Northeast Fisheries Science Center Reference Document 07-19
Allocating Observer Sea Days to Bottom Trawl and Gillnet Fisheries
in the Northeast and Mid-Atlantic Regions to Monitor and Estimate Incidental Bycatch of Marine Mammals
by Marjorie C. Rossman
National Marine Fisheries Serv, Woods Hole Lab, 166 Water St, Woods Hole MA 02543-1026
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publication date November 2007;
web version posted November 8, 2007
Citation: Rossman MC. 2007. Allocating Observer Sea Days to Bottom Trawl and Gillnet Fisheries in the Northeast and Mid-Atlantic Regions to Monitor and Estimate Incidental Bycatch of Marine Mammals. U.S. Dep. Commer., Northeast Fish. Sci. Cent. Ref. Doc. 07-19; 17 p.
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Abstract: Estimating marine mammal bycatch mortality with low levels
of uncertainty is important in attaining the conservation goals of the Marine
Mammal Protection Act. The Protected Species Branch at the Northeast
Fisheries Science Center allocates fishery observer sea days to achieve a 30%
precision on estimates of incidental marine mammal bycatch in US gillnet and bottom
trawl fisheries conducted off the Atlantic coast of the United States. A three-step
process is used to: (1) project sample sizes; (2) proportionally allocate
projected sea days to fishery strata; and (3) assign allocated sea days to
ports and months within strata proportional to fishing effort. Under
conditions of limited observer funding, a set of decision rules is followed to
establish which fisheries will receive observer coverage. The
amount of observer coverage (in sea days) needed to achieve a 30% precision in
the bycatch estimates varies widely among fishery strata. In the 2007
fiscal year (October 2006 – September 2007), funding was only available for 802
observer sea days, although nearly 57,000 sea days are required provide a 30%
precision in the bycatch estimates of harbor porpoise (Phocoena phocoena) and bottlenose dolphin (Tursiops truncatus) in the
Northeast and Mid-Atlantic gillnet fisheries, and for pilot whales (Globicephala spp.), white-sided dolphin (Lagenorhynchus acutus), and common
dolphin (Delphinus delphis) in the Northeast and Mid-Atlantic bottom
trawl fisheries. After the decision rules were applied in FY07, all 802
observer sea days were allocated to coverage of the gillnet fisheries;
47% (377 days) was allotted to the Northeast gillnet fishery
and 53% (425 days) to the Mid‑Atlantic gillnet fishery. Due
to the very large amount of fishing effort in the Northeast and Mid-Atlantic
bottom trawl fisheries, a substantial increase in annual sampling coverage is
required for monitoring what are essentially rare marine mammal bycatch events
in these fisheries.
Introduction
The Northeast and Mid-Atlantic regions of the Northeast
continental shelf are home to around 30 stocks of marine mammals (Figure
1;
Waring et al. 2007). Documentation of annual marine mammal mortality estimates
attributed to commercial fishing operations in these regions dates back to the
late 1980s (Blaylock 1995; Bravington and Bisack 1996; Bisack 1997; Belden et
al. 2006; Belden 2007).
The 1994 amendments to the Marine Mammal Protection Act
(MMPA) require the monitoring of bycatch mortality of marine mammals in U.S.
commercial fisheries. In 2004, NOAA National Marine Fisheries Service
(NMFS) published a report (NMFS 2004), which recommended that benchmark levels
of precision on bycatch estimates should be about 30%.
This report documents the analytical approach that the
Protected Species Branch (PSB) at the Northeast Fisheries Science Center
(NEFSC) uses to allocate fishery observer coverage (in sea days) to facilitate
precise estimation of bycatch mortality estimates of marine mammals in the
Northeast (Maine to Connecticut) and Mid-Atlantic (New York to North Carolina)
gillnet and bottom trawl fisheries (Figure
1).
Methods
Several marine mammal species are incidentally captured in
U.S. commercial gillnet and bottom otter trawl fisheries operating off the
Atlantic coast (Waring et al. 2007). In this region, marine mammal
species are considered for observer coverage based on either their MMPA status
(endangered, threatened, strategic, or depleted), or because of Take Reduction
Plan (TRP) monitoring requirements. Five species typically qualify
for observer coverage: harbor porpoise (Phocoena phocoena); coastal
bottlenose dolphin (Tursiops truncatus); pilot whale (Globicephala spp.); white-sided dolphin (Lagenorynchus acutus); and common
dolphin (Delphinus delphis).
The analytical approach used annually in allocating observer
coverage involves a three-step process:
(1) The
number of observed fishing trips needed to achieve a 30% coefficient of
variation (CV) for a bycatch estimate (within an appropriate fishery/time/area
stratum) is determined from data collected in previous years (i.e., the number
of observer trips and the CV of previous bycatch estimates).
(2) The
number of sea days to be observed for each species (within each
fishery/time/area stratum) is proportionally allocated based on:
(a) the projected number of trips needed to achieve a 30% CV (step
1); (b) the average duration of a trip (in days); the amount of observer
funding available; and (d) the cost of a sea day.
(3) The
available sea days (step 2) are allotted to port groups and months (within a
fishery/time/area stratum) proportional to the number of fishing trips (based
on dealer records) previously reported in each port group/month stratum.
Step 1
For each of the five cetacean species, the first step
projects the number of sampled fishing trips required to achieve a 30% CV for
the bycatch estimate. The fishing trip is the sampling platform upon
which the fisheries observer is deployed, and where fisheries bycatch data are
recorded.
The observer sea day projections rely heavily on the
mortality analyses available for each of the five cetacean species. These
have been developed using different time frames for each species and subject to
varying types of data constraints. Hence, the methods used to estimate
bycatch rates differ among the five species. For example, harbor porpoise
bycatch rates in the gillnet fisheries are estimated using a traditional ratio
estimator (Cochran 1963; Rossman and Merrick 1999), while cetacean and coastal bottlenose
dolphin bycatch rates in the bottom trawl and gillnet fisheries are estimated
by regressing observed takes on significant covariates using generalized linear
models (Palka and Rossman 2001). As bycatches of cetaceans are rare
events, pooling data over years was necessary in some cases due to sparse data.
The methods used to estimate bycatch mortality for pilot whales, white-sided
dolphins, and common dolphins in U.S. Atlantic bottom trawl fisheries have been
documented but not published.[1]
All the bycatch analyses use stratified data. The
stratification scheme (Table
1) is an implicit way to optimally allocate
observer sea days, as it accounts for the inherent spatial/ temporal
variability characteristic of the bycatch interactions.
The stratified bycatch rates and their associated levels of
precision (referred to as the ‘input baseline data’) are used in the sea day
projection equation. The input baseline data are derived from observations
recorded by staff of the Northeast Fisheries Observer Program (NEFOP).
The projected CV for a fishery/time/area stratum is defined
as the product of the observed (baseline) CV and the inverse square root of n from that fishery/time/area [2]:
(1)
where cvprojected = projected CV, cvobserved = observed (baseline) CV, and nrelative = relative
change in the number of sampled trips (= nprojected/nobserved).
In the Mid-Atlantic gillnet fishery, the projected number of
trips needed to achieve a 30% CV for the coastal bottlenose dolphin bycatch
rates was limited to fishing trips within state waters in the coastal
bottlenose dolphin habitat (Palka and Rossman 2001). However, it is also
necessary to observe gillnet trips fishing in federal waters outside of coastal
bottlenose habitat, because harbor porpoise interactions typically occur here (Belden
2007). This area, however, is restricted to a single stratum in
the entire Mid-Atlantic region. Therefore, the projected number of trips
in FY07 needed in federal waters outside of the coastal bottlenose dolphin
habitat was defined to be the same coverage as occurred in 2005; that is, the
projected number of observed trips in 2007 was calculated as the product of the
total number of Mid-Atlantic observed gillnet trips fished in 2005 multiplied
by the proportion of trips observed fishing in federal waters only.
In the Northeast gillnet fishery, harbor porpoise bycatch
rarely occurs during the summer months (Belden et al. 2006; Belden 2007) As a
result, only the winter (January–May) and fall (September–December) harbor
porpoise strata were included in this analysis.
Step 2
In step 2, observer sea days are allocated to strata (those
defined in step one), within the constraints of the annual available marine
mammal observer funds. To accomplish this, the projected number of sea
days needed to obtain a 30% CV are estimated by multiplying the projected
number of trips needed in each stratum (from step 1) by the average length of a
trip within that stratum. Average trip duration is derived from observed
trip length data (reported as days absent) recorded by the NEFOP. The total
amount of sea days available (defined by the available funds and the cost of an
observer sea day) are then allocated to strata proportional to the projected
number of sea days in each stratum needed to obtain a 30% CV.
From a practical perspective, there have never been
sufficient funds to allocate observer coverage at the level required to achieve
30% precision in each of the 23 marine mammal bycatch strata (Table
2). Overall,
the number of observer trips would be need to be doubled (15,315 to 34,001
trips), and some strata would require greater than a 5-fold increase in the
number of observer trips to achieve a 30% CV (i.e., strata 6, 8, 11, 13, and 18).
Hence, apart from allocating coverage within existing financial
constraints, it has also been necessary to prioritize the selection of the
protected species requiring observer coverage for monitoring bycatch. For the
five cetacean species considered in this report, three decision rules are used
to determine which strata have priority for annual monitoring of their
incidental takes. First, if a species is managed under a TRP (because the
stock has been classified under the MMPA as endangered, threatened, depleted,
or strategic), the species is treated as a high priority for bycatch monitoring
using observer coverage. Second, if annual commercial bycatch mortality
of a species is 10% or greater than its Potential Biological Removal (PBR)
level, the species is also considered a high priority for monitoring. The
third decision rule considers whether other sampling programs exist that could
supplement or enhance those already in place to monitor marine mammal bycatch.
The total quantity of available observer sea days is
determined by dividing the total amount of observer funds available by the cost
of a sea day (which is determined by the NEFOP.)
In FY07, the resulting total number of sea days available
for observing protected species interactions was 802 days.
Step 3
The third and final step allocates the total number of
available observer sea days (from step 2) across port group/month strata
in proportion to the number of fishing trips in each port group/month stratum
as reported in dealer records during the most recent year for which the dealer
data are complete (in FY07, data from 2005 were used). The Northeast Region
(NER) dealer database contains records of all seafood transactions from
commercial fishing trips landing federally regulated species. This
database is considered to represent a complete census of commercial fishing
trips, and therefore is used to determine the proportion of fishing effort
(trips) by port and month. Because some fishing effort in North Carolina is not
reported in the NER dealer database due to non-federally regulated fisheries
and confidentiality requirements, North Carolina Division of Marine Fisheries
trip ticket data are used (in conjunction with the NER dealer data) to quantify
the number of trips from North Carolina.
After the annual quantity of available observer sea days
have been allocated across port group/month strata, fisheries observer coverage
is assigned concordant with this schedule.
Results
Table
1 is a summary of the baseline data used to project
the annual observer coverage (in sea days) needed to estimate the bycatch rates
of the five cetacean species in the U.S. Atlantic bottom trawl and
gillnet fisheries (Figure
1). Table
2 compares the baseline coverage and
associated bycatch CVs in each sampling stratum with the coverage required to
achieve a 30% CV on the estimated bycatch rate.
The relative change in the number of observer trips to
achieve 30% precision differs widely among the strata (Table
2). Some
strata already have a precision level better than 30% (strata 14 and 16), so
projected observer trips in these strata can actually be reduced. Stratum
8, which has the poorest baseline CV (1.61) requires the largest percentage
increase in number of trips (28.8 fold relative change), but is a stratum
with relatively low absolute baseline observer coverage (27 trips). This
is in contrast to stratum 23 where the baseline observer effort is large (3,291
trips), and only a 4-fold increase in observer trips is needed to achieve a 30%
CV. However, in this stratum, this increase translates into a very large
number of trips (13,592).
The projected number of observer sea days needed to achieve
a 30% CV also differs widely among fisheries, ranging from 825 sea days for the
bycatch estimate of harbor porpoise in the Northeast gillnet fishery to 28,627
days for the bycatch estimate of pilot whales in the Northeast and Mid-Atlantic
bottom trawl fisheries (Table
3).
Both harbor porpoise and coastal bottlenose dolphin are
presently being managed by TRPs because of their MMPA status as strategic and
depleted stocks, respectively. Recent annual bycatch mortality estimates
for both species are also within 10% of PBR (Table
4). Although
pilot whale, white-sided dolphin, and common dolphin mortality is greater than
10% of PBR they are not considered endangered, threatened, depleted, or
strategic (Table
4).
Hence, based on the three decision rules in step 2, all of
the 802 observer sea days available in FY07 were allocated to the monitoring
harbor porpoise and coastal dolphin takes in the Northeast and Mid-Atlantic
gillnet fisheries (Table
3). Forty-four percent of the observer days (352
days) was allotted to the Northeast gillnet fishery to monitor harbor porpoise
bycatch, and 56% (450 days) was allocated to the Mid‑Atlantic gillnet
fishery to monitor bycatches of harbor porpoise and coastal bottlenose dolphin
(Table
3). These allocations represent only 2-3% of the projected number
of observer days needed to obtain a 30% CV on the bycatch rates for each of
these species in these two fisheries.
Tables 5 through 8 provide further details to illustrate how
the observer sea days for the Northeast and Mid-Atlantic gillnet fisheries
in FY07 (Table
3) were assigned to port and month strata.
In the Northeast gillnet fishery, harbor porpoise bycatches
occur in both winter (March– May) and fall (September–December). During
winter, three ports account for the majority of fishing trips in the Northeast
gillnet fishery: New Bedford/Westport/Fall River, MA (26%); Little Compton, RI
(23%); and Gloucester/Marblehead/Rockport, MA (14%). During the fall,
Gloucester, MA (46%),Portsmouth ,NH (17%), and New Bedford, MA (9%) account for
most of the Northeast gillnet trips (Table
5). Based on the
proportion of Northeast gillnet fishing trips in 2005 that occurred in
each month/port stratum (Table
5), the 377 available observer sea days
(193+184) for covering the Northeast gillnet fishery in FY07 were subdivided
within each season in each month/port strata (Table
6).
In the Mid-Atlantic fishery 38% of the fishing trips came
from North Carolina counties, followed by Virginia (30%) and New Jersey ports
(21%; Table
7). Of the 425 observer sea days allocated in FY07 to the
Mid-Atlantic gillnet fishery, 144 days were allotted to monitoring coastal
bottlenose dolphin bycatch and 281 days to monitoring bycatches of harbor
porpoise (Table
3). Of the latter amount, 22 days were set aside to observe theNorth
Carolina beach seine fishery. Based on the proportion of Mid-Atlantic
gillnet fishing trips in 2005 that occurred in each month/port stratum (Table
7), the 403 available observer sea days (144 + 281 – 22) for covering the
Mid-Atlantic gillnet fishery in FY07 were subdivided across month/port strata
(Table
8).
Discussion
Rare Events
Unlike most finfish bycatch, the bycatch of marine mammals
is a rare event. Rare events can generate large CVs depending on the
fishery sampled, the amount of observer coverage, and the frequency of the rare
events. To address the high uncertainty often associated with marine
mammal bycatch mortality estimates, observer data are frequently pooled over
several years before analysis. However, this can be problematic with
respect to generating observer coverage deployments for annual sampling
programs. Due to the very large amount of fishing effort in the Northeast
and Mid-Atlantic bottom trawl fisheries, a substantial increase in annual
sampling coverage is required to obtain precise and accurate bycatch estimates
of what are essentially rare marine mammal bycatch takes in these fisheries. This might be addressed by implementing rotational sampling programs where
selected fisheries are observed on a non-annual, but periodic basis (Didier et
al. 1999). Under this scenario, a high level of observer coverage is
deployed once every several years to a fishery where bycatch is very rare, but
intensive sampling is necessary to accurately assess stock recovery (decline in
bycatch mortality) under a TRP. However, this approach may conflict with
current MMPA policy requirements (which require annual mortality estimates for
strategic stocks), unless it is deemed acceptable to assume that fishery
bycatch rates are identical between observed and non-observed years.
Funding
Observer sea day allocations are affected by a number of
factors, and observer funding can vary widely from year to year. Sometimes
these effects can be buffered by utilizing other bycatch programs (e.g., those
observing finfish bycatches) to monitor marine mammal interactions.
Occasionally, observer funding to achieve a very specific
objective becomes available. In 2006, for example, funds were provided by
the National Observer Program (NOP) to observe 100 sea days in the Illex squid bottom trawl fishery. Similarly, in 2006 and 2007, dedicated
funding was made available for 114 observer sea days to monitor the bycatch of
coastal bottlenose dolphin off the coast ofNorth Carolina .
Decision Rules
New decision rules to prioritize observer coverage among
fisheries may be required in the future. If additional protected species meet
the first and second criteria for monitoring, one or more additional decision
rules may need to be developed. For example, if the white-sided dolphin stock
was deemed to be strategic, new decision rules would have to be developed to
select which strata (Table
2) would receive observer coverage, as it would be
neither fiscally nor logistically possible to deploy sufficient coverage in all
bottom trawl fishery sampling strata to achieve the desired 30% precision.
One way to address such constraints is to evaluate the relative contribution of
individual strata to the overall magnitude and imprecision of the bycatch
mortality estimate for a particular species. That is, if a stratum
contributes a large proportion of mortality to the total estimate and also has
a high CV, this stratum should be considered a priority for receiving observer
coverage. In contrast, if a stratum contributes only a small proportion of
mortality to the total estimate but also has a high CV, allocation of observer
coverage to this stratum might be considered as a lower, second priority. Such
decisions would have to be evaluated in the context of the status (and
available biological information) for the stock in question. Stocks with
low PBRs are typically at more risk. In these cases, even strata
contributing low mortality and possessing high uncertainty should be considered
a priority for observer coverage.
Strata Overlap
The current approach to allocating observer sea days does
not account for the temporal and spatial overlap in sampling requirements among
species across strata. For example, the projected number of observer days for
monitoring pilot whale bycatch in the Northeast and Mid‑Atlantic bottom
trawl fisheries is independent of the projected observer coverage for white‑sided
and common dolphin bycatches, even though these projections are for the same
fisheries, both in time and space. More work is needed to determine the extent
of habitat and fishery strata overlap among these three species to optimize
observer sea day allocations in the future.
As well, protected species observer coverage should be
compared to observer coverage deployed in monitoring finfish bycatch.
Potential spatial/temporal overlaps in coverage could result in the
redirection of the duplicative observer days to other strata.
Sampling Projections
Differences in projected observer coverage requirements
largely reflect differences in the magnitude of fishing effort among the
various fisheries in which marine mammals are incidentally taken. The
bottom trawl fishery has many more vessels (and accomplishes many more fishing
trips) than the gillnet fishery, and the average trip duration is much longer.
Hence, from a cost perspective, increasing observer coverage to
improve the precision of bycatch estimates in the bottom trawl fisheries is far
more expensive than for the gillnet fisheries.
Comparing Approaches to Observer Effort Allocation
The approaches for allocating observer coverage discussed in
this report are not the only ones used in determining and assigning observer
effort among fisheries. A holistic approach to monitoring bycatch (fishes,
marine mammals, sea turtles, etc.) in all fisheries is presented in Wigley et
al. (2007). However, it remains unclear whether a single, standardized
approach is appropriate to monitor the bycatch of different taxa, and which
types of estimators are best to employ in deriving bycatch values (several of
these issues were discussed at an April 2006 joint meeting of the Science and
Statistical Committees of the New England and Mid-Atlantic Fishery Management
Councils[3]). As bycatch issues are now
receiving increased attention world-wide, ongoing and future research will
likely provide new insights on these approaches in the future.
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Acronyms
CV |
= |
coefficient of variation |
MMPA |
= |
Marine Mammal Protection Act |
NOP |
= |
National Observer Program |
NEFOP |
= |
Northeast Fisheries Observer Program |
NEFSC |
= |
Northeast Fisheries Science Center |
NER |
= |
Northeast Region |
NMFS |
= |
National Marine Fisheries Service |
NOAA |
= |
National Oceanic and Atmospheric Administration |
PBR |
= |
Potential Biological Removal |
PSB |
= |
Protected Species Branch |
TRP |
= |
Take Reduction Plan |