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Population estimates of northern squawfish, Ptychocheilus oregonensis, at Bonneville Dam First Powerhouse, Columbia RiverMichael H. Gessel, Benjamin P. Sandford, Bruce H. Monk, and Dean A. BregeNational Marine Fisheries ServiceNorthwest Fisheries Science Center Coastal Zone and Estuarine Studies Division 2725 Montlake Blvd. E. Seattle WA 98112-2097 December 1994
U.S. DEPARTMENT OF COMMERCE Ronald H. Brown, Secretary
National Oceanic and Atmospheric Administration
National Marine Fisheries Service |
Purse seines, trap nets, gill nets, electrofishing, and angling have been used to sample northern squawfish populations in Columbia River reservoirs. With the exception of angling, these methods must be modified when used near hydroelectric dams. Trap nets and purse seines cannot be used in strong currents, but they have been used successfully in tailrace and forebay areas with reduced currents. Gill nets may injure or kill adult salmonids entering or exiting fishways. Electrofishing is usually more effective in shallow, slow current areas. Angling, although not generally a sampling method of choice, is an effective method for fish capture when target species are concentrated, as is the case with northern squawfish at hydroelectric dams. Thus, for this study, we used angling to derive a 1989 population estimate of northern squawfish in the Bonneville Dam First Powerhouse forebay pool.
Angling was conducted from the forebay deck at Bonneville Dam First Powerhouse during summer 1989, from 5 to 19 July, and again on 4 August. A crew of three to six fished light sport-tackle with artificial lures (rubber worm lures of various types and colors). Captured northern squawfish were placed in 500-L holding tanks supplied with river water and then moved to a tagging station, generally in less than 1 hour. Fish were measured to fork length (centimeters), tagged with a numbered Floy anchor tag, and marked with a hole punch on the left opercle. They were then released into a recovery net-pen in the forebay to exit volitionally. The catch and length of fishing time were recorded for each angler to derive catch per unit of effort (CPUE). Tag number, date, and location of recaptures were also recorded.
Three different methods were used for estimates of population (N) using the following notations:
m | = | number of periods | |
---|---|---|---|
Mi | = | total marked fish in forebay at the start of the ith sampling period (i = 1,..., m). | |
Ci | = | total sample taken in period i. | |
Ri | = | number of recaptures in the sample Ci. | |
R | = | (sum of) Ri | total recaptures during the experiment. |
Method 1: Schnabel (adjusted)
Schnabel's (1938) approximation to the maximum likelihood
estimator of population, N, from multiple censuses (Ricker 1975),
as adjusted by Chapman (1952, 1954) was
Approximate 95% confidence limits for this estimator were obtained by treating R as a Poisson variable and substituting limits found in Ricker (1975) for R in (1) above.
Method 2: Schumacher-Eschmeyer
Schumacher and Eschmeyer (1943) used the regression slope
estimator in the plot of recovery rate versus the number of
marked fish to obtain the following estimator:
Approximate 95% confidence limits for N were obtained by first calculating limits for 1/N and then inverting those limits. The confidence limits for 1/N were based on a t-value with m-1 degrees of freedom and the standard error (S.E.) of 1/N [see (3) below].
Method 3: Peterson (adjusted)
The 5-19 July marking periods were considered one marking
period and 4 August as a single sampling period. The Schnabel
estimator used in Method 1 was therefore reduced to a Peterson
estimator and used for N (adjusted for bias) (Ricker 1975, Seber
1982) as follows:
N = | (Ci + 1) Mi + 1) -------------------------------- Ri + 1 |
where i = 4 August.
Approximate 95% confidence limits for this estimator were again obtained by treating Ri as a Poisson variable and substituting limits found in Ricker (1975) for Ri in (4) above.
The choice of sampling period length for population estimates in Methods 1 and 2 was subjective, so data were grouped into different-sized sampling periods to determine an "optimum" grouping (John Skalski, Quantitative Science Department, University of Washington, Seattle, Pers. commun., September 1989). Four groupings were considered (4 August was excluded since it was separate from the other days): 1) individual days, 2) days grouped in twos (first three together), 3) days grouped in threes, and 4) days grouped in fours (first three together).
Grouping days reduced the inherent sampling variation (binomial or Poisson) and increased the number of recaptures per sampling period which reduced bias in the estimators (Ricker 1975). However, information was lost each time data were pooled. The optimum grouping was determined by comparing plots for observed recovery rate of tagged fish versus number of tagged fish in the population at the time of sampling. The smallest grouping which provided a good linear fit for this plot was selected (a good fit corresponded to a small sampling variation). Further grouping improved the fit but not enough to outweigh the loss of information.
