A study of possible causes for extensive mortality of oysters in the Upper Chesapeake Bay was taken on by year-round monitoring of conditions during a two-year period

Beaven, G. Francis

English

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CHESAPEAKE BIOLOGICAL LABORATORY 
MARYLAND 
DEPARTMENT OF RESEARCH AND EDUCATION 
EFFECT OF SWSQUEHANNA RIVER STREAM FLOW 
ON CHESAPEAKE BAY SALINITIES AND HISTORY 
OF PAST OYSTER MORTALITIES ON UPPER BAY 
BARS 
G. FRANCIS BEAVEN 
Reprinted from 1.946 Annual Report, Maryland Board of Natural Resources 
State of Mary1and 
DEPARTMENT OF RESEARCH AND EDUCATION 
COMMISSIONERS: 
...................................................................... B, H. WILLIER, Chairman.. .Baltimore 
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .  H. R. BASSETT.. Crisfield 
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .  LLOYD &I. BERTHOLF Westminster 
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .  E. N. CORY College Park 
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .  FRANKLIN D. DAY ..CentrevilIe 
DIRECTOR: 
................................................................................ R. V. TRUITT .Solomons Island 
STATE WEATHER SERVICE: 
G. N. BRANCATO, Meteorologist in Charge . . B a1 timore 
CHESAPEAKE BIOLOGICAL LABORATORY: 
EDWIN M. BARRY, B.S., Education Assistant 
G. F. BEAVEN, M.A., Biologist I ,  Oyster Investigations 
COIT &I. COKER, M.A., Biologist I I ,  Fishery Investigations 
ALICE W. CRONIN, A.B., Chemist III 
L. EUGENE CRONIN, PhD., Biologist I I ,  Crab Investigations 
GEORGE B. GRAY, B.S., Administrative Assistanf 
HARRY A. HENSEL, JR., Investigator 
HARVEY MISTER, Captain, Fish Cultz~rist 
LOLA M. PARKS, A.B., Librarian 
RICHARD E. TILLER,, M.S., Biologist I I ,  Fishery Investigations 
R. H. THOMPSON, PhD., Biologi.st I 
R. V. TRUITT, PhD., Biologist 
DORIS M. %TOODBURN, Secretary 
EFFECT OF SUSQUENANNA RIVER STREAM FLOW 
ON CHESAPEAKE BAY SALINITIES AND HISTORY 
OF PAST OYSTER MORTALITIES 'ON UPPER BAY 
BARSV1 
G. FRANCIS BEAVEN 
I t  has long been recognized that an area of extensive oyster rocks 
in Upper Chesapeake Bay, north of Kent Island, is characterized 
by erratic production, slow growth and occasional heavy oyster 
mortality. These Upper Bay bars once supplied large quantities of 
small stock for the steam houses in Baltimore when Cove Oysters 
were canned. Quantities of the small round single oysters, which 
a t  times are abundant, have been utilized by both private industry 
and the State as seed oysters for planting further down the Bay. 
At rare intervals, the upper Bay oysters make good growth and pro- 
duce a quantity of acceptable market oysters for the shucking houses. 
The proper management of these bars presents different prob- 
lems frcm those encountered in the rest of the State and has evoked 
considerable discussion and debate. Although the adverse effect 
of fresh, water in this area is generally reccgnized, the fact that an 
extensive mortality in 1943 occurred at  a time when local precipita- 
tion was known to be deficient, as was true also in other instances, 
gave rise to other theories concerning the cause of the oyster losses. 
A "blight," pollution frcm Baltimore, and the opening of the bars 
to  ccmmercial dredging in certain years have been suggested as 
possible reasans for oyster losses accurring on the bars. 
Far too little factual data is available concerning past oyster 
mortalities. Even as late as 1943 no observations on these bars were 
made until several months after a severe mortality had occurred. 
Since that year,' the bars have been kept under continuous observa- 
tion and extensive studies have been made during the past two 
years by the Fish and Wildlife Service. 2 
The possibility of a "blight" or oyster disease being a causative 
agent in an extensive mortality extending scme distance down belaw 
Kent Island during 1916 was investigated by the Bureau of Fish- 
--- 
1 Read at the meeting of the National Shellfisheries Association, New York City, June 5-7, 1946. 
2 Engle James B 1946 Commercial Aspects of the U per Chesapeake Bay Oyster Bars in Light 
of th; Recent 6kter  hortalities. U. S. Fish and ~iydl i fe  Service. 
eries. At that time no conclusions as to the exact agency responsible 
for the oyster deaths were reached. Examination of oysters then 
and in other more recent instances failed to disclose any recognized 
parasite in unusual abundance. Observations of the dispersal of 
industrial pollutants from Baltimore and the fact that the intensity 
of the mortalities observed has been greater on bars above and to the 
eastward than on those nearest to the approaches to Baltimore offer 
no support to the theory that pollution from Baltimore has caused 
the extensive losses observed. Continuous examination for the 
presence of both parasites and pollutants must, nevertheless, be 
continued since they constitute a potential danger which may vary 
from year to year. 
The proximity of the upper Bay bars to the entrance of the 
Susquehanna River (Fig. 1) and the frequent references in past 
conservation reports to destructive floods from that River have 
prompted a study of available records for the purpose of organizing 
the data and determining what relationship exists between Susque- 
hanna stream flow, Chesapeake salinities and past oyster mortal- 
ities. 
The Chesapeake Bay is a drowned valley and  comprise^ the largest 
inland waterway along the Atlantic Coast of the United States. 
I t  has been formed by the flooding of the 1 ower stream system of the 
Susquehanna River as a result of coastal subsidence. Its salinity 
varies from slightly below that of the open ocean at the Virginia 
Capes to fresh water on the Susquehanna flats. An average outward 
flow of about three tenths of a knot was calculated by Wells, Bailey 
and Henderson in their 1929 publication. The outward flow of the 
fresher and lighter water at the surface is accompanied by a slow 
movement of denser salt water up the Bay in the deeper channels. 
Deflection of currents toward their right by the earth's rotation to- 
gether with greater stream flow from the western shore of the Bay 
