Warming oceans may shrink the habitat of many marine animals — but not all

Brad Seibel remembers 20 years ago that sounded like a B-rated sci-fi movie: "Giant Squid Invades Monterey Bay" or something
.
At the time he was a postdoctoral scholar
at the Monterey Bay Aquarium Research Institute (MBARI).


This is by no means fiction
.
This gluttonous animal, which has historically lived in more tropical latitudes, has seen a record number of central California waters — feeding its belly with cod, rockfish and other important commercial species, much to the dismay
of local fishermen.
Scientists believe their arrival is linked to climate change and overfishing, but details are unclear
.


Now a professor at the USF School of Marine Sciences and an expert in marine physiology, Seibel recently published a paper explaining those long-ago headlines
.
It links information about animal metabolism that he has gathered over 20 years during seven research cruises in Mexico's Gulf of California, adding a new chapter
to the story of how some animals are coping with ocean warming.


"The basic narrative in recent years is that as the oceans warm and lose oxygen, animals in the oceans will be driven out of their native habitats and migrated to cooler waters
farther north latitudes," Seibel said.
"But it's
an oversimplification.
"

Not all marine animals respond equally
to environmental changes.


Seibel co-authored the book
with his former graduate student Matt Birk.
Burke is now a professor at
St.
Francis University in Pennsylvania.
This study is the first to delve into the relationship between
oxygen, temperature, and metabolic requirements in vertically migrating animals.
Vertical migratory animals include billions of marine animals, from tiny crustaceans known as krill to giant 6-foot-long squid
.
Seibel and Birk used modeling to understand how six krill species and giant squid respond metabolically to different parameters close to diurnal
habitats.


"Vertical migrants run counter to the basic narrative that is largely based on the study
of coastal fauna," Seibel said.


As the oceans warm, squid and other vertically migratory animals living in the tropics may expand their habitat north — but not necessarily out of their native tropics
.


Seibel said this could have been what happened
in Monterey 20 years ago.
El Niño temporarily brings warm waters
to the coast.
(Think of it as a relatively short-lived model
of climate change.
) The warm waters allowed the squid to expand their range northward, where they took advantage of new food sources — severely affecting local fisheries — despite plentiful
food in tropical latitudes.


"It's not because they don't have enough oxygen, or it's not because it's too hot for them in the south; Before El Niño, the climate in the north was too cold for them" — which has to do with their metabolic demands, Seibel said
.


Vertically migratory species live very differently from coastal species, which experience a fairly stable supply
of oxygen in waters that mix well with the atmosphere.
They live in the deep sea during the day, where it is cold and dark with less oxygen, and at night they swim hundreds of meters to the relatively warm surface of the sea to feed, where oxygen is plentiful and foraging is safer
.


"This study is a great example of what we often draw from well-researched and easily captured organisms that may not apply to the greater diversity
of species and lifestyles found in the ocean," Birk said.


The results show that temperature has an effect of 4-5 times
higher on the metabolic rate of vertically migrating species than most coastal species.
In the deep sea, for example, squid doesn't do anything at all
.
When migrating to shallower waters to feed, Seibel said, their metabolic rate rises
rapidly.


Models that enhance the effect of temperature on metabolic rate in vertically migratory animals suggest that climate change will expand the available habitat of vertically migratory animals by 10-20 degrees
north and south by the end of the century, Seibel said.


"We really need to delve into animal physiology to better understand the way
various species evolve and adapt to environmental conditions," Seibel said.