wgwap 19/22 rev. western gray whale population assessment … · 2019. 2. 7. · hypothesis 6b...

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WGWAP 19/22 rev.

Western Gray Whale population assessment update with reference to historic range and recovery prospects

Justin G. Cooke1

ABSTRACT The assessment of the Sakhalin Gray Whale aggregation is updated using photo-id collected up to the 2017 season. Data were combined from two ongoing photo-id projects off NE Sakhalin (Burdin et al. 2018; Yakovlev et al. 2018), along with data from Kamchatka collected during 2004-12 (Yakovlev et al. 2013) and 2006-12 from Mexico (Urbán et al. 2012, 2013). Abundance estimates and population trajectories are derived that correspond to each of the various stock structure hypotheses developed by the IWC Scientific Committee’s Range-wide Review of the Population Structure and Status of North Pacific gray whales (IWC 2018, 2019), by fitting an individually-based population model to the photo-id data, supplemented with sex determination data from biopsies (Lang 2010) and results of satellite tagging. (Mate et al. 2015). Abundance estimates for the Western Feeding Group (that migrates in winter to the eastern North Pacific but feeds in the west) in 2017 range from about 150 to 320 whales (aged 1 yr and over) depending on stock structure hypothesis, while abundance estimates for the putative Western Breeding Stock are all less than 100. The abundance of the Western Feeding Group is estimated to have been increasing over the period 1995-2017 for all stock structure hypotheses. The number of gray whales (excluding calves) which regularly visit Sakhalin in summer (regardless of stock affiliation) is estimated to be about 170 (cv 0.07) in 2017, while the number of gray whales which regularly or occasionally visit Sakhalin is estimated to be about 225 (cv 0.04) in 2017. These latter two estimates are insensitive to the choice of stock structure hypothesis.

The areas of potential gray whale feeding habitat (taken as waters 5-50m deep) is computed for the eastern and western North Pacific, and also for the Pacific Arctic and subarctic (Bering and Chuckchi Seas) . The latter region is assumed to contain habitat potentially available in summer to whales from both the eastern and western breeding stocks. The conventional density-dependent assumptions imply that the pre-whaling equilibrium abundances would be proportional to the relative capacities of winter habitats. The historical evidence suggests significant catches by pre-modern whaling in Asian countries over a long period which depleted the western population. The result of fitting the population model to all available data suggests that the western population was large in the past (~25 000 whales) but that the eastern population has expanded as a result of the depletion of the western population. The implications for measures to support the recovery of the western gray whale population have yet to be determined.

1 INTRODUCTION

This document contains two parts. The first part is an update of the annual population assessment of whales feeding off northeastern Sakhalin (Cooke et al. 2017) and, as appropriate, SE Kamchatka, taking account of the various stock structure hypotheses developed by the IWC Rangewide Workshops on North Pacific Gray Whales (IWC 2018). Population trajectories are fitted by applying an individually-based model to photo-id data collected during 1995-2017 from Sakhalin Island (Burdin et al. 2018; Yakovlev et al. 2018), during 2004-12 from SE Kamchatka (Yakovlev et al. 2013) and during 2006-12 in the lagoons of Baja California, Mexico (Urbán et al. 2012, 2013). Sex determinations from biopsies (Lang 2010) and the results of tracking of 3 individuals from Sakhalin Island to the eastern North Pacific are also used (Mace et al. 2015). The technical specifications of the individually-based population model structure and the method of fitting to data are given by Cooke (2018).

The second part places the results in a range-wide and historical perspective. Omura (1988) pointed out that the eastern and western North Pacific present very different environments for gray whales. Potential gray whale feeding habitat (shallow water) is rather limited in the eastern North Pacific, such that the gray whales there have little feeding opportunity in winter, and are dependent on the migration to (sub-)Arctic waters in summer. In the western North Pacific, there are substantial shallow waters that permit year-round feeding of gray whales. A simple three-habitat

1 Centre for Ecosystem Management Studies, Höllenbergstr. 7, 79312 Emmendingen, Germany. Email: [email protected].

WESTERN GRAY WHALE ADVISORY PANEL19th meeting

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population model is fitted to the historical data to reconstruct the pre-whaling abundance in the eastern and western North Pacific and hence throw light on the scope for population recovery.

2 ASSESSMENT OF THE SAKHALIN/KAMCHATKA SUBPOPULATION

2.1 Stock structure hypotheses The stock structure hypotheses developed by the 4th IWC Rangewide Workshop on North Pacific Gray Whales (IWC 2018) which were still under consideration for consideration after the 5th Workshop (IWC 2019) are hypotheses 3a, 3b, 3c, 3e, 5a, 6b listed in Table 6 of JCRM 19(Suppl.):528.

