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                                                  Details of Status Assessment for the Eastern North Pacific gray whales

Gray whales (Eschrichtius robustus) in the North Pacific have complex stock structure, with multiple breeding stocks comprised of various feeding aggregations. This status assessment focuses on Eastern North Pacific Gray whales (i.e., the Eastern Breeding Stock, EBS), which likely include at least two feeding groups: the large Northern Feeding Group (NFG), and the small Pacific Coast Feeding Group (PCFG). However, due to the complex stock structure, the status assessment models simultaneously consider other gray whale stocks. Notably, it is possible that another sub-unit of gray whales that feed around Sakhalin Island and southern Kamchatka, the Western Feeding Group (WFG), sometimes intermixes, temporarily, with the EBS (and is counted during coastal surveys in North America). The PCFG interbreeds with NFG, but WFG likely breeds separately. Aboriginal subsistence hunting by Russia and the US has a combined annual limit of 140 strikes, currently all taken from the northern feeding ground.

Status assessment models and methods coincide with those used in the development and testing of the Gray Whale Strike Limit Algorithm. Details are provided by IWC (2019). The base-case trials for the assessment are two Implementation Simulation Trials (Table 1), extracted from the full set of trials specified by IWC (2019), where the technicalities of this table are explained.

Table 1: Base-case trials for assessment

 

 

 

 

MSYR1+

 

PCFG

 

Trial

Description/stock hypothesis

PCFG or WFG

in Chukotka

NFG

PCFG

WFG

Imm.

Pulse

Bycatch

Ref 4a

Two breeding stocks (WFG and EBS) and two feeding groups of EBS (PCFG and NFG)

       No

4.50%

4.50%

4.50%

2

20

D x 4

Ref 7a

Three breeding stocks (WBS, WFG and EBS) and two feeding groups of EBS (PCFG and NFG)

       No

4.50%

4.50%

4.50%

2

20

D x 4


In these trials, MSYR1+ is pre-specified for the base-case trials (at 4.50%) and the stock does not start at carrying capacity. The trials include spatial strata, but these are used only to remove catches and non-hunting human caused mortality by feeding group/population and to fit to data. Results by area are ignored. Rather, results are shown for EBS, and separately for the NFG and PCFG components. The estimates of 1+ depletion are expressed relative to current carrying capacity because carrying capacity may have changed for some populations.

Table 2 summarizes summary statistics by trial for the base-case trials. The following statistics are shown:

  • ·       current (2023) 1+ abundance;
  • ·       current (2023) depletion (number of animals aged 1+ and older relative to current 1+ carrying capacity);
  • ·       20-year total change in 1+ abundance, 2003-2023, expressed as a percentage
  • Results include the median value and 5th and 95th percentiles for each trial.

Table 3 provides the overall assessment estimates, i.e., the average medians across trials, and the pooled 90% intervals for each statistic, where values are pooled over trials and simulations within trials.

Table 2: Summary statistics, by sub-stock and overall, by trial.

Trial

Statistic

Stock

5%

Median

95%

Reference 3a

Abundance

NFG

14,301

15,763

17,510

   

PCFG

210

222

234

   

Total EBS

14,861

16,303

18,111

 

Depletion

NFG

0.555

0.611

0.711

   

PCFG

0.763

0.791

0.824

   

Total EBS

0.561

0.616

0.714

 

Percent Change, 2003-2023

NFG

-19

-10

3

   

PCFG

2

11

16

   

Total EBS

-18

-9

4

Reference 5a

Abundance

NFG

14,348

15,800

17,553

   

PCFG

210

222

234

   

Total EBS

14,808

16,272

18,044

 

Depletion

NFG

0.551

0.614

0.706

   

PCFG

0.766

0.794

0.819

   

Total EBS

0.555

0.618

0.707

 

Percent Change, 2002-2023

NFG

-21

-10

2

   

PCFG

4

11

18

   

Total EBS

-20

-9

3



Table 3: Summary statistic medians averaged over trials
and upper and lower bounds for pooled 90% intervals for the summary statistics

Statistic

Stock

5%

Median

95%

Abundance

NFG

14,348

15,791

17,510

 

PCFG

210

222

234

 

Total EBS

14,861

16,280

18,020

Depletion

NFG

0.555

0.614

0.706

 

PCFG

0.765

0.794

0.818

 

Total EBS

0.561

0.618

0.707

Percent Change, 2003-2023

NFG

-20

-10

2

 

PCFG

3

11

16

 

Total EBS

-19

-9

3

 

References:

IWC. 2019. Report of the Fifth Rangewide Workshop on the Status of North Pacific Gray Whales. Journal of Cetacean Research & Management 20 (Supplement): 569-599.