11/25/13. Population Ecology of Fishes. I. Population structure

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1 Population Ecology of Fishes I. Population Structure II. Demographic Rates III. Population Dynamics & Regulation IV. Intrapopulation Structure V. Determinants of Population Size in Fishes The central question of population ecology: What determines population abundance or density? Abundance vs. density: abundance = number of individuals density = number / unit area or volume What is a population? Ecological definition: a group of individuals of the same species in a specified area I. Population structure Closed: no exchange of individuals among populations X Evolutionary / Genetic definition: a group of interbreeding individuals Open: individuals move among populations Most marine reef fishes live in open populations because... adult habitat is patchy adults are sedentary there is dispersal of planktonic larvae Many fishes live in metapopulations Metapopulation: a collection of spatially isolated open subpopulations connected by dispersal among them Most freshwater fishes live in closed populations because... habitats are physically isolated (lakes, ponds, streams) generally lack planktonic larvae the metapopulation is a closed population at a large enough spatial scale, all populations are closed 1

2 II. Demographic rates Population size is set by the rates of gain & loss: demographic rates Gains: births immigration Losses: deaths emigration Settlement is the demographic equivalent of birth in demersal fishes with planktonic larvae settlement occurs when a larvae leaves the water column and associates with benthic habitat closed population births + deaths open population (demersal fishes) settlement + deaths reproduction Bipartite life cycle of benthic marine fishes with pelagic larvae Recruitment: not quite settlement recruitment is a proxy for settlement -- used because settlement is very difficult to measure recruitment = # settled # dead before recruitment census Fishery definition: Recruitment = the addition of new individuals into the fishable population III. Population Dynamics and Regulation Population regulation: bounded fluctuations in abundance 1. What causes differences in abundance over and space? 2. What keeps populations from going extinct or increasing without bounds? abundance tight regulation loose regulation 2

3 without regulation populations go extinct or increase to infinity (theoretically) Causes of fluctuations and regulation no regulation no regulation two levels of explanation: abundance demographic = changes in demographic rates ecological = the factors that cause the demographic rates to change Population Growth (per capita rate) per capita rate Demographic causes of population regulation: Dependence 1 Birth Death Population K Time Ecological causes of population regulation Intraspecific Competition Disease/Parasitism Predation Intraspecific Competition resources become limited mortality increases or birth/settlement declines potentially limiting resources: Space/Shelter Food (Forrester & Steele 24).4.3 Shelter abundance low medium high Mortality Goby density (# per m 2 ) 3

4 Experiment: vary refuge abundance and goby density on small plots (4 x 4 m) Ambient refuge abundance Disease/Parasitism increased rates of transmission at high densities causes density-dependent mortality Refuge Addition salmon with sea lice copepods Predators can cause competition for shelter and densitydependent mortality 4 types of predatory responses that cause density-dependent mortality: 1. functional response 2. aggregative response 3. developmental response 4. numerical response 1. Functional response how rate of prey capture / predator changes with prey density 3 components to prey capture Search Capture Handling 3 general types of functional responses only one can regulate prey populations Type I Type II Type III Type I functional response not density-dependent prey per capita death rate Type II functional response not density-dependent (inverse density dependence) prey per capita death rate # Prey killed / Predator # Prey killed / Predator 4

5 Type III functional response density-dependent at low prey densities prey per capita death rate Can get type III functional response by: switching # Prey killed / Predator search images limited prey refuges 2. Aggregative response 3. Developmental response # of predators or spent by each predator Predator size IV. Intrapopulation Structure Predator density 4. Numerical response Not all individuals in a population are the same! Structure: relative abundance of individual traits among individuals in a population 1. age 2. size 3. stage (e.g., larvae, juveniles, adults) 4. sex 5. genetic (distribution of genotypes throughout population) 6. spatial (distribution and interaction of individuals within and among populations) all of these can influence per-capita rate of mortality (d) and reproduction (b) 5

6 Size Structure is very important in fishes variation in size typically large (e.g., blue marlin: 2 mm - 5, mm & 2, lb.) competitive abilities typically set by size (although very different sizes of same species may use completely different resources) cannibalism is common V. Determinants of population size in reef fishes 1. Pre-Settlement Processes reproductive output larval dispersal set larval supply larval mortality 2. Settlement Processes settlement cues prior residents 3. Post-Settlement Processes predation competition Importance of larval mortality mortality of larvae in plankton high (>99.999%) slight change in mortality rate = big change in survival to % mortality =.1 to.2% change in survival i.e., survival doubles Thus, fishes with a planktonic larval stage are inherently likely to have variable settlement Causes of variability in larval mortality 1. Critical stage need high food abundance at first feeding 2. Larval advection get carried away 3. Larval predation variation in predation variation in recruitment not mutually exclusive no consensus on which is most important difficult to study larvae What controls dynamics of populations of reef fishes? 1. Larval supply 2. Competition 3. Predation Determination by larval supply predictions: add more young fish, get more adults add resources get no extra fish density-independent mortality Mortality of settlers of adults 6

7 Determination by competition predictions: add more young fish, get no extra adults add resources get more fish density-dependent mortality Determination by predation predictions: depend upon whether mortality is densitydependent or density-independent -independent -independent Mortality of settlers 1. of adults K Mortality of settlers 1. of Adults -dependent -dependent Do predators cause density-dependent mortality in reef fishes? a case study (Forrester & Steele 2) blackeye goby bluebanded goby bridled goby Usually, many factors affect populations simultaneously need to measure their relative importance case study: temperate reef gobies (Steele 1997, 1998) Mortality No Yes No + P - P Bluebanded goby Blackeye goby Predation Larval Supply Competition Competition Larval Supply Predation Importance of fish 7