disko Bugt, a large marine embayment in West Greenland, are used to assess the deglacial chronology and palaeoceanography of the area during the early Holocene. Deglaciation of the main part of Disko Bugt is directly dated for the first time to a minimum of ca 10.2 ka cal. BP. By this time the Jakobshavn Isbrae ice stream had retreated to the eastern margins of Disko Bugt. Initial deglaciation took place before the relatively warm West Greenland Current (WGC) was initiated. A strengthening in the WGC is identified from ca 9.2 ka cal. BP accompanied by retreat of the ice stream to a shallow bank immediately west of the present day Jakobshavn Isfjord. While in this position the calving and melting ice stream produced large volumes of sediment and meltwater. The large volumes of meltwater prevented the relatively warm waters of the WGC reaching the eastern margins of Disko Bugt. The ice stream remained grounded on this bank for over a thousand years until ca 7.9 ka cal. BP when reduced sedimentation rates and an increase in boreal fauna point to its retreat inside the present day fjord system.

Which ecological indicators can robustly detect effects of fishing?

Many ecological indicators have been proposed to detect and describe the effects of fishing on marine ecosystems, but few have been evaluated formally. Here, simulation models of two marine systems off southeastern Australia (a large marine embayment, and an EEZ-scale regional marine ecosystem) are used to evaluate the performance of a suite of ecological indicators. The indicators cover species, assemblages, habitats, and ecosystems, including quantities derived from models such as Ecopath. The simulation models, based on the Atlantis framework, incorporate the effects of fishing from several fishing gears, and also the confounding impacts of other broad-scale pressures on the ecosystems (e.g. increased nutrient loads). These models are used to provide fishery-dependent and fishery-independent pseudo-data from which the indicators are calculated. Indicator performance is quantified by the ability to detect and/or predict trends in key variables of interest (“attributes”), the true values of which are known from the simulation models. The performance of each indicator is evaluated across a range of ecological and fishing scenarios. Results suggest that indicators at the community level of organization are the most reliable, and that it is necessary to use a variety of indicators simultaneously to detect the full range of impacts from fishing. Several key functional groups provide a good characterization of ecosystem state, or indicate the cause of broader ecosystem changes in most instances.

Habitat partitioning among four elasmobranch species in nearshore, shallow waters of a subtropical embayment in Western Australia

There is a need to explore, in an integrated and statistical manner, how the number of species, relative abundance, species composition and life-cycle stages of elasmobranchs in nearshore waters vary among habitat types and during the year. Therefore, four sites in a large marine embayment, each representing a different habitat type, were sampled at regular intervals. These sites were: (1) unvegetated, with no vegetation within at least 200 m; (2) unvegetated, immediately adjacent to sparse mangroves; (3) unvegetated, immediately adjacent to dense mangroves; and (4) vegetated, with seagrass (Posidonia australis) throughout and in the vicinity. Gill netting caught 10 shark species (5 families), 5 ray species (4 families) and 12 teleost species (10 families). Carcharhinus cautus, which contributed approximately 60% to the numbers of elasmobranchs caught, completed its life cycle in nearshore, shallow waters. Negaprion acutidens, Carcharhinus brevipinna, Carcharhinus limbatus and Rhizoprionodon acutus used these waters as a nursery area. C. cautus was caught mainly in the unvegetated sites, particularly in those near mangroves. N. acutidens was caught entirely in unvegetated sites, while R. acutus, C. brevipinna and Chiloscyllium punctatum were caught predominantly or exclusively in seagrass. The mean number of species and mean catch rate of elasmobranchs were greatest for the seagrass site and least for the unvegetated site with no vegetation within at least 200 m and were significantly less for the latter site than for the unvegetated site immediately adjacent to dense mangroves (P<0.05). The numbers of species and catch rates of elasmobranchs were significantly greater in summer and autumn than in winter (P<0.05) and, in the case of number of species, also than in spring (P<0.05). We conclude that the spatial and food resources in the nearshore, shallow waters of Shark Bay are partitioned among elasmobranch species, thus reducing the potential for competition among these species for the resources in those waters.

A Bayesian approach for overcoming inconsistencies in mortality estimates using, as an example, data for Acanthopagrus latus

Reliable estimates of natural (M) and total mortality (Z) are essential for effective fisheries management. However, estimates of M, which are frequently determined from life history parameters, are imprecise and often inconsistent with the values of Z derived from life history parameters and other analyses. This is exemplified by the mortality estimates derived for Acanthopagrus latus in a large marine embayment. Thus, such estimates, calculated for M for this population from a growth parameter and from growth parameters and water temperature, were both 0.70·year–1, whereas those for Z, calculated from maximum recorded age, relative abundance analysis, and a simulation based on maximum age and sample size, ranged from 0.18 to 0.30·year–1. These results are clearly inconsistent. A Bayesian approach was therefore developed that combines the posterior probability distributions of the various mortality estimates and thereby produces integrated and consistent estimates for M and Z. The application of our Bayesian approach to the data for A. latus yielded lower values for M than for Z. Our approach is equally applicable to other fish species.