have resulted in slightly higher salinities on the eastern side than on 
the western. The normal heavier run-off from the land during the 
spring months causes an annual salinity fluctuation with lowest 
salinities occurring3in the spring and highest in the fall. Seasonal 
variations in evaporation, precipitation, and wind direction and 
velocity all influence the salinity pattern of the Bay. 
The drainage area of the Bay system (Fig. 2) is about 64,900 
square miles with the Susquehanna River comprising approximately 
43% of the total. Stream flow from the Susquehanna represents over 
85% of all contributions north of the Potomac and over 95% of that 
above the Patapsco. Variations in the flow of the Susquehanna 
River would thus be expected to have a major influence on salinities 
in the upper part of Chesapeake Bay, with its effects being marked 
as far down as the entrance of the Potomac. 
Fig. 1. Charted Oyster Bars of the Chesapeake Bay and Tributaries. 
Pig. 2. Drainage Basin of the Chesapeake Bay. Susquehanna River drainage 
basin shown in darker shading. 
No continuous daily record of saIinities in the upper Bay proper 
is available. At Solomons, some 65 miles below the affected bars, the 
average daily surface salinity is a little less than 14 p.p.t. or approxi- 
mately double the normal salinity on the upper Bay oyster bars. It 
ranges from a normal of 10.3 about May 1 to 17.3 in early November. 
A comparison of intermittent salinity records from the upper Bay 
with those a t  Solomons shows that both follow the same general 
trend with abrupt fluctuations more smoothed a t  Solomons than 
up the Bay. Extensive surface and bottom samples on oyster bars 
have further shown that surface salinity fluctuations correlate 
closely with those occurring in the slightly higher salinities of the 
bottom water. 
Salinities a t  Solomons have been plotted and compared with graphs 
of the precipitation recorded for the Maryland-Delaware section 
by the U. S. Weather Bureau. Monthly average salinities a t  Solo- 
mons and monthly average precipitation during the past five years 
are shown by the accompanying graph. (Fig. 3). There is some 
general relationship shown between recorded salinities and local 
SALINITY AT SOLOMONS 
PREGIPITATION, MARYLAND-DELAWARE 
Fig. 3. Relationship between the Maryland-Delaware precipitation and the 
salinity at Solomons, Maryland. 
precipitation but no well-defined correlation exists. The marked low 
salinity which occurred in 1943 is not accompanied by ,above normal 
precipitation for the same period. This lack of correlation might 
be expected from the relatively small portion of fresh water which is 
contributed to the upper Bay by local run-off. 
Accurate records of Susquehanna River stream flow have been 
kept ,near its mouth a t  Conowingo Dam since 1933 by the Susque- 
hannh Electric Company. These have been studied and plotted in 
several ways for the entire period. Relationship of stream flow, 
precipitation and salinity at Solomons are shown graphically for 
the year 1945 (Fig. 41, a year of marked precipitation and stream 
flow peaks. No noticeable effect of local rainfall on salinity can be 
found except a slight dip following record breaking rains in mid- 
July. This same period also showed a moderate rise in stream flow 
from the Susquehanna. The graph of daily stream flow a t  Cono- 
wingo, however, shows a definite relationship to daily salinity. 
Each marked peak of flow is followed by a trough of low salinity. 
The interval of time ranges from five to about fourteen days and is 
usually slightly less than one week. However, the effect of periods 
of high stream flow is cumulative so that when salinity is depressed 
it does not recover fully for a period of weeks or months. 
Exposure to a brief period of low salinity seems to have little 
permanent effect upon oysters, but long or frequently repeated ex- 
posure may result in serious damage. Thus, salinities averaged 
over a monthly period are more significant than the daily extremes. 
At the bottom of the graph, the monthly average sal.inity at  Solo- 
rnons is plotted with low figures a t  the top and high ones at  the 
bottom so that peaks of low salinity will parallel peaks of high 
stream flow. In ordef to smooth out and show the cumulative effect 
of stream flow, the monthly average, three month progressive 
average and six month progressive average daily flow are plotted. 
The six month progressive average curve appears to correspond more 
closely with that of monthly salinity than do the others. Several 
years were plotted in similar manner and the same general relation- 
ships were found to hold. Other periods of progressive average flow 
were tried, but the six month period seemed to follow general salinity 
trends best as illustrated by the five-year graph shown (Fig. 5). 
Salinity records at Solomons do not extend back enough years 
to cover earlier periods of recorded oyster mortalities a t  the Head- 
of-the-Bay nor do s the stream flow records at  Conowingo. Daily 
salinity records at  Fort McHenry in Baltimore Harbor have been 
kept by the Coast and Geodetic Survey since 1914. Solomons rec- 
ords were considered best for preliminary analysis since they are 
a t  the mouth of a long and broad tidal estuary and are little affected 
by local stream flow while those a t  Baltimore are likely to be more 
influenced by local Patapsco River conditions. When both, however, 
are plotted together, a high degree of correlation is shown generally. 
Records of Susquehanna stream flow extending back to 1890 have 
been kept a t  Harrisburg by the Coast and Geodetic Survey. When 
daily flows a t  Harrisburg and Conowingo are plotted in parallel a 
very high correlation is found. Records of peak floods showed that 
DAILY AND MONTHLY PREKlPLTAllON, SOLOHOMS 
DAILY SURFACE SALINITY, SOLONONS 
4 
8 
12 
16 
2 0 
6 
10 
14 
18 
JAN FEB nm. IPR MY JUNE JULY AUO. SEP. OGT NO* DEG. 
(0) DAILY, (M I  MONTHLY AVERAGE DAILY, (3) THREE AHO (6) SIX MONTH 
PROORESSIVE AVERAOE OAILY FLOW, COWOWlNQO 
(9) MOHTULY AVLRAQE SALINITY, SOLOMONS 
Fig. 4. Daily, monthly, &month and &month progressive average daily fla 
at Conowingo and the monthly average salinity at Solomons. 

