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Journal of Great Lakes Research 38 (2012) 514–523

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Journal of Great Lakes Research

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Relative demand by double-crested cormorants and anglers for ﬁsh production from lakes on Manitoulin Island, Lake Huron

Mark S. Ridgway a,⁎, Warren I. Dunlop b, Nigel P. Lester a, Trevor A. Middel a a Harkness Laboratory of Fisheries Research, Aquatic Research and Development Section, Ontario Ministry of Natural Resources, Trent University, 2140 East Bank Drive, Peterborough, Ontario, Canada K9J 7B8 b Fisheries Policy Section, Biodiversity Branch, Ontario Ministry of Natural Resources, 300 Water Street, Peterborough, Ontario, Canada K9J 8M5 article info abstract

Article history: The magnitude of angler harvest (kg·ha−1·yr−1) and cormorant consumption (kg·ha−1·yr−1) were com- Received 3 November 2011 pared for a set of lakes (N=11) on Manitoulin Island. Empirical models relating total phosphorus to total Accepted 21 June 2012 fish production as well as production to body mass were used to scope the possible range of fish production Available online 24 July 2012 and to partition production among small, medium and large size segments of fish populations, respectively. Medium (66–112 g) and large (>200 g) size segments were defined as size categories targeted by cormo- Communicated by Francesca Cuthbert rants (stomach diet analysis) and anglers (creel interviews), respectively. Angling effort and cormorant den- Index words: sity were estimated from aerial surveys of the lake set during the open water season and for anglers during Manitoulin Island the winter ice-fishing season. Results showed that anglers harvested almost all large fish production, assum- Lake Huron ing the mean total fish production model, and 43% of large fish production under the more optimistic upper Aerial survey 80% prediction limit of total fish production. Cormorant consumption of medium fish production was less Fish production (39% using mean regression model; 15% using upper 80% prediction model) than angler consumption of Phalacrocorax auritus large fish production. Anglers therefore imposed more population stress on their preferred sizes of fish than cormorants imposed on their preferred sizes of fish. Population stress was increased when cormorant consumption of medium size fish was discounted from contributing to large fish production. Angler harvest near (or above) sustainable yield levels will be exacerbated and appear as a fish collapse when cormorants consume fish production destined for fish size segments preferred by anglers. Crown Copyright © 2012 Published by Elsevier B.V. on behalf of International Association for Great Lakes Research. All rights reserved.

Introduction cormorants consumed large quantities of subadult yellow perch (Perca flavescens) and walleye (Sander vitreum) compared to larger fish sizes Anglers generally view double-crested cormorants (Phalacrocorax taken by anglers (VanDeValk et al., 2002), and this consumption auritus) as competitors for fish production in the Laurentian Great accounted for reduced fish populations (Rudstam et al., 2004). Con- Lakes with the relative demand for fish by cormorants defining the sumption of yellow perch occurred at sizes beyond the size range ‘cormorant issue’ in the region (Muter et al., 2009). Technical and policy where compensatory mechanisms operate (Rudstam et al., 2004). Cor- information needed to help resolve the apparent conflict is extensive morant consumption of subadult fish therefore reduced fish available (Behrens et al., 2008; Cowx, 2003; Harris et al., 2008). Addressing for anglers. cormorant effects on fish populations should include some combination Differences between anglers and cormorants in prey size have been of long-term studies of fish population dynamics, trophic relationships observed in other locations in the Great Lakes. Mortality of sub-adult among birds and fish, understanding the role of compensatory pro- smallmouth bass (Micropterus dolomieu), ages 3–5, increased in eastern cesses in fish populations, and use of total production indexes relative Lake Ontario following cormorant population increases (Lantry et al., to cormorant and fishery consumption of fish production (Wires et al., 2002), and in the Les Cheneaux Islands, Lake Huron, multiple fishery 2003). metrics pointed to increased sub-adult mortality for yellow perch Most of these requirements were met in a study of cormorant–fish (Fielder, 2008). In both cases, increased mortality resulted in reduced interactions on Oneida Lake through a combination of long-term study fish targeted by the fishery. of population dynamics of fish and predator–prey relationships among Angler and cormorant preferences for fish of different sizes can aid cormorants and fish (Rudstam et al., 2004). There, double-crested in assessing relative prey demand at sites with less detailed long-term data than Oneida Lake. Two general observations facilitate using pro- fi ⁎ Corresponding author. Tel.: +1 705 755 1550. duction indexes relative to angler and cormorant demand for sh. E-mail address: [email protected] (M.S. Ridgway). First, cormorants consume smaller fish (most fish≤25 cm length;

