10

Data Collection at Sea

Work is conducted on a 24-hour basis with two watches of 5-6 individuals
standing a 6-hour on and 6-hour off schedule. Upon arriving at a station, the
position (latitude, longitude and Loran bearings) is recorded, and a surface
to bottom temperature profile may be taken. From 1963 to 1970 temperature
profiles were recorded on glass slides with mechanical bathythermographs.
Since 1970 an expendable bathythermograph (XBT) system has been used, which
produces analog chart recordings. In 1985, the XBT system was modified to
permit transmission of seawater temperature profile data to shore-based
facilities on a real-time basis; XBT analog chart recorders were replaced by
SEAS (Shipboard Environmental Acquistion System). This is a computer based
system that records digital information and transmits it via satellite.
Current procedures require collection of XBT data at 1/3 to 1/2 of the total
number of stations, depending on location and biological sampling requirements
(for example, XBT probes are launched at stations where plankton collections
are taken). Plankton tows are made at selected stations employing a 61 cm
bongo frame fitted with 0.505 mm and 0.333 mm mesh plankton nets.

Observations on weather, sea state and position are also recorded.

After the above information is recorded, the trawl] is set with the amount
of wire out (or scope) dependent upon depth. For the "36 Yankee", a 3:1 scope
is used except in depths greater than 183 meters (100 fathoms) when the scope
is 2.1/2:1. The net is towed at 6.5 kilometers/hour (3.5 knots) relative to
the bottom for thirty minutes from the time the brake drums are set. A
fathometer trace is recorded during each tow. Acoustically linked mensuration
gear is now used during many tows to measure headrope height, wing spread and
bottom temperature. Periodic readings are taken and averaged for the duration

of the tow. Information relative to position, depth, environmental



31

There is also an obvious potential. for improving NEFC survey efficiency
by eliminating coverage of some areas in the Canadian Economic Zone, e.g.,
Browns Bank, Bay of Fundy stations. The Bay of Fundy (Stratum 35) has been
sampled infrequently and the Group recommended discontinuing coverage in this
area. For Browns Bank, the Group concluded that a final decision should be
deferred subject to further negotiations with Canada (note, however, that
biological sampling in this area. is being discontinued wherever possible).
Elimination of the area from Browns Bank - Bay of Fundy would save 3-4 days of

vessel time, not to mention trawl gear.


27

EFFECTS OF CHANGES IN SAMPLING INTENSITY
OR AREAS SAMPLED
Sampling Intensity

The effects of reductions (or increases) in sampling intensity were of
primary interest in this analysis, given the need for vessel time and the
potential for future losses associated with funding constraints or mechanical
breakdowns. Since it is more convenient to deal with "sea days" rather than
stations from an administrative standpoint, the Group worked directly with sea
days whenever possible, assuming an average requirement of 50 days for
covering the region from Cape Hatteras to the Scotian Shelf. (Note that
coverage of the region south of Cape Hatteras would require an additional 5-5
days, discussed below).

The Group first examined the relationship between sea days and sampling
intensity expressed as number of stations that could be occupied per survey
cruise (percentage increase or decrease as compared to current levels).
Potential changes in sampling intensity of up to +50% were considered, since
it was deemed unlikely that sampling intensity could be increased by as much
as 50% given current vessel and funding constraints or conversely that the
survey could remain viable with a reduction exceeding this amount. Several
hypothetical scenarios were examined within this range based on vessel speed,
average time on station, and time requirements for port calls; results are
given in Figure 29, It can be seen that within this range the impact of
reductions in vessel time on survey coverage can be quite significant, e.g.,
loss of 15 sea days (30% of our current 50-day requirement to survey from the
Scotian Shelf to Cape Hatteras) would result in a 45% reduction in the number
of stations that could be sampled. The disparity reflects the relative

increase in steaming time (relative to time on station) needed to maintain the


82

BUTTERFISH
Middle Atlantic-Georges Bank
Spring

8

Stratified Mean Woeight/Tow (kg)
8

Legend
ear eee aT
° PYM...
@e
“
s
3 %
§
$
i”
3
a”
Legend
fo aT
oF PTT...

Figure 32. Comparison of stratified mean weight/tow (Delta distribution)
with and without strata of depths greater than 110 m for
butterfish.



36

Table 1. Bottom trawl surveys conducted by the Northeast
Fisheries Center, 1963-1987.

SPRING
1968 6 Mar-22 Apr ALBATROSS IV 36 NS-CH
1969 5 Mar-10 Apr ALBATROSS IV 36 NS-CH
1970 12 Mar-29 Apr ALBATROSS IV 36 NS-CH
1971 9 Mar- 1 May ALBATROSS IV 36 NS-CH
1972 8 Mar-24 Apr ALBATROSS IV 36 NS-CH
1973. 16 Mar-15 May ALBATROSS IV 41 NS-CH
& DELAWARE II
1973. 8 May- 4 Jun ATLANTIC TWIN 3/4 BI-CH
1974 12 Mar- 4 May ALBATROSS IV 41 NS-CH
1974 1 Apr- 2 May ATLANTIC TWIN 3/4 NT-JF
1975 4 Mar-12 May ALBATROSS IV 41 NS-CH
1975 18 Mar-24 Mar ATLANTIC TWIN 3/4 BI-DB
1976 3 Mar- 8 May ALBATROSS IV 41 NS-CH BI-CH
& DELAWARE IT
1977 19 Mar-20 May ALBATROSS IV 41 NS-CH BI-CH
& DELAWARE II
1978 20 Mar-23 May ALBATROSS IV 41 NS-CH BI-CH
1979 21 Mar-12 May ALBATROSS IV 41 NS-CF GM-CF
& DELAWARE II
1980 16 Mar- 8 May ALBATROSS IV 41 NS-CF GM-CF
& DELAWARE II
1981 19 Mar-24 May DELAWARE II 41 NS-CF GM-CF
1982 9 Mar- 8 May DELAWARE II 36 NS-CF GM- CF
1983 7 Mar- 6 May ALBATROSS IV 36 NS-CF GM-CF
1984 24 Feb-25 Apr ALBATROSS IV 36 NS-CF GM-CF
1985 25 Feb-13 Apr ALBATROSS IV 36 NS-CF GM-CF
1986 3 Mar-27 Apr ALBATROSS IV 36 NS-CL GM-CL
1987 23 Mar-29 Apr ALBATROSS IV 36 NS-CL GM-CL

& DELAWARE II


western Nova Scotia (Figure 1, strata 1 to 49) in depths of 27 to 366 m (15 to
200 fm). This area was surveyed in summer, autumn, and winter from the summer
of 1963 (the first survey) through the winter of 1966. Beginning in autumn
1967, survey coverage was extended south to Cape Hatteras, North Carolina
(Figure 1, strata 61-76). A spring survey time series was initiated in

1968. The autumn survey has continued uninterrupted since 1963 and forms the
longest time series (Table 1).

In 1972, coverage was expanded into inshore areas at depths shallower
than 27 m (15 fm). Few stations have been occupied shallower than 9 m (5 fm)
due to vessel safety considerations. Inshore strata from Eastport, Maine to
Buzzards Bay, Massachusetts are shown in Figure 2; inshore strata from Cape
Cod Bay to Cape Hatteras are shown in Figure 3. These cruises have provided
at least partial coverage of inshore areas from New England to the Carolinas
since 1972 (Table 1).

In autumn of 1974, the NEFC provided funds to the State of South Carolina
to survey the inshore and offshore areas from Cape Fear, North Carolina to
Jacksonville, Florida (Figure 4, inshore strata 66-91, offshore strata 78-
93). See Azarovitz 1981. As NMFS coverage extended only as far south as Cape
Hatteras, a small gap in coverage was created between Cape Hatteras and Cape
Fear (Figure 4, inshore strata 51-64, offshore strata 50-77) which was
surveyed beginning in autumn of 1978. Shortfalls in vessel time, and limited
applicability of data collected from south of Cape Hatteras for NEFC research
Purposes have since resulted in more limited coverage of that region, i.e.,
only the region from Cape Hatteras to Cape Lookout, North Carolina (Figure 4,
inshore strata 51-52, offshore strata 50-53) is now being sampled. For
similar reasons South Carolina no longer conducts a broadscale ecological

survey but concentrates on estuarine and hard bottom/reef areas.


64

GEORGES BANK COD SPRING 1985

UNLOGGED WITH ZEROS
Z
.
=
ge Z|
nv IZ)
|
° ote Joon 2
Haedorige oe oa
‘20.
LOGGED WITH ZEROS
A
5 ig] J
3 Z
é
: wg
Z|
: BOLL

5. a.
i I
1 _. Fa ek oo

Figure 13. Observed distribution of Atlantic cod (catch per tow in weight)
during the NEFC 1985 autumn -survey.


46

Table 7, Results of tests for normality for 26 species/stocks
(log-transformed non-zero tows only) sampled during
the NEFC 1985 bottom trawl survey.

Sample Shapiro-Wilk

Species Stock! size (N) Test Statistic
Atlantic Cod GB 24 0.9690
GM 30 0.9602
Haddock GB 41 0.9280*
GM 15 0.9089
Pollock GB-GM-SS 30 0.8484**
Redfish GB-GM 38 0.9482
Yellowtail Flounder MA 8 0.9061
SNE 9 0.9483
GB 15 0.9174
cc 7 0.7321*
Summer Flounder MA i 0.9337
SNE 9 0.8461
Winter Flounder MA 6 0.9157
SNE 18 0.9330
GB an 0.8622
GM 8 0.9190
Atlantic Herring GB 5 0.9555
GM-SS 12 0.8549*
Atlantic Mackerel MA-SS 6 0.9042
Scup SNE-MA 38 0.9646
Bluefish CH-CC 23 0.9432
Black Sea Bass SNE-MA 14 0.8411
American Lobster MA 15 0.7985
SNE 49 0.9113*
GB 28 0.9264
GM 22 0.9696

1 CH=Cape Hatteras; MA=Middle Atlantic; SNE=Southern New England;
GB=Georges Bank; SGB=Southern Georges Bank; NGB=Northern
Georges Bank; CC=Cape Cod; GM=Gulf of Maine; SS=Scotian Shelf.

2 Shapiro-Wilk W for N<51.

* Hypothesis of normal distribution rejected at 0.05 level.
**Hypothesis of normal distribution -rejected at 0.01 level.



44
Table 5. (contd)

Atlantic Herring

outhern New England

Georges Bank

Gulf of Maine - Scotian Shelf
Atlantic Mackerel

Middle AtTantic - Scotian Shelf

Butter fish
MiddTe AtTantic - Georges Bank

Scu

Southern New England - Middle Atlantic

Bluefish
Cape Hatteras - Cape Cod

Black Sea Bass

Southern New England - Middle Atlantic

American Lobster
Middle AtYantic
Southern New England
Georges Bank

Gulf of Maine
Scotian Shelf

Northern Shrimp
GuTf of Maine

Shortfin squid
fiddle Atlantic - Georges Bank
Longfin Squid
‘iddle Atlantic - Georges Bank

All_ Species
Total Survey Area

1-12, 61-76
13-23, 25
24, 26-30, 36-49

Spring 1-25, 61-76
Autumn 1-42, 49

Spring 1-12, 61-76
Autumn 1-14, 16, 19-20,
25, 61-76

Spring 2-3, 61-63,
© 65-67, 70-71,
74-75
Autumn 1-2, 5-6, 9, 61,
65, 69, 73-74

1-25, 61-76

Spring 61-76
Autumn 1-2, 5-6, 9-10,
61-76

61-76

1-12

13-23, 25

24, 26-30, 36-40
31-35, 41-49

24, 26-28, 37-40
1-25, 61-76
1-25, 61-76

1-40,.61-76



50

Table 11. Stratified mean weight per tow for silver hake
taken in NEFC spring and autumn bottom trawl
surveys from the Southern Georges Bank - Middle
Atlantic area compared to corresponding Delta
distribution estimators.

Linear Scale Delta Distribution
Year Mean Std. Error Mean Std. Error
Spring
68 3.627 1.501 3.756 1.616
69 2.131 0.378 2.202 0.430
70 1.223 0.161 1.233 0.176
71 2.200 0.322 2.192 0.302
72 1.372 0.182 1.399 0.210
73 7.420 0.928 7.999 1.142
74 5.728 0.930 5.594 0.889
75 9.278 1.550 9.882 1.956
76 7.202 1.723 6.618 1.167
77 6.825 1.013 7.237 1.130
78 8.675 1.593 8.547 1.501
79 3.469 0.688 3.771 0.904
80 4.432 0.785 4.475 0.763
81 5.585 0.635 6.055 0.897
82 1.982 0.392 2.018 0.459
83 1.291 0.187 1.376 0.241
84 2.282 0.636 2.209 0.550
85 2.657 0.521 2.642 0.464

Autumn



54

+ 44° aac

42° 42° 4

70° 68°

Figure 2. Strata sampled on NEFC inshore bottom trawl surveys from
Eastport, Maine to Buzzards Bay, Massachusetts.


Table 4. (Contd)

42

Areal

Depth

Zone (m)

No. of
Stations

Offshore Strata

Depth
Stratum Area! Zone (m)
1 2516 27-55
2 2078 56-110
3 566 111-183
4 188 >183
5 1475 27-55
6 2554 56-110
7 514 111-183
8 230 >183
9 1522 27-55
10 2722 56-110
ll 622 111-183
12 176 >183
13 2374 56-110
14 656 111-183
15 230 >183
16 2980 56-110
17 360 111-183
18 172 >183
19 2454 27-55
20 1221 27-55
21 424 56-110
22 454 111-183
23 1016 56-110
24 2569 111-183
25 390 27-55
26 1014 56-110
27 720 111-183
28 2249 >183
29 3245 >183
30 619 >183
31 2185 111-183
32 655 56-110
33 861 56-110

a
PAONUWOYNEON RANE EHMDOWEROWROWWDONWWONWW YX

34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76

1766
1097
4069
2108
2560
730
578
1570
156
860
934
150
247
1159
1184
198
1318
243
86
60
2832
555
86
52
2433
1024
281
105
2145
1273
139
60

111-183
111-183
>183
111-183
111-183
56-110
56-110
111-183
56-110
111-183
56-110
56-110
56-110
111-183
111-183
>183
27-55
56-110
111-183
>183
27-55
56-110
111-183
>183
27-55
56-110
111-183
>183
27-55
56-110
111-183
>183

square nautical miles



14

same time provide the only monitoring capability we have for many species for
which adequate commercial information is lacking.

The autumn survey has been relied upon most extensively for development
of abundance indices (Figure 8). For 23 of the 28 species considered (82%)
autumn data are used in preference to spring data or at least appear to be of
comparable utility (Figures 8 and 9). To a large degree this reflects the
availability of a longer autumn time series and gear changes in spring as
described earlier; conversion factors are not available for many species and
the task of developing them is not a trivial one. Of the five monitored by
spring indices, those of major commercial or recreational significance
(pollock and mackerel) can be reliably tracked by virtual population analysis
(VPA) or commercial catch per unit effort (CPUE). In the absence of a spring
survey, alternative monitoring tools would be lacking for dogfish, skates, and
ocean pout; and while these species are of minor commercial significance at
present, dogfish and skates are major ecosystem components with potential
significant impacts on a number of fisheries. It is probable that alternative
procedures, e.g., CPUE or autumn survey modifications could provide this
capability.

Although the autumn survey time series has been clearly preferable for
this purpose, it should be noted that spring data are often no less important
as a "backup" tool. Short-lived species, e.g., squids, also require seasonal
monitoring. The capacity for "backup" monitoring becomes particularly
important in the case of intensively utilized species under strict management
regimes as well as for species for which no alternative measures are
available, e.g., certain flounders, white hake, and cusk, which collectively
comprise an important component of the New England and Mid-Atlantic fishery

resource.



32

CONCLUSIONS

Results of this project have been useful in documenting assessment-
related uses of survey data, for evaluating precision including techniques for
increasing it, and for evaluating gains or losses in precision associated with
changes in sampling intensity. With respect to assessment-related
applications, the autumn survey is clearly the primary data source, although
the spring survey is important for corroborating autumn survey trends
(including recruitment estimates) and for development of biological parameter
estimates including maturity information. Discontinuation of the spring
survey would result in significant reductions in our monitoring and predictive
capabilities and in our ability to evaluate biological parameters, Other
programs would also be affected.

Precision of the Delta distribution estimators was found to be highest
for demersal species and was generally higher in autumn. Precision for
flounders was generally poor. Comparable levels of precision were achieved
for test cases in which direct comparisons were made between Delta
distribution estimators and indices calculated from untransformed data. It
was determined that, for general management applications, levels of precision
achievable through use of the Delta distribution estimators were reasonable.

The SWG smoothed the Delta distribution estimators for selected
species/stocks using time series (single-season) models. Trends for the

“resulting indices agreed well with those evidenced by fishery-dependent
estimates. Attempts to maximize benefits from the surveys by combining spring
and autumn data and fitting time series models to the combined data sets did
not result in consistent improvements, probably due to seasonal differences in
catchability.

Analyses to evaluate relationships between sampling intensity (in terms



63

YELLOWTAIL AUTUMN 1985

In(Variance)
°
°

-14

-24 .

n4

-3 T T T T T T
. o 0s 1 15

0
~ e
.
$4
> .
5 ?
§ ? fee
of
= z*.
.
. e e
-5
e e
-10 “T T T
a a H y
In(Meon)

Relationship between variance and mean survey catch per tow
for yellowtail flounder and silver hake taken during the
NEFC 1985 autumn bottom-traw] survey (data transformed to

natural logarithms).

Figure 12.


| Preceding p page blank v

EXECUTIVE SUMMARY

“In March of 1986 the Survey Working Group (SWG), consisting of personnel from

the Population Dynamics and Population Biology Branches of the Conservation and
Utilization Division as well as staff from the Fisheries Ecology Division,
began an evaluation of the Northeast Fisheries Center (NEFC) bottom trawl
survey program. ~ Objectives were to: >) . °

“1 )document significant aspects of the Woods Hole bottom trawl
survey program, including its history, areas covered, and vessels,
gear, and procedures used;

2\document current assessment-related uses of survey data;
aluate current levels of precision attained;

4)investigate methods of improving survey precision and
reliability; and

\
Study the implications of changes in sampling intensity

including modifications in existing survey coverage.

Results are intended for use by personnel within NEFC and other organizations
for scientific research purposes and by agencies responsible for design and
implementation of resource surveys. The following report summarizes the results
of this evaluation.

I. OVERVIEW OF THE NEFC BOTTOM TRAWL SURVEY PROGRAM

The SWG reviewed major features of the NEFC bottom trawl] survey program.
Significant aspects are as follows:

1) The program was initiated in Summer of 1963. To date, 62 offshore
surveys at depths >27 m (15 fm) and 40 inshore surveys at depths <27 m
have been completed. Offshore spring and autumn surveys (dating back to
1968 and 1963, respectively) provide the longest time series; winter and
summer surveys have also been run intermittently. Inshore survey coverage

was initiated in 1972.

2) Spring and autumn survey coverage has included the region from Cape
Hatteras to the Scotian Shelf since 1967. Additional coverage of areas
south of Cape Hatteras was provided in most spring and autumn surveys from
1978-1985 but since the latter year most of this coverage has been
discontinued. Inshore coverage has been more variable and has generally
been most intensive in the Middle Atlantic region.


Figure 15.

66

YELLOWTAIL FLOUNDER
Southern New England
Autumn

25

Stratified Mean Weight/Tow (kg)

62 66 70 cl 78 62 86
Year
SILVER HAKE
Gulf of Maine — Northern Georges Bank
Autumn

30

254

20-5

Stratified Mean Weight/Tow (kg)

Stratified mean catch per tow in weight for yellowtail
flounder and silver hake taken in NEFC autumn bottom
trawl] surveys plotted against the corresponding Delta
distribution estimators, 1963-1985.



Figure 14.

5
~
o
<
z
cc}
& 104
S
o
=
c
&
=
27]
=
&
a

0

62 66 70 cod 78 82 86

Year
HADDOCK
Georges Bank
Autumn

80
~
RS
x 7
S
B=)
= 404
8 A
= Linear
3
= 24
&
G

o — —

62 66 70 aT 78 82 86

Year

Stratified mean catch per tow in weight for Atlantic cod
and haddock taken in NEFC autumn bottom trawl surveys
plotted against the corresponding Delta distribution
estimators.


68

DEMERSALS

35
04 Spring
254

204

54
104 GY oA

54

A)
304 Autumn

254
20+

154

Percent

10-4

54
0 ZAI

304 Spring and Autumn

25-4

204

5

10+

54

3 LA A

Interval

Figure 17. Frequency distribution of coefficients of variation by
season for demersal species.


25

Bank cod, Georges Bank haddock, and redfish (see Figures 22 and 23). The
relationship between the smoothed survey index and yellowtail flounder CPUE
was generally good although a notable discrepancy occurred in the latter part
of the series in both cases (Figure 24). This appears to be due to an upward
bias in CPUE estimates for more recent years when stock size was declining
rapidly on Georges Bank and off southern New England. CPUE was computed for
only those trips in which yellowtail comprised 50% or more of the catch; this
would result in a positive bias when stocks were declining since lower CPUE
levels would not be represented, Recent CPUE data for the southern Georges
Bank - Middle Atlantic silver hake stock may be similarly biased (Figure 25).

The conclusion of the Group was that time series modelling approaches are
very promising. There is also a marked improvement in precision of the
estimators when using the smoothing technique, as a result of more information
being used in computing the smoothed index (Pennington 1985). We rely not
only on the point estimate for a given year but the estimates from adjacent
years (full benefits therefore do not apply to the ends of the series where
there is less information).

The Group next attempted to achieve maximum benefit from the surveys by
combining spring and autumn data and fitting time series models to the
combined data sets (1968-present; two points per year). Combining information
in this way increases the number of data points available for analysis and
provides a basis for consistent interpretation of trends in abundance (it is
not uncommon for the two seasonal series to exhibit differences in short-term
trends due to differences in sampling variability).

Examples of time series models fitted for spring and autumn surveys (two
observations per year) are provided in Figures 26 through 28. Comparisons in

residual mean square error for the seasonal models to that for the


ll

parameters, and trawl setting and performance is collectively referred to as
"station data."

After haulback, the codend contents are sorted by species (spiny dogfish,
lobsters and crabs are further separated by sex) and weighed to the nearest
0.1 kg. Most fish species are measured to the nearest whole centimeter (fork
length). Other measures include wing width for rays, carapace length for
lobsters, carapace width for crabs, shell height for scallops, and mantle
length for squids. Shrimp are weighed only. For catches that are too large
to sort completely, a subsample by weight or volume is taken and later
expanded to represent the entire catch. All catch and station information is
recorded on a two sided waterproof log (Figure 5) which serves as an original
written record of all data obtained at a station.

After initial processing, ageing samples (i.e., scales or otoliths) are
collected for approximately 26 species for later processing and age
determinations at the laboratory. Sex and maturity information is also
recorded, as well as a variety of disease observations, Stomach contents of
selected species are routinely examined for food habits studies. Whole
specimens or parts thereof are also preserved and documented as needed by
researchers in other agencies and academic institutions. Detailed
observations on marine mammals and birds are also recorded throughout the
cruise, usually by experts from other organizations, e.g., Manomet Bird
Observatory. A schematic outlining the general flow and disposition of this
material and resulting data.is given in Figure 6.

Data Processing

Initial data processing is accomplished at sea by watch chiefs and the

chief scientist, who review and code all logs prior to returning to port.

Weights and total number at length by species are determined; a numerical code


17

data are, provided by VPA's but the surveys provide the only basis for
prediction. They also provide the only historical data source we have for the
species indicated (Figures 8 and 10).

It is extremely important to note that these evaluations were
intentionally directed towards species of major commercial or recreationa
importance for which alternative sources of data are likely to be available.
If all species in the ecosystem are considered, the surveys become over-
whelmingly important as a scientific data source.

Other Data

In addition to the assessment information mentioned above, NEFC spring
and autumn surveys have provided a basis for studying distribution, fecundity,
and food habits; monitoring prevalence of fish disease and anthropogenic
impacts, and other miscellaneous studies. Biological samples are also
collected for a wide range of studies at numerous colleges, universities,
state or federal research agencies and the private sector. Normally, such
studies would be far too expensive because of the high cost of chartering and
equipping vessels to make such collections. In addition, oceanographic and
ichthyoplankton sampling is routinely conducted for other Center research
programs, e.g. studies of recruitment variability. The necessary seasonal
coverage for such programs can be provided in part by "piggy-backing" on
spring and autumn surveys. In addition, the need to develop an adequate
understanding of ecological relationships for multispecies management insures
the need for continued seasonal monitoring of species assemblages. Such work

can best be accomplished through intensive seasonal surveys.


13

SURVEY DATA APPLICATIONS
Stock Assessment Data

The relative utility of the NEFC spring and autumn bottom trawl] surveys
as a source of data for assessment-related applications has been of primary
interest due to possible redundancy in these time series and potential future
losses in vessel time. Accordingly, the Group documented the uses to which
these data have been put.

Both surveys are important in assessing the multispecies complex off the
northeast coast of the USA. Most species in this region undergo seasonal
shifts in distribution, some more pronounced than others. Thus, areas of
concentration for many species differ seasonally, often with a marked
influence on both amount and size/age composition of the survey catch.
Maturation, spawning, and recruitment also differ seasonally. It follows that
the relative utility of these surveys will vary considerably depending upon
species and application.

The Group considered the following categories of assessment-related
information for 28 species of major commercial or recreational significance:
(a) relative abundance, (b) growth, (c) population age/size composition, (d)
maturity, (e) mortality, and (f) recruitment. Figure 8 documents the relative
utility of each survey by species and category as reflected by the best
judgement of the assessment scientist concerned (summarized in Figure 9.) and
also indicates whether NEFC surveys provide the only source of such
information (summarized in Figure 10). Specifics for each category are
discussed below.

Spring and autumn survey data have been of primary importance in
monitoring trends in relative abundance. The bottom trawl surveys provide the

only source of statistically valid unbiased indices of abundance and at the


26

corresponding single season model revealed no consistent differences. For
southern New England yellowtail flounder, the seasonal model was characterized
by a 23% reduction in mean square error relative to the single season model;
for Georges Bank yellowtail, residual error for the seasonal model was nearly
identical to that of the model based on the above autumn series, and for
Georges Bank haddock, an increase of 13% residual error was observed for the
combined model relative to the single season model. This result was not
unexpected given the potential for seasonal differences in catchability. An
alternative approach such as averaging spring and autumn survey data and

modeling the averaged series may be more effective in such cases.



been assigned a minimum of two stations to permit variance computations.
Certain strata off Southern New England and on Georges Bank have also been
sampled more heavily because of assessment priorities. Areas of individual
strata sampled in this program, together with the average number of stations
historically sampled in each, are given in Table 4. (See also Figures 1-4,
depicting individual strata).

Following Cochran (1977) the stratified mean catch per tow and its

variance are expressed as:

- 2 Nn
Vot = = oy
a
and 2 2
- h Sh
Wy) 2b th
st ne hel ny,
where N = total area of all strata;
Nn = area of Stratum h;
In = sample mean in Stratum h;
sie = sample variance in Stratum h;
Mh = number of sample observations in Stratum h, and
& = number of strata in the strata set.

An approximate confidence interval about the stratified mean (using a large-
sample approximation) may be calculated as:

- - Yo

Yet t L.96LV (Fey )P
The exact expression for the confidence interval is given by Cochran (1977:

95-96).



60

% CHANGE IN NUMBER OF STATIONS
s
o

° 2 a 2 6 6 DO

s a 0
CHANGE IN NUMBER OF SEA DAYS

Figure 29. Relationship between change in number of sea days and
percentage change in number of stations that can be
occupied during NEFC spring and autumn bottom trawl
survey.

% CHANGE IN STANDARD DEVIATION

+26 PARES.

18-6 -2 9 6-30 3 6 @ 2 WB
CHANGE IN NUMBER OF SEA DAYS

Figure 30. Relationship between changes in number of sea days and
percentage change in standard deviation for demersal,
pelagic and flounder species taken during the NEFC 1985
autumn bottom trawl survey.


67

ALL SPECIES

35
304 Spring
254

204

54

10 4
54 Gi

304 Autumn
254
204
64
104

Y —_
54
0 AANA
304 Spring and Autumn

Percent

254

204

154

10+
sd
oO T T f y + y + y an

0 1 20 30 40 50 60 70 80 90 100
Interval

Figure 16. Frequency distribution of coefficients of variation by
season for all species combined.


52

Table 13. Changes in sampling precision (expressed as percentage change in standard
deviation) resulting from adjustments to number of sea days. Calculations
based on data collected during the NEFC 1985 autumn bottom trawl survey.

CHANGE (+/-) 1N NUMBER OF SEA DAYS
SPECIES STOCK -18 -15 +12 “9 -6 +6 +9 +12 +15 +18
HADDOCK ce 52.3. 35.3 20.2 14.0 4.300 -3.4  -9.9 12.5 -17.2 0 -20.3
HADDOCK cM 58.5 30.0 29.2 11,8 11.9 -8.8 -8.8 -15.6 -15.8 -21.2
cop cB 58.5 34.5 21.3 13.3 5.4 -4.5 -9.5 -13.0 -16.9 -21.2
cop cM 58.2 30.3 28.3) 11,6 11.5 78.5 -8.6 -15.3 -15.9 -21.2
YELLOWTAIL GB 56.6 30.3 19.8 12.2 5.30 -4.6 -9.0 -12.5 -15.9 -20.9
YELLOWTAIL SNE 47.30 41.8 18.4 15.6 1.70 1.30 -10,7 -11.7) -18.6  -19.3
YELLOWTAIL CC 52.6 36.1 23.5 13.7 6.7 -4,8 +98 -13.5 -17.4 -20.4
YELLOWTAIL MA 43.1 41.9 15.9 15.4 0.4 =-0.3 +106 -10.8 -18.6 -18.8
WINTER FUN GB 61.4 40.0 25.4 15.1 6.6 -5.5 -10.4 -14.4 -18.2 -21.5
WINTER FLN SNE 43.3 41.9 16.2 15.7 0.4 =-0.3° -10.8 -11.0 -18.8 -19.0
WINTER FLN GM 58.7 29.6 29.6 11,9 12.1 -8.9 -8.8 -15.8 -15.8 -21.3
WINTER FLN MA 52.1 26.6 25.2 10.7 10.2 77.6 8.1 -14,1  -15.3 -20.0
SUMMER FLN GB 57.6 38.8 24.8 14.8 6.4 -5.4 -10.3  -14.3  -17.9  -21.0
SUMMER FLN SNE 41.9 41.2 15,6 15.4 0.2 -0.1 -10.6 -10.7 -18.8 -18.6
SUMMER FLN MA 51.4 28.6 22.4 9.2 9.6 -7.4  -7.1 13,1 -15.3 19.9
SILVER HAKE SGB-HA 62.0 40.9 26.2 15.4 6.8 -5.7 -10.5 -14.7 -18.3  -21.5
SILVER HAKE GM-NGB 63.9 37.7 23,3 10.8 9.0  -6.9 -8.1 -13,6 -17.6 -21.6
RED HAKE SGB-MA 64.7 38.2 24.8 15.5 6.1 -5.0 -10,5 -14.2 -17.6 =21.8
RED HAKE GM-NGB 1035.30 28.00 13.1 7.60 -5.5 -9.4 ©1367 -17.0  -20.5
WHITE HAKE © GB-GM 3037.30 26.2 12.8 S48 67.30 9.0 -14.6  -17.4 -21.7
POLLOCK GB-GM-SS. 58.9 32,6 27.3 12.5 10.1 -7.6 9.1 14.9 -16.4 -21.2
BUTTERFISH  GB-MA 55.4 36.5 22.8 14.4 5.300 -6.30 --9.8 -13.0 -17.1 -19.5
REDFISH GB-GM 57.8 30,0 28.1 12.5 11.6 -8.5 -8.5  -15.2 -15.8 -21.0
ATL MACKEREL MA-SS 58.6 29,5 29.4 12,0 11.9 -8.8 -8.9 -15.8 -15.8 -21.3
BLUEFISH cC-CH 51.9 36.6 19.4 11.7 5.7 94,5 +85 12.1 -17.4 20.1
BLK SEA BASS SNE-MA 39.6 26.9 13.8 2.4 5.0 -3.9  -5.6 -8.8 -12.8 -15.1
ScUP SNE-MA 52.1 35.4 18.0 11.4 5.300 4.20 -8.2 -11.7 17.1 -20.3
ATL HERRING GB 52.3 34.8 24.3 13.6 7.4 8.30 69.7 -13.8 -17.0 -20.4
ATL HERRING GM-SS 58.7 29.7 29.8 11.9 12.0 -8.9 8.8 -15.8 -15.8 -21.3
AM LOBSTER SNE 37,4 30.8 14.2 11.4 1.70 -1,3 0 -7.5 0-844 --12.8  -13.6
AM LOBSTER SS 31,5 0 18.4 1.1 5.4 3.8 0 -2.7 -3.8 5.8 -7,8 8.9
AM LOBSTER = MA 36.4 26.1 13.8 7.0 5.0 -4,0 -5.4 -8.7 -12.6 -16.9
AM LOBSTER © GM 58.6 30.0 28.9 12,0 11.6 -8.6 -8.8 -15.5 -15.9 -21.2
AM LOBSTER GB 56.3 35.0 21.9 13.6 5.6 4.7) --9.7 413.2 -17.0 -20.9
SFIN SQUID MA-GB 49.0 25.2 15.5 9.1 4.60 -3.4  -6.1  -8.8 -11.1 14.2





23

The lower precision for flounders deserves further attention. It is
believed that this problem relates primarily to gear performance, i.e. use of
roller gear, which causes the net to pass over individuals lying on or
burrowed into the substrata. For the same reason, availability may vary to a
greater extent for this group due to diel activity changes. Efficiency could
be increased by use of a chain sweep but this would restrict surveys to smooth

bottom area (thus in effect requiring development of a special survey for this

group).


59

CRUISE LOGS
COMPLETED AT SEA!

CATCH DATA STATION DATA TEMPERATURE]
NO.. WT.. SIZE LOCATION, DEPTH, ‘SALINITY
COMPOSITION | TEMP..SAL, DATA

WEATHER PROCESSING

DATA ENTAY DATA ENTAY

PRELIMINARY FISHERMEN'S | PRELIMINARY
DATA FILES REPORTS —[ DATA FILES
STATION
DATA AUDIT
MERGE
CORRECTED
TEMPERATURE
STATION/CATCH SALINITY
DATA PREAUDIT DATA
CATCH DATA
AUDIT
AGE AND
MATURITY

DATA FINAL
CALCULATIONS,
PACKING, KEYING

MASTER UNKS TO OTHER

DATA BASE NEFC OATA BASES
SVSTA.DAT
SVCAT-DAT

Figure 7. Flow diagram indicating NEFC survey data entry procedures
and outputs.



61
SURVEY ASSESSMENT INFORMATION SOURCE

100

S

WY 3 > 5
5 Se Se oss oe sy
foe] =
S| Bese FRET SE ZZ BEM 257
ir] Sy 4 os =x S &
a cay go = = S
x 8 = BY
Yy, //, 7/,
YW,

OY) Legend
Vi Ui e:

wma

0 as

Figure 9. Relative utility of NEFC spring and autumn survey data by category
(type of assessment-related information) summarized over 28 finfish
and invertebrate species (see Figure 8 for species involved).
Percentages for each category were based on number of species for
which surveys provide information.

100

so

PERCENT

25
neey
YSy
n Sh

yj
J

Legend
Wm ONLY SURVEYS
CZ ALL sources

Figure 10. Importance of NEFC spring and autumn surveys as a source of assessment
information for 28 finfish and invertebrate species. Percentages
for each category based on number of species for which information
specified is available.


In(Variance)

Figure 11.

In(Variance)

62

ATLANTIC COD AUTUMN 1985

10 LJ
e
ry
.
s4
.?
o%
fe
oo
o4
°
-5 T T —
° 1 2 3 ‘ 5
In(Mean)
HADDOCK AUTUMN 1985
a
64 ce
a .
oe
Se
24 °°
.
04
-24
°
4 —— T
-2 = ° 2 3 4

in(leor)

Relationships between variance and mean survey catch per tow
for Atlantic cod and haddock taken during the NEFC 1985
autumn bottom-trawl survey (date transformed to natural

logarithms) .



20

England, Mid-Atlantic, Scotian Shelf, Gulf of Maine, etc., see Table 7.- It
should be noted, however, that tests of this type should be conducted at the
stratum level, since the operational level is the individual stratum in actual
practice. Thus, results must be taken as suggestive but not definitive.

Since few observations were available for most of these tests, the Group
used the Shapiro-Wilk statistic for small sample sizes (N<50). This statistic
is robust and provides one of the most powerful tests available for normality
in such cases (Shapiro and Wilk 1965). The hypothesis of a normal
distribution for the log-transformed catch values could be rejected for few of
the species/stocks tested (Table 7). These results support the general
application of the Delta distribution to NEFC bottom trawl survey data.

The Delta distribution estimator (c) for stratified mean catch per tow in

numbers or weight is defined as:

c= r Ws C(ms/ns)exp(y;) G (£s.2)] m>1
ey hE ENTE i) Sn Si

The variance of this estimator (Pennington 1983) is:
m,

n ms
var eb a/R emai of G8) - Gab 6H sf] ms
ie i i

where W; = weight assigned to stratum i;

my number of nonzero tows in Stratum i;

n; = total number of tows in Stratum i:

Y; = mean of the log-transformed data in Stratum i;

3,2 = variance of the log-transformed data in Stratum i;
G&, = an infinite series (used to correct for bias during

retransformation),



72

YELLOWTAIL FLOUNDER
Southem New England
Autumn

Stratified Mean Weight/Tow (kg)

SILVER HAKE
Gulf of Maine - Northern Georges Bank
Autumn

Stratified Meen Weight/Tow (kg)
e

e ” 4 ET a -
Yeer

Figure 21. Effect of applying time series models to stratified mean
catch per tow values (Delta distribution estimators, solid
lines) for yellowtail flounder and silver hake.


vil.

3) As a rule, data from both surveys are used for estimating biological
parameters (growth, mortality, maturity, age/size composition) because of
a need for the widest possible seasonal time span. Spring survey data
provide an important source of maturity information for many species.

4) Surveys provide an exclusive source of assessment-related information
for anywhere from 30-55% of the species considered, depending upon
category.

III, PRECISION OF SURVEY RESULTS

The SWG evaluated the underlying statistical characteristics of NEFC survey
data and determined the degree of precision and reliability that could be
achieved using appropriate transformations. Results were as follows:

1) For most species, analyses indicated an aggregated form of
distribution. The SWG partitioned catches into zero and non-zero values
and took logarithms of the latter, which were found to be normally
distributed. Estimators for the mean and variance of the mean for the
partitioned data (Delta distribution) have been shown to be more efficient
than the corresponding sample statistics in such cases. Accordingly, the
SWG developed Delta distribution estimators which generally agreed closely
with the corresponding sample statistics (linear scale). Standard errors
were not necessarily less than those calculated from untransformed data.

2) As a general rule, precision of the Delta distribution estimators was
found to be highest for demersal species, and higher in autumn than in
spring. Precision for flounders tended to be generally poor, however.

3) Examination of confidence intervals about the mean suggested that for
general management, applications levels of precision achievable through use
of the Delta distribution were reasonable for demersal species. This is
less true for pelagics although we have reasonable information on pelagic
species of major importance from other sources.

4) Lower precision achieved for flounders suggests the need for alter-
native procedures, e.g., specialized surveys to monitor trends in
abundance.

IV. IMPROVEMENT OF SURVEY PRECISION

The SWG attempted to smooth random variability by fitting time series models to
the Delta distribution estimators. Results were as follows:

1) Time series models proved to be very effective in filtering out random
variability in the data sets examined. They also appear to provide
considerable insight into reliability of individual data points and of the
time series as a whole.


Il.

3) Vessels and gear have been standardized insofar as possible so as to
provide consistent results. Offshore surveys and most inshore surveys
have been conducted aboard R/V ALBATROSS IV and R/V DELAWARE II. A
standard "36 Yankee" trawl] has been used in all autumn surveys and in
spring surveys conducted from 1968 to 1972 and since 1982, while a
modified "41 Yankee" trawl was used in spring surveys from 1973 to 1981.
Both trawls are equipped with a 1.25 cm (0.5 inch) stretched mesh liner in
the codend and upper belly for sampling juvenile fish and roller gear to
make them suitable for use on rough bottom.

4) A stratified random sampling design has been used in these surveys,
with stations allocated to strata roughly in proportion to area and
assigned to specific locations within strata at random. Between 350 and
400 stations (one for every 200 square nautical miles) are routinely
occupied during a given survey.

5) Data collected at sea includes both "station" data (location, depth,
meteorological observations and sea state, expendable bathythermograph or
XBT profiles, and similar information, and "biological" data (numbers,
weight, size composition and sex and maturity information). Biological
samples are also collected for ageing and other purposes. Plankton
samples are collected at selected stations for use in other Center
programs.

6) Following completion of the cruise, data are entered into preliminary
data files for use in industry reports and for meeting immediate
assessment. and management needs. Data files are then audited, corrected as
needed and merged into the master survey data base.

SURVEY DATA APPLICATIONS

The SWG documented assessment-related applications (relative abundance, growth,
population size/age composition, mortality, maturation patterns, and
recruitment indices) for each survey. Results were as follows:

1)Because of the availability of a longer and more consistent time series
the autumn survey is used preferentially to provide indices of relative
abundance. Spring survey data are important for providing corroboration
of population trends, particularly for species for which alternative
sources of information are not available.

2) The autumn survey is also used to a greater extent for developing
recruitment indices. In this case, however, the importance of the spring
survey is proportionally greater because of increased availability of
juveniles of several species/stocks to the survey gear.


Station Selection

Sampling stations are selected as follows. Each stratum (Figures 1
through 4) is divided into rectangular units of 5 minutes of latitude by 10
minutes of longitude. Each rectangular unit is further subdivided into 10
sampling locations of 2 YW minutes of latitude and 2 minutes of longitude
which are numbered consecutively. Each location is considered to be
homogeneous, requiring only one station to characterize it. A random number
generator is used to locate the appropriate number of stations within each
stratum. If two stations are selected within a 5 x 10 minute unit, the second
is eliminated and another selection is made. Stations are plotted on nautical
charts before each cruise. Between 350 and 400 stations (approximately one
station for every 200 square nautical miles) are routinely occupied during
seasonal survey cruises.

In recent years, increased activity by fishermen employing fixed gear in
both inshore and some offshore strata has made it necessary to relocate
stations or to drop them altogether. In order to avoid this gear, these
stations are often occupied only during daylight hours, possibly introducing
bias to the data base. Known hard bottom areas may also be avoided with
stations being moved by as much as several nautical miles.

Cruise tracks are designed to provide synoptic coverage of major species/
stock areas and to facilitate travel between stations in as short a time as
possible. The direction of each tow is generally made toward the next
station, although. exceptions may be made due to strong currents, inclement
weather, known hard bottom areas and attempts to follow depth contours. A

survey cruise requires from 45-55 days.


ACKNOWLEDGEMENTS

The following personnel of the Conservation and Utilization Division (CuD)
and the Fisheries Ecology Division (FED) of the Northeast Fisheries Center,
National Marine Fisheries Service authored or otherwise contributed to sections
of this report: Frank Almeida, Thomas Azarovitz, Stephen Clark, Donald
Flescher, Michael Fogarty, Janice Forrester, Josef Idoine, Linda Patanjo,
Michael Pennington, and Gary Shepherd. Nancy Andrade and Ruth Prouty typed the
manuscript. Frank Almeida, Donald Flescher, and Brian O'Gorman assisted with
Preparation of the tables and computer graphics work. Phyllis Hartley prepared
charts of inshore survey strata and survey strata south of Cape Hatteras.
Primary responsibility for organization and direction of analytical work was
assumed by Michael Fogarty. Overall editing and coordination was provided by
Stephen Clark.

Special appreciation is expressed to Emory D. Anderson, formerly of the
Northeast Fisheries Center, for his contributions and guidance during early

phases of this work (1983-1985).


Figure 31.

81

RED HAKE
Southern Georges Bank — Midde Atlantic
Spring

Stratified Mean Weight/Tow (kg)

24
Legend
a
;
Ss
i
z
5
Legend
FOR...

Comparison of stratified mean weight per tow (Delta
distribution) with and without strata of depths greater
than 110 m for red hake.


74

HADDOCK
Georges Bank

500 60
4 [50
400:
>
€ Hy 40
8 300 ‘ 2+ Biomass x
s i 3
S 30 =
2 L
: 3
5 200 =
a
+ | 20
N
100:
F 10
o 0
62 66 70 oo 78 82 86
Year
REDFISH
Georges Bank — Gulf of Maine
150 40
1+ Biomass
> | 30
c
& 100
2 - x
8 3
S £
Sw OT index eR + 20
4 3
2 =
§ “
& so
+
= fr 10
oO T T T T — 0
62 66 70 7m 78 #2 86

Year

Figure 23. Comparisons between fitted (smoothed) survey indices and
fishery dependent population estimates for haddock and

redfish.


Figure 26. Stratified mean catch per tow (Delta distribution estimators.
solid lines) for Georges Bank haddock taken in NEFC spring
and autumn surveys, 1968-1986, smoothed by time-series
modeling.


15

NEFC survey data have been widely used for evaluating growth and
maturation. For growth, there tends to be more balanced usage of the spring
and autumn surveys than for developing relative abundance indices. Seasonal
coverage provides considerably more information, particularly for juvenile
fish, since many commercial species do not recruit fully to the survey gear
for a full year (spring at age 1}. Such surveys also provide representative
sampling of the population in space and time and data for other species of
lesser importance that are not readily available from other sources.
Accordingly both spring and autumn data have generally been used for growth
studies (Figures 8 and 9). The spring survey has also been of considerable
importance as a source of maturity data. Spring data are collected
immediately before or during the spawning season of many important species,
thus providing the best opportunity for determination of sexual maturity.
Consequently, there has been more of a tendency to rely on spring survey data
for maturation studies. These considerations indicate the importance of
adequate seasonal coverage in providing biological data required for
management, e.g. for development of mesh size regulations keyed to growth and
maturation.

The NEFC survey data base has been utilized less for evaluation of
mortality rates (Figure 8) since greater reliance has typically been placed on
analysis of commercial data for this purpose. This capability has, however,
been useful for intensively utilized species for which VPA's or similar
analyses are lacking and for preliminary assessments. Both surveys have been
used for this purpose (Figure 9).

Perhaps the most important application of survey data in general lies in
determining population size and age composition with particular reference to

predicting the strength of recruiting year classes. For age/size composition,


titi
NOAA Technical Memorandum NMFS-F/NEC-52 :

This TM series is used for documentation and timely communication of
preliminary results, interim reports, or special purpose Information, and
has not received complete formal review, editorlal control, or detailed
editing,

‘An Evaluation
‘of the
Bottom Trawl Survey Program
of the
Northeast Fisheries Center

Survey Working Group, Northeast Fisheries Center

U.S. DEPARTMENT OF COMMERCE
C. William Verity, Secretary

National Oceanic and Atmospheric Administration
J. Curtis Mack Il, Assistant Secretary for Oceans and Atmosphere
National Marine Fisheries Service
William E. Evans, Assistant Administrator for Fisheries
Northeast Fisheries Center
Woods Hole, Massachusetts

March 1988

‘Springfeld, Virginia 22464

mepnopucep sy: |

US. Department of Commerce
National Technical Information Service


78

Figure 27. Stratified mean catch per tow (Delta distribution estimators,
solid lines) for Georges Bank yellowtail taken in NEFC spring
and autumn surveys, 1968-1986, smoothed by time-series modeling.



Table 1. (contd)

Dates Vessel Yankee Area

WINTER
1964 16 Jan-15 Feb ALBATROSS IV 36 NS=HC
1965 1 Feb- 8 Apr ALBATROSS IV 36 NS-HC
1966 18 Jan-23 Feb ALBATROSS IV 36 NS-HC
1981 6 Jan-28 Jan DELAWARE II 36 CC-CH
1982 18 Jan-12 Feb DELAWARE II 36 GM-DB = GM-DB
1983. 14 Feb- 9 Mar DELAWARE II 36 GM-HC = GM-DB
1984 13 Feb-24 Feb ALBATROSS IV 36 GM-HC = GM-DB
& DELAWARE II
1985 13 Feb-22 Feb ALBATROSS IV 36 BI-NT

1 = Geographic features referred to abbreviated as follows:

BI = Block Island
CA = Cape Canaveral
CB = Chesapeake Bay
CC = Cape Cod

CF = Cape Fear

CH = Cape Hatteras
CL = Cape Lookout
CN = Charleston, SC
DB = Delaware Bay
GM = Gulf of Maine
HC = Hudson Canyon
JF = Jacksonville, FL
NS = Nova Scotia

NT = Nantucket Shoals



reference to those that can be controlled, and to evaluate survey design and
analytical methods. Subsequent efforts under this program have included
documentation of survey gear, design, and procedures; evaluation of the
impacts of changes in survey gear and procedures on catchability; re-
assessments of sampling priorities; and development of at-sea data entry and
auditing procedures. Particular interest, however, has been focused on
sources of variability (i.e. spatial, temporal, and measurement error with
particular reference to between-survey components), survey design, and methods
of analysis (transformation, modelling using time-series approaches, etc.).
The present study, initiated in March of 1986, represents an extension of this
work and focuses directly on survey precision and efficiency.

Objectives of the present study were (1) to provide an overview of the
NEFC survey data base and time series, documenting survey procedures and uses
of survey data, and (2) to evaluate the precision and efficiency of NEFC
bottom trawl surveys with particular reference to levels of precision
attained, methods for improving survey precision and reliability, and
implications of changes in sampling intensity. Analyses have been undertaken
cooperatively by a working group consisting of staff members from the
Population Dynamics Branch and the Population Biology Branch of CUD and the
Fisheries Ecology Division (FED), hereafter referred to as the Survey Working
Group (SWG), or "Group." This technical memorandum represents an interim

report on this work.


INTRODUCTION

Since 1963, the Northeast Fisheries Center (NEFC) has conducted an
intensive multispecies bottom-trawl survey program off the northeast coast of
the USA. An autumn survey was initiated in 1963; a spring survey was
initiated in 1968, and summer and winter surveys have also been conducted
intermittently. These surveys are designed to monitor trends in abundance and
distribution, to determine population age/size composition, and to evaluate
the biology and ecology of a broad suite of finfish and invertebrate
species. Over the years these surveys have become important multipurpose
research tools, providing both an essential component of the assessment data
base and opportunities to collect information used in many NEFC programs.

While these surveys have provided unbiased and generally comparable
results over time, procedures used are not. above question and in recent years
increasing attention has been focused on the precision and efficiency of these
surveys and potential methods for improvement. Questions relating to
precision and accuracy for these surveys have been considered in papers by
Grosslein (1971), Pennington and Grosslein (MS 1978), Pennington and Brown
(1981), Pennington (1983), and Pennington and Berrien (1984). Papers by
Collie and Sissenwine (1983) and Pennington (1985, MS 1985) examine the
problem of between-survey variability. These studies have been extremely
useful in developing the tools necessary for a comprehensive analysis of
survey precision and efficiency.

In 1983, the Resource Assessment Division (now Conservation and
Utilization Division, CUD) began a series of research projects designed to
define and document procedural and gear changes and their impacts on the

assessment data base, to examine sources of variability with particular


51

Table 12. Species/group designations used in analyses of survey
precision.

Species included in the DEMERSAL category:

Atlantic Cod
Haddock
Redfish

Silver Hake
Red Hake

White Hake

Scup
Black Sea Bass
Anerican Lobster
Northern Shrimp
Shortfin Squid
Longfin Squid

Species included in the PELAGICS category:

Pollock
Atlantic Herring
Atlantic Mackerel
Bluefish
Butter fish

Species included in the FLOUNDERS category:

Yellowtail Flounder
Winter Flounder
Summer Flounder



34

REFERENCES

Azarovitz, T. R. 1981. A brief historical review of the Woods Hole
Laboratory trawl survey time series, p. 62-67. In: Doubleday, W. G. and
D. Rivard, eds. Bottom trawl surveys. Can. Spec. Publ. Fish. Aquat.
Sci. 58, 273 p.

Cochran, W. G. 1977. Sampling techniques. John Wiley and Sons, Inc., New
York, 422 p.

Collie, J. S., and M. P. Sissenwine. 1983. Estimating population size from
relative abundance data measured with error, Can. J. Fish. Aquat. Sci.
40:1871-1879.

Grosslein, M.D. MS 1968. Results of the joint USA-USSR groundfish
studies. Part II. Groundfish survey from Cape Hatteras to Cape Cod.
Int. Comm. :Northw. Atlant. Fish., Res. Doc. 68/87, 28 p.

«+ 1969. Groundfish survey program of BCF Woods Hole. Comm.
Fish. Rev. 31 (8-9): 22-30.

- 1971. Some observations on accuracy of abundance indices
derived from research vessel surveys. Int. Comm. Northw. Atlant. Fish.,
Redbook 1971 (III) :249-266.

Pennington, M. 1983. Efficient estimators of abundance for fish and plankton
surveys. Biometrics 39:281-286.

- 1985. Estimating the relative abundance of fish from a series
of trawl surveys. Biometrics 41:197-202.

- MS 1985. Some statistical techniques for estimating
abundance indices from trawl surveys. Northw. Atl. Fish. Org., SCR Doc.
85/81, 12 p.

» and P, Berrien. 1984. Measuring the precision of estimates of
total egg production based on plankton surveys. J. Plank. Res. 6: 869-
879.

» and B. —. Brown, 1981. Abundance estimators based on
stratified random trawl] surveys, p. 149-153. In: Doubleday, W. G. and
D. Rivard, eds. Bottom-traw] surveys. Can. Spec. Publ. Fish. Aquat.
Sci. 58, 273 p.

> and M. D. Grosslein. MS 1978. Accuracy of abundance indices
based on stratified random trawl] surveys. ICES C.M. 1978 D:13, 33 p.

Shapiro, S. S., and M. B. Wilk. 1965. An analysis of variance test for
normality (complete samples). Biometrika 52:591-611.



75

YELLOWTAIL FLOUNDER
Georges Bank
6 10
sd
re
e4
RS) be &
a 3
es
& 24
b2
i”
0 T °o
62 66 70 Le 78 a2 86
Year
YELLOWTAIL FLOUNDER
Southern New England
6 20
34
bis
D4
g 3
§ 34 Ln =
3 2
8 2 ~
ts
1
o — — 0
62 66 70 i 78 82 86

Comparisons between fitted (smoothed) survey indices and
fishery dependent population estimates for yellowtail

flounder.

Figure 24,


22

group is provided in Table 12. Precision was highest (lowest CV's) for the
demersal complex (Figure 17). This result is not unexpected since a bottom-
tending gear is used. Precision was higher in autumn than in spring for this
group. Pelagic species were characterized by some very high CV's although
again, the CV's were somewhat lower in autumn (Figure 18). Interestingly, the
flounders also had generally high CV's although in this case, the autumn
surveys were characterized by higher CV's than the spring surveys (Figure 19).

The Group evaluated the relative importance of these results in terms of
confidence intervals about the mean. When calculated and summarized for these
species stocks as frequency distributions, the modal 80% confidence interval
for demersal species was 40% of the mean. For pelagic species and flounders,
however, the modal value was considerably higher. Modal values for higher
confidence intervals, e.g. 95% would, of course, be shifted to the right as
well, While the levels of precision necessary for management applications
must, of course, be subject to final determination by the managers, it is
suggested that 80% confidence limits - that is, accepting a probability level
of 80% that the interval includes the true population mean - is probably
adequate for assessment and management.

The Group concluded that levels of precision achievable through use of
the Delta distribution estimators were reasonable. Estimates for pelagic
species and flounders were less precise; this result was not unexpected for
pelagic species as they tend to be distributed in schools rendering survey
catch per tow extremely variable. Fortunately, we have reasonable information
on pelagic species of primary importance from the commercial fishery and are

not dependent on the survey alone.



Table 2. Specifications for the NOAA research vessels

39

ALBATROSS IV and DELAWARE II.

Length

Displacement.

Shaft Horsepower
Number of Main Engines
Propeller

Rudder

Main Winch, Line Pull
Main Winch, Line Rate
Trawl Warp Diameter

Towing Gear

57.0 m

987.9 m tons
1,130

2

Variable pitch
Kort Nozzle
7,257 kg

65.5 m/min
22.2 mm

Hydraulic Gantry

47.2 m
687.6 m tons
1,230

1

Fixed Pitch
Standard
9,072 kg
36.3 m/min
25.4 mm

Fixed Gallows


21

Comparisons of the Delta distribution estimators with corresponding
stratified mean catch-per-tow values (linear scale) for Atlantic cod, haddock,
yellowtail flounder, and silver hake are provided in Tables 8-11 and Figures
14 and 15. The Delta distribution estimators of the mean are generally in
close agreement with the linear values; more pronounced differences are
evident for the standard errors although no consistent trends are evident.
The Delta distribution estimator for the standard error is not necessarily
less than the traditional estimator (S); this result is due to the
inefficiency of S which may result in artificially low estimates of sample
variability. Pennington (1983) has shown that Delta distribution estimators
for large sample sizes are more efficient than the corresponding traditional
estimators, if the nonzero observations are log-normally distributed.

The Group examined precision of the Delta distribution estimators for the
1984 and 1985 spring and autumn surveys. These two years were chosen because
they are representative of current conditions with respect to both sampling
intensity and resource status. Relative precision for the species/stocks
listed in Table 5 was examined by computing and plotting distributions of the
coefficient of variation of the mean (standard error/mean * 100) for each
survey. Frequency distributions were constructed using 1984 and 1985
estimates for each species/stock as an individual observation (no differential
weighting was applied).

The modal values for the coefficient of variation (CV) for both spring
and autumn surveys for all species ranged from 30-40% (Figure 16). Autumn
survey precision tended to be higher, i.e. the observed proportion of
coefficients of variation exceeding 50% was higher in spring. The group also
computed CV's for demersal species only, pelagic species only, and flounder

species only (Figures 17, 18, and 19). The list of species included in each


‘SPECIAL BIOLOGICAL OBSERVATIONS ON
COLLECTIONS SAMPLES: MAMMALS, BIRDS
ANO DATA AND TURTLES
“|
AGEING SAMPLES | FOOD HABITS PATHOLOGY BLANKTON.
SCALES, OTOLITHS DATA SAMPLES SAMPLING |
PROCESSING PROCESSING PLANKTON
AND AN
AGEING ARCHIVING

DATA ENTRY. DATA ENTRY DATA ENTRY. DATA, SAMPLES,
‘AND SUM- ‘AND SUM- ‘AND SUM- TRANSMITTED
MARIZATION MARIZATION MARIZATION TO NEFC

‘AUDITING ‘AUDITING DATA
‘AND ARCHIVING

‘AND NI
VERIFICATION VERIFICATION

MASTER MASTER MASTER MASTER
DATA BASE DATA BASE DATA BASE DATA BASE

Figure 6. Flow diagram indicating disposition of biological samples and
observations collected or recorded during NEFC bottom trawl]
surveys.


viiié

2) Correspondence between the smoothed indices and alternative population
measures was generally very good. Discrepancies observed are thought to
have resulted primarily from biases in reporting and/or analysis of
commercial data.

3) Attempts to achieve maximum benefit from the surveys by combining
spring and autumn data and fitting time series models to the combined data
sets were less successful; no consistent differences in residual mean
square error were detected between the single-season and combined season
models. This result may relate to seasonal catchability differences.

V. EFFECTS OF CHANGES IN SAMPLING INTENSITY OR AREAS SAMPLED

Evaluations of relationships between vessel time, sampling intensity, and
sampling precision provided the following:

1) The impact of reductions in vessel time on survey coverage can be quite
significant, e.g., a 30% reduction in sea days would necessitate a 45%
reduction in number of stations that could be occupied.

2) For modest losses in vessel time (<20% of current levels) reductions in
precision would be minor, but precision drops rapidly if vessel time is
further reduced. Reductions in the order of 35% increased the standard
deviation over 50% for most species-stocks examined.

3) No consistent differences in precision were detectable between species-
stocks or species groups with changes in sampling intensity.

The following modifications to existing survey coverage could be made with
minimal losses in information:

a) Survey coverage south of Cape Hatteras
could be eliminated.

b) Sampling intensity in offshore strata (>110 m)
from Georges Bank to Cape Hatteras could be
reduced by perhaps 50%.

c) Sampling can be reduced or eliminated in some
inshore areas and in certain areas within the
Canadian Economic Zone.


12

is assigned to each species, and entry of station data (position, depth, time,
etc.) is completed.

Upon returning to port, cruise logs are subjected to a second review for
omissions and miscalculations, after which a station plot is created. Data
are then entered into computer format and a preliminary “Fishermen's Report"
is produced providing catch data (number and weight) for 24 commercially
important species and environmental information for each location sampled.
Catch data are also plotted by species. Similar information, together with
length frequency data, is also generated by species for priority assessment.
requirements. This information is produced within a week of the final cruise
leg (Figure 7).

Auditing of the preliminary data files is the next step (Figure 7). In
the station data audit, a total of 56 parameters are compared to a
computerized master data file to check for gross errors and error diagnostics
are reviewed and data corrected. Temperature data recorded from XBT traces or
shipboard sensors (SEAS System) are checked, verified, and merged with the
audited station data when available. (Salinity data can be handled in similar
fashion). A “preaudit" then compares the cruise, station and strata-tow
numbers on both the station and biological records to identify inconsistencies
in these four fields. Corrections are made, and the catch data audit is then
submitted (Figure 7) which checks length data by species and employs length-
weight equations to check observed and calculated weights. Subsampling,
coding and length recording errors are most often discovered through this
audit. Once audits are complete, the data are appended to the master data

files.



76

SILVER HAKE
Gulf of Maine — Northern Georges Bank

20 10
rs
154
>
8 104 =
© 3
liz
8
s4
2
O 0
62 66 70 ma 73 82 86
Year
SILVER HAKE
Southern Georges Bank — Middle Atlantic
S7— 4
3
|
c
=
&
by
\ crue
° oO
62 66 70 7 78 82 86

Year

Figure 25. Comparisons between fitted (smoothed) indices and fishery
: dependent population estimates for silver hake.



49

Table 10. Stratified mean weight per tow for yellowtail
flounder taken in NEFC spring and autumn bottom
trawl surveys from the Southern New England area
compared to corresponding Delta distribution
estimators.

Linear Scale Delta Distribution
Year Mean Std. Error Mean Std. Error

68 18.624 4.654 21.015 6.866
69 13.340 2.836 13.998 3.795
70 11.721 2.204 11.762 2.485
71 10.693 1.948 11.068 2.247
72 10.728 2.977 10.735 3.078
73 14.678 2.497 18.401 5.124
74 5.040 1,105 5.058 1.161
75 1.984 0.423 1.973 0.414
76 2.452 0.559 3.231 1.275
77 1.993 0.613 1.965 0.575
78 5.146 0.833 5.570 1.305
79 2.147 0.495 2.349 0.662
80 5.949 0.683 6.566 1.018
81 6.846 1.680 6.974 1.756
82 6.001 1.940 6.569 2.392
83 4.641 0.851 4.868 1.109
84 1.625 0.392 1.637 0.414
85 0.666 0.130 0.675 0.138
Autumn
63 16.842 4.057 18.638 6.106
64 19.030 3.981 22.786 7.966
65 12.675 2.831 13.061 3.622
66 9.431 1.884 10.663 3.186
67 14.057 2.570 14.899 3.193
68 10.062 2.598 10.804 3.091
69 14.401 §.272 13.520 4,093
70 10.965 3.499 11.524 4.424
71 9.186 3.655 9.594 3.663
72 20.114 8.504 21.569 10.977
73 2.264 0.973 2.415 1.193
74 2.141 0.979 2.087 0.902
75 0.715 0.437 0.715 0.437
76 2.962 1.063 3.047 1.228
77 1.501 0.604 1.422 0.519
78 3.057 0.794 3.491 1.189
79 2.565 0.547 2.614 0.618
80 1.957 0.778 1.789 0.643
81 3.789 1.088 3.763 1.060
82 8.126 3.483 9.469 4.953
83 6.515 2.151 6.869 2.630
84 1.365 0.447 1.300 0.420

85 0.438 0.167 0.439 0.167


73

ATLANTIC COD
Gulf of Maine
6 12
54 10
=4 He
S
xz x
Yr 3
w =
§ LS
ie b2
oO o
62 66 70 mm 78 ‘82 36
Year
ATLANTIC COD
Georges Bank
6 2
s4 f 10
p44 be
g i
5 34 Ls =
< 3
3 ir
S 24 Le
1 L2
0 r oO
a 6 7 7 7 a 36

Yeor

Figure 22. comparisons between fitted (smoothed) survey indices and
fishery dependent population estimates for cod.


79

Figure 28. Stratified mean catch per tow (Delta-distribution estimators,
solid lines) for Southern New England yellowtail taken in
NEFC spring and autumn surveys 1968-1986, smoothed by time-
series modeling.


56

76°

Figure 4. Strata sampled on NEFC inshore and offshore bottom
trawl surveys between Cape Hatteras, North Carolina
and Cape Canaveral, Florida.


29

days, however, precision drops off rapidly (Table 13); a reduction of 18 sea
days (36% of the current total) increased the standard deviation in most cases
by over 50%. On the other hand, the addition of 18 days to the current
schedule would only reduce the standard deviation by 20% or so (Table 13). An
evaluation of the acceptability of such reductions (or the need for increased
sampling levels) from an assessments standpoint is beyond the scope of the
present report.

Areas Sampled

The Group.considered alternative methods of reducing sea time and
possible implications including (1) eliminating all sampling south of Cape
Hatteras, (2) reducing or eliminating sampling in deep water strata (>110 m
from Georges Bank south), and (3) eliminating sampling in other areas of lower
priority or for which alternative coverage exists, e.g., inshore strata
surveyed by the State of Massachusetts.

The Group examined data collected south of Cape Hatteras and found that
although many species have been taken, species of commercial or recreational
significance have generally been poorly represented. In addition, there has
been little use of these data for stock assessments and related work. These
data will doubtless be useful for other applications, e.g., biological or
distribution studies at some future date but for such uses the existing data
base is probably sufficient. The Group accordingly concluded that survey work
‘from Cape Hatteras to Cape Fear should be discontinued. This would provide a
savings of 5-6 days of vessel time as indicated above.

Deep water strata (>110 m) from Georges Bank south have been difficult to
sample due to the large amounts of fixed gear that must be avoided, difficulty
in following depth contours (resulting in numerous water hauls when the net

fails to touch bottom) and encounters with "ghost" gear which may affect trawl


69

PELAGICS

35

304 Spring

254
20+
154

10-4

54 Y

304 Autumn

254
204

54 yy

10-

Percent

54

304 Spring and Autumn

25-4

20-4

is

104

54

r ONLY
0 10 20 $0 40 50 60 70 80 90 100
Interval

Figure 18. Frequency distribution of coefficients of variation by
season for pelagic species.


30

performance. Accordingly, the effort which must be expended in such areas is
usually considerably out of proportion to the value of such tows from an
assessment standpoint, particularly when these strata are being oversampled in
proportion to their area.

To evaluate the implications of eliminating these strata altogether, the
Group computed survey indices with and without these strata for species of
known importance in these areas. Little or no impact was observed in autumn
(Figures 31-33); a more pronounced effect was evident in spring although for
the species in question autumn survey data have been used preferentially (or
are at least adequate) for assessment purposes. The Group concluded that
sampling in these strata should be reduced, either by (1) reducing the number
of samples in these strata to one per stratum and assigning variances based on
historical relationships between the variance and the mean, or (2) by
combining adjacent deepwater strata and sampling each enlarged stratum in
proportion to total area. The latter would appear preferable given the fact
that relationships between the variance and mean would be subject to change
over time.

Inshore stations are typically close together and tows in such areas are
often characterized by high catches of juvenile fish. This often necessi-
tates "laying to" while:the catch is processed. The Group considered possible
alternatives for reducing sample coverage in these areas including (1)
combining inshore strata with each other (or possibly with adjacent offshore
strata) to permit reductions in sample intensity as suggested for offshore
strata above, or (2) eliminating coverage in areas now surveyed by other
agencies, e.g., Massachusetts Division of Marine Fisheries survey. The latter
course of action would require analyses to verify the comparability of the two

data sets as a source of assessment-related information.



45

Table 6. Results of tests to evaluate the mean-variance
relationships for 1985 autumn bottom trawl survey
catches using Bartlett's three group method for Model
II regression.

ween nnn n nnn n ene e eee ee Sample
Species Slope Upper Lower Distribution!
Atlantic Cod 1.933 2.215 1.649 22
Haddock 1.844 2.030 1.647 19
Pollock 1.716 2.034 1.375 13
Redfish 1.898 2.207 1.583 12
Silver Hake 1.797 2.032 1.567 49
Red Hake 1.741 1.919 1.546 39
White Hake 1.462 1.743 1.114 23
Yellowtail Fidr 1.739 1.966 1.540 18
Summer Flounder 1.712 2.881 1.233 8
Winter Flounder 1.521 1.782 1.203 18
Atlantic Herring 1.773 2.037 0.005 9
Atlantic Mackerel 2.000 2.003 1.998 6
Butterfish 1.842 1.997 1.696 41
Scup 1.763 1.921 1.568 15
Bluefish 1.800 2.205 1.258 13
Black Sea Bass 1.737 2.356 -0.121 7
American Lobster 1.633 2.599 0.654 34
Shortfin Squid 1.620 1.837 1.411 39
Longfin Squid 1.831 2.038 1.626 42

1 Number of strata containing the species indicated.


18

PRECISION OF SURVEY RESULTS

The SWG examined relative levels of sampling precision that are currently
attained with existing survey design and sampling intensity. Studies of
survey precision have been previously conducted for several species but this
is the first comprehensive analysis. The Group chose an assemblage of 41
species-stocks, representing 20 species, for detailed analyses (Table 5).

The Group first examined the relationship between the stratum mean and
variance for each species over all strata sets used to evaluate general
distribution patterns, Taylor (1961) derived the following empirical
relationship between the mean and variance of a sample:

v= ax?
where X and V are the mean and variance, respectively, and a and b are
coefficients. On a logarithmic scale, the relationship is linear. The slope
(b) is a measure of the amount of aggregation; for b > 1.0 a contagious
distribution is indicated. The appropriate transformation (Taylor 1961) is
y = x(l-b/2)
Note that if b = 2.0, the log, transform is indicated.

The Group tested the null hypothesis that the slope of the log, mean -
log, variance relationship was 2.0, using data for 19 of the above species
collected during the 1985 autumn bottom-trawl survey (Table 6). A functional
regression (Bartlett's three group method; see Sokal and Rohlf 1981) was used,
since both the mean and variance are measured with error. Slopes were
significantly greater than 1.0 for 16 of the 19 species examined (Table 6),
indicating aggregated distribution patterns. For three species (Atlantic
herring, American lobster, and black sea bass), the confidence interval of the
slope included 1.0; for herring and sea bass this result can be attributed to

low sample sizes. In the majority of cases, one would fail to reject the null



48

Table 9. Stratified mean weight per tow for haddock
taken in NEFC spring and autumn bottom trawl
surveys from Georges Bank compared to
corresponding Delta distribution estimators.

Linear Scale Delta Distribution
Year Mean Std. Error Mean Std. Error
Spring
68 13.611 3.047 13.483 3.046
69 11.212 3.289 11.534 3.807
70 11.343 7.77 8.967 5.61
71 3.308 0.782 3.290 0.772
72 4.887 0.954 5.234 1.265
73 10.182 3.471 9.472 2.656
74 11.727 4.033 12.450 4.43
75 5.438 2.334 6.047 3.198
76 10.408 3.388 15.483 8.814
77 17.599 6.313 18.538 9.725
78 20.710 5.855 21.316 6.317
79 13.088 2.495 16.458 5.157
80 35.712 12.337 35.442 15.631
81 31.945 7.224 33.423 8.865
82 11.015 2.236 11.706 2.682
83 8.750 2.274 10.437 3.657
84 4,931 1.27 6.290 2.437
85 11.143 3.569 12.902 5.35
Autumn
63 52.840 9.024 60.680 13.72
64 64.069 11.689 66.805 13.611
65 48.197 7.41 53.768 10.621
66 19.777 3.773 20.415 4.165
67 16.873 3.701 17.759 4.283
68 10.203 2.987 10.137 2.876
69 5.589 1.523 5.555 1.333
70 8.946 3.337 8.961 3.342
71 3.706 1.095 3.755 1.131
72 5.614 1.108 5.583 1.154
73 6.481 1.767 7.099 2.489
74 2.647 0.711 2.842 0.939
75 10.004 §.21 9.243 4.268
76 23.683 10.218 37.841 25.771
77 23.135 7.782 22.937 7.713
78 15.181 2.891 19.269 6.744
79 26.873 12.177 23.076 8.622
80 18.474 3.984 20.729 4.497
81 11.772 2.535 11.899 2.788
82 4.838 1.063 4.865 1.345-
83 3.808 0.858 4.034 1.108
84 2.965 0.879 2.914 0.874

85 3.684 0.656 3.862 0.799



T SHIP OPERATIONS
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it 3 H — a GEAR TYPE tO
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a te +t ot +t + H (SITART {€)NO co
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= * 7 + — a LaT($) LONG(S) Gyo
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x cr Te = “at + ce ve) § o 8° Oe)
i a wl 4 rT + = (so aot W: TRE)
or = =r — 7 7 }Oorner
: z ee 3 3 fiewaien PITCH [aun}
z 7 7 — i 7 OFFICER ON waten ||HOG cRSE RPEMET CUCU
2 | + iE + + af GEAR PROBLEM: eerteicakten 2 ath Coc)
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fronsuns| ore) wee Gx] toes” [oe |e eee
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— 7
i +
+ z
Tro Ty | vonme # iters | Sampung, +
(tana eb | ens an :

Figure 5. Cruise log form used for recording data during NEFC bottom

trawl surveys.




37
Table 1. (contd)

Dates Vessel Yankee Areal
Traw] No. Offshore Inshore
SUMMER
1963 18 Jul-19 Aug ALBATROSS IV 36 NS-HC
1964 27 Jul-22 Aug ALBATROSS IV 36 NS-HC
1965 7 Jul-10 Aug ALBATROSS IV 36 NS-HC
1969 14 Jul-28 Aug ALBATROSS IV 36 NS-CH
1977, 27 Jul-31 Aug ALBATROSS IV 36 GM-CH GM-CH
& DELAWARE II
1978 25 Jul-20 Aug ALBATROSS IV 36 GM-CF GM-CF
& DELAWARE II
1979 25 Jul- 1 Sep ALBATROSS IV 36 GM- CF GM-CF
& DELAWARE II
1980 11 Jul-22 Aug ALBATROSS IV 36 GM-CF GM-CF
& DELAWARE II
1981 23 Jun-24 Jul DELAWARE II 36 GM-CB GM-CB
AUTUMN
1963 13 Nov-16 Dec ALBATROSS IV 36 NS-HC
1964 22 Oct-25 Nov ALBATROSS IV 36 NS-HC
1965 6 Oct- 9 Nov ALBATROSS IV 36 NS-HC
1966 12 Oct-13 Nov ALBATROSS IV 36 NS-HC
1967. 17 Oct- 9 Dec ALBATROSS IV 36 NS-CH
1968 10 Oct-26 Nov ALBATROSS IV 36 NS-CH
1969 8 Oct-23 Nov ALBATROSS IV 36 NS-CH
1970 3 Sep-20 Nov ALBATROSS IV 36 NS-CH
& DELAWARE II
1971 30 Sep-19 Nov ALBATROSS IV 36 NS-CH
1972 27 Sep-20 Nov ALBATROSS IV 36 NS-CH CH-CA
& DELAWARE IT
1972 31 Oct- 5 Dec ATLANTIC TWIN 3/4 BI-CN
1973. 26 Sep-20 Nov ALBATROSS IV 36 NS-CH
1973, 1 Oct- 7 Nov ATLANTIC TWIN 3/4 BI-CF
1974 20 Sep-14 Nov ALBATROSS IV 36 NS-CH BI-DB
1975 15 Oct-18 Nov ALBATROSS IV 36 NS-CH CC-CH
1976 28 Sep-23 Nov ALBATROSS IV 36 NS-CH BI-CH
1977 26 Sep-15 Dec DELAWARE II 36 NS-CH BI-CH
1978 5 Sep-22 Nov DELAWARE II 36 NS-CH GM-CF
1979 12 Sep-19 Nov ALBATROSS IV 36 NS-CF CC-CF
& DELAWARE II
1980 17 Sep-15 Nov DELAWARE II 36 NS-CF GM-CF
1981 16 Sep- 7 Nov ALBATROSS IV 36 NS-CF GM-CF
1982 13 Sep-12 Nov ALBATROSS IV 36 NS-CF GM-CF
1983 12 Sep-10 Nov ALBATROSS IV 36 NS-CF GM-CF
1984 10 Sep- 9 Nov ALBATROSS IV 36 NS-CH GM-CH
1985 9 Sep-16 Nov ALBATROSS IV 36 NS-CH GM-CH
& DELAWARE II
1986 13 Sep- 6 Nov ALBATROSS IV 36 NS-CL GM-CL

1987 10 Sep- 6 Nov ALBATROSS IV 36 GM-CL GM-CL


35

Sissenwine, M. P., and E. W. Bowman. 1978. An analysis of some factors
affecting the catchability of fish by bottom trawls. Int. Comm. Northw.
Atl. Fish., Res. Bull. 13:81-87.

Sokal, R. R., and F. J. Rohlf. 1981. Biometry. W. H. Freeman and Co., New
York, 859 p.

Taylor, L. Re 1961. Aggregation, variance, and the mean. Nature, 189: 732-
735.


SPECIES

ATLANTIC COD ZA

HADDOCK
YELLOWTAIL FLOUNDER | e
ATLANTIC HERRING
ATLANTIC MACKEREL
SILVER HAKE

RED HAKE e
REDFISH
POLLOCK
LONGFIN SQUID
SHORTFIN SQUID
BUTTERFISH
AMERICAN LOBSTER
NORTHERN SHRIMP.
SUMMER FLOUNDER
WINTER FLOUNDER
SCUP

BLUEFISH

SPINY DOGFISH
WEAKFISH

‘SKATES

WHITE HAKE

BLACK SEA BASS
WITCH FLOUNDER
AMERICAN PLAICE __|
OCEAN POUT
ATLANTIC WOLFFISH
CUusK

Figure 8. Relative utility of NEFC spring and autumn survey data by category
(type of assessment-related information) for 28 finfish and
invertebrate species. Spring = black; autumn = cross-hatched;

solid circles indicate whether the surveys provide the sole source
of information.


A summer survey was initiated in 1963 and conducted from 1963-1965 and
again in 1969 (Table 1). Amore specialized summer survey time series was
initiated. in 1977 to provide additional data with special reference to species
of recreational interest (Azarovitz 1981). Coverage of areas shallower than
110 m (60 fm) was stressed on this survey since many economically important
species tend to concentrate in these areas during summer. This survey was
discontinued in 1981 after further analyses showed that many important species
actually tended to concentrate in estuaries and were, therefore, unavailable
for capture. A winter survey time series was also conducted during 1964-1966;
and a specialized winter survey time series was initiated in 1981
concentrating primarily on areas shallower than 27 m (15 fm). The primary
intent of this survey was to collect biological data on Atlantic herring.

This survey was discontinued in 1985.

The NEFC survey data base has also been augmented by cooperative cruises
with foreign nationals. Intensive exploitation of fishery resources in USA
continental shelf waters during the mid-to-late 1960's and increasingly
restrictive management under the International Commission for the Northwest
Atlantic Fisheries or ICNAF led to cooperative surveys with the Soviet Union,
the Federal Republic of Germany, France, the German Democratic Republic,
Japan, Poland, Spain, and Canada. Since passage of the Magnuson Fisheries
Conservation and Management Act (MFCMA) such activity has been much reduced
and in recent years only Poland has participated in cooperative surveys. Data
from these surveys have been highly valuable in specific instances, e.g.,
initial gear trials employing the USA "36 Yankee" trawl (described below) and
Soviet commercial gear were instrumental in demonstrating the utility of the
NEFC survey data base for monitoring trends in abundance and species

composition (Grosslein MS 1968). Joint USA - Soviet studies were also


47

Table 8. Stratified mean weight per tow for Atlantic cod
~  taken.in NEFC spring and autumn bottom trawl
surveys from the Gulf of Maine compared to
corresponding Delta distribution estimators.

Linear Scale Delta Distribution
Year Mean Std. Error Mean Std. Error
Spring
68 11.051 1.886 11.049 1.894
69 8.145 2.956 7.782 2.517
70 6.838 1.944 6.735 1.758
71 4.319 1.040 4.486 1.144
72 4.956 1.255 5.341 1.544
73 11.609 5.234 10.322 3.512
74 4.579 0.993 5.247 1.560
75 3.728 1.063 3.709 1.033
76 4.664 0.830 4.868 0.972
77 5.272 1.207 5.393 1.491
78 4.751 1.092 4.712 1.059
79 5.860 1.166 5.933 1.129
80 5.702 1.159 5.678 1.163
81 9.927 2.252 10.533 2.693
82 7.938 1.964 7.574 1.720
83 6.766 1.665 6.806 1.632
84 3.792 1.423 3.360 0.995
85 7.645 1.799 8.354 2.460
Autumn
63 11.080 4.562 10.536 3.871
64 14.073 7.541 14.178 7.579
65 7.411 2.281 7.385 2.225
66 7.973 2.052 10.290 4.889
67 5.695 1.397 5.866 1.559
68 11.998 2.603 12.312 3.039
69 9.486 2.332 9.280 2.175
70 10.149 2.690 10.226 2.729
71 10.202 3.408 11.485 4.471
72 8.017 1.841 8.654 2.561
73 5.406 1.629 5.417 1.629
74 5.530 1.221 5.515 1,183
75 5.320 0.858 5.463 1.015
76 4.161 1.015 4.114 0.987
77 9.397 1.238 10.313 1.781
78 11.884 1.929 13.080 2.544
79 10.819 1.509 11.582 1.987
80 13.085 2.187 13.595 2.746
81 4.961 1.222 5.551 1.936
82 9.827 7.313 7.051 4.065
83 5.195 1.103 5.186 1.102
84 5.392 1.906 5.445 1.935

85 8.264 3.332 7.638 2.517


>183

56-110

DEPTH ZONES (M)

EES 27-55

INNS

C1 w-ws

KA

Figure 1. Strata sampled on NEFC offshore bottom trawl surveys.


doors weighing 545 kg (1,200 1b). In the spring of that year these were
replaced by Portuguese-type polyvalent doors of approximately the same weight
since it was becoming increasingly difficult to obtain BMV oval doors built to
standard specifications. Studies are presently underway to determine effects
of this door change on fishing power.
Sampling Design

The sampling design used in this program is stratified-random, with
stratification being based on depth, latitude, and historic fishing

patterns. Seven primary depth zones have been recognized, as follows:

Inshore Strata Offshore Strata
Meters Fathoms Meters Fathoms
<9 <5 27-55 15-30
9-17 5-9 56-110 31-60
18-26 10-14 111-183 61-100
>183 >100

This system has been used for all offshore strata and all inshore strata
from Cape Hatteras to eastern Long Island. Further north and east, different
depth zones have been used on occasion due to irregularities in bottom
topography (Table 4).

Given. that the original stratification scheme is valid, this design can
be expected to provide increased precision relative to simple random sampling
and has other desirable features, e.g., it insures a fairly uniform
distribution of stations throughout the survey area yet provides flexibility
for increased sampling intensity in critical areas if needed. In the present
program, sampling stations are allocated to strata roughly in proportion to
area (which facilitates post-stratification if necessary). Some inshore and

deeper offshore strata have been sampled more intensively, because they have


cr

| Preceding Page Blank |

ir

TABLE OF CONTENTS
EXECUTIVE SUMMARY. .ceecccccccccccccccececcccescsererscecssserssrsscee el
ACKNOWLEDGEMENTS... eseeecceccccerceee ere rere rr reer ere rere errr!
INTRODUCTION. cceecevccscccccccccrcccccescescercssssesscsceesersesscerel

OVERVIEW OF THE NEFC BOTTOM TRAWL SURVEY PROGRAM... .ceccccccesceeeveeed
Historical Aspects....sc.sseeeee ereeeeeeeree ee eccccecccccceeces
Vessels and Gear....
Sampling Design...
Station Selection.
Data Collection at Si
Data ProcesSing.sccocccccccvccccccccccccrecesccssccssesvessoessell

SURVEY DATA APPLICATIONS. ..cccceccccccccnccccccceccecesccaseessessereld
Stock Assessment Data..ccecsscecseee
Other Datar.cccsccccccccccccvccccccccccccccescscsserscvscessesseels

PRECISION OF SURVEY RESULTS... scceccescccccecccccsccssceserstesscseseld
IMPROVEMENT OF SURVEY PRECISION. sceccccscececscccccsscecsccssscessseld
EFFECTS OF CHANGES IN SAMPLING INTENSITY OR AREAS SAMPLED....sceeceeee 27

Sampling Intensity. .cccccccccccscccccveccssceececes weed]
Areas SaMPledsseeseccecesevcceecceccecvenseesceecs

CONCLUSIONS. wecececccrccccccssccccesersccneercesesensenseesersseseseede
REFERENCES co seescesccscencesccscccccrsccsncsssescsnscrassersssensesee dd

TABLES. cece ccesecescverersesecsnccercsecesscsceesesesesteescescescss 30

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28

basic integrity of the survey region. The disparity becomes proportionally
lower with further reductions in vessel time, e.g., loss of 40 sea days or 80%
of the current total would necessitate a 90% reduction in station coverage.
Obviously such a scenario would have no practical significance from a
statistical standpoint; indeed, losses in the order of 20 days would probably
necessitate significant reductions in areal coverage to maintain acceptable
levels of sampling precision for at least part of the data base.

Since the relationship between vessel time (sea days) and number of
stations that can be covered is reasonably well defined over a practical range
(Figure 29) and since changes in precision can be related to changes in
sampling intensity (stations) it follows that a direct relation can be
developed between number of sea days and sampling precision. Such a
relationship has been developed between number of sea days and percentage
change in standard deviation for 35 species/stocks. Increase or decrease in
standard deviation in relation to sample size was calculated for the 1985
autumn bottom trawl survey in terms of conventional sampling theory (Cochran
1977). Calculations were based on the above relationship between vessel time
and station coverage applied proportionately to each stock. Results are given
in Table 13.

The most striking result in Table 13 is the relative uniformity over all
species/stocks; no consistent differences in precision can be detected as sea
days are decreased or increased (see Figure 30 for combined plots of
percentage change in standard deviation in relation to reductions or- increases
in sea days for demersals, pelagics and flounders). Reductions in precision
associated with modest losses in vessel time (6-9 sea days or 12-18% of the
present total) are generally minor; no case was observed in which the standard

deviation was increased by more than 16%. With greater reductions in sea



70

FLOUNDERS

35
so Spring

254

204

54

104
54

; WALA

30 4 Autumn

254

204

Percent

IS

104
s4

LBWAVA

304 Spring and Autumn

254
204

54

54
ua

oO T T T y + + + y y t

0 10 20 30 40 50 60 70 80 90 100
Interval

Figure 19. Frequency distribution of coefficients variation by
season for flounders.


16

use of these time series is again relatively well balanced (Figures 8 and

9). In the case of recruitment, use of autumn data again reflects the
availability of a longer standard series which facilitates development of
empirical relationships. On the other hand, many important species become
fully available to the survey gear in spring at age 1 thus providing the
earliest reliable indicator of year-class size. In addition, reliable
predictions cannot be made for certain key species, e.g. mackerel and
yellowtail, based on autumn survey results. The "backup" capability is
equally important for other species, e.g. for haddock, only one reliable
index-autumn survey catch per tow at age 1 - would be available prior to
recruitment to the commercial fishery if the spring survey were to be
discontinued. With the spring survey, three index values - spring and autumn
survey catch per tow at age 1 and spring survey catch per tow at age 2 - are
available. Given the inherent variability in survey data it is possible that
we might fail to predict a large incoming haddock year class if only autumn
survey data were available.

Figure 10 summarizes the relative importance of the surveys as a sole
data source by category, expressed as a percentage of the total number of
species in that category for which the information indicated is available.
For relative abundance, the surveys provide the only monitoring capability we
have for 13 of the 28 species included (46%) as well as for others of lesser
importance not included in Figure 8. For growth, survey data can be
supplemented by other sources although again about half our information for
the species considered is supplied by this source alone (and obviously,
surveys provide the only consistent source for age groups not yet recruited to
the commercial fishery). Surveys are also heavily relied on in the case of

the other categories of information. In the case of recruitment, historical





40

Table 3. Specifications for the "36 Yankee" and "41 Yankee" trawls.

#36 Yankee #41 Yankee
Trawl Trawl

Opening Height of Trawl 3.2m 4.6m
Opening Width of Traw1 10.4 m 11.8 m
Overall Length of Trawl 28.4 m 28.6 m
Codend Length 5.7m 5.7m
Foot Rope Length 24.4 m 30.5 m
Head Rope Length 18.3 m 24.4 m
Opening Mesh? 12.7 cm 12.7 cm
Average Body Mesh 12.7 cm 12.7 cm
Codend Mesh 11.4 cm 11.4 cm
Codend Liner 1.3 cm 1.3 cm
Number of Floats 36 53

Float Diameter 20 cm 20 cm
Roller Gear Yes Yes

Length of Bridles 9.1m 18.3 m
Length of Doors? 2.4m 2.5m
Width of Doors? 1.4 m 1.4m
Weight of Doorsé 545 kg 682 kg
Type of Doors? BMV Oval BMV Oval

1a11 mesh measurements given as "stretched" values.

Beginning in 1985, 450 kg polyvalent doors have been used (length,
2.5m; width, 1.4m).


24

IMPROVEMENT OF SURVEY PRECISION

The SWG next considered ways of improving the precision of the
estimators. The first analysis attempted was to smooth random variability in
the estimates by fitting time series models to the survey data (Pennington
1985). This approach is based on the concept that full advantage should be
taken of the survey time series; the rationale being that biomass of multi-age
class stocks would not be expected to change radically from year to year
unless a causative agent could can be identified. (In other words, there is
"memory" in the system). This is less true for short-lived species with high
mortality rates. The time series models considered in these analyses were
built on the supposition that much of the interannual variability in catch per
tow is due to random variation in catchability. The objective was, therefore,
to filter out this random variation to provide better estimates of population
trends.

Results of applying this technique to four species/stocks are given in
Figures 20 and 21. It is evident that the technique is very effective as a
smoothing function and provides insight into reliability of the time series as
a whole, e.g., for Georges Bank haddock the autumn survey index was obviously
inflated upward in 1976 by some anomalously high catch per tow values (Figure
20) as was the index for southern New England yellowtail in autumn of 1972
(Figure 21). Similarly, the population increase observed in the early 1980's
for yellowtail, while real, was apparently not of the magnitude evidenced by
untransformed data (Figure 21). The utility of the procedure in filtering out
random "noise" in the system is also evident.

Comparisons for species/stocks for which alternative measures of
abundance exist appear in Figures 22-25. Correspondence between the smoothed

or adjusted survey index and the alternative measure was very good for Georges



33

of vessel time) and sampling precision revealed that for modest losses in sea
time (<20% of current levels) losses in precision would be relatively minor;
precision drops off rapidly, however, with further losses in sea time. The
Group determined that significant savings could be achieved with minimal loss
in information by 1) eliminating survey coverage from Cape Hatteras to Cape
Fear, 2) reducing sample coverage in offshore deepwater strata (>110 m) from
Georges Bank south, and 3) modifying coverage in the Canadian Economic Zone.
Coverage for inshore strata probably can also be reduced, especially in
coastal waters now being surveyed by the states, with minimal effects.

Plans for future work include the following:

1) Further evaluation of existing survey coverage, particularly for
inshore strata and strata in the Canadian Economic Zone, to permit reductions
wherever possible;

2) Studies to improve precision and accuracy by time series modeling and
by modifications to sampling design and intensity (for example, survey
precision may be increased by sampling more intensively in key strata, using
combinations of inshore and deepwater strata, and the addition of fixed
stations);

3) Further evaluations of relationships between sampling intensity and
sampling precision by species and species groups; and

4) Examination of the nature of catchability in the survey data base,
with specific reference to interannual variation. Stock size estimates
derived from VPAs and/or spawning biomass estimates as derived from egg

surveys could provide a basis for such evaluations.


Stratified Mean Weight/Tow (kg)

Legend
‘mDLpAT yrusre
ote’ wImO
o mn OR TM EY 7% 0 8
Yeor

Stratifled Mean Weight/Tow (kg)

Year

Figure 33. Comparison of stratified mean weight/tow (Delta distribution)
with and without strata of depths greater than 110 m for
longfin squid.

FEU. 5. GOVERNMENT PRINTING OFFICE: 1985--500-986--60171


19

hypothesis that the slope is 2.0. Examples of log, mean - log, variance plots
are provided in Figures 11 and 12.

For bottom trawl survey data it is axiomatic that for any given species a
high proportion of "zero catches" will be encountered over a broad
geographical area, i.e. the species in question will be present in only a
fraction of the tows. This results in a highly skewed distribution of catch
per tow. Development of confidence intervals about mean catch-per-tow values
will be complicated by this asymmetric distribution, and at the same time the
occurrence of zero catches complicates the development of an effective
normalizing transformation. To illustrate this point the observed
distribution of catch per tow in weight for Georges Bank cod taken during the
1985 autumn survey is provided in Figure 13, The observed distribution on a
linear scale is markedly asymmetrical, with a high proportion of zero
catches. If we attempt. to normalize the data using the log, (X+1) transform
the data remain highly skewed; there is still a pronounced peak representing
the zero catches (log, (X+1) = 0). For the original and log, (X+1)
transformed data, the null hypothesis of a normal distribution is rejected
{Kolmogorov-Smirnov test; P<0.01).

An alternative approach involves partitioning the catches into zero and
-nonzero values and taking natural logarithms of the latter set of values
(Pennington 1983). The transformed nonzero catches are more nearly normally
distributed (Figure 13); the hypothesis of a normal distribution is not
rejected (Shapiro-Wilk test; P>0.05). Accordingly, the Group tested the
distribution of the transformed nonzero tows for 26 of the species/stocks
identified earlier (Table 7). To obtain a sufficiently large sample size, the
distribution of catch-per-tow indices for relatively broad geographical

regions was considered, e.g. Strata sets for Georges Bank, southern New




OVERVIEW OF THE NEFC BOTTOM TRAWL SURVEY PROGRAM

Historical Aspects

In the early 1950's, staff members of the Bureau of Commercial Fisheries!
Woods Hole Laboratory in Woods Hole, Massachusetts initiated surveys to gather
information on the distribution and biology of commercially important fish
species aboard the R/V ALBATROSS III. Cruises were conducted on Georges Bank
and the Gulf of Maine to locate concentrations of redfish, Atlantic cod,
haddock, and Atlantic herring in order to determine their abundance and
Spawning capacity. While these cruises provided useful data for a variety of
purposes, procedures and temporal and spatial coverage were variable,
detracting considerably from the value of the data base. The need was obvious
for a repeatable survey employing standardized procedures and equipment which
would not only provide comprehensive data on biology and distribution but.
would also provide a fishery-independent source of data for monitoring and
predictive purposes.

Accordingly, a comprehensive bottom trawl survey program was initiated in
1963 employing standard gear and sampling procedures which has been continued
to the present day. Objectives of this program were to monitor population
fluctuations, to assess the fish production potential off the northeast coast,
to determine environmental factors controlling the distribution and abundance
of fish species, and to provide the basic ecological data required to
understand interrelationships between fish and their environment (Grosslein
1969). The resulting data base has been extremely valuable for fish stock
assessments and has also been widely used for other applications.

The first survey cruises covered the region from the Hudson Canyon to

lNow National Marine Fisheries Service (NMFS).


55

Figure 3. Strata sampled on NEFC inshore bottom trawl surveys
from Cape Cod Bay, Massachusetts to Cape Hatteras,
North Carolina.


71

ATLANTIC COD
Gulf of Maine
Autumn

Stratified Mean Weight/Tow (kg)

HADDOCK
Georges Bank
Autumn

80

Stratified Mean Weight/Tow (kg)
8

a oe 70 7 70 ” *
5 Year

Figure 20. Effect of applying time series models to stratified mean
' catch per tow values (Delta distribution estimators, solid
lines) for Atlantic cod and haddock.


41

Table 4, Strata used during NEFC bottom trawl surveys from western
Nova Scotia to Cape Hatteras, North Carolina including
area, depth zone and average number of stations fished.
Depth No. of Depth No. of
Stratum Area! Zone (m) Stations Stratum Areal Zone (m) Stations

Inshore Strata

1 44 <18 T 46 273 18-26 2
2 62 18-26 2 47 45 <18 1
3 13 <9 1 48 113 <9 0
4 26 9-17 2
5 62 18-26 2 50 15 <9 0
6 26 <9 1 51 117 9-17 0
7 35 9-17 2 52 521 9-17 4
8 150 18-26 2 53 142 <9 0
9 40 <9 1 54 277 9-17 0
10 48 9-17 2 55 495 18-26 4
11 242 18-26 2 56 57 9-26 1
12 44 <9 1 57 34 <9 0
13 88 9-17 2 58 88 9-17 1
14 110 18-26 2 59 93 18-26 1
15 22 <9 1 60 126 27-41 2
16 62 9-17 2 61 133 42-55 2
17 238 18-26 2 62 62 <9 2
18 97 <9 1 63 78 9-17 1
19 216 9-17 2 64 90 18-26 1
20 356 18-26 2 65 75 27-41 1
21 22 <9 1 66 151 42-55 2
22 154 9-17 2 67 5 <9 0
23 167 18-26 2 68 40 9-26 1
24 53 <9 1 69 57 27-55 1
25 172 9-17 2 70 10 <9 0
26 154 18-26 2 71 72 9-26 1
27 35 <9 1 72 129 27-55 2
28 220 9-17 2 73 31 <9 0
29 185 18-26 2 74 68 9-26 1
30 75 <9 1 75 76 27-55 1
31 299 9-17 2 76 20 <18 0
32 106 18-26 2 77 34 18-55 1
33 92 <9 1 78 44 18-55 1
34 167 9-17 2 79 34 <18 0
35 88 18-26 2 80 58 18-55 1
36 119 <9 1 81 38 18-55 1
37 312 9-17 2 82 209 <18 0
38 224 18-26 2 83 80 “18-55 1
39 36 <9 1 84 137 <18 0
40 176 9-17 2 85 106 18-55 1
41 383 18-26 2 86 60 <18 0
42 40 <9 1 87 153 18-55 2
43 172 9-17 2 88 34 <18 0
44 304 18-26 2 89 59 18-55 1
45 170 18-26 2 90 125 56-110 2


43

Table 5. Species/stocks and strata set definitions for
finfish and invertebrate species used to evaluate
precision and efficiency of survey design.

Atlantic Cod
Southern New England - Middle Atlantic Spring 1, 5-6, 9-10
Autumn 1-2, 5-6, 9-10,
65, 69-70, 73-74

Georges Bank 13-25
Gulf of Maine 26-30, 36-40
Haddock
Georges Bank 13-25, 29-30
Gulf of Maine 26-28, 36-40
Pollock
Georges Bank - Gulf of Maine -

Scotian Shelf 13-40
Redfish
Georges Bank - Gulf of Maine 24, 26-30, 36-40

Silver Hake
Southern Georges Bank - Middle Atlantic 1-19, 61-76
Gulf of Maine - Northern Georges Bank 20-30, 36-40

Red_Hake
Southern Georges Bank ~ Middle Atlantic 1-19, 61-76
Gulf of Maine - Northern Georges Bank 20-30, 36-40

White Hake

Georges Bank - Gulf of Maine 21-30, 33-40
Yellowtail Flounder

Middle AtTantic 1-2, 69-70, 73-74
Southern New England 5-6, 9-10
Georges Bank 13-21

Cape Cod 24-26

Gulf of Maine 27-28, 37-40
Summer Flounder

Middle AtTantic 61-76
Southern New England 1-12
Georges Bank 13-25
Winter Flounder

MiddTe AtTantic 61-76
Southern New England 1-12, 25
Georges Bank 13-22

Great South Channel 23

Gulf of Maine 24, 26-30, 36-40


valuable in development of conversion factors for USA gear which have since
been widely used (Sissenwine and Bowman 1978). As a rule, however,
cooperative surveys with foreign nationals have not been as valuable as NEFC
surveys because of the variety of vessels, gear and procedures involved.
Vessels and Gear

In the above time series, vessels and gear have been standardized insofar
as possible to provide consistent results. Offshore and most inshore surveys
have been conducted aboard the National Oceanic and Atmospheric Administration
(NOAA) R/V ALBATROSS IV, a 57 meter (187 foot) stern trawler and R/V DELAWARE
II, a 47 meter (155 foot) stern trawler. Specifications for both of these
vessels are given in Table 2. Several inshore survey cruises during 1972-1975
were conducted aboard R/V ATLANTIC TWIN, a smaller vessel designed for inshore
studies.

Three bottom trawls have been used in this program. A "36 Yankee" trawl
has been used in al] autumn surveys and in spring surveys conducted between
1968 and 1972 and from 1982 to the present (Table 1). From 1973 to 1981,
however, a modified high opening "41 Yankee" trawl was used during spring
surveys in an attempt to increase fishing power for pelagic species. Both
trawls are equipped with roller gear to make them suitable for use on rough
bottom and a 1.25 cm (0.5 inch) stretched mesh liner in the codend and upper
belly for sampling juvenile fish. A "3/4 Yankee" trawl rigged with a chain
sweep and ground cables was used aboard R/V ATLANTIC TWIN during several
inshore cruises between 1972 and 1975. Again, the "3/4 Yankee" was equipped
with a 1.25 cm stretched mesh liner. Specifications for the "36 Yankee" and
the "41 Yankee" trawls are given in Table 3.

The "41 Yankee" trawl was fished with BMV oval doors weighing 682 kg

(1,500 1b). Until 1985, the "36 Yankee" was fished with Tighter BMV: oval