Population dynamics and monitoring fish populations Flashcards
Are fish a renewable resource?
Yes as they naturally replace themselves by breeding to produce offspring that replace the adults that have died
How have all species breeding rates evolved?
to have a natural breeding rate which exceeds the necessary rate to sustain the population
What will the breeding surplus over carrying capacity not do?
increase the populaiton longterm as the carrying capacity of the environment will have been exceeded and the surplus individuals will die
What is carrying capacity?
The maximum number of individuals from a certin species that can be provided for by the ecosystem
What will control any population that exceeds carrying capacity?
density dependant factors will increase the mortality rate especially intra-specific competition for food
What does intra-species mean?
When individuals of the same species compete against each other
What happens if the adult mortality rate increases?(i.e. becuase of fishing)
more juvenile fish will survive and be recruited to the adult population as more food will be avaliable and intra-specific competition will have reduced (maintain population)
What happens if an excessive number of the adult population are caught? (fishing)
Adult populaiton may decline becuase there are not enough young to replace them
Fewer young produced reducing ability for population to recover (rapid popuation decline)
What means overfishing might not be evident in reduced catches?
as the total weight may be maintained by catching a larger number of smaller fish
What can the catching of smaller fish lead to?
adult breeding population may be reduced to the point where evry few young are produced resulting in total populaiton collapse
What is the main problem with fish size and population management?
Some fish breed before reaching a catchable size there will always be an adult breeding population
If a fish reaches catchable size before breeding all breeding adults will be caught destroying the population
What is maximum sustainable yield (MSY)?
for fish it is the greatest biomass that can be removed from the population each year without the populaiton suffering long term decline
Why is it important that MSY is not excceded?
would cause overfishing and population decline
What is an example of a fish that is easily overfished?
the orange roughy
Why is the orange roughy so easily overfished?
K-selected species
Lives for 150years
Only reaches sexual maturity at 30yrs old
What is needed to estimate MSY? (General)
reliable data
What formula can be used to estimate MSY?
Russell Formula
What is the Russell formula? (Actual formula)
S2=S1+(A+G)-(C+M)
What is S1 in the Russell formula?
biomass of stock at the begining of the year
What is S2 in the Russell formula?
biomass of stock at the end of the year
What is A in the Russell formula?
biomass of young fish added to stock
What is G in the Russell formula?
Biomass added by growth of all fish in the stock
What is C in the Russell formula?
Biomass caught by fishing
What is M in the Russell formula?
Biomass lost through natural mortality
what is the only factor in the Russell formula that is easy to measure?
the biomass caught by fishing as the catch is landed t ports and the mass caught can be recorded
Why might it be difficult to collect data about fish in the sea?
Fish populations are often mobile and move long distances often between different countries’ territories
Distribution of populations uneven
Representative data of large enough proportion of the total area occupied by the fish may be impractical
What more practical thing can be done instead of just collecting fish data from the sea?
collect data from fish that are caught which indicates whether the structure of the population is changing (fishing above or below MSY)
What data can you get from fish catches?
catch size
catch per unit of fishing effort
Mean fish size
Mean age
Why is catch size not always a good measure of the biomass or population size of an exploited species?
Technological changes may make it possible to maintain catches while the population is declining
What technological advances are allowing catches to remain stable with a reducing population?
Bigger boats
Bigger nets
Improved fish detecting systems (sonar)
What is the problem with catch per unit fishing effort?
difficult to measure
Why is catch per unit fishing effort better than catch size?
better measure of fishing sustainability
How do you know if a population is getting overfished with catch per unit fishing effort?
if it is becoming harder to maintain catches then over fishing is occurring
Why would you you mean fish size as an indicator of overfishing?
catch biomass may be maintained but this can hide the fact that smaller fish are being caught
Reducing fish size may indicate that the older adults are being caught faster then replaced
How can the age of a fish be determined?
growth of rings on the scales that grow on their skin
Otoliths - bones in their ears
How do you know if overfishing is occurring with mean age?
if the mean age is declining
What do the people fishing only have knowlegde of fish populaitons?
fish that are large enough to be caught in their nets
What can data from fish catches be sued to estimate?
estimation of the current catchable population
Why are the fish in the younger year groups important?
as they will be recruited to the adult populaiton in the future and replace those that are caught or lost
What is the problem if there are too few fish in the younger year groups?
population could suddenly collapse
When do most fish eggs and larvae die?
in the first few weeks when they are planktonic
Why do fish eggs/ larvae die in the early stages?
eaten by predators
fail to find food
suffer accidental deaths (storms, carried to unsuitable area by currents)
What are density dependant limiting factors?
factors that depend on population size
(competition, predation,food, diseases and paratism)
What are density independant limiting factors?
Affect all populations in a similar way independant of population size
(drought, wild fires, volcanic eruptions, hurricane and deforestation)
What can vary with young fish year in year?
survival rate so knowing the populaiton of breeding adults is not that accurate to number of young produced
What can help to predcit the number of young fish that are ikely to be recruited to the adult populaiton in future years?
Collecting data on planktonic fish egg and larval populations
What is the issue if data on planktonic eggs and larvae isnt collected?
possible several years of poor egg survival may lead to poor recruitment and populaiton collapse as fishing amount couldnt be reduced to match MSY
How must information abot breeding success and survival of juvinile fish collected?
scientific research programmes
