Introduction

Estuaries provide essential habitat for many commercially and ecologically important animals during at least one phase of their life cycle. Most of these estuarine-dependent animals produce tremendous numbers of microscopic larvae that disperse away from parents before returning to suitable settlement sites. Because population dynamics of such organisms may be regulated by factors that affect either the larval or adult stages of their life histories, fluctuations in population sizes oftenmaybegreat.Mostmortalityisbelievedtooc-cur in the plankton because eggs and larvae have less control over their fates in this dynamic environment than do larger juveniles and adults that live on stable substrates. According to this line of thinking, few larvae survive long enough to settle into adult habitats except during years when favorable planktonic conditions prevail (Thorson, 1950; Morgan, 1995,2001). The shear numbers oflarvae produced, poor swimming capabilities of tiny larvae, and episodic settlement events have led to the belief that various sources of mortality overwhelm larvae.

Advection of larvae by currents may be one of the most important sources of mortality and may generate considerable variation in the timing and magnitude of recruitment. Indeed, one of the earliest mysteries that intrigued investigators throughout the twentieth century was how weakly swimming larvae avoid being flushed from estuaries and lost to thepopulation (Young, 1990). Net flow in estuaries is seaward, which would seemingly sweep weakly swimming larvae out to sea. Once larvae are adrift in a vast ocean, the probability of them finding their way back into an estuary would seem to be small. At the very least, larvae should be transported downstream from the parental population, raising the question of how they return upstream to recruit to adult habitats. These sorts of problems have led many to believe that marine organisms play the lottery; they produce thousands or millions of larvae, increasing the odds that a few will return by chance. In contrast to this traditional view of marine life histories, mounting evidence suggests that larvae of estuarine-dependent animals

Figure 12.1. Sampling scheme consisting of three 48hour stations in the upper Hudson River estuary sampled during spring and neap tides, multiple stations to map the location of the estuarine plume, a transect across the estuarine plume sampled for several days four times during spring and neap tides, two transects across the continental shelf sampled weekly for four months (June-Sept), two pairs of48-hour stations on the inner shelf, two pairs of 48-hour stations on the outer shelf and two moorings with current meters on the inner and outer shelf between the transects. For each sampling scheme, the vertical and horizontal distributions of larvae relative to water column structure and current velocity were documented duringtwo consecutive summers. The dotted line at the seaward end of the two transects represents the shelf break.

make true migrations between adult and larval habitats by exploiting predictable oceanographic features (Epifanio and Garvine, 2001, Strathmann etal., 2002).

Most of the previous research on larval transport in the Hudson River estuary has been conducted on commerciallyimportant species of anadromous fishes that spawn and develop in the upper reaches ofthe river. However, manyotherimportantspecies develop in the estuary and adjacent coastal waters and their larval distributions are barely known. With the help of my students and colleagues, I launched an oceanographic research program to provide an initial understanding of how larvae are transportedbetween the Hudson River Estuary and NewYork Bight. The research program consisted of three sampling schemes reaching from the upper estuary near the George Washington Bridge to the edge of the continental shelf (Fig. 12.1). The sampling program coupled extensive horizontal surveys of the study area with intensive vertical profiles of the water column to determine how this differential transport is accomplished.

The purpose of this chapter is to provide an overview of this comprehensive research program. I will highlight (1) larval transport patterns of species that spend at least part of their life cycle in the Hudson River Estuary, (2) the mechanisms enabling differential larval transport, (3) management implications of these transport patterns, and (4) directions for future research.

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