Early Online Version
Pike, D.G., Gunnlaugsson, T., Desportes, G., Mikkelsen, B., Víkingsson. G.A., Bloch, D.
(2019). Estimates of the relative abundance of long-finned pilot whales (Globicephala
melas) in the Northeast Atlantic from 1987 to 2015 indicate no long-term trends.
NAMMCO Scientific Publications, Volume 11. https://doi.org/10.7557/3.4643
Creative Commons License
Estimates of the relative abundance of
long-finned pilot whales (Globicephala
melas) in the Northeast Atlantic from 1987
to 2015 indicate no long-term trends
Daniel G. Pike1,2, Thorvaldur Gunnlaugsson3, Geneviève Desportes2, Bjarni
Mikkelsen4, Gísli A. Víkingsson3 and Dorete Bloch4*
1Esox Associates, 1210 Ski Club Road, North Bay, Ontario, Canada. Corresponding author
2North Atlantic Marine Mammal Commission, Tromsø, Norway
3Marine and Freshwater Research Institute, Reykjavík, Iceland
4Faroese Museum of Natural History, Faroe Island
* This paper was completed after the death of Dorete Bloch, who was responsible for organising the
Faroese part of the North Atlantic Sightings Surveys from 1987 to 2007.
North Atlantic Sightings Surveys (NASS) and associated surveys, covering a large but
variable portion of the North Atlantic, were conducted in 1987, 1989, 1995, 2001, 2007 and
2015. Previous estimates of long-finned pilot whale (Globicephala melas) abundance,
derived using conventional distance sampling (CDS), are not directly comparable to one
another because of differing survey coverage, field methods and, in the case of the 1989
NASS, different survey timing. CDS was used to develop indices of relative abundance to
determine if pilot whale abundance has changed over the 28-year period from 1987 to 2015.
The varying spatial coverage of the surveys is accommodated by delineating common regions
that were covered by: i) all 6 surveys, and ii) the 3 largest surveys (1989, 1995, and 2007).
These “Index Regions” were divided into East and West subregions, and post-stratification
was used to obtain abundance estimates for these index areas only. Estimates are provided
using the sightings from the combined platforms for surveys that used double platforms or
the primary platform only.
Total abundance in the Index Regions, uncorrected for perception or availability biases,
ranged from 54,264 (CV=0.48) in 2001 to 253,109 (CV=0.43) in 2015. There was no
significant trend in the numbers of individuals or groups in either the 6 or 3 Survey Index
Regions, and no consistent trend over the period. Power analyses indicate that negative
annual growth rates of -3% to -5% would have been detectible over the entire period. The
Index Regions comprise only a portion of the summer range of the species and changes in
annual distribution clearly affect the results. Operational changes to the surveys, particularly
in defining pilot whale groups, may also have introduced biases. Recommendations for future
monitoring of the long-finned pilot whale population are provided.
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North Atlantic Sightings Surveys (NASS) are a series of internationally co-ordinated
cetacean surveys that have been conducted in 1987, 1989, 1995, 2001, 2007 and
2015. The initial surveys were organized under the auspices of the Scientific
Committee of the International Whaling Commission (IWC), while surveys after
1989 were planned by the Scientific Committee of the North Atlantic Marine
Mammal Commission (NAMMCO), with formal oversight by the IWC Scientific
Committee. Although sightings of all cetacean species were recorded, the target
species of the surveys have been: fin whales (Balaenoptera physalus) (Iceland and
Spain), common minke whales (Balaenoptera acutorostrata) (Iceland, Norway,
Greenland, Faroe Islands), sei whales (Balaenoptera borealis) (Iceland 1989) and
long-finned pilot whales (Globicephala melas) (Faroe Islands). The spatial and
temporal extent of the surveys, and to some extent the survey and analytical methods
employed, were optimized to the extent feasible for the target species. Ships were
used in most areas, however the coastal areas of Iceland and Greenland were covered
by aircraft. In 2007 the Cetacean Offshore Distribution and Abundance in the
European Atlantic (CODA) survey was conducted in offshore European waters
(Hammond et al., 2009). This survey was planned in conjunction with the 2007
Trans-NASS (T-NASS) so that the survey areas were contiguous and the survey
methodologies were compatible.
The long-finned pilot whale is an oceanic species that occurs in offshore as well as
coastal areas (Buckland et al., 1993). They are very widely distributed in the North
Atlantic, from about 35o - 65o N in the west and from about 40o - 75o N in the east
(ICES, 1996; NAMMCO, 1998a, 1998b). While there is little evidence of extensive
migrations, their distribution does change on a seasonal basis, probably in relation
to the abundance of their principle prey, which consists mainly of squid of several
species (ICES, 1993, 1996; Payne & Heinemann, 1993; Zachariassen, 1993; Hátun
& Gaard, 2010; Sigurjónsson, Víkingsson & Lockyer, 1993).
The long-finned pilot whale has been the object of a drive hunt in several areas of
the North Atlantic, including the Faroe, Shetland and Orkney Islands, Iceland,
Greenland, the eastern USA and Newfoundland in Canada (Joensen, 1976; Bloch,
1994a; Nelson & Lien, 1996). This has most notably occurred in the Faroe Islands
where the hunt has been sustained for several hundred years and continues to this
day (Hoydal, 1985; Bloch, 1994b; Faroe Islands, 2017).
The most recent assessment of the long-finned pilot whale in the northeast and
central North Atlantic was conducted by NAMMCO in 1997 (NAMMCO, 1998b).
At that time, it was concluded that the Faroese drive hunt is likely sustainable at
current levels, given the estimated abundance and evidence that the population
exploited in the hunt was recruited from a large area, rather than an insular stock.
This evidence included high inter-annual variability in distribution and catch around
the Faroe Islands, and the variation in pollutant loads and parasite burdens between
schools of long-finned pilot whales taken in the Faroese drive fishery (NAMMCO,
1998b). This assessment was based heavily on the abundance estimate from the 1989
NASS, the only survey so far to cover a large proportion of the summer distribution
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of the species. Given that both the survey and the assessment are now dated, this
paper reports a response to NAMMCO Scientific Committee recommendations in
2008 and in 2011 that indexes of relative abundance be developed and applied to the
area that is common to all or several surveys, with the aim of determining trends in
abundance over the full period of the NASS.
MATERIALS AND METHODS
Survey design and field methodology
The survey design and field methods used in the NASS have been described
elsewhere (Sigurjónsson, Gunnlaugsson & Payne, 1989; Sigurjónson,
Gunnlaugsson, Ensor, Newcomer & Víkingsson, 1991; Sigurjónson, Víkingsson,
Gunnlaugsson & Halldórsson, 1996; Joyce, Desportes & Bloch, 1990; Desportes et
al., 1996, 2001; Desportes & Halldórsson, 2008; Gunnlaugsson et al., 2002;
Víkingsson, Gunnlaugsson, Halldórsson & Ólafsdottir, 2002; Víkingsson,
Ólafsdottir & Westerberg, 2008; Gunnlaugsson, 2008; Mikkelsen, 2008a; Øïen
2009; Pike et al., in press-a, in press-b) and will not be repeated in detail here.
Stratification for all surveys is shown in Figure 1 and Table 1. Surveys up to and
including 1995 were stratified into smaller blocks than later surveys. For example,
the 1987 survey had 17 strata compared to 9 in 2007 for a somewhat greater area. As
knowledge of the distribution of the target species was accumulated, there was no
justification for retaining some of the finer divisions.
