Juvenile American Shad Alosa Sapidissima Research Articles

Findings on American Shad and Striped Bass in the Hudson River Estuary: A Fish Community Study of the Long‐Term Effects of Local Hydrology and Regional Climate Change

AbstractIt has been hypothesized that climate change is an underlying factor in determining fish abundances in the Hudson River estuary (HRE). To study the effects of hydrology and climate on the HRE fish community, we applied multivariate statistical methods to data on 20 species–life history stages collected from 1974 to 2005. We confirm that the HRE fish community has changed over this period. These changes are correlated with local hydrology (freshwater flow and water temperature) and regional climate (the Atlantic Multidecadal Oscillation [AMO] and North Atlantic Oscillation). We found that the abundances of striped bass Morone saxatilis larval stages are positively correlated with high freshwater flows and those of juvenile American shad Alosa sapidissima negatively correlated with the AMO or warmer sea surface temperatures. Our findings suggest that climate‐related variability affects HRE juvenile shad abundances and that management strategies for this declining species should include the implications of climate change.Received March 19, 2011; accepted January 10, 2012

Cohort‐Specific Growth and Mortality of Juvenile American Shad in the Pamunkey River, Virginia

AbstractWe estimated the variation in instantaneous rates of growth (G) and mortality (M) between intraannual cohorts of juvenile American shad Alosa sapidissima in the Pamunkey River, Virginia. The ages of juveniles captured by push net during the juvenile abundance index surveys in 1998 and 1999 were estimated by counting daily rings on the sagittal otoliths. Weight‐at‐age and abundance‐at‐age data were used to generate instantaneous daily rates of growth and mortality for 5‐d cohorts. In 1998, the peak hatch date lagged behind the peak spawning periods that had been inferred from collections of American shad broodstock and occurred after peak spring flow events. In 1999, the correspondence between the hatch date distribution and the peak spawning periods was greater than in 1998. The instantaneous daily growth rate was relatively constant between cohorts, ranging from 0.037 to 0.064 in 1998 and from 0.046 to 0.066 in 1999. The instantaneous daily mortality rate was more variable between cohorts, ranging from 0.047 to 0.084 in 1998 and from 0.044 to 0.093 in 1999. The physiological mortality rate, or M/G, was calculated for all cohorts. Most cohorts in 1998 and 1999, including the largest cohorts in both years, had an M/G value close to 1.0, indicating that these cohorts were barely maintaining or losing biomass during the early juvenile stage. The results of this study indicate that in both 1998 and 1999 the year‐class declined in biomass during the early juvenile stage and underwent demographic structuring that affected the composition of the population that ultimately migrated to sea.

Late migration and seawater entry is physiologically disadvantageous for American shad juveniles

Juvenile American shad Alosa sapidissima were subjected to isothermal transfers into sea water (salinity 24)‘early’(1 September; 24° C) and ‘late’(10 November; 10° C) in the autumn migratory season. Early acclimation resulted in a modest osmotic perturbation that recovered rapidly. Haematocrit declined by 14% at 24 h, recovering within 48 h. Plasma osmolality increased by 6% at 4 h, recovering within 8 h. Early acclimation caused a two‐fold increase in gill Na+, K+‐ATPase activity by 24 h and a four‐fold increase by 4 days. The number of chloride cells on the primary gill filament increased two‐fold by 4 days. Chloride cells on the secondary lamellae rapidly decreased from 22 cells mm−1 to <2 cells mm−1 within 4 days. Late acclimation resulted in a severe and protracted osmotic perturbation. Haematocrit levels declined by 23% at 4 days, recovering by 14 days. Plasma osmolality increased by 36% by 48 h, recovering by 4 days. Initial gill Na+, K+‐ATPase activity was two‐fold greater than in ‘early’ fish and did not change during acclimation. Initial numbers of chloride cells on the primary filament were two‐fold greater than ‘early’ fish and did not increase during acclimation. Initial number of chloride cells on the secondary lamellae was five‐fold greater than ‘early’ fish (116 v. 22 cells mm−1) and declined to negligible numbers over 14 days. Differences between initial measures for ‘early’ and ‘late’ fish reflect previously described physiological changes associated with migration. These data indicate that late migrants face a greater physiological challenge during seawater acclimation than early migrants. Physiological performance apparently limits the observed duration of autumnal migration.

