Michael J. Van Den Avyle

ZOOPLANKTON VARIABILITY AND LARVAL STRIPED BASS FORAGING: EVALUATING POTENTIAL MATCH/MISMATCH REGULATION

We quantified temporal and spatial variability of zooplankton in three potential nursery sites (river, transition zone, lake) for larval striped bass (Morone saxatilis) in Lake Marion, South Carolina, during April and May 1993–1995. In two of three years, microzooplankton (rotifers and copepod nauplii) density was significantly greater in the lake site than in the river or transition zone. Macrozooplankton (>200 μm) composition varied among the three sites in all years with adult copepods and cladocerans dominant at the lake, and juvenile Corbicula fluminea dominant at the river and transition zone. Laboratory feeding experiments, simulating both among-site (site treatments) and within-site (density treatments) variability, were conducted in 1995 to quantify the effects of the observed zooplankton variability on foraging success of larval striped bass. A greater proportion of larvae fed in the lake than in the river or transition-zone treatments across all density treatments: mean (x), 10x and 100x. Larvae also ingested significantly more dry mass of prey in the lake treatment in both the mean and 10x density treatments. Field zooplankton and laboratory feeding data suggest that both spatial and temporal variability of zooplankton influence larval striped bass foraging. Prey density levels that supported successful foraging in our feeding experiments occurred in the lake during late April and May in 1994 and 1995 but were never observed in the river or transition zone. Because the rivers flowing into Lake Marion are regulated, it may be possible to devise flow management schemes that facilitate larval transport to the lake and thereby increase the proportion of larvae matched to suitable prey resources.

Comparison of Catch Rate, Length Distribution, and Precision of Six Gears Used to Sample Reservoir Shad Populations

Abstract Hydroacoustics, trawling, gillnetting, electrofishing, shoreline seining, and cove rotenoning were used concurrently in August 1991 at Lake Texoma, Texas–Oklahoma, to compare sampling efficiency for gizzard shad Dorosoma cepedianum and threadfin shad D. petenense. A simple random-sampling design was used at nearshore and offshore stations in each of three 400–2,000-ha sites in the reservoir. Most gears provided similar evidence of spatial patterns of shad abundance among sites, but length distributions and sampling precision varied among methods. At offshore transects, catch-per-unit-effort (CPUE) data for hydroacoustics, trawling, and gillnetting were positively correlated (r = 0.45–0.80; P < 0.05) when data from all sites were combined. Gears differed in proportions of small (age-0) shad versus larger (age-1 and older) fish. Gears that collected mainly age-0 shad were trawls, surface-set gill nets, and seine, whereas catches of age-1 and older shad were greater with bottom-set gill nets, electr...

Genetic Analysis of Reproduction of Hybrid White Bass x Striped Bass in the Savannah River

Abstract Hatchery-reared F1 hybrids of male white bass Morone chrysops x female striped bass M. saxatilis are stocked routinely in the Savannah River drainage by agencies of Georgia and South Carolina. Concern has arisen that these F1 hybrids themselves may be reproducing, and that possible increases in abundance of F2 hybrids or backcrosses could disturb white bass and striped bass parental stocks through ecological competition or genetic introgression. We identified and employed genetic markers at four protein-electrophoretic systems to search for recombinant genotypes indicative of successful hybrid reproduction. The genetic basis and reliability of the allozyme markers were confirmed by analysis of hatchery-reared F2 hybrids, and by examination of geographic variation in allele frequencies in populations from several southeastern states. Only 6 of 642 fish assayed from the Savannah drainage had recombinant genotypes. However, in two of four test cases involving Morone samples from other locales, the g...

Effects of Saltwater Intrusion and Flow Diversion on Reproductive Success of Striped Bass in the Savannah River Estuary

Abstract Spawning by striped bass Morone saxatilis in the Savannah River estuary was monitored in 1986–1989 to determine effects of elevated salinity and altered channel morphology on reproductive success. A tide gate and its attendant channel modifications displaced the salt wedge (salinity >0.5‰) upriver and altered the pathways and rates of water flow in the estuary. Average densities of striped bass eggs at sites sampled in 1989 were 0.5–11.0% of levels reported in the late 1970s. All recently spawned eggs (< 10 h old) were found where salinity was 0.5‰ or less, an indication that adults select freshwater sites for spawning. A hypothesis of consistent recruitment failure in the 1980s is supported by parallel declines in abundance of eggs and adults on the spawning grounds, the lack of any detectable upriver displacement of spawning activity, and the low abundance of eggs in 1988–1989 despite the freshwater conditions prevalent at the historic spawning sites. Studies that used floats to simulate rates ...

Interactive Effects of Stocking Site Salinity and Handling Stress on Survival of Striped Bass Fingerlings

Abstract We evaluated the effects of prestocking handling and stocking site salinity on survival and stress response during the first 48 h after stocking for three size-classes of cultured striped bass Morone saxatilis (mean sizes: 20, 65, and 230 mm total length). Treatments were routine handling, tagging, and minimal handling. Routinely handled and tagged fish experienced normal hatchery procedures for a large-scale stocking program. Minimally handled fish were transported directly from rearing ponds to stocking sites at low densities. Fish were stocked at freshwater or brackish (salinity, 0–7‰) sites. Survival in cages was monitored for 48 h after stocking, and cortisol concentration was measured to indicate stress levels. Handling before stocking, stocking site salinity, and the interaction of these factors significantly affected survival and cortisol levels. For the two larger size-classes, minimally handled fish had high survival rates (>99%) and low cortisol levels (≤18 μg/dL) at both stocking site...