The hypotheses posit the existence of up to two breeding stocks: an eastern breeding stock (EBS) which winters off Mexico, and possibly a western breeding stock (WBS) which winters at one or more locations in Asian waters. The hypotheses also posit the existence within the eastern breeding stock of a western feeding group (WFG) which visits Sakhalin and Kamchatka in summer, and a northern feeding group (NFG) which feeds in the Bering and Chuckchi Seas and possibly also off SE Kamchatka.

Of these, hypotheses 3a, 3c, 3e posit only a single breeding stock, and are mutually indistinguishable with respect to the data collected on western North Pacific gray whales.

Hypothesis 6b posits a western breeding stock (WBS) but it only occurs in areas for which no data are available, and is therefore effectively invisible with respect to the available datasets. With regard to observed whales in the western North Pacific, 6b is indistinguishable from the hypotheses with just one breeding stock (3a/3c/3e).

In order to implement the hypotheses, whales occurring off Sakhalin and Kamchatka were divided into four feeding subgroups, whose numbers are to be estimated from the data:

Table 1. Feeding subgroups defined for the purpose of implementing stock structure hypotheses

The hypotheses and their definitions are listed in Table 2. In all the hypotheses considered by IWC (2018), whales seen off Sakhalin include WFG whales and possibly also WBS whales. Whales seen of SE Kamchatka include WFG whales and possibly also WBS and NFG whales. The hypotheses considered by IWC (2018) do not allow for the possibility that all whales seen off Kamchatka belong to the WFG. In view of the lack of evidence that any Kamchatka whales belong to the NFG, two further hypotheses were added (8 and 9) in which there are no NFG whales off Kamchatka.

Table 2. Definitions of stock structure hypotheses in terms of affiliations of feeding subgroups

Hypothesis Description WBS WFG NFG

3a/3c/3e One breeding stock, some NFG in K S, SK, KS K

3b Two breeding stocks, WBS in K K S, SK, KS

5a Two breeding stocks, some NFG in K S, SK S, SK, KS K

8 One breeding stock, no NFG in K S, SK, KS, K

9 Two breeding stocks, no NFG in K S, SK S, SK, KS, K

2.2 Data

2.2.1 Photo.identification Sampling with photo-identification was conducted in the summer feeding grounds off NE Sakhalin during 1995-2017 and off SE Kamchatka during 2004-12. There were two research teams working off Sakhalin and one off Kamchatka. Off Sakhalin, the Russian Gray Whale Project (RGWP, Pacific Geography Institute, Petropavlovsk, Kamchatka; a continuation of the former Russia-US project) has been collecting data since 1995((Burdin et al. 2018), and the Marine Biology National Research Center , (MBNRC, Vladivostok; formerly Institute of Marine Biology, IBM) has been collecting data since 2002 and cross-matched the results with photos from Kamchatka (Yakovlev et al. 2013, 2018).

Subgroup

S

SK

KS

K

Whales that visit Sakhalin but not Kamchatka

Whales that visit Sakhalin and occasionally Kamchatka

Whales that visit Kamchatka and occasionally Sakhalin

Whales that visit Kamchatka but not Sakhalin

Description



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A total of 287 distinct whales had been identified through the 2011 season, based on cross-matching of the individuals observed by each team. During 2012-2017, a further 65 whales were seen by the RGWP team and 82 whales by the MBNRC and Kamchatka teams, which have not been cross-matched.

Sampling also occurred in the wintering grounds off Baja California, Mexico. Because theses grounds are shared with the more numerous eastern North Pacific gray whales, only individuals matched with the western North Pacific were included in the analysis. Cross-matching was conducted between whales identified in Mexico during 2006-12 and all whales in the Sakhalin and Kamchatka catalogues up to 2011 (Urbán et al. 2012, 2013).

Because the matching rate in Mexico appeared to be strongly age-specific, only animals known to be at east 6yr old were considered as potentially samplable in Mexico, and only matches from those animals were used, for the reasons explained by Cooke (2016). Of 17 whales matched between Sakhalin and Mexico, 16 met this criterion. Of a further 4 whales matched between Kamchatka and Mexico, 2 met this criterion.

2.2.2 Additional features recorded The additional features recorded for at least some identified animals were: sex; mother/calf status; breeding stock affiliation.

