Status of Lake Whitefish Populations In Lake Superior Mike Petzold Upper Great Lakes Mgmt Unit, Ontario, M.N.R. For Mar. 2002, Annual Meeting of the Lake Superior Committee Title: Status of lake whitefish in Lake Superior The lake whitefish harvests in Lake Superior are currently approaching the historic highs observed in the late 1800s. Populations had undergone progressive declines into the 1920’s, when hatchery programs were implemented to increase whitefish abundance. Stocks apparently rebuilt in the late 1940’s, but harvest declined sharply in the late 1950s, concomitantly with the collapse of the Lake trout fishery. The subsequent withdrawal of large amounts of effort permitted populations to progressively increase over the next four decades. However, calibration of effort over this time span is required, given the changes in gear quality (e.g., mesh size and mesh depth), in order to assess changes in abundance. The nominal commercial catch-per-unit of effort in Ontario waters currently exceeds 100 kg/km in most zones, and is at the highest level since the beginning of the record (1950). Increases in abundance have been associated with decreases in growth rate, and increase in the mean age of the catch, varying between eight and ten years of age. Increased fishing pressure in south-eastern Lake Superior, since the mid 1990’s, has been followed by progressive decreases in CPUE, with concomitant increases in growth. To adequately determine the trajectory of these populations, assessment and simulation models, will need to incorporate changes in growth with density, as well as find objective measures for adjusting for changes in catchability.

Acknowledgments M. Ebener (CORA) B. Mattes (GLIFWC) Members of LSTC

Introduction Habitat Harvest and Effort History Management Guidelines Current Status Recommendations In this talk I will provide an overview of the habitat of Lake Whitefish in Lake Superior, review factors affecting the historical trends in abundance, including effort and harvest, discuss management guidelines and assessment of populations with regard to guidelines. I will discuss current status and prognosis for whitefish, as well as provide recommendations for research, management and assessment.

Known and Historic, Lake Whitefish Spawning and Nursery Areas Thunder Bay Kaministikwia R. Michipicoten R. St. Louis Estuary Isle Royale Bete Grise Bay Figure. Locations of known historic spawning and nursery grounds of lake whitefish in Lake Superior (from Coberly and Horral 1980; Goodier 1981; Goodyear et al. 1982). Lake whitefish nursery grounds are shown surrounding Isle Royale. Figure from Draft Fish Community Objectives, 2001 Spawning and nursery habitat is in sand to rock substrate in areas <9m. Early reports also indicated the existence of river-spawning populations, such as those in the St. Mary’s River rapids above the control gates, the St. Louis River in the U.S., and the Michipicoten, Dog, and Kaministikwia Rivers in Ontario (Lawrie and Rahrer 1972). There are extensive spawning grounds on shallow gravel in Whitefish Bay. Many of the river stocks were adversely affected by early logging practices. The St. Louis estuary stock was eliminated near the early part of the 20’th century. Keewenaw Bay Apostle Is. Whitefish Bay From Bronte et al, unpublished manuscript

Ontario Annual Whitefish Catch Distribution Whitefish Habitat Figure. Mean annual (1990-1995) Ontario whitefish catch by grid in relation to whitefish habitat (water less than 80 m). Adult whitefish occupy water less than 80 m in depth in Lake Superior. This habitat represents less than 23% of Lake Superior surface area. Ontario gillnetters concentrate effort in this depth zone. A large portion of the harvest comes from relatively few grids, in Whitefish Bay and Thunder Bay. CATCH (T) . 5 1 . 2 . 4 . 8 . 1 6 . 3 2 .

Total Harvest in Metric Tons 2 CATCH (T) 1 Figure. Annual harvest of Lake Whitefish in metric tons, from 1989 to 1999, from all jurisdictions. Harvest records from all jurisdictions begin around 1989, which was also the highest reported extraction on record, at 2,200 metric tons. At the early part of the period, whitefish were the primary species harvested in Lake Superior, but were replaced by lake trout and herring by 1900. Harvest declined continuously until the mid 1920’s at around 300 tons. The initial decrease in harvest was due to a variety of factors, but the “fishing up” processe (Lawrie and Rahrer, 1972), was the likely cause of the initial decline, together with the destruction of habitat by deposition of woody debris from extensive logging. Harvest gradually increased to another peak in 1950 at 700t, when it declined sharply with the collapse of the lake trout fishery. 1 1 1 1 1 1 1 1 1 1 1 1 8 9 9 9 9 9 9 9 9 9 9 9 8 1 2 3 4 5 6 7 8 9 9 9 9 YEAR MI MN ON WI

