Location: Coral reefs around the world

Recruitment in Coral Reef Fish Populations

Coral reefs are extremely rich in marine life, including vast numbers of different species of fish. The questions then arise: Why are there so many fishes on coral reefs, where do they come from, and how do they all get there? Why do fish communities on reefs in Cuba look very similar in species composition to reef communities in the Bahamas or throughout the Caribbean, but different from those in the Indo-Pacific region?

To address these questions, one must first understand the processes of recruitment in coral reef fishes. Broadly, recruitment is defined as the addition of new individuals to populations or to successive life history stages within populations. In marine systems, recruitment is defined more restrictively as the addition of individuals to local populations following settlement from the pelagic (planktonic) phase to the benthic (demersal) early juvenile phase. Recruitment is usually measured at or shortly after the time of settlement from the plankton to reef habitat. It can be quantified over a period of time by swimming transects along the reef and counting the numbers of recently settled individuals that were not present the previous day. Species that show the greatest recruitment variation tend to be highly fecund, laying thousands to millions of eggs in one spawning event, but tend to suffer very high larval and juvenile mortality. In such species, a period in which there is a pronounced increase in the survival rates of larval and juvenile fish has the potential to create a pronounced peak in the number of recruits reaching the reef. Locating the source of the larvae, and understanding their transport by currents and tides and the eventual recruitment process is one of the most basic of marine ecological objectives, yet one of the most challenging to follow and quantify.

Figure 1 A depiction of the concept of open populations for marine organisms.

Closed vs. Open Systems

For years, coral reef ecologists purported some wrong assumptions based on what was known in terrestrial ecology. In terrestrial ecology, most species are perceived to be somewhat long-lived, provide extensive parental care to offspring, and show limited movement among local populations, with the young growing up and staying within their group or family (for example, deer, squirrels, or resident bird species). It has now become widely accepted that these features of terrestrial systems (typically closed populations), are very different from oceanic systems (typically open populations). Coral reef fish species, in contrast to terrestrial systems, are relatively short-lived. Depending on the species, they rarely show parental care and juveniles are dispersed into different geographic locales away from their parents (see Figure 1). A fish spawned in Puerto Rico for example, could hypothetically end up as a juvenile fish on a coral reef off the coast of Florida!

For closed populations (like most terrestrial populations), birthrates and death rates are dependent solely on population density, with little emigration and immigration taking place between local populations. For open populations, replenishment is largely or exclusively dependent on an exterior supply of juveniles. In the case of reef fishes, their populations are dependent on a supply of larvae from the plankton to replenish their populations. The local production of offspring has little or no direct role in setting local population size because larval recruitment from elsewhere provides the only substantial input of new individuals. If recruitment fails, the local population will decline to extinction, regardless of local fecundity. Conversely, a different local population could persist as long as recruitment continues, even if the adults produce no viable offspring.

Equilibrial vs. Nonequilibrial Communities

During the late 1970s and early 1980s, there was a resurgence of interest in the factors that influence community organization in marine ecosystems. This arose partly because of a growing discontentment with the conventional understanding of communities. Prior to this time, communities were generally perceived to be influenced primarily by biotic interactions among community members (for example, predation and competition). They were modelled as equilibrial systems in which past history (including most recent reproductive periods) was irrelevant. By taking past history into account, and particularly reproductive outputs of previous generations, marine ecologists finally became aware of the importance of recruitment and its variability in influencing the structure of populations and communities. Concurrently, fisheries managers, although already aware of recruitment variation at this time, realized that they could no longer base their estimates on average levels of recruitment. Recruitment levels were too unpredictable. Overall, scientists have become more aware that processes like competition and predation tend to drive equilibrial communities but comprise only part of the picture of the nonequilibrial nature of marine communities. Nonequilibrial communities do not usually return to the same community composition after a disturbance occurs. Good examples of nonequilibrial communities include coral reef fishes and pelagic fishes. Manipulative experiments with coral reef fishes and observations of exploited pelagic fish populations demonstrate these factors. When fishes were removed with anaesthetics and hand nets from patches of coral reefs, the fish community did not predictably return to the same community structure it had prior to the manipulation. In the fishing industry, managers were failing miserably at attempting to manage fish stocks, exemplified by the crash of anchovy fisheries in the 1970s. They were commonly observing decreased stock numbers in spite of a reduced fishing effort. Fishery managers started to realize the importance of recruitment or "year-class strength" (= numbers of juveniles that get recruited into the adult population). Even in areas where fishing is low or nonexistent, typical populations tend to suffer sporadic, or in some cases, periodic, blooms, reflecting short runs of good recruitment conditions. This suggests that a better focus for monitoring populations would be the physical/ecological requirements for successful early life history patterns.

