Final Paper: Fish Recruitment to Reefs

This discussion topic submitted by Jacob Massoud ( jakemassoud@yahoo.com) at 10:49 am on 6/8/00. Additions were last made on Tuesday, March 26, 2002.


Recruitment is an important component of population dynamics. It plays an essential role in the life history of marine organisms, because the survival of juveniles is linked to adult populations. In order to survive juveniles must overcome competition, predation, and habitat availability (Pile et al. 1996). Typically, recruitment is defined by researchers as the survival of individuals that settle to an arbitrarily chosen point in time (Pile et al. 1996). It involves two life history phases, settlers and recruits. Many marine organisms produce pelagic larvae that begin their development in the open ocean. When such larvae reach the first adult-like form and they begin to inhabit the adult environment, they become known as settlers. Once they have reached a post-settlement time period that is defined by the researcher, they are known as recruits (Pile et al. 1996). Thus, recruitment is important in population structure, because it is linked to survival to the adult stage.


Recruitment has been well studied in many benthic invertebrates and reef fishes. Coral reefs are dynamic environments that contain a myriad of living organisms. These systems provide habitat for a diversity of reef fishes and serve as adult environments for recruits. This is the result of the presence of photosynthetic organisms of the reef close to the surface, efficient recirculation of nutrients, and various symbioses between producers and consumers (Spanier et al. 1985). According to Jones (1990), variation in the composition of coral reef fish assemblages and the abundance of their corresponding populations is the result of recruitment patterns. Many studies have addressed recruitment, density dependence, and density independence on adult population size. Nonetheless, there is no clear answer. Mechanisms such as density-dependent growth and density-independent mortality could potentially alter the patterns established during recruitment to the population (Jones 1990). Additionally, Jones (1990) contends that the effects of recruitment on population dynamics ultimately cannot be determined by short-term studies, because coral reef fishes often possess extensive lifespans. For example, P. moluccensis can live up to eighteen years (Connel 1997). In natural populations, recruitment to the adult population may result from numerous cohorts during a year or multiple years. In some years, recruits may provide enough input to saturate the habitat. Under such circumstances, juvenile growth rates may be reduced through social interactions due to density (Jones 1990). Juveniles would then fill vacancies in the habitat, as they become available. Jones (1990) found that in patch reefs, above certain levels of recruitment, adult numbers are independent of the number of recruits. On experimental reefs, adult numbers increased as a function of recruitment when the number of recruits per meter2 was between 0 and 1, but did not increase at higher levels of recruits (Jones 1990). Reefs may have a carrying capacity for the number of reproductively mature adults that they can support. Therefore, one cannot assume that recruitment is the only important factor regulating adult populations, but rather a combination of other parameters such as food and shelter resources, predation pressure, competition, and natural disturbances.


During the first weeks of settlement, it is believed that juvenile reef fish experience high rates of mortality (Connell 1997). Early juvenile mortality can lead to changes in patterns of recruitment. Large predatory fish are considered to be a major cause of juvenile mortality (Carr and Hixon 1995, Connel 1997). A number of predator fish inhabit reefs and many fish become piscivorous shortly after settling (Carr and Hixon 1995). Connel (1997) found that coral reef fish encountered predators at a much higher rate on natural reefs than on artificial reefs. Other studies also indicated that predation pressure decreased with increasing distance from natural reefs. This may be due to the strong association of predatory fish with natural reefs, or increased shelter for prey on artificial reefs. Structural refuge has been shown to enhance the survival of new recruits (Carr and Hixon 1995). Additionally, predation pressure on natural patch reefs was lower than continuous reef (Connel 1997). To test out the effects of predators on new recruits, Carr and Hixon (1995) translocated coral reef patches and observed the effects of resident predators. Their results indicated that piscivores such as moray eels, groupers, and snappers significantly altered the survival rate of new recruits. Carr and Hixon (1995) concluded that early post-settlement predation plays an important role in regulating population sizes in Bahamian reef fishes, because many piscivorous fishes that can easily consume small recruits inhabit such coral reefs. However, survivorship was highly variable amongst fish species. For example, resident piscivores had no effect on bluehead wrasse, Thalassoma bifasciatum. Another interesting finding was that blue chromis (Chromis cyanea) survivors were larger in patch reefs in which predators were present. This suggests that predators either consumed smaller recruits, or recruits grew faster after predators reduced their density (Carr and Hixon 1995). However, in other investigations predators more frequently consumed larger recruits.


Generally, distance from coral reefs is an important factor in recruitment. Several studies indicated that recruitment of tropical reef fishes decreases as the distance from a large reef increases (Shulman 1985). However, Shulman (1985) found that this pattern is not always the case for artificial reefs. Instead, recruitment was greater at reefs built further from the natural reef. In Caribbean backreef habitat, juvenile recruitment occurred at a higher rate 10-40 meters into the lagoon than along the edge of the reef (Shulman 1985). Recruitment was high in the algal and seagrass areas of the lagoon. Shulman (1985) attributes this to better shelter conditions within the structure of algae and seagrass. Additionally, at greater distances into the lagoon predator-prey encounters are less abundant, because piscivorous fishes rely heavily on the major reef (Shulman 1985). Furthermore, algal-grazers reduce the algal cover closer to reefs. In accordance with this data, the majority of reef recruits were older and larger juveniles than those found in the algal/seagrass areas. Eventually, small juveniles probably settle to the reef, because they have grown larger than the prey size range of many predators, and they can out-swim and out-maneuver many of them (Shulman 1985). For others juveniles, adult diets are better served on the reefs. Thus, when coral reefs occur in conjunction with seagrass beds, the seagrass provides an important nursery habitat for recruits. The spatial distribution of herbivores, predators, and habitat types greatly affect recruitment within this type of reef fish assemblage.


