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The blue shark Prionace glauca is a top predator with one of the widest geographic distributions of any shark species, yet classified as critically endangered in the Mediterranean Sea, and Near Threatened globally. Previous genetic studies did not reject the null hypothesis of a single global population across the worldwide species range. Blue shark situation was proposed as a possible archetype of the ‘grey zone of population differentiation’, coined to designate cases where population structure may be too recent or too faint to be detected using a limited set of markers. Here, blue shark samples collected throughout its global range were sequenced using a specific ddRAD method (DArTseq; Georges et al. 2018), which recovered 37,655 genome-wide single nucleotide polymorphisms (SNPs). Two main groups emerged, with Mediterranean Sea and Northern Atlantic samples significantly differentiated from the Indo-west Pacific samples. Significant pairwise FST values indicated further genetic differentiation within the Atlantic Ocean, and between the Atlantic Ocean and the Mediterranean Sea. Reconstruction of recent demographic history suggested the divergence between northern and southern oceanic populations emerged about 500 generations ago and revealed a drastic reduction in effective population size from a large ancestral population. Our results illustrate the power of high-density genome scans to detect population structure and reconstruct demographic history in highly migratory marine species. As the management of the blue shark fishery, either as target or as bycatch, does not account for this delineation, we strongly recommend that the results presented here be considered in future stock assessment and management plans.

Large scale migrations are a key component of the life history of many marine species. We quantified the annual migration cycle of juvenile southern bluefin tuna (Thunnus maccoyii; SBT) and spatiotemporal variability in this cycle, based on a multi-decadal electronic tagging dataset. Behaviour-switching models allowed for the identification of cohesive areas of residency and classified the temporal sequence of movements within a migration cycle from austral summer foraging grounds in the Great Australian Bight (GAB) to winter foraging grounds in the Indian Ocean and Tasman Sea and back to the GAB. Although specific regions within the Indian Ocean were frequented, individuals did not always return to the same area in consecutive years. Outward migrations from the GAB were typically longer than return migrations back to the GAB. The timing of individual arrivals to the GAB, which may be driven by seasonality in prey availability, was more cohesive than the timing of departures from the GAB, which may be subject to the physiological condition of SBT. A valuable fishery for SBT operates in the GAB, as do a number of scientific research programs designed to monitor SBT for management purposes; thus, understanding SBT migration to and from the area is of high importance to a number of stakeholders.

As part of a national research program studying the sources, distribution, and effects of litter entering the ocean, we established a national citizen science program engaging nearly 7000 primary and secondary students, teachers and corporate participants in collecting marine debris data around Australia's coastline. Citizen scientists undertook a one-day training program, which addressed data collection skills and academic topics in the national science curriculum. A subset of teachers and corporate sponsor staff participated in an intensive multi-day training program with researchers before venturing into the field. Data collected by citizen scientists were compared with data collected by researchers at nearby locations. We found the citizen science data were of equivalent quality to those collected by researchers, but there were differences among students. Primary school students detected more debris than did older secondary students. Students detected small items (

Marine debris is recognized as an important global issue that can negatively affect wildlife, habitats, environmental processes, ecosystem services, and human activities including tourism, fishing, and navigation. To improve understanding of the sources and impacts of marine debris, we carried out a national litter survey at 175 sites around Australia using a stratified random sampling approach. Litter from land- and sea-based sources is ubiquitous, and sampling effects related to coastline shape, substrate characteristics, gradient, and backshore type were highly significant. Source effects related to land-based sources (eg population density and distance to road) were also highly significant. Of the total debris sampled, approximately 75% was plastic; 2% was related to recreational fishing. Litter density significantly increased with proximity to urban areas, suggesting a domestic origin; statistical patterns suggest that illegal rubbish disposal is a major driver. By quantifying debris at a large scales and distinguishing potential litter sources, we can better develop scale-appropriate solutions to reduce debris inputs to the environment.

The impact of seismic surveys on the catchability of marine fish is a contentious issue, with some claims that seismic surveys may negatively affect catch rates. However little empirical evidence exists to quantify the impacts or identify the mechanisms of such impact. In this study, we used a 2-D seismic survey in the Gippsland Basin, Bass Strait, Australia in April 2015 as an opportunity to quantify fish behaviour (field-based) and commercial fisheries catch (desktop study) across the region before and after airgun operations. Three species found in abundance (gummy shark, swell shark, tiger flathead) were acoustically tagged and released within one of two acoustic arrays (experimental and control zone) and monitored before, during and after the seismic survey. In the field study, only 35% of the gummy sharks and 30% of the swell sharks were subsequently detected two days after release, suggesting movement outside the study area. Various tagged individuals returned sporadically over the monitoring period, including during the seismic survey operations. Behaviour consistent with a possible response to the seismic survey operations was observed for flathead which increased their swimming speed during the seismic survey period and changed their diel movement patterns after the survey. We also investigated the potential impacts of the seismic survey on catch rates using Commonwealth fisheries logbook data from Jan 2012-Oct 2015. Fifteen species and two gear types (Danish seine, gillnet) were modelled to examine differences in catch rates before and after the seismic survey. The catch rates in the six months following the seismic survey were significantly different than predicted in nine out of the 15 species examined, with six species (tiger flathead, goatfish, elephantfish, boarfish, broadnose shark and school shark) showing increases in catch following the seismic survey, and three species (gummy shark, red gurnard, and sawshark) showing reductions. With the exception of flathead movement, we found little evidence for consistent behavioural or catch rate changes induced by the seismic survey in the targeted species, although behavioural data were limited because many sharks left the acoustic receiver array prior to the commencement of the seismic survey.

Signature lipid and fatty acid analysis were used to discriminate the diet of swordfish ( Xiphias gladius , orbital fork length: 60–203 cm) from waters off eastern Australia. The fatty acid (FA) composition of a range of known prey (squid, myctophids, and other fishes) of swordfish, taken from stomach samples and from net tows, was compared with that of the white muscle tissue (WMT) of swordfish from the same region. Swordfish muscle was lipid rich (average 24–42% dry weight), as was the skeleton (28–41%). The robustness of the approach was also tested by comparison against a key squid prey species that was collected and stored using different protocols: (i) fresh frozen, (ii) fresh frozen, then thawed, and (iii) stomach content collection. The FA profiles were generally similar, with the ratio of docosahexaenoic acid (DHA) and palmitic acid (16:0) in particular showing no significant difference. Major fatty acids in swordfish WMT were 18:1ω9c, 16:0, 22:6ω3, and 18:0. Multidimensional scaling showed that the swordfish WMT grouped closely with small fish prey including myctophids, and not with squid. Squid contained markedly higher 22:6ω3 than swordfish. Individual prey species of the myctophidae could also be separated by the same technique. These results were supported by traditional stomach content analyses (SCA) that showed fish were the dominant prey for small swordfish sampled from southern waters whereas squid were the main prey in more northern waters, matching the FA patterns we found for the two regions. We propose that where general diet patterns are established, signature FA analysis has good potential to compliment or in some cases, replace temporal and spatial monitoring of trophic pathways for swordfish and other marine species.