Lake Wanapitei Summer Profundal Index Netting (SPIN) Technical Report

Transcription

1 Lake Wanapitei Summer Profundal Index Netting (SPIN) Technical Report Kimberley Tremblay Fisheries Biologist September Wallace Road, Unit 5 North Bay, ON P1B 8G4 Phone: (705) Fax: (705) aofrc@aofrc.org Report WNP-0625

2 ii T a b l e o f C o n t e n t s List of Tables... List of Figures... iii iii Introduction... 1 Methods... 1 Results... 3 Discussion Conclusion References... 16

3 iii Lis t o f T a b l e s Table 1: 2006 SPIN catch composition for Lake Wanapitei... 4 Table 2: Table 3: Table 4: Mean ages, fork lengths, and weights for lake trout male, female and sexes combined, Lake Wanapitei Distribution of lake trout catch over the depth stratums, Lake Wanapitei, For lake trout with fork lengths between mm Catch per Unit Effort of lake trout, lake whitefish and lake herring over three depth stratums in Lake Wanapitei, L i s t o f F i g u r e s Figure 1: Lake Wanapitei 2006 SPIN sample sites... 3 Figure 2: Percent catch distribution, by age, for lake trout, Lake Wanapitei Figure 3: Percent catch distribution, by fork length for lake trout, Lake Wanapitei Figure 4: Percent catch distribution for immature and mature male lake trout, Lake Wanapitei Figure 5: Percent catch distribution for immature and mature female lake trout, Lake Wanapitei Figure 6: Von Bertalanffy growth rate for lake trout from Lake Wanapitei. L(t) =791[1-e -0.05(t ) ], R and Lake Temagami, 2006L(t) =683[1-e (t ) ], R Figure 7: Weight-length relationship for lake trout in Lake Wanapitei and Lake Temagami Figure 8: Diet composition of lake trout from Lake Wanapitei SPIN Figure 9: Lake trout spatial catch distribution, Lake Wanapitei,

4 1 I n t r o d u c t i o n Wahnapitae First Nation is located approximately 29 km northeast of the Greater City of Sudbury, off Hwy 17, on the shores of Lake Wanapitei. One of the largest inland lakes in the province, Lake Wanapitei, has a surface area of ha. with a maximum depth of 113 m. The fish community includes walleye (Sander vitreus), northern pike (Esox lucius), smallmouth bass (Micropterus dolomieui), lake herring (Coregonus artedi), lake whitefish (Coregonus clupeaformis) and lake trout (Salvelinus namaycush). For generations, Lake Wanapitei has supported a modest subsistence fishery for the Wahnapitae First Nation. The community is concerned about rising pressure on the fishery from non-native angling, shoreline development and fluctuating water levels from the operation of a hydroelectric dam. The Anishnabek/Ontario Fisheries Resource Centre (A/OFRC), along with the Wahnapitae First Nation, have completed a variety of fisheries assessments starting with a Fall Walleye Index Netting Assessment (FWIN) in 2004, a Spring Littoral Index Netting (SLIN) survey in 2005, and finally a Summer Profundal Index Netting Survey (SPIN) in The SLIN conducted in 2005 did not provide conclusive results on the stock status of the lake trout population; therefore, a new assessment method, Summer Profundal Index Netting (SPIN) survey, was adopted in M e t h o d s The SPIN method was developed by the Ontario Ministry of Natural Resources (OMNR) for their State of the Resource initiative. In 2002, the OMNR started an initiative that involved state of the fisheries resource (SOR) monitoring and modernization of the province s recreational fisheries regulations (OMNR 2004). This included changing fishing regulation management zones and exploring different methods to determine stock status (i.e. SPIN) of fish populations (i.e. lake trout). A/OFRC staff were trained in the application of the SPIN methodology by Steve Sandstrom, Supervisor, Muskoka Lakes Fisheries Assessment Unit, OMNR, and Nigel Lester, Scientist, Aquatic Resource and Development Section, OMNR. The MLFAU provided gillnets and the SPIN sampling guidelines (Sandstrom Unpublished 2006). For the SPIN assessment, a lake is spatially divided into a number of strata, based on lake area, to ensure netting effort is equally allocated. The lake is also divided into three different depth strata (10 m 20 m, 20 m 30 m, 30 m 40 m), with an optional stratum for depths greater than 40 m (Figure 1). Lake Wanapitei was divided into 4 spatial strata with 4 depth strata, based on these criteria. A daily sampling regime was devised using the SPIN Support Spreadsheet (Sandstrom Unpublished 2006). In the beginning nets are randomly set across all depth stratums. The daily lake trout catch is then entered into the SPIN spreadsheet. This program determines how many nets should be set in which depth stratums based on the number of sets, the proportional area for each depth stratum, the sampling effort, and the relative catch success. This spreadsheet helps target the depth stratum where the lake trout are located, and also ensures that all stratums are being sampled accordingly.

