Estuarine, Coastal and Shelf Science 60 (2004) 409e429 www.elsevier.com/locate/ECSS Distribution and transport of bay anchovy (Anchoa mitchilli) eggs and larvae in Chesapeake Bay ) E.W. North , E.D. Houde1 University of Maryland Center for Environmental Science, Chesapeake Biological Laboratory, USA Received 28 February 2003; accepted 8 January 2004 Abstract Mechanisms and processes that influence small-scale depth distribution and dispersal of bay anchovy (Anchoa mitchilli) early-life stages are linked to physical and biological conditions and to larval developmental stage. A combination of fixed-station sampling, an axial abundance survey, and environmental monitoring data was used to determine how wind, currents, time of day, physics, developmental stage, and prey and predator abundances interacted to affect the distribution and potential transport of eggs and larvae. Wind-forced circulation patterns altered the depth-specific physical conditions at a fixed station and significantly influenced organism distributions and potential transport. The pycnocline was an important physical feature that structured the depth distribution of the planktonic community: most bay anchovy early-life stages (77%), ctenophores (72%), copepod nauplii (O76%), and Acartia tonsa copepodites (69%) occurred above it. In contrast, 90% of sciaenid eggs, tentatively weakfish (Cynoscion regalis), were found below the pycnocline in waters where dissolved oxygen concentrations were !2.0 mg lÿ1. The dayenight cycle also influenced organism abundances and distributions. Observed diel periodicity in concentrations of bay anchovy and sciaenid eggs, and of bay anchovy larvae O6 mm, probably were consequences of nighttime spawning (eggs) and net evasion during the day (larvae). Diel periodicity in bay anchovy swimbladder inflation also was observed, indicating that larvae apparently migrate to surface waters at dusk to fill their swimbladders. Overall results suggest that wind-forced circulation patterns, below-pycnocline dissolved oxygen conditions, and diel changes in vertical distribution of larvae and their copepod prey have important implications for potential transport of bay anchovy early-life stages. Ó 2004 Elsevier Ltd. All rights reserved. Keywords: biologicalephysical interactions; larval transport; predatoreprey interactions; bay anchovy; zooplankton; Chesapeake Bay 1. Introduction stages, including those of bay anchovy (Anchoa mitchilli) in Chesapeake Bay. The mechanisms by which organisms respond or Bay anchovy is the most abundant fish in Chesapeake react to the biological and physical environment are Bay and in many coastal areas of the western North critical in estuaries, the site of important spawning Atlantic. It is a pelagic, small (!110 mm), short-lived grounds and nursery areas for many fish species. Many (!3 years) fish that plays an important ecological role in factors, including physics, larval development, food Chesapeake Bay as a major prey for piscivores such as abundance, and predation act and interact to affect the striped bass (Morone saxatilis), bluefish (Pomatomus small-scale distributions and dispersal of fish early-life saltatrix), and weakfish (Cynoscion regalis)(Baird and Ulanowicz, 1989; Hartman and Brandt, 1995). It is ) Corresponding author. University of Maryland Center for a pelagic, serial spawner with a reproductive season in Environmental Science, Horn Point Laboratory, P.O. Box 775, Chesapeake Bay that extends from May to September Cambridge, MD 21613, USA. peaking in July (Luo and Musick, 1991; Zastrow et al., 1 Present address: University of Maryland Center for Environmen- 1991). Spawning occurs at salinities from 0 to 32 psu tal Science, Chesapeake Biological Laboratory, P.O. Box 38, Solo- mons, MD 20688, USA. (Dovel, 1971; Olney, 1983; Houde and Zastrow, 1991) E-mail addresses: [email protected] (E.W. North), ehoude@ and peaks at temperatures from 26 to 28 (C(Houde and cbl.umces.edu (E.D. Houde). Zastrow, 1991). Bay anchovy spawns between 1800 and 0272-7714/$ - see front matter Ó 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.ecss.2004.01.011 410 E.W. North, E.D. Houde / Estuarine, Coastal and Shelf Science 60 (2004) 409e429 0100 hours (Luo and Musick, 1991; Zastrow et al., position in the water column in relation to currents 1991), producing daily cohorts of eggs that hatch into rather than by swimming horizontally (Norcross and yolk-sac larvae at 18e27 h after fertilization, depending Shaw, 1984; Miller, 1988). Several transport mecha- upon temperature (Houde and Zastrow, 1991). Larvae nisms have been documented for larval fish including begin feeding about 2 days after hatching (Houde and tidally-timed vertical migration, also referred to as Zastrow, 1991). Larval anchovy first feed on microzoo- selective