(large mesh gillnet equivalents) LARGE MESH GILLNET EFFORT (KM) Total Effort (large mesh gillnet equivalents) 3 2 LARGE MESH GILLNET EFFORT (KM) 1 Figure. Total annual large-mesh effort ‘equivalents’ for Lake Superior, from 1950-1999. US data series commences in 1972. Michigan data is missing for 1999. Effort includes gillnet, trapnet and angling effort from state, provincial, tribal fishermen. All effort was “standardized” in terms of large mesh gillnet effort. Lawrie and Rahrer (1972) suggested decline in whitefish harvest in late 1950’s was due to lamprey mortality; however, compilation of effort statistics for Ontario waters indicate a large removal of effort after the collapse of the lake trout fishery, which could account for the reduction in lake whitefish harvest. 1 1 1 1 1 1 9 9 9 9 9 9 5 6 7 8 9 9 9 YEAR MI MN ON WI

ONTARIO LARGE MESH GILLNET, BY MESH DEPTH 1 9 8 7 6 5 PERCENT 4 3 2 Figure. Mesh Depth distribution of Ontario commercial gillnets from 1950 to 2000. Gear quality and catchability have increased along with increasing effort. Relatively shallow gillnets used in the early 1950s (23 mesh or less) have been replaced by deeper gillnets. Most nets are now greater than or equal to 50 meshes deep. 1 5 6 7 8 9 YEAR OTHER MESH DEPTH < 18 18 19-25 30-36 50 >50

COMMERCIAL CATCH PER KM OF LARGE MESH GILLNET 1 9 1 8 1 7 1 6 1 5 1 4 1 3 1 2 CPUE KG/KM 1 1 1 9 8 7 Figure. Commercial CPUE data from 1975-2000. Data was pooled from all agencies, including tribal fisheries, where reported. Note: CPUE data has not been adjusted for changes in gear quality; therefore, CPUE trend likely overestimates abundance. In Ontario, most zones outside of Whitefish Bay (ON-33, ON-34) follow the lake-wide trend of increasing CPUE through time. In Eastern Michigan (CORA) zones (WFS-6, WFS-7 and WFS-8) CPUE follows the statewide trend, although those of WFS-5 follow the Ontario and Wisconsin trends. Lakewide biomass increase has also been confirmed by bottom trawls, which increased from 1.0 kg/km (1978-83) to 2.5 kg/km (1984-88), and to 6.5 kg/km (1989-94; SCOL2 Bronte et al, unpublished). There have been significant changes in quality of effort, however. 6 5 4 3 1970 1980 1990 2000 YEAR MI ON WI

Catch per km of gillnet for Western Ontario Zones 6 5 4 CPUE (KG/KM) 3 2 1 Figure. Trends in annual commercial CPUE data for western Ontario waters of Lake Superior. Trends in CPUE vary among management units within jurisdictions, but generally follow the overall trend of increasing CPUE through time. For example, CPUE in western Ontario waters have increased to a high level in Thunder Bay (Zone 1); there has also been a gradual increase to high catch rates in Black Bay (Zone 7). However, the CPUE outside of the Black Bay (Zone 9), has been declining since the early 1980s. 7 7 7 7 7 8 8 8 8 8 8 8 8 8 8 9 9 9 9 9 9 9 9 9 9 5 6 7 8 9 1 2 3 4 5 6 7 8 9 1 2 3 4 5 6 7 8 9 YEAR Zone 1 7 9

KG PER KM Geometric Mean CPUE Whitefish Bay (April-Sept.) 1 1 1 9 8 7 1 9 8 7 6 KG PER KM 5 4 3 Figure. Geometric mean CPUE from spring and summer commercial catch-effort data for southeastern Ontario waters of Lake Superior. In Whitefish Bay (ON-34 and ON-33) CPUE reached a peak in in the late 1980’s, exceeding 80 kg/km, but gradually fell by the late 1990’s. The pattern is similar to that observed in the nearest US zones (WFS-7 and WFS-8). 2 1 7 4 7 6 7 8 8 8 2 8 4 8 6 8 8 9 9 2 9 4 9 6 9 8 00 YEAR ZONE ON-3 3 ON-3 4 Data plotted where effort >25 km

ESTIMATED SPAWNING BIOMASS - EASTERN MICHIGAN Figure. Spawning biomass estimated for eastern Michigan waters, estimated by statistical catch-at-age analysis (Ebener, unpublished data) . Spawning stock biomass estimates (WFS-6, WFS-7 and WFS-8) correspond to the the south-eastern Ontario commercial CPUE trend with a peak in the late 1980s followed by a gradual decline to the year 2000. However, the increasing spawning biomass in WFS-5 corresponds with the overall CPUE pattern for Ontario (excluding Whitefish Bay) and Wisconsin. Data from M. Ebener, CORA

Management Guidelines/Objectives Extractions of 0.51 kg/ha (water < 73 m) Busiahn(1990) Total annual mortality (adults) between 60 and 65% Busiahn(1990) Maintain self-sustaining populations within range of abundance observed between 1990-1999 (Draft Fish Community Objectives, 2001).

