Marine biology- part 3
Bycatch- animals that are accidentally caught while fishermen are trying to capture something
Reasons for being caught in net:
o Curious, attracted to large fish, don’t notice net, not understanding danger.
o 20,000,000 tonnes
o 25-40% total global catch in a single year bycatch
There are two types of nets:
o Still nets- stationary
Weir- moderate bycatch, but porpoise common among herring weirs.
V-shaped fence- like structures that channel fish in a pen dug in a riverbed.
Gill nets- fish trapped by gills when they try to force through net
Pingers were developed in Canada and are acoustic beacons that emit and
intermittent, short, high pitched noise which are attached along nets to prevent
marine mammals from being trapped.
Expensive, scare away fish, get use to and ignore
Mammals particularly threatened are: harbour porpoises(north American
coastline), north Atlantic right whale (the Bay of Fundy), Humpback whale
o Mobile nets- moving
“Backdown procedure”- vessel reverses which allows part of the gear to sink
below the waterline and dolphin can escape.
New Zealand there have been changes to gear, a large grid of netting is placed
near the rear of the netting which allows the fish to pass through but forces
marine mammals to surface where they can escape.
Purse seine tuna fishery (Pacific Ocean) # of dolphins entangled was in the
2000-4000 dolphins die each year as bycatch and 1000 marine animals per day
die from being trapped.
Entanglement in nets in the main reason why the Vaquita (small porpoise) and
new Zealand sea lion are threatened with extinction and why the Chinese river
dolphin in extinct.
Habitat disruption/ destruction
o Grey whales give birth and raise their young in tropical lagoons while seals need ice
packs to give birth.
o Sirenians feed in shallow seagrass beds while dolphins feed in the seas and oceans for
o Polar bears roam the Artic looking for food while baleen whales swim the oceans
looking for krill.
Habitat destruction can be grouped into the following categories:
o Physical damage or changes to their environment o Disturbances from introduced human activities (noises)
o Competition for food sources
o Ship strikes
Physical damage or changes to their environment
Creating industrial harbours where natural estuary or marshland, wetlands drained for farming
or tourism (resorts) and alterations of rivers for drinking sources.
Indus River in Pakistan has been diverted by dams and into canals which has resulted in ove4r
half the historical range of Indus river dolphins being lost.
Extinct Chinese River Dolphin or Baiji was seriously affected by building of dams along the
Yangtze/Kiang river system and polluting their habitat.
Deforestation of the tropical forests affects freshwater supply and habitats to Amazon manatee
and Amazon River dolphins.
Disturbances from introduced human activities (noises)
These activities produce frequencies in the range of 10 to 500Hz with water craft noises in the
range of 2- 20kHz and military sonar ranging between 2-500 kHz
Baleen whales more sensitive to low frequencies below 5kHz (larger water crafts, oil/gas
activities, dredging and construction)
Dolphins and toothed whales more sensitive to above 10kHz (active sonar, high speed water
These noises affect marine mammals in two ways: causes changes in behaviour and physical
Competition for food sources
Fisheries- 2 types of interactions between marine mammals and humans. Biological and
o Competition btwn fishers and marine mammals for same prey species as well as
transmission of parasites btwn fishery resource and marine mammals.
o Pinnipeds commonly act as intermediate host for parasites (ex. Codworm, the host
being the gray seal, larvae infest the muscle tissue of many fish (mostly the highly prized
cod fish) and render the flesh less marketable.
o Direct biological events are when the same pray fish is hunted directly by both marine
mammal and human.
o Indirect biological marine mammals hunt prey that serves as food for others species that
are commercially hunted.
Operational interaction occurs in three different types of situations:
o Depredation or when marine mammals take bait or fish from fishing gear o Disturbance when marine mammals disturb the fishing operation
o Damage caused to fishing gear
o Fishing operations concentrate the prey which makes them easier to hunt
o New species are available that would normally be outside of the normal hunting range
o Estimates of fish loss is 1-8% of total catch, however with some species such as the
California sea lion catch loss can range from 25-100% per attack.
o Occurs when marine mammals cause the prey disperse and dive rapidly thereby
reducing the amount netted.
o Pinnipeds may be caught in the nets and be brought aboard which disrupts operations.
o Occurs when marine mammals try and remove the prey from the nets or hooks which
causes holes in the netting or loss of hooks.
o When animals become trapped in the gear, damage usually occurs.
o Repairs are costly or replacement of the gear can be lengthy, lost income.
The competition for food between humans and marine mammals may result in an imbalance in
the complex predator-prey relationship.
During the 20 century when baleen whales were commercially harvested, their total biomass
(total body weights of the species) went from 43 million to 6.6 million tons.
This resulted in a surplus of 153 million tons of krill which caused the remaining whales to grow
faster, reach sexual maturity faster and lead to an increase of pregnancy rates.
These effects were also seen in the Antarctic crabeater seal and several seabirds.
In the North Atlantic, there has been drastic drops in fish stocks of mackerel, herring and cod,
which hunted other fish species such as sand lance, sprat and gadoid species.
Harbour porpoise and Humpback whales (cetceans) switched to these now abundant species
and not only ate more of those species but changes geographical locations to hunt for them.
The grey seal population has increased by approx. 7% per year eating the very abundant sand
Sand lance ate more krill which caused local numbers of right whales to decline but Atlantic
white sided dolphins became more abundant.
The Florida manatee and the North Atlantic right whale have been affected. 50 manatees are killed each year by water craft and numerous others injured or harassed.
Since 1970’s new legislation has managed to reduce the risk of strikes or disturbances by having
designated protected areas, reducing speed limits of water craft and even prohibiting human
activities in sensitive habitats.
New environmental hazards are (perinatal deaths, cold stress)
Ship strikes have been a serious threat to the North Atlantic right whale approx. 300 left
Ecology and Biogeography
Ecology is the study of the relationship between animals and their environment.
Biogeography is the study, both past and present, of the pattern of geographic distribution
of organisms and all the factors that influence those patterns.
These 2 areas of biology allow for the description, interpretation and understanding of
patterns of distribution of the various marine mammal species.
This results in having to unders