SELLFIS
3O:CPROGRESS REPORT NO.
Oregon
Juay, ?950
orpoi't
OBSERVATIONS OI HORSECW
oppor'u
/L'I
io o
OF ThE 1949 SET
re effordod bo observe growth of horaoolazs
yer-p1ue e1aa vJu i.t wa3 diuc.overed April 7, 1950, that a mewly
ord nA fla
in Ycqi.
fomc'
op embor and October 1948 by fill from
dredging operations, had thousands c
img horseolama embedded in it.
the hosoc1ame ipawu in Jebruary eid
roh principally, these clams would
Bocause
cowity have to be of the 1949 cpavn, or one-yearo1d class.
, hundred seventy cemn young clams taken April 7, 1950, were held
ever rn.ght in the aquar.a and then dr?ldod .into three groups according to
increasing size
aside.
AU dead or apparently weak elaine were removed and set
Of the 177 specimens brought In 143 ware acceptable for length weight
relationship.
(oup I (200 to 29.9 mm in length
49 clams averaged 2,39 grams per clams;
averaging 25,6 mm) with
(oup II (30.0 to 39.9 mm in lengbh
averaging 34.0) with 70 clams aveagod 5.69 grams per clam; and Group III
(40.0 to 49.9 ui in length
grar
per clam.
averaging 43.2 ann) with 24 olamna averaged 12.83
The average length and. aorage weighb was 32.8 mm and 6.67
grane respectively.
Terminology usod in the following paragraphs concerning age deterinimtion
and methods of
aauring may need some explanation.
Whenever ring is used
it shE4l be construed as moaning that ring which is formed during the slack
season of growth, usually during winter months, by the slowing down of depo.
aition of shell matter of the clam, this showing up as a rixg
shell end as s.
ring' on the ligament bed (scar).
on the outer
The ligament bed is a mir-
ror immge of the ligament and is affected by good and poor growing periods an
LI
is the outside shell,
On this ligament bed can be found the annual growth rings.
.
Depth of the shell (valve) is the stn.right line distance from the tip of
the unibo to the outer edge of t
shell
this line being approximately
perpendicular to the length line
The bodies wore removed from the valves and the following measurements
made on the shel1
1.
Overall length of the shell
2.
Overall depth of the shell.
3.
Length at maxnualfl ring
4.
Depth at "annualt ring
5.
Distance from the annual ring to the edge of the sholl
6.
Number of ligament rings showing in the ligament ooavity
7.
Distance from rings to outer edge of the ligaunt bed
Graphoal representation of oirerall length versur overall depth can be
seen in graph #1, the average depth being 21.7 um and the average length
being 33.7 ims for the sample.
Location of growth rings were based on the following criteria which are
listed in order of importanoe
1.
Definite changes in oontovr of the sho11
2.
A close cuocession of lines causing growth rings.
3.
Dark rings formed simultaneously with thø growth rings by the
periostracui.
Length of the shell at the ring was recorded as was depth of the shell
at the ring.
Length at the ring versus depth at the ring for the same
amp1e
is plotted in graph #2, the average depth being 17.4 um and the average
length being 273 imn.
Graphs I and 2 when superimposed show identioal slopes
and airi1ar 3pread further strengthening the length depth data.
Length frequency for both overall length and length at ring (graph #3)
6
*
showed the average overall
peak at
2 mm.
length (smoothed by 3 s) to be 33 9 mm with one
The average length at ring was 27.3 with a peak at 21 mm.
Superimposing these two and comparing peaks showa they compare in magnitude
and is an indication that at lea.t the rings were being read with a fair
degree of accuracy.
It is important to note that the growth rings were deposited prior
April 7, 1950.
Just how
ny days or
to
weeks or months before is imposaiblâ to
say hut it appears from observation that they are formed around January
or
February.
As a follow up on the formation of ligament rings within the ligament
bed each ligament bed was examined for rings. The rings have a bluish color
and are easy to locate in the majority of oases. It is the contention of the
writer that these rings are laid down the same time the annual ring is laid
down on the outside of the shell, The fact that the rings were found in thee.
one year old plus clams is strong evidence that they are formed every year.
