Evidence of Decreasing Spatial Diversity in British ...

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Evidence of Decreasing Spatial Diversity in British Columbia Salmon Stocks Carl I. Walters and Peter Cahosn Institute of Animal Resource Ecology, The University of British Columbia, Vancouver, B.C. V6T 3 W5

Walters, C. J., and P. Cahoon. 1985. Evidence of decreasing spatial diversity in British Columbia salmon stocks. Can. 1. fish. Aquat. Sci. 42: 1033-1037. Escapement records indicate a severe decline since 1950 in the number of streams contributing significantly to salmon production in southern British Columbia. To account for 90% of the escapement for chinook (Oncoshynchus tshawytscha), coho (8. kisutch), pink (0.gorbuscha), and chum (8.keta), one need examine only about half as many streams now as in 1950. The deterioration has been most pronounced for coho; only 40 of 350 south coast streams now account for 80% of the recorded escapement. The apparent changes are due partly to reduced monitoring effort rather than actual stock declines, but deterioration in the monitoring system is itself a cause for major concern. Les donnees sur la remonte revelent un declin important, depuis 1950, du nombre d e cours d'eau cone tribuant de faeon significative a la production salmonicole dans le sud de la Colornbie-Britannique. A I'heure actuelle, 90 % d e la remonte d e saumons quinnat (Bncoshynchus tshawytscha), coho (8.kishstch), rose (0.gorbuscha) et keta ( 0 . keta) peut &re observee dans environ la moitie seulernent du nombre d e cours d'eau par rapport a 1950. Cette det4rioration est beaucoup plus prononcke pour le coho ; ainsi, 40 des 350 cours d'eau de la cbte sud representent 80 % de la remonte enregistree. Ces variations evidentes sont en partie attribuables a un effort de surveillance reduit et non a des baisses reelles du stock ; la degradation du systeme de surveillance est cause d'inquietude importante. Received july 3, 1984 Accepted February 6, 1985 (J785.5)

acific salmon are produced in hundreds of spawning streams in British Columbia. Most of these streams cannot be managed separately, and harvests are mainly taken in large mixed fisheries such as the Georgia Strait sport, coast wide troll, and Johnstone Strait net fisheries. Crude indices of escapement levels have recently been assembled from field surveys conducted by fishery officers since 1948 and recorded on so-called BC 16 records. These records are too inaccurate to permit much quantitative analysis, such as stock-recruitment relationships, but even a casual glance at them suggests some a l m i n g , qualitative changes in the spatial distribution of escapements. Many small runs have disappeared since 1950, and even some major stocks have declined substantially. On the other hand, some stocks have increased, and it is difficult to get an intuitive feeling for net change over the period of record. This note summarizes evidence that spatial structure of the production system is indeed deteriorating, as evidenced by declines in the number of streams contributing significantly to total escapement in southern British Columbia statistical areas 11-27. Escapements have been recorded for approximately 350 streams in these areas, which surround Vancouver Island and Can. 9. Fish. Aquat. Sci., Vol. 42, 1985

the Georgia Strait (excluding the Fraser River and a few streams to the north of it). The data were made available to us in a computer data base by A. Gould, Department of Fisheries and Oceans, Nanaimo, B .C.

A Diversity Measure For four species (chum, Oncorhynchus keta; coho, 0. kikisutch; chinook, 8.tshawytscha; pink, 0.gsrbuscha), we calculated 6-yr average escapements for each of the area 11-27 streams for the y e a s 1948-83. For years when a stream was not visited, the observed escapement was taken to be zero. Then for each species and 6-yr period, we ranked the streams by average escapement (1 = highest average, 350 = lowest average). The rank orders were not always consistent over time, i.e. a particular stream might rank high in 1948-53 and then low in 1978-83. Larger rivers tended to be higher and relatively more stable in rank than small streams, but this pattern was not completely consistent. Next we cumulated the average escapements by stream rank order, so rank order I was assigned the highest escapement, rank order 2 was assigned the highest and second highest


