Growth and Mortality

• Lecture Outline

– Growth

– Mortality

 

 

• Assignments

– Pp. 497 – 506 in Fisheries Techniques; pp. 140 – 155 Inland Fisheries Management 

Growth

1) Change in size or calories over time

2) Fishes have indeterminate growth

3) Growth = I – M – E

a) I = ingested materials

b) M = metabolic losses

c) E = excretory losses

4) Factors affecting growth

a) Temperature, dissolved oxygen, ammonia, salinity, competition, food quality, age, photoperiod, exercise

Measuring growth

1) Raise in controlled environment

2) Mark-recapture

3) Length-frequency

4) Back-calculated from hard structure

von Bertalanffy Model

1) Rate of change in length per unit of time (dl/dt) will get smaller and eventually become zero as fish reaches its maximum size

Mortality

1) Natural mortality

a) Predation, diseases, weather, etc…

2) Fishing mortality

a) Harvest

3) Total Mortality

a) Natural plus fishing mortality

4) Recruitment – the addition of fish to the exploited portion of the stock

5) Prerecruitment mortality

a) Only natural mortality

6) Postrecruitment mortality

a) Both fishing and natural mortality

Estimation of Total Mortality: Exponential model

1) Assumptions

a) Reproduction is constant from year to year

b) Survival is equal among age-groups

c) Survival is constant from year to year

2) Are these assumptions realistic?

Fishing mortality

1)     Rate of exploitation – proportion of fish dying from fishing in a given time period

2) Z = M + F

3) Assume F = amount of fishing effort (qf)

4) Z = M + qf

a) Does this look familiar?

5) Considerations

a) Catchability coefficient is constant over time and age-groups

b) Requires several years of data