Coho salmon (Oncorhynchus Kisutch) COSEWIC assessment and status report: chapter 9

Population sizes and trends

Aboriginal traditional knowledge

Aboriginal Traditional Knowledge (ATK), sometimes referred to as Traditional Ecological Knowledge, describes the knowledge originating with First Nations peoples pertaining to their immediate environments, and the cultural practices that build on that knowledge (Ford and Martinez 2000). Communities with a long history of resource use can acquire a deep but qualitative knowledge about the resource that they depend upon (Kurien 1998).

Interior Fraser coho return to spawn primarily within the traditional territories of the Secwepemc people (North and South Thompson and Clearwater rivers) and of the Nlaka’pmux, Sce’exmx and Okanagan people of the upper Fraser canyon and Nicola valley. Some coho spawning also takes place within the traditional territories of the St’at’imc, (Lillooet/Bridge River areas) and Tsilhqot’in (Chilcotin river system). The Secwepemc Fisheries Commission (SFC) and the Nicola Valley Stewardship and Fisheries Authority (NWFSA) represent bands with knowledge of traditional fisheries. In addition, there are various bands not affiliated with these organizations that also possess ATK. ATK pertaining to some natural resources in the interior Fraser has been assembled (e.g. Turner et al. 2000) but no thorough review of ATK has been undertaken for salmon. Irvine et al. (2000) review issues concerning the role of ATK in the assessment of interior Fraser coho salmon.


Abundance estimates

We have no estimates of the abundance of coho salmon in the interior Fraser prior to the arrival of Europeans. Northcote and Burwash (1991) calculated that the average annual abundance (catch plus spawners) for Fraser River coho salmon in the 1920’s to early 1930’s was approximately 1.2 million. Assuming ~1/3 of these fish were from the interior Fraser, the abundance of interior Fraser coho during this period was ~400 000 and assuming as Northcote and Burwash did that 50% of these fish were harvested, the annual escapement of coho was in the order of 200 000 fish. In their report on fish of the Fraser River basin, Northcote and Burwash (1991) estimated that coho salmon in the Fraser watershed underwent a 7.7 fold decrease between the 1920’s and the period between the 1950’s and the 1980’s. However, they cautioned that data for coho were the least reliable of the salmon data.

Spawning coho salmon are notoriously difficult to count. Although escapement estimates (number of salmon escaping marine fisheries and returning to freshwater to spawn) exist for some streams in the interior Fraser as far back as 1951, older estimates are of unknown accuracy and precision. Consequently they are of little use for time series analysis, other than to confirm species presence in a watershed.

A variety of techniques are currently used to estimate coho salmon escapements for ~71 streams in the interior Fraser River watershed. Although essentially all known spawning streams in the Thompson watershed are assessed, many in the upper Fraser are not.

Precision is more important than accuracy in time series analysis, and high precision requires that field and estimation methods should be consistent through time.Two approaches were used to examine recent trends in spawner numbers, an escapement indicator approach and an approach using an adjusted escapement series.

The escapement indicator approach relied on escapement estimates to unenhanced North and South Thompson streams with reasonably consistent monitoring. Total escapements to the 10 North Thompson escapement indicator streams followed similar temporal patterns as escapements to the 16 South Thompson escapement indicators (Irvine et al. 2001). Numbers appeared to be relatively stable during the late 1970’s, increased during the early 1980’s, peaking in the latter part of the decade. Since then, escapements declined rapidly until 1996, after which a small increase may have occurred. Using procedures documented in Simpson et al. (2001), the adjusted escapement series consisted of total numbers of coho salmon spawners returning to the major basins within the Thompson watershed, and included hatchery-origin fish (Table 1). Similar to the wild indicator data set, the adjusted escapement estimates for the North and South Thompson also peaked in the mid-1980’s, declined until about 1996, and were then stable or increasing (Fig. 6). It was difficult to reconstruct the time series for lower Thompson/Nicola and non-Thompson populations. A significant portion of returns to the lower Thompson/Nicola is enhanced, and many estimates are unreliable. Escapements to the lower Thompson appear less variable than to other parts of the Thompson, although they also may have increased in recent years (Fig. 6). The time series for non-Thompson streams is too short to assess temporal patterns.

