A total of 68,926 American shad passed the Conowingo Dam East Fish Lift in 2005. An additional 3,896 shad were collected
in the West Fish Lift at Conowingo Dam. American shad passage at Holtwood, Safe Harbor and York
Haven Dams was 34,189, 25,425, and 1772 respectively. Since 1997, fish passage efficiency at
Holtwood , Safe, Harbor and York Haven Dams averaged 30%, 74% and 14%, respectively. Passage efficiency must improve
if self-sustaining stocks are to be established.
A total of 277 adult American shad otoliths were processed from adult
shad sacrificed at the Conowingo Dam West Fish Lift in 2005. Based on tetracycline marking, 35%
of the 274 readable otoliths were identified as wild and 65% were identified as hatchery in origin.
Otoliths were also extracted from adult American shad collected at Lapidum, at the head of tide, three miles downstream
from Conowingo Dam. Based on 158 readable otoliths, shad from that site were 41% wild and 59% hatchery.
Using age composition
and otolith marking data, the lift catch was partitioned into its component year classes for
both hatchery and wild fish. Results indicated that for the 1986-1997 year classes, stocking
of approximately 181 hatchery larvae was required to return one adult to the Conowingo fish lifts.
to restore American shad to the Susquehanna River have been conducted by the Susquehanna River
Anadromous Fish Restoration Cooperative (SRAFRC). Primary restoration approaches consisted of:
1) trapping of pre-spawn adults at Conowingo Dam and transfer to areas above dams (1972 to 1999),
2) direct fish passage (1997 to the present), and 3) planting of hatchery-reared fry and fingerlings.
order to evaluate and improve the program, it was necessary to know the relative contribution
of the hatchery program to the overall restoration effort. Toward that end, the Pennsylvania
Fish Commission developed a physiological bone mark which could be applied to developing fry prior to release (Lorson
and Mudrak, 1987; Hendricks et al., 1991). The mark was produced in otoliths of hatchery-reared fry by immersion in
tetracycline antibiotics. Analysis of otoliths of outmigrating juveniles allows discrimination of "wild" vs.
hatchery reared fish. The first successful application of tetracycline marking at Van Dyke was
conducted in 1984. Marking on a production basis began in 1985. This report presents results
of evaluation of otoliths from adult American shad collected in 2005.
shad passing the Conowingo Dam East Fish Lift were counted by a single observer as they passed
the viewing window in the fish lift exit trough. A representative sample of adult shad returning
to Conowingo Dam was obtained by sacrificing every 50th shad to enter the West Fish Lift. Each
sampled fish was sexed, measured and decapitated. Whole heads were frozen and delivered to the
Van Dyke Hatchery. Otoliths (sagittae) were extracted and one otolith was mounted for mark analysis
in Permount® on
a microscope slide, while the other was mounted for ageing on clear tape in two part rod-building
For mark analysis, otoliths were ground on both sides to produce a thin sagittal section
and the specimen examined under UV light for the presence of a tetracycline mark.
were aged by viewing with a dissecting microscope and a fiber optic light. The best contrast
was obtained by directing the light from the side, parallel to the sagittal plane of the otolith.
Ageing was done by a single researcher. After initial ageing, length at age was analyzed and
apparent outliers were re-examined. We have assembled a collection of several hundred otoliths
from known aged shad based on the presence of a unique tetracycline mark. These were used as
reference material. Historical fish lift catch data was compiled from SRAFRC Annual Progress
Reports for the years 1972 through 2004.
Recruitment to the lifts by year class was determined
for hatchery fish by partitioning the lift catch for each year into its component year classes
based upon age composition and otolith marking data. Total recruitment by year class was determined
for hatchery groups by summing the data for each year class over its recruitment history. The number of larvae required
to return one adult to the lifts (L/A) was determined for each year class by dividing the number of larvae stocked
above dams by the total recruitment of adults which originated as hatchery larvae. Overall L/A was calculated by dividing
the sum of the number stocked by the sum of the total recruitment of the group, for the cohorts in question.