Sex. Sex was determined by biopsy for a total of 167 whales, including 156 by the RGWP project, 23 by the MBNRC project, and 12 by both projects. Of the 12 individuals sexed by both projects, 11 agreed with respect to sex. The one whale without agreement was deemed to be female, because it had been biopsied as female by the RGWP project on four separate occasions. Sex determination is assumed for modelling purposes to be 100% accurate.

Cow/calf status. Of 3,696 encounters of living whales off Sakhalin and Kamchatka (where “encounter” means an individual identified at least once by a team in a season) 211 were of cow-calf pairs and 77 were unaccompanied calves. It is assumed that all cow-calf pairs and accompanied calves were correctly identified by the RGWP team from 1995 and by the MBNRC team from 2003. The mother/calf status was not recorded by the MBNRC team in 2002. It is assumed that all unaccompanied calves were correctly identified by the RGWP team from 1995 and by the MBNRC team from 2007, which used the same criteria from this date. For the years 2003-06, MBNRC unaccompanied calf identifications were not used (they were treated as indistinguishable from other whales).

Of the 23 matching identifications from Mexico that were used in this analysis, 8 were of cows accompanied by calves, but the calf identifications were not available for this analysis.

Breeding stock affiliation

Three individuals sighted off Sakhalin were successfully tagged and tracked to the eastern North Pacific (Mace et al. 2015). It is assumed that the successful long-term tracking of an individual reveals its breeding stock affiliation (eastern or western). Individuals sighted in Mexico are also assumed to belong to the Eastern Breeding Stock (EBS).

2.3 Population model The generic population model structure and the method of fitting to data is described by Cooke (2018). specified in the Appendix. The specific implementation choices used for western North Pacific gray whales are described here.

2.3.1 State space The following attributes are represented in the state space:

Life stage (sex, age and reproductive status, mortality status)

Feeding subgroup affiliation

Breeding stock affiliation

Sampling availability (reflection of individual heterogeneity in capture probability)

The life stages and the possible transitions are shown in Table 3. Animals in any of these stages can die by entering a “carcass” state, where they have a (typically small) probability of being recovered and identified. From the following year they are assumed to be unobservable (“buried”).



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Table 3. List of living stages in the gray whale model

The youngest allowed age for a pregnant whale is 7 years and the youngest age for a lactating whale is 8 years. In the summer season, the “Age x” stage contains animals aged approximately x + ½ .

Feeding group and breeding stock affiliations are assumed to be maternally inherited and to remain constant throughout life.

Individual heterogeneity in availability was modelled using three availability classes. Each whale is born into a random class with equal probability. The individual availability class is assumed to be retained throughout life but not to be heritable. The purpose of including these classes is to reduce the bias in abundance estimation arising from capture probability heterogeneity.

2.3.2 Reproduction, mortality and other transitions Each animal in the lactating stage has exactly one calf, assumed to be male or female with equal probability.

The calf mortality is modelled as a constant plus an optional annual random effect. The non-calf mortality rate is assumed to be constant.

The transition probability to the maturing class is assumed to be constant. The transition probability to the pregnant stage depends on the source stage (maturing, lactating or resting) plus an optional annual random effect.

2.4 Sampling model

2.4.1 Primary and secondary sampling occasions The sampling in a season by a research team is treated as sampling occasion. Up to and including the 2011 season, each season of sampling by each team off Sakhalin and Kamchatka is treated as a primary sampling occasion (see Appendix section 2.1 for the definition). From 2012, two options were considered: (i) RGWAP team as primary team throughout, MBNEC and Kamchatka teams secondary from 2012; (ii) MBNRC and Kamchatka teams are primary throughout, RGWAP secondary from 2012.

Individuals sighted up to 2011 by a non-primary team only were treated as “identity lost” (see Appendix section 2.4) from 2012 onwards, because they might, unknown to the analysis, match whales seen by the primary team for the first time in 2012 or later. Sampling occasions in Mexico during 2006-12 are treated as secondary, because only whales matched with Sakhalin or Kamchatka were used.

2.4.2 Capture probability models The capture probability model for the summer sampling includes a free parameter for each sampling occasion (team by year), and a parameter for each combination of feeding subgroup and location, where “location” means Sakhalin or Kamchatka. That is, each feeding subgroup may be differentially catchable in each location.