Commercial Fish Regulations Individual Transferable Quotas (Ontario) Seasons and/or Licence Limits (Tribal Fisheries, States) Gear Restrictions (all agencies)

Mean Annual Yield in kg/ha, Lake Superior, 1998-2000 DATA FROM B. MATTES, 2001 REPORT TO THE GLFC Figure. Current extractions (averaged for years1998-2000) of whitefish (kg/ha) for water designated as whitefish habitat. In Ontario, surface areas of each management, considered to be suitable whitefish habitat, are based on charts with the 91 m depth contour, and in water less than 73m in the US. The highest extractions, on a per ha basis, occur in Thunder Bay at 2.1 kg/ha (ON-1). Other zones that exceed 0.51 kg/ha are Black Bay Penn (ON-9, 0.68 kg/ha), Pukaskwa South (ON-24), Whitefish Bay (ON-34, 0.74 kg/ha; WFS-8, 0.574 kg/ha). Despite the high extractions from Thunder Bay, nominal CPUE has been increasing since the mid-1970s. In Ontario, zones were designed for the management of discrete stocks of any species (e.g. lake trout, walleye and lake whitefish) in order to prevent progressive elimination of discrete stocks. Zone boundaries were also established to accommodate historical fishing grounds of individuals, or groups of individuals, and to minimize conflicts among users. Some zones may be aggregated for assessment purposes. Kg/ha . - . 2 . 2 - . 9 . 9 - . 2 5 . 2 5 - . 3 1 . 3 2 - . 5 3 . 5 4 - 2 . 1 3 9

Mean Age (top number) and Annual 6.8 11.5 7.7 10.0 8.3 8.7 63% 7.8 55% 7.9 59% 9.4 55% 8.0 49% 7.6 69% [33%] 7.7 35% [36%] 7.4 38% [51%] 8.7 49% Figure . Total annual mortality and mean age of lake whitefish from large mesh commercial gillnets (>113 mm) from various management units within lake Superior. Data source: COTFMA, GLFWIC and OMNR. Mortality estimates were calculated by linear regression for six ages following the modal age. Estimates were only included where precision of the estimate was within 10% at the 95% confidence level. Due to bi-modal distributions, mortality could not be calculated for ON-1 and ON-28. The current average age in the harvest varies considerably across the lake. The higher density areas (as indicated by CPUE or biomass estimates) have older age distributions than those with lower CPUE. The average age varies from a low of 6.8 years in ON-07 to a high 11.5 years in ON-01. “Except for some very lightly fished stocks, ages 5-9 whitefish comprise 80% of the harvest in most areas “(Bronte et al, SCOL2 unpublished report). Annual mortality, as calculated from catch-curves of commercial large-mesh samples (for one year beyond the modal age), varies from a low of 35% in WFS-6, to a high of 69% in WFS-5. All management units, with the exception of WFS-5, are below the maximum target guideline of 65%. These estimates are likely biased upwards due to gillnet selectivity. Estimates based on catch-at age analysis are available from Michigan zones WFS-5, WFS-6, WFS-7 and WFS-8 - the age of maximum catchability was determine to be age 8. 8.2 56% Mean Age (top number) and Annual Mortality (bottom), 1997-2000 from Catch Curve 7.5 47% [62%] Note: Number in brackets is mortality of age 8 fish from Catch at Age Analysis

Age Distribution in Commercial Gillnets THUNDER BAY (ON-1) 2 1 9 1 8 1 7 1 6 1 5 1 4 1 3 AGE 1 2 1 1 1 9 8 7 6 Figure. Age distribution of whitefish from commercial large mesh gillnet harvest, Thunder Bay (Zone 1). Thunder Bay exhibits the oldest age distribution in Lake Superior, despite the fact that extractions are the highest., and CPUE is the highest. The trend to older fish has been on-going since the early 1980’s 5 4 3 2 7 7 7 7 7 8 8 8 8 8 8 8 8 8 8 9 9 9 9 9 9 9 9 9 9 5 6 7 8 9 1 2 3 4 5 6 7 8 9 1 2 3 4 5 6 7 8 9 YEAR

