Bay Anchovy Larvae Research Articles

Responses of bay anchovy (Anchoa mitchilli) larvae under lethal and sublethal scenarios of crude oil exposure

Bay anchovy (Anchoa mitchilli) is an ecologically important zooplanktivorous fish inhabiting estuaries of the Gulf of Mexico and eastern North America from Maine to Florida. Because they have a protracted spawning season (spring through fall) and are abundant at all life stages in coastal estuaries, their eggs and larvae likely encountered oil that reached the coast during the Deepwater Horizon oil spill. We compared responses to oil exposure at different life stages and at lethal and sublethal conditions using acute, 24h exposures. In a series of experiments, bay anchovy larvae were exposed to high energy water accommodated fractions (HEWAF) and chemically-enhanced WAF (CEWAF) at two stages of larval development (5 and 21 days post hatch, dph). HEWAF oil exposures induced significantly greater life stage dependent sensitivity at 5 dph than at 21 dph but chemically dispersed (CEWAF) exposure mortality was more variable and LC50s were not significantly different between 5 and 21dph larvae. Acute exposure to two low-level concentrations of CEWAF did not result in significant mortality over 24h, but resulted in a 25–77% reduction in larval survival and a 12–34% reduction in weight specific growth after six days of post-exposure growth following the initial 24h exposure. These results show that younger (5 dph) bay anchovy larvae are more vulnerable to acute oil exposure than older (21 dph) larvae, and that acute responses do not accurately reflect potential population level mortality and impacts to growth and development.

Foraging behavior of bay anchovy larvae, Anchoa mitchilli

Foraging behavior of the bay anchovy larvae, Anchoa mitchilli was characterized by silhouette video photography. Swimming speed, pause frequency and duration, burst duration, reactive distance (RD), horizontal and vertical reaction angle (HRA, VRA), and length of the predation cycle were estimated over a range of sizes and stages of development. First feeding larvae displayed a distinct pause-travel search behavior that included horizontal rotation of their body to the right or left to scan a new volume of water. Pause duration was affected by light intensity in small larvae and significantly decreased in length of time as larvae grew (ANOVA, p < 0.01). Duration of swimming bursts between pauses was short (mean = 0.13 ± 0.03 s). Average swimming speeds increased with size from a mean of 0.50 ± 0.39 BL · s - 1 (body lengths) at first feeding to 2.25 ± 1.27 BL · s - 1 at 12 mm SL. Mean RD of bay anchovy larvae at 10 lux and corrected for vertical angle was 0.65 BL and increased to 0.92 and 1.00 BL at 100 and 250 lux, respectively. The average RD increased in the presence of turbidity from 0.65 to 0.83 BL at 10 lux. Reducing light (250, 100, 10 lux) affected the foraging behavior of the largest larvae (14-19 mm SL) especially at 10 lux where only one attack was observed with or without turbidity. The Horizontal attack angles (HRA) of larvae ranged from 0-114° with 78% of attacks taking place at angles between 20 and 70° while vertical reaction angles (VRA) of larvae ranged from 12-90° with 77% of attacks occurring at < 50°. The average length of the predation cycle decreased with increasing larval size from 2.4 s down to 1.34 s for larger larvae. Bay anchovy larvae frequently aborted attacks (33%) after reacting and approaching prey. Attack success was high even for small larvae (mean = 83%) and approached 100% in the largest size class.

Distribution and transport of bay anchovy ( Anchoa mitchilli) eggs and larvae in Chesapeake Bay

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 (>76%), 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 l −1. The day–night cycle also influenced organism abundances and distributions. Observed diel periodicity in concentrations of bay anchovy and sciaenid eggs, and of bay anchovy larvae >6 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.

Mechanisms promoting upriver transport of larvae of two fish species in the Hudson River estuary

