Chapter 8-9 - Growth

Absolute population size

total number of all animals or total biomass

Closed population

population that remains unchanged during the study period (migration, mortality, and recruitment negligible)

Relative population size

estimates based on data assumed to be correlated with absolute abundance size

often called an index of abundance

(ex. number of cougar tracks)

Relative population estimates are easier and less expensive to obtain, BUT it only works if…

the metric is proportional to absolute abundance

N = qU

(U = relative abundance)

(N = absolute abundance)

(q = a scalar)

Mark-recapture theory

the principle that the proportion of marked individuals in a second sample (R/M2) equals the original marked proportion of the total population (M1/N)

CPUE

a relative index of population abundance developed from fishery-dependent observations

U = C/f

Catchability coefficient (q)

the proportion of the population captured by a standard unit of fishing effort, relating CPUE to true population size

Describe what environmental factors (both biotic and abiotic) might affect CPUE. 

Abiotic - water temperature, salinity, oxygen levels, weather/climate, habitat structure, currents/tides

• All of the above effect animal density, how they behave, and how effective fishing gear will be. For example, strong currents and tides will influence how fish move and might create CPUE peaks. 

Biotic - prey availability, predators, population size, migrations or spawning, disease

• All of the above will also affect density and behavior. For example, prey availability might increase or decrease CPUE, dependent on how well-fed and willing fish are to take bait. 

Explain when/why electrofishing may be used to estimate fish population.

• very quick and efficient for rapid testing

• works in shallow, low salinity habitats (streams, lakes, etc.)

• good for understanding current fish stock, predators, and forage availability 

Explain when/why various distance methods (such as line transects or visual surveys) may be used to estimate fish population.

• good for long term monitoring of large areas and collecting absolute density without the need for physical capture

• works best in clear waters for reef fish or large pelagic species

Explain when/why mark-recapture may be used to estimate fish population.

• for when we need true population estimates/models and not just an index

• when fish are hard to count directly

• when CPUE is unreliable

• good for tracking movement and life cycles

Principle of Parsimony / “Ockham’s razor”

the most acceptable explanation of an occurrence, phenomenon, or event is the simplest, involving the fewest entities, assumptions, or changes

Variables VS Parameters (in models)

Variables - entities that can be defined and associated to observations from nature

Parameters - help define the relationships between variables

Life history

major events relating to an organism’s growth, development, reproduction, and survival

Life history strategy

timing and nature of life history events, averaged across all the individuals of a species or population

Why do we study life history?

To understand and model population dynamics

To develop quantitative relationships for factors such as: growth, fecundity, weight/length, gender balance, natural mortality, etc.

Growth is the combination of…

anabolism (A) and catabolism (C)

for growth to occur, the biomass added by anabolism must exceed that lost by catabolism (A > C)

Anabolism is a function of the _____ of an animal (ie. area), while catabolism is a function of the _____ of an animal.

surface (S)

volume (V)

How do we age fish?

• Otoliths

• Spines

• Vertebrae

Marginal increment analysis

measure from last annulus to edge

Fecundity

the number of fertilized eggs produced by a mature female

True or False: Fish have higher fecundity compared to other species.

True

Fecundity in fish tends to _____ (increase/decrease) with length.

increase

Total spawners / total fecundity

fish that release one batch of eggs per breeding season

Batch spawners / batch fecundity

release multiple batches of eggs in one season

Compensatory density dependence

decrease in recruits per spawner as spawning stock increases (i.e. can be due to limited shared resources)

Depensatory density dependence

increase in recruits per spawner as spawning stock increases (i.e. can be due to compensating for predation)

Allee / Founders effect

decreased reproductive success at low population densities

Sequential hermaphroditism

the ability to switch genders

Protogynous hermaphrodites

female → male

ex. groupers

Protandric hermaphrodites

male → female

ex. snooks

What are the two most commonly used stock-recruitment relationships?

1. Beverton and Holt

2. Ricker

Beverton and Holt (stock recruitment)

incorporates density-dependent survival rates reflecting intracohort competition for resources

predicts that recruitment saturates at high stock levels

contest competition

Ricker (stock recruitment)

density dependence mortality term for eggs and juvenile fish relates to total stock size rather than cohort size

predicts that recruitment declines at high stock levels

scramble competition

Spawning stock biomass

a measure of the reproductive productivity of the mature population (optimal measurement is number of eggs spawned) – can be estimated from average fecundity

Growth overfishing VS recruitment overfishing

Growth - stock is fished so hard that most individuals are caught at a relatively small size – classic yield-per-recruit problem of balancing stock loss against stock gains from growth (both individual animal growth and population growth)

Recruitment - occurs when a stock is fished so hard that the stock size is reduced below the level at which it can produce enough new recruits to replace those dying

Spawning potential ratio (SPR)

the impact that fishing has on the ability of each recruit to contribute to spawning

compares the spawning ability of a stock in the fished condition to the stock’s spawning ability in the unfished condition