Sevengill sharks
Project Leader: Adam Barnett
Background
Shark populations worldwide are being placed under escalating pressure through both targeted fisheries and as bycatch from other fisheries, with many shark species being depleted at an alarming rate. Few countries have implemented management strategies of shark fisheries or conservation plans to protect dwindling shark populations. Australia does maintain and manage fisheries for some target species – e.g. school (Galeorhinus galeus) and gummy (Mustelus antarcticus) sharks. However, for the majority of non-targeted species there is a distinct lack of biological, ecological or catch data.
The sevengill shark (Notorynchus cepedianus) is an excellent example of just such a species: large, coastal, and common in most temperate seas of the world, very little is actually known about many aspects of the sevengill shark’s biology, ecology or even how many are caught annually in Australia.
They are probably the most significant top-order predator in the bays and estuaries of southern Tasmania where they are believed to be both specialist elasmobranch (shark & ray) predators, and also significant predators of seal pups around seal colonies. They are also the dominant bycatch species in southern commonwealth shark fisheries and are becoming increasingly important as a target species.
The sevengill shark (Notorynchus cepedianus) is an excellent example of just such a species: large, coastal, and common in most temperate seas of the world, very little is actually known about many aspects of the sevengill shark’s biology, ecology or even how many are caught annually in Australia.
They are probably the most significant top-order predator in the bays and estuaries of southern Tasmania where they are believed to be both specialist elasmobranch (shark & ray) predators, and also significant predators of seal pups around seal colonies. They are also the dominant bycatch species in southern commonwealth shark fisheries and are becoming increasingly important as a target species.
In the field
Adam Barnett
Using the sevengill shark as a case study and with financial support from SOSF, Adam Barnett is now undertaking a PhD that explores the role of higher trophic level predators on inshore ecosystems and their interactions with fisheries. This is a collaboration between the University of Tasmania through Tasmania Aquaculture and Fisheries Institute (TAFI) and CSIRO Marine & Atmospheric Research, Tasmania.
Aims & objectives
This study has 3 specific aims: to increase the knowledge of a species that is poorly understood; to expand our knowledge of the interactions of apex predators with their ecosystems; to address fisheries management issues, with a focus on fishing pressures when applied to sevengill sharks.
In addition, the study will examine the degree to which sevengills feed on early year classes of commercially-fished shark species within areas that have been set aside as shark refuges. It is hoped that this will provide a broader understanding of more general ecosystem issues in the management of large predators that interact with commercial fisheries.
In addition, the study will examine the degree to which sevengills feed on early year classes of commercially-fished shark species within areas that have been set aside as shark refuges. It is hoped that this will provide a broader understanding of more general ecosystem issues in the management of large predators that interact with commercial fisheries.
Project Update: 2009
This summer (2008/09) has seen the completion of the field work for a number of the components of this study.
1. The second batch of acoustic tags were deployed on both sevengill sharks and their main prey species. In total, 88 chondrichthyan individuals have been tagged, including 45 sevengill sharks and 43 prey (species tagged are listed below). Unfortunately, elephant fish proved hard to catch this season and therefore the southern eagle ray, which is prominent in sevengill sharks diet, was included instead. The species tagged are a good representation of the sevengill sharks diet and, with the exception of the eagle ray, are all strongly associated with fisheries in southern Australia. Thus, we should have a good sample size to study predator prey interactions as well as gauge the movement patterns of a number of chondrichthyan species that are affected by fisheries and, in particular, investigate the seasonal use of protected areas in south east Tasmania. An interesting result to come from the acoustic tagging study thus far was the detection of two male sevengill sharks by receivers near Batesmans Bay southern New South Wales from August to October 2008 (Fig. 1). This coincides with another male being tracked with a PAT (archival) tag from to the same region (Jervis Bay) in July 2008 (Fig.1), indicating that male sevengill sharks may migrate north from Tasmania during winter.
Individuals tagged as part of this study:
3. The collection of dietary information, i.e. gut samples gained from flushing the sharks stomachs (before being released) and tissue biopsy for stable isotope analysis has also been completed. The laboratory analysis and the writing of the results are presently underway.
4. Another active track was completed this season. This track incorporated the use of the business card tag (see 2008 report) to investigate species interactions. However, due to equipment difficulties we failed to retrieve the business card tag, which was very disappointing.
Continued work for 2009
Due to the success of the last 2 PAT (archival) tags (see September 2008 report), this May will see three more PAT tags deployed. This will take place on a course teaching students about using tracking techniques in shark research. June will see the majority of the Vr2 receivers collected for data analysis, a small number of receivers will remain in the water until December 2009 to see if tagged sharks return to the coastal areas after winter once again. The second half of this year will be an intensive period for data analysis and writing up of the results.
Figure 1. Locations were sevengill sharks PAT tags were released (King Island and Jervis Bay) and the region where sevengill sharks were detected on another studies VR2 receivers (Batemans Bay).
Figure 2. Seasonal catch rates of sevengill sharks in both the Derwent River and Norfolk Bay. CPUE = Catch per unit effort, which in this case is number of sharks caught per 50 hooks.