0380-1330/$ – see front matter. Crown Copyright © 2012 Published by Elsevier B.V. on behalf of International Association for Great Lakes Research. All rights reserved. doi:10.1016/j.jglr.2012.06.013 Author's personal copy

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Johnson et al., 2002, 2006; Seefelt and Gillingham, 2006; VanDeValk Methods et al., 2002) than larger fish sizes preferred by anglers (most fish≥ 25 cm length; VanDeValk et al., 2002). Second, there is an allometric Study area relationship relating body mass (W) of aquatic organisms to production (P/B) with the exponent −0.4 (i.e., W−0.4; Banse and Mosher, Manitoulin Island is the largest freshwater island in the world 1980; Dickie et al., 1987; Randall and Minns, 2000). We can therefore with an area of 2766 km2 (Chapman and Putnam, 1973). It is located expect fish size classes preferred by cormorants to be more productive in northern Lake Huron and separated from the mainland by the than fish size classes preferred by anglers. An estimate of total fish North Channel (Fig. 1). Lakes on Manitoulin Is. are generally small production in an aquatic ecosystem could be partitioned into relevant (geometric mean surface area=90.2 ha, mean maximum depth= size classes as a means of assessing demand by anglers and cormo- 5.4 m; Jackson and Harvey, 1989). Mean depths of lakes in this rants. Use of fish production indexes in this manner is along the study ranged from 2 to 15.2 m and surface area ranged from 229 ha lines requested by Wires et al. (2003). to 10,613 ha (see Appendix 1 in the Supplementary data). Most The purpose of this study is to determine the relative magnitude lakes contain warm water fish assemblages with Lake Manitou also of fish production removed by double-crested cormorants on inland containing lake trout (Salevinlus namaycush) and a coldwater assem- lakes of Manitoulin Island and to evaluate whether this represents blage (Harvey, 1978). A comparison of fish assemblages and environ- an additional burden of sustainability on recreational fisheries on mental parameters showed that Manitoulin Island and neighboring these lakes. Cormorants feed extensively on lakes of Manitoulin Is. Bruce Peninsula represent distinct fish assemblage regions due mostly where up to a third of cormorants in the northern Lake Huron region to the underlying dolomitic limestone (relatively high pH; Harvey and can be found on a seasonal basis (Ridgway and Middel, 2011). Prey Coombs, 1971) and lake size (Jackson and Harvey, 1989). demand (kg∙ha−1∙yr−1) by cormorants from large lakes on Manitoulin Lake selection for this study followed guidelines outlined in Is. is nearly identical to levels observed in the North Channel of Lake McGuiness et al. (2000) that included a size stratified random sub- Huron indicating the importance of these lakes for cormorants in the sample of 10 lakes selected from all lakes greater than 50 ha (N=32). region (Ridgway and Middel, 2011). If selected lakes were unsuitable for sampling (e.g. no access, too We incorporate the concept of population indicators of stress to shallow, etc.) they were substituted with lakes of a similar fish commu- describe demand for fish production (see Shuter, 1990). Stressors nity and surface area (Harvey, 1978). Lake Wolsey, an embayment of on fish populations are those that change the environmental carrying the North Channel, was also added to the lake list. capacity of a population (ecosystem level) or those that affect survival and reproduction of members of populations (population level; Shuter, Water samples 1990). Depending on what fish sizes are exploited, the stress imposed on populations due to demand for fish production can affect different Water samples were collected in the first week of June 2005 fish life stages. For anglers, targeting mature adults stems from the size following spring overturn when lakes were thermally mixed and a bias or preference for a catch of larger fish. For cormorants, capture of surface water sample represented whole-lake conditions. Parameters smaller fish largely constitutes their diet which can include smaller included total phosphorus, pH, dissolved organic carbon, conductivity, sizes of fish targeted by the recreational fishery. Cormorant consump- and alkalinity. The total phosphorus value used in the Downing et al. tion could remove production for recreational fisheries prior to the (1990) model was the average of two samples taken at each lake. action of the fishery itself. Because population-level stress is expressed Lake location, surface area, mean depth and chemical limnology as a decrease in the absolute abundance of adults (Shuter, 1990, parameters are listed in Appendix 1 in the Supplementary data. p. 149), we use the demand for fish production in medium (cormorant consumption) and large,(angler harvest) size segments of fish relative Cormorant density and consumption to total fish production of these size groups (i.e., percent consumed or harvested), as an indicator of stress. Aerial line transect distance sampling was used to estimate density of Fish production is the population-level elaboration of biomass double-crested cormorants (Buckland et al., 2001; Ridgway, 2010b,c). accumulated each year through the trade-off between growth of Distance sampling is employed widely in estimating animal abundance individual fish and losses due to mortality (Haddon, 2001). A fish pop- and has received recent attention as a preferred method relative to ulation in a steady state exhibits no net production because the accumulated growth of individuals is balanced by losses of individuals through annual mortality. Total fish production is the cumulative production of all fish populations in a lake. Direct measurement of fish production in a lake is laborious, requiring intensive sampling of all fish species and multiple years of data. Minimum requirements for direct estimates require average weight by age to assess growth for each population combined with estimates of biomass stemming from estimates of population size. A direct method of estimating fish production was not feasible in our study because we required production estimates in many lakes from a single year of study. More impor- tantly, anglers and cormorants move among lakes so a multi-lake examination of demand for fish production required a model approach. The Downing et al. (1990) empirical model predicting total fish production from total phosphorus concentration was used to estimate total fish production among a set of lakes ranging in surface area from approximately 200 ha to >10,000 ha on Manitoulin Island. We evaluated how the stress imposed by cormorants would exacerbate angling stress if angling harvest was not reduced to accommodate cormorant consumption. The degree of change in angling stress registers the extent to which combined cormorant and angler harvest threatens Fig. 1. Map of Manitoulin Island identifying survey lakes for estimating cormorant density the sustainability of a fishery. and angling effort. Insert map shows location of Manitoulin Island in Lake Huron. Author's personal copy

