Transcript SharksSp15 - St. Olaf Pages

Key Points from Shark
Article!
Group 1: Lark, Sam, Emily, Erin
Ecological Features
● Inhabit all areas of ocean, from fully oceanic to being
limited to shallow reefs and continental shelves, and
some that transition between the two.
● Life History - Life cycle of growth, sexual maturity,
reproduction, and offspring rearing
o Slow growers and reproducers. This prevents
sharks from recovering quickly from population
depletion since they reach sexual maturity late in
life, and do not have large numbers of offspring.
● Sharks occupy different levels of the food chain- some
are top predators while others are mesopredators
● Generalist feeders- mostly carnivores, but some
planktivores
● Because their feeding is not gape-limited, they are
often the major or exclusive predators of megafauna
Group 2: Eva, Robin, Miles, Thea
Figure 2. Global fisheries trends and conservation status of chondrichthyans.
Time series refer to landings of sharks, rays and chimeras in thousands of metric tonnes km -2 of shelf area since 1950,
until 2007.
Red bars=% of species in certain area that are globally assessed as critically endangered (CR), endangered (EN), and
vulnerable (VU)
Yellow bars=yellow indicates near threatened (NT) status and green the percentage of species assessed as least concern (LC).
Green bars= species that are the least concern
Transparent bars= species that are assessed data deficient (DD) or that have not been assessed yet
n = total species in area ,
A = number of species assessed
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For the most part, all areas have upward trends, except for China, and areas in Northern Europe... which
appear to be pretty constant.
Many areas have drops around the 1970’s and 1980’s.
In South America, there is a lot of variation between years.
In Western Africa, a huge increase in the mid-1990’s.
Ferretti et al.
Caitlin
Coastal Ecosystems (Closest to the shore, no more than 200 m deep):
● Highly accessible, lots of human activity→ exploitation
● High biological productivity
● Been fished since humans lived in coastal regions
Demersel Ecosystems (Continental shelf and slopes):
● Home to 90% of elasmobranch species (Sharks, rays, and skates)
● Trawl fishing→ dragging a big net
● Shrimp fisheries with a lot of elasmobranch by-catch
● Fished by bottom trawling since the 1970s
Pelagic Ecosystems (Open ocean, deep waters):
● Industrial fishing since the 1950’s
● Long-line fishing for tuna, swordfish, and other large fish
● Known for large amounts of sharks as by-catch (1 shark for every 2 yellowfin)
Group 4: Effects on Elasmobranch Mesopredators
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Mesopredator: a medium-sized predator which often increases in abundance when large predators
decline.
Decreases in large sharks usually lead to increases in elasmobranch mesopredators (i.e. dogfish
and other small sharks).
This is better documented in coastal habitats than in pelagic (open sea) habitats.
o Some increases reported in pelagic habitats but effects have also been attributed to:
 Declines in other open sea apex predators (i.e. tuna)
 Changes in habitat use, allowing mesopredators to move into waters previously
dominated by larger sharks.
Elasmobranchs are highly sensitive to fishing, so any increases in fishing would seriously impact
mesopredator populations despite their recent increases in abundance.
Example: dogfish have increased 20- and 17-fold in the Gulf of
Alaska and Prince William Sound, and S. megalops has increased
fivefold in Australian waters from ’76–’97.
Toby, Gabe, Andrew, Ruth
http://www.elasmodiver.com/Spiny%20
Dogfish%20Pictures.htm
Group 5: Dani, Evan, Lisa
Effects of Shark Exploitation on Marine Mammals and Sea Turtles:
● Decrease of sharks increase range of marine mammals and sea turtles
● Reduced shark predation on juveniles increase marine mammal and sea turtle populations
● Increased population sizes of marine mammals and sea turtles create competition within and
between species (trophic cascade)
● Increased population sizes of marine mammals and sea turtles decreases the amount of seagrass
left (trophic cascade)
● Marine mammals tend to avoid areas of high predation risk, but sea turtles tend to remain in
seagrass beds with high predation risk
https://wavemakersrq.wordpress.com/2012/10/08/module-3-the-first-four-kingdoms-part-2-2/
http://animals-pics.com/tag/seals-monk-animal-picture/
Group 6: Jake, Emma, Emilee
● Removal of large sharks results in trophic cascades
o Large sharks eat the rays and small sharks, the small sharks then eat
the bony fish (see figure on the next slide).
● Large sharks were netted off the coast of South Africa for 50 year
● When large sharks were originally netted, that resulted in an increase in small
sharks and a decrease in bony fish
● When small sharks began to succomb to angling pressure, the population of
bony fish increased again
● Sharks can also influence other marine ecosystems, such as those that
contain seagrass and grazers like turtles and dugongs
● More research needs to be done to determine if this is an isolated effect or if
it might be seen in another ecosystem; there simply isn’t much research on
non-commercial sharks.
Group 7: Lily, Kristen, Amy
:Conclusion
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visibility/public opinion in terms of conservation
sensitivity to ecosystem change, even ‘sustainable’ fishing
trophic cascade: increase in mesopredators
low reproductive rates increase risk
wide range mobility makes observation difficult
because the ocean is a complex system, it is difficult to discern
causes/effects
● similarly, it is difficult to pinpoint the specific effect of fishing