Recruitment is the process by which young individuals (e.g., fish and coral larvae, algae propagules) undergo larval settlement and become part of the adult population. The rate, scale, and spatial structure of larval dispersal drive population replenishment, and therefore have significant implications for population dynamics, marine reserve orientation, and resilience of a system.
Steps for Recruitment
Conceptual model of processes leading to coral recruitment to the benthos. Arrows to the right represent possible recruitment limitations. Arrows down indicate a coral larva’s successful progression through the steps to recruitment. Modified from Arnold et al. 2010.
Fundamental steps required for successful recruitment include: 1) the availability of competent larvae (dependent on connectivity); 2) the ability of larvae to settle—often aided by chemical cues that induce settlement and metamorphosis; and 3) the availability of suitable settlement substrate where post-settlement survival is high. ref
Settlement and recruitment of coral larvae occur only if certain conditions are met, and the behavior of coral larvae controls their ability to settle. For example, for coral larvae to settle, the larvae must move to specific depths, seeking specific light intensities that favor settlement. Once the larvae contact the benthos (i.e., seafloor), some organisms such as coralline algae provide chemical cues that trigger coral metamorphosis and settlement. ref
Guidance for Managers
Exploring the following questions pertaining to recruitment can provide managers with a better understanding of recruitment at their sites and can inform management strategies, such as placement of MPAs, fishing restrictions or watershed management. In each of the following slides specific management guidance is highlighted.
What physical oceanographic conditions characterize the site?
Large-scale physical oceanographic processes, such as ocean currents, upwelling, and eddies can cause considerable mixing and affect long-distance transport of pelagic larvae. These large-scale processes also affect recruitment patterns at smaller scales (site-level); currents and areas of upwelling will have a direct effect on the extent of larval transport to distant locations and the movement of larvae over particular sites, and thus overall patterns of recruitment. At a smaller scale, other physical processes can either enhance or inhibit larval dispersal and recruitment patterns such as micro-currents, small eddies, light, areas of flow constriction, salinity, depth, and sedimentation.
- To better understand these large-scale physical processes and how they affect local areas, managers should examine oceanographic currents within the area. Information on surface ocean currents and tides provide managers with the general movement patterns and expected larval distribution.
- With recruitment pattern information, managers can make informed decisions about MPA size, location and distance between MPAs within a network. However, analyzing physical oceanographic conditions and modeling larval transport and dispersal patterns to inform MPA design is a relatively new field and can be very challenging (See Cowen et al. 2006 and Steneck et al. 2009 for examples of how oceanography can affect larval transport and dispersal). It is likely to require significant technical expertise, access to complex models, and partnerships between oceanographers and managers.
Larval Sources and Behavior
Where are the sources of larvae for the site?
The production, settlement, and survival of larvae depend upon the availability of source areas. The source of larvae can be an external location, or the source can be local if larval production and settlement occur on the same reef. If the source is local, the system is considered self-recruiting and is not dependent on outside sources of larvae for replenishment. The pattern of larval exchange, and the degree to which larvae originate from outside populations, helps to explain connectivity between and among coral reefs. A large amount of self-seeding leads to low connectivity, while high rates of larval exchange with other populations generate high connectivity.ref
Understanding coral recruitment patterns such as where larvae originate and settle is a challenge. In the field of larval ecology, scientists used to consider that larvae were passive particles carried by ocean currents to locations far from their birth site. However, studies suggest that some reef fish populations are actually self-recruiting, and larvae and juveniles are able to intentionally return to their birth sites. ref The authors found higher than expected (possibly as high as 60%) self-recruitment in reef fish populations, and more recent studies ref support these original findings. Scientists are finding that most corals recruit relatively locally ref because corals have relatively short larval durations (days to weeks), and many corals actually recruit closer to their source than reef fish. ref Studies demonstrating local retention of larvae are important because they suggest that marine reserves can provide recruitment benefits not only beyond, but within their boundaries.