1. The second batch of acoustic tags were deployed on both sevengill sharks and their main prey species. In total, 88 chondrichthyan individuals have been tagged, including 45 sevengill sharks and 43 prey (species tagged are listed below). Unfortunately, elephant fish proved hard to catch this season and therefore the southern eagle ray, which is prominent in sevengill sharks diet, was included instead. The species tagged are a good representation of the sevengill sharks diet and, with the exception of the eagle ray, are all strongly associated with fisheries in southern Australia. Thus, we should have a good sample size to study predator prey interactions as well as gauge the movement patterns of a number of chondrichthyan species that are affected by fisheries and, in particular, investigate the seasonal use of protected areas in south east Tasmania. An interesting result to come from the acoustic tagging study thus far was the detection of two male sevengill sharks by receivers near Batesmans Bay southern New South Wales from August to October 2008 (Fig. 1). This coincides with another male being tracked with a PAT (archival) tag from to the same region (Jervis Bay) in July 2008 (Fig.1), indicating that male sevengill sharks may migrate north from Tasmania during winter.
Individuals tagged as part of this study:
- 45 sevengill sharks (Notorynchus cepedianus)
- 18 gummy sharks (Mustelus antarcticus)
- 8 school sharks (Galeorhinus galeus)
- 9 spiny dogsharks (Squalus acanthias)
- 5 southern eagle rays (Myliobatis australis)
- 3 elephantfish (Callorhinchus milii)
3. The collection of dietary information, i.e. gut samples gained from flushing the sharks stomachs (before being released) and tissue biopsy for stable isotope analysis has also been completed. The laboratory analysis and the writing of the results are presently underway.
4. Another active track was completed this season. This track incorporated the use of the business card tag (see 2008 report) to investigate species interactions. However, due to equipment difficulties we failed to retrieve the business card tag, which was very disappointing.
Continued work for 2009
Due to the success of the last 2 PAT (archival) tags (see September 2008 report), this May will see three more PAT tags deployed. This will take place on a course teaching students about using tracking techniques in shark research. June will see the majority of the Vr2 receivers collected for data analysis, a small number of receivers will remain in the water until December 2009 to see if tagged sharks return to the coastal areas after winter once again. The second half of this year will be an intensive period for data analysis and writing up of the results.
Figure 1. Locations were sevengill sharks PAT tags were released (King Island and Jervis Bay) and the region where sevengill sharks were detected on another studies VR2 receivers (Batemans Bay).
Figure 2. Seasonal catch rates of sevengill sharks in both the Derwent River and Norfolk Bay. CPUE = Catch per unit effort, which in this case is number of sharks caught per 50 hooks.
Project Update: 2008
The original aims have been expanded to include:
- Determine the relative number, size classes and sex ratios of sevengill and other shark species using specific coastal habitats.
- Investigate spatial dynamics of sevengill sharks and their prey species.
- Determine the trophic role of sevengill sharks.
- Examine predator prey relationships.
- Determine if sevengill sharks use the coastal systems for reproductive purposes.
- Investigate interactions between commercial fisheries and top order predators.
Project Update: 2007
Good progress has been made during the year, with almost 200 sharks having been tagged and released with conventional fishery tags. Two sharks have also been acoustically tracked, showing that these sharks are capable of moving relatively large distances (more than 30 kilometers) over short periods of time. The tagging data thus far has produced some interesting preliminary results with one particular habitat being identified as a potential hot spot for this species. Over the next year this will be validated and hopefully show why this particular habitat is so heavily used.
These preliminary results have assisted in the planning of the next phase of the project. Funding from SOSF has facilitated the acoustic tracking component of the study that will be implemented towards the end of 2007 and early 2008. 55 VR2 receivers will be deployed with tagging planned for 60 sharks, including both sevengill sharks and significant prey species such as gummy and school sharks. Coded transmitters will be used to investigate movement patterns, seasonality and site fidelity, hopefully providing insights on predator prey interactions. The receivers will be deployed for the next 15 months to allow monitoring of the sharks over all seasons and, during the peak season of tagging, a VRAP system will also be used to investigate fine scale movements of both predator and prey. In addition 10 sevengill sharks will be tagged with archival tags to investigate larger scale movements and a further 3 sharks will be actively tracked to enhance fine scale movement data.
These preliminary results have assisted in the planning of the next phase of the project. Funding from SOSF has facilitated the acoustic tracking component of the study that will be implemented towards the end of 2007 and early 2008. 55 VR2 receivers will be deployed with tagging planned for 60 sharks, including both sevengill sharks and significant prey species such as gummy and school sharks. Coded transmitters will be used to investigate movement patterns, seasonality and site fidelity, hopefully providing insights on predator prey interactions. The receivers will be deployed for the next 15 months to allow monitoring of the sharks over all seasons and, during the peak season of tagging, a VRAP system will also be used to investigate fine scale movements of both predator and prey. In addition 10 sevengill sharks will be tagged with archival tags to investigate larger scale movements and a further 3 sharks will be actively tracked to enhance fine scale movement data.