The open, nonequilibrial nature of reef fish communities arises because: 1) their habitat is patchy over a gradient of spatial scales. Inhabitants are restricted due to the presence of inhospitable patches at one spatial scale or another; thus coral reefs are comprised of local patches of fishes. 2) These local patches are connected through pelagic larvae. Reef fishes are typically highly fecund, producing numerous clutches of many eggs. Indeed, there are some reef species that are known to spawn daily throughout most of the year! (for example, the Caribbean bluehead wrasse, Thalassoma bifasciatum). 3) The eggs hatch into larvae that are pelagic (drift about in the ocean with some motility and orientation abilities) for periods of days to months depending on the species. Considering that the adult fishes are confined to local habitat patches and spawn numerous larvae with a high mortality rate in the pelagic phase, it is not too surprising to find that dynamics of fish populations are not necessarily closely related to reproductive success of the fishes in the immediate local area.

Figure 2 The cycle of recruitment in reef fish populations. The various life history stages in reef fishes are depicted, and the main causes of death in each stage are listed in the circle.

Early Life History of Fishes and the Pelagic Larval Phase

The early life history of fishes is considered to include the period from fertilization through the embryonic and larval periods and extending into the early portion of juvenile life (see Figure 2). This is a dynamic interval of the life cycle, during which individuals undergo rapid changes in behavior, morphology, ecology, and habitat use. Consequently, it is a very risky life stage and scientists have estimated that more than 99.9% of fishes die in their larval stage.

Most coral reef fish have a bipartite life history characterized by a pelagic larval phase followed by transformation and settlement into a benthic existence. During the pelagic stage, ocean currents disperse planktonic eggs and larvae over periods ranging from days to months before settlement. A few species of coral reef fish settle directly onto adult habitats, whereas others may use different habitats before assuming an adult existence due to ontogenetic (changing needs as the organism grows) or environmental factors. The return of larvae or juveniles to reefs can be highly variable in space and time.

During the early years when larval transport was studied, physical oceanographers and marine ecologists considered larvae to be passive particles that are transported solely by water currents. This is certainly true; however, over the past decade or so, we have begun to realize that larvae have greater sensory and motility abilities than was originally believed.

Pelagic fish eggs and larvae have several adaptations to survive these early stages. For example, most pelagic eggs contain oil droplets that make them buoyant. Adult fishes tend to spawn at locations on the reef where larvae will likely be carried away from the reef, and away from demersal predators on the reef. Larvae rarely look like their adult counterparts, until they undergo extensive metamorphosis; they are typically very small and lack pigment, which makes it difficult for pelagic predators to see them. They sometimes have defensive structures such as spikes or spines that deter predators and often possess large eyes that enable them to feed actively at night and be quiescent during the day when diurnal predators are active. They also can undergo vertical migration (toward the surface at night, migrating deeper during the day) to escape predation. Despite these advantageous traits, most eggs and larvae succumb to the rigors of a pelagic existence (see Figure 2). In addition to the threat from numerous predators, the larvae may fail to find adequate supplies of food because food resources for pelagic larvae can be very patchy. Furthermore, physical processes of the water mass may carry them to places from which they are unable to return to suitable adult habitat (known as the process of expatriation).

Delayed Metamorphosis/Slowing Growth Rate Down to Delay Settlement

While in the pelagic stage, larvae go through a metamorphosis prior to being competent, or able, to settle on the reef environment. There is recent evidence to suggest that some larvae are capable of delaying the process of metamorphosis. This may reduce the risk of expatriation by controlling the timing of metamorphosis until a suitable settlement site is presented; or more importantly, when such a site is not available, to continue in the larval phase until such an area is located. On the other hand, delay of metamorphosis eventually results in a decrease in larval condition, substratum selectivity, or actual ability to metamorphose. Some widely dispersing larvae apparently experience a marked reduction in their growth rate after becoming competent to settle, perhaps because a larger size in the plankton would reduce survival. Bigger size usually means greater risk of predation, especially in the planktonic situation where larvae are exposed to an open environment with no shelter.