Most reef ecosystems are not exclusively self-recruiting or dependent on outside sources. Proportions of larvae originating from internal or external locations can vary widely within and between reef systems. If an exclusively self-recruiting coral community suffers mass mortality from a disturbance event, there is little prospect for recovery, since all of the sources of larvae were impacted. Likewise, recovery of a coral community that depends solely on external sources is completely dependent on the arrival of coral larvae that have survived the disturbance event. A reef that both provides and receives larvae is more likely to be resilient to disturbance for two reasons because: 1) it has multiple sources of larvae to enhance recovery and 2) access to external larvae may increase the potential for greater genetic diversity.
- To support recovery from disturbance (e.g. bleaching), it may be optimal to have a combination of both self-recruitment and external sources of recruitment.
- Recording of recruits should be included in site monitoring protocols to help determine a reef’s recovery potential.
- Management actions should be prioritized that reduce algal biomass on coral reefs because algal overgrowth reduces the survival of newly settled corals.
- Management actions that reduce algal biomass caused by overfishing of herbivores or eutrophication (e.g., establishing no-take areas, improving water quality) will help improve coral recruitment and consequently, coral recovery following a disturbance.
- Herbivores should be managed at the scale of the entire reef because herbivores play such an important role in coral reef ecosystems. Examples of such management strategies include policies that outlaw the take of herbivorous fishes across entire countries (e.g., Bonaire, Belize) and policies that outlaw the export of reef fish (e.g., in Palau).
Is there suitable habitat for recruits?
Finding unoccupied space and suitable habitat for settlement on coral reefs is a competitive process for coral larvae. If a habitat is unfavorable to settling corals, then recruitment will not be successful.
Larval behavior (controlling movement, metamorphosis, and settlement), environmental conditions (e.g., ocean currents that affect larval transport), and availability and type of substrate (e.g., presence of coralline algae and/or absence of macroalgae) all influence the ability of larvae to settle.
Substrate type is an important factor influencing coral larvae settlement and a possible determinant of coral community structure. Substrates, such as live coral, sediment, macroalgae, encrusting sponges, and loose, unconsolidated substrate are unsuitable for coral recruit settlement. Suitable recruitment habitat includes a stable bottom type, limited sedimentation in water column, and absence of large macroalgae.
The suitability of a surface for coral settlement is determined by chemical or biological properties of the surface. The presence of algae can greatly reduce survivorship and settlement success of coral planulae. A recent study of the common Hawaiian reef-building coral Montipora capitata found a negative relationship between density of early life history corals (1-3 polyps) and fleshy algae. Planulae that do settle in algal-dominated areas not only suffer from increased indirect, algal-induced mortality, but also experience lower recruitment success as algae are unlikely to serve as stable substratum for future coral growth. ref
The presence of chemical properties in crustose coralline algae (CCA) and other substrates, such as dead coral, have been shown to encourage coral larvae settlement. ref Coral larvae appear to be able to recognize and respond to chemical signatures in CCA in the selection of settlement habitat location. ref
- Monitoring programs should evaluate the availability of suitable habitat, looking specifically for areas of CCA or patches of dead coral which may provide adequate settlement substrate for new coral recruits.
- Managers can use this information to ensure that areas with an adequate supply of recruits and suitable substrate habitat are included in protected areas.
What is the herbivorous fish assemblage at the site?
Herbivores play a key role in influencing competitive interactions between corals and macroalgae (i.e., herbivores consume algae and create space for coral recruits to settle). Therefore, the abundance and community structure of herbivorous fish and invertebrate grazers can greatly influence coral recruitment. The presence of grazing reef fish, such as parrotfish, can reduce macroalgal cover and thus facilitate coral recruitment. ref
- Management that is designed to enhance coral recruitment should include strategies that reduce algae cover through the maintenance of high levels of grazing reef fish and invertebrates. To do this, managers need to evaluate the types and extent of grazing occurring within the area.
- Prohibiting or limiting the take of herbivorous species is critical for maintaining reef resilience in areas where herbivores are targeted, and should be a high priority for reef management in algal-dominated systems.