Coral reef fishes exhibit highly variable fluctuations in recruitment. Spatial variability in recruitment also exists among adjacent reef patches, widely separated sites within a reef zone, reefs located miles apart, and regionally (Planes et al. 1993). Planes et al. (1993) found that different sites had significant differences in recruit abundance. Some of the species were ubiquitous, whereas others displayed clear site preferences. For example, the two Mugilidae species were present only near the beach and Thalassoma hardwicki was found only in the outer fringing and barrier reefs (Planes et al. 1993). Reproduction seemed to be correlated with the abundance of new recruits, and 70% of the total number of fish and 65% of the species diversity were recruited to the fringing reef (Planes et al. 1993). It was also evident that the fringing reef acts as a nursery for the barrier reef. Temporal variation also occurs in recruitment. Usually, seasonal variability is evident in patterns of recruitment with peak recruitment occurring in the summer for most reef fishes (Planes et al. 1993). At a larger scale, recruitment levels can vary from year to year.


Artificial reefs are frequently utilized to enhance local fish abundance to provide resources for both commercial and recreational fishermen (Spanier et al. 1985, Beets 1989, Danner et al. 1994). Typically, artificial reefs can be constructed of a variety of materials. Beets (1989) used 300 queen conch shells to construct a reef, whereas Spanier et al. (1985) employed steel and fiberglass bars to stabilize used car tires. Spanier et al. (1985) found that the complexity of the artificial reefs (additional spaces) and the enhancement with frozen fish flesh resulted in a higher carrying capacity than control sites. Their study was conducted in one of the least productive zones in the Mediterranean Sea, yet recruitment was rapid and significant with regard to commercial species. However, after the first month it began to level off. If the intention of an artificial reef is to replace or enlarge portions of natural communities, Danner et al. (1994) recommend placing the artificial reef within 600 meters of existing natural habitats. This permits the immigration of invertebrates and the recruitment of algae and fishes to both the natural and artificial reefs (Danner et al. 1994). Beets (1989) reported that fish aggregating devices and benthic artificial reefs, when used together, successfully increase species richness, postlarval recruitment, and abundance than either structure used individually. However, shelter limitation may have been an important factor in limiting the abundance of large benthic fishes. Beets (1989) concluded that the presence of structure is a critical cue for fish larvae to settle. As fish grow, the size of structures needed for shelter increases. The most abundant recruits were haemulids such as C. cyanea (Beets 1989). A different study showed that gobiids were the most abundant recruits. Clearly, there is site specific variation.

In conclusion, fish recruitment plays a crucial role in the dynamics of coral reef fish communities. New recruits rely heavily on reefs for refuge from predators, food, and growth. Nonetheless, given the complexities of coral reefs and fish population dynamics, it is difficult to determine the actual role that recruitment plays in adult abundance. It is likely that abundance is the effect of numerous variables such as competition, predation, and available habitat. Because recruitment is important for the survival of juveniles to the adult stage, artificial reefs can be utilized to enhance fishing and damaged reef systems.


References
Beets J. (1989) Experimental evaluation of fish recruitment to combinations of fish aggregating devices and benthic artificial reefs. Bulletin of Marine Sciences 44: 973-983.

Carr MH, and MA Hixon (1995) Predation effects on early post-settlement survivorship of coral-reef fishes. Marine Ecology Progress Series 124: 31-42.

Connell, SD (1997) The relationship between large predatory fish and recruitment and mortality of juvenile coral reef-fish on artificial reefs. Journal of Experimental Marine Biology and Ecology 209: 261-278.

Danner, EM, TC Wilson, and RE Schlotterbeck. (1994) Comparison of Rockfish recruitment of nearshore artificial and natural reefs off the coast of central California. Bulletin of Marine Science 55: 333-343.

Jones, GP (1990) The importance of recruitment to the dynamics of a coral reef fish population. Ecology 71: 1691-1698.

Pile, AJ, RN Lipcius, J Van Montfrans, and RJ Orth (1996) Density-dependent settler-recruit-juvenile relationships in blue crabs. Ecological Monographs 66: 277-300.

Planes S, A Levefre, P Legendre, and R Galzin. (1993) Spatiotemporal variability in fish recruitment to a coral-reef (Moorea, French-Polynesia) Coral Reefs 12: 105-113.

Shulman, MA (1985) Recruitment of coral reef fishes: effects of distribution of predators and shelter. Ecology 66: 1056-1066.

Spanier E, M Tom, and S Pisanty. (1985) Enhancement of fish recruitment by artificial enrichment of man-made reefs in the southeastern Mediterranean. Bulletin of Marine Science 37: 356-363.


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