Females Males

Calf Calf

Age 1 Age 1

Age 2 Age 2

Age 3 Lactating Age 3 (from all live states)

Resting

Age 4 Pregnant Age 4 Carcass

Age 5 Maturing Age 5 Buried

Age 6 Age 6

Age 7 Age 7

Age 8 Age 8

Age 9 Age 9

Age 10 Adults Age 10



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The summer capture probability model also contains optional random-effect terms for life stage, life stage team interaction, and availability class. The life stages for this purposes were summarized into five stages: calves, subadults, lactating mothers, other mature females, and mature males.

The capture probability for the Mexican wintering grounds was estimated externally of this model, because the great majority of the animals there are eastern North Pacific gray whales. The average annual capture probability was estimated to be 0.054 (Cooke 2016).

Because only three identified carcasses were observed, a single value for the capture probability of carcasses was estimated. Unidentified carcasses were not used: each year, the observation of carcasses is treated as a secondary sampling occasion.

2.4.3 Additional features recorded Sex is treated as a deterministically observed feature for those animals where it was determined. Any gender bias present in the sampling of individuals is modelled through the capture probability model, which allows for sex-biased sampling.

The probability that a calf of the year was still with its mother when first sampled by a team in a season was modelled as a team-specific parameter, because the MBNRC team tended to observe the animals later in the season than the RGWP team, after more calves had been weaned.

Identifications of calves were assumed to be deterministic in all years in which they were used (see above).

Individuals sighted in Mexico are automatically assigned belong to the eastern breeding stock (EBS), in the sense that the probability of sighting an animal from the western breeding stock (WBS) in Mexico is assumed to be zero.

Three individuals were successfully tagged and tracked to the eastern North Pacific were likewise assigned to EBS. It is assumed that, conditional on the location where they were originally sampled and tagged (in this case, Sakhalin) the probability of successfully tagging and tracking a sampled animal is independent of their breeding stock affiliation (that is, had the individuals been western breeding stock animals, this would have been verified).

2.5 Model selection Model selection for the purpose of determining which nuisance covariates need to be included was performed by Cooke (2018) using the AIC criterion. Because only one additional year of data has been added since that analysis, the model selection exercise is not repeated here: the same selected model was used.

2.6 Results Fig 1 shows a comparison of the mean population trajectory for all Sakhalin whales (groups S, SK and KS in Table 2) using (a) the RGWP data; (b) the MBNRC data as the primary data set for the years 2012-17 (when no cross-matching was conducted). The difference in trajectories is seen to be negligible, and the RGWP data were treated as the primary data set for all further model fits.

The population trajectories for the aged 1+ population size for the WFG, the WBS and their sum, were extracted from the posterior distributions of the individual-based population trajectories for each stock structure hypothesis. The estimates of abundance for 2017 are shown in Table 4 by a) breeding stocks (WBS and WFG); and b) by feeding subgroups. The mean abundance estimates for the western feeding group (WFG) in 2017 range from 149 to 321 depending on the stock structure hypothesis. The estimates for the western breeding stock (WBS) range from 75 to 96. When broken down by feeding subgroups without regard to breeding stock affiliation (Table 4b) the abundance estimates are virtually independent of the stock structure hypothesis. Figs 2a-e show samples of population trajectories from the posterior distributions for each hypothesis.

Table 4a. Mean and cv’s of population estimates (animals aged 1+ in 2017) by stock

Hypothesis WFG cv WBS cv WFG+WBS cv

3a 225 0.04

3b 226 0.04 97 0.13 323 0.05

5a 157 0.12 78 0.23 224 0.04

8 323 0.05

9 245 0.09 78 0.23 326 0.05

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Table 4b. Mean and cv’s of population estimates (animals aged 1+ in 2017) by feeding subgroup

Fig. 1. Comparison of estimated population trajectories for Western feeding Group (WFG) for Hypothesis 3a using RGWP and MBNRC data sets as primary data sets during 2012-17.

Hypothesis

3a 166 0.06 225 0.04 323 0.05

3b 167 0.07 226 0.04 323 0.05

5a 170 0.07 225 0.04 323 0.05

8 166 0.06 225 0.04 323 0.05

9 170 0.07 225 0.04 323 0.05

Regular Sakhalin whales

(S, SK)

All Sakhalin whales (S,

SK, KS)

All Sakhalin and Kam‐chatka

whales (S, SK, KS, K)



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Fig. 2a-e. Posterior samples of population trajectories for WFG and WBS by stock structure hypothesis.

