INTRODUCTION
The majority of marine animals produce thousands if not million of eggs each year. Larvae remain in the water column for periods of days to months before they settle or transform to adult forms. During this time, they may move long distances by actively swimming or by passively drifting in currents. One possible outcome of larval behavior is the "open system" scenario. Scientists have suggested that larvae are broadcast over long distances passively and that populations are maintained by recruitment from many locations. In this scenario, the effective population size is very large.
The production of many small offspring is a common life history strategy for organisms in which the survival of offspring to adulthood is highly variable. Life history theory says that ecological conditions will occasionally favor excellent survival, creating populations with strong year classes. This mode of thinking fits with the random process of dispersal by ocean currents and the domination of larvae survival by the vagaries of ocean currents and turbulence and predation processes.
Nonetheless, an evolutionary perspective suggests that larvae might want to settle where their parents have been successful. This second possibility is the "closed system" scenario, in which populations are maintained solely by local recruitment. Larvae modify their swimming and dispersal behavior and take advantage of ocean gyres to remain near parental populations. Most systems probably operate between the open and closed system extremes -- allowing for a mixture of local and distance recruitment that depends on species, geography, and oceanography.
CHALLENGE
While marine biologists have identified the importance of the larval life stage, it has been difficult to gather data. The lack of information leads fisheries managers to ignore the larval stages altogether. Classic models in fisheries biology such as the Ricker and Beverton-Holt formulations, do not consider the biology of the larvae or the ecology of the adults. It is not surprising, then, that these classic models are poor predictors of the number of young adults that are recruited into the population.
Conservation and fisheries biologists would like to know where the larvae are coming from because it would allow them to protect source populations (Ogden 1997). Even for such important commercial species, such as lobster and tuna, the scientific community knows little about the sources and dispersal of larvae.
NEW RESULTS
A clear need exists for marine population biologists and larval ecologists to develop new techniques for studying larval recruitment, but progress has been slow because of the vast number of larvae, the spatial scale of dispersal and the lack of information about larval ecology. New advances in modeling of populations (Dixon et al. 1999) and ocean currents (Roberts 1997, Bellwood et al,. 1998, Cowen 2000) as well as the marking of the otoliths (ear bones) of larval fish (Jones et al. 1999, Swearer et al. 1999, commentary by Palumbi 1999), provide new insights about dispersal and its implications for marine conservation.
Important questions are: "can dispersal be mostly local?" and if so, "can we understand the ecological conditions that lead to successful recruitment?"
Jones et al. (1999) cite five lines of evidence that suggest local recruitment may be important: 1) some nearby marine populations are genetically distinct, 2) endemic species persist on isolated islands, 3) introduced species persist locally, 4) populations persist with no apparent current source, and 5) larvae near reefs behave so as to reduce transport.
Two recent papers that have used hydrodynamic models to infer disperal patterns have modelled ocean currents and larval biology (Roberts 1997, Cowen et al. 2000). Bellwood at el. (1998) challenged Roberts (1997) conclusions that larvae are transported long distances. Additionally, Cowen et al. (2000) presented good reasons to think that larvae are not always being dispersed long distances. Genetic methods will be important to determine population structure, but marine metapopulation models will be needed to help assess if larval dispersal is large enough to sustain a population.
In more direct demonstrations, Swearer et al. (1999) and Jones et al. (1999) have evidence that larval fish may not disperse long distances. Swearer et al. (1999) analyzed ototoliths for natural geochemical markers. Jones et al. (1999) used tetracycline dye which was taken up in the egg during development. These first studies are from tropical systems; however scientists from tropical and temperate systems alike will now try to duplicate and generalize these results.
The Dixon at al. (1999) paper is remarkable because the authors have identified three ecologically sensible factors that, with a nonlinear model, allow them to effectively predict recruitment of a population of damsel fish on the Great Barrier Reef. These factors are: 1) the moon's phase, determining the time of egg release; 2) turbulence around the reef, impacting opportunities for feeding; and 3) winds blowing over the water, affecting transport of larvae away form the island.
CONSERVATION IMPLICATIONS
What are the consequences of larval recruitment for marine conservation? At present, we do not know the correct spatial scale(s) for populations of marine organisms. In general, we believe dispersal can range over distances much larger than those for terrestrial organisms. However, if dispersal is local, then management decisions that provide many local marine protected areas would be the key to maintaining populations (Palumbi 1999). As techniques evolve for studying the ecology of marine larvae, scientists will be able to provide hard answers about the correct spatial scale(s) or mixture of local and distance dispersal of marine organisms. Also, scientists will be able to identify sources and sinks at the population level. Data about both dispersal and sources are needed for effective conservation management.
Literature cited:
Bellwood David R., Jeffrey M. Leis, Ilona C. Stobutzki;, Peter F. Sale, Robert K. Cowen, and Callum M. Roberts. 1998. Fishery and reef management. Science 279: 2019e-2025e
Cowen, Robert K., Kamazima M. M. Lwiza, Su Sponaugle, Claire B. Paris, and Donald B. Olson. 2000. Connectivity of marine populations: Open or closed? Science 287: 857-859.
Dixon, Paul A., Maria J. Milicich, and George Sugihara. 1999. Episodic fluctuations in larval supply. Science 283: 1528-1530.
Jones, G. P., Milicich, M. J., Emslie, M. J. & Lunow, C. 1999. Self-recruitment in a coral reef fish population. Nature 402: 802-804.
Ogden, John C. 1997. Marine managers look upstream for connections. Science 278: 1414-1415
Palumbi, Stephen R. 1999. Population biology: The prodigal fish. Nature 402: 733 - 735
Roberts, Callum M. 1997. Connectivity and management of Caribbean coral reefs. Science 278: 1454-1457.
Swearer, S. E., Caselle, J. E., Lea, D. W. & Warner, R. R. Larval retention and recruitment in an island population of a coral-reef fish. Nature 402: 799-802 (1999).