MEAN FORK LENGTH(CM) FROM LARGE MESH GILLNETS 5 3 5 2 5 1 5 4 9 Mean Fork Length (cm) 4 8 4 7 4 6 4 5 4 4 Figure. Average fork length of lake whitefish in large mesh commercial harvest. Despite increasing harvests, the average size of whitefish in the harvest has been increasing in many zones, e.g. (ON-1, ON-7 and WFS-7). Note means plotted where sample size exceeded 100 fish. 4 3 7 7 7 7 7 7 8 8 8 8 8 8 8 8 8 8 9 9 9 9 9 9 9 9 9 9 4 5 6 7 8 9 1 2 3 4 5 6 7 8 9 1 2 3 4 5 6 7 8 9 YEAR Mgmt. Unit ON-1 WFS-7 ON-33

Percent Mature in Commercial Catch, Whitefish Bay 1 9 9 9 8 9 7 9 6 9 5 PERCENT MATURE 9 4 9 3 9 2 9 1 Figure ? Percent of female whitefish mature in eastern Michigan waters of lake Superior. Maturity data (Figure ?) in Michigan waters (WFS-7 and WFS –8) indicate that most (>90%)of the harvest is composed of mature fish. The relationship between percent maturity and length, however, varies considerable among Management Units, and is likely density dependant. 9 8 9 8 8 9 1 9 2 9 3 9 4 9 5 9 6 9 7 9 8 9 9 YEAR WFS-7 WFS-8

PROPORTION FEMALES MATURE BY LENGTH 1 . . 9 . 8 . 7 . 6 PROPORTION MATURE . 5 . 4 . 3 . 2 Figure. Percent maturity of female whitefish as a function of total length. Michigan data is from from CORA (Mark Ebener), MI -7 ( pooled 1980-2000), MI -6 and MI-8 from 1985-2000. The relationship between percent maturity and length, however, varies considerably among Management Units, and is likely density dependant. Lake whitefish have been found to be resilient to exploitation (Smith 1972), and can sustain annual mortalities as high as 65% (Clark 1985). Mortality compensation is primarily through a decrease in the average age-at-first maturity (Jensen 1993), and by increased fecundity and growth (Healey 1975). . 1 . 4 5 6 7 TOTAL LENGTH-CLASS (CM) Mgmt. Unit WFS-7 WFS-6 ON-33 WFS-8

MEAN TOTAL LENGTH - AGE 7 TOTAL LENGTH (CM) 6 2 6 1 6 5 9 5 8 5 7 5 6 5 5 5 4 TOTAL LENGTH (CM) 5 3 5 2 5 1 5 4 9 Figure ? Mean total length of lake whitefish at age seven in the commercial large mesh gillnet harvest. The average size at age seven is a surrogate for growth. The decline in mean size from 1974 to 1990 is an indication of density dependant growth at a time when the commercial CPUE was increasing. As CPUE began to decline in the early 1990’s mean size began to increase in Whitefish Bay (WFS-7 and ON-33). 4 8 4 7 4 6 7 7 5 8 8 5 9 9 5 YEAR ON-1 ON-33 WFS-7

Stock Status Summary Abundance is high in most zones, likely a consequence of stocks rebuilding after removal of effort when lake trout fishery collapsed in 1950s Mortality below 60-65% guideline in most zones Harvest composed mainly of mature fish Average harvest weight has been increasing or is static in many zones Prognosis is good for lake whitefish in most areas

However, Extractions, lake-wide, approach Historic maximum Exceed 0.51 kg/ha in Whitefish Bay, Thunder Bay, Black Bay, and S. Pukaskwa Abundance declining in Whitefish Bay (WFS-7, WFS-8, ON-33, ON-34)

Recent Applications Statistical Catch-at-age-analysis (CORA) Simulations and Projections (CORA) Habitat Mapping (MN, CORA-Whitefish Bay) Food Web Modelling (ECOSIM) As we approach historic maximum harvests levels, empirical harvest criteria need to be refined by more detailed models that are sensitive to changes in recent recruitment. Stock recruitment relationships need to be determined and incorporated into the model. CORA has recently applied these model to 1836 ceded waters of Michigan.

Recommendations Implement: Index Fisheries to Acquire relative abundance estimates maturation rates, fecundity, growth Methods to Delineate Stock Habitat Mapping Spawning and nursery areas Refine and Develop Models Seek ways to reduce lake trout by-catch Index Fisheries are required because the changing commercial fishing gear and practices makes it difficult to assess abundance from commercial data alone. Secondly, samples from commercial gear give biased growth rate parameters.