Stratum areas were re-estimated in the Albers Equal Area Conic projection using
MapViewer GIS software (version 8, goldensoftware.com).
Figure 1. Base stratification, survey effort (BSS<5) and sightings of long-finned pilot whales. Symbol
size varies with group size from 1 to 500. Sightings outside of the survey area in 2007 were made by
extension vessels. The Index Areas are outlined in blue (6-SIR) and red (3-SIR).
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Table 1. Features of NASS 1987 - 2015. K – number of transects, GM – long-finned pilot whales,
certain sightings, GM? – long-finned pilot whales, uncertain sightings, GMP – sightings from primary
Up to 1995, an equal-spaced zig-zag transect design was used in all strata except the
large Faroese block EA in 1995. In 2001, 2007 and 2015, the large strata west of
Iceland and the Icelandic shelf (in 2015) were covered by vessels that were
simultaneously carrying out a fish survey. An equal-spaced parallel transect design
was used in these areas. There was also some deviation from designed tracklines in
all years, but especially in 2001 to the north and east of Iceland, primarily due to
adverse weather and time constraints.
Field methodology changed over the course of the surveys as new methods were
developed and tested. The main focus of methodological evolution has been to
account for the bias associated with visible whales being missed by observers
(perception bias) and with whales approaching or fleeing from survey vessels
(responsive movement). The early surveys (1987 and 1989) were conducted with a
single combined observer platform incorporating observers on the bridge roof and 1
in the crow’s nest, in passing mode with delayed closing on some sightings to
confirm species identification and group size estimates. In 1995, the Faroese vessel
used a Buckland and Turnock (BT) survey mode (Buckland & Turnock, 1992) which
uses asymmetric platforms with one-way independence (i.e. the tracker platform is
aware of primary platform sightings but not vice-versa) with the tracker platform
surveying farther ahead of the vessel. Duplicate identification was performed in the
field by a dedicated observer on the tracker platform, based on coincidence in
sighting times, angles, species ID and group size. This method was used to estimate
the proportion of visible whale groups missed by the primary observers, and to
determine bias due to responsive movement, however, we do not make use of these
data in this way since they are not available for all surveys. Other vessels in 1995
used the same methods as used in earlier surveys. In 2001 and 2007, the BT method
was used on all survey vessels. A full description of the methodology is provided in
Pike et al. (in press-a). In 2015, all vessels again used double platforms, but the
platforms were symmetrical and independent from one another and tracking was not
carried out. On the Icelandic vessel, the platforms were stacked vertically, while they
were side-by-side on the Faroese vessel. A complete description of the methodology
used in 2015 is provided in Pike et al. (in press-b).
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The switch to double platform methods after 1995 does have implications for the
interpretation of these data even if the sight-resight data are not utilized because the
later surveys had more observers and thus greater observing power than the earlier
ones. Up to and including 1995, the survey vessels operated with 3 or 4 observers
operating as a single platform (except on the Faroese vessel in 1995, which had 5
observers divided between 2 platforms). After 1995, all vessels had 5 or 6 observers
divided between 2 platforms. We therefore derived index abundance values from the
later surveys in 2 ways: 1) using all unique sightings from both platforms, and 2)
using sightings from the primary platform only. For the 2015 survey, in which both
platforms were equivalent, we averaged the single platform estimates from both
platforms to obtain a primary platform estimate.
Group size estimation can be problematic for this species because long-finned pilot
whales can form large dispersed aggregations that contain many sub-groups.
Therefore, defining a “group” for the purpose of making a sighting and measuring
distance to the group centroid can be challenging. In general, there was a greater
emphasis on defining sub-groups as sightings in surveys conducted after 1989. As
this might influence the estimate of line transect abundance of individuals, we also
looked at the abundance of schools (clusters) as an index of relative abundance.
The general approach was to define the largest possible area that was covered by all
surveys (Figure 1), hereafter referred to as the Survey Index Region (SIR). This area
in turn was divided into East (E) and West (W) sub-regions. Existing strata that
overlapped with these index regions were divided into portions inside the index
region and portions outside.
The size of the SIR covered by all 6 surveys (6-SIR) was limited in the west by the
extent of the 1987 survey and in the east by the extent of the 2001 survey. Therefore,
an additional post-stratification using only the 1989 and 1995 NASS and the
combined 2007 T-NASS and CODA surveys was carried out (3-SIR, Figure 1). This
required the addition of block 1 from the CODA survey to the NASS effort. As the
CODA survey was conducted using a field method slightly different from that used
in the concurrent T-NASS 2007, the CODA data were analysed separately from the
T-NASS data, using methods identical to those described below.
For some sightings there was uncertainty in species identification. Sightings were
categorized according to the degree of certainty as High (GM), Medium (coded with
one question mark: GM?) and Low (coded with two question marks: GM??). As
there were very few sightings (17) of lower certainty than GM?, these were omitted
from the analysis.
For the 1995 (Faroese), 2001 and 2007 surveys that used BT methodology, duplicate
identification was performed in the field by a dedicated observer on the tracker
platform, based on coincidence in sighting times, angles, species ID and group size.
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In high density areas, duplicate identification was performed post-survey based on
the recorded data. Duplicate certainty was classified as definite (90% likely),
possible (>50% likely) or remote (<50% likely), with only the first 2 categories
included as duplicates in the analysis.
For the 2015 survey, which used symmetrical platforms, duplicates were sometimes
identified in the field if the vessel closed on the sighting. Otherwise, duplicates were
identified later in the day or post-survey. This was done by: 1. Similarity of sighting
location taking into account the time interval between the sightings, and; 2.
Similarity of species identification and group size. Duplicates were classified as
definite (D) or remote as above. When one platform had a low confidence species
identification while the other had a high or medium confidence identification, the
duplicate was classified as L. When one platform had an undefined species or a
different species from the other platform, the duplicate was classified as B. For the
purposes of abundance estimation, only D and L duplicates were retained.
In some cases, Beaufort Sea State (BSS) was recorded as a range (e.g. 1-2) or as a
decimal value (e.g. 2.5). Range values were converted to decimal values at the
midpoint of the range. Only effort and sightings of BSS less than or equal to 4 were
retained for this analysis, in conformity with most previous abundance estimates for
this species (Buckland et al., 1993; Burt & Borchers, 1997; Pike, Gunnlaugsson,
Vikingsson, Desportes & Mikkelsen, 2003; Paxton, Gunnlaugsson & Mikkelsen,
Visibility was recorded variably among surveys but was converted to a common
scale as follows: 0=>5 nm, 1=2-5 nm, 2=1-2 nm, and 3=<1 nm. Only effort and
associated sightings made at visibility 0-2 were retained for this analysis.
As noted above, for surveys that used double platforms, analyses were performed
using combined unique sightings from both platforms (C), and using only primary
platform sightings (P). In cases of duplicate sightings between the tracker and
primary platforms, distance measurements from the tracker platform were
considered more reliable and therefore preferred. For the 2015 survey, what were
considered to be the most reliable measurements were used, such as where one
platform had higher confidence in species identification or noted more cues.
In 2015, in strata covered by the combined cetacean/fisheries research vessel, some
cetacean survey effort was maintained while ferrying between transects, resulting in
some transects that paralleled the coast of Iceland or Greenland. As these transects
were aligned with suspected gradients in long-finned pilot whale density, their
inclusion could result in positively biased estimates (Pike et al., in press-b).