the average time interval for a peak to travel from Harrisburg to 
Conowingo is eight hours. The narrow gorge of the Susquehanna and 
the maintenance of a continuous high head of water back of the 
dams gives them little if any flood control effect. Stream flow rec- 
ords at  Harrisburg thus furnish a reliable index of the water dis- 
charge into the upper Bay by the Susquehanna River (Fig. 6). 
Publications of various agencies in Maryland dealing with oys$ers 
have been searched for records of general mortalities occurring a t  
the Head-of-the-Bay. It is found that such losses when occurring 
in spring or summer were sometimes unreported until the rocks 
were visited after Christmas so that reco~ds of such loss may be 
given in the following year. There is also evidence that oysters 
weakened by spring floods, followed by below-normal salinities dur- 
ing the summer, may succumb the following winter when environ- 
mental conditions are unfavorable. The following major mortalities 
and no others were found to have been reported during the period " 
for which records are available: 1908-1909, estimated at  55% on 
the Tea Tables and 62% on Man O V a r  Shoals; 1916, only an 
occasional living oyster could be found above Swan Point and the 
Patapsco River; 1928, an 80% mortality of up-Bay oysters; 1936, 
a heavy mortality from freshets down to Swan Point and Sandy 
Point; 1943, 97% loss on Tea Tables ranging to little loss at  Swan 
Point and Sandy Point; 1945-46, extensive mortality decreasing 
down the Bay below Love Point.2 
The six-month progressive average daily flow of the Susquehanna 
River at  Harrisburg for the period of recorded mortalities has 
been plotted together with the existing monthly average salinity 
records from Baltimore (Fig. 7). Mortality years are marked on 
the graph by an " M". These six mortality periods correspond with 
the six highest sustained periods of cumulative run-off from the 
Susquehanna. The five mortality periods reported since Baltimore 
salinities were available correspond with five of the seven recorded 
periods when salinities remained below five for three months or 
longer. These records thus afford excellent evidence that the rec- 
orded major oyster mortalities at  the Head-of-the-Bay have all 
been associated with and probably are principally the direct result 
of low salinities caused by periods of high run-off from the Susque- 
hanna River. 
1 Loc. cit. Eagle, James B. 1946. 
-.- SIX MONTH PROGRESSIVE AVERAGE DAILY FLOW CONOWINGO 
- SIX MONTH PROGRESSIVE AVERAGE DAILY FLOW HARRISBURG 
Fig. 6. Six-month progressive average daily flow of the Susquehanna River. 
S I X  M O N T H  P R O G R E S S I V E  A V E R A G E  D A I L Y  FLOW AT H A R R t S B U R G  
S A L I N I T Y  AT B A L T I M O R E  
Fig. 7. Relationship between the six-month progressive average daily flow at 
Harrisburg and the salinity at Baltimore, 1907 to 1946. 


Effect of Susquehanna River stream flow on Chesapeake Bay salinities and history of past oyster mortalities on upper Bay bars

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Effect of Susquehanna River stream flow on Chesapeake Bay salinities and history of past oyster mortalities on upper Bay bars

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