Northeast Fisheries Science Center Reference Document 01-06
Defining
Triggers for Temporary Area Closures
to Protect Right Whales
from Entanglements:
Issues and Options
by Phillip J. Clapham and
Richard M. Pace, III
National Marine Fisheries Service, 166 Water Street, Woods Hole,
MA 02543
Print
publication date April 2001;
web version posted April 24, 2001
Citation: Clapham, P.J.; Pace, R.M., III. 2001. Defining triggers for temporary area closures to protect right whales from
entanglements: issues and options. Northeast Fish. Sci. Cent. Ref. Doc. 01-06; 28 p.
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EXECUTIVE SUMMARY
The remaining population of the North Atlantic right
whale (Eubalaena glacialis) is small, critically endangered,
and appeared to be declining in the 1990s. Human-related mortality
from ship collisions and fishing gear entanglements is known to be
a major factor in this failure to recover. Among measures being considered
by NOAA Fisheries to reduce or eliminate entanglement mortality are
temporary area closures, in which a specified area would be closed
to fishing when aggregations of right whales are observed there.
A key issue concerns the number or density of right whales that must
be observed to initiate or "trigger" such a closure. Here,
existing data on right whale occurrence and distribution are analyzed
to evaluate criteria for triggering temporary area closures. Specific
criteria are then applied to existing aerial survey data sets to
assess the effectiveness of the closures, as well as the frequency
with which closures would have been enacted in past years had triggers
been in place. Analyses are based upon the assumption that feeding
right whales are at highest risk of entanglement; conversely, it
is assumed that transiting whales, while certainly not at zero risk
of entrapment, do not constitute sufficient grounds to close an area
to fishing.
Data from Cape Cod Bay and Stellwagen Bank
from April through October, 1980-1996, were used to assess whether
there was any connection between the number of animals in an initial
sighting and the magnitude and duration of the sighting events that
followed. An event was defined as two or more right whale sightings
separated by an interval of not more than 10 days. Conversely, a non-event is
any right whale sighting which was not followed by another within 10
days. The data produced a total of 42 events (with durations ranging
from two to 95 days) and 21 non-events. There were 50 initial sightings
in which the number of right whales involved was either one or two.
Of these sightings, 29 (58%) began an event, while 21 (42%) were non-events.
In contrast, all initial sightings involving three or more whales (n =
13) began an event (i.e., they were followed by one or more
subsequent sightings within ten days). Of these 13 events, 5 (38%)
lasted less than a week and 8 (62%) for a week or more. The 13 events
ranged in duration from two to 33 days (mean = 15 days, median = 12
days). Thus, the data suggest that an initial sighting of three or
more right whales is a reasonably good indicator of an event, and the
average duration of such events is about two weeks.
A trigger density of 4.16 right whales (rounded off here
to 4 whales) per 100 nm2 was calculated from the 13 events
described above. Additional analysis indicated that a buffer of about
15 nautical miles placed around the sightings from the first day of
an event will in most cases encompass the movements of right whales
during the entire course of that event.
Data from aerial surveys in 1999 and 2000 were used to
retrospectively evaluate the closure triggers. Two trigger approaches
were assessed. The Capture Radii Method combined all sightings
in a survey day to produce an overall survey density, and compared
this density against the proposed trigger value of 4 whale/100 nm.
Using this method, 19 of 54 daily aerial surveys produced capture radii
with right whale densities > 4 whale/100 nm2 (note that
because of overlap among areas, this does not equal 19 closures). All
but one of these 19 events involved three or more whales.
In contrast, the Local Area Density Method used
equal-density circles centered on each whale sighting; if a contiguous
set of circles encompassed at least three whales, that local set of
sightings was used to construct a closure area. This method triggers
closures based on density of whales in a local area and is less affected
by the particular spatial coverage of an individual aerial or ship-based
survey. Use of this method circumvents a major problem with the capture
radii method, which is that the density of whales in a concentration
may be artificially diluted by isolated distant sightings. Therefore,
use of the local area density method to assess closures is recommended,
notably when such assessments are based upon data from aerial surveys.
