//
you're reading...

Behavior

Tracking the movements of a heavily fished Fijian shark

Bond, M. E., Tolentino, E., Mangubhai, S., & Howey, L. A. (2015). Vertical and horizontal movements of a silvertip shark (Carcharhinus albimarginatus) in the Fijian archipelago. Animal Biotelemetry3(1), 1-7. DOI: 10.1186/s40317-015-0055-6

Figure 1. Silvertip reef shark by David Hall, Seaphotos.com.

Figure 1. Silvertip reef shark by David Hall, Seaphotos.com.

Introduction

Shark finning is a practice wherein nearly every fin is removed from the body to be sold for use in various foods.  The South Pacific, including Fiji, exports vast quantities of shark fins to the market in Hong Kong, China.  Based on the types of fishing gear used (longlines and gillnets), it is assumed that most of the fins come from pelagic (open ocean) species.  Silvertip sharks are one of several reef-associated shark species in the Fijian archipelago.  They are a slow-growing species that reach a maximum length of about 3 meters.  Silvertip sharks are considered apex predators, though they have a varied diet including benthic and pelagic fish, rays, other sharks and cephalopods (like squid).  The species is currently classified as ‘near-threatened’ by the International Union for Conservation of Nature (IUCN).  The horizontal movement of silvertips has not been well studied to date though they are known to be ‘habitat-faithful’ meaning silvertips do not migrate far from home reefs.  This habitat-faithful trait means that isolated populations (those that do not mix with other populations of silvertips) are particularly at risk of overexploitation by the shark finning trade.  No data on the population status of silvertips in the Fijian archipelago exists though the tuna longline fishery notes high levels of by-catch.  This study published in the journal Animal Biotelemetry documented a silvertip shark’s depth and thermal habitat use as an important step towards learning more about this understudied species.

Figure 2. The study site from Bond et al. (2015).  Deployment location indicated in red, and first Argos location after detachment indicated in yellow (Figure 1, Bond et al. 2015).

Figure 2. The study site from Bond et al. (2015). Deployment location indicated in red, and first Argos location after detachment indicated in yellow (Figure 1, Bond et al. 2015).

Methods

Satellite telemetry was used to track the movements (both horizontal and vertical) of silvertip sharks along the Great and North Astrolabe Reefs (Figure 2).  Together, they form one of the world’s largest barrier reefs.  These reefs are in healthy condition and contain a variety of habitats.  Lagoon depths extend to 50 meters with areas of rock, seagrass and coral.  The outer reef, with soft sediment and rubble ranges from 35 to over 65 meters.  Beyond that, the reef wall drops to the ocean floor, over 1,600 meters.

On April 7, 2014, an immature female silvertip shark 115 cm in length was tagged.  On April 18 the tag released due to attachment problems.  Throughout this time, the tag recorded 15-minute interval measurements of depth, temperature, and light-level.  All of the data collected on the tag was transmitted to the Argos satellite system.  There were 2,182 records of temperature and depth.  The shark ranged in depth from 0 to 381.9 m with a mean depth of 59.9 (+/- 38.5 m).  The shark spent most of the time at depths shallower than 150 m and more than half of the time shallower than 50 m.  Less than 3.1% of the time was spent in the upper 10 m (figure 3).  Daytime depths were significantly deeper than night depths (Figure 4).  And daily temperatures were significantly cooler than nighttime temperatures (Figure 5).  The 3 deepest 15-minute records were during the night.  There were no consecutive depth records below 200 m.  This means that the trips to deeper water could not have lasted longer than 30-minutes as the tag collected data at 15-minute intervals.

Figure 4. Depth distribution frequency (Figure 3, Bond et al. 2015).

Figure 4. Depth distribution frequency (Figure 3, Bond et al. 2015).

Figure 5. Temperature distribution frequency (Figure 4, Bond et al. 2015).

Figure 5. Temperature distribution frequency (Figure 4, Bond et al. 2015).

Discussion

The shark’s depth varied considerably more during the day than at night, which indicates that silvertips may use a greater range of depth habitat during the day.  Daily depth migrations, also called ‘diel vertical migrations’ (DVM), to deeper water are also seen in other shark species.  Silvertips may follow this depth pattern to follow cephalopods, which comprise ~13% of their diet.  Observed DVM behavior may also be explained by following preferred temperatures.  Laboratory studies have found the some strategies maximize energy efficiency by foraging in warmer water (warm temperatures = increased metabolism) and resting in cooler temperatures (cold temperatures = reduced metabolism).  Silvertips may also be avoiding predators such as other, larger shark individuals.  Predator avoidance probably does not explain the deeper, short duration dives as they would likely be longer.    Because this study tagged just one silvertip and the tag lasted a short time (12 days), the authors state that conclusions about the species should not be drawn from this study.  Though it may be considered a model for future studies to develop an understanding of this species.

Discussion

No comments yet.

