population dynamics and monitoring fish populations Flashcards
what kind of resource are fish?
renewable
they naturally replace themselves by breeding to replace the adults that have died
what do all species have because of evolution?
a natural breeding rate
that exceeds the rate necessary to sustain the population
the natural breeding rate exceeds the necessary to sustain the population. why will a breeding surplus not increase the long term adult population?
surplus individulas will die because the carrying capacity of the environment has been exceeded
what will increase in a population where the carrying capacity of an environment has been exceeded?
density dependent factors which will increase mortality rate
e.g. intraspecific competition for food
how is a fish population maintained?
if adult mortality rate increases, more juvenile fish will survive
more will be recruited to the adult population because food is available & intraspecific competition will be reduced
what may increase the adult fish mortality rate?
fishing
explain how a reduced adult population can result in population decline
if an excessive number are caught, number of adults declines because there isnt enough young to replace them
less adults, less young produced
this further reduces the ability of a population to recover
how does catching fish, not reduce the total population?
as long as the catch doesnt exceed the populations ability to replace the losses
why might overfishing not be obvious?
the total catch weight may be maintained by catching a larger number of smaller fish
why is overfishing dangerous?
the fishing may continue if it is not obvious
this can result in total population collapse because the breeding population is exploited so very few young are produced
what happens if a fish reaches catchable size after it has started breeding?
means there will always be a breeding population
overfishing however may remove all the large fish
what happens if a fish reaches a catchable size before it is large enough to breed?
it is possible to catch all the breeding adults and destroy an entire population
MSY
Maximum sustainable yield
what is the MSY for a fish population?
the greatest biomass that can be removed from the population each year without the population suffering long term decline
what could cause a fish population to exceed the MSY?
overfishing
3 examples of fish that have been exploited above the MSY
orange roughy
atlantic cod
tuna
what is the orange roughy?
a deep water fish species
where is the orange roughy found?
off coasts of new zealand, SE australia, the west of ireland and NW scotland
when was the orange roughy first exploited?
late 1970s
many populations soon overfished
why is the orange roughy so vulnerable to over-exploitation?
it is a K selected species
the fish live to 150 but dont start breeding till over 30 and produce few eggs
why are atlantic cod so vulnerable to ovefishing?
they reach a catchable size at age 2 but not maturity until 4
overfishing can remove most of the breeding population
why are tuna vulnerable to overfishing?
tuna start breeding once they become large enough to catch so fishing can remove the breeding population
what formula is used to calculate the MSY of fish
russel formula
russell formula
S2=S1+ (a+g)-(c+m)
requires reliable data in all categories for an estimate
S1 in the Russell formula
biomass of stock at the beginning of the year
S2 in the russell formula
biomass of stock at the end of the year
A in Russell formula
biomass of young fish added to stock
G in Russell formula
biomass added by the growth of all fish in the stock
C in Russell formula
biomass caught by fishing
M in Russell formula
biomass lost through natural mortality
which factor of the russell formula is easiest to measure?
the biomass caught by fishing because the catch mass landing in ports can be recorded
the other factors of the russell formula require data about the fish in the sea. give 3 reasons this is difficult to obtain?
- fish populations are mobile and move long distances
- the distribution of the populations is uneven
-collecting data on a large enough proportion of the total area is impractical
what can be done instead of data being collected from fish in the sea- because that is impractical?
more practical to collet it from fish caught
can indicate whether population structure is changing and if fishing is above or below MSY
give 4 types of data available from fishing catches
catch size
catch per unit fishing effort
mean fish size
mean age
