Virginia Institute of Marine Science

Sampling Methods

  • Ageing scales
    Ageing scales  This is a scale impression from an American shad. The annuli are counted to determine the age of the fish, this is a 5 year old. The freshwater zone is also marked, this forms when juveniles transition from freshwater to saltwater.  
  • Removing otoliths
    Removing otoliths  Otoliths(ear bones) are carefully removed from each American shad and placed in numbered trays. Otoliths are used to determine hatchery prevalence.  Brian Watkins
  • Hatchery Mark
    Hatchery Mark  Hatchery fish are immersed in an oxytetracycline (OTC) bath within a few days of hatching. The chemicals are deposited in the otoliths(ear bones). When exposed to UV light the OTC mark will illuminate.  Ashleigh Rhea
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VIMS used historical records of fishing locations to choose the current monitoring sites. One historical site from each river was chosen to monitor catch rates using staked gill nets (SGN). To ensure that current monitoring was comparable to historical records nets were constructed of the same mesh size and length of netting. One SGN, 900 feet in length with 4 7⁄8″ mesh was set on the James and York rivers. Another SGN, 912 feet in length with 5″ mesh was set on the Rappahannock River. A SGN is set and fished 2 consecutive days a week on each river during the spring spawning run. Scientists accompany commercial fishermen during each sampling trip. All American shad are returned to the laboratory for analysis. All other species caught are counted and recorded on log sheets and released. Surface water temperature and salinity are also recorded at every sampling event.

Data Collection

All American shad collected are weighed and measured using an electronic fish measuring board. Measurements from the board are recorded and stored in a data file for later analysis. Each fish is measured for fork length, total length, sex, gonad stage and weight. Scales are then taken from each fish to be used for age determination. Sagittal otoliths (ear bones) are removed and scanned for hatchery marks.

Ageing scales

Scales are removed from the mid-lateral area just below the dorsal fin on the left side of each fish. Before scales can be processed they first have to be cleaned. Scales are soaked in water and scrubbed to remove any debris that can interfere with reading. Several scales from each fish are mounted on a 3-inch x 5-inch acetate sheet. The sheets are then pressed in a hydraulic press with a force of 16,000 pounds for 2 minutes. The scales are removed from the sheet leaving an impression of the scale. The sheets are then read on a microfilm projector using Cating's Method, a method introduced by James Cating  in 1953 that uses scales to determine the age of American shad. Fish scales have annuli or rings, much like the rings on a tree. Every year during periods of slow growth annuli are formed. By counting these marks scientists can determine the age of the fish.

Otolith scanning

In 1994 the Department of Game and Inland Fisheries (VDGIF) and the US Fish andWildlife Service (USFWS) began a hatchery restocking effort in the James and Pamunkey rivers. Adult shad from the Pamunkey River are used as brood stock; eggs are stripped and fertilized in the field. The larvae are then reared in the VDGIF and USFWS hatchery facilities. After the fry have hatched they are immersed in an oxytetracycline (OTC) solution that marks the otoliths with a distinctive epi-fluorescent ring. This allows scientists to distinguish  hatchery fish from wild stock when they return to the river for spawning. Since fry are released from many sites along the east coast, different marking sequences are used for each river. In 2003 VDGIF began a new hatchery-release program on the upper Rappahannock River. The goal is to restore American shad to historical spawning areas that were previously blocked by Embrey Dam.