Post a Comment

Instagram

  • by oceanbites 3 months ago
    Happy Earth Day! Take some time today to do something for the planet and appreciate the ocean, which covers 71% of the Earth’s surface.  #EarthDay   #OceanAppreciation   #Oceanbites   #CoastalVibes   #CoastalRI 
  • by oceanbites 4 months ago
    Not all outdoor science is fieldwork. Some of the best days in the lab can be setting up experiments, especially when you get to do it outdoors. It’s an exciting mix of problem solving, precision, preparation, and teamwork. Here is
  • by oceanbites 5 months ago
    Being on a research cruise is a unique experience with the open water, 12-hour working shifts, and close quarters, but there are some familiar practices too. Here Diana is filtering seawater to gather chlorophyll for analysis, the same process on
  • by oceanbites 6 months ago
    This week for  #WriterWednesday  on  #oceanbites  we are featuring Hannah Collins  @hannahh_irene  Hannah works with marine suspension feeding bivalves and microplastics, investigating whether ingesting microplastics causes changes to the gut microbial community or gut tissues. She hopes to keep working
  • by oceanbites 7 months ago
    Leveling up - did you know that crabs have a larval phase? These are both porcelain crabs, but the one on the right is the earlier stage. It’s massive spine makes it both difficult to eat and quite conspicuous in
  • by oceanbites 7 months ago
    This week for  #WriterWednesday  on  #Oceanbites  we are featuring Cierra Braga. Cierra works ultraviolet c (UVC) to discover how this light can be used to combat biofouling, or the growth of living things, on the hulls of ships. Here, you
  • by oceanbites 7 months ago
    This week for  #WriterWednesday  at  #Oceanbites  we are featuring Elena Gadoutsis  @haysailor  These photos feature her “favorite marine research so far: From surveying tropical coral reefs, photographing dolphins and whales, and growing my own algae to expose it to different
  • by oceanbites 8 months ago
    This week for  #WriterWednesday  on Oceanbites we are featuring Eliza Oldach. According to Ellie, “I study coastal communities, and try to understand the policies and decisions and interactions and adaptations that communities use to navigate an ever-changing world. Most of
  • by oceanbites 8 months ago
    This week for  #WriterWednesday  at  #Oceanbites  we are featuring Jiwoon Park with a little photographic help from Ryan Tabata at the University of Hawaii. When asked about her research, Jiwoon wrote “Just like we need vitamins and minerals to stay
  • by oceanbites 8 months ago
    This week for  #WriterWednesday  on  #Oceanbites  we are featuring  @riley_henning  According to Riley, ”I am interested in studying small things that make a big impact in the ocean. Right now for my master's research at the University of San Diego,
  • by oceanbites 8 months ago
    This week for  #WriterWednesday  at  #Oceanbites  we are featuring Gabby Stedman. Gabby is interested in interested in understanding how many species of small-bodied animals there are in the deep-sea and where they live so we can better protect them from
  • by oceanbites 9 months ago
    This week for  #WriterWednesday  at  #Oceanbites  we are featuring Shawn Wang! Shawn is “an oceanographer that studies ocean conditions of the past. I use everything from microfossils to complex computer models to understand how climate has changed in the past
  • by oceanbites 9 months ago
    Today we are highlighting some of our awesome new authors for  #WriterWednesday  Today we have Daniel Speer! He says, “I am driven to investigate the interface of biology, chemistry, and physics, asking questions about how organisms or biological systems respond
  • by oceanbites 10 months ago
    Here at Oceanbites we love long-term datasets. So much happens in the ocean that sometimes it can be hard to tell if a trend is a part of a natural cycle or actually an anomaly, but as we gather more
  • by oceanbites 10 months ago
    Have you ever seen a lobster molt? Because lobsters have exoskeletons, every time they grow they have to climb out of their old shell, leaving them soft and vulnerable for a few days until their new shell hardens. Young, small
  • by oceanbites 11 months ago
    A lot of zooplankton are translucent, making it much easier to hide from predators. This juvenile mantis shrimp was almost impossible to spot floating in the water, but under a dissecting scope it’s features really come into view. See the
  • by oceanbites 11 months ago
    This is a clump of Dead Man’s Fingers, scientific name Codium fragile. It’s native to the Pacific Ocean and is invasive where I found it on the east coast of the US. It’s a bit velvety, and the coolest thing
  • by oceanbites 12 months ago
    You’ve probably heard of jellyfish, but have you heard of salps? These gelatinous sea creatures band together to form long chains, but they can also fall apart and will wash up onshore like tiny gemstones that squish. Have you seen
  • by oceanbites 12 months ago
    Check out what’s happening on a cool summer research cruise! On the  #neslter  summer transect cruise, we deployed a tow sled called the In Situ Icthyoplankton Imaging System. This can take pictures of gelatinous zooplankton (like jellyfish) that would be
  • by oceanbites 1 year ago
    Did you know horseshoe crabs have more than just two eyes? In these juveniles you can see another set in the middle of the shell. Check out our website to learn about some awesome horseshoe crab research.  #oceanbites   #plankton   #horseshoecrabs 
WP2Social Auto Publish Powered By : XYZScripts.com