3 RANGEWIDE HISTORICAL ASSESSMENT

3.1 Potential Gray Whale Habitat Figs 3a-b map waters between 5 and 50m depth (based on mean high water) in the North Pacific. Thuis is assumed to represent potential gray whale feeding habitat. The area of accessible habitat in the common northern area depends on ice cover. Estimates of monthly ice cover from 1850 to the present based on the method of Titschner and Rayner (2014) were downloaded from the Hadley site (metoffice.gov.uk/hadobs/hadisst2/). Fig. 3a overlays the areas of over 50% average ice cover during May-October as a proxy for areas considered largely inaccessible to gray whales, based on average pre-1980 ice cover. Fig. 3b overlays the corresponding areas based on 2017 ice cover. Fig. 4 shows the time trajectory of the effective available habitat, using this criterion, from 1850-2017. This shows that the decrease in ice cover has accelerated since 1980. Prior to 1980, spring ice cover remained similar to that encountered by mid-19th century whalers (Mahoney et al. 2011). The later part of the Little Ice Age does not appear to have been associated with higher ice cover in this region. The proportion of the total potential northern habitat that has become accessible under this criterion has increased from about 35% pre-1980 to about 60%. It may become effectively all available by the late 21st century.

Table 5 gives the area of potential gray whale feeding habitat in the eastern and western North Pacific and in the Arctic. The eastern and western North Pacific area shave been arbitrarily divided into “winter” and “summer” at 35°N. Whether or not the “summer” part is included, there is much less habitat area in the eastern North Pacific than in the western North Pacific.



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Fig. 3a. Potential gray whale feeding habitat based on 5-50m water depth, with pre-1980 median summer ice cover

Fig. 3b. Potential gray whale feeding habitat based on 5-50m water depth, with 2017 summer ice cover



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3.2 Multi-habitat population modelling

The analysis is based on a simple schematic three-habitat population model where the historic eastern and western gray whale populations had a common feeding habitat in the Arctic and separate winter habitats in the eastern and western North Pacific.

Under classic density-dependent population models, the net recruitment rate of a population, r, is a function of P/K, the ratio of population size (P) to the environmental carrying capacity (K). If the three habitats (east, west and north) have capacities KW, KE and KN, and the population sizes of the two breeding populations are PW and PE, then the two per capita growth rates, rE and rW are such that:

rW depends on PW/KW and on (PE + Pw)/KN

rE depends on PE/KE and on (PE+Pw)/KN

Table 5. Areas of potential gray whale feeding habitat

Region Area km²

Northern area (Bering and Chuckchi Seas) 1 818 829

Available (pre‐1980) 651 357

Available (2017) 1 231 082

Western North Pacific

"Summer" (north of 35°N) 397 671

"Winter" (south of 35°N) 454 888

Eastern North Pacific

"Summer" (north of 35°N) 70 926

"Winter" (south of 35°N) 39 532

0 000

200 000

400 000

600 000

800 000

1 000 000

1 200 000

1 400 000

1 600 000

1 800 000

2 000 000

1840 1860 1880 1900 1920 1940 1960 1980 2000 2020 2040

Fig. 4. Potential ice‐free gray whale habitat (km²) in Bering & Chuckchi Seas

Ice‐free area May‐Oct Total habitat area



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In the literature, various different choices have been made for the specific functional form (shape) of the density-dependent relationships. Regardless of the choice of functional form, this kind of density-dependence implies that at the pristine equilibrium, where rE = rW = 0, the ratio of population sizes PE/Pw is equal to the ratio KE/Kw, regardless of the capacity, KN, of the common shared northern feeding habitat.

For the purpose of fitting a population model, the functional forms assumed in the analysis below are:

0

0

1 exp 1

1 exp 1

NEE

E E W

W NW

W E W

KKr r

P P P

K Kr r

P P P

As noted by Omura (1988), the situation for gray whales in the North Pacific is asymmetric between east and west. The area of potential winter feeding habitat is much smaller in the eastern North Pacific than in the western North Pacific, i.e. KE ≪ KW hence at equilibrium PE ≪ Pw. Thus in the original state, the western breeding population will have been the larger of the two.

The carrying capacity of the northern feeding grounds appears to have been subject to some fluctuation in recent times, there having been a marked downward variation around the end of the 20th century, as evidenced by unusually high observed mortality of gray whales (Gulland et al. 2005) and a sudden drop in annual census numbers followed by a gradual recovery towards previous levels (Laake et al. 2012; Durban et al. 2015). Perryman et al (2002) shows a relationship between feeding ground carrying capacity and seasonal ice cover. For the purpose of this analysis, such fluctuations are assumed to average out and are ignored. Because the overall decline in ice cover appears to have become significant only since 1980, the area of available habitat has been treated as constant for the purpose of the historical population modelling in this paper.

