Whether larvae settle near parental estuaries or disperse far away has important implications for the management of estuarine resources, as well as the evolution of estuarine life histories. From a management perspective, stocks in the Hudson River Estuary may be managed far more easily if local conditions regulate recruitment into adult populations than if many recruits originated from estuaries in neighboring states. Identifying the larval transport mechanisms is critical if we are to understand how variation in oceanographic and meteorological conditions affects the location, timing, and magnitude of recruitment. Ultimately, the aim is to understand the underlying causes of variation in larval recruitment in order to forecast harvests of commercially and recreationally important species and to model fundamental ecological processes that regulate the abundance of marine populations.
This study has provided an initial picture of larval transport patterns of species that recruit to the Hudson River Estuary and the underlying transport mechanisms that enable differential transport between the estuary and NewYork Bight. The coupling of simple behaviors with conservative oceanographic features may enable larvae to reliably migrate between adult habitats and larval nursery areas. Understanding how larvae routinely accomplish these migrations is necessary before we can fully appreciate how episodic changes to prevailing oceanographic conditions affect larval recruitment to adult populations. Predictions of year class strength will be greatly improved by monitoring the key physical variables that affect larval transport processes.
Monitoring river runoff may be especially important to species that develop within estuaries. A nine-year record of settlement variation by barnacles in Narragansett Bay, Rhode Island, was explained by variation in runoff, which affected the number of larvae that were retained in the bay or flushed into coastal waters; settlement was low when flushing rates were high (Gaines and Bertness, 1992). Monitoring larval supply of estuarine-dependent species and correlating it with variation in river runoff would be the first step toward determining whether this potentially important factor affects recruitment success. If it does, then the second step would be to determine how the larval transport mechanism weakened by conducting vertical and horizontal plankton surveys.
Monitoring internal wave formation and wind velocity may be particularly important to estuarine-dependent species that develop on the continental shelf. Concurrently monitoring both larval supply and postsettlement mortality is necessary to fully understand recruitment dynamics; only by doing so did it become apparent that early postsettlement mortality rather than larval supply was the bottleneck in blue crab populations, even though postlarvae had to cross the shelf to reach the estuary. Blue crabs prefer to settle in submerged vegetation, and packing more recruits into this limited area leads to very highly postsettlement mortality from cannibalization (Pile et al., 1996; Heck, Coen, and Morgan, 2001). The connection between larval transport, larval supply, settlement, and recruitment into adult populations will become clearer as concurrent monitoring of benthic and pelagic phases of the life cycle continue and our understanding of coastal transport processes improves.
The recruitment studies conducted in Long Island Sound and elsewhere along the Atlantic and
Gulf coasts suggest that a high proportion of larvae may recruit back to parental estuaries. The proportion of larvae that return to the parental estuary cannot be known with certainty until we can effectively tag and recapture them. However, if self-recruitment eventually proves to be the case, then local conditions will have a greater impact on recruitment than conditions in neighboring estuaries. This knowledge of transport mechanisms is important in understanding stock discreteness and size. Without such information, managers are limited in their estimates of stock abundance. Armed with a model of transport mechanisms, predictions can be made about the timing and relative strength of recruitment events. Moreover, management practices implemented in the Hudson River Estuary will have a large effect on local species that do not migrate as adults between estuaries, even if managers in adjacent estuaries choose not to implement them. Thus, greater local control may be exerted over stocks in the Hudson River Estuary, rendering policy easier to enact and stocks easier to manage effectively.
Finally, monitoring plankton populations revealed surprisingly few larvae of several taxa, including fiddler, blue, and marsh (Sesarma retic-ulatum) crabs in the Hudson River estuary and estuarine plume, even though larvae of these species abounded in neighboring Long Island Sound (Hovel and Morgan, 1997) and bays and estuaries along the coast of Delaware (Epifanio et al., 1984, Epifanio et al., 1988). Destruction of intertidal adult habitat, perhaps compounded by pollu-tionmayhavegreatlyreduced the number offiddler and marsh crab larvae in the Hudson River estuary. Dense breeding populations in Fire Island Inlet on the southernshore of Long Island, also virtually disappeared about a decade before this study was conducted (J. Ebert, Natural Resources Manager Fire Island, personal communication), perhaps due to pollution or localized disease. First stage blue crab larvae may have been uncommon, because New York Bight may be nearing the northern limit of the reproductive range of this subtidal crab (Williams, 1984). Rather than living there year-round, females migrate along the coast from farther south to release larvae inlateAugustandSeptember (P. Briggs, NewYorkDepartment of Environmental Conservation, personal communication). The implications of the loss of the large input of crab larvae into estu-arine and nearshore plankton communities during the summer when many fishes are feeding and developing there is unknown.
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