There will be, of course, a follow up through examination of this same stock of
clams throughout this year and especially next winter to find out if end vthen
a ring will be formed.
It was hoped that the distance from the ring to the edge of the
shell
would show a close correlation with the measurements made on the distance from
the ligament ring to theedge of the ligament. However, when plotted against
one another (graph #1) the correlation was poor (r
can be explained in most
part1
0.42). ' This law correlation
if not entirely, in the method of measuring the
distance from the ligament ring to the edge of the ligament bed. An ooulsr micrometer was used in this case. The ring itself is anywhere from 0.05 to 0.20
ocular unite wide (0.07 mm to 0.29 mm) when using lOX oculara and OJX object'
ivee, and becomes ill defined with an increase in
.4.,
magnification power
Tb.
j.
t
ourvature of the edge of the ligament bed presents another problem in the
definition of limits. A tilting of the shell a few degrees can mean a dif
ferenco of 0,20 ocular units (0,29 imn)o These variables account in the
min
for the poor correlation.
From auperficial examinatIon of older shells where the annual rings can
be compared with the ligament rings there appears to be a close correlation
between the two as far as growth and time of formation is concerned.
It has
been shown pretiously that the total nulnber8 of ligament rings and growth
rings are the same for any one clam.
Among almost any set of shells to be read for age determination there
there appears to be no annual
are a certain number which defy reading.4 i.e
rings formed.
This is especially true in climates where the winters are mild
However, after
and growth progresses relatively uninterrupted.
examination
of ligament rings it was noted that a ligament ring occurred in 97.7% of the
oases, whereas rings on the outer
shell could be found in 93.2% of the cases
for tho same sample.
St
1. A study area for horseclam growth has
been set up in Yaquina Bay.
This area was formed in the fall of 1948 by dredging aotivity and was aubso'
quently seeded by natural spawning of adult horseolame the spring of 1949.
2. Of the first sample of clams ta]n from this area the average size
for the first year of life
at the
time the ring was formed - was 27.3
in length and 17.4 imn in depth
3.
The growth ring was formed prior to April 7, 1950.
4.
April 7, 1950 these clams averaged 32.8 sin in length and 5.67 grams
in weight.
5.
Ligament rings appeared on ligament beds of shells even when growth
rings on the outer shell were not distinguishable.
This report is intended as a eupplonientto Progress Report #18, August
1949, which dealt with the phy3ioal condition of primarily the gonad of the
horeeolam throughout the various mcnths of the year as related to the present
regulation of a olosod season from January let through June 30th.
Spawning
season of the horeeolsin has previously been designated for the Oregon Bays
as chiefly February end March.
This report will further diaou
this matter
with additional notes on the spawning season this year.
Yaquina Bay because of its central locality and accessibility has been
the primary subject of oonoentated research with Coos and Tillamook Bays
being secondary.
The following points are talcan from Progress Report #18:
1.
Standard length-weight relationship for Taquina Bay horseolame of
the lengths
for the month of November has been established
as a basis for comparison for other months of thó year.
These length-weight
figures are used for computations in the following formula.
2.
The derived standard formula for determining Ferent edible meat
recovery from unoleanod whole weights is
Wt. of
Uncleaned necks /
Wt. of
Cleaned bodies
(Yaq. Bay Stand. Wt) X (f spec,,
for given length
3.
% Edible Meat Recovery
from Unoleanod Yholo
ighta
in sample
Horaeclsms reach their poorest condition in Meroh and April and
their best condition in November and December.
The
unusua1l? cold wInter and spring had its effect on the spawning
of horseclams in Yaquina Bay.
Very few spawned-out gonads were noticed this
season and it appeared that those clams that did spawn did not do so completely.
(A spawned-out gonad is very watery and dark in color and is easily detected).
p
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50
Yaquina Bay - April 7, 1950
ffor..claas of on.-yr-pJ.is age
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