Population dynamics and monitoring fish populations

Question 1

Are fish a renewable resource?

Answer 1

Yes as they naturally replace themselves by breeding to produce offspring that replace the adults that have died

Question 2

How have all species breeding rates evolved?

Answer 2

to have a natural breeding rate which exceeds the necessary rate to sustain the population

Question 3

What will the breeding surplus over carrying capacity not do?

Answer 3

increase the populaiton longterm as the carrying capacity of the environment will have been exceeded and the surplus individuals will die

Question 4

What is carrying capacity?

Answer 4

The maximum number of individuals from a certin species that can be provided for by the ecosystem

Question 5

What will control any population that exceeds carrying capacity?

Answer 5

density dependant factors will increase the mortality rate especially intra-specific competition for food

Question 6

What does intra-species mean?

Answer 6

When individuals of the same species compete against each other

Question 7

What happens if the adult mortality rate increases?(i.e. becuase of fishing)

Answer 7

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)

Question 8

What happens if an excessive number of the adult population are caught? (fishing)

Answer 8

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)

Question 9

What means overfishing might not be evident in reduced catches?

Answer 9

as the total weight may be maintained by catching a larger number of smaller fish

Question 10

What can the catching of smaller fish lead to?

Answer 10

adult breeding population may be reduced to the point where evry few young are produced resulting in total populaiton collapse

Question 11

What is the main problem with fish size and population management?

Answer 11

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

Question 12

What is maximum sustainable yield (MSY)?

Answer 12

for fish it is the greatest biomass that can be removed from the population each year without the populaiton suffering long term decline

Question 13

Why is it important that MSY is not excceded?

Answer 13

would cause overfishing and population decline

Question 14

What is an example of a fish that is easily overfished?

Answer 14

the orange roughy

Question 15

Why is the orange roughy so easily overfished?

Answer 15

K-selected species
Lives for 150years
Only reaches sexual maturity at 30yrs old

Question 16

What is needed to estimate MSY? (General)

Answer 16

reliable data

Question 17

What formula can be used to estimate MSY?

Answer 17

Russell Formula

Question 18

What is the Russell formula? (Actual formula)

Answer 18

S2=S1+(A+G)-(C+M)

Question 19

What is S1 in the Russell formula?

Answer 19

biomass of stock at the begining of the year

Question 20

What is S2 in the Russell formula?

Answer 20

biomass of stock at the end of the year

Question 21

What is A in the Russell formula?

Answer 21

biomass of young fish added to stock

Question 22

What is G in the Russell formula?

Answer 22

Biomass added by growth of all fish in the stock

Question 23

What is C in the Russell formula?

Answer 23

Biomass caught by fishing

Question 24

What is M in the Russell formula?

Answer 24

Biomass lost through natural mortality

Question 25

what is the only factor in the Russell formula that is easy to measure?

Answer 25

the biomass caught by fishing as the catch is landed t ports and the mass caught can be recorded

Question 26

Why might it be difficult to collect data about fish in the sea?

Answer 26

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

Question 27

What more practical thing can be done instead of just collecting fish data from the sea?

Answer 27

collect data from fish that are caught which indicates whether the structure of the population is changing (fishing above or below MSY)

Question 28

What data can you get from fish catches?

Answer 28

catch size
catch per unit of fishing effort
Mean fish size
Mean age

Question 29

Why is catch size not always a good measure of the biomass or population size of an exploited species?

Answer 29

Technological changes may make it possible to maintain catches while the population is declining

Question 30

What technological advances are allowing catches to remain stable with a reducing population?

Answer 30

Bigger boats
Bigger nets
Improved fish detecting systems (sonar)

Question 31

What is the problem with catch per unit fishing effort?

Answer 31

difficult to measure

Question 32

Why is catch per unit fishing effort better than catch size?

Answer 32

better measure of fishing sustainability

Question 33

How do you know if a population is getting overfished with catch per unit fishing effort?

Answer 33

if it is becoming harder to maintain catches then over fishing is occurring

Question 34

Why would you you mean fish size as an indicator of overfishing?

Answer 34

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

Question 35

How can the age of a fish be determined?

Answer 35

growth of rings on the scales that grow on their skin
Otoliths - bones in their ears

Question 36

How do you know if overfishing is occurring with mean age?

Answer 36

if the mean age is declining

Question 37

What do the people fishing only have knowlegde of fish populaitons?

Answer 37

fish that are large enough to be caught in their nets

Question 38

What can data from fish catches be sued to estimate?

Answer 38

estimation of the current catchable population

Question 39

Why are the fish in the younger year groups important?

Answer 39

as they will be recruited to the adult populaiton in the future and replace those that are caught or lost

Question 40

What is the problem if there are too few fish in the younger year groups?

Answer 40

population could suddenly collapse

Question 41

When do most fish eggs and larvae die?

Answer 41

in the first few weeks when they are planktonic

Question 42

Why do fish eggs/ larvae die in the early stages?

Answer 42

eaten by predators
fail to find food
suffer accidental deaths (storms, carried to unsuitable area by currents)

Question 43

What are density dependant limiting factors?

Answer 43

factors that depend on population size
(competition, predation,food, diseases and paratism)

Question 44

What are density independant limiting factors?

Answer 44

Affect all populations in a similar way independant of population size
(drought, wild fires, volcanic eruptions, hurricane and deforestation)

Question 45

What can vary with young fish year in year?

Answer 45

survival rate so knowing the populaiton of breeding adults is not that accurate to number of young produced

Question 46

What can help to predcit the number of young fish that are ikely to be recruited to the adult populaiton in future years?

Answer 46

Collecting data on planktonic fish egg and larval populations

Question 47

What is the issue if data on planktonic eggs and larvae isnt collected?

Answer 47

possible several years of poor egg survival may lead to poor recruitment and populaiton collapse as fishing amount couldnt be reduced to match MSY

Question 48

How must information abot breeding success and survival of juvinile fish collected?

Answer 48

scientific research programmes