3.3 Historic catches

3.3.1 Western North Pacific 3.3.1.1 PRE-HISTORIC Although there was some pre-historic hunting for gray whales, as suggested by petroglyphs near Ulsan, Korea (Park 1995), there is little information on how substantial it was. Gray whales could in principle have been vulnerable to pre-historic hunting anywhere in its known range, including Vietnam, China, Korea, Japan, and the northern regions. 3.3.1.2 JAPANESE TRADITIONAL The documentation of Japanese traditional whaling is very incomplete. Information from the few surviving sources has been summarized by Tønnessen (1965), Hawley (1958) and Omura (1986). Omura (1986) notes that whaling was already taking place in the Genki era (1570-73) in Ide Bay on the south-facing Pacific coast of Honshu, and that probably gray whales were the main species taken, but there are no estimates of numbers. This kind of whaling, using hand harpoons, spread throughout southern Japan. Five whaling companies were founded in 1606 at Taiji under the control of the Wada clan. According to Omura (1986) one of the Wada clan invented net whaling in 1675; under this method, when a whale was sighted by an onshore lookout, boats were launched which held out a net, while other boats drove the whale into the net, where it could be made fast and killed. The net method apparently spread rapidly and replaced the older methods in all areas by the end of the 1600s. Catch figures have only been available for a few locations. Omura (1986) lists 15 locations where whaling is documented, but notes that these were only some of the locations where whaling occurred. Tønnessen (1965) after consulting with Omura suggested there were about 30 whaling locations. Of the few locations for which catch figures by species are available for some years, gray whales were taken in each one except Ine, on the Sea of Japan coast of central Honshu. Given the takes of gray whales recorded in the Yamaguchi and Fukuoka prefectures, Gray whales clearly were present in Japanese waters of the Sea of Japan, but it may be that the migration from there did not follow the Honshu coast all the way north. Other than the approaches to the Inland Sea, there is little shallow water on the Sea of Japan coast of Japan (Fig 3).



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The catch figures by Omura (1986) indicate an average of about 4 gray whales per location per year for the 19th century up to the start of modern whaling. Some decline is apparent towards the end of the 19th century, but not as marked as for right and humpback whales. There is little information on struck-and-lost rates. Omura (1974) discusses old accounts of gray whales in the Inland Sea, between Kyushu, Shikoku and Honshu. Annual winter aggregations of 60-70 whales with calves, apparently gray whales, were hunted by a local whaling group until 1877. Dead calves were sometimes stranded. The only definitely confirmed gray whales were two taken in February 1864 which Omura identified definitely as gray whales from drawings. Omura cites various authors as believing there was a calving ground here. Most of the Inland Sea is shallow and would provide feeding opportunities. At the end of the 19th century, Japanese net whalers were operating also in Korean waters, but catch figures are available only for three, non-consecutive seasons, totalling 44 gray whales (Park 1995 cited by Kato and Kasuya 2002). If there were 30 locations and an average of 4 gray whales per year were taken at ¾ of them, that would yield a total that would imply 100 gray whales per year. If there was an additional 30% struck -and-lost mortality, the total kill could have been about 130 per year, which could have continued from the late 1600s to the late 1800s and at a declining level until the end of traditional whaling in 1913. 3.3.1.3 CHINA A report (Anon 1843) of whaling activities along the Chinese coast was published in the Chinese Repository, a 19th century journal devoted to describing aspects of Chinese life to a western readership. The description of the whales strongly suggests gray whales: covered in barnacles, frequent breaching, entering very shallow water, and stirring up the sea bed. The whales are said to appear in January and February, “in great numbers” and accompanied by calves. The described method of hunting seems to be gravity-driven tethered harpoons. The vessel manoeuvres up to the whale so that the harpooner standing in the bows can drop the heavy iron barbed harpoon, pointing vertically downwards, into the back of the whale. It seems to have been a large fishery, involving hundreds of vessels, and the author appears to have witnessed it first hand, but gives no indication as to how many years this fishery had been active. The geographical information is also vague: the author reports that the whales occur along the coast ”southward” of Hailing Island, but the coast runs initially westward from Hailing Island and then turns southward. The hunters supposedly belonged to “Hainan and the neighbouring islands”. Hainan is itself a large island but has no “neighbouring islands” worth mentioning. Possibly by “Hainan” the author was referring loosely to southern Canton, or specifically to the Leizhou peninsula where there are a number of islands including those in Leizhou Bay. The article talks of whaling vessels filling the bay and that the water was very shallow with many shoals. This is suggestive of Leizhou Bay. It seems likely that the fishery was well-established operation and may have continued until few whales were left. 3.3.1.4 AMERICAN AND EUROPEAN PRE-MODERN WHALERS US and European sail whalers reached the western North Pacific around 1820 and the Okhotsk Sea by 1845. Gray whales were also taken in the Bering Sea and Chuckchi Seas. From a sample of logbooks, Smith et al (2012) plotted positions of catches by species. The gray whale positions in the Okhotsk Sea are nearly all in May-September and are in the northeast (Shelikov Bay), north (Magadan area) and west (north of Sakhalin and in the Shantar region). There are none recorded on the NE Sakhalin shelf where gray whales occur today, but this could simply reflect the small number of logbooks sampled. There was substantial overlap between gray whale and bowhead whale catches, with both species being present in the areas where they occurred, except at the northern extremity of the whaling in the Chuckchi Sea. There was little overlap with right whales catches, which were large in the southern Okhotsk Sea, Sea of Japan, Kurile Islands and eastern Kamchatka and the adjacent offshore areas. The right whale areas were almost exclusively too deep for gray whale feeding. Foreign whalers were not allowed to operate in Japanese territorial waters, thus could not take gray whales migrating close to the coast. The operations of US (and some European) sail whalers in the Okhotsk Sea have not yet been satisfactorily quantified, but it is clear that they were very substantial, based on the information summarized by Ivashchenko and Clapham (2010). According to Lindholm (1888) 438 vessels operated there during 1855-57. There were also some Russian operations during the 1850s, which Lindholm worked for before starting his own whaling venture, which operated from 1865-85 from a land station in the Shantar region. Lindholm’s operation apparently took many carcasses lost by foreign whalers, but also killed whales using bomb lances and rifles. Catches declined from the 1860s but American