Reproductive biology of Rhabdosargus sarba (Sparidae) in Western Australian waters, in which it is a rudimentary hermaphrodite

The reproductive biology of the tarwhine Rhabdosargus sarba has been studied in three very different environments in Western Australia, namely the lower reaches of the Swan River Estuary and marine waters at the same latitude, i.e. ≈32°S, and a large subtropical marine embayment (Shark Bay) approximately 800 km further north. A macroscopic and histological examination of the gonads demonstrated that R. sarba is typically a rudimentary hermaphrodite in Western Australian waters, i.e. the juveniles develop into either a male or female in which the ovarian and testicular zones of the gonads, respectively, are macroscopically undetectable. This contrasts with the situation in the waters off Hong Kong and South Africa where R. sarba is reported to be a protandrous hermaphrodite. Although R. sarba spawns between mid-late winter and late spring in each water body, the onset of spawning in the estuary is delayed until salinities have risen well above their winter minima. Although males and females attain sexual maturity at very similar lengths in the Swan River Estuary and Shark Bay, i.e. each L50 for first maturity lies between 170 and 177 mm total length (TL), they typically reach maturity at an earlier age in the former environment, i.e. 2 vs 3 years old. During the spawning period, only 25 and 12% of the males and females, respectively, that were caught between 180 and 260 mm TL in nearshore marine waters were mature, whereas 94 and 92% of the males and females, respectively, that were collected in this length-range over reefs, were mature. This indicates that R. sarba tends to move offshore when it has become ‘physiologically’ ready to mature. The L50s at first maturity indicate that the minimum legal length in Western Australia (230 mm TL) is appropriate for managing this species.

New observations on the relative sea level and deglacial history of Greenland from Innaarsuit, Disko Bugt

Relative sea level (RSL) data derived from isolation basins at Innaarsuit, a site on the south shores of the large marine embayment of Disko Bugt, West Greenland, record rapid RSL fall from the marine limit (ca. 108 m) at 10,300–9900 cal yr B.P. to reach the present sea level at 3500 cal yr B.P. Since 2000 cal yr B.P., RSL rose ca. 3 m to the present. When compared with data from elsewhere in Disko Bugt, our results suggest that the embayment was deglaciated later and more quickly than previously thought, at or slightly before 10,300 cal yr B.P. The northern part of Disko Bugt experienced less rebound (ca. 10 m at 6000 cal yr B.P.) compared with areas to the south. Submergence during the late Holocene supports a model of crustal down-warping as a result of renewed ice-sheet growth during the neoglacial. There is little evidence for west to east differences in crustal rebound across the southern shores of Disko Bugt.

Photosynthetic responses of two temperate seagrasses across a water quality gradient using chlorophyll fluorescence

Photosynthetic characteristics of seagrasses measured by pulse amplitude modulation (PAM) fluorometry have demonstrated acute responses of seagrasses to short-term doses of high light irradiance, UV radiation, osmotic stress and desiccation. In this work, we use PAM fluorometry to examine photosynthetic rates of the seagrasses Zostera tasmanica and Zostera capricorni across a light gradient within a large temperate marine embayment in Westernport, southern Australia. Three sites were studied across the bay representing a gradient in water quality, with low light conditions at Charing Cross, intermediate light conditions at Crib Point and high light availability at Newhaven. In the field electron transport rates (ETRs) were measured both as rapid light curves (RLCs) by in situ yield measurements under ambient daylight. Z. capricorni showed a high photosynthetic activity compared with Z. tasmanica, a response consistent with the adaptation of Z. capricorni to high light conditions. The low midday photosynthetic rate (ETR max) and photosynthetic efficiency ( α) of both species at Charing Cross where photon fluxes were low suggests a photoadaptive response. Diurnal responses of ETR, photosynthetic efficiency ( α) and I k for both seagrass species were consistent with diurnal responses reported for other seagrass species in Australia. Low early morning photochemical quenching (qP) of Z. tasmanica and Z. capricorni at Charing Cross (low light site) also may be a function of reduced ETR and photosynthetic efficiency. A key finding was the persistent low dark-adapted maximum quantum yield ( F v/ F m) of Z. tasmanica at Charing Cross in both winter and summer due to a decrease in F m relative to F o. Such a finding suggests photosynthetic downregulation and closure of photosystem II (PSII) reaction centres possibly in response to light deprivation. The response suggests seagrasses at Charing Cross showed physiological signs of stress characterised by low quantum yields and low rates of electron transport. Low photosynthetic rates may in part explain the twofold to threefold lower leaf biomass of Z. tasmanica shoots at this site compared with the other two sites.

Late Weichselian deglacial history of Disko Bugt, West Greenland, and the dynamics of the Jakobshavns Isbrae ice stream

New relative sea‐level (RSL) data from Disko Bugt, a large marine embayment in West Greenland, are used to examine the deglacial history of the Jakobshavns Isbrae ice stream. RSL data show rapid deglaciation after 10.3 ka cal. yr BP. Once deglaciation began, a bedrock high in the west of the bay exerted no discernible influence on the deglacial chronology. Following initial rapid retreat, ice stream recession slowed as it approached the eastern shores of the bay. Seabed elevations increase here and the ice stream terminus lingered for several thousand years before retreating into the narrow bedrock‐confined Jakobshavns Isfjord. The seabed topography of Disko Bugt includes several deep channels which probably record the former course of the ice stream. Using a simple water depth/calving velocity relationship it is estimated that the maximum calving velocity on deglaciation was c. 4.8 km a‐1. This is less than the present rate (6–7 km a‐1), although ice discharge was two to four times that observed today. Initiation of rapid ice stream retreat was probably caused by ice stream thinning and increased surface melting. A critical point in time was the retreat of the ice stream from shallow continental shelf waters (c. 400 m) into the deep bedrock trough (&gt;800 m) which marks the entrance to Disko Bugt.