At the peak of our time series of reliable estimates, escapements to the interior Fraser were in the order of 100 000 and abundance in two years appeared to exceed 300 000 (Table 1). The size of the total population spawning more recently was estimated by averaging escapement estimates for each of the five subpopulations (i.e. North Thompson, South Thompson, lower Thompson/Nicola, Fraser canyon, and upper Fraser) during 1998-2000 (Table 1). All estimates will be biased low to a small degree simply because it is not possible to see all fish. Of greatest concern is the degree of bias for upper Fraser streams. Results from a fishwheel mark recapture program imply that this bias may be in the order of 57% (Irvine et al. 2001). The total population of interior Fraser coho spawning annually in recent years is probably in the order of 24 000 (23 914) fish (Table 2), of which ~15% are of hatchery-origin. The North and South Thompson populations collectively include slightly more than half the total population of coho in the interior Fraser


Figure 6: Adjusted historical escapement estimates for Coho Salmon (Wild and Hatchery Fish) returning to the South, Lower, and North Thompson Watersheds

Figure 6: Adjusted historical escapement estimates for coho salmon.

Data are in Table 1.

 

Table 1 (South Thompson): Estimated fishery exploitation rates, escapements, marine fishery catches, and total abundances for interior Fraser coho salmon (hatchery and wild fish combined)
Return
Year
Estimated fishery
exploitation rates
Escapements Marine fishery
catches
Total
abundances
1975 0.68 5 864 12 490 18 354
1976 0.68 3 920 8 349 12 268
1977 0.68 8 490 18 082 26 572
1978 0.68 7 996 17 032 25 028
1979 0.68 10 198 21 720 31 918
1980 0.68 7 025 14 964 21 989
1981 0.68 4 120 8 775 12 895
1982 0.68 5 849 12 459 18 308
1983 0.68 6 196 13 196 19 392
1984 0.68 15 394 32 789 48 183
1985 0.68 16 998 36 205 53 204
1986 0.66 16 521 31 665 48 186
1987 0.54 21 087 24 478 45 564
1988 0.71 24 426 60 376 84 802
1989 0.65 17 208 31 288 48 496
1990 0.74 8 609 24 069 32 677
1991 0.68 4 160 8 737 12 896
1992 0.81 11 886 52 239 64 125
1993 0.88 1 873 13 172 15 045
1994 0.43 4 485 3 430 7 915
1995 0.56 3 622 4 639 8 261
1996 0.83 1 760 8 906 10 667
1997 0.40 2 034 1 384 3 418
1998 0.07 4 946 375 5 321
1999 0.09 3 074 305 3 379
2000 0.03 3 785 134 3 919
2001 0.07 13 239 996 14 235

Data updated from Irvine et al. 2001. Exploitation was not measured prior to 1987 but was assumed to be the same as the average exploitation during 1987-1996. 2001 estimates are preliminary.

 

Table 1 (continued - North Thompson): Estimated fishery exploitation rates, escapements, marine fishery catches, and total abundances for interior Fraser coho salmon (hatchery and wild fish combined)
Return
Year
Estimated fishery
exploitation rates
Escapements Marine fishery
catches
Total
abundances
1975 0.68 22 286 47 468 69 754
1976 0.68 20 675 44 037 64 713
1977 0.68 42 804 91 171 133 975
1978 0.68 39 095 83 269 122 364
1979 0.68 47 819 101 851 149 670
1980 0.68 10 542 22 454 32 996
1981 0.68 20 615 43 909 64 524
1982 0.68 42 295 90 087 132 382
1983 0.68 35 086 74 731 109 816
1984 0.68 69 552 148 141 217 692
1985 0.68 45 160 96 188 141 349
1986 0.66 104 267 199 846 304 113
1987 0.54 54 884 63 710 118 594
1988 0.71 70 612 174 539 245 150
1989 0.65 30 677 55 779 86 455
1990 0.74 25 697 71 844 97 542
1991 0.68 14 585 30 633 45 217
1992 0.81 22 042 96 875 118 917
1993 0.88 9 669 67 999 77 667
1994 0.43 10 031 7 671 17 702
1995 0.56 22 477 28 794 51 272
1996 0.83 12 319 62 325 74 645
1997 0.40 6 722 4 573 11 295
1998 0.07 9 125 685 9 810
1999 0.09 8 916 885 9 801
2000 0.03 7 032 250 7 282
2001 0.07 26 429 1 989 28 418

Data updated from Irvine et al. 2001. Exploitation was not measured prior to 1987 but was assumed to be the same as the average exploitation during 1987-1996. 2001 estimates are preliminary.

