FISH POPULATION DYNAMICS Fish Population Dynamics includes temporal (seasonal or year-to-year) variation in: population numbers age structure biomass

FISH POPULATION DYNAMICS Information on Fish Population Dynamics is used to: Determine the current status of a fishery Develop fisheries management plans Evaluate management success and failure

FISH POPULATION DYNAMICS 2 Major Subcategories Population Assessment Modeling Population Trends (dynamics of a population under different management scenarios)

Key Demographic Processes that Cause Populations to Change over Time Births Immigrants Deaths Emigrants Population Change = B + I – D – E

Factors that Cause a Population to Change can be the result of either Density-Dependent (D-D) Density-Independent (D-I) Processes

Density-Dependent Processes Demographic rates (b,d,i,e) are related to population density. Forces: Food availability, availability of spawning habitats, predation, cannibalism, disease, parasites, exploitative vs. interference competition Example: With increasing fish density, there is a reduction in the amount of food per fish. This results in reduced fish growth and condition. With reduced condition, there is an increase in fish mortality rates and a reduction in fish reproductive rates. This causes the rate of population increase to decrease with increasing population density.

Density Dependent Mortality and Recruitment (simple linear)

Density Independent Processes Demographic rates are variable from year-to-year but not in relation to density. Forces: water temperature, flow extremes, water chemistry variability, demographic stochasticity Example: year-to-year variation in the severity of spring time flows causes scour of stream bottoms. This results in high mortality rates of trout eggs and larvae, and mortality rates are independent of the number of eggs present to start with.

Density Independent Mortality and Recruitment

D-D and D-I Interaction

Relative Importance of D-D and D-I In General: Ponds, Lakes, Oceans are dominated by D-D processes with D-I processes important but secondary. Rivers and Streams are dominated by D-I processes with D-D processes important but secondary. Most populations regulated in part by both D-D and D-I processes.

Fish Population Modeling Births Immigrants Deaths Emigrants Population Change = B + I – D – E

EXPONENTIAL MODEL OF POPULATION GROWTH Nt+1 = Nt x (1+R) Nt = N0 x (1+R)t Where R = b – d = “Finite Rate of Population Increase”

EXPONENTIAL MODEL R=0.15 R=0.13 R=0.10

Logistic Model of Population Growth Nt+1 = Nt + Nt x R x (1- Nt / K) Where: K = Carrying Capacity Maximum population size that can be supported in a particular environment. Encompasses many potential limiting factors: food, space, shelter, mates

LOGISTIC MODEL OF POPULATION GROWTH Nt = K; N Nt < K; N Nt > K; N

LOGISTIC MODEL OF POPULATION GROWTH

Characteristic Dynamics of Fish Populations Equilibrium Concept – Populations tend to stay at or near a certain level

Complementary vs Supplementary Habitats Complementary Habitat: necessary for the completion of an individual’s life cycle and maintenance of the population Supplementary Habitat: unnecessary but results in increased population productivity (density and/or biomass)

Complementary vs Supplementary Habitats: Steelhead Example Small Stream Ocean Reproduce Forage Refuge Forage Small Streams COMPLEMENT Ocean Ocean SUPPLEMENTS Small Streams

Scale of Spatial Links is Determined by Movement Rates Max Distance = 225 m Mottled Sculpin Brook Trout Max Distance = 6.5km

Scale of Spatial Links 2m 2km R R F Sculpin F Re Re R Brook Trout R Re

Good for reproduction groundwater stable temp stable flow bed-moving flows rare Good for eating high light high productivity lots of small fishes

Headwaters Larger Tributaries Mainstem

Despite higher summer temperatures in the mainstem

High productivity leads to high growth rates Mainstem

Brook trout survive summer by finding coldwater “pockets” in the mainstem