Population structure

• Lecture Outline

– PSD and RSD

• Assignments

– None

Why study population structure?

1) Evaluate processes that regulate abundance and biomass

a) Reproduction – Production of viable offspring

b) Recruitment –Number of fish born in a year that survive to reproductive size

c) Growth – addition of biomass by individuals

2) Assessment of the harvest potential of a fishery

Population Balance

1) Swingle (1950) defined a balanced population

a) Sustain a satisfactory harvest of good-size fish in proportion to the productivity of the habitat

b) I.e., a balance between catchable-size fish and small fish for replacement

Age-frequency histograms

1) Used to describe status of successful cohorts

2) Used to identify excessive mortality

3) Can be tracked over time to evaluate growth

Proportional Stock Density

1) Stock size – minimum length at maturity (Anderson, 1980), effectively sampled by fishing/fisheries gear, or that provides recreational value

2) Quality size – That which most anglers like to catch

3) PSD Values from Table 7.1 (IFM)

a) Largemouth 40 – 70 %

b) Bluegill 20-40 %

c) Walleye, N. pike, Muskellunge, smallmouth bass -30 – 60%

Relative Stock Density

1) 5 cell model by Gabelhouse (1984)

a) Based on percentage of world record length

2) Typical length-groups

a) Stock, quality, preferred, memorable, trophy

Considerations when using PSD and RSD

What do stock indices tell us?

1) Quantifies length frequency histogram

2) Has been shown to be related to

a) Population density

b) Growth

c) Mortality

d) Predator-prey dynamics

Pros and Cons of Stock Density Indices

1) Effective for rapid assessment of many populations

2) Universal across all states and widely accepted

3) Is only ONE tool used for population assessment

4) Need other pieces of information for a more complete picture of the population

a. CPUE, condition, growth, etc.