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whalers continued visiting the area in decreasing numbers until 1913. There were also some Russian operations during 1896-1912 (Tønnessen 1965). Apart from the logbook samples examined by Smith et al 2012, the species composition of the catches in the Okhotsk Sea is poorly known, and gray whales would not have been a preferred species. Bowheads were the preferred quarry, followed by right whales. However, given that the whalers had apparently taken virtually all the bowhead whales in the Okhotsk Sea by the time pre-modern whaling ended in the early 20th century, it is likely that, regardless of the initial number of gray whales in the Okhotsk Sea, few would have been left alive by the end of the 19th century. 3.3.1.5 EARLY MODERN WHALING (1889-1911) Modern (Norwegian-style) whaling was started by Russian operators in 1889, but documentation remained poor until 1911 when the Japanese government issued regulations on whaling in Japan and territories under Japanese control. According to Akaishi (1910) Russian modern whaling operated along the coast of Korea until the Russo-Japanese war of 1904, and took an estimated 130 gray whales based on species proportion in the few years with data. Japanese modern whaling began, also in Korean waters, in 1899, and expanded rapidly from 1905. Catch figures are not split by species, and not always by area. Kasuya (2002) and Park (1995) provide figures totalling 2,625 whales in Korean waters during 1899-1910. For the first years of whaling in Korea with catches broken down by species (1911-13), approximately 50% were gray whales (567 out of 1238). On this basis, the number of gray whales taken by Japanese whalers in Korean waters during 1900-10 is estimated at 1,300, but the proportion of gray whales in the catch of the preceding decade may have been higher than 50%, because this species would have been depleted quite rapidly. Applying the conventional struck-and-lost adjustment of 30% for early modern whaling (IWC 2015) , the estimated kill would be about 1,700 gray whales. Of the catches off southern and western Japan during 1911-13, only about 1% (17 out of 1,517) were gray whales, hence the catch in the preceding decade was probably also small. 3.3.1.6 FULLY MODERN PERIOD (1911-66) Proper catch statistics were collected from 1911 onwards. Gray whales were already quite rare in Japanese waters by this time. Only 55 gray whales were taken in Japan, all between 1911-21 and off NW Kyushu (apart from three taken in the northern territories). By contrast, 1,403 gray whales were taken off Korea during 1911-68, mainly during 1911-31. Of these, only 7 were taken in the Yellow Sea, the remainder on the eastern and southern coasts. 3.3.1.7 IMPLICATIONS OF ASIAN CATCH HISTORY FOR STOCK STRUCTURE The near extirpation of whales in Japan before the start of the modern period, contrasted with substantial modern catches of gray whales (over 3,000) off Korea, suggests that there were Japanese and Korean substocks of the western gray whale population, as suggested by Mizui (1951) and Omura (1974). The Korean gray whales likely migrated along the Korean and Russian coasts or the offshore Sea of Japan rather than along the Pacific coast of Japan. The summer destination of the 3,000 or so gray whales taken off Korea in the early 20th century is unknown. However, if their main feeding ground had been in the Okhotsk Sea, it seems unlikely that so many gray whales, even as a less preferred species, could have survived the intensive 19th century whaling in the Okhotsk Sea. Given that the vast majority of gray whales alive today feed in the Bering and Chuckchi Seas, the most likely explanation is that this was also the summer destination of many of the whales exploited off Korea, and that eastern and western breeding stocks of gray whales had overlapping feeding grounds.

3.3.2 Eastern North Pacific 3.3.2.1 PREHISTORIC

Mitchell and Reeves (1990) estimated an annual kill prior to 1850 of approaching 200 by aboriginal hunting, on the west coast of what is today USA and British Columbia, Canada, declining around 50 per year following the depletion of gray whales by European-American whalers. However later archaeological findings suggest many of these whales were humpbacks (e.g. Hielsebeck 1988) and the number of gray whales taken may have been less.

3.2.2. HISTORIC

Pre-modern whaling in the eastern North Pacific began in 1846 with the operations of Scammon in the breeding lagoons of Baja California, Mexico. Catches averaged about 500 per year during 1855-65 and the lagoons were largely exhausted by 1875. Shore-based whaling continued at a lower level in California until the late 19th century (Reeves and Smith 2010). There were some pelagic catches off California and Mexico by Norwegian and American vessels in the 1920s and 1930s, and further 320 Gray Whales were taken under scientific permit in the 1960s (Rice and Wolman 1971).



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3.3.3 Arctic and subarctic There were some catches in the Bering Sea during 1846-75, where bowheads were the main species taken (Allison 2017). Catches off Chukotka are documented from the 1940s and continue today (subject to an annual average catch limit of 140). There may have been some aboriginal catches prior to the 1940s (Allison et al. 2011).

3.4 Fitting historical population trends The data used to fit the population model are:

Relative ENP/WNP initial population sizes inferred from habitat area

Estimated and recorded historic catches 1600-present: eastern, western and northern

Demographic parameters from Sakhalin photo-id 1995-2017 (section 2, this paper)

Estimated size of Western Breeding stock in 2017 (section 2, this paper)

Abundance estimates of the eastern breeding stock 1968-2016 (Laake et al. 2012; Durban et al. 2015, 2017)

Although not designed for this purpose, it was found easier to apply the model of this paper than to develop a new model for the purpose of fitting historical population trends. The results of a preliminary fit are shown in Fig. 5. Some numerical issues arose in relation to the model fitting over such a long time period, that could not be resolved in the time available. Hence the fit shown here is considered illustrative only, until the fitting process has been refined to permit proper characterisation of the uncertainty in the fit.

4 DISCUSSION

Various authors (e.g. Lankester and Beddington 1986; Butterworth et al. 2002) have noted that that 19th century catches of gray whales in the eastern North Pacific were not sufficiently large to explain the increase observed during 1968-86 as a recovery from past exploitation. An increase in carrying capacity has been hypothesized, but as discussed above, there is evidence only for an increased carrying capacity since about 1980 period, based on declining ice cover. The range-wide population model fitted here explains the increase as a recovery of the range-wide population that shares summer feeding grounds in the Arctic, rather than recovery of a purely eastern North Pacific breeding population.

The results presented here suggest that the western gray whale population was large in the past at ~25,000 whales, but was substantially depleted before the advent of modern whaling. Modern whalers reduced the population further, taking about 3,000 whales from a group that migrated past the Korean peninsula. Today, gray whales from the eastern North Pacific are also found in Asian waters, including the northeastern Sakhalin shelf. It us unclear whether this was always the case or whether the presence of eastern (Californian) gray whales in the Okhotsk Sea is a more recent incursion following the depletion of western gray whales.

0

5 000

10 000

15 000

20 000

25 000

30 000

1600 1700 1800 1900 2000

Western (Asian)

Eastern (Californian)



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The potential scope for recovery of western gray whales is very high, but further work is needed to determine the prospects for recovery of the western population in the presence of anthropogenic hazards including fishing and vessel traffic.

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