Grouping recoveries into 3-day blocks to define sampling periods appeared optimum. Population estimates derived from the three methods were fairly close in agreement, ranging from 54,480 to 63,017 northern squawfish (Table 3). Wide confidence intervals made the differences between the three estimates insignificant. However, we consider the derived values only as estimates of the general order of magnitude of the northern squawfish population.
Date | Number caught | Number tagged | Cumulative tagged | Number recaptured | Cumulative recaptured |
---|---|---|---|---|---|
Jul 5 | 58 | 58 | 58 | 0 | 0 |
Jul 6 | 112 | 112 | 170 | 0 | 0 |
Jul 7 | 81 | 81 | 251 | 0 | 0 |
Jul 8 | 424 | 423 | 674 | 1 | 1 |
Jul 9 | 622 | 616 | 1,290 | 6 | 7 |
Jul 10 | 106 | 106 | 1,396 | 0 | 7 |
Jul 11 | 149 | 147 | 1,543 | 2 | 9 |
Jul 12 | 125 | 121 | 1,664 | 3 | 12 |
Jul 13 | 67 | 66 | 1,730 | 1 | 13 |
Jul 14 | 68 | 66 | 1,796 | 2 | 15 |
Jul 15 | 265 | 228 | 2,024 | 8 | 23 |
Jul 16 | 179 | 173 | 2,197 | 6 | 29 |
Jul 17 | 53 | 52 | 2,249 | 1 | 30 |
Jul 18 | 115 | 112 | 2,361 | 3 | 33 |
Jul 19 | 40 | 38 | 2,399 | 2 | 35 |
Aug 4 | 226 | 0 | 2,399 | 9 | 44 |
Tag number | Date tagged | Location1 | Date recaptured | Location1 | Days after tagging |
---|---|---|---|---|---|
00150 | July 6 | N | July 8 | N | 2 |
00066 | July 6 | N | July 9 | N | 3 |
00122 | July 6 | N | July 9 | 3 | |
00143 | July 6 | N | July 9 | N | 3 |
July 9 | N | ||||
004752 | July 8 | N | July 9 | S | 1 |
00239 | July 7 | N | July 9 | S | 2 |
00032 | July 5 | N | July 11 | N | 6 |
00348 | July 8 | N | July 11 | N | 3 |
00391 | July 8 | N | July 12 | N | 4 |
01103 | July 9 | N | July 12 | N | 3 |
00531 | July 8 | S | July 12 | N | 4 |
006363 | July 8 | S | July 10 | 2 | |
01645 | July 12 | N | July 13 | N | 1 |
00865 | July 9 | N | July 14 | S | 5 |
01153 | July 9 | N | July 14 | N | 5 |
01252 | July 9 | July 15 | N | 6 | |
01454 | July 11 | July 15 | S | 4 | |
01810 | July 14 | July 15 | N | 1 | |
01262 | July 9 | July 15 | S | 6 | |
01548 | July 11 | July 15 | S | 4 | |
01135 | July 9 | July 15 | S | 6 | |
00087 | July 6 | N | July 15 | N | 9 |
00803 | July 9 | July 15 | S | 6 | |
00115 | July 6 | N | July 16 | S | 10 |
00736 | July 9 | S | July 16 | S | 7 |
01435 | July 11 | N | July 16 | N | 5 |
00564 | July 8 | N | July 16 | S | 8 |
00186 | July 7 | N | July 16 | S | 9 |
00263 | July 8 | N | July 16 | S | 8 |
01288 | July 9 | N | July 17 | N | 8 |
00180 | July 7 | July 18 | N | 11 | |
00608 | July 8 | July 18 | N | 10 | |
01590 | July 12 | N | July 18 | 6 | |
01269 | July 9 | N | July 19 | N | 10 |
01680 | July 12 | N | July 19 | N | 7 |
016584 | July 12 | S | August 1 | ||
02393 | July 19 | N | August 4 | N | 16 |
01707 | July 13 | S | August 4 | N | 22 |
00134 | July 6 | N | August 4 | N | 29 |
00721 | July 9 | S | August 4 | N | 26 |
01009 | July 9 | N | August 4 | 26 | |
00094 | July 6 | N | August 4 | S | 29 |
00725 | July 9 | S | August 4 | N | 26 |
02416 | July 19 | N | August 4 | N | 16 |
01523 | July 11 | N | August 4 | N | 24 |
2 - Fish was hooked, but lost into the ice and trash sluiceway which empties downstream from the dam.
3 - Fish was captured by dip net at Cascade Locks, Oregon.
4 - Fish was double tagged; recaptured by electrofishing gear in tailrace area below Bonneville Dam Second Powerhouse.
The Schnabel and Peterson estimators were both adjusted to be relatively unbiased. The Schnabel and Schumacher-Eschmeyer estimators were reasonably unbiased for sample size since all Ri > 3 (Ricker 1975). Also, since R > 7 on 4 August, sample bias was negligible for the Peterson estimate (Seber 1982).
Estimator | N | Confidence limits |
---|---|---|
Schnabel (adjusted) | 58,891 | (44,022, 80,599) |
Schumacher-Eschmeyer | 63,017 | (53,475, 76,703) |
Peterson (adjusted) | 54,480 | (30,099, 108,960) |
There are three key assumptions on which these estimators depend:
Assumptions 1 and 2 were very difficult to examine with these data. Sustained mark-recapture sampling (individual tags) may have provided data to test these assumptions (Otis et al. 1978; John Skalski, personal communication); however, statistical methods have been developed to compensate for assumed bias (Pollock et al. 1984). Failure of Assumptions 1 and 2 would have led to serious bias in the Schnabel and Peterson estimates (Otis et al. 1978), and somewhat less serious bias in the Schumacher- Eschmeyer estimate. If fish did not intersperse well, the bias would probably have been negative, leading to an underestimate of the true population. If short-term interspersion did not occur (within the 2-week tagging period), the 4 August Peterson estimate would have been larger than the other two estimates. Since it was not, short-term interspersion did not appear to be a serious problem. If tagged fish had become "hook-shy" the bias would have been positive, leading to an overestimate of the population. We are uncertain whether this occurred. However, since we recaptured some fish the day after tagging, it is reasonable to assume that the bias was minimal.
Assumption 3 must only be approximately met for the usefulness of these methods (Ricker 1975). The Peterson estimate calculated N at the time of the second sample (4 August) and so required closure of the population (for inmigration) only during the sampling period; in this case, 1 day. Outmigrating fish would have produced a positive bias in the Peterson estimate. Overton (1965) described a method to account for known removals from the population estimate.
A sampling period of only 1 month was sufficient to ensure that Assumption 3 was reasonable for the Schnabel estimate (Ricker 1975). A good linear fit for the plot of recovery rate versus number of tagged fish indicated that it was reasonable to assume population closure (Seber 1982). Inmigration would have made the Schnabel and Schumacher-Eschmeyer estimates larger than N on 5 July, and smaller than N on 4 August (Table 2). Outmigration would have had the opposite effect. The effect of both would have created negative or positive bias depending on the magnitude of each. The relative agreement of the Peterson estimate with the other two population estimates provided evidence that large-scale inmigration or outmigration was not occurring.
The Schumacher-Eschmeyer estimate was utilized to provide a robust population estimate under the three key assumptions. The estimate of standard error for 1/N is quite robust (Seber 1982) and the estimate of N is somewhat robust. The Schumacher- Eschmeyer estimate should be used along with other estimates (Seber 1982).
Northern squawfish concentrations at hydroelectric projects on the Columbia River are a product of an artificial condition. Decreases in salmon populations may be partially related to the apparent increased northern squawfish population between 1980 (Uremovich et al. 1981) and 1989 (18,000 to > 50,000) at Bonneville Dam. The timing of northern squawfish spawning and changes in juvenile salmonid migration periods in the vicinity of Bonneville Dam may exacerbate predation problems. Northern squawfish in the Columbia River Basin spawn when water temperature is about 16°C (Jepsen and Platts 1959, Patten and Rodman 1969). Patten and Rodman (1969) described northern squawfish spawning behavior in Merwin Reservoir on the East Fork of the Lewis River in Clark County, Washington (approximately 70 km downstream from Bonneville Dam). They indicated that spawning probably occurs throughout June and July and that large concentrations of northern squawfish are in the spawning area for only a few days.
Hydroelectric dams delay juvenile salmonid migrations (Raymond 1988). Concurrent with dam construction, average river temperatures increased markedly at Bonneville Dam between the 1950s and 1980s (Fig. 2).
Both the delayed summer juvenile salmonid migrations and the increased water temperatures have altered this relationship. In the 1980s, most northern squawfish spawning probably occurred between 5 and 25 June (based on average water temperatures), while the subyearling chinook salmon migration occurred from 1 June through 15 August.
Predator/prey relationships are of necessity very delicately balanced. Any situation that substantially favors the predator must be short-lived or eventually both species will suffer. It appears that the construction of these hydroelectric sites has provided a setting that maybe upsetting this balance. In selected areas (forebays and tailraces of the dams) northern squawfish (that have completed spawning) may now concentrate predation at the protracted peak of the juvenile subyearling chinook salmon outmigration. This situation may substantially impact the Snake River fall chinook salmon, recently listed by the National Marine Fisheries Service (NMFS) as a threatened species under the Endangered Species Act (NMFS 1992).
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