Settlement Patterns

Following the dispersive larval phase, if an individual survives all of the hazards of the pelagic existence and finds suitable habitat at an appropriate time, it must undergo the process of settling onto the reef habitat. The intensity of local settlement is determined by the number of larvae that reach a location and settle. While larval supply can be influenced by current patterns, settlement of strongly swimming larvae can be influenced by substrate selection behavior. There is ample evidence to suggest that settling benthic larvae show substratum preferences. Settlement patterns can strongly affect the composition of reef fish communities and the spatial distributions and sizes of populations of individual species. Subsequent post-settlement movements and mortality can also affect the distribution of demersal fishes and mask the initial settlement patterns. One key factor defining the influence of settlement on population dynamics is the nature of any interaction between settling larvae and established residents of the benthic habitat. Variation in settlement and subsequent recruitment rate has been shown to affect competitive interactions, predation, and other community level processes.

Settlement rates in coral reef fishes are sometimes timed with phases of the moon. Several researchers have found lunar periodicity in settlement rates, with high pulses of new settlers occurring at full and dark moons. Some researchers have only speculated as to why this occurs. One reason may be that the tides are closely linked with phases of the moon, and the tidal cycle may simply facilitate the movement of larvae from the water column onto the reef. It may also occur because environmental cues from the lunar cycles (for example, peak tides and brighter nights from full moons) simultaneously cue all larvae waiting just outside the reef to settle; and a larva settling in a group may have a greater chance of surviving than a larva settling alone.

Recruitment Limitation and Supply-side Ecology

Recruitment is typically quantified as the initial sighting of a recently settled juvenile in the adult habitat. The amount of time a settler must remain on the reef before it is considered a successful recruit is usually set arbitrarily by the researcher. Survival of settlers and recruits in the reef environment is highly variable and can be affected by several factors (for example, predation and fierce competition for space or resources). Adults can potentially influence recruitment by altering habitat availability during settlement or by influencing the survival of recruits. Adults of the same species may be a good cue for settlers as they indicate appropriate habitat and resources for settlers. On the other hand, they may be a deterrent, as there will be greater competition for resources among individuals of the same species compared with individuals of different species. In some species, adults actively chase conspecific settlers (same species) and juveniles from their home range.

Conclusions

Taking all of this into consideration, we should consider the implications of our current perception of recruitment in oceanic systems and how it may affect the distribution and community structure of organisms. Many marine ecologists now concede that no single factor adequately explains the distribution and abundance of marine organisms with complex life histories. Instead, the cumulative effects of a number of interacting factors cause pre-settlement losses (predation, starvation, and expatriation) of pelagic larvae and post-settlement losses (predation and competition) of recruits and juveniles. For reef fishes, researchers have demonstrated that stochastic variation in larval success can affect recruitment strength, but that high recruit and juvenile mortality can also modify recruitment patterns. Marine scientists and fisheries managers now believe that events in the early life stages of fishes (eggs, larvae, recruits, and juveniles) are critical to the fluctuations of fish populations in marine environments.

Our current understanding of why recruitment varies in space and time is still limited. In particular, we need to come to a better understanding of the causes of variation in production and survival of larvae, the processes that transport them, and mechanisms of habitat selection.

Key Principles

1. Open populations vs. closed populations
2. Equilibrial vs. equilibrial systems
3. Pelagic life stage; bipartite life cycles

Ethical Considerations

1. How should the perception of recruitment variability assist coral reef fisheries managers?
2. What can be done to enhance the numbers of individual fishes reaching harvestable size?
3. Is closing off areas (marine protected areas) from fishing a viable practice in developing countries, considering the fact that many locals depend on fishing for income?

Suggested References for Further Reading

Hunt, H. L., and R. E. Scheibling. 1997. Role of early post-settlement mortality in recruitment of benthic marine invertebrates. Marine Ecology Progress Series. 155:269-301.

Leis, J. M., and B. M. Carson-Ewart. 1997. In situ swimming speeds of the late pelagic larvae of some Indo-Pacific coral reef fishes. Marine Ecology Progress Series 159:165-74.

Sale, P. F. 1991. The ecology of fishes on coral reefs. Academic Press, Inc.

Wellington, G.M., and B. C. Victor. 1989. Planktonic larval duration of one hundred species of Pacific and Atlantic damselfishes (Pomacentridae). Marine Biology. 101:557-567.

Internet Links

FishBase- an international, searchable fish encyclopaedia

Authors

K. Martha M. Jones
Department of Biological Sciences
University of Windsor
Windsor, Ontario N9E 3G5
Canada

Dr. Iain J. McGaw
Department of Biological Sciences
University of Nevada, Las Vegas
Las Vegas, Nevada 89154
U.S.A.