escapements, and so forth. The resulting curve of cumulative escapement versus escapement rank order is a direct diversity measure. Below we shall call this measure the "rank order curve." Simila eequitabilitylhetersgeneity measures (Peet 1974) have been widely used in ecology. Economists call it (with axes reversed) the ""Lorenz curve" ( L o ~ n z1905; Taillie 1979) or "concentration curve9' and use it to describe distribution of. wealth and concentration of industrial activity. If d l streams had equal average escapement, the rank order curve would be a straight line with slope equal to that average, i.e. each stream would add the same amount to cumulative escapement. If all the escapement were concentrated in one or a few streams, the rank order curve would rise very steeply and then plateau sharply, with most streams not adding at all to the cumulative escapement. Temporal decrease in diversity should be reflected by a steepening of the curve over time, that is by movement of the curve away from the straight kine representing maximum possible diversity with all streams contributing equally. To eliminate effects of temporal trends in total escapement, we normalized the rank order curves by dividing each by its maximum (total escapement for the period). This convention also makes it easier to extract qualitative information from the curves, such as the number of streams that would have to be monitored in order to account for 98% of the total escapement.

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Normalized rank order curves as defined above are shown in Fig. 1, and temporal trends in overall escapements are shown in Fig. 2. For all four species, it is clear that observed escapements became more concentrated (less spatially diverse) between 1948-53 and 1978-83. Within the limits of the very crude escapement estimates used, it appears that the change has occurred rather smcmthly over time. The number of streams needed to account for 80% of total recorded escapement has been cut roughly in half since 1950. The top ranking 10 streams for each species and period are listed in the Appendix. These listings show that the rank order of dominant streams has remained relatively stable over time, at least in the sense that some streams have consistently been top contributors. The most striking deterioration has apparently been with coho salmon, for which only 40 (out of 350) streams now account for nearly 80% of the observed escapement. However, field personnel have noted that coho are particularly difficult to enumerate, and that enumeration effort (time spent counting, number of field employees) has fallen in recent yeas. Also, coho exploitation rates increased during the 197O's, mainly due to growth in the sport fishery (Argue et al. 1983); this change would be expected to hurt some stocks more than others. Another factor is noted in the Appendix: 3 of the top 10 coho escapements for 1978-83 were for stocks that had previously not been major contributors, but were subject to major enhancement projects in the 1970's (Puntledge, Quinsam, Qualicum); these thee stocks contributed 29% of the total recorded coho escapement for the 1978-83 period.

Discussion

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The changes shown in Fig. 1 may be attributed to a variety of factors, the most obvious being (1) local stock declines due to habitat deterioration, (2) widespread increases in fishing mortal-

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Can. J . Fish. Aqucat. Sci., V d . 42, 1985

1 . Cumulative relative escapement as a function of stream rank order for six time periods, British Columbia statistical areas 11-27. I


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FIG. 2. Escapements of four salmon species to British Columbia statistical areas 11-27, averaged by 6-yr periods. The contribution sf each stream to each perid average was estimated from only those years in the period for which the stream had a recorded estimate (this convention corrects for decreases in reported escapement dame to lack of field surveys in some years, especially 1980-82).

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FIG. 3. Number of streams in British Columbia statistical areas 1 1-27 for which escapement estimates were reported by field staff as "not recorded" or "not available." For recent years these reporting codes were mainly used to indicate that no field s w e y had been made. Can. 9. Fish. Aqueat. Sci., V01. 4 2 , 1985


I

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FIG. 4. Cumulative relative escapemnt as a function of stream rank order as in Fig. I, except that the average escapementfor each stream in each period is estimated from only those years in the period for which the stream had a recorded estimate. Compare with Fig. I, where nonsuweyed stream - y e a cases were assigned an observed escapement of zero.

