Luxuriant schools of mackerel and bass weave through the currents of the North Atlantic. Gobies, snappers, and grouper navigate the labyrinthine roots of mangrove trees in the Florida Keys. Over 3,000 species of polychromatic reef fish traverse the waters of the Coral Triangle. These scenes paint a picture of the extraordinary biodiversity that exists in the world’s oceans. The global seas host nearly half of all species and 99% of living space on Earth. Yet, while it is vast and resilient, the ocean cannot buffer all impacts. Today, the biodiversity of our oceans faces imminent threats, ranging from unsustainable fishing practices and coastal development to anthropogenic-induced climate change and eutrophication. The declining state of marine ecosystems calls for more comprehensive management strategies. One management practice that has received much attention from scientists and policy-makers is the designation of marine reserves.
What is a marine reserve?
Marine reserves are prescribed areas of the ocean where extractive activities such as fishing are strictly forbidden. Also known as “no-take areas”, marine reserves prohibit the removal or disturbance of any resources within their boundaries, with the exception of samples for monitoring or research. Over the past couple decades, they have gained popularity as a means of achieving conservation and fisheries management. Despite the scientific consensus that marine reserves increase the abundance and diversity of marine organisms, these protected zones make up less than one percent of the world’s oceans today.
The establishment of a marine reserve is not a trivial undertaking. Those involved must consider the costs and mechanisms of enforcement, the interests of diverse stakeholders, and spillover ecological effects, with the ultimate goal of reaping maximum ecosystem benefits at the lowest expense to fishermen and local communities. The design of marine reserves is a nuanced process and represents an ongoing, active area of research. Determination of key attributes such as reserve location, size, shape, and distribution necessitates an understanding of properties that are complex and specific to each system. This includes the dispersal patterns of larvae that replenish adult fish populations within marine reserves.
Does larval supply determine adult population sizes?
Because larvae face unique challenges and occupy spatially distinct areas, their success is frequently decoupled from that of adult populations, and it is important for scientists to study their ecology separately. The study of larval dispersal requires an understanding of fish reproductive strategies, physical oceanography, stages of larval development, predation on larvae, threats such as food limitation and disturbances, and factors that govern settlement and recruitment.
In the 1980s, work done by Underwood and Denley (1984) and Gaines and Roughgarden (1985) pioneered a trend in “supply-side ecology”, which posited that larval supply can represent a more important determinant of community structure than post-recruitment biotic interactions. An offshoot of this theory, termed the recruitment limitation hypothesis, postulates that varying recruitment levels predict subsequent population sizes. The alternative hypothesis states that initial variations in population due to differential recruitment will level out due to density-dependent processes that take place after recruitment, such as competition.
Multiple studies have demonstrated recruitment limitation within fish communities. Through a long-term study in the southern Great Barrier Reef, Doherty and Fowler (1994) found that differing recruitment levels explained over 90% of the variation in damselfish numbers across seven different coral reef communities. Furthermore, population age structures showed that these recruitment signals persisted for at least ten years, implying that, at the scale of this study, any density-dependent mortality that occurred post-recruitment did not significantly alter damselfish abundances. A study of bluehead wrasses in Panama (Victor 1985) similarly found that demographic patterns that were established by larval recruitment characterized later adult populations. He inferred that juvenile mortality for these fish was predominantly density-independent, rather than density-dependent.
While one can debate the merits of either hypothesis and try to appoint an ultimate driver of community structure, both recruitment and post-recruitment processes ultimately contribute to population patterns, and it is important to not exclude either one. Indeed, Caley et al (1996) argue that the debate between recruitment limitation and density-dependent mortality as determinants of demography creates an unnecessary dichotomy, and that the two processes are not mutually exclusive. This represents a more recent trend towards pluralism that emphasizes the study of the combined effects of recruitment and post-recruitment factors. Nevertheless, Caley and colleagues conclude that, for most marine systems, density-dependent interactions cannot completely abolish trends established by recruitment. They believe that, in most cases, population sizes will reflect recruitment levels, with density-dependent mortality partially diminishing the effect of large recruitment spikes. These studies indicate that external processes controlling recruitment, such as ocean climate and hydrodynamics, or larval competition and predation, play a critical role in configuring population patterns within marine ecosystems.