Evaluation of Gill-Net Sampling for Estimating Abundance and Length Frequency of Reservoir Shad Populations

Abstract Horizontal and vertical gill nets were used to intensively sample gizzard shad Dorosoma cepedianam and threadfin shad D. petenense in Lake Texoma, Texas–Oklahoma, in August 1991. We evaluated spatial patterns, sampling precision, mesh-size selectivity, and sample sizes required to detect differences in shad abundance. Abundance patterns differed among species, length-groups, and sampling sites. Age-0 shad occurred mainly near the surface, whereas older fish were more evenly distributed over depth. Spatial variation in abundance was more pronounced among embayments than between nearshore and offshore sites within embayments. Spatial aggregation of shad was similar to that reported for a variety of other aquatic organisms, suggesting that experimental designs for gill-net surveys should incorporate statistical approaches similar to those used in other sampling programs. With horizontal gill nets, investigators can estimate median catch rate (number per net) within 25% of the true median with 95% ce...

Special Section: Sampling Shad in Reservoirs: Overview of Gear Evaluation Study for Sampling Gizzard Shad and Threadfin Shad Populations in Reservoirs

Abstract To develop effective sampling strategies for gizzard shad Dorosoma cepedianum and threadfin shad D. petenense, six gears—including hydroacoustics, electrofishing, gill nets, rotenone, seines, and trawls—were used concurrently to sample shad on Lake Texoma, Texas–Oklahoma, in August 1991. Density estimates, precision, and length distributions were compared among gears. Sample design recommendations were made in light of gear biases, sample-size requirements, spatial patterns of shad abundance, and labor and cost limitations. This paper provides an overview of the study, and specific results are reported in the articles that follow.

Assessing Shad Abundance: Electrofishing with Active and Passive Fish Collection

Abstract We compared electrofishing samples collected with a push net to those collected by dipnetting. Catch per unit effort (CPUE) of threadfin shad Dorosoma petenense and of gizzard shad D. cepedianum 150 mm total length or shorter did not differ between collection methods, although more gizzard shad larger than 150 mm were collected by dipnetters than were collected with the push net. The precision of CPUE estimates differed between gears and between species. To obtain a target coefficient of variation of the mean (CV X = 100 × SE/mean) of 20% for threadfin shad, dipnetting required 5–7 more person-hours than did electrofishing with the push net. However, for gizzard shad 150 mm or shorter the push net required up to 1.0–1.4 more person-hours than dipnetting. Results for gizzard shad longer than 150 mm were mixed between sites. The push net may provide an acceptable alternative to dipnetting for sampling inshore shad populations, particularly in systems dominated by small shad.

Effects of Zooplankton Spatial Variation on Growth of Larval Striped Bass: An Experimental Approach

Abstract We quantified growth and mortality rates of larval striped bass Morone saxatilis in laboratory experiments simulating variability of prey abundance and composition along the riverine to lentic gradient in Lake Marion, South Carolina. Larvae were reared from 4 to14 d posthatch in three treatments: (1) low-prey, which simulated average prey abundance and composition in riverine and transitional habitats; (2) medium-prey, which simulated average abundance and composition in lentic habitats; and (3) high-prey, which simulated peak (patch) abundance and composition in lentic habitats. Larvae did not grow (weight-specific growth, G = −0.80 to −0.062/d; −0.02 to −0.01 mm/d) in the low-prey treatment but grew in the medium- and high-prey treatments (G = −0.035 to 0.105/d; 0.02–0.17 mm/d). Additionally, mortality also varied significantly among prey treatments. Larvae in the medium- and high-prey treatments experienced minimal mortality (Z = 0.041–0.085/d), whereas mortality was greater in the low-prey tr...

A Comparison of Diets of Blueback Herring (Alosa aestivalis) and Threadfin Shad (Dorosoma petenense) in a Large Southeastern U.S. Reservoir

ABSTRACT The objective of this study was to determine dietary overlap between blueback herring and threadfin shad in J. Strom Thrumond Reservoir, South Carolina/Georgia. We also evaluated prey selectivity for each speices and diet differences between two size categories of blueback herring. Diet and zooplankton samples were collected every other month from April 1992 to February 1994. We examined stomachs containing prey from 170 large blueback herring (>140mm), 96 small blueback herring (<140mm), and 109 threadfin shad, and we also examined 45 zooplankton samples. Large blueback herring diets differed significantly from threadfin shad diets on 11 of 12 sampling dates, and small blueback herring diets differed from threadfin shad diets on all sampling dates. In general, blueback herring consumed proportionally more copepods and fewer Bosmina sp. and rotifers than threadfin shad. Large and small blueback herring diets were significantly different on five of eight sampling dates, primarily due to the tendency of small blueback herring to eat proportionally more Bosmina sp. than large blueback herring. Both blueback herring and threadfin shad fed selectively during some periods of the year. Diet differences between the species may contribute to their coexistence; however, both blueback herring and threadfin shad showed a strong preference for Bosmina sp., increasing the chance that they may negatively influence one another.