ity rates, (3) selective increase in a few stocks due to salmonid enhancement activities, and (4) changes in counting effort and methods. The last of these factors cannot be ignored, as noted above for coho. F i g m 3 shows time series of the number of streams in areas 11-27 for which escapement estimates were reported as 'knavailable" (usually meaning no visit by field staff), '"unknown," or "not recorded-" For coho and churn, the majority of streams fell in these categories for 1980-82, and field staff have argued that this is due to loss of funding for seasonal field staff. However, this sampling problem does not fully explain the patterns shown in Fig. 1; a trend toward decreasing diversity is evident even when the average escapements for each time period are calculated using only the number of years (usually three rather than six for the 1978-83 period) for which estimates were actually recorded on each stream (Fig. 4). Comparing Fig. 1 md 4: it appears that roughly half of the decrease in apparent diversity for coho in the 1978-83 period can be explained by the sampling rate change for 1980-82. Fishery officers have reported increased difficulty seeing fish in many streams, due to turbidity and flooding that they

associate with logging and other habitat disturbances. However, such comments are temporally scattered throughout the records, and the most common disturbances involve mainly transient effects (lasting a few years) on turbidity and flow. Thus, the physical conditions for counting have probably not become cumulatively worse since 1950. Likewise, the records do not show a clear pattern of irreversible stock loss following reports of transient disturbance. There have been very few irreversible losses attributable to permanent habitat changes such as construction of hydroelectric dams. Even if the changes shown in Fig. 1 can be attributed largely to decreases in counting effort rather than changes in real abundance, there is still considerable cause for a l m . Deterioration in the escapement monitoring system will eventually leave the Department of Fisheries and Oceans without an adequate data base to assess stock trends and impacts of regulatory measures. Also, habitat protection measures are now justified largely by arguments about loss in potential production rather than current, realized performance; such arguments may be challenged by other watershed users in the future, with demands that fisheries interests demonstrate the actual economic value of fish produced. Such demonstrations will require data on historical escapement levels. Decreases in diversity have not been consistently associated with decreases in total escapement (Fig. 2). For pinks and chums, there have been deliberate efforts to rebuild escapements following a period of heavy exploitation in the 1950's (D. Anderson, DFO, Nanairno, B .C., pers. comrn.); it appears that these efforts have led to the selective recovery sf a few more productive stocks, while the majority of (small) stocks have failed to respond. This failure may reflect the pressure of depensatory mortality agents that have caused reduced stocks to keep declining even after exploitation rates have been reduced. For chinook and coho, exploitation rates have been increasing at Beast since 1978 (Argue et al. 19831, and most escapements have declined accordingly; the main exceptions are systems subject to major salmon enhancement projects. It should be noted that decreased stock diversity does not imply decreased total production in mixed fisheries. When the stocks cannot each be managed seperately, Paulik et al. (1 9671, Ricker (1973), and Hilborn (1985) have noted that the loss of some less productive units is practically inevitable. However, the price bf this loss may include increased variability of catches, and increased vulnerability of the mixed fishery to disturbances of the remaining stocks. Beyond such statistical effects and risks, decreased diversity will mean lost opportunities for improved management in the future through measures such as temind fisheries (where each stock can be harvested at a rate appropriate to its productivity) m d culture of stocks with unique and desirable genetic characteristics.

References ANONYMOU~. 1982. Salmonid Enhancement Program, Annual Report for 1982. Available from Department of Fisheries and Oceans, 1090 W. Pender, Vancouver, B.C. 2141s. ARGUE,A. W., W. HILBORN, I%. M. PETERMAN, M. J. STALEY, AND C . J. WALTER$.1983. Strait s f Georgia chinook and coho fishery. Can. Bull. Fish. Aquat. Sci. 21 1: 91 p. HILBORN, Ha. 1985. Apparent stock recruitment relationships in mixed stock . fisheries. Can. J. Fish. Aquat. Sci. 42: 718-723. LOWENZ, M. C. 1905. Methods of measuring the concentration of wealth. J. Am. Stat. Assoc. 9: 209-219. BAULIK, G . J . , A. S. HOURSTON, A N D P. A. LARKIN.196'7. Exploitation of multiple stocks by a common fishery. J. Fish. Res. Board Can. 24: 2527-2537. Can. J . Fish. Aqedat. Sci., bl.42, 698.5

PEET,R. K. 1974. The measurement sf species diversity. Annu. Rev. Ecol.