Recruitment limitation implies that marine communities are undersaturated and could support higher abundances at greater recruitment levels; in other words, most marine populations have not yet reached their carrying capacity. Threats such as overfishing, habitat destruction, climate change, and pollution only reinforce this trend. The undersaturation of marine communities highlights the potential for recuperation by enhancing larval recruitment rates. Furthermore, marine reserves can represent an important means of protecting larvae. Although humans do not directly fish larvae, their presence can threaten larval survival through pollution or habitat destruction, and the establishment or marine protected areas can alleviate these pressures.
How can we study larval dispersal?
Having established the importance of recruitment as a pivotal determinant of population size, it becomes clear that an understanding of larval dispersal must play into the designing of marine reserves. Models can yield insight into how factors that influence larval transport impact community structure. Scientists can combine ocean circulations models with field data to determine the relative effects of currents, turbulence, light, and prey on larval dispersal and development. They can also use model flow-fields to compute particle trajectories that simulate larval dispersal, and then combine these particle trajectories with individualized responses to physical, chemical, or biological cues for each larva.
Researchers can test their models by comparing the patterns of larval supply that they generate against ecological experiments or population genetic analyses. In addition to models, Moran et al (2005) have experimented with direct means of observing dispersal patterns, such as by using fluorescent markers that can be directly embedded into calcified larval structures. Microchemical analysis of fish otoliths, a calcareous structure in the inner ear of all vertebrates, can also disclose dispersal movements. Scientists can measure the Sr/Ca ratios in otoliths to get an idea of a fish’s past life history. Otolith composition can provide information about numerous factors, including physiology, environmental stress, food availability, ambient temperature, and fish activity level. Ultimately, these strategies provide scientists with tools to predict recruitment success under different marine reserve scenarios.
Implications for marine reserve design:
Over the past decade, scientists have begun to flesh out the interactions between larval recruitment and marine reserve design. In 2000, Warner et al investigated the effect of larval accumulation and retention patterns on the success of marine reserves. Their study holds important implications for the use of marine reserves in fisheries management. Current designs often employ models that assume passive-particle larval behavior. However, Warner and colleagues point out that these models may oversimplify larval behavior, and that larvae may actually assemble in certain areas before settlement and return to nearshore source populations in large quantities, even after long larval durations.
Models that currently focus on maximizing larval export to restock fisheries place reserves in non-retention areas, where the mass export of larvae may prove unsustainable for target fish populations. Meanwhile, reserves assigned to regions where all of the larvae are locally retained will experience enriched biodiversity but achieve little in the way of replenishing fisheries. Furthermore, marine reserves in high retention areas may prove more vulnerable to catastrophic disturbances that lower biodiversity because they cannot rely on external sources of larvae for recovery. Past research demonstrates that the degree of local retention differs drastically between marine communities, even for those that are spatially adjacent to one another. Thus, effective marine reserve design should incorporate larval retention and accumulation patterns in the context of reserve’s priorities and objectives. Especially in situations of recruitment limitation, the preservation of nearshore retention areas may prove as crucial as that of offshore adult habitats.
Botsford and colleagues (2001) also showed that the protection of nearshore habitats could enhance fish populations. Their consideration of marine reserves focused on the dispersal distances of larvae and highlighted the need to consider the connectivity of protected sites within marine reserve networks. They observed that fish populations demonstrated reduced viability within habitats that contained noncontiguous marine reserves. From their study they inferred that, to ensure sustainability, marine reserves should either incorporate far more coastline or encompass an area greater than the mean larval dispersal distance of their target species. They also speculated that noncontiguous marine reserves could drive changes in community composition by favoring species with larvae that dispersed across shorter distances.