The Stress Response of Juvenile American Shad to Handling and Confinement is Greater during Migration in Freshwater than in Seawater

The physiological responses of juvenile American shad Alosa sapidissima were evaluated during the period of downstream migration in freshwater (FW) and after seawater (SW) acclimation associated with postmigration. Fish were subjected to a standardized, acute handling and confinement stress (3 h). Changes in plasma cortisol, plasma chloride, and hematocrit were monitored for 24 h. Basal levels of plasma cortisol were 5 times as great in FW as in SW fish (34 and 7 ng/mL, respectively). Within 0.5 h, both groups exhibited significant increases in plasma cortisol. The increase in FW fish was 4.5 times as great as that in SW fish. While the cortisol levels in SW fish returned to their basal values within 24 h, those of FW fish remained more than 25 times as high as basal values. Changes in plasma chloride occurred in fish after the initial stress, decreasing in FW fish and increasing in SW fish. This perturbation was overcome within 24 h in SW fish but not in FW fish. Hematocrit increased in FW fish 3 h after the initial stress and returned to normal within 24 h; fish stressed in SW exhibited no change in hematocrit. Significant mortalities were observed in the FW group but not in the SW group. During the period of downstream migration in FW, fish exhibited a heightened sensitivity to acute stress compared with that of SW-acclimated postmigrants. Handling and confinement associated with fish transport and fish passage structures are therefore likely to impact the performance scope and survival of juvenile American shad.

Effect of Water Acceleration on Downstream Migratory Behavior and Passage of Atlantic Salmon Smolts and Juvenile American Shad at Surface Bypasses

Abstract Behavior and passage rate of smolts of Atlantic salmon Salmo salar and juvenile American shad Alosa sapidissima were compared between a standard (sharp-crested) and a modified surface bypass weir that employs uniform flow velocity increase (1 m·s−1·m−1 of linear distance). Within the first 30 min after release, significantly more smolts passed the modified weir than the standard weir, but no differences in passage rate between weir types were noted for juvenile American shad. More Atlantic salmon smolts and juvenile American shad were passed by the modified weir in groups of two or more than were passed by the standard weir. Mean lengths of passed and nonpassed smolts were not significantly different between weir types, but American shad passed by the sharp-crested weir were significantly smaller than nonpassed fish. Most individuals of both species that passed the modified weir maintained positive rheotaxis and strong swimming throughout the length of the weir. In addition to acceleration, visua...

Notes: Turbine-Passage Mortality of Juvenile American Shad at a Low-Head Hydroelectric Dam

The immediate (1-h) turbine-related mortality of juvenile American shad Alosa sapidissima at the Hadley Falls Hydroelectric Station on the Connecticut River, Holyoke, Massachusetts, was estimated to be 0% ± 14.5% (95% confidence interval) at the 35% wicket gate opening and 2.7% ± 16.2% at the 100% opening. We used the HI-Z TurbˈN tag-recapture technique, which helped minimize control mortality and maximize recapture rates. Earlier literature estimates of turbine-related mortality (up to 82%) of juvenile alosids in passage through Kaplan turbines, in our view, were substantially overstated due to either low recapture rate, high control mortality, or both.

Notes: Group-Size-Mediated Metabolic Rate Reduction in American Shad

The relation of oxygen consumption to ambient temperature and group size was studied in juvenile American shad Alosa sapidissima at three group sizes: 5 or 6 (small), 9–13 (medium), and 41–48 (large) fish per 500-L tank. Oxygen consumption rates, water temperatures, and behavior were measured concurrently in the three group-size treatments. Schooling predominated in large groups, whereas aggregating behavior dominated medium and small groups. Fish in small or medium groups consumed oxygen at two to four times the rate of fish in large groups (0.36 or 0.25 versus 0.11 mg O2/[g biomass·h]). This group-size-related respiratory rate reduction could be caused by the effect of calming, type of swimming, swimming speed, or hydrodynamic advantage.

Behavior, Length, and Sex Ratio of Seaward-Migrating Juvenile American Shad and Blueback Herring in the Connecticut River

The fall downstream migrations of juvenile American shad Alosa sapidissima and blueback herring Alosa aestivalis were monitored for 3 years in the Connecticut River at Holyoke Dam, Massachusetts. Decreasing water temperature—not increasing river flow—determined the time migration began and ended for both species. The migration of blueback herring began in September as water temperature declined to 21°C, peaked at 15–14°C, and ended in late October or early November at 10°C. The American shad migration began at 19°C, peaked at 14–9°C, and ended at 10–8°C. Most movement peaks were centered on quarter-moon periods; seven were centered on a new moon and none was centered on a full moon. The diel pattern of migration varied among the 3 years. In general, however, blueback herring were active throughout the 24-h period (peak activity at 1800 hours), and American shad moved in the afternoon and evening (peak activity between 1800 and 2200 hours). The migration of juvenile American shad may be mediated by decreasing temperature such that, as a threshold of 19°C is reached in the fall, the behavioral tendency to maintain position against the current in low light or at night is curtailed and the juveniles drift downstream. Sex ratios of American shad were not 1:1; females dominated in 1980 and males in 1981 and 1982. Sex ratios of blueback herring were 1:1 in 1980 and 1981, but males significantly outnumbered females in 1982. The mean total fish lengths increased significantly during the course of the migration for American shad in 1982 and for blueback herring in 1981.