5 2 The SPIN gillnets were 64 m long (8 mesh sizes x 8 m panels) and consisted of a random series of mesh sizes (57 mm, 64 mm, 70 mm, 76 mm, 89 mm, 102 mm, 114 mm, 127 mm). To increase the effort two gillnets were tied together. Nets were set between 0800 and 1700 hours for a minimum period of two hours, with a maximum of ten sets a day. The SPIN survey was conducted from July 24, 2006 to August 11, 2006, when surface water temperatures were 21 o C. A total of 96 two hour sets and 2 overnight sets were completed (Figure 1). Depth was determined with a depth sounder, and net location was recorded using a Global Positioning System (GPS) unit. Surface water temperature was recorded with a pocket thermometer upon lifting each net. All lake trout and lake whitefish were biologically sampled for: fork length, total length, weight, aging structures (otoliths), and stomach content. Annual adult mortality (A), the proportion of fish that die in a given year, was estimated for fish ages 7 to 20 using the following Robson and Chapman equation (Ricker 1975 p. 31): S = T/ ΣN + T 1 where S is annual adult survival, T = N1 + 2N2 + 3N3, N = N0 + N1 + N2, and N is the number of fish whose age is greater than or equal to age 7. Annual adult mortality (A) was then calculated as A=1-S and expressed as a percent. Lake trout density, catch corrected for net selectivity, percent chance to detect change, and power (the probability that the test will reject a false null hypothesis), were determined using the SPIN Support Spreadsheet (Sandstrom Unpublished 2006). Lake trout with fork lengths greater than 300 mm and less than 1500 mm are considered in the SPIN support spreadsheet calculations (Sandstrom Unpublished 2006) because this is the size range that is fully recruited to the gear. Growth was determined using the Von Bertalanffy equation L(t) = L 1990). (1 - exp[-k(t-to)]) (Lester

6 3 Figure 1: Lake Wanapitei 2006 SPIN sample sites R e s u l t s The overall target of the SPIN protocol is to ensure that the sample size provides a relative standard error (RSE) of After 96 two hour net sets the arithmetic relative standard error (RSE) was 0.17 and the project was complete. A total of 112 lake trout (RSE 17.8%) and 26 lake whitefish (RSE 24.8%) were caught (Table 1).

7 Catch ( % of Lake Trout) 4 Table 1: 2006 SPIN catch composition for Lake Wanapitei Species Catch (no) Percent of Total Catch (%) 98 nets Percent Relative Standard Error (% RSE) Lake Trout Salvelinus namaycush Lake Herring Coregonus artedi Burbot Lota lota Lake Whitefish Coregonus clupeaformis Longnose Sucker Catostomus catostomus White Sucker Catostomus commersonl Rainbow Smelt Osmerus mordax Walleye Sander vitreus The Robson and Chapman (Ricker 1975) annual mortality and survival rates, for lake trout are A = 16 % and S = 84% (0.84 ± CI 0.038), respectively, with 95% confidence intervals. The weight of lake trout, sexes combined, ranged from 160 g (0.4 lbs) to 3900 g (8.6 lbs). The mean weight was g ± (SE) (~2 lbs) (Table 2). A wide range of age classes, from 4 to 29 years, was represented in the catch (Figure 2). The mean age, for sexes combined, was 10 years (Table 2) Age (years) Figure 2: Percent catch distribution, by age, for lake trout, Lake Wanapitei 2006.

8 Catch (% of Lake Trout) 5 Table 2: Mean ages, fork lengths, and weights for lake trout male, female and sexes combined, Lake Wanapitei Sex Mean Age (yrs) Mean Fork Length > 400mm (Spawners) Combined (N=107 SE 0.47) (N=51 SE 9.46) Female (N=48 SE 0.52) Male 11 (N=53 SE 0.78) (N=22 SE 11.60) (N=29 SE 14.27) Mean Fork Length (mm) (N=111 SE 8.95) (N=49 SE 12.62) (N=55 SE 12.94) Mean Weight (g) (N=111, SE 71.38) (N=49, SE 96.78) (N=55, SE ) The distribution of the catch according to fork length, for sexes combined, ranged from 250 mm to 678 mm (Figure 3) The mean fork length for all lake trout was mm ± 8.9 (SE)(Table2). When correcting for net selectivity (Standstrom Unpublished 2006), the adjusted mean fork length for all captured lake trout, is 385 mm ± 95 (SE) Fork Length (mm) Figure 3: Percent catch distribution, by fork length for lake trout, Lake Wanapitei 2006.

9 6 Male lake trout start to mature at a fork length of 275 mm with the majority of lake trout maturing around 425 mm in length (Figure 4). The mean fork length of spawning male lake trout (> 400 mm) was mm ± (SE) (Table 2). Although female lake trout begin maturing at a fork length of 425 mm, the majority do not reach maturity until 525 mm (Figure 5). The mean fork length of spawning female lake trout (> 400 mm) was mm ± (SE) (Table 2). When correcting for net selectivity the mean fork length for lake trout, sexes combined, is 494 mm Catch (% of Lake Trout) Immature Males Mature Males Fork Length (mm) Figure 4: Percent catch distribution for immature and mature male lake trout, Lake Wanapitei 2006

10 Catch (% lake trout) Immature Females Mature Females Fork Length (mm) Figure 5: Percent catch distribution for immature and mature female lake trout, Lake Wanapitei 2006 For comparative purposes, results of the 2006 SPIN conducted on Lake Temagami, a lake of similar size, latitude, and productivity, is included in the following figures. The Von Bertalanffy equation that best describes the rate of lake trout growth in Lake Wanapitei for 2006 is L(t) =791[1-e -0.05(t ) ], R For comparison, the growth rate of lake trout in Lake Temagami, which has a surface area of ha, is represented by the equation L(t) =683[1-e (t ) ], R (Figure 6).

11 Total Length (mm) Mean Length Temagami Mean Length Wanaptei VonBertalanffy Wanapitei VonBertalanffy Temagami Age (Years) Figure 6: Von Bertalanffy growth rate for lake trout from Lake Wanapitei. L(t) =791[1-e -0.05(t ) ], R and Lake Temagami, 2006L(t) =683[1-e (t ) ], R

12 Weight (g) Temagami LogW= Log FLEN Wanapitei LogW= Log FLEN Fork Length (mm) Figure 7: Weight-length relationship for lake trout in Lake Wanapitei and Lake Temagami 2006 Lake trout condition can be expressed by the weight-at-length relationship, using the equation W=aL b (Figure 7). There is a strong relationship between weight and length for lake trout in both Lake Wanapitei (P=0.0001, R 2 =0.97) and Lake Temagami (P=0.0001, R 2 =0.84).

13 10 Smelt Empty Fish remains Minnow Herring Invertebrates Stickleback Sucker Sculpin Figure 8: Diet composition of lake trout from Lake Wanapitei SPIN 2006 Smelt makes up a large proportion of the summer diet of lake trout in Lake Wanapitei (Figure 8). Lake trout in Lake Wanapitei consumed organisms ranging from macro-invertebrates to fish species (Figure 8).

14 11 Table 3: Distribution of lake trout catch over the depth stratums, Lake Wanapitei, For lake trout with fork lengths between mm. Stratum (m) # Sites # Nets Total Catch % Mean CUE (fish/net) Relative Standard Error (RSE) Figure 9: Lake trout spatial catch distribution, Lake Wanapitei, 2006

15 12 When only considering the lake trout that are fully recruited to the SPIN net (fork length mm) a total of 63 lake trout were caught. The observed catch per unit effort (CUE), for fish with fork length between mm, was 0.37 fish/net (n= 96, sd = 0.62). When comparing each stratum, the majority of lake trout were caught in the m strata (Table 3). Spatially the lake trout catch was distributed on the northeastern portion of the lake, which coincides with the deeper water (Figure 9). An estimated density of 1.9 lake trout per ha, giving a population estimate of fish (68% prediction level) (Sandstrom Unpublished 2006). There is a 36% chance that a declining change in the Wanapitei lake trout population will be detected with a statistical power (probability that the test will reject a false null hypothesis) of 80%. Table 4: Catch per Unit Effort of lake trout, lake whitefish and lake herring over three depth stratums in Lake Wanapitei, 2006 Stratum Lake Trout (fish/net) Lake Whitefish (fish/net) 2 (10 20 m) (20 30 m) (30 40 m) Lake Herring (fish/net)