More recently, O’Connor et al (2007) noted that latitudinal variations in water temperature could hold implications for the spacing of marine reserves. The scientists observed that planktonic larval durations were almost always inversely linked to ocean temperature. Thus, larvae tend to disperse farther in colder waters because they stay in the planktonic state for a longer period of time. These findings imply that the optimal spacing between individual marine reserves depends on ambient water temperatures. To ensure connectivity, marine reserves may need to be closer in the tropics than in high-latitude oceans.
In 2003, Gaines et al investigated the effects of currents on larval dispersal and population dynamics within reserves. They observed that most of the few models that incorporated larval dispersal did so based on diffusion and failed to consider horizontal larval transfer through directional ocean currents. Gaines et al discovered that the optimal number and location of marine reserves differ depending on whether diffusion or advection dominates the local environment. Reserve location matters much more in regions characterized by diffusion than those characterized by advection. In advective systems, most of the larvae produced within a reserve disperses beyond the confines of the protected area. Therefore, under high advection, single reserves tend to accomplish little, regardless of location. Flows with directional variability benefit multiple reserve systems because all of the reserves occasionally receive larvae from other nearby reserves.
Based on models, Gaines and colleagues determined that if any advection exists within a system, reserve schemes outperform policies that set fishing quotas. Advection enhances the effectiveness of marine reserve networks because directional currents promote connectivity between individual reserves. These differences between diffusive and advective affects can generate different larval dispersal patterns, and therefore should be taken into account when considering the spacing and scaling of marine reserves.
Where do we go from here?
While the studies described above explore various facets of recruitment limitation in the context of marine reserves, they only scratch the surface of a topic that extends much deeper. Interactions between larval dispersal and marine reserves are both pervasive and nuanced. Trying to account for all of the factors that govern dispersal and recruitment is somewhat of a juggling act. Models based on simple flow fields cannot account for factors such as variations in larval accumulation and retention, temperature effects on planktonic larval duration, directional bias from advection, and species-specific responses to environmental cues. Gaines et al (2003) point out that their study neglects multiple aspects of larval behavior, such as variations in larval stage duration and larval transport via vertical flow structures. Furthermore, because there are so many factors at play, it is difficult to establish any conclusive causal relationships between reserves and abundance or biomass increases. Paddack and Estes (2000) noted that the relationship between the size of existing reserves and spatial scale of larval and adult dispersal patterns remains unclear.
Ultimately, larval behaviors are complex, multi-dimensional, and dependent on numerous variables. While the task of decrypting the mechanisms underlying larval recruitment is certainly daunting, it is an important practice that wields immense potential for improving the effectiveness of marine reserves. Scientists can create models that incorporate flow dynamics, observed dispersal distances, and environmental cues unique for each larva, and then test their results with ecology and population genetics. While our knowledge of how to include larval recruitment patterns in marine reserve design is still nowhere near exhaustive, scientists are paying more attention to these interactions.
Marine reserves represent a powerful emerging means of achieving conservation and fisheries management. In the face of escalating human impacts, marine reserves can provide a much-needed refuge from anthropogenic disturbances for marine biota. Because they are expensive to maintain and enforce, it is in our best interests to determine how to maximize the effectiveness of marine reserves while minimizing losses to fishermen and local users of resources. This necessitates a better understanding of larval dispersal, settlement, and recruitment processes. Studies attempting to discern these patterns represent an important step towards sustaining the wealth and biodiversity of our oceans.
More reading on marine reserves and larval recruitment:
Allison, G.W., J. Lubchenco, and M.H. Carr. 1998. Marine reserves are necessary but not sufficient for marine conservation. Ecological Applications 8: S79–S92.
Botsford, L.W., Hastings, A. and Gaines, S.D. 2001. Dependence of sustainability on the configuration of marine reserves and larval dispersal distance. Ecology Letters 4: 144–150.