Therefore, sightings from these “compromised” transects were not included in the
encounter rate. However, sightings from these transects were included in the
estimation of the detection functions.
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Density and abundance were estimated using stratified line transect methods
(Buckland et al., 2001) using the DISTANCE 6.2 (Thomas et al., 2010) software
package. Abundance was estimated first using the original strata (Figure 1), then
using the post-stratified blocks and the same model for the detection function. The
perpendicular distance data were truncated such that 10% of the greatest distances
were discarded. This was maintained in all analyses for consistency across surveys.
Past abundance estimates optimized for individual surveys (e.g. Buckland et al.,
1993; Borchers, Burt and Desportes, 1996; Burt and Borchers, 1997; Paxton, et al.,
2009; Pike et al., in press-a, in press-b) may have used different truncation distances.
The Hazard Rate and Half Normal functions were considered for modelling the
detection function, and the final model was chosen by minimization of Akaike's
information criterion (AIC) (Buckland et al., 2001). Covariates affecting only the
scale component of the detection function were considered for inclusion in the model
to improve precision and reduce bias (Thomas et al., 2010) and AIC was again used
to compare models. The following covariates were considered: BSS (as recorded and
in 2 (0-2, 3+) and 3 (0-1, 2, 3+) level classifications), vessel identity (actual and with
Faroese and Icelandic vessels combined), weather code, visibility, number of
observers, and platform ID. For the post-stratified data, index area (in or out) was
also tried. Additional covariates for sea swell and cue type were available for the
Regression of the natural log of group size (ln(s)) against estimated detection
probability was used to determine if there was size bias in group detectability. If this
regression was significant at the P<0.15 level, the detection of groups was
considered to be size biased and the estimate of mean group size was adjusted using
this regression; otherwise, the simple mean of group size was used.
Abundance estimates for the index areas were generated operationally by zeroing the
surface areas of strata lying outside of the index areas.
Rates of change in school and animal abundance in the 3-SIR and 6-SIR were
calculated using log-linear regression, and confidence intervals for the rates of
change were estimated using a parametric bootstrapping procedure, assuming a log-
normal distribution for the abundance estimates. This procedure was also used as a
power analysis by simulating a series of survey estimates with a known growth rate
and the observed variance for each survey to determine the least negative growth
rate that could be detected.
Sightings and distribution
Long-finned pilot whale sightings by stratum are summarized in Table 1. The total
number of sightings ranged from 191 in 2015 to 66 in 1995. The distribution of long-
finned pilot whales varied considerably between surveys, but they were widespread
in offshore areas throughout the survey area south of 66º N (Figure 1). The 1989
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survey, which was conducted relatively later in the season and extended further south
than the others, revealed an area of high density directly south of Iceland, south of
56º N. This area was not well covered in any other survey.
Long-finned pilot whales were most commonly sighted in the southern portion of
CODA block 1 (Rogan et al., 2017), south of 55° N. This area borders on the
southernmost Faroese block surveyed the same year, however, this block was very
Figure 2. Frequency distribution of long-finned pilot whale school size by survey.
The early surveys, particularly those carried out in 1987 and 1989, tended to record
larger groups more frequently than did the later ones (Figure 2). Mean group size
generally ranged from 1 to 50 in most strata except in 1987 when it was higher in
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some areas. In 1987 the Faroese vessel recorded a very high average group size of
118 (Figure 3), significantly (t-test, P<.05) greater than that observed in any other
year or vessel classification. Group size was not significantly correlated with
detection probability so mean group size was used in analyses. There was
significant variation in mean group size among strata in every survey, so group size
was not pooled over strata.
Figure 3. Mean group size for Faroese (F), Icelandic (I), and all (ALL) survey vessels.
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Estimation of effective strip half-width
Truncation of the greatest 10% of distances resulted in an absolute strip width (W)
ranging between 1,200 and 2,073 m, except for 2001, which was higher at 2,744 m
(Table 2). Restricting the dataset to primary platform sightings only reduced W
substantially in some years.
Table 2. Features of detection function models. PLAT – Platform, C – Combined, P – primary only;
W – right truncation distance; esw – effective strip half-width; HZ – hazard rate; HN – half-normal;
COS – cosine; BSS – Beaufort sea state; BSS2 – Beaufort sea state, 2 levels (0-2, 3+) ; VESS – vessel
identity; VESS2 – vessel, Faroese or Icelandic; VIS – visibility.
Specifications of the models used to fit the detection functions are given in Table 2.
Of the covariates tested, only BSS (as recorded and in a 2-level classification), vessel
identity (by ship or classified as Faroese or Icelandic) and visibility improved the fit
of some models. For the NASS and T-NASS, effective strip half-width (esw) for the
combined platform models ranged from a low of 459 m in 1995 to a high of 794 m
in 2001 and did not show any trend with survey year (Figure 4). Restricting the data
to primary platform sightings only resulted in a reduction of esw. Effective strip half-
width for the CODA survey was lower than that for any NASS or T-NASS, and
restriction to primary platform sightings only reduced esw still further.
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Figure 4. Effective strip half-width (m) for a) all sightings and b) primary platform
sightings only from the 1995, 2001, 2007, and 2015 surveys.
Regional estimates and trends
Estimates of long-finned pilot whale abundance for the 6-SIR are provided in Table
3 and Figure 5, while those for the 3-SIR are provided in Table 4. For the 6-SIR,
abundance was greater in the western (W) subregion than in the eastern (E) in all
survey years except 1995. Abundance in the E subregion was lowest in 2007 and
highest in 1995 or 2015 (using combined platforms) but there was no significant
difference between survey years. Abundance in the W subregion was highest in 2015
and lowest in 2001 but again there was no significant difference between survey
years. Total regional abundance was lowest in 2001 and highest in 2015, almost 5
times higher than in 2001 but nevertheless not significantly so (P>0.05).
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Table 3. Abundance estimates for the 6 Survey Index Regions. PLAT – platform, C (combined) or P (primary); n – number of sightings; esw – effective strip half-width;
DS– density of schools; DI – density of individuals; N – number of individuals; LCL and UCL – upper and lower 95% confidence limits.
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Figure 5. Long-finned pilot whale abundance by 6 Survey Index Region (E, W, ALL). C – Combined platforms
used on 1995-2007 surveys. P – Primary platforms only used on 1995-2007 surveys.
Restriction of the data to only primary platform sightings had a variable effect on the estimates.
For 2015, the estimate for the primary platform was 38% lower than that for the combined
platforms, while it was 16% higher in 1995.
Point estimates of population growth rates for the E, W, and Total regions of the 6-SIR were
variable but not significantly different from 0 over the period for any area (Table 5). Power
analysis indicated that decrease rates from -0.02 to -0.04 would have been detectable in all
areas over the period.
Estimates of long-finned pilot whale school density are provided in Table 3 and Figure 6.
While school density was generally higher in 2015 than any other year in most cases, there
was no monotonic trend over the period. The rate of growth in school density ranged from
2.2% to 4.5% and was significantly different from 0 in most cases (Table 5). Power analysis
indicated that school density decrease rates from -0.03 to -0.04 would have been detectable
over the period.
For the 3-SIR (Table 4 and Figure 7), abundance was higher in the E subregion than in the W
in all years. Abundance generally declined over the period in the E, W, and Total regions at a
rate ranging from -0.02 to -0.04 (Table 5) for both the combined and primary platform
estimates. However, in no case was the rate of decline significantly different from null
(P>0.05). Power analysis indicated that a rate of decline ranging from -0.03 to -0.05 would
have been detectable by these surveys over the period.
