Blue sharks
Project Leader: Dr David Sims
Background
The population structure of the blue shark (Prionace glauca) in the Atlantic appears complex, with anecdotal evidence for migrations, age and sexual segregation, and spatially discrete nursery areas. Underlying this apparent complexity is the likelihood that fisheries are significantly impacting populations. The purpose of this SOSF supported research in the north-east Atlantic is to determine the movements, behaviour and critical habitat of blue shark populations for direct use in conservation assessment of fishery interactions.
In the field
The aim is to focus on obtaining longer term satellite trackings of mature male and female blue sharks together with those of juveniles. This will help identify whether adults segregate by sex, a behaviour which has important wider implications for assessing the impacts on blue shark population trends of spatially restricted fisheries. It will also examine when and where mature males and females occur together, to delineate likely mating areas for this species.
Long-term movements of juveniles/subadults are especially important to resolve because we have discovered both philopatry and long-range dispersal among individuals tagged within days of each other at the same site. This suggests complex movement patterns that may pose a particular risk to younger stages from both coastal and high seas fisheries; that are, they may experience little respite from fisheries activity during much of their annual movements. The SOSF research already carried out on blue shark has now achieved tracking durations greater than 3 months for adults and juveniles. The present aims build on this success to better understand critical components of blue shark population structuring and spatial dynamics over the annual cycle.
Long-term movements of juveniles/subadults are especially important to resolve because we have discovered both philopatry and long-range dispersal among individuals tagged within days of each other at the same site. This suggests complex movement patterns that may pose a particular risk to younger stages from both coastal and high seas fisheries; that are, they may experience little respite from fisheries activity during much of their annual movements. The SOSF research already carried out on blue shark has now achieved tracking durations greater than 3 months for adults and juveniles. The present aims build on this success to better understand critical components of blue shark population structuring and spatial dynamics over the annual cycle.
Aims & objectives
This project aims to conduct a detailed analysis of blue shark movement from the satellite data collected, including movement paths and dive behaviour.
The specific objectives will be to:
Though at an early stage of analysis, some initial results show a high degree of sexual segregation among blue sharks in the northeast Atlantic. Using the tags Dr David Sims and his team have tracked the female-only migration within and away from the English Channel and identified specific behaviours associated with foraging and migrating. Male blue sharks were more abundant further west and a male tracked from the Azores showed directed movement towards an isolated seamount; juvenile blue sharks tracked on the continental shelf of southern Portugal were also tracked moving to oceanic seamounts, suggesting that these topographic features are probably important blue shark habitats.
Sadly, two of the six sharks tracked with SPOT tags were captured by long-lining fisheries within a month of release and tracked to ports in southern Spain, where one was physically retrieved. The death of these sharks reinforces the need for further research into the intersection of migratory routes and fisheries.
Our aim is to track the longer term movements of key life-history stages of blue shark in the NE Atlantic over the annual cycle.
Sadly, two of the six sharks tracked with SPOT tags were captured by long-lining fisheries within a month of release and tracked to ports in southern Spain, where one was physically retrieved. The death of these sharks reinforces the need for further research into the intersection of migratory routes and fisheries.
Our aim is to track the longer term movements of key life-history stages of blue shark in the NE Atlantic over the annual cycle.
The specific objectives will be to:
- Determine the movements of mature male and female blue shark within and away from the putative mating ground off southern Portugal. This will examine the degree and timing of sexual separation in the population and will provide further definition of the putative mating ground for this species in the NE Atlantic.
- Investigate the dispersal behaviour of young (juvenile/sub-adult) males and females over longer timescales to identify whether return migrations to preferred habitats by individuals occur within an annual period. This will test whether residence and migration are components of a repeated annual cycle of behaviour.
- Map spatially and temporally the satellite-derived, fishery independent geolocations of different sizes and sexes of blue shark on maps of habitat and physical variables (bathymetry, sea surface temperature, primary productivity, altimetry). This will identify and describe the critical habitat of different life-history stages and visualise spatial population structuring through time to aid interpretation for the efficacy of future spatial conservation initiatives (OOMPAs).
Project Update: 2008
Dr David Sims
To date 12 pop-up archival transmitting (PAT) tags have been attached to blue sharks in three locations in the northeast Atlantic: seven in the English Channel, three in southern Portugal and two in the Azores. These tags archive pressure (depth), sea temperature and light intensity data and pop-off at pre-programmed times to report summary data via polar-orbiting satellites.
By September 2007, seven out of nine tags had achieved their pop-up dates and were able to relay large movement and behavioural datasets. Two of the tags deployed on female sharks in English Channel have also been physically retrieved, yielding full archival datasets of depth, temperature and light at 10-second resolution, providing an unparalleled insight into the migration behavior of blue sharks.
In addition to PAT tags, 11 smart position-only tags (SPOT) were attached to the first dorsal fins of blue sharks in the same three locations. These tags accurately pinpoint the locations where sharks break the surface during normal swimming. These have provided less reliable data with only four of 11 tags reporting positions on a regular basis.
By September 2007, seven out of nine tags had achieved their pop-up dates and were able to relay large movement and behavioural datasets. Two of the tags deployed on female sharks in English Channel have also been physically retrieved, yielding full archival datasets of depth, temperature and light at 10-second resolution, providing an unparalleled insight into the migration behavior of blue sharks.
In addition to PAT tags, 11 smart position-only tags (SPOT) were attached to the first dorsal fins of blue sharks in the same three locations. These tags accurately pinpoint the locations where sharks break the surface during normal swimming. These have provided less reliable data with only four of 11 tags reporting positions on a regular basis.