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traditional index count methods (Rosenstock et al., 2002). Details of The midday activity bias factor values (Km) ranged from 1.1 to 1.3 and the cormorant aerial surveys are provided in Ridgway (2010b,c) and were selected for each lake from values presented in Parker et al. Ridgway and Middel (2011). Analysis of cormorant sighting data was (2006) based on fish community (lake trout, walleye or multi-species conducted using Distance 3.5 (Thomas et al., 2010). targeted) and day-type (weekend vs weekday; Kaufman et al., 2009). Flights took place from mid-June to the end of August and occurred For the ice-fishing season, parameter values used were: N=90 every 2–3 weeks. The sampling flights occurred on June 14–17, July (64 weekdays; 26 weekend days); T =10 h; p =2; Km ranged from 6–13, July 25–27, Aug 15–17 and Aug 29–31, 2005. Cormorants were 1.0 to 1.2. categorized as on the water, flying or loafing on shore with density Angler interviews were conducted on the study lakes throughout in each behavioral category combined for an overall density estimate the regular fishing seasons (open-water and ice fishing) to determine for each lake (Ridgway, 2010b,c). The overall density estimate for target species, catch per unit effort (CUE), harvest per unit effort lakes from each flight were assumed to represent bird density for (HUE), and to collect biological data from harvested fish (length, specific time periods that included Apr 20–June 30; July 1–21; July weight, aging structures). Surveys were conducted on weekdays and 22–Aug 12, Aug 13–Aug 31, and Sept 1–30. weekend days at times selected to maximize angler encounter rates. Data for cormorant density on large lakes were combined (>2000 ha; Despite this sampling strategy, biological sample sizes were small or Lakes Manitou, Kagawong, Mindemoya, and Wolsey) and summarized in samples were unavailable for some lakes where anglers were rarely Ridgway and Middel (2011). For this study, that aggregated estimate was encountered with harvested fish (e.g., Bass, Loon, Sucker, Tobacco, partitioned into lake-specific estimates of cormorant density to facilitate Windfall). lake comparisons. For small lakes, aircraft speed did not allow a distance Angler interview data were used to calculate the proportion of sampling approach because of the fast sampling speed relative to a small effort (effort fraction) directed toward each target species. Total esti- lake surface area. A single flight was flownovereachsmalllakeduring mated effort was multiplied by the effort fraction to estimate targeted each sampling date and the number of birds detected on the lake was effort. Mean harvest per unit effort (HUE) values from species specific recorded. Lakes where this protocol was followed have only one survey anglers were combined with targeted effort estimates to estimate transect line. total harvests (in numbers) of target species (Malvestuto, 1983). For each lake, total consumption (kg∙ha−1∙yr−1) was based on com- Harvest weights by species were calculated from estimated harvest bining density estimates of double-crested cormorants (number∙km−2) numbers multiplied by the mean weight (kg) of fish sampled from with daily food requirements for the period of nest construction angler harvest. For lakes with small sample sizes or missing data, through fledging (Apr 20–July 21; 0.542 kg∙d−1; Ridgway, 2010a)and mean weights estimated from other lakes were applied. after fledging (July 22–Sept 30; 0.436 kg∙d−1; Ridgway, 2010a). Harvest by weight (kg∙yr−1) was divided by the surface area of each lake to calculate annual yields (kg∙ha−1∙yr−1). Yields for Angler harvest open-water and ice-fishing seasons were summed to estimate annual yield for each species and then summed across species to estimate Aerial surveys were used to collect midday activity count (MAC) total annual yield. These values were compared with estimates of data to estimate angler effort following standard protocols developed cormorant consumption. for provincial fisheries monitoring programs (Kaufman et al., 2009; Size selectivity of fish by anglers and cormorants was compared Lester et al., 1991; McGuiness et al., 2000). Aerial surveys of anglers on Kagawong Lake. This fishery is dominated largely by catches of were distinct from aerial surveys of cormorants. Aerial surveys were yellow perch and smallmouth bass. Forty cormorants were lethally conducted during the fishing seasons (open-water and ice fishing) sampled after being observed foraging on Kagawong Lake and subse- to count fishing parties and estimate total annual angling effort quently roosting on the lakeshore. Fork lengths of identifiable fish still (angler∙h·ha−1·yr−1) for each of the study lakes. Midday flights intact were recorded from cormorant stomachs and compared to the (i.e., between 1000 and 1400 h) were conducted weekly, alternating length of angler caught fish in the open water and winter fishery. Fish between weekdays and weekends. sizes harvested by anglers in other lakes were also summarized for Eighteen (9 weekday and 9 weekend day) open water season comparison with fish sizes consumed by cormorants on Kagawong flights were conducted between June 7, 2005 and Sept 29, 2005. Lake. Twelve (6 weekday and 6 weekend day) ice-fishing season flights were conducted between January 8, 2006 and March 21, 2006. Total Fish production effort estimates were calculated for weekday and weekend strata, and summed by season. Annual estimates were obtained by summing We used an empirical model to predict fish production in the across seasons. lakes, (Equation 4, Downing et al., 1990). The model demonstrates −1 −1 −1 Estimates of annual angling effort (Ej;h∙yr ) were calculated for that total fish production (FP,kg∙ha ∙yr ) increases with total each lake as: phosphorus concentration (TP, μg∙L−1):