 

Table 1 (continued - Lower Thompson): Estimated fishery exploitation rates, escapements, marine fishery catches, and total abundances for interior Fraser coho salmon (hatchery and wild fish combined)
Return
Year
Estimated fishery
exploitation rates
Escapements Marine fishery
catches
Total
abundances
1984 0.68 5 155 12 050 17 205
1985 0.68 1 913 4 060 5 973
1986 0.66 2 211 4 300 6 511
1987 0.54 4 208 4 945 9 153
1988 0.71 4 013 9 830 13 843
1989 0.65 3 423 6 340 9 763
1990 0.74 4 421 12 600 17 021
1991 0.68 3 794 8 825 12 619
1992 0.81 4 905 21 000 25 905
1993 0.88 8 416 61 500 69 916
1994 0.43 5 252 3 965 9 217
1995 0.56 1 984 2 525 4 509
1996 0.83 1 209 5 900 7 109
1997 0.40 4 217 2 820 7 037
1998 0.07 2 628 200 2 828
1999 0.09 5 007 495 5 502
2000 0.03 4 459 157 4 616
2001 0.07 9 828 740 10 568

Data updated from Irvine et al. 2001. Exploitation was not measured prior to 1987 but was assumed to be the same as the average exploitation during 1987-1996. 2001 estimates are preliminary.

 

Table 1 (continued - Non-Thompson/Fraser): Estimated fishery exploitation rates, escapements, marine fishery catches, and total abundances for interior Fraser coho salmon (hatchery and wild fish combined)
Return
Year
Estimated fishery
exploitation rates
Escapements Marine fishery
catches
Total
abundances
1998 0.07 8 147 610 8 757
1999 0.09 5 389 535 5 924
2000 0.03 4 723 144 4 867
2001 0.07 13 515 1 018 14 533

Data updated from Irvine et al. 2001. Exploitation was not measured prior to 1987 but was assumed to be the same as the average exploitation during 1987-1996. 2001 estimates are preliminary. The Fraser Canyon and the Upper Fraser subpopulations have been merged into Non-Thompson Fraser.

 

Table 2: Mean number (percentages) of spawners escaping to the major basins of the interior Fraser during 1998-2000
Origin South Th. North Th. Lwr Th./
Nicola
Fraser
Canyon
Upper
Fraser
Total
Wild + hatchery 3 935 (18) 8 358 (37) 4 031 (18) 4 092 (18) 1 994 (9) 22 410
Wild + hatchery 3 935 (16) 8 358 (35) 4 031 (17) 4 092 (17) 3 498 (15) 23 914
Wild only 3 904 (19) 7 202 (35) 1 617 (8) 4 092 (20) 3 498 (17) 20 313

In the lower two rows, the number of spawners estimated in the upper Fraser was divided by 0.57 to account for fish in streams that were not surveyed.


Enhancement

Hatchery production of coho salmon began in the early 1980’s, peaking during the mid- to late 1980’s when enhancement strategies were being tested for coho in the Eagle, Salmon, and Coldwater systems. The main objectives of enhancement during this period were to evaluate the effectiveness of different strategies and to assess the impact of enhanced production on natural stocks (Perry 1995, Pitre and Cross 1993). These authors concluded that fry releases might be a useful supplementation strategy when progeny from natural spawning do not fully occupy available habitat. However, negative interactions between wild and hatchery coho salmon can occur, particularly when the release of hatchery coho fry results in the carrying capacity of a stream being exceeded.

There are no large production facilities for coho salmon in the interior Fraser. There are ~13 small enhancement projects producing coho as well as habitat restoration at various sites. Enhancement efforts currently focus on rebuilding depressed stocks and obtaining assessment information that can be used for both wild and enhanced stocks. Since temporal patterns for the adjusted escapement series (Fig. 6) that included hatchery fish were similar to those of the wild indicator series, it appears that enhancement had a relatively minor effect on overall population trends. Bradford and Irvine (2000) reached a similar conclusion. They found that the presence of hatchery activities had no significant effect on rates of decline. Enhancement activities are described in more detail by Irvine et al. (1999a, 2000).

