Methods

FISH SAMPLING TECHNIQUESThe techniques used to sample the fish and estimate the population density in the cascadelakes, vary according to the species and life-stage of interest. The following techniquesfor the different fish populations have been used from 1984 to present, except whennoted.1. Minnows (Dace, Mudminnows, Sticklebacks and assorted small cyprinids) andBass (young-of-year):The gear most used to sample these small-bodied species is the minnow trap. Thetraps are commercially available under the name Gee s Minnow Trap. They aremade of 1or4" galvanized hardware cloth with approximately 1" openings in eitherend. The only modification made to the commercial trap, is treatment with acidprior to use to remove the shine of the new trap. Shiny traps have been found tobe less effective. Twenty-four minnow traps are set in the littoral zone and 18 inthe pelagic zone of each lake. The littoral traps are set from stakes approximatelyuniformly spaced around the lake at depths of .5 m and on the bottom alternately.The pelagic traps are set at 1 and 3 m across the deepest portion of the lake on asingle transect with 9 stations (one 1 m and one 3 m trap at each station). Thetraps are set bi-weekly for 24 hr. All fish are counted and identified to species.The traps are normally set unbaited. Bait (bread) is used when large numbers ofminnows must be caught, such as for a Delury estimate (Ricker, 1975).A subsample of approximately 400 individuals is measured. These 400 fish areanesthetized in small batches with MS-222. The anesthetized fish are placed on aclipboard covered with a sheet of transparency plastic. The nose of the minnow isplaced against a retaining board and a hole is punched in the plastic at the end ofthe minnow s tail. The distance from the edge of the plastic to hole is measuredback at the lab to obtain total length in millimeters.Both mark-recapture and Delury population estimation are used to estimate theabundance of minnows. With mark-recapture, a large subsample, greater than1000individuals, from a minnow-trap capture are finclipped and released. Minnowtraps are then reset after waiting at least 1 but not more than 7 days. Thepopulation is then estimated using the ratio of marked to unmarked fish caught inthe second set. The Delury estimate is done by depleting the population usingrepeated minnow trapping and recording the catch and cumulative catch. Theminnows are held in floating net boxes at densities of 1000-2000 per cubic meteruntil at least four trap sets have been made. The population is estimated by theintercept of the regression between catch and cumulative catch (the level ofcumulative catch where catch equals zero). The Delury and mark-recaptureestimates provides an independent population estimate to correlate the catch-per132unit-effort (CPUE) of the regular minnow-trapping with known populationdensities.1332) Bass and perch (age 1plus and older):The larger fishes are sampled for population estimation twice each year, once inmid-May and once in mid-August. The primary technique used is nightelectrofishing with mark-recapture population estimation.A 16 foot Cofelt electrofishing boat, with dual booms (3 4-ft electrodes perboom), is used. Electroshocking is done largely perpendicular to shore,shocking from approximately 3 meters water depth to the shore. DC current isused to minimize damage to the fish. 600 volts provides adequate current, 2-6amps, to stun the fish.All fish are placed in a live-well on board the shock boat and are taken to ashore station for processing. On shore, the first 25 fish of each species arestomach pumped for gut analysis. Scales are also taken from a subsample of thefish for age analysis. All fish are identified to species, counted, measured (totallength in millimeters), and weighed with either a Pesola spring-scale or anOhaus electronic pan balance. If the spring-scale is used then the fish is simplyheld by the lip with the clip on the scale; if the electronic scale is used the fish iswrapped in a wet cloth to restrain the fish and the fish and cloth are weighedtogether. The weight of the cloth is removed by taring the scale with the clothprior to weighing the fish.If a markorrecapture estimation of the population is to be done, all fish sampledon the first night of electrofishing are marked. The fish are tagged withindividually numbered anchor tags (Wydoski and Emery, 1983) if the fish isgreater than 150mm total length and has not been previously tagged. If the fishis smaller than 150mm, the dorsal lobe of the caudal fin is clipped. The markedfish are then placed in a holding net until the first sampling is complete. Thefish are released at the end of the first night of electrofishing. To increase thepower of the mark-recapture technique, the number of marked fish is increasedby angling and marking fish on the day prior to electroshocking at night forrecapture. The population is sampled again the following night to estimate theratio of marked to unmarked fish. For the Delury estimation, fish are removedfrom the lake using several days of sampling effort using both angling andelectroshocking.3) Scale Samples:Scale samples are taken at least once a year, from at least 50 randomly selectedfish of each species. Large fish are usually sampled for scales when they arecaught. At least 5 scales are taken from each fish from the area below the originof the dorsal fin and above the lateral line. Scales are permanently mounted ona plastic slide later for aging and individual growth determination (Summerfeltand Hall, 1987).1344) Larval Perch (and other pelagic larvae), 1989-present:Two techniques are used to sample pelagic fish larvae: purse seining, and sonar.a. Purse seining is a method of enclosing a volume of water in the pelagiczone with a net and filtering that water to obtain the larval fish. The netdesign and technique are described in Evans and Johannes (1988). Thenet used in the Cascade project is 33 meters long and 6 meters deep, madeof polyester net material with 1.6mm openings and dyed green.b. Sonar is used in conjunction with purse seining to obtain the sizefrequency,species composition, and spatial distribution of icthyoplankton.The Cascade project uses both 70 and 200 khz sonar. The transducer istowed approximately 20cm under the surface. The technique is outlined inThorne (1983). The HADAS acoustics processing hardware and softwareis used to analyze the recorded signal (see Rudstam, 1988, for adescription of the analysis).STATISTICAL POPULATION ESTIMATIONThe two techniques used in the Cascade project to estimate population density are themodified Peterson mark-recapture and Delury estimations (Ricker 1975).a. Mark-recapture is used for populations that are not being intentionallydepleted in a lake such as the bass population in Paul Lake.b. Delury estimates are used when a population is being removed from a lakesuch as for the bass in Peter Lake in the fall of 1989 or for minnows whichare easily handled.135REFERENCES:Evans, D.O. and P.R. Johannes. 1988. A bridle-less trawl and fine-mesh purse seine forsampling pelagic coregonine larvae with observations of the spatial distributionand abundance. Ontario Fish. Tech. Rep. no 27:1-19.Ricker, W.E. 1975. Computation and interpretation of biological statistics of fishpopulations. Department of the Environment, Fisheries and Marine Service,Fisheries Research Board of Canada Bulletin 191, Ottawa, Canada.Rudstam, Lars G. 1988. Patterns of zooplanktivory in a coastal area of the northernBaltic proper. Doctoral thesis at the University of Stockholm.Summerfelt, R.C. and G.E. Hall. 1987. Age and Growth of Fish. Iowa State UniversityPress, Ames. Iowa.Thorne, R.E. 1983. Hydroacoustics. pp. 239-260. In: L.A. Nielsen and D.L. Johnson ed.Fisheries Techniques. American Fisheries Society, Bethesda, Maryland.Wydoski, R. and L. Emery. 1983. Tagging and Marking. pp. 215-237. In: L.A. Nielsenand D.L. Johnson ed. Fisheries Techniques. American Fisheries Society,Bethesda, Maryland.136

Methods

please see abstract for methods description