Local whale densities exceeded 4 whale/100 nm2 in
45 of 54 surveys analyzed. The local area density approach would have
triggered eight closures during both 1999 and 2000. Several of the
closure areas overlapped regions already targeted for gear restrictions
such as Cape Cod Bay and the Great South Channel. Based on these results,
it appears that extending the Great South Channel restricted area to
the northeast would reduce the need for recurring temporary area closures
and offer considerable protection to right whales during April and
May.
INTRODUCTION
The North Atlantic right whale (Eubalaena glacialis)
is among the most endangered of all marine mammals (Clapham et
al. 1999, IWC 2001). Following ten centuries of hunting, the
remaining population is today believed to number only about 300 whales.
Although right whales were once distributed across the North Atlantic
from North America to Europe, the primary range of the population
now extends only from calving grounds off the southeastern United
States to feeding areas off New England and eastern Canada (Kraus et
al. 1986).
No recovery has been evident in this population despite
several decades of protection from hunting, and recent research indicates
that the population declined during the 1990's (Caswell et al.
1999). Human-related mortalities, from ship collisions and entanglements
in fishing gear, are thought to be the major causes of this decline
(Knowlton & Kraus 2001). Reducing entanglement mortality has
been the focus of efforts by the Atlantic Large Whale Take Reduction
Team (AWLTRT), and through the team's efforts various mitigation
measures have been enacted. However, entanglements continue to occur.
Hamilton et al. (1998) estimated that more than 60% of individually
identified right whales had been entangled at some point in their
lives. While many whales rid themselves of entangling gear, some
sustain serious injuries and eventually die (Knowlton & Kraus
2001).
Several measures are being considered by NOAA Fisheries
to reduce and eliminate entanglement mortality in right whales. One
of these is temporary area closures, in which a specified area is
temporarily closed to fishing following confirmed sightings of right
whales in that area. A key issue in using this approach is the number
or density of right whales that must be observed to initiate or "trigger" such
a closure. Based upon analyses of existing data, this document evaluates
various criteria to "trigger" temporary area closures.
Specific criteria are then applied to existing aerial survey data
sets to assess the effectiveness of the closures, as well as the
frequency with which closures would have been enacted in past years
had triggers been in place.
Although this document primarily presents scientific
analysis relating to triggers, it is important to recognize that
management measures such as temporary area closures must be considered
in the broader framework of risk assessment. This is outlined below.
Levels of risk
Discussions of area closure triggers have occurred
both within and outside the ALWTRT, and various "triggers" have
been informally proposed. However, no reliable data exist to develop
statistically sound models to predict probabilities of right whale
entanglement. As such, the analyses presented in this report are
based on perceived risks of entanglements. It is generally believed
that whales foraging in an area are at higher risk of entanglement
than those transiting through the area. Thus, the pertinent question
is: what is the minimum number of whales that reliably indicates
the presence of foraging whales? Alternatively, a behavioral approach
might be used based upon behavior which demonstrates or implies that
foraging is occurring.
Under either approach, it is assumed that feeding
equates to a high risk of entanglement. Conversely, it is assumed
that transiting whales, while certainly not at zero risk of entanglement
(notably if they are engaging in V-shaped "prospecting" dives
even while traveling in a straight line), do not constitute sufficient
grounds to close an area to fishing.
No amount of analysis of existing survey data will
produce a result separating whales at risk from whales that are
not. For example, while one might define a trigger as (say) "four
right whales observed in a 100 nm2 area", it would
be incorrect to assume that three right whales seen in the same
area would be at zero risk of entanglement. Scientifically, there
is no
doubt that a single right whale feeding in an area with fishing
gear present would be at a distinctly non-zero risk of entanglement
(and
thus potentially of death). Given this, the fundamental question
is how much risk of entanglement is acceptable? This issue is ultimately
one of policy rather than science.
Triggers: general issues
At one extreme, the question of the number of right
whales to use as a closure trigger is easy. For example, it is unlikely
that a reasonable argument could be mounted against using a sighting
of thirty right whales in a relatively small area as a trigger, since
this many whales would unequivocally indicate the existence of a
significant prey resource (and thus of the feeding behavior that
poses a high risk of entanglement). So would twenty whales, and so
(probably all whale biologists would argue) would ten. The problem
lies in selecting the smallest number that could be advanced as a
reliable indicator of the same state.