Since many of the coho salmon released from hatcheries are marked, and most of the salmon returning to these systems are examined for marks, it was possible to estimate the proportion of the escapement that are of hatchery origin. During 1998-2000, hatcheries contributed a significant proportion of the fish returning to streams in the lower Thompson/Nicola watersheds (Table 2), but relatively few fish elsewhere.


Survivals and fishery exploitations

Temporal patterns of salmon abundance are often analysed by partitioning survival into freshwater and marine components. Freshwater survival estimates are not available for interior Fraser coho salmon. Marine survivals declined during the decade following the late 1980’s, a pattern that has been documented for many coho south of northern British Columbia (Coronado and Hilborn 1998). While marine survivals may have improved in the last two years, they are still generally less than 3%, much less than they were in the late 1980’s (Irvine et al. 2000).

Estimates of fishery exploitations up to 1997 are available through the MRP, and since then from an analysis of DNA from tissue samples from fish taken in fisheries (Irvine et al 2001). Unprecedented restrictions in Canadian salmon fisheries commencing in 1997 and increasing in 1998-2000 are apparent in the time series of exploitation rates (Table 1). Exploitation rates (i.e. catch/catch plus escapement) during 1998-2000 averaged 6.5% (Canada and USA combined). In contrast, the mean annual exploitation during the ten-year period from 1987-1996 was 68% (Table 1).


Rates of decline

Rates of decline were computed based on:

  1. The escapement time series consisting of data from the 10 North Thompson and 16 South Thompson indicator streams, which have relatively few missing data and are unaffected by hatchery activities.4
  2. The adjusted total escapement estimates for the North and South Thompson watersheds (Table 1).

Declines were estimated in two ways. The first approach used annual estimates of abundance and the standard COSEWIC formula for calculating declines.5 The second (smoothed) used running averages calculated over 3-year periods, the normal generation time for these fish. This approach reduced year to year variations in abundance, which are common with semelparous animals including coho salmon that reproduce primarily at one age.

Each approach resulted in relatively large rates of decline (Table 3). Estimates for the 10-year period from 1990 – 2000 ranged from 27 – 73% (overall mean 60%).6

The lowest estimated decline resulted when the smoothed approach was used to interpret escapement indicator data from the North Thompson watershed. Differences between methods and data sets were not consistent.

Annual estimates of productivity for Thompson coho were estimated as:

ran = ln [Rt/St-3]

where Rt is recruitment (i.e. catch plus escapement) and St-3 is the abundance of parent spawners (i.e. escapement). Thus r is a measure of survival from spawners to returning (i.e. prefishery) adults. The time series of ran for the mean of the 10 North and 16 South Thompson indicator streams is presented (Fig. 7).

 

Table 3: Rates of decline (percentages) for South and North Thompson coho salmon escapement indicator data (wild coho only) and adjusted estimates of total escapements (wild and hatchery) during 1990-2000
Method S. Thompson

Esc. Indic.
S. Thompson

Adjusted
S. Thompson

Mean
N. Thompson

Esc. Indic.
N. Thompson

Adjusted
N. Thompson

Mean
Overall Mean
COSEWIC 56.5 61.2 58.9 56.0 72.6 64.3 61.6
Smoothed 79.3 65.6 72.4 27.2 57.7 42.5 57.5

Estimates were calculated using the standard COSEWIC formula, as well as using 3 year smoothed data.


Figure 7: Time series of ran, the annual rate of population growth of thompson Coho Salmon

Figure 7: Time series of ran, the annual rate of population growth of Thompson coho salmon.

From Irvine et al. 2001. Each point is the average (±SE) of two time series (North and South indicator stream aggregates). When r<0, populations are unable to replace themselves, even in the absence of fishing.

There was an overall decline in ran from the mid-1980’s until 1997-1998 (Fig. 7). During some years (1991 and possibly 1995, 1997, and 1998) ran may have been < 0, meaning that some populations were unable to replace themselves, even if fishing mortality was zero. Fortunately, the average ran for the 1999 and 2000 returns was positive.


Reference points

Biological reference points are benchmarks against which the status of fish populations can be measured (Collie and Gislason 2001). Irvine et al. (2001) computed provisional reference points for coho from the North Thompson watershed (see also Chen et al. 2002) which are used here to identify somewhat arbitrary categories of abundance. Results are presented in terms of numbers of adult female coho salmon per km of habitat accessible to them.