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 move- ments 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 apart, coinciding with spring and neap tides. We sampled every 2 h with an ichthyoplankton trawl to permit tests of time, depth, and lateral position on larval concentration. We also collected data on water-column structure with a CTD, and cur- rent velocity with an acoustic Doppler current profiler (ADCP). We briefly sampled at several sites over a distance of 25 km along the river, and found that larvae of both species were uniformly abundant along this section although salinity decreased sharply with increasing distance upriver. Bay anchovy larvae were more abundant than goby larvae (median concentration 234 vs 6.6 ind. 100 m -3 ). Most sampling was conducted at an oligohaline location (mean salinity = 3 to 5 psu). Larvae were typically more concentrated at greater depths; among anchovy larvae during neap tide conditions, and goby larvae during neap and spring tide conditions, larvae were more concentrated at 6 and 8 m than at the surface by a factor of 2 to 9. Large larvae showed a stronger depth preference than small larvae. During spring tide, the water column was less stratified, and anchovy larvae under these conditions were uniformly distributed vertically. There were slight lateral differences in larval concentration, with fewer larvae in shallow water over the shoals than in similar depths in the main channel. We evaluated periodic cycles in flow and larval dis- tributions via harmonic regression. Tidal constituents of the depth-averaged current flow included the K1 (period = 23.9 h), M2 (period = 12.4 h), and the M4 (period = 6.2 h) tides. Harmonic regression explained >95% of the observed variability in mean flow. Diel periodicity in depth-averaged larvae con- centration was evident, particularly among large anchovy and goby larvae during neap tide conditions. Larvae were more abundant in the sampled depths at night than during the day by a factor of 3 to 10. There was also diel periodicity in the mean depth of goby larvae, such that larvae were about 2 m shal- lower at night than during the day. There was no periodic variability in the mean depth of anchovy larvae. We suggest that diel periodicity in larval concentration and mean depth reflects diel migration to shallower water at night, noting that temporal variability in net avoidance may also contribute to the periodicity. We conclude that anchovy and goby larvae exhibit a depth distribution and vertical migra- tion behavior that promotes upriver transport. Transport should be most rapid during neap tide periods.

Ichthyoplankton Abundance along a Recruitment Corridor from Offshore Spawning to Estuarine Nursery Ground

Two temporally separate larval fish assemblages were identified from ichthyoplankton samples collected between November 1987 and January 1990 along a transect from offshore to Oyster Bayou tidal pass into Fourleague Bay, Louisiana. The warm water species (e.g. bay anchovy, naked goby, Microgobius sp., spotted seatrout and skilletfish) dominated in water temperatures >23 °C and the cooler water species (e.g. gulf menhaden and Atlantic croaker) dominated in water temperatures <23 °C. These assemblages were characterized by different life-history strategies, distributions, and seasonal abundances suggesting that they utilize resources in different locations and times. Analysis of Oyster Bayou tidal pass data indicated that larvae from all species were more abundant on flood than ebb tides and larval fish densities were often statistically greater near bottom, suggesting tidal transport and/or retention. In addition, larger larval bay anchovy and gulf menhaden were taken in the middle of the tidal pass during flood tides. During ebb tides, however, larger bay anchovy larvae were taken along the pass edges, suggesting behaviourally mediated transport into and/or retention within the estuary.

Ichthyoplankton abundance along a recruitment corridor from offshore spawning to estuarine nursery ground

Two temporally separate larval fish assemblages were identified from ichthyoplankton samples collected between November 1987 and January 1990 along a transect from offshore to Oyster Bayou tidal pass into Fourleague Bay, Louisiana. The warm water species (e.g. bay anchovy, naked goby, Microgobius sp., spotted seatrout and skilletfish) dominated in water temperatures >23 °C and the cooler water species (e.g. gulf menhaden and Atlantic croaker) dominated in water temperatures <23 °C. These assemblages were characterized by different life-history strategies, distributions, and seasonal abundances suggesting that they utilize resources in different locations and times. Analysis of Oyster Bayou tidal pass data indicated that larvae from all species were more abundant on flood than ebb tides and larval fish densities were often statistically greater near bottom, suggesting tidal transport and/or retention. In addition, larger larval bay anchovy and gulf menhaden were taken in the middle of the tidal pass during flood tides. During ebb tides, however, larger bay anchovy larvae were taken along the pass edges, suggesting behaviourally mediated transport into and/or retention within the estuary.

Growth and survival of bay anchovy Anchoa mitchilli larvae in mesocosm enclosures

Factors that affect sunrival and growth of bay anchovy Anchoa mitchilli larvae in the Chesapeake Bay, USA, were studied in situ in 3.2 m3 mesocosm enclosures. Experiments tested the effects of variable prey level and anchovy egg and larvae densities on growth and survival of larvae to at least 16 d post-hatching. Eggs were stocked at 100 m-3 with microzooplankton (< 150 ~m) prey added at nominal levels of 50, 500, 1500, or 5000 1-l. In a second series of experiments, eggs were stocked at 10, 100, or 2 1000 m-3 and prey level was maintained near 500 1-l. Larvae growth was rapid (0.39 to 0.63 mm d-l), even at the lowest prey levels and highest initial egg stocking densities. Growth rates tended to decrease as egg density increased. Instantaneous mortality rates were high (0.08 to 0.23 d-l) but lower than those reported for bay anchovy larvae from estuaries with gelatinous zooplankton and fish predators present. Highest mortality unexpectedly occurred in mesocosms with the highest prey level and lowest mortality occurred in enclosures with the highest initial egg stocking densities. Relative production decreased as prey level increased but Gw:Z ratios were above unity for all except the highest prey level experiment. In the stocking density experiments, highest production occurred in enclosures with the lowest initial egg densities. Some unexpected results may have been induced by effects of the mesocosm enclosures.