In the absence of sightings of "many" whales
on a single survey, residency of a smaller number over a period of
two such surveys (closely spaced in time) has been suggested as an
alternative trigger approach. However, residency per se is
not the issue; rather, residency simply provides an indication that
feeding is occurring.
Given that foraging whales are assumed to be at risk
of entanglement, it may be irrelevant whether or not whales are resident
in a particular area for a protracted period. If whales are feeding,
they are at risk, irrespective of whether or not they quickly consume
the resource and move elsewhere. In other words, the potential for
entanglement does not depend upon the prey resource remaining in
place for an extended time. This is important, since it means that
any trigger (numerical or behavioral) based upon observed or implied
feeding behavior would require data from only a single survey.
Indeed, given the many logistical and regulatory problems
involved in identifying a right whale aggregation "event",
and expeditiously removing gear from an area, it is of paramount
importance that triggers be based upon single rather than repeated
surveys. This principle underlies all of the analyses presented below; i.e.
triggers have been defined that reliably indicate the presence of
non-transient whales using data from a single survey. For those cases
where right whales are observed in densities below the trigger threshold,
additional (prioritized) survey coverage would be needed to determine
whether the animals sighted were in residence or merely transiting
through.
DATA
ANALYSIS AND DEFINITION OF CLOSURE TRIGGERS
The following sections of this document provide analyses
of right whale sighting data for use in defining triggers and areas
for spot closures. The first set of analyses looks at events in which
right whales were observed repeatedly over periods of time in an
area, and evaluates whether there is any connection between the number
of animals in the initial sighting and the magnitude and duration
of the sighting events that followed. The results of these analyses
are subsequently translated into a density of whales (whales per
X square miles) which would trigger a temporary area closure; a modification
of this approach is also proposed to avoid a problem which became
apparent during the analysis. The trigger criterion is then applied
retrospectively to aerial survey sighting data from 1999 and 2000
to investigate how frequently closures would have been effected,
and how large the areas concerned would have been, had the trigger
been in place in these years.
ANALYSIS 1: TRIGGERS - MINIMUM NUMBER OF WHALES
Because of large gaps in temporal and spatial coverage,
NOAA Fisheries aerial survey data from 1999 and 2000 were not sufficient
to address the question of how many whales need to be seen to reliably
predict that other sightings will occur. What was needed was daily
effort over long periods in a single area; data from the Center for
Coastal Studies' (CCS) vessel cruises (both directed trips and whalewatching
cruises) provide exactly this type of consistent coverage and were
therefore used. CCS whalewatching cruises made up the bulk of the
effort; these provided daily coverage of Cape Cod Bay and southern
Stellwagen Bank from mid-April to the end of October each year. Although
daily effort varied from one to as many as nine cruises, and while
a few days a year were typically missed due to bad weather, daily
coverage was effectively continuous during this 6.5-month annual
time period.
The issue of spatial stability - how much a concentration
of right whales moves over the period that it remains together -
is addressed in a separate analysis in the second section of this
report.
Methods
CCS sighting data from 1980 to 1996 were analyzed.
The months of January, February, March, November and December were
excluded to restrict the data to months of continuous daily coverage
from whalewatching vessels. Directed cruise data from the April-October
period were also used. We used data from only the first trip each
day, to avoid counting the same animals two or more times in a day.
Data were sorted by area according to the following
three categories: Cape Cod Bay (CCB), Stellwagen Bank, and Other. "Other" was
excluded from all analyses. Cape Cod Bay was defined as anything
within the Bay south of the 44120 Loran line. Stellwagen Bank was
defined as a box north of this to the 44280 line and bounded by Loran
lines 13675 to the east and 14000 to the west.
Sightings were then grouped by date such that an "event" was
defined as any two or more sightings of right whales, separated by
an interval of no more than X days. We ran this program separately
for intervals of 4 days, 7 days, 10 days and 14 days, and ran each
of these for CCB/Stellwagen together, and Stellwagen alone.
The choice of interval is important in defining the
event. Short intervals (e.g. 4 days) can result in the breakup of
events which are actually longer. Long intervals (e.g. 14
days or more) risk grouping as a single event sightings which do
not belong together (e.g. a single whale seen on day 1 and
a single whale seen on day 14, with nothing in between). Results
using different intervals are given in Appendix A (Tables 1, 2a and
2b). Comparisons of results indicated that the best interval is 10
days, and this was used in all analyses.
For each event, we calculated: (a) the number of whales
in the initial sighting (the total number observed on the first day);
(b) the number of separate days on which sightings were made; (c)
the total duration of the event (first day to last day); and (d)
the mean number of whales sighted overall.
We also looked at cases where an initial sighting of
a right whale was not followed by another within the specified interval
(termed "non-events" here). In this analysis, we excluded
any sighting that was the last record in the series for an event,
since by definition the last such sighting could not be followed
by another within 10 days (or the event would not have ended).
Thus, in what follows:
an "event" is two or more right
whale sightings separated by an interval of not more than
10 days
and:
a "non-event" is any right whale
sighting which was not followed by another within ten days
(unless the sighting concerned represented the end of an
event).
Results
Events
Limiting the data to Stellwagen Bank, analysis of the
CCS daily sightings data produced 17 events under the 10-day definition.
Adding in the Cape Cod Bay data increased this to 42 events, many
of which involved sightings from both areas. Differences between
the two treatments highlighted the potential problem of relying on
data from a smaller marginal area that may involve spillover from
a nearby, higher-use habitat. A list of events from the combined
data sets is given in Tables 1a and 1b (sorted chronologically, and
by the number of whales in the initial sighting). Events from just
the Stellwagen Bank area are presented in Tables 2a and 2b.
Non-events
Right whale non-events (i.e. sightings not followed
by subsequent sightings within ten days) totalled 21 for the combined
Cape Cod Bay/Stellwagen data set (Table 3), and 16 for the Stellwagen-only
data set (Table 4).
Initial sighting size as a predictor
Table 5 summarizes the frequency of events and non-events,
by number of right whales recorded in the initial sighting in the
combined Cape Cod Bay and Stellwagen Bank data set. There were 50
initial sightings in which the number of right whales involved was
either one or two. Of these sightings, 29 (58%) began an event, while
21 (42%) were not followed by a subsequent sighting within ten days
(i.e. they were non-events). In contrast, all initial sightings involving
three or more whales (n = 13) began an event (i.e. they were followed
by one or more subsequent sightings within ten days).
Of the 16 non-events noted in the Stellwagen-only data
set (Table 4), all but one involved initial sightings of either one
or two right whales. The exception, a sighting of four whales on
30 May 1992, was a non-event only because Cape Cod Bay data were
excluded; this sighting was actually part of an event lasting two
days which included sightings from both Stellwagen Bank and Cape
Cod Bay.
Event duration
The duration of the 42 events in the combined Cape
Cod Bay/Stellwagen data set (Tables 1a and 1b) ranged from 2 to 95
days (the latter being the unusual summer residency recorded in 1986).
Of these events, 19 (45%) lasted for less than a week, and 23 (55%)
for a week or more. Of 29 events beginning with initial sightings
of one or two right whales, 14 (48%) lasted less than a week and
15 (52%) for a week or more. Of 13 events beginning with initial
sightings of three or more right whales, 5 (38%) lasted less than
a week and 8 (62%) for a week or more. The 13 events ranged in duration
from two to 33 days (mean = 15 days, median = 12 days).
Conclusions
The analysis above indicates that an initial sighting
of three or more right whales in an area appears to be a reasonably
good indicator of an event. The average duration of an event is about
two weeks.
ANALYSIS 2: CLOSURES - TRIGGER DENSITY, BUFFERS AND
AREA DEFINITION
In the following section, the "three or more whales" trigger
initiating an event is converted into a density of whales which can
be uniformly applied in any area. A method is then developed for
establishing a closure area that is robust to movements of whales
over the course of an event. Finally, the trigger criterion is applied
to existing aerial survey data to examine how frequently closures
would have occurred in the past.
In other words, the questions being addressed are:
- What density of whales (taken from analysis
of past events) would trigger a closure?
- How much of an areal
buffer would there have to be around the sightings from the
first day of an event to
make sure that all whales present over the course of
that event were protected by the closure area?
- Applying these methods
to data from 1999 and 2000, how many closures would have been
triggered?
Definitions
The following definitions are used in this analysis:
Event - An event is two or more right whale
sightings separated by an interval of not more than 10 days. We focus
on events triggered by an initial sighting of three or more right
whales because initial sightings of one or two whales were more frequently
associated with non-events (see Analysis 1).
Event epicenter - The geographic center of all
sightings on the first day of an event.
Capture radius - The distance between an event
epicenter and the most distant sighting from that center. This is
the radius of the smallest circle, centered at the epicenter, which
captures a particular set of sightings (e.g. day one of an
event or all sightings in an event).
Density - A measure of distribution expressed
as the number of whales per 100 nm2.
Proximity - A function of density; if the minimum
density to trigger an event is D, then proximity is the radial distance
that circumscribes a point location of sighted whales equal to D.
Methods
For the 42 events identified in the combined Cape Cod
Bay and Stellwagen Bank data set (Tables 1a and 1b), distance relationships
among whales sighted during the first day of the event were
calculated (using the statistical software package SAS), as well
as relationships among all whale sightings for the entire duration of
that event. For each event, distance epicenters were determined and
plotted (together with geographic reference features) using the software
package ARCVIEW. Then, distances were calculated between the epicenter
and the most distant whale seen during the first day (r1),
and the most distant sighting during the entire event (r2).
The difference between these two radial distances was considered
to be the buffer (Figure 1).
A density for the first day of an event was also calculated
as D = n[1]/[ r1)2, where n[1] was the number
of whales seen on the first day of an event. Essentially, this density
represents the number of whales divided by the area of a circle,
centered at the epicenter, that enclosed all whale sightings on the
first day of an event.
Results
The 42 analyzed events ranged in duration from 2 to
95 days and included between two and 449 total sightings (Table 1a).
Buffers and trigger densities were calculated only for the 13 events
which were initiated by an initial sighting of three or more animals;
the other 29 events were ignored.
Buffer size
For the 13 events triggered by an initial sighting
of three or more right whales, first-day capture radii ranged from
0 (i.e. all whales seen on the first day were together in the same
place) to 10.4 nautical miles (nm). Total capture radii (i.e.
for all sightings over the duration of an event) ranged from 1.6
to 25.2 nm. The largest buffers were slightly larger than 15 nm;
thus, a buffer of 15 miles around the first day's sightings will
usually enclose all of the right whales in an event, irrespective
of their movements during the course of the event (Table 6).
Whale density
The minimum first-day density for the 13 events was
4.16 whales/100 nm2, rounded off here to 4 whales in a
100-square mile area (roughly equivalent to 3 whales in a 75 nm2 area).
This density is therefore considered a reasonable trigger for a closure.
However, when a large area is surveyed on one day (i.e. with
an aircraft), a situation can occur where a concentrated cluster
of sightings (one that would normally trigger a closure) falls below
the trigger density through inclusion of another right whale sighting
a long way away, creating an artificially large circle for that days'
sightings (and consequent low overall whale density). To address
this problem, an alternative trigger approach called the Local Area
Density Method was developed (see Retrospective Analysis section
below).
The inverse of the 4 whales/100 nm2 density
is 24 nm2/whale, which is equivalent to a radial distance of 2.77
nm for a single whale sighted (3.91 nm for 2 whales, 4.79 nm for
3 whales, etc). When whale locations are plotted together with their
proximity circles, local whale densities above or below closure triggers
are identifiable (Figure 2).
ANALYSIS 3: RETROSPECTIVE ANALYSIS
Methods
Right whale sightings from four data sets were used
in a retrospective evaluation of the closure triggers: the 1999 and
2000 NOAA Fisheries Northeast Fisheries Science Center (NEFSC) aerial
surveys, and the 1999 and 2000 NOAA Fisheries Northeast Regional
Office/Sighting Advisory System (NERO/SAS) surveys (Figure 3). These
data were analyzed to examine the frequencies and sizes of area closures
that would have resulted from using the density trigger proposed
above. The four data sets included all documented right whale sightings
observed during aerial surveys conducted by one or two aircraft on
30 different days during the period 10 March to 27 June 1999, and
on 24 different days during the period 25 March to 6 July 2000. The
aerial surveys sighted 461 right whale groups involving 652 whales.
The surveys covered several areas, including the Great South Channel,
Georges Bank, Jeffreys Ledge, Cashes Ledge and adjacent areas, Platts
Bank and Wilkinson Basin.
Two trigger approaches were used:
The Capture Radii Method combines all sightings
in a survey day to produce an overall survey density and compares
this density against the proposed trigger value of 4 whale/100 nm2.
The Local Area Density Method uses equal-density
circles centered on each whale sighting. If a contiguous set of circles
encompasses at least three whales, that local set of sightings is
used to construct a closure area. This method triggers closures based
on density of whales in a local area and is thus less affected by
the particular spatial coverage of an individual aerial or ship-based
survey. Use of this method circumvents the problem noted above regarding
dilution of density caused by isolated sightings that are distant
from a concentration of whales.
Results
Capture Radii method
Of the 54 daily aerial surveys, 19 produced capture
radii with right whale densities > 4 whale/100 nm2 (Figure 4). All but one of these 19 events involved three or more whales.
Note that the 19 events would not translate into 19 closures, because
many of the events would be part of the same closure. The recognized
high concentration of whales observed near Cashes Ledge during June
2000 would not have triggered a closure under this approach because
one or two whales observed on the western edge of that day's survey
would have diluted the density of the other 20+ whales to <4 whale/100
nm2 (Figure 5). Because of this shortcoming, use of the
capture radii method is not recommended; instead, the local area
density method should be employed in all assessments of potential
closures.
Local Area Density Method
Local whale densities exceeded 4 whale/100 nm2 in
45 of 54 surveys analyzed. The local area density approach would
have triggered 8 closures during both 1999 and 2000 (Figure 6,
Figure 7). Several of the closure areas overlap areas already targeted
for gear restrictions such as Cape Cod Bay and the Great South Channel
(GSC). Based on these results, it appears that extending the GSC
restricted area to the northeast would reduce the need for recurring
temporary area closures and offer considerable protection to right
whales during April and May.
SUMMARY
1) An initial sighting of three or more right whales,
with a minimum density of about 4 whales per 100 nm2,
is a reliable indicator that additional sightings will occur in an
area (i.e. that an event will take place).
2) Establishing a buffer of 15 nm will in most cases
demarcate an area that will include all right whales sighting throughout
the duration of an event.
3) Where aerial survey data are used to assess closures,
the Local Area Density Method should be applied to avoid the problem
of density dilution that occurs when sightings distant from a concentration
of whales are included.
ACKNOWLEDGMENTS
We thank Moira Brown, Jooke Robbins and the Center
for Coastal Studies for kindly providing right whale data for this
analysis. We thank Richard Merrick and Fred Serchuk for helpful comments
on both the analyses and the text of the manuscript.
LITERATURE
CITED
Caswell, H., Brault, S. and Fujiwara, M. 1999. Declining
survival probability threatens the North Atlantic right whale. Proc.
Natl. Acad. Sci. USA 96: 3308-3313.
Clapham, P.J., Young, S.B. and Brownell, R.L. Jr. 1999.
Baleen whales: conservation issues and the status of the most endangered
populations. Mammal Review 29: 35-60.
Hamilton, P.K., Marx, M.K. and Kraus, S.D. 1998. Scarification
analysis of North Atlantic right whales (Eubalaena glacialis)
as a method of assessing human impacts. Paper SC/M98/RW28 presented
to the IWC Scientific Committee, March 1998. 39 pp.
IWC. 2001. Report of the Workshop on status and trends
in western North Atlantic right whales. Rep. int. Whal. Commn.,
Special Issue 18 (in press).
Knowlton, A.R. and Kraus, S.D. 2001. Mortality and
serious injury of North Atlantic right whales (Eubalaena glacialis)
in the North Atlantic Ocean. J. Cetacean Res. Manage. (in
press).
Kraus, S.D., Moore, K.E., Price, C.E., Crone, M.J.,
Watkins, W.A., Winn, H.E. and Prescott, J.H. 1986. The use of photographs
to identify individual North Atlantic right whales (Eubalaena
glacialis). Rep. int. Whal. Commn., Special Issue 10,
145-151.