MAC T p N ¼ j j j j ¼ − : þ : − : ðÞ2 Ej log10 FP 0 319 1 441 log10 TP 0 209 log10TP Km;j : R2 ¼ 0:79; n ¼ 14; SE ¼ 0:296 where j identifies a season (open water vs. winter), MAC is the midday activity count from aerial surveys, T is the length of the fishing day (hours), p is mean party size, N is the number of fishing days in Over the range of phosphorus levels in Manitoulin lakes, this the season, and Km is the midday bias factor. Km is the sum of all equation (solid line in Fig 2)impliesfish production increases (approx- daily angler counts on a lake divided by the hourly count of anglers imately linearly) with phosphorus. Our main diagnosis of possible (Lester and Dunlop, 2004; Parker et al., 2006). The ratio adjusts cormorant effects was based on the assumed fish production predicted total effort based on mid-day flights used for detecting anglers on by the middle line in Fig. 2. Using indicators to predict total fish produc- lakes. Instantaneous angler counts from roving creel surveys (N= tion, whether TP or other parameters (e.g., morphedaphic index, Ryder, 228 lakes) spanning Ontario, including the Manitoulin Is. region, 1965; macrobenthos, Hanson and Leggett, 1982; Matusek, 1978;ther- were used to estimate Km (Parker et al., 2006). mal habitat volume, Christie and Regier, 1988), provides point esti- For the open-water season, the following parameter values were mates of production. The 80% prediction limits (SE×1.2815) of the TP used: N=121 (84 weekdays; 34 weekend days); T=14 h; p =2. model were used to capture uncertainty in this relationship for the Author's personal copy

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because angling stress can be measured in the same manner: the ratio of angling harvest to potential fish production serves as an index. When comparing these stress indicators, it is important to recognize that fish production available to cormorants and anglers depends on the size of fish they harvest. Our stress analysis includes data from Oneida Lake (New York) for comparative purposes where the impact of cormorants is well-documented (cormorant consumption= 2.88 kg/ha; Table 3 in Rudstam et al., 2004). Oneida Lake TP was 17.5 μg∙L−1 in recent years (Hall and Rudstam, 1999). Harvest and length-at-age data were gathered from publications summarizing the walleye and yellow perch fishery on Oneida Lake from the late 1990's (Rose et al., 1999; VanDeValk et al., 2002; VanDeValk et al., 2005; Xe et al., 2005). Based on this information angler yield was approximately 3.17 kg∙ha−1∙yr−1 from that time period and was used in calculations of angler harvest relative to production of fish. Fig. 2. Predicted fish production based on phosphorus concentration (Downing et al., 1990). The solid line is the predicted mean and dotted lines are 80% prediction limits. These analyses supplied measures of population stress resulting from cormorants and anglers. To assess the impact of cormorants on the angling fishery, we re-calculated measures of angling stress Manitoulin Island lake set (dashed lines in Fig. 2). Given management given the level of cormorant stress. Large fish production is potentially and policy concerns on the issue of angler and cormorant competition reduced in the presence of cormorants because cormorants consume for fish production, we used these prediction limits to scope the relative medium-size fish that in turn support the production of larger fish demand for fish production by anglers and cormorants. When we ex- harvested by anglers. Consequently, measures of angling stress are in- amined estimates of total fish production based on the lower 80% pre- creased. Because production generally offsets losses due to mortality, diction limit (Fig. 2) we found total fish production was low enough the proportion of medium fish production that transfers to the large to question whether sustainable harvesting or consumption could fish category (=0.5) was based on interpolating the relationship be- occur at all. Because of these results we did not pursue any further anal- tween instantaneous mortality and size of large prey in cormorants ysis of the lower estimate. We focused on the upper 80% prediction limit (approx. 250 mm; Fig. 1 in Randall and Minns (2000); see Results). to provide relatively optimistic estimates of total fish production for The impact of cormorants on the angling fishery can be assessed by comparison with Equation 4 in Downing et al. (1990). the extent to which cormorants increase measures of angling stress. Production was partitioned among size classes of fish using the To account for this effect, we subtracted cormorant consumption Shuter method (Leach et al., 1987). This method builds on the rela- from medium fish production and multiplied this value by 0.5 to arrive tionship between P/B and body mass of organisms where production at a revised estimate of large fish production incorporating losses is allometrically related to body mass with an exponent of −0.4 due to cormorants. The proportion of angler harvest relative to the re- (i.e., W−0.4), whether for fish or aquatic invertebrates (Banse and vised fish production estimate was calculated based on the mean Mosher, 1980; Dickie et al., 1987). The exponent value is within the predicted total fish production for Manitoulin Is. lakes (i.e., Equation 95% confidence limits (−0.23, −0.47) of the exponent relating fish 4inDowning et al., 1990) as well as the upper 80% prediction limit. P/B to weight (Randall, 2002; Randall and Minns, 2000). The average The phosphorus model was not used to predict fish production in production generated by a size range of fish can be calculated from Lake Wolsey. It is connected to the North Channel so fish production the geometric mean of the bounds on any size interval (Dickie et al., is not derived solely from this lake. Fish production estimates for Lake 1987; Leach et al., 1987). We chose size intervals based on cormorant Huron (14.8 kg∙ha−1∙yr−1) derived by Leach et al. (1987) were used diet items sampled from Kagawong Lake and angler catches from creel as a fish production estimate for Lake Wolsey. Because Lake Wolsey is surveys. The geometric mean body mass of fish caught by anglers an embayment of the North Channel, it was not used in any figures was 353 g (1 SD, 153–795 g) and the geometric mean body mass of summarizing our results. Data summaries for cormorant abundance fish in cormorant diets was 58 g (1 SD, 21–158 g) (see Results). We and angler effort for Lake Wolsey are included in table summaries. partitioned total fish production into three size classes based on geometric mean body mass of fish in angler catch (358 g=88.25 g dry Results mass assuming 25% wet mass), cormorant diet (58 g=14.5 g dry mass assuming 25% wet mass) and small fish that appeared to be Double-crested cormorants and anglers smaller than prey normally taken by cormorants (5–20 g=5 g dry mass assuming 25% wet mass). We labelled these categories as large Yellow perch and smallmouth bass were the most common two fish (body mass in angler catch range), medium fish (body mass in species contributing consistently to annual yield in the Manitoulin cormorant consumption range) and small fish, respectively. The per- Is. lake set (Table 1). Estimates of annual yield by other recreational centage of total production generated by each size group x was calcu- fish to anglers were divided among walleye, northern pike (Esox −0.4 −0.4 −0.4 −0.4 lated from dry mass as Wx /[Wlarge +Wmedium +Wsmall]⋅100. The lucius) and where available lake trout (Salvelinus namaycush) and percentages for large, medium and small fish were 16.14%, 33.14% lake whitefish (Coregonus clupeaformis) (Table 1). and 50.72%, respectively. Total production (based on Downing et al., Cormorants and anglers on Kagawong Lake selected fish of differ- 1990) was partitioned into the three size classes by these proportions ent sizes among species for consumption or harvest, respectively, to estimate potential production in each size group. Dividing cormo- with cormorants selecting smaller fish than anglers in the open rant consumption by medium fish production (×100) provided an water and winter fishery (Fig. 3). The mean length of fish selected by estimate of cormorant stress. Similarly, dividing angler harvest by cormorants was 15.9 cm (95% CI, 14.5–17.3 cm) with a mean mass large fish production (×100) provided an estimate of angler stress. per fish of 89.2 g (95% CI, 66.2–112.2 g). Cormorant diet consisted of The level of consumption relative to production provides a useful a number of fish species with rock bass (Ambloplites rupestris) and yel- lake ecosystem-level stress indicator. If consumption is low relative low perch the most common by frequency of occurrence and biomass to production, then one concludes that cormorant stress is low or, (Table 2). Cormorants consuming either rock bass or yellow perch did alternatively, if cormorant consumption is high relative to production not consume the other species. Sixteen cormorants that contained then cormorant stress is high. This approach is particularly useful rock bass did not include yellow perch in their diet. In contrast, Author's personal copy

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Table 1 Angling effort (total, rod∙h∙yr−1; standardized, h∙ha−1·yr−1) and species yield for the Manitoulin Island lake set. Lakes are ordered by surface area (see Appendix 1 in the Supplementary data). Summer and winter angling effort and yield are listed in Appendices 2 and 3 in the Supplementary data, respectively. Dashed symbols represent no yield recorded during survey.

Lake Total angling effort Annual yield (kg∙ha−1∙yr−1) (rod∙h∙yr−1) Angler effort (h∙ha−1·yr−1) Yellow perch Smallmouth bass Walleye Northern pike Lake trout Lake whiteﬁsh

Manitou 44,565 4.2 0.02 0.08 – 0.02 0.80 – Kagawong 41,265 7.3 0.51 0.60 –– – – Mindemoya 32,417 8.5 0.54 0.01 0.58 ––0.14 Silver 2890 5.5 – 0.68 –– – – Windfall 1674 53.9 0.08 0.28 –– – – Bass 2093 7.9 0.31 0.50 0.05 ––– Loon 50 0.2 0.01 0,01 –– – – Tobacco 1335 5.6 0.01 0.65 –– – – Pike 4315 19.0 0.08 – 2.31 1.88 –– Sucker 637 2.9 – 0.36 –– – – Wolseya 14,611 6.3 5.87 ––––– Perch 1792 9.2 0.40 0.64 –– – –

a Winter yield only. thirteen cormorants that contained yellow perch did not include rock Demand for fish production by cormorants and anglers was similar bass. Only three cormorants contained both rock bass and yellow in magnitude based on the aerial surveys of cormorants, anglers and perch in their diet while four cormorants consumed fish species angler interviews (Table 3). Among lakes, the mean cormorant con- other than rock bass or yellow perch. This pattern was a significant sumption was 0.92 kg∙ha−1 (90% CI, 0.56–1.28 kg∙ha−1) and angler departure from an assumption of a common diet among all birds harvest was 1.49 kg∙ha−1 (90% CI, 0.48–2.50 kg∙ha−1). Lake Wolsey is (x2 test; p=0.0028) indicating that most cormorants on Kagawong open to the North Channel allowing fish passage so total fish production Lake consume rock bass or yellow perch but not both. Smallmouth at this site is not based solely on productivity within the bay. Excluding bass were present in the diet of Kagawong Lake cormorants but repre- Lake Wolsey, the mean cormorant consumption on Manitoulin Island sented a relatively small percentage of total consumption (Table 2). lakes was 0.88 kg∙ha−1 (90% CI, 0.48–1.28 kg∙ha−1) and angler harvest In contrast, anglers in the open water and winter fishery harvested was 1.05 kg∙ha−1 (90% CI, 0.36–1.74 kg∙ha−1). The confidence intervals larger fish than cormorants (Fig. 4). In the open water fishery, the for cormorant consumption are contained within those for angler mean fork length of the fish was 25.3 cm for yellow perch and harvest pointing to greater variation in angler harvest among lakes. smallmouth bass combined. When separated by species the mean The total yield of fish production (cormorants+anglers) varied fork length of yellow perch harvested by anglers was 23.6 cm (mean among lakes. In some cases, cormorant consumption exceeded angler mass=179 g) while the mean length of smallmouth bass harvested harvest on a per area basis (e.g., Manitou, Silver and Bass lakes) ver- was 28.0 cm (96% CI, mean mass=343 g). In contrast cormorants sus lakes where angler harvest exceeded consumption by cormorants consumed yellow perch with a mean FL=17.0 cm (mean mass= (e.g., Kagawong, Mindemoya, Tobacco and Pike Lakes; Table 3). This 95.3 g; N=23) and smallmouth bass with a mean FL=17.6 cm was not a consistent pattern based on lake size. In Lake Manitou, (mean mass=89.5 g; N=4). Therefore, the basic premise of this cormorant consumption exceeded angler harvest while the reverse study that cormorants consume medium size fish (10–200 g) and occurred in Lakes Kagawong and Mindemoya. Among smaller lakes, anglers select larger fish (>200 g) was confirmed. the predominance of cormorant consumption or angler harvest in For all lakes combined, anglers harvested larger fish than those total fish yield varied depending on whether lakes were receiving selected by foraging cormorants (Fig. 4). Among the recreational fish high levels of angler exploitation (e.g., Pike Lake) or not (e.g., Loon harvested by anglers in the lake set yellow perch mean FL=22.9 cm, Lake; Table 3). smallmouth bass mean FL=27.9 cm, walleye mean FL=38.7 cm and the combined species of northern pike (Esox lucius), lake white- Fish production and stress estimates fish (C. clupeaformis) and lake trout mean FL=50.2 cm (Fig. 4). Total fish production on Manitoulin Is. lakes ranged from approximately 3.6 kg∙ha−1∙yr−1 (Tobacco Lake; Table 4) to over 10 kg∙ha−1∙ yr−1 (Bass and Pike Lakes; Table 4) based on the total phosphorus and mean total fish production relationship (Fig. 2). When partitioned into size classes mean medium fish production was 2.55 kg∙ha−1∙

Table 2 Summer diet composition of cormorants from Kagawong Lake. Summary based on examination of 40 cormorants with a total of 108 food items weighing 8.49 kg.

Species Percent occurrence

Frequency Biomass

Rock bass (Ambloplites rupestris) 31.5 38.3 Smallmouth bass (Micropterus dolomieu) 7.4 8.4 Bluegill (Lepomis macrochirus) 0.9 0.3 Yellow Perch (Perca flavescens) 25.0 30.3 Stickleback (Gasterosteus spp.) 1.9 0.1 Darