NOAA Technical Memorandum NMFS NE 156
Proceedings of the Summer
Flounder Aging
Workshop,
1-2 February 1999, Woods Hole,
Massachusetts
by George R. Bolz1,
James Patrick Monaghan, Jr.2,
Kathy L. Lang1,Randall W. Gregory2,
and Jay M. Burnett1
1National
Marine Fisheries Serv., Woods Hole Lab., 166 Water St., Woods Hole,
MA 02543
2North
Carolina Div. of Marine Fisheries, P.O. Box 769, Morehead City, NC 28557
Print
publication date May 2000;
web version posted August 16, 2001
Citation: Bolz GR, Monaghan JP, Lang KL, Gregory RW, Burnett JM. 2000. Proceedings of the Summer
Flounder Aging
Workshop,
1-2 February 1999, Woods Hole,
Massachusetts. US Dep Commer, NOAA Tech Memo NMFS NE 156; 15 p.
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Abstract
A workshop to review summer flounder aging techniques was conducted
at the National Marine Fisheries Service’s Northeast Fisheries Science
Center (NEFSC) in Woods Hole, Massachusetts, on 1 and 2 February 1999. The
workshop convened to address: 1) the poor results of comparative
readings from a 1997 scale sample exchange between age readers at the
NEFSC and North Carolina Division of Marine Fisheries (NCDMF); and 2)
concerns over aging accuracy expressed in recent stock assessments and
in a review of those assessments by the National Research Council.
The workshop identified three factors which contributed to the poor
results of comparative readings. The majority of age determination
disagreements were related to differing age-reading conventions used
by each agency with regard to the marginal increment (i.e., growth
beyond the last annulus) on the scales. Correcting for this discrepancy
in convention increased agreement rates from about 53% to over 80%. The
origin of the convention problem appears to be differences in the timing
of annulus formation between fish inhabiting the Cape Hatteras region
(i.e., during January-March) and fish in the Southern New England
region (i.e., during May-July).
The second source of disagreement between agencies was related to the
choice of a first annulus in some, but not all, fish. The third
source of disagreement occurred when NEFSC age readers determined a scale
event to be a “check” as opposed to NCDMF age readers determining it
to be an annulus.
Workshop participants concluded that future exchanges should occur on
a regular basis to ensure uniformity of aging conventions and age interpretations. Additionally,
it was agreed that analysis of samples and/or data for young-of-the-year
summer flounder (possibly archived by state or university researchers)
might resolve uncertainties regarding first-annulus location.
INTRODUCTION
Difficulties
in aging summer flounder, Paralichthys dentatus, have been ongoing
for many years. Aging disagreements for this species were noted
in an October 1979 literature review by the Mid-Atlantic Fishery Management
Council’s State/Federal Summer Flounder Scientific and Statistical Committee,
and were discussed in a May 1980 workshop convened by that committee
in Woods Hole, Massachusetts (Smith et al. 1981), to resolve the
noted discrepancies. Investigators from state, federal, and university
laboratories participated in the 1980 workshop. Two conclusions
reached at that 1980 workshop were: 1) mean length at age 1 was
calculated to be 17-18 cm; and 2) great latitudinal variation was found
with respect to edge type, length at age, and growth rate. Although
six types of scale growth patterns were categorized, the 1980 workshop
determined that the limited number of scales available for examination
did not allow firm conclusions about growth pattern differences among
areas.
In 1989, at the Ninth NEFC Stock Assessment Workshop, members of that
workshop’s Summer Flounder Working Group raised concerns again regarding
differences in aging protocols between state and federal laboratories
(Northeast Fisheries Center 1989). A workshop was convened in June
1990 (Almeida et al. 1992) to reach consensus regarding the interpretation
of scale growth patterns. Several studies (Able et al. 1990;
Szedlmayer et al. 1992) which had been conducted between the 1980
and 1990 workshops determined that summer flounder growth during the
first year was much greater than previously thought and could explain
some of the confusion in determining the first annulus. Participants
at the 1990 workshop examined approximately 150-200 scale samples and
agreed upon aging criteria. Noteworthy was the conclusion that
annulus formation in the more northerly waters (i.e., Southern
New England and New Jersey) occurred in late spring - early summer, while
further south it occurred as early as January or February. The
1990 workshop participants recommended that formal age structure exchanges
be initiated to achieve consistency.
Accordingly, an exchange of scale impressions occurred in June 1997
between the National Marine Fisheries Service’s Northeast Fisheries Science
Center (NEFSC) and the North Carolina Division of Marine Fisheries (NCDMF),
the only two agencies actively performing production aging of summer
flounder at that time. The exchange consisted of two parts: 1)
a sample of 50 fish exhibiting temporal and spatial variation which had
been selected from NEFSC and Massachusetts Division of Marine Fisheries
(MDMF) bottom trawl surveys and from the commercial fishery throughout
the Northeast; and 2) a 120-fish sample from the winter commercial fishery
in the North Carolina - Virginia region.
Comparative reading results from the exchange indicated a poor level
of agreement (i.e., approximately 53%; Table
1 and Table 2) between the agencies. Concern
regarding the implications of potential aging problems on future stock
assessments was raised during a review of the most recent summer flounder
assessment at a National Research Council committee meeting on improving
the collection and use of fisheries data, held on 25 and 26 January 1999
in Washington, D.C. To determine the cause of, and to resolve,
these aging disagreements, age readers from the NEFSC and NCDMF conducted
a workshop during 1-2 February 1999 in Woods Hole, Massachusetts. Also
present at the 1999 workshop were representatives of Old Dominion University’s
(ODU’s) Fish Age & Growth Laboratory, which had recently been contracted
to perform production aging of summer flounder from the Chesapeake Bay
region (Table 3).
The objectives of the 1999 workshop were to: 1) identify factors
resulting in age determination discrepancies between the NEFSC and NCDMF;
2) resolve, to the extent possible, disagreements in the exchange samples;
and 3) initiate a course of action to ensure consistency in aging methods
and future age interpretations. This report summarizes the activities
and results of the 1999 workshop, and offers recommendations for further
consideration.
METHODS
PRECISION
In preparation for the 1999 workshop, NEFSC age readers conducted a
blind reading (i.e., sex and length of fish unknown, but date
of collection known) of the 120-fish NCDMF samples constituting the 1997
exchange materials to determine levels of precision with respect to the
original age determinations. The NCDMF age readers conducted a
blind reading of the 50-fish NEFSC sample during the course of the workshop.
AGE DETERMINATIONS
Aging criteria described by Dery (1988) and accepted by the 1990 Summer
Flounder Aging Workshop participants (Almeida et al. 1992) were
reviewed in detail during the 1999 workshop. Annuli generally consist
of “cutting-over” marks which must be continuous through the scale’s
lateral field and into the ctenii (Figure 1). The
age of a fish is determined first by counting the number of annuli present
and then by considering the amount of growth beyond the last annulus
(i.e., “edge” or “ + ” growth) at the time of sample collection. Age
is then determined relative to the convention of the 1 January birthdate
used for assessment purposes. For example, a fish which has formed
three annuli and has additional growth beyond the third annulus would
be considered age 3+ if collected on 31 December, but age 4 on 1 January.
During the 1999 workshop, summer flounder scale impressions (see Penttila et
al. 1988 for details regarding preparation of scale impressions)
were viewed using a Leitz TP 300 Contour Microprojector at a magnification
of 50X and occasionally 20X. It was noted that the NCDMF normally
views and measures images of summer flounder scale impressions at a
magnification of 24X.
Workshop participants agreed to view in its entirety the 50-fish NEFSC
exchange sample as the basis for the workshop (hereafter referred to
as the “workshop sample”), and as much of the 120-fish NCDMF exchange
sample as time allowed.
For each fish, five or six scale impressions were viewed silently by
participants until they indicated that an age determination had been
made. These age determinations were then compared to those provided
by the NEFSC and NCDMF during the exchange exercise, and any discrepancies
were discussed until either a consensus age was obtained or an impasse
was reached. Additional time was budgeted during the workshop to: 1)
view summer flounder otoliths for consideration as an alternative aging
structure; 2) view other scale sample images as necessary to illustrate
specific points; and 3) to generate annotated images illustrating workshop
examples and results using an OPTIMAS Image Analysis System at magnifications
of 12.5X and 20X.
RESULTS
PRECISION
A re-reading of the workshop sample by NEFSC age readers resulted in
a precision of 92% when compared with their original reading. After
further review of this sample, age determinations by the NEFSC age readers
were adjusted upward by 1 yr for three fish. One other fish was
adjusted downward 1 yr; however, this fish was subsequently discarded
from further analysis at the workshop when consensus with NCDMF age readers
could not be reached.
Precision of NCDMF age readers was 96% (Table
4). Age determinations by the NCDMF age readers were adjusted
downward for two fish.
AGE DETERMINATIONS
Results for the 50-fish workshop sample are listed in Table
4 and Table 5, with workshop resolution of
disagreements or the specific reason for unresolved disagreements noted
in the “Workshop Resolution” column of Table 4.
Scale impression quality was similar in each exchange sample and was
not a factor for any disagreements in age determinations. Three
factors were identified, though, which contributed to age determination
disagreements. The factor which occurred most frequently and had
the greatest impact on exchange results was a difference in conventions
used by each agency with respect to the marginal increment (i.e.,
growth beyond the last annulus). NEFSC age readers assume that
annulus formation occurs during May-July (Dery 1988); therefore, scale
samples collected on winter and spring surveys or during the first and
second quarters of the calendar year should have a wide marginal increment
prior to annulus formation. The algorithm invoked by the NEFSC
for this portion of the year – prior to 1 July – is to count the number
of annuli present and also to count the edge. Thus, a fish whose
scale contains three annuli would be assigned age 4 during 1 January
- 30 June. However, since annuli form in January-March for fish
in the North Carolina - Virginia region, NCDMF age readers assign an
age based on the number of annuli present, assuming that the last-formed
annulus occurred after 1 January of the collection year.
How this observed difference in annulus formation results in age determination “disagreements” is
best illustrated through the use of some 1999 workshop samples. Figure
1 shows a scale image from a 34-cm male summer flounder captured
in May 1995 during the MDMF spring inshore bottom trawl survey (Table
4, ID 6). Each agency agreed that one annulus was present;
however, NEFSC age readers assigned an age of 2 yr based on the width
of the marginal increment and the expectation that a second annulus would
be forming in the next month or so. NCDMF age readers, who typically
age fish which have formed annuli during January-March, interpreted the
increment to be current-year growth, and assigned an age of 1 yr. The
consensus of workshop participants was that the width of the marginal
increment represented nearly 1 yr of growth beyond an annulus formed
during the previous May-July, rather than 3-4 mo of growth beyond an
annulus formed during January, and thus the scale edge should be counted,
resulting in an age of 2 yr. Correcting for this difference in
convention accounted for 40% of the age determination disagreements in
the original 50-fish NEFSC exchange sample (Table
4).
As the workshop progressed to the 120-fish NCDMF exchange sample, the observed
difference in annulus formation discussed in the preceding paragraph
was identified as resulting in an opposite type of increment interpretation
discrepancy. Figure 2 shows a scale image
from a 33-cm fish collected on 4 March 1997 from the Cigar Shoal region
off the North Carolina - Virginia border by the winter commercial fishery
(Table 2, ID 7). Age readers from the
NEFSC assigned an age of 2 yr, and NCDMF 1 yr, for the same reasons stated
earlier. However, in this instance, the width of the marginal increment
is relatively small, supporting a January annulus formation, and the
consensus of the workshop was that growth beyond the annulus represented
current year growth and should not be used to increment the age as done
by the NEFSC. Although time constraints did not allow the workshop
to view the entire NCDMF exchange sample, it is likely that 26 of 51
age determination disagreements (51%; Table 2)
resulted from the NEFSC practice of counting the edge during this period
of the calendar year.
Given the importance of assumptions about the timing of annulus formation
to the 1999 workshop results, participants reviewed samples to verify
the observed variability in this phenomenon. A 6-mo range for annulus
formation was documented. A 25-fish sample commercially caught
in NEFSC Statistical Area 521 (Figure 3) and
obtained in Chatham, Massachusetts, on 6 July 1997 (NEFSC commercial
fishery weighout document #275249) contained numerous scales on which
an annulus had just formed, or was forming, on the scale edge. Similarly,
annulus formation just inside scale edges was observed in a 25-fish sample
commercially caught in NEFSC Statistical Area 635 and obtained in Virginia
on 7 January 1997 (weighout document #600007).
A second source of disagreement between agencies occurred in the choice
of a first annulus in some, but not all, fish. In seven instances
(14%), NCDMF age readers increased the total age by 1 yr by selecting
a first annulus considered by NEFSC age readers to be too small to be
a true annulus, and to be a “check” (i.e., temporary halt in growth)
marking an early life history event. For example, Figure
4 shows the scale image of a 32-cm male summer flounder captured
in March 1996 during the NEFSC spring bottom trawl survey (Table
4, ID 17), in which the NCDMF observed two annuli, but which the
NEFSC aged as 1 yr. Figure 5 shows the
scale image of a 50-cm fish captured in June 1995 by the Southern New
England commercial fishery (Table 4, ID 38),
in which, again, the NCDMF observed two annuli, but which the NEFSC aged
as 1 yr. In other cases, each agency selected different scale events
as the first annulus, which did not alter agreement on a total age, but
would bias length-at-age estimates determined from back-calculation analyses.
The third source of age determination disagreements occurred when the
two agencies differed as to whether certain scale events constituted
an annulus or a check. The NCDMF tended to count such marks if
they continued into the lateral scale field, whereas the NEFSC rejected
these marks if they were weaker in the lateral field than adjacent marks,
or, if their spacing from preceding annuli was not within normal expectations.
CONCLUSIONS
AND RECOMMENDATIONS
Workshop participants concluded that, in general and relative to other
species, summer flounder is difficult to age, due in part to its
extensive range and migratory habits. Annuli in scales are often
weak or ambiguous. Review of whole and sectioned otoliths prepared
by ODU suggested that otoliths may be useful in some instances, particularly
for older fish. However, due to the unavailability of commercial
samples because of dealers’ preference that fish not be cut, otoliths
have not been routinely used. Despite these difficulties, workshop
participants agreed that summer flounder can be reliably aged
using scales, given adequate precautions discussed in succeeding paragraphs.
Workshop participants concluded that the majority of aging disagreements
in the two exchange samples arose from the interpretation of marginal
scale increments relative to the highly variable timing of annulus formation
observed for summer flounder throughout its range. The protracted
period of annulus formation, 6 mo or more, is atypical of most species. Additionally,
it appears that the biological mechanism for annulus formation in different
groups of fish may differ as well; the formation of annuli coincides
with the late spring inshore migration of fish in the northern portion
of the species’ range, but, for fish forming annuli in January-March,
the event appears to occur when the fish are offshore. If the phenomenon
occurred along a latitudinal gradient, then decisions about growth beyond
the last annulus could be simplified to some extent, but the observation
from NCDMF tagging studies that fish from different geographical regions
are mixing offshore during winter months confounds any simple approach
to setting up aging conventions. This confounding poses less of
a problem for the NCDMF, for which production aging is based primarily
on samples obtained from the winter fishery in that region. However,
the NEFSC must exert extreme care in evaluating marginal increment width
in each individual sample, given the spatial and temporal nature of the
samples they receive from research vessel surveys and the commercial
fishery.
The workshop also concluded that more attention must be given to consideration
of first-year growth and the location of the first annulus. The
NCDMF consistently applies criteria of “cutting-over” and intersection
with the ctenii in selecting the first annulus. In order to help
locate the first annulus, NEFSC age readers rely more on a pattern of
checks thought to mark early life history events. Workshop participants
agreed that analyses of material from juvenile fish, possibly archived
by state and university researchers, might assist age readers in interpreting
first-year growth patterns and first annulus selection.
The following recommendations were made at the conclusion of the workshop:
- Sample exchanges should continue on a regular basis between the NEFSC
and the NCDMF, beginning with samples from North Carolina’s 1998 winter
fishery and the NEFSC’s 1998 winter, spring, and autumn bottom trawl
surveys. It was suggested that age readers from ODU also participate
in the exchanges to ensure uniformity of aging methods for the species.
- Appropriate
representatives from the NEFSC, NCDMF, and ODU should inquire as to
the availability of samples and/or data for young-of-the-year summer
flounder in order to conduct analyses of first-year growth.
REFERENCES
CITED
Able, K.W.; Matheson, R.E.; Morse,
W.W.; Fahay, M.P.; Shepherd, G. 1990. Patterns of
summer flounder, Paralichthys dentatus, early life history
in the Mid-Atlantic Bight and New Jersey estuaries. Fish.
Bull. (U.S.) 88(1):1-12.
Almeida, F.P.; Castaneda, R.E.; Jesien, R.; Greenfield, R.E.;
Burnett, J.M. 1992. Proceedings of the NEFC/ASMFC
Summer Flounder, Paralichthys dentatus, Aging Workshop,
11-13 June 1990, Northeast Fisheries Center, Woods Hole, Mass. NOAA
Tech. Memo. NMFS-F/NEC-89; 7 p.
Dery, L.M. 1988. Summer flounder, Paralichthys
dentatus. Chapter 15 in: Penttila, J.; Dery, L.M.,
eds. Age determination methods for Northwest Atlantic
species. NOAA Tech. Rep. NMFS 72:97-102.
[Northeast Fisheries Center.] 1989. Report of the
Fall 1989 NEFC Stock Assessment Workshop (Ninth SAW). Northeast
Fish. Sci. Cent. Ref. Doc. 89-08b; 71 p. Available
from: National Marine Fisheries Service, 166 Water St., Woods
Hole, MA 02543.
Penttila, J.; Nichy, F.; Ropes, J.; Dery, L.M.; Jearld, A.,
Jr. 1988. Methods and equipment. Chapter 3
in: Penttila, J.; Dery, L.M., eds. Age determination methods
for Northwest Atlantic species. NOAA Tech. Rep. NMFS 72:7-16.
Smith, R.W.; Dery, L.M.; Scarlett, P.G.; Jearld, A., Jr. 1981. Proceedings
of the Summer Flounder (Paralichthys dentatus) Age and
Growth Workshop, 20-21 May 1980, Northeast Fisheries Center,
Woods Hole, Massachusetts. NOAA Tech. Memo. NMFS-F/NEC-11;
30 p.
Szedlmayer, S.T.; Able, K.W.; Rountree, R.A. 1992. Summer
flounder, Paralichthys dentatus, growth and temperature
induced mortality during the first year in southern New Jersey. Copeia 1992:120-128.
Acronyms
MDMF= Massachusetts
Division of Marine Fisheries
NCDMF= North
Carolina Division of Marine Fisheries
NEFSC=Northeast
Fisheries Science Center
ODU=Old
Dominion University