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Table 4. Abundance estimates for the 3-SIR. 1esw for CODA is 403 m (CV 0.12). 2esw for CODA is 277 m (CV 0.34). PLAT – platform, C (combined) or P (primary); n
– number of sightings; esw – effective strip half-width; DS – density of schools; DI – density of individuals; N – number of individuals; LCL and UCL – upper and lower
95% confidence limits.
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Table 5. Annual growth rate (G) of abundance estimates for the 3 and 6 survey index regions.
SURVEYS – 6 or 3 survey post-stratification; OBJECT – I – individual whales, S – whale schools;
PLAT – platform, C (combined) or P (primary); LGR – Least negative growth rate detectable.
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Figure 6. Long-finned pilot whale school density (no. nm-2) by 6 Survey Index Region (E, W, ALL).
C – Combined platforms used on 1995-2007 surveys. P – Primary platforms only used on 1995-2007
Figure 7. Long-finned pilot whale abundance by 3 Survey Index Region (E, W, ALL). C – Combined
platforms used on 1995-2007 surveys. P – Primary platforms only used on 1995-2007 surveys.
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School density (Table 4, Figure 8) showed even less change over the period and in
no case was the growth rate significantly different from null (P>0.05) (Table 5).
Power analysis indicated that a rate of decline ranging from -0.04 to -0.05 would
have been detectable at the P<0.05 level over the period.
Figure 8. Long-finned pilot whale school density (no. nm-2) by 3 Survey Index Region (E, W, ALL).
C – Combined platforms used on 1995 and 2007 surveys. P – Primary platforms only used on 1995-
and 2007 surveys.
DISCUSSION AND CONCLUSIONS
The main focus of this paper is to develop and use an index of relative abundance to
determine if long-finned pilot whale abundance in the eastern and central North
Atlantic may have changed over the 28-year period from 1987 to 2015, a period over
which 6 large-scale surveys have been conducted. Over this period, field methods
have changed considerably to accommodate the data demands of new analytical
techniques that eliminate some of the biases associated with earlier methods. We use
Conventional Distance Sampling to provide a “lowest common denominator”
estimate of relative abundance, as the data available from all the surveys supports
this approach. Mark-Recapture Distance Sampling would be possible using data
from surveys carried out after 1995, but surveys prior to and including 1995 did not
use independent double platforms, precluding this approach. Bias-corrected
estimates from the 2007 and 2015 surveys are provided by Pike et al. (in press-a, in
In addition to the analytical issues, the spatial extent of the survey coverage varied
greatly from survey to survey. This necessitated the delineation of a common index
region that was covered by all the surveys, here referred to as the 6 Survey Index
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Region or 6-SIR. A second and larger 3 Survey Index Region (3-SIR) was covered
by 3 of the surveys spanning the period 1989-2007.
All of our estimates are negatively biased because we did not account for visible
whales that were missed by observers (perception bias) or whales that were
submerged and invisible to observers (availability bias). A central assumption of our
trend analysis is that these biases remain constant or at least do not have a temporal
trend. However, changes in survey methodology and experience gained over 28
years and 6 surveys may have changed the efficiency of the surveys, and it is worth
exploring what effects this may have had on the apparent trends in relative
Number of observers and perception bias
It is reasonable to assume that increasing the number of observers will increase the
proportion of visible whales that are sighted (i.e. reduce perception bias). The early
NASS (1987 and 1989) used 3 or 4 observers, with 1 in a high barrel and 2 or 3 on
the primary platform. Both of these stations were combined into 1 platform for the
purposes of analysis. In 1995 the Icelandic vessels used 4 observers while the
Faroese vessel used 5, 3 on a tracker platform and 2 on the primary platform). In
2001 and 2007, all dedicated vessels used 5 or 6 observers, 3 or 4 on the tracker
platform and 2 on the primary platform. Although 1 or 2 of the observers on the
tracker platform were not dedicated observers but acted as data recorders and
duplicate identifiers, they likely improved observer performance by minimizing
distractions for the dedicated observers on the tracker platform. In 2015, each
platform used the same protocol and had at least 2 observers on effort at all times.
Therefore, while the total number of observers has increased over the course of the
NASS from 3 or 4 in the early surveys to as many as 6 in the later surveys and the
CODA survey, the number of observers on the primary platform has actually
decreased from at least 3 in the early surveys to 2 in the 2001, 2007, and 2015
surveys. All other factors excluded, we might therefore expect an increase in survey
efficiency over time for the combined platforms and perhaps a decrease for the
primary platforms only. The single platform estimates were lower for all surveys
except 1995, suggesting that including additional observers does reduce perception
bias. Trends in regional abundance followed roughly the same pattern using either
combined or single platform estimates (Figure 5).
Pike et al. (in press-b) estimated perception bias for the combined platforms in the
2015 survey as 0.74 (CV=0.09). Including platform identity in the conditional
detection function (which was not supported in the final model as determined by
AIC) resulted in single platform bias estimates of 0.43 to 0.52. Similarly, Pike et al.
(in press-a) estimated perception bias as 0.52 (CV=0.44) for the primary platforms
of the 2007 survey, and Rogan et al. (2017) estimated a primary platform bias of
0.52 for the CODA survey in the same year. These results suggest that perception
bias is relatively high for this species, with half or more visible sightings missed by
a single platform. It also suggests that using sightings from the combined platforms
will result in estimates 20-30% higher than using a single platform estimate.
Unfortunately, using these data, we can reach no conclusion as to whether the
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sighting efficiency of the primary platforms has increased or decreased over the
course of the surveys, as bias is impossible to estimate for surveys up to and
Group size estimation
If it is correct that a) the estimation of group size had been consistent for all surveys,
and b) actual average long-finned pilot whale group size had not changed over the
period, we would expect the temporal patterns of long-finned pilot whale group
density and individual density to be the same. That they are not (see Figures 6 and
7) suggests that one or both of these assumptions is false.
Group size estimation is problematic for this species as they form large dispersed
groups (Figure 9), often with many apparent “sub-groups”, depending on the
operational definition employed. The first whales from such an aggregation that are
detected by observers will naturally tend to be close to the vessel and hence the centre
of the aggregation might be measured as being closer to the vessel than it actually is.
Mean school size may tend to be overestimated because larger schools are seen more
easily than smaller ones. The usual method of correcting this bias, by adjustment
using the regression of school size and detection probability is not effective because
the distances to the group centres
are not accurate, and this leads to
a lack of correlation between
group size and sighting distance
(Buckland et al., 1993). In this
study there was in no case a
significant (P<0.15) correlation
between group size and detection
probability. Both of these factors
(underestimation of distances to
actual group centres and
overestimation of mean group
size because of greater visibility
of large groups) could lead to
positive bias in the abundance
Another related issue arises because of the difficulty in counting the number of
individual long-finned pilot whales in a group. Long-finned pilot whales exhibit non-
synchronous diving behaviour at sea and therefore the number visible to an observer
at any one time will be less than the real group size. Group size can be accurately
estimated only by viewing the group at close range for an extended period of time.
Various strategies have been employed to overcome these difficulties. After 1987,
more effort was put into recording accurate distances to long-finned pilot whale sub-
groups, which were designated as “sightings” in the datasets. While sub-groups’
affinities to larger aggregations were also noted, features of these larger aggregations
were not used in abundance estimation. In addition, greater emphasis was put on
Figure 9. Long-finned pilot whales often occur in large
tightly-packed schools, making the estimation of group
size challenging. Photo credit: Faroese Museum of
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recording distances to all sub-groups that were relatively close (i.e., within about
1,000 m) to the vessel.