16 13 D i s c u s s i o n This SPIN index netting assessment project gives a snapshot view of the lake trout population in The results show a well rounded population that has good recruitment and retention of older individuals. Lake Wanapitei supports a healthy but low density lake trout population, in comparison with other lakes from across the province (Sandstrom Unpublished 2007). Density of lake trout can be affected by multiple factors including a lakes carrying capacity, recruitment problems, or high mortality (i.e. fishing pressure). Carrying capacity is determined by abiotic (i.e. habitat characteristics, water clarity, depth) and biotic factors (i.e. fish community composition, aquatic invertebrate density). Lake trout in Lake Wanapitei have good recruitment and an acceptable rate of mortality. The following discussion compares results from the SPIN assessment for Lake Wanapitei with those conducted in the same year on Lake Temagami and Cold Lake (Sandstrom Unpublished 2007). Both Lake Temagami and Lake Wanapitei are large lakes with similar surface areas, ha and ha, respectively. These lakes are in similar geographic locations, have the same fish species, and they are easily accessible by similar sized human populations, so fishing pressure would be comparable. The age and length distributions of lake trout, based on the 2006 SPIN results, were similar for both lakes, as were the survival rates, mean sizes, and mean lengths. However, the density of lake trout is almost four times greater in Lake Temagami (6.8 fish/ha) than in Lake Wanapitei (1.9 fish/ha). What factors contribute to such a difference in lake trout density for these two similar lakes? Lake trout diet influences lake trout growth rates and spatial distribution (Sellers et al. 1998). The shapes of these two lakes are distinctive; Lake Temagami is a sprawling lake with many islands and Lake Wanapitei is shaped like a deep bowl. Lake Temagami has considerably more preferred habitat in the m depth range thereby possibly supporting a greater density of fish. In Lake Wanapitei lake herring and lake whitefish were found to be more abundant in the 10 to 20 m depth stratum, which was the same depth range that lake trout dominated. The fish community composition is quite different; notably, there is a higher density of lake whitefish in Lake Temagami compared to Lake Wanapitei. Common forage items for lake trout include pelagic coldwater fish species such as lake herring and lake whitefish, as well as pelagic invertebrates (Hassinger and Close 1984, Pazzia et al 2002). If forage fish are not available, invertebrates make up the bulk of the lake trout diet and growth rates are reduced. Lake whitefish are believed to be native to Lake Wanapitei and lake herring were introduced into the lake in 1951 as an additional food for lake trout (Bragg et al. 1962). Rainbow smelt (Osmerus mordax), an exotic species of marine origin, was introduced in recent decades (1970 s) into lakes throughout the Sudbury region (Hallman. and Gunn 1996). Based on the stomach content analysis, the common diet item for Lake Wanapitei lake trout was the rainbow smelt. Lake trout in Lake Temagami have a more rapid rate of growth than those in Lake Wanapitei, which may be closely related to diet. Condition can be an indicator of food resource availability and development of gonads (Guy and Brown 2007). When comparing condition between the two lakes, Lake Wanapitei lake trout are shorter and fatter at older ages than Lake Temagami lake trout. Lake Temagami, however, has a larger available forage base of lake whitefish which might contribute to the observed differences in growth and condition, between the two lakes.

17 14 Cold Lake, with a surface area of ha, is located in a provincial park in northern Alberta. The deep (100 m) bowl shape of Cold Lake is very similar to Lake Wanapitei, yet Cold Lake has a high density of large bodied lake trout (6.1 lake trout /ha) (Sandstrom Unpublished 2007). Cold Lake also has a high density of lake whitefish and lake herring. Lake trout and lake herring are caught at all depths because their populations are so large they are forced into non preferred habitat (Sandstrom Unpublished 2007). Lake Wanapitei does not support the high biomass of fish species that Cold Lake can support, even though the habitats are similar. All three of these lakes have similarities in size and species present; however, there are other contributing factors (e.g. habitat availability, aquatic invertebrates, coregonid species, chemical parameters) that make each lake have a distinct carrying capacity. Maximum sustainable yield (MSY) is a theoretical value of fish harvest which can occur safely, without leading to a collapse in fish stocks to the point where future production is compromised. Fisheries scientists have realized that this definition is not completely accurate, since using MSY as a target level for harvest can make fish stocks vulnerable to over fishing and collapse (Lester et al. 2002). MSY estimates are better used as limit reference points, defining thresholds which should never be exceeded. MSY for lake trout in Lake Wanapitei was 0.62 kg/ha/year using the Shuter et al. (1998) equation (with a total dissolved solids (TDS) of mg/l and lake area of ha). The next step is to gain an understanding of the harvest rates for lake trout in Lake Wanapitei. A creel survey would give an indication of the recreational fishing pressure that the fishery is experiencing. It would also provide a population estimate that can be compared with the SPIN estimate. The implementation of a subsistence and commercial catch sampling program, within the Wahnapitae First Nation, would provide additional information regarding harvest numbers. A long term monitoring plan should be put in-place where the same netting methodology (SPIN) would be conducted every five years, or two consecutive years every 10 years, to monitor changes in density. This plan should also include survey designs for creel surveys, commercial harvest surveys, and other fish community indices.

18 15 C o n c l u s i o n In 2006, the lake trout population in Lake Wanapitei was healthy. The population estimate for 2006 is approximately 1.9 lake trout/ ha, or fish. This is a low population density but it is to be expected in large lakes. Different factors affecting the carrying capacity of fish species (lake whitefish, lake herring and lake trout) in Lake Wanapitei limit the productivity of this lake. Monitoring the change in density over the years and determining the amount of lake trout harvested yearly are important questions to answer in the future.

19 16 R e f e r e n c e s Braggs, E., C. Edwards, and D. Gillespie A progress report on the lake Wanapitei. OMNR Sudbury District. Guy, C.S. and M. L. Brown Analysis and Interpretation of Freshwater Fisheries Data. American Fisheries Society. 458p. Hallman, A. and J. Gunn Nepahwin Lake Watershed: Its past, present, and future. Co-operative Freshwater Ecology Unit, Laurentian University Hassinger, R.L. and T. L. Close Interaction of Lake Trout and Rainbow Smelt in Two Northeastern Minnesota Lakes. Division of Fish and Wildlife Department of Natural Resources. No. 379 Lester, N.P., M. M. Petzold, W.I. Dunlop, B.P. Monroe, S.D. Orsatti, T. Scaner, and D.R. Wood Sampling Ontario Lake Trout Stocks: Issues and Standards. Lake Trout Synthesis Sampling Issues and Methodology Working Group Ontario Ministry of Natural Resources. 117p. Ontario Ministry of Natural Resources 2004 State of Resource Reporting and Fisheries Regulation Streamlining Workshop Proceedings Queen s Printer for Ontario ISBN Pazzia, I., M. Trudel, M. Ridgway and J.B. Rasmussen Influence of food web structure on the growth and bioenergetics of lake trout (Salvelinus namaycush). Can. J. Fish. Aquat. Sci. 59: Ricker, W.E Computation and interpretation of biological statistics of fish populations. Bull. Fish. Res. Board. Can. 191:382p. Sandstrom, S. Unpublished 2006 SPIN Support Spreadsheet Ver 7.0 BETA Muskoka Lakes Fisheries Assessment Unit Ontario Ministry of Natural Resources. Sandstrom, S. Unpublished 2007 Lake Temagami and Cold Lake SPIN Support Spreadsheet Ver 7.0 BETA Muskoka Lakes Fisheries Assessment Unit Ontario Ministry of Natural Resources. Sellers.T.J., E.R. Parker, D.W. Schindler, and W.M. Tonn Peiagic distribution of lake trout (Salvelinus namaycush) in small Canadian Shield lakes with respect to temperature, dissolved oxygen, and light. Canadian Journal of Fisheries and Aquatic Sciences 55: Shuter, B.J. M.L. Jones, R.M. Korver, and N.P Lester A general, life history based model for regional management of fish stocks: the inland lake trout (Salvelinus namaycush) fisheries of Ontario. Can. J. Fish. Aquat. Sci. 55: