Science and Information Branch NORTHWEST SCIENCE AND INFORMATION Aquatics Update 2002-1 Fall Walleye Index Netting (FWIN) and Trap Netting Summary for Niobe Lake, 2000 by T. Cano Introduction Since 1981, the Quetico Mille Lacs Fisheries Assessment Unit (QMLFAU) has been studying Niobe Lake gathering long-term biological and limnological information. This includes water quality data, as well as periodic estimates of population sizes, growth rates, and age distribution for fish species in the lake. The QMLFAU s most recent effort, monthly trap netting in the spring and summer of 2000, followed by Fall Walleye Index Netting (FWIN) meets these goals in two ways. The most direct result is the gathering of a variety of biological information including: sex, maturity, age, length, and weight distributions. Based on the number of fish marked and recaptured during the 2000 field season, population estimates for walleye and smallmouth bass were calculated. By examining these population attributes, the state of Niobe Lake s sport fishery was assessed. In addition, data from netting programs is used to fine-tune assessment methods. By comparing trap net catches each month with population estimates for a large number of lakes, a method of indexing fish abundances based on simple rapid assessment methods can be tested. However, because fish tend to prefer different areas in a lake as water temperature and oxygen levels change through the summer, trap net catches vary throughout the summer months (e.g. Wright 2000). The QMLFAU, through netting programs in various lakes, will help to determine the best time to perform this type of netting so that catches are reflective of actual fish abundance. In addition, trap netting provides a non-lethal method to acquire some of the data needed to manage fisheries resources, such as population estimates. Methods Trap nets were set overnight for 25 nights in May and August, and 26 nights in June and July, according to Nearshore Community Index Netting (NSCIN) provincial sampling standards as described by Stirling (1999). A total of four 1.82 m (6 foot) trap nets with 45.7 m (150 foot) leads were set randomly throughout the lake each day. All nets were set perpendicular to shore at a maximum depth of 5 m. Fish were sampled for fork and total length and marked by clipping the left ventral fin before live release (used to identify recaptures and facilitate Schnabel-Schumacher population estimates). In addition, all walleye and smallmouth bass were fitted with a coloured plastic sew-on disk tag applied in front of the dorsal fin. A FWIN program was carried out in September on Niobe Lake according to the Manual of Instructions Fall Walleye Index Netting (FWIN) (Morgan 2002). A total of 15 gillnets were set in the lake and left overnight. Nets consisted of eight panels of gill net, each 1.8 m (6 feet) high by 7.6 m (25 feet) long, ranging in size from one to six inches (stretched mesh). All fish captured were biologically sampled for fork and total length, round weight, sex, and maturity. In addition, tissues (scales, dorsal spines, otoliths, cleithra) were taken for aging purposes. Data were entered into FISHNET and submitted to the provincial database as well as being used to examine fish population status in Niobe Lake. 1
Results A grand total of 1,550 fish were caught in trap nets set during the summer of 2000. The most commonly captured species was white sucker, primarily because of large catches during May, followed by smallmouth bass and walleye. Catches during the three subsequent months were roughly equal (Table 1). Northern pike, yellow perch, and lake herring were also noted in catches to a lesser degree. Although lake trout are present in Niobe Lake, the summer field program did not target this species. Catches of smallmouth bass peaked in July, then fell off to lower levels by August. Catches of walleye oscillated through the program, and were highest in May and July. Smallmouth bass captured during the trap netting program numbered 302 fish ranging in size from 168 to 468 mm and averaging 342 mm. The average weight of smallmouth bass was 636 g (60 1750 g, n=156) (Figure 1). Based on multiplemark recapture information, a Schnabel-Schumacher population estimate for smallmouth bass was calculated at approximately 4,000 fish (range 2,700 10,000). Walleye population characteristics were acquired from both trap net and FWIN programs. Fork length data from both programs showed walleye to range from 116 to 661 mm fork length and average 379 mm. Round weights taken during the FWIN ranged from 40 to 3,750 g and mean weight was 1,104 g (Figure 2). FWIN netting resulted in a catch of 57 walleye, four of Table 1. Trap net catches by month and species May June July August Total FWIN (Recaptures) Smallmouth bass 73 65 118 46 302 2(1) Walleye 91 50 102 30 273 57(4) Northern pike 69 13 32 17 131 22(1) Lake herring 0 0 1 0 1 42(0) Yellow perch 2 0 0 1 3 4(0) Lake trout 0 0 0 0 0 17(0) White sucker 464 158 94 124 840 30(2) Figure 1. Smallmouth bass weight and length distribution 2
Figure 2. Walleye length and weight distribution which had been captured during summer trap netting. FWIN catch per unit effort (CUE) was 3.8 walleye per net, placing Niobe with below average population size (less than 5 walleye per net) for Ontario s walleye lakes (T. Marshall, Aquatics Biologist, NWSI, Thunder Bay). Ages ranged from 0 to 20 years and averaged 4.5 years (Figure 3). The small sample size and number of recaptures during the FWIN prevented calculation of population estimates using this data. Therefore, a Schnabel-Schumacher estimate was performed using trap net data and the population estimated at 1,250 fish (range 950 1,900). Figure 3. Walleye FWIN age distribution 3
Discussion Data acquired during FWIN and trap netting conducted during the 2000 field season indicate that healthy populations of both smallmouth bass and walleye exist in Niobe Lake. CUE in the FWIN indicates that walleye abundance is below the provincial average; however, the population appears healthy. Data acquired during the FWIN supplies population information from which indicators of stressed fish populations can be assessed. These indicators include: fewer age classes, early age at maturity, increased growth rates, and a high proportion of small fish with few larger individuals (OMNR 1982a). It was not possible to determine whether walleye exhibited early maturity due to the small sample size. All walleye sampled were mature at age 5 but because of a small sample size of age 3 and 4 walleye, it was difficult to determine the age when fish began to exhibit maturity. The sample demonstrates a wide range of ages, from young of the year (YOY) fish to age 20. Strong 1, 2 and 5 year classes also exist in Niobe Lake, indicating that spawning and recruitment into the fishable population are not problems. The presence of fish in year classes to age 20 also implies a healthy population of brood stock in the lake. The combination of these factors indicates that walleye in Niobe Lake are spawning and recruiting into the population successfully. Niobe Lake also supports a large number of smallmouth bass, estimated at 4,000 fish. Similar to walleye, smallmouth bass can be found in a wide range of sizes. Although ageing was not conducted on smallmouth bass, the broad size distribution found in trap net catches is suggestive of both young and older year classes. Length and weight data indicates that a high proportion of these fish are of a size to provide a quality angling experience. Niobe Lake supports several species of sportfish, including walleye, smallmouth bass, lake trout and northern pike. Netting conducted during the 2000 field season indicates that smallmouth bass and walleye populations are healthy at this time. Although walleye CUE during the FWIN placed the lake in the below average category (less than five walleye per net) relative to all Ontario walleye lakes, this finding needs to be put into the proper context. The physical attributes of Niobe Lake likely limit walleye yield. Both walleye and smallmouth bass were introduced to the lake in the 1960s. Compared to a lakes with conditions more suitable for walleye production, it is not surprising that Niobe Lake falls below the provincial average. It must also be noted that any given lake can only produce a finite amount of fish each year based on nutrient availability, climate, and other physical factors. In Niobe, this finite production is partitioned amongst lake trout, walleye, smallmouth bass and northern pike, as well as a wide variety of other fish species. Sixteen fish species have been reported in Niobe Lake (OMNR 1987) which imposes a ceiling on production by any single species (OMNR 1982b) and likely limits production of walleye. In addition, small fish are heavily preyed upon, resulting in low catch per unit effort but highly desirable sized fish. Thus, further addition of any walleye through stocking programs would likely have no positive effect on the population. At present, there is no need for additional management actions such as habitat mitigation or stocking. Current populations of fish species sampled during the summer of 2000 are healthy, as indicated by age structure, size distribution and other factors. Niobe Lake appears to be limited mainly by the boundaries of physical characteristics of the lake itself and not problems associated with stressed fish populations. 4
References Morgan, G.E. 2002. Manual of Instructions - Fall Walleye Index Netting (FWIN). Report of the Percid Community Synthesis Diagnostics and Sampling Standards Working Group. 20 pp + appendices. Ontario Ministry of Natural Resources. 1982a. The identification of overexploitation. Report of SPOF Working Group Number Fifteen. Ontario Ministry of Natural Resources. 1982b. Partitioning yields estimated from morpheodaphic index into individual species yields. Report of SPOF Working Group Number Twelve. Ontario Ministry of Natural Resources. 1983. The physical and chemical limnology of Niobe Lake, Ontario 1981-1982. Quetico Mille Lacs Fisheries Assessment Unit Report 1983-1. Ontario Ministry of Natural Resources. 1987. Lake Synopses. Report of Quetico Mille Lacs Fisheries Assessment Unit. Stirling, M.R. 1999. Manual of Instructions: Nearshore Community Index Netting (NSCIN). Ontario Ministry of Natural Resources, Lake Simcoe Fisheries Assessment Unit. Wright, J. 2000. Seasonal variation in trap net catches of smallmouth bass in two northwestern Ontario lakes. Ont. Min. Nat. Resour., Northwest Science & Tech., Aquatics Update 2000-1. 4 pp. This publication should be cited as: Cano, T. 2002. Fall Walleye index netting (FWIN) and trap netting summary for Niobe Lake, 2000. Ont. Min. Nat. Resour., Northwest Science & Information, Aquatics Update 2002-1. 5 pp. Ontario Ministry of Natural Resources Northwest Science and Information R.R.#1, 25th Side Road Thunder Bay, Ontario P7C 4T9 Phone: (807) 939-2501 Fax: (807) 939-1841 E-Mail: nwst@mnr.gov.on.ca MNR 51700 ISBN 0-7794-3524-9 (electronic edition) (07 30 02) 5