A total of 68,926 American shad passed the Conowingo Dam East Fish Lift in 2005.
An additional 3,896 shad were collected in the West Fish Lift at Conowingo Dam. American shad
passage at Holtwood, Safe Harbor and York Haven Dams was 34,189; 25,425; and 1,772 respectively.
Since 1997, fish passage efficiency at Holtwood , Safe, Harbor and York Haven Dams averaged 30%,
74% and 14%, respectively. Passage efficiency must improve if self-sustaining stocks are to be established.
A total of 274 shad was sacrificed for otolith analysis from Conowingo Dam in 2005
(Table 1). No samples were collected from the East Lift since it was operated in fish passage
mode. There were three unreadable otoliths. A total of 96 (35%) otoliths exhibited wild microstructure
and no tetracycline mark. One hundred and seventy-eight (65%) exhibited tetracycline marks including
single, double, triple, quadruple, and quintuple immersion marks. Adult shad were also sacrificed from egg collection
efforts at Lapidum, MD, three miles downstream from Conowingo Dam. Sixty-four (41%) of the 158 readible otoliths were
from wild shad, while 94 (59%) were from hatchery shad. It is not surprising that the wild contribution is higher
at Lapidum since it is a prime spawning site for the indigenous upper Chesapeake Bay stock.
samples of adults have been collected since 1989 and the results of the classifications are summarized
in Table 2. The contribution of wild (naturally produced) fish to the adult population entering
the Conowingo Dam fish lifts during 1989-2005 ranged from 10 to 71% (Table 2, Figure 2). Although
the proportion of wild fish in the Conowingo Lift collections was low prior to 1996, the numbers
of wild fish showed an increasing trend from 1989 to 2000 and have decreased since 2000 (Figure 3).
Fish lift catch, age composition and origin of sacrificed shad are presented
in Table 3. Analysis of otoliths to assess hatchery contribution was not conducted prior to 1989.
As a result, the catch for year classes prior to 1986 could not be partitioned into hatchery
and wild and are not presented. Year classes after 1997 are not fully recruited and are not included
in the analysis. For the period 1986-1997, the number of hatchery larvae required to produce one returning adult (L/A)
ranged from 60 to 620, with an overall value
of 181 (Table 4). L/A was highest (431-620) for the early cohorts (1986 – 1989). During 1990 to 1997, L/A improved
to 60-289, presumably due to improvements in fish culture practices.
L/A was surprisingly low in comparison to the reproductive potential of wild fish. If
fecundity of wild females is assumed to be 200,000, then 2 of 200,000 eggs must survive to maturity
to replace the spawning pair in a stable population. If we assume a fertilization rate of 60%
(comparable to strip-spawning), 60,000 fertilized eggs would be required to produce one wild adult at replacement.
This suggests that mortality in the wild is extremely high during incubation and/or for the first
week after hatch.
Hendricks, M. L. 2006. Job III. American shad hatchery operations.
In: Restoration of American shad to the Susquehanna River, Annual Progress Report, 2005. Susquehanna
River Anadromous Fish Restoration Committee.
Hendricks, M. L. 2006. Job V, Task 2. Analysis of
adult American shad otoliths. In: Restoration of American shad to the Susquehanna River, Annual
Progress Report, 2005. Susquehanna River Anadromous Fish Restoration Committee.
T.R. Bender, and V.A. Mudrak. 1991. Multiple marking of American shad otoliths with tetracycline
antibiotics. North American Journal of Fisheries Management. 11: 212-219.
Hendricks, M.L., D.
L. Torsello, and T.W.H. Backman. 1994. Use of otolith microstructure to distinguish between wild
and hatchery-reared American shad (Alosa sapidissima) in the Susquehanna River. North American
Journal of Fisheries Management.
Lorson, R.D. and V.D. Mudrak. 1987. Use of tetracycline to mark
otoliths of American shad fry. N. Am. J. Fish. Mgmt. 7:453-455.