Caley, M.J., M.H. Carr, M.A. Hixon, T.P. Hughes, G.P. Jones, and B.A. Menge. 1996. Recruitment and the local dynamics of open marine populations. Annu. Rev. Evol. Syst. 27: 477-500.
Chesson, P. 1998. Recruitment limitation: A theoretical perspective. Australian Journal of Ecology 23: 234-40.
Doherty, P. 1983. Tropical territorial damselfishes: is density limited by aggression or recruitment?. Ecology 64: 176-90.
Doherty, P. and T. Fowler. 1998. An empirical test of recruitment limitation in a coral reef fish. Science 263: 935-39.
Gaines, S.D., B. Gaylord, and J.L. Largier. 2003. Avoiding current oversights in marine reserve design. Ecological Applications 13: S32-S46.
Gaines, S.D., J. Lubchenco, S. Palumbi, and M. Dethier. “Scientific Consensus Statement on Marine Reserves and Marine Protected Areas.” Annual Meeting of the American Association for the Advancement of the Sciences. 2001
Gaines, S. and J. Roughgarden. 1985. Larval settlement rate: a leading determinant of structure in an ecological community of the marine intertidal zone, Proc. Nat. Acad. Sci. 82: 3707–3711.
Halpern, B.S. “Marine reserves.” Encyclopedia of Earth. Washington D.C.: National Council for Science and the Environment, 2008.
Marko, P.B. 2004. ‘What’s larvae got to do with it?’ Disparate patterns of post-glacial population structure in two benthic marine gastropods with identical dispersal potential. Molecular Ecology 13: 597-611.
Moran, M.L. and P.B. Marko. 2005. A simple technique for physical marking of larvae of marine bivalves. Journal of Shellfish Research 24: 567-71.
O’ Connor, M.I., J.F. Bruno, S.D. Gaines, B.S. Halpern, S.E. Lester, B.P. Kinlan, and J.M. Weiss. 2007. Temperature control of larval dispersal and the implications for marine ecology, evolution, and conservation. PNAS 104: 1266-71.
Paddack, M.J. and J.A. Estes. 2000. Kelp forest fish populations in marine reserves and adjacent exploited areas of central California. Ecological Applications 10: 855-870.
Peterson, C.H., and H.C. Summerson. 1992. Basin-scale coherence of population dynamics of an exploited marine invertebrate, the bay scallop: implications of recruitment limitation. Mar.Ecol. Prog. Ser. 90: 257-72.
Radtke, R.L. and D.J. Shafer. 1992. Environmental sensitivity of fish otolith microchemistry. Australian Journal of Marine and Freshwater Research 43: 935-51.
Shen, K.N., Y.C. Lee, and W.N. Tzeng. 1998. Use of otolith microchemistry to investigate the life history pattern of gobies in a Taiwanese stream. Zoological Studies 37: 322-29.
Sousa, W.P., P.G. Kennedy, B.J. Mitchell, B.M. Ordonez. 2007. Supply-side ecology in mangroves: do propagule dispersal and seedling establishment explain forest structure?. Ecological Monographs 77: 53-76.
Spalding, Mark. Interview by Nature.org. 2010. The Nature Conservancy.
Underwood, A.J. and E.J. Denley. 1984. Paradigms, explanations, and generalities in models for the structure of intertidal communities on rocky shores. In: D.R. Strong, D. Simberloff, L. Abele and A.B. Thistle, Editors, Ecological Communities: Conceptual Issues and the Evidence, Princeton University Press, Princeton (1984), p. 151-130.
Victor, Benjamin C. 1986. Larval settlement and juvenile mortality in a recruitment-limited coral reef fish population. Ecological Monographs 56: 145-60.
Warner, R.R., S.E. Swearer, and J.E. Caselle. 2000. Larval accumulation and retention: implications for the design of marine reserves and essential fish habitat. Bulletin of Marine Science 66: 821-30.
Wiebe, P., R. Beardsley, D. Mountain, and A. Bucklin. 2001. U.S. GLOBEC Northwest Atlantic/Georges Bank Program. Oceanography 15: -17.