Fig 7 - uploaded by Eric T Schultz
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Anchoa mitchilli and Gobiosoma bosc. Temporal variability in larval abundance and depth distribution. Symbols represent observed values, and lines the fitted relationship based on harmonic regression, including only those predictors that were nominally (p < 0.05) significant. (A) Anchovy larval abundance, for clarity, values for small larvae have been shifted up 1 increment on the y-axis, and values for large larvae have been shifted down 1 increment. (B) Anchovy larvae mean depth, values for small larvae have been shifted up (shallower) 1 increment on the y-axis, and values for large larvae have been shifted down (deeper) 1 increment. (C) Goby larval concentration, values for small larvae have been shifted up 1 increment. (D) Goby larvae mean depth, values for small larvae have been shifted up (shallower) 2 increments on the y-axis. Key applies to all graphs
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Bay anchovy Anchoa mitchilli and naked goby Gobiosoma bosc larvae have been reported to move up-estuary. In the present study, we examined depth preferences and periodic vertical movements that might promote such along-estuary transport in these 2 species. We conducted 2 cruises of 3 d each in the Hudson River estuary, USA. The cruises were 1 wk ap...
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Citations
... An effect of season was tested by standardizing date to a mean of 0 and including date and date-squared (date 2 ) as predictors. The effect of moon phase was tested via harmonic regression (Batschelet 1981;Lorda and Saila 1986;Schultz et al. 2003) wherein the 328-h cycle is partitioned into two trigonometric variables that can be used as predictors in linear regression models. Collinearity among the regressors was assessed via correlation tables and eigenanalysis of the design matrix ( Belsley et al. 1980). ...
We analyzed juvenile anadromous alewife migration at Bride Lake, a coastal lake in Connecticut, during summer 2006 and found that migration on 24-hour and seasonal timescales was influenced by conditions of the environment and characteristics of the individual. To identify environmental cues of juvenile migration, we continuously video recorded fish at the lake outflow and employed information-theoretic model selection to identify the best predictors of daily migration rate. More than 80% of the approximately 320,000 juveniles that migrated from mid-June to mid-August departed in three pulses lasting one or two days. Pulses of migration were associated with precipitation events, transient decreases in water temperature and transient increases in stream discharge. Diel timing of migration shifted over the summer. Early in the season most migration occurred around dawn; late in the season migration occurred at night. To identify individual characteristics associated with migratory behavior, we compared migrating juveniles that we collected as they were exiting Bride Lake to non-migrating juveniles that we collected from the center of the lake. Migrants were a non-random subset of the population; they were on average 1 – 12 mm larger, 2 – 14 d older, had grown more rapidly (11% greater length-at-age), and were in better condition (14% greater mass-at-length) than non-migrant fish. We infer that the amount of accumulated energy has a positive effect on the net benefit of migration at any time in the migratory season.
... It is a predominant member of the estuarine fish assemblage, ranging from tropical and subtropical estuaries in the Gulf of Mexico and the Atlantic coast of the southeastern US (Deegan and Thompson 1985;Castillo-Rivera et al. 1994;Snelson and Johnson 1995;Ayala Perez et al. 1998) to the temperate estuaries of the North Atlantic Bight and southern New England (Dovel 1981;Monteleone 1992;Keller et al. 1999;Dorfman 2000). Early-stage (larvae and juvenile) anchovy are often the most abundant members of the summer ichthyoplankton assemblage (Olney 1983;Setzler-Hamilton 1987;Monteleone 1992;Keller et al. 1999;Schultz et al. 2003). In northern latitudes, anchovy migrate out of estuaries to coastal waters in the autumn, returning in spring as reproduction commences (Dovel 1981;Vouglitois et al. 1987;Schmidt 1992;Wang and Houde 1995). ...
... The behavioral mechanisms underlying migration of early-stage anchovy have been recently studied in the Hudson River (Schultz et al. 2000;Schultz et al. 2003). ...
... Temporally-averaged depth distributions of early-stage anchovy have often been nonuniform but have not been consistent. In some circumstances anchovy exhibited preference for greater depth (see also Loos and Perry 1991;Schultz et al. 2003). During one three-day period of sampling in neap tide conditions at Croton Point ( Fig 4B) anchovy were two to four times more concentrated at depth in the channel than at the surface. ...
We review three areas of recent research on Hudson River bay anchovy. One focus has been the along-estuary movement of early life stages. A cohort analysis of samples collected in a spatiotemporally extensive monitoring program has confirmed that early-stage anchovy migrate up-estuary, at an estimated rate of 0.6 km/d. Complementary fine-scale field sampling was designed to clarify behaviors that effect the migration. This work found that early-stage anchovy can show preferences for depth and can conduct periodic vertical migration. To determine whether these behaviors were sufficient to produce up-estuary migration, larval flux and velocity were estimated. These estimates were consistent with local retention rather than concerted migration. High priority should be given to examining individual migration histories through analysis of otolith microchemistry. A second focus of research on Hudson anchovy has been on local population structure, permitting comparison to anchovy in other locations. The demography of the Hudson River anchovy appears to be unique. Anchovy that spawn in the Hudson River are larger than those spawning in the Chesapeake Bay region and are mostly two years old, whereas yearlings predominate in other estuaries. Batch fecundity was lower and egg mortality higher in Hudson River than in Chesapeake Bay. A key issue arising from these recent findings is the degree to which the Hudson anchovy pool is connected with other large anchovy pools, such as Narragansett Bay and Chesapeake Bay. A third focus of research on Hudson anchovy has been analysis of interannual variability in early-stage abundance. A >20 year time series of juvenile bay anchovy abundance shows that juvenile abundance has varied over one order of magnitude. There has been no significant change in abundance over the entire time series, but abundance has declined 10-fold since a peak in the late 1980s. Anchovy abundance was negatively associated with the abundance of early-stage striped bass, and positively associated with the abundance of early-stage tomcod. We suggest that these associations reflect direct interactions among the species and urge further work on the ecological role of striped bass in the estuary.
... Generally migration (or retention) is promoted by periodic vertical migration such as selective tidal stream transport, or in some cases a simple depth preference ( Grioche et al. 2000). Anchovy larvae in some situations migrate on a daynight cycle and exhibit a preference for deeper water (Schultz et al. 2003). The extent to which these behaviors promote up-estuary migration remains to be quantified. ...
Larvae and juveniles are often most abundant at some distance from where spawning occurs. Such apparent distribution shifts
can be the result of larval migration, seasonal shifts in the distribution of spawners, or spatial heterogeneity in mortality.
In the present study, a cohort-based method was adopted to test for larval migration of bay anchovy (Anchoa mitchilli) and to estimate its rate and timing. A spatially and temporally extensive series of ichthyoplankton samples was collected
in the lowest 122 km of the Hudson River in summer 1998. Bay anchovy eggs and young-of-year individuals were abundant (mean
density=1.6 m−3 and 0.95 m−3, respectively). Daily age used to define cohort membership was determined from otoliths. Age and length were strongly related
(R2=0.74). The age-length relationship varied over the season such that early-season bay anchovy hatched in June grew more rapidly
than later-season anchovy hatched in August. Cohorts were defined as week-long birth-data classes, and net migration patterns
of 11 cohorts were analyzed. Mean location shifted upriver as cohorts increased in age to 6 wk, at a net rate of about 0.6
km d−1. Cohort abundance usually decreased over time in downriver regions, but usually increased in upriver regions. The cohort
analysis confirmed that the upriver shift in distribution of early-stage anchovy is not entirely due to changes in adult spawning
behavior or to clines in larval mortality, and must be partly the result of larval migration.
... Short-term (tidal), seasonal, and longer-term (interannual to decadal) changes in fish distribution and abundance can, at least partially, be ascribed to changes in the salt distribution. Vertical distribution is also affected by salinity preference (e.g., Schultz et al. 2003) and vertical distribution in turn affects predator-prey interactions and alongriver transport, among other processes. ...