Some effort was made to better quantify sub-group size by closing on a random sub-
sample of groups. This practice was carried out in 1989 (Joyce, Desportes & Bloch,
1990) and in 1995 (Desportes et al., 1996). In many cases, closing resulted in
revision of the number of groups sighted, usually increasing from one to two or more
groups. Usually the “confirmed” group sizes were larger than those estimated in
passing mode. For example, in 1995, in those 5 cases where only 1 group was sighted
both before and after closing, the mean ratio of confirmed group size to initial group
size was 1.86 (CV=0.77). However, it is less than clear whether or not these data
were used to correct group sizes estimated in passing mode. There is no indication
that Buckland et al. (1993) used data from closings to correct group size estimates in
their work on the 1987 and 1989 surveys. While Borchers et al. (1996) used group
size estimates from closings in their abundance estimate for the Faroese blocks from
1995, Burt & Borchers (1997) apparently did not. We did not use these data as they
are not available consistently from all surveys, or even for all vessels within surveys.
In 2015, an attempt was made to use a drone aircraft equipped with a video camera
to record some long-finned pilot whale groups, for comparison of group size
estimation methods, however, results from this experiment have not been reported.
Group sizes in the 1987 survey were strikingly larger than in any other year, and this
difference is primarily attributable to the very high group sizes recorded by the
Faroese survey vessel in that year (Figures 2 and 3). Group size estimates by the
Icelandic vessels show a slight declining trend up to 2007 with an increase in 2015,
and overall group sizes declined up to 1995 and exhibited no trend thereafter. The
main contributing factor to this decline is the decreasing frequency of recording very
large groups (Figure 2). Assuming that the lower group sizes recorded by the Faroese
participants after 1987 did not reflect an actual decrease in long-finned pilot whale
group sizes in the area, at least 2 factors may have contributed to the difference.
Firstly, the Faroese observers likely defined groups differently in 1987 than in other
years, as larger aggregations rather than smaller sub-groups. Evidence for this is that
the density of groups was lower in the Eastern (primarily Faroese) index region in
1987 than in any other year (Figure 6) whereas the density of animals was not. This
means that the observers in this area recorded fewer larger groups than in other years,
which probably indicates that they defined groups differently at that time.
Secondly, it appears that the Faroese observers may have used a traditional heuristic
derived from the drive fishery, which assumes that the number of whales on the
surface - multiplied by ten - approximates the number of whales in the school
(Desportes et al., 1996). While this “rule of thumb” may hold at least proximately
for compact groups of agitated whales being driven into shore, it appears not to work
for the much more dispersed aggregations seen at sea. In this respect it is interesting
to note that the mean group size observed by the Faroese vessel in 1987 is roughly
ten times that observed in other years. Some of the observers used in 1987 were
experienced whalers, but none had much experience in whale surveys because few
had been conducted in the area prior to that time. After 1987, a much greater
emphasis was placed on getting more accurate group size estimates and group size
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estimates presumably improved as the observers gained experience. This may
account for the overall decline in group size estimates seen over the course of the
NASS surveys, as observers became more adept at and committed to discriminating
smaller long-finned pilot whale sub-groups within overall aggregations.
Taken together, the difficulties in estimating group size inherent with this species
probably did not have much influence on the trend analysis of individual long-finned
pilot whale abundance estimates presented here. Group size estimates are reasonably
consistent except in 1987 as noted. Field methods used since that time have not
changed greatly, so the inherent biases should be relatively constant. Assuming that
the differences observed are mainly the result of different levels of splitting of
subgroups among observers, vessels and surveys, they should be compensated by
concomitant changes in the size of the groups in the estimates of abundance.
Nevertheless, the problems observed may mask real trends in long-finned pilot whale
group structure and size. It is obvious that further work needs to be done to obtain
accurate group size estimates for long-finned pilot whales. Some recommendations
are as follows:
1. Future surveys should rigorously define the meaning of “group” in the field
protocol. For the 2007 survey, a group was defined as “... the smallest unit
that can be tracked. A convenient rule is to define a group as containing
individuals not more than 2-3 animal lengths from each other. The group
may be exhibiting the same swimming pattern and/or general behaviour such
as travelling, milling or resting, although not necessarily with a synchronised
surfacing pattern” (Anonymous, 2007). Unfortunately, this definition does
not appear to have been documented before 2007 so we are uncertain how
groups were defined prior to that.
2. The importance of discriminating and counting all groups, especially those
close to the vessel, should be reiterated and emphasized to observers. Groups
far from the vessel should be ignored if they interfere with the accurate
recording of closer groups.
3. All vessels should close on a random subsample of sightings to obtain
accurate counts of group sizes, and the correction factors so obtained, with
associated variance, should be used in abundance estimates. Alternatively, a
subsample of “true” group sizes could be obtained using a drone or a
simultaneous aerial survey in the same area.
As long-finned pilot whale density is likely to vary over large spatial scales and
distribution does evidently vary from year to year (Figure 1), increasing the size of
the Index Region should provide a more accurate reflection of trends in abundance
over the species’ range. We therefore attempted to maximize the extent of the Index
Region by limiting the analysis to 3 surveys only. This provided a substantial
southern extension of the E subregion, and a more modest extension in the W (Figure
1). The overall effect of this was to decrease the magnitude of the trend in observed
abundance in the 6-SIR (up to 2007), showing a negative but non-significant trend
over the period (Figures 7 and 8).
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Ideally, if detecting trends in abundance is a major objective of the NASS survey
series, all surveys should cover a common core area and that area should be as large
as possible to capture the summer range of the target species. However, we recognize
that effort is limited by available funding and that extending the covered area without
a proportional increase in effort would result in a decrease in precision for abundance
All NASS (except that of 1989) have been conducted within the period from late
June until the first week of August. This was considered optimal for the target species
of common minke and fin whales based primarily on whaling records and other
observations (e.g., Rorvik, Jónsson, Mathiesen & Jonsgard, 1976; Sigurjónsson,
1982; Sigurjónsson & Víkingsson, 1997). In 1989 the participants decided to extend
the coverage area farther south and conduct the survey later in the season, primarily
in the hope of obtaining a better estimate for sei whales, which tend to arrive later in
the season in northern areas (Sigurjónsson et al., 1991). That survey began on 10
July and concluded on 13 August, about 1-3 weeks later than the other NASS.
It is difficult to say what effect, if any, the later timing of the 1989 survey had on the
relative abundance of long-finned pilot whales in the Index Regions. Much of the
index region is at the northern limit of the range of long-finned pilot whales in this
area, so one might expect more whales in the area later in the summer. This
expectation is supported by the seasonal distribution of long-finned pilot whale catch
events in the Faroes (referred to as grind in Faroese), which peaks in August
(Zachariassen, 1993; Bloch, 1994b). However, the estimate for 1989 for the 6-SIR
(Figure 5) is not exceptionally large and is very similar to that for 1987. The estimate
for the 3-SIR for 1989 (Figure 7) is greater than those for other years, but this is
attributable mainly to larger numbers seen in the southern part of the index area.
Overall the evidence for a seasonal effect on abundance is equivocal.