The estimated number of female spawners that would produce maximum sustained yield (24.9 females/km of accessible habitat) was selected to identify the transition between zones of moderate and high abundance (Fig. 8). The mean of two lower reference points was chosen as the boundary between critical and poor categories of abundance. One lower reference point was the minimum escapement that the population has recovered from (6.1), and the second was a value computed as the theoretical 10% probability of extinction for a single brood line in one generation (4.3) (Irvine et al. 2001; Chen et al. 2002). The zone between poor and moderate abundance was two times this mean value (10.4).

As can be seen in Fig. 8, with one exception spawner numbers were in the moderate and abundant categories from 1975-1990, were in the poor or low moderate zones from 1991-1996, and were near but generally below the poor-critical boundary from 1997-2000 (Fig. 8).


Figure 8: Annual estimates of numbers of adult female Coho Salmon (Wild and Hatchery) per kilometre within the North Thompson watershed

Figure 8: Annual estimates of numbers of adult female coho salmon (wild and hatchery) per kilometre within the North Thompson watershed.

Adapted from Irvine et al. 2001). Horizontal lines indicate boundaries between abundance categories.


Future scenarios

The future for interior Fraser coho is highly uncertain and depends on impacts due to two categories of human-induced activities, fishing and habitat perturbations, and one category that is largely out of our control, climate-related changes in salmon survival. The human population in the Pacific Northwest (including British Columbia) is expected to increase by 2-7 fold this century (Lackey 2001). Hartman et al. (2000) discuss how human activities affect salmon at local, regional, and global levels and conclude that human population growth likely represents the greatest threat to Pacific salmon.

In Fig. 9, the outlook for coho in the North Thompson watershed is forecast beginning in 2001 under three simplistic scenarios. In the most optimistic scenario, survivals are assumed to improve to levels recorded during 1978-1997 (recruits/spawner (R/S) = 3.31); in the average scenario, survivals remain the same as during 1998-2000 (R/S = 1.47); and in the worst case scenario, survivals are the same as recorded for fish returning in 1998, a poor survival year (R/S = 0.96).7 It is assumed that there will be no additional impact of habitat development and that fishing will remain at the same low levels as during 1998-2000 (i.e. ~7%). In addition, spawners are assumed to be all 3 years old, and spawner numbers cannot exceed 50 female coho per kilometer, a level reached only once in the previous 25 years (Fig. 8).

For North Thompson and presumably other populations of coho, a return to survival levels experienced historically, combined with continued low fishing pressures and no additional habitat impacts, would theoretically produce rapid increases in escapements and rebuilding within two generations. If survivals continue at recent average levels, populations will take 5-6 generations to reach the abundant category. However, if survivals are as poor as they were in 1998, numbers will decrease, eventually resulting in extinction (Fig. 9).

These forecasts are simplistic and probably overly optimistic for several reasons: the capacity of the environment to produce coho salmon has probably been reduced in recent years due to habitat changes and this has been ignored in the model; the extremely low fishing pressures assumed have come at a large socioeconomic cost and may be difficult to maintain; and, finally, no variability in survival is assumed in the model. Routledge and Irvine (1999) found that even small increases to the amount of chance variation in recruitment can significantly reduce population survival rates.




Footnotes

4 North Thompson watershed index streams are: Barrièrre, Blue, Cook, East Barrièrre, Fennel, Lion, N. Thompson, Raft, Reg Christie, and Tumtum, while South Thompson watershed index streams are Adams (lwr), Adams (up), Bessette, Blurton, Bolean, Canoe, Hunakwa, Kingfisher, Scotch, Shuswap (lwr), Shuswap (mid), South Pass, Tappen, Trinity, Wap, and Sinmax.
5 Formula provided by COSEWIC (http://www.cosewic.gc.ca/COSEWIC/authors/).
6 Preliminary escapement estimates for 2001 became available as this report was being revised. When the smoothed approach was used to compute declines during 1991-2001 using the adjusted escapement data set, estimates for the South and North Thompson aggregates were 12 and 34% respectively. These estimates were more positive than estimates for 1990-2000 for two reasons: 2001 escapements were much higher than in recent years, and the 1991-2001 data set excluded the relatively large escapements of 1990 (relative to 1991).
7 Spawner recruitment data all from the North Thompson indicator stream data set.

 

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