EXECUTIVE SUMMARY

A total of 109,360 American shad passed the Conowingo Dam East fish Lift in 2004. An additional 3,426 shad were collected in the West Lift at Conowingo. American shad passage at Holtwood, Safe Harbor and York Haven Dams was 3,428, 2,109, and 219 respectively. Since 1997, fish passage efficiency at Holtwood , Safe, Harbor and York Haven Dams averaged 27%, 74% and 16%, respectively. Passage efficiency must improve if self-sustaining stocks are to be established.

A total of 163 adult American shad otoliths were processed from adult shad sacrificed at the Conowingo Dam West Fish Lift in 2004. Based on tetracycline marking, 28% of the 158 readable otoliths were identified as wild and 72% were identified as hatchery in origin. No double marked fish (released below Conowingo Dam) were collected in the Conowingo West Lift samples.

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 178 hatchery larvae was required to return one adult to the Conowingo fish lifts.

INTRODUCTION

Efforts 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.

In 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 2004.

METHODS

Adult American 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 lift. Because the West Fish Lift was under repair during much of the season, these collections were supplemented with fish collected from anglers and used in tank-spawning trials. 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 epoxy.

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.

Whole otoliths 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. Aging 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 2003.

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.

RESULTS AND DISCUSSION

A total of 109,360 American shad passed the Conowingo Dam East Fish Lift in 2004. An additional 3,426 shad were collected in the West Lift at Conowingo. American shad passage at Holtwood, Safe Harbor and York Haven Dams was 3,428, 2,109, and 219 respectively. Since 1997, fish passage efficiency at Holtwood , Safe, Harbor and York Haven Dams averaged 27%, 74% and 16%, respectively. Passage efficiency must improve if self-sustaining stocks are to be established.

A total of 163 shad was sacrificed for otolith analysis from Conowingo Dam in 2004 (Table 1). No samples were collected from the East Lift since it was operated in fish passage mode. There were five unreadable otoliths. A total of 45 (28%) otoliths exhibited wild microstructure and no tetracycline mark. One hundred and thirteen (72%) exhibited tetracycline marks including single, triple, quadruple, and quintuple immersion marks. No specimens exhibited double marks or feed marks.

Random 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 1989-2004 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 178 (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.

REFERENCES

Hendricks, M. L. 2004. Job III. American shad hatchery operations. In: Restoration of American shad to the Susquehanna River, Annual Progress Report, 2003. Susquehanna River Anadromous Fish Restoration Committee.

Hendricks, M. L. 2004. Job V, Task 2. Analysis of adult American shad otoliths. In: Restoration of American shad to the Susquehanna River, Annual Progress Report, 2003. Susquehanna River Anadromous Fish Restoration Committee.

Hendricks, M.L., 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.

Back to Top

American Shad Index -- Fisheries Management -- PFBC Home