The spatial extension of the 1989 survey farther south of Iceland did have a large
effect on the estimate of abundance, as the far southern blocks contributed heavily
to the abundance estimate for that year (Buckland et al., 1993). However, this did
not affect the estimates for the Index Regions. The estimate for 2015 is nearly as
large and that survey did not extend far to the south (Pike et al., in press-b). This
confirms that the seasonal distribution of long-finned pilot whales varies
substantially from year to year.
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Whales that are submerged and not visible to observers are not counted. This
“availability bias” was thought to be small for long-finned pilot whales because they
occur in groups that usually do not dive synchronously (Buckland et al., 1993).
However, more recent data from satellite tagging experiments (Figure 10) indicates
that long-finned pilot whales stay submerged below 7 m depth for a substantial
proportion of their time around the Faroes. Heide-Jorgensen et al. (2002) found that
long-finned pilot whales were submerged below this depth an average of 40% of the
time in July, while Mikkelsen (2008b) observed that the whales spent 75% of their
time below 16 m in fall and early winter. While a group may be detected if diving is
asynchronous and some members are visible at the surface, group size will be
underestimated as some
members will not be visible.
If diving is synchronous,
entire groups will be missed.
Either scenario leads to a
negative bias in estimated
abundance, the magnitude of
which will depend on the
diving cycle of the whales
and the time they are
potentially in the viewing
field of observers on a
passing ship. However, it is
unlikely that the magnitude
of this bias would change
over time, making
uncorrected estimates useful
for detecting trends in
For some species, such as blue whales, uncertainty in species identification is a
serious issue that must be accounted for in analyses (Pike et al., in press-a, in press-
b). However, only 8% of long-finned pilot whale sightings were recorded as
uncertain, suggesting that they are relatively easy to distinguish from other species
at sea. Nevertheless, this proportion has increased from 0% in 1987 to 24% in 2007,
probably because a greater emphasis has been put on recording uncertainty in later
surveys. This probably has little or no effect on estimates of abundance, as almost
all sightings identified as long-finned pilot whales are included in the estimates.
Trends in relative abundance
Relative abundance in the E, W and combined 6-SIR’s appeared to decline over the
18-year period from 1989 to 2007 (Figure 5, Table 3), thereafter recovering to the
highest levels yet seen in 2015. The observed pattern was similar whether combined
or primary platform estimates were used. As a result, there was no significant
population growth rate, positive or negative, in the region (Table 5). Similarly, no
Figure 10. Tagging long-finned pilot whales in the Faroe
Islands. Satellite tags provide valuable information on spatial
and temporal distribution, as well as diving behaviour. The
latter can be used to correct surveys for bias caused by
animals that are diving while the survey vessel passes
(availability bias). Photo credit: Faroese Museum of Natural
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NAMMCO Scientific Publications, Volume 11 ̶ Early Online Version 24
monotonic trend in group density (Figure 6, Table 5) was seen over the period, but
again density in 2015 was higher than in other years.
For the larger 3-SIR used in 1989, 1995, and 2007, a slight and non-significant
decline in animal abundance was observed over the period (Figure 7, Table 5). No
such trend was observable in group density (Figure 8, Table 5). The 2015 estimates
from the western 6-SIR are close to the estimate for the larger western 3-SIR in 1989,
eliminating any possibility of a negative trend in that part of the survey area if the
larger area had been surveyed in 2015.
The analysis of relative abundance in the index areas therefore provides no evidence
of any change in the numbers of long-finned pilot whales over the period 1987-2015.
Power analyses suggest that an abundance decrease rate of -3% to -5% per year
would have been detectable (Table 5). This is, however, optimistic as it requires a
monotonic change over the entire period. The survey series is therefore not very
powerful in detecting trends in abundance for this species. By way of illustration, a
3% annual rate of decrease in the index area over the period would have resulted in
a population size in 2015 nearly 60% lower than that seen in 1987.
The index areas encompass only a relatively small proportion of the summer range
of long-finned pilot whales as revealed by the NASS series (Figure 1), and most of
the NASS do not cover their full range in the northeast and central North Atlantic.
The 1989 NASS showed that long-finned pilot whales occur in large numbers farther
south than surveyed in other years, and even in 1989 pilot whales occurred on the
southern edge of the survey area, suggesting that their range might extend still further
south, as has also been demonstrated by the movements of tagged animals
(Mikkelsen, 2008b). Moreover, it is obvious that the spatial distribution of long-
finned pilot whales does change from year to year. For example, long-finned pilot
whales were concentrated farther north in the western part of the survey area in 1987
than in other years. Such annual variation is also seen in the Faroese catch series,
assuming it reflects the availability of long-finned pilot whales in the local area
(Table 6). This in turn is probably influenced by changes and variability in the marine
climate (Hátun & Gaard, 2010). The flying squid (Todarodes sagittatus), a major
prey species for long-finned pilot whales off the Faroes and Iceland (Desportes &
Mouritsen, 1993; Sigurjónsson, Víkingsson & Lockyer, 1993), occurs in large
aggregations around Iceland, the Faroe Islands, and off the north-western coasts of
Norway in so-called “squid years”. For example, squid were abundant in the area
from the late 1970s to the mid-1980s but have been virtually absent after the mid-
1980s (Hátun & Gaard, 2010). Therefore, the proportion of long-finned pilot whales
occupying the index area may vary greatly from year to year, making the detection
of trends in abundance less powerful. Only surveys that consistently encompass all
or most of the summer distribution range could overcome this problem.
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Table 6. Annual takes of long-finned pilot whales in the Faroese drive hunt. Source: NAMMCO.
While many factors could affect long-finned pilot whale numbers in the area, 1 on
which we have good information is the annual drive hunt conducted in the Faroe
Islands. Catch records from the Faroes go as far back as 1584, and are unbroken
since 1709 (Bloch, 1994a). Catch, corrected for hunting effort, shows a cyclic pattern
with a period of 100-120 years, with peaks in catch occurring in 1720-1730, 1840-
1850, and 1935-1985 (Hoydal & Lastein, 1993). There is no long-term indication of
declining or increasing catch over the period (Hoydal & Lastein, 1993; Zachariassen,
1993; Bloch & Lastein, 1995). Catch since 1987 has varied from 0 in 2008 to 1,738
in 1988, with an average take of 808 (CV=0.55) over the period (Table 6). Given the
minimum size of the population, as indicated by the 1989 survey of over 600,000
animals (Buckland et al., 1993), it seems very unlikely that an annual harvest of
around 1,000 whales could have caused the population to decline. However, the
stock delineation of long-finned pilot whales is uncertain. In 1997, the Scientific
Committee of NAMMCO concluded that it was likely that there was more than 1
stock of long-finned pilot whales in the North Atlantic, and more than 1 stock subject
to harvesting in the Faroe Islands (NAMMCO, 1998b). Therefore, the stock unit or
units that is/are subject to harvesting in the Faroes could be smaller than that
indicated by the maximum total survey abundance. The Eastern sub-Index Regions
may be most relevant to the Faroese harvest and there is no evidence of decline in
either the 3-SIR or 6-SIR (Table 5). The fact that the population has been subject to
approximately the same level of harvest for at least 300 years, with apparently little
change in availability (Hátún & Gaard, 2010), suggests that the Faroese harvest is
probably not causing the stock to decline. The NAMMCO Scientific Committee
concluded in 2012 that a minimum stock size of 50,000 animals would be required
to sustain recent Faroese harvest levels (NAMMCO, 2012).