tidal stream transport (Rowe and Epifanio, plankton such as copepod nauplii, rotifers, and tintin- 1994), migration around the depth of null velocity nids, and shift to larger copepodites and adult copepods (Fortier and Leggett, 1983), and migration in relation to as they grow (Houde and Zastrow, 1991; Auth, 2003). both time of day and tides (Weinstein et al., 1980). In Their growth rate is temperature-dependent (Houde, estuaries with two-layer circulation, such as Chesapeake 1974) and is about 0.5e0.8 mm dÿ1 in Chesapeake Bay Bay, diel vertical migration could result in up-estuary (Rilling and Houde, 1999b; Auth, 2003). transport if larvae move into landward-flowing waters Physical conditions may influence the spatial and during the day in summer when days are longer than vertical distribution of bay anchovy early-life stages. nights. It is also possible that random movements of The occurrence and survival of early-life stages below larvae, coupled with frequent spawning by adults, could the pycnocline may depend on dissolved oxygen (DO) lead to retention of a sizable fraction of larvae in the concentrations because larvae avoid waters with low DO estuary. (Breitburg, 1994; Keister et al., 2000) which can limit the The objective of this research was to identify the viability of bay anchovy early-life stages (Chesney and mechanisms and processes influencing small-scale verti- Houde, 1989; Houde and Zastrow, 1991). Temperature cal distributions and potential transport of bay anchovy also potentially could influence the distribution of eggs and larvae. The study was designed to determine bay anchovy early-life stages, especially in highly how currents, time of day, physics (temperature, sali- stratified conditions. The reported range of occurrence nity, DO), ontogeny (egg and larval stages), food abun- for temperature is 13.0e30.0 (C for eggs and 15.0e dance, predation, and weather act or interact to affect 30.0 (C for larvae in Chesapeake Bay (Houde and egg and larval distributions. The temporal scale of sam- Zastrow, 1991). In addition, circulation patterns such as pling was designed to detect diel and tidally-timed residual eddies (Hood et al., 1999) and plume fronts vertical migrations of fish larvae. The research consisted (Peebles, 2002) can form retention areas that affect of: (1) an initial survey in Chesapeake Bay to determine early-stage distributions. areas of maximum egg and larvae abundance; (2) depth- Prey and predator concentrations may influence the stratified sampling at a fixed location to describe vertical spatial occurrence and vertical distribution of bay anch- distributions of early-life stages in relation to physical ovy eggs and larvae. Adult bay anchovy may spawn and biological factors; (3) length measurements of larvae where food abundance is high, leading to an association to classify larvae by ontogenetic stage; and (4) an anal- between high concentrations of early-life stages and the ysis of environmental data to evaluate factors that in- copepods that serve as prey for adult anchovy (Peebles fluenced physical conditions and organism distributions. et al., 1996; Peebles, 2002). The location of larval prey may also influence larval distributions, and larvae may follow the vertical migrations of copepod prey. Pre- 2. Methods dation may affect the distribution of bay anchovy eggs and larvae by: (1) causing direct mortality; (2) stimulat- Data were collected in Chesapeake Bay from 18e27 ing predator-avoidance movements by larvae; and/or June 1996 on the 120 ft RV Cape Henlopen. The first two (3) influencing adult spawning-site selection because days of the cruise consisted of an ichthyoplankton and adults may avoid spawning in areas of high gelatinous CTD survey along the axis of Chesapeake Bay (Fig. 1). zooplankton abundance (Dorsey et al., 1996). Major After the initial survey, sampling effort was concen- predators include the gelatinous zooplankton scypho- trated at a fixed station located in mid-Bay (37( 45# N) medusan (Chrysaora quinquecirrha) and the lobate from 20 to 23 June and from 26 to 27 June (Fig. 1). ctenophore (Mnemiopsis leidyi)(Purcell et al., 1994). Ontogenetic changes in swimming ability, buoyancy regulation, and larval behavior also may influence larval 2.1. Axial survey distributions (Boehlert and Mundy, 1988). Potential for buoyancy regulation increases when the swimbladder The initial survey along the axis of Chesapeake Bay first inflates, although precise control may not be pos- was conducted to determine the spatial distributions sible until later in the development process (O’Connell, and abundances of fish early-life stages and gelatinous 1981). zooplankton and to locate suitable areas for inten- Bay anchovy larvae could affect their transport sive depth-stratified collections. Twelve