Northeast Fisheries Science Center Reference Document 04-09
Estimation of Striped Bass Discards
in the Multispecies Groundfish Fishery,
during the 2002 Fishing Year
(May 2002 – April 2003)
by Gary R. Shepherd
National Marine Fisheries Serv., Woods Hole Lab., 166 Water St., Woods Hole MA 02543
Print
publication date June 2004;
web version posted June 21, 2004
Citation: Shepherd, G.R. 2004. Estimation of striped bass discards in the multispecies groundfish fishery during
the 2002 fishing year (May 2002 - April 2003). Northeast Fish. Sci. Cent. Ref. Doc. 04-09; 15 p.
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Estimation of discards in commercial fisheries is generally dependent
on a subset of information collected by at-sea observations, which is
then expanded to the total fishery.� The method of expansion depends
on the type of fishery. General methods for expansion include use of
the ratio of discard weight of species A to kept weight of species A
(Dspp a / Kspp a); the ratio of discard weight
of species A to the aggregate kept weight of other species (Dspp /
Kagg); or the ratio of discard weight of species A per unit
of fishing effort.� For striped bass fisheries which are not legal in
federal waters, estimation of discards based on discard to kept ratio
is not appropriate. Estimation of discards based on using a fishing effort
ratio is difficult because the duration of commercial fishing trips varies
among fisheries and ports, and measurement of effective effort is difficult
to quantify consistently. Therefore, the most appropriate estimator for
striped bass discards in the multispecies groundfish fishery is the ratio
of striped bass discards to aggregate kept weight of species targeted
by the fishery.
A simple ratio of cumulative discard to aggregate kept weight on observed
multispecies groundfish trips is not appropriate due to seasonal and
geographic variations in multispecies landings between Maine and North
Carolina. To account for this variability, the ratios were stratified
by month and statistical area. Statistical area information (Figure
1)
for landings was only available from vessel logbook data (VTR data).� All
trips reporting landing of multispecies in the 2002 VTR logbook database
were initially included. Species landed weight was summed within trip,
then across trips by month, 3-digit statistical area and gear type. The
data were limited to otter trawl and sink gillnets since most of the
multispecies groundfish observer trips were made on vessels using these
gear types; these gear types are also the most likely to capture striped
bass.� The VTR results were compared to the reported dealer landed weight
data for species comprising the multispecies group. With the exception
of white hake (which are landed in a variety of configurations, e.g.
headed, or gutted) and halibut (which comprise a very small % of the
total), the VTR data averaged 5% less than weighout data (Table
1). Therefore,
the VTR landings were adjusted upward by 5% to account for all landings.� Monthly
landings that did not have area reported were re-distributed based on
proportion of landings within each area. The 2002 VTR and observer data
sets were subset to May through December to correspond to the fishing
year beginning May 1st.
The observer data consist of haul specific information
from trips targeting multispecies groundfish. The aggregate weight of
multispecies groundfish kept per haul was summed across trip, gear, area
and month. The data were also limited to sink gillnet and otter trawl
gear. In addition, the weight of striped bass discarded was summarized
by month, area and gear type. The data were insufficient to stratify
at any finer geographic level than statistical area.� A ratio of striped
bass discards to aggregate landed weight of the multispecies complex
was then calculated by month, area and gear.� Observer data for the period
January to April 2003 contained no record of striped bass caught or discarded;
therefore the expanded estimate of striped bass discarded by the multispecies
groundfish fishery in these months was zero (0).� VTR reported landings
from cells (month, area, gear) with observer coverage accounted for 89%
of total landings in the otter trawl fishery and 73% from sink gillnets
(Table 2 and Table 3).� A monthly summary of landings and observed striped
bass discards is provided in Table 4.
In cells (gear, month, area groups) with observer coverage
(Table 5 and Table 6), the discard ratio was applied to expanded VTR aggregate
landings data.� In cells with no observer coverage for the multispecies
fleet, a ratio for the general geographic area was applied. Areas were
grouped as follows:� Gulf of Maine (511-515); Rhode Island to the Great
South Channel (521,526-539); western Georges Bank (522,525); eastern
Georges Bank (542-543,551-562) ; Long Island south (611 and higher).� An
overall ratio for an area group was calculated as the sum of observed
striped bass discards in the group/sum of observed multispecies landings
within the same group. The resulting ratio was applied to VTR landings
for each area in the group (Tables 5 and 6). The resulting striped bass
discard estimates (Table 7 and Table 8) were summed across cells by gear type,
with the assumption of 100% discard mortality in both gear types.
The result was a total of 289,808 lbs of striped bass discards
in the multispecies groundfish fishery: 287,019 lbs. from otter trawls
in May-December 2002 (Table 7); 2,789 lbs from sink gillnets in May-December
2002 (Table 8); and no discards during January to April for either gear.� Beginning
in June-July, there appears to be a north to south progression of striped
bass discards in the trawler fleet.� The majority of discards occurred
in statistical area 521, which corresponds to the Great South Channel.� Discards
of striped bas in September and October in Great South Channel accounted
for 84% of the total annual estimated trawl discards of bass in the multispecies
groundfish trawl fishery. The NEFSC research vessel bottom trawl survey
data (Figure 2 and Figure 3) indicate that the Channel has a seasonal concentration
of striped bass (Figure 2), particularly in the autumn when migrating
bass spatially overlap with spawning Atlantic herring.
This analysis was intended to examine striped bass discards
in the multispecies groundfish fishery.� Striped bass encounters within
this fishery are a relatively rare event (Figure 4).�� However, on occasion
seasonal/geographic aggregations of striped bass result in high levels
of discards. Since statistical area was the lowest resolution for geographic
stratification, discards may actually be over-estimated for some areas.
Expansion by all landings within broad areas may encompass groundfishing
locations which are not in the migratory pathway of striped bass and
therefore unlikely to result in striped bass discards. In contrast, discards
may also be under-estimated in areas with limited or no observer coverage,
such as inshore locations where the majority of the striped bass population
occur.� Since the analysis did not include all fisheries potentially
discarding striped bass, the estimate should not be considered a total
estimate of commercial striped bass discards.� Nonetheless, striped bass
discards in the multispecies groundfish fishery appear to be a localized,
seasonal event.