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Utility of the NASS series for indexing long-finned pilot whale abundance
While long-finned pilot whales have been a specific target of the Faroese component
of the NASS, they have not been for Iceland or other participants. As a consequence,
the surveys have never covered the full geographic range of the species in the
northeast and central North Atlantic. The 1989 survey covered a larger part of this
range, and as a result produced a higher estimate than any of the other surveys.
However, this estimate is now 30 years old and its use as the basis of a conservation
management program is questionable. The more recent 2015 survey covered a
smaller area of potential long-finned pilot whale habitat, however the resultant
abundance estimate is the highest observed since 1989 (Pike et al., in press-b).
The spatial extent and stratification of the NASS have been highly variable. As a
result, the area that has been covered by all surveys equates to the smallest area
covered by any of the surveys. While there may have been valid reasons for these
changes, they are clearly detrimental to monitoring and interpreting trends in
distribution and abundance in the area. In contrast, the Norwegian minke whale
survey has used a common design since 1995 (Skaug, Schweder & Bothun, 2004),
and the Icelandic aerial survey has used the same design since 1987 (Pike et al., in
press-c). If the NASS series is to be continued, maintaining a standard design should
be seriously considered.
A synoptic survey covering the entire range of the long-finned pilot whale in this
area would have to extend even farther to the south than the 1989 survey. Given the
resources available to the participating countries, this is probably not feasible in the
near future. “Mosaic” surveys, in which the total survey area is covered over several
years, are used by Norway for common minke whales (Skaug et al., 2004) and could
be successful for this species. However, the NASS experience suggests that there
may be extreme temporal variation in the distribution of long-finned pilot whales
which would limit this approach.
The authors would like to thank the captains, crews, and observers of all the vessels
used in the surveys for their dedication and hard work. We would also like to thank
Dr. Philip Hammond for allowing us to use the CODA data. Funding for this analysis
was provided by the Faroese Ministry of Fisheries. We thank the Scientific
Committee of NAMMCO for inspiring and encouraging this work.
We dedicate this paper to the memory of our colleague and friend Dorete Bloch,
who led the pilot whale research program in the Faroe Islands with dedication and
enthusiasm. We fondly remember her welcoming and warm nature and not least
her wonderful hospitality and cooking skills.
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NAMMCO Scientific Publications, Volume 11 ̶ Early Online Version 27
Anonymous. (2007). T-NASS: Trans North Atlantic Survey: Information for
Observers. (Available from the NAMMCO Secretariat).
Bloch, D. (1994a). Grindadráp í øðrum londum. Frøði 3, 30-31.
Bloch, D. (1994b). Pilot whales in the North Atlantic. Age, growth and social
structure in Faroese grinds of the long-finned pilot whale, Globicephala
melas. (Unpublished Doctoral dissertation), Lund University, Sweden.
Bloch, D. & Lastein, L. (1993). Morphometric segregation and long-finned pilot
whale in the eastern and western North Atlantic. Ophelia 38, 55-68.
Bloch, D. & Lastein, L. (1995). Modelling the school structure of pilot whales in
the Faroe Islands, 1832-1994. In A. S. Blix, L. Walløe, & Ulltang, U.
(Eds.), Whales, seals, fish and man (pp.499-508). Amsterdam - Lausanne -
New York - Oxford - Shannon –¬ Tokyo: Elsevier.
Borchers, D. L., Burt, M. L. & Desportes, G. (1996). Preliminary estimates of
abundance of long-finned pilot whales (Globicephala melas) from the
Faroese component of the NASS-95 survey. Available from the
NAMMCO Secretariat (unpublished).
Buckland, S. T. & Turnock, B. J. (1992). A robust line transect method. Biometrics
48, 901-909. https://doi.org/10.2307/2532356
Buckland, S. T., Anderson, D. R., Burnham, K. P., Laake, J. L., Borchers, D.L. &
Thomas, L. (2001). Introduction to Distance Sampling. New York: Oxford
Buckland, S. T., Bloch, D., Cattanach, K. L., Gunnlaugsson, T., Hoydal, K., Lens,
S. & Sigurjónsson, J. (1993). Distribution and abundance of long-finned
pilot whales in the North Atlantic, estimated from NASS-1987 and NASS-
89 data. Report of the International Whaling Commission (Special Issue
Burt, M. L. & Borchers, D. B. (1997). Pilot whale abundance in the North Atlantic,
estimated from NASS-95. Document SC/5/AE/3 for the NAMMCO
Scientific Committee (unpublished).
Desportes, G., Bloch, D., Allali, P., Barbaud, C., Egholm, J., Joensen, M., Leroy,
B., Mougeot, F., Mouritsen, R., Reinert, A. & Sequeira, M. (1996). NASS-
95 - preliminary report of the Faroese cruise. Document SC/4/16 for the
NAMMCO Scientific Committee. (unpublished).
Desportes, G. (1996). Super schools. WP7 submitted to the ICES Study Group on
Long-Finned Pilot Whales, Cambridge, UK, April 22-26, 1996.
Desportes, G., Mikkelsen, B., Bloch, D., Danielsen, J., Hansen, J. & Mouritsen, R.
(2001). Survey report from the Faroese shipboard survey of NASS 2001.
SC/9/17 for the NAMMCO Scientific Committee. (unpublished).
Desportes, G. & Mouritsen, R. (1993). Preliminary results on the diet of long-
finned pilot whales off the Faroe Islands. In G. P. Donovan, C. H. Lockyer,
& A. R. Martin (Eds) IWC Special Issue 14: Biology of Northern
Hemisphere Pilot Whales (pp. 305-324). Cambridge: International
Desportes, G. & Halldórsson, S. D. (2008). Cruise report for Venus & Co.
Pike et al. (2019)
NAMMCO Scientific Publications, Volume 11 ̶ Early Online Version 28
SC/15/TNASS/30 for the NAMMCO Scientific Committee. (unpublished).
Gunnlaugsson, T. (2008). T-NASS 2007 cruise report for vessel Jákup B.
SC/15/TNASS/31 for the NAMMCO Scientific Committee. (unpublished).
Faroe Islands. (2017). Whales and whaling in the Faroe Islands: Catches 2000-
2017. Retrieved from http://www.whaling.fo/en/regulated/450-years-of-
Gunnlaugsson, T., Halldórsson, S. D., Ólafsdóttir, D. & Víkingsson, G. A. (2002).
NASS-2001 Icelandic shipboard survey report. SC/10/AE 10 for the
NAMMCO Scientific Committee. (unpublished).
Hammond, P. S., Macleod, K., Gillespie, D.,...and Castro, R. (2009). Cetacean
Offshore Distribution and Abundance in the European Atlantic (CODA).
Retrieved from http://biology.st-
Hátún, H. & Gaard, E. (2010). Marine climate, squid and pilot whales in the
northeastern Atlantic. In S-A. Bengtson, P. Buckland, P. H. Enckell, & A.
M. Fosaa, A.M. (Eds.), Dorete - her book – being a tribute to Dorete Bloch
and to Faroese nature (pp. 50-68) (Annales Societatis Scientiarum
Færoensis; Vol. 52). Faroe University Press.
Heide-Jørgensen, M. P., Bloch, D., Stefansso, E., Mikkelsen, B., Ofstad, L. H. &
Dietz, R. (2002). Diving behaviour of long-finned pilot whales
Globicephala melas around the Faroe Islands. Wildlife Biology 8, 307-313.
Hoydal, K. (1985). Attempts to use the 274 years' Faroese time series of catches of
pilot whales (Globicephala melaena, Traill) to assess the state of the stock.
