Coral reef fish population ecology

  1. Coral reef fishes: population processes
    1. An important 'model system'
      1. accessibility of habitat to investigators
      2. high diversity of fishes
        1. 30% to 40% of all fish species are reef associated.
        2. accounting for this diversity has kept many ecologists busy for decades
        3. global diversity highest in Philippines (fig 1 Sale 1980).
        4. what permits coexistence of so many species?
    2. Life history characteristics
      1. broadcast or demersal spawners
      2. daily or biweekly or monthly spawning
      3. larvae may be highly dispersive
        1. larvae period is weeks or months long
      4. adults do not disperse: once recruited, stay in an area (Sale 1991 fig 1 p 565)
      5. survivorship (fig. 23.5):
        1. At least 99.9% of larvae die before settling
        2. even after settlement, type III survival curve
    3. Processes that might limit abundance (fig. 23.6)
      1. One way to divide up is pre-recruitment vs. post-recruitment
        1. prerecruit: starvation of larvae, predation, transport away from reefs
        2. postrecruit: influences on juveniles and adults
      2. Another way: deterministic/stochastic
        1. generally, postsettlement processes expected to be more density-dependent, deterministic
          1. are resources limiting?
        2. presettlement processes are stochastic?
          1. competition among larvae unlikely, limiting resources not likely
  2. Studies focusing on limiting resources
    1. Robertson 1991: food limitation
      1. urchins died off because of disease. This species known to control biomass of turf algae
      2. Robertson worked with long-term datasets he keeps
      3. adult numbers of two species of herbivorous surgeonfishes does increase after die-off. Delayed response in one sp. (fig 1). Third species feeds off-reef.
      4. no change in recruitment
      5. so surgeonfish do compete with urchins, and so are limited by food resources.
      6. In this case, some evidence for regulation of populations and community by resources
    2. Testing for competition for sleeping shelters: Robertson and Sheldon. 1979
      1. when fish go to sleep at night, some species go into shelter holes, and aggressive interactions at this time. Do species compete over shelters?
      2. bluehead wrasse holes cemented up: individuals find another hole nearby, suffers no additional mortality
      3. removed some blueheads and natural vacancies: holes vacated were used only 7% of time.
      4. Add blueheads: all find holes, no contests.
      5. shelter does not limit population size
  3. Testing for limits on population size:
    1. Doherty 1983
      1. : 2 spp. of pomacentrid: P. wardi and P. flavicauda
      2. Expts. performed on patch reefs: isolated, no migration of post-recruits.
      3. Expt. 1: Fig. 2 all resident wardi removed. Recovery not complete after 2 yr. Avg. recovery 39%.
      4. Expt. 2: Fig. 3 Resident wardi removed from expt. patches and every month new arrivals removed. So have estimate of rect. rate, compare with control. Removal had no effect.
      5. Presence of adults does not limit recruitment rate; no density dependent influence on abundance
    2. Doherty and Fowler 1994
      1. Surveyed 7 reefs for 9 years
      2. Some reefs get consistently high recruitment, some get low recruitment
      3. Population size is largely a function of recruitment rate (fig 2). (but note possible levelling off; maybe at higher levels of recruitment there is some compensation).
    3. Test of density-dependent mortality: Hixon and Carr 1997
      1. Blue chromis on artificial reefs
      2. different predator exclusion expts
      3. Mortality rate higher when settler density higher and predators present
    4. Bottom line: generally reef fish population dynamics seem to be dominated by stochastic processes acting during the larval stage. There is some evidence for post settlement density dependence.