Syst. 5: 285-307. WICKER,W. E. 1973. Two mechanisms that make it impossible to maintain peak-period yields from stocks of Pacific salmon and other fishes. J . Fish. Res. Board Can. 30: 1295- 1286.

TAILLIE, C. 1979. Species equitability: a comparative approach, p. 5 1-62. In %.F. Grassle, G . P. Patil, W. Smith, and C. Taillie [ed.] Ecological diversity in theory and practice. International Cooperative Publishing House, Fairland, MD. 365 p.

APPENDIX. The top 18 streams contributing to escapements for the species-time combinations plotted in Fig. 1. Asterisks in the 1948-83 column indicate streams where escapements may have been substantially influenced by salmonid enhancement projects (from Anonymous 1982).

Chinook

Nimpkish Somass Southgate Cowichan Nahmint Klinaklini hntledge Hornathko Marble Tahsish

Cowichan Klinaklini Nimpkish Somas Wantledge Homathko Southgate Kingcome Marble Nanaimo

KBinakfini Somass Nimpkish Homathko Southgate Cswichan Campbell Kingcome Toba Puntledge

Somass Cowichan Toba Campbell Klinaklini Nimpkish Homathks Southgate Klite Toba

Somass Cowichan Campbell Nimpkish Toba Klinaklini Southgate Homathko Toba Wakeman

Somass* Cowichan Qualicm* Southgate Homathko Marble Campbell* Nanaimo Gold KBinaklini

Cshs

Somass Cowichan Kaleweiken Ni~~apkish Kokish Anuhati Kingcome Oyster Quatse San Josef

Cswichan Somass Kingcsme Toba Nirnpkish Oyster Wakeman Toquart Keogh Anuhati

Cowichan Somass San Juan Nimpkish Kingcome Koksilah Toba Tsolum Oyster Black

Somass Cowichan San Juan Nimpksh Kingcome Toba Black Marble Oyster Tsolum

Somass Cowichan Qualicum Toba Kingcome San Juan Demamiel Black Koksilah Kakweiken

Qudicum* Cowichan Somass* Quinsam* Kakweiken Wantledge* Marble Homathko Toba Black

Pink

Keogh Kakweiken Tsolum Quatse Oyster Amor de Cosmos Skwawka Adams Toba Deserted

Skwawka Glendale Tsolum Kakweiken Keogh Oyster Toba Adms Amor de Cosmos Deserted

Glendale Skwawka Amor de Cosmos Adams Keogh Phillips Burman Toba Anuhati Wakeman

Glendale Burman Keogh Quatse Grassy Kakweiken Amor de Cosmos Nahwitti Phillips Stranby

Kakweiken Wteman Glendde Kingcome Phillips Burman Adams Amor de Cosmos Grassy Keogh

Kakweiken Glendale Wortley Wakeman Anuhati Grassy Phillips Keogh Cluxewe Ahta

Chum

Cowichan Nimpkish Qualicum Chemainus L Qualicum Nanaimo Klinddini Nahmint Puntledge Kakweiken

Viner Cowichan Nimpkish Nanaimo Nahrnint E Qualicum Sarita hntledge Qualicum Toquart

Qualicum Cowichan Puntledge L Qualicum Viner Sound Toba Nitinat Nimpkish Tzoonie Glendale

Qualicum Cowichan I, Qualicum Sarita Nahmint Nitinat Nanaimo Puntledge Sooke Viner

Qualicum Nitinat Orford Cowichan Saita L Qualicum Puntledge Viner Nanaimo Sooke

Southgate Cowichan Qualicum* Orford hntledge* L Qualicum* Sarita Nanaimo* Chemainus* Nahmint

Can. J . Fish. Aquaf. Sci., Vod. 42, I985

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