International Council for the Exploration Sea Cooperative Research Report
Hoydal, K. & Lastein, L. (1993). Analysis of Faroese catches of pilot whales
(1709-1992), in relation to environmental variations. Report of the
International Whaling Commission (special issue 14), 89-106.
ICES. - International Council for the Exploration of the Sea. (1993). Report of the
study group on long-finned pilot whales. ICES CM 1993/N:5.
ICES. (1996). Report of the study group on long-finned pilot whales. ICES CM
Joensen, J. S. (1976). Pilot Whaling in the Faroe Islands. Ethnologia Scandinavica
Joyce, G. G., Desportes, G. & Bloch, D. (1990). The Faroes NASS 1989 sightings
cruise. SC/42/O11 for the IWC Scientific Committee. (unpublished).
Mikkelsen, B. (2008a). TNASS 2007 - Faroe Islands. Thor Chaser. Preliminary
cruise report. SC/15/TNASS/32 for the NAMMCO Scientific Committee.
Mikkelsen, B. (2008b). Movements and diving of pilot whales in autumn and early
winter. SC/15/PW/07 for the NAMMCO Scientific Committee.
NAMMCO - North Atlantic Marine Mammal Commission. (1998a). Report of the
Scientific Committee Working Group on Abundance Estimates. In
NAMMCO Annual Report 1997 (pp.173-202), Tromsø: NAMMCO.
NAMMCO. (1998b). Report of the fifth meeting of the Scientific Committee. In
NAMMCO annual report, 1997 (pp.85-202), Tromsø: NAMMCO.
Pike et al. (2019)
NAMMCO Scientific Publications, Volume 11 ̶ Early Online Version 29
NAMMCO. (2012). Report of the nineteenth meeting of the Scientific Committee.
In NAMMCO Annual Report 2012 (pp. 263-548). Tromsø: NAMMCO.
Nelson, D. & Lien, J. (1996). The status of the long-finned pilot whale,
Globicephala melas, in Canada. Canadian Field Naturalist 110, 511-524.
Øïen, N. (2009). Distribution and abundance of large whales in Norwegian and
adjacent waters based on ship surveys 1995-2001. NAMMCO Scientific
Publications 7, 31-47. https://doi.org/10.7557/3.2704
Paxton, C. G. M., Gunnlaugsson, T. & Mikkelsen, B. (2009). Density surface
fitting of the T-NASS 2007 pilot whale sightings. SC/17/AE/13 for the
NAMMCO Scientific Committee. (unpublished).
Payne, P.M. & Heinemann, D. W. (1993). The distribution of pilot whales
(Globicephala spp.) in shelf/shelf-edge and slope waters of the northeastern
United States, 1978-1988. Report of the International Whaling
Commission (special issue 14), 51-68.
Pike, D. G., Gunnlaugsson, T., Mikkelsen, B., Halldórsson, S.D., Vikingsson,
G.A., Acquarone, M., & Desportes, G. (in press a). Estimates of the
abundance of cetaceans from the T-NASS Icelandic and Faroese ship
surveys conducted in 2007. NAMMCO Scientific Publications 11.
Pike, D. G, Gunnlaugsson, T., Mikkelsen, B., & Vikingsson, G.A. (in press b).
Estimates of the abundance of cetaceans from the NASS Icelandic and
Faroese ship surveys in 2015. NAMMCO Scientific Publications 11.
Pike, D, G., Gunnlaugsson, T. & Vikingsson, G. A. (in press c). Estimates of the
abundance of cetaceans from T-NASS aerial surveys around Iceland in
2007 and 2009. NAMMCO Scientific Publications 11.
Pike, D. G., Gunnlaugsson, T., Víkingsson, G. A., Desportes, G. & Mikkelsen. B.
(2003). An estimate of the abundance of long-finned pilot whales
(Globicephala melas) from the NASS-2001 shipboard survey.
SC/11/AE/10 for the NAMMCO Scientific Committee. (unpublished).
Rogan, E., Ca-adas, A., Macleod, K., Santos, M. B., Mikkelsen, B., Uriarte, A.,
Van Canneyt, O., Vázquez, J. A., & Hammond, P. S. (2017). Distribution,
abundance and habitat use of deep diving cetaceans in the North-East
Atlantic. Deep Sea Research Part II: Topical Studies in Oceanography 141,
8–19. Retrieved from https://doi.org/10.1016/j.dsr2.2017.03.015
Rorvik, C. J., Jónsson, J., Mathiesen, O. A. & Jonsgård, Å. (1976). Fin whales,
Balaenoptera physalus (L.), off the west coast of Iceland - distribution,
segregation by length and exploitation. Rit Fiskideildar 5(5), 1-30.
Sigurjónsson, J. (1982). Icelandic minke whaling 1914-1980. Report of the
International Whaling Commission 32, 287-95.
Sigurjónsson, J., Gunnlaugsson, T. & Payne, M. (1989). NASS-87: Shipboard
sightings surveys in Icelandic and adjacent waters Jun-July 1987. Report of
the International Whaling Commission 39, 395-409.
Sigurjónsson, J., Gunnlaugsson, T., Ensor, P., Newcomer, M. & Víkingsson, G.
(1991). North Atlantic Sightings Survey 1989 (NASS-89): Shipboard
surveys in Icelandic and adjacent waters July-August 1989. Report of the
International Whaling Commission 41, 559-572.
Sigurjónsson, J. & Víkingsson, G. A. (1997). Seasonal abundance of and estimated
food consumption by cetaceans in Icelandic and adjacent waters. Journal of
Northwest Atlantic Fishery Science 22, 271-287.
Pike et al. (2019)
NAMMCO Scientific Publications, Volume 11 ̶ Early Online Version 30
Sigurjónsson, J., Víkingsson, G., Gunnlaugsson, T., & Halldórsson, S.. (1996).
North Atlantic Sightings Survey 1995 (NASS-95): Shipboard surveys in
Icelandic and adjacent waters June-July 1995. Preliminary cruise report.
SC/4/17 for the NAMMCO Scientific Committee. (unpublished).
Sigurjónsson, J., Víkingsson, G., & Lockyer, C. (1993). Two mass strandings of
pilot whales Globicephala melas on the coast of Iceland. Report of the
International Whaling Commission (Special Issue 14), 407-423.
Skaug, H. J., Øien, N., Schweder, T., & Bøthun, G. (2004). Abundance of minke
whales (Balaenoptera acutorostrata) in the Northeastern Atlantic. Canadian
Journal of Fishery and Aquatic Science 61, 870-886.
Thomas, L., Buckland, S.T., Rexstad, E.A., Laake, J. L., Strindberg, S., Hedley, S.
L. J., Bishop, R.B., Marques, T. A. & Burnham, K. P. (2010). Distance
software: design and analysis of distance sampling surveys for estimating
population size. Journal of Applied Ecology 47, 5-14.
Víkingsson, G. A., Gunnlaugsson, T., Halldórsson, S. D., & Ólafsdóttir, D. (2002).
NASS-2001 Icelandic shipboard survey report. SC/54/O9 for the IWC
Scientific Committee. (unpublished).
Víkingsson, G. A., Ólafsdóttir, D. & Westerberg, U. (2008), Cruise report for R/V
Árni Friðriksson, RE 200. SC/15/TNASS/29 for the NAMMCO Scientific
Zachariassen, P. (1993). Pilot whale catches in the Faroe Islands, 1709-1992.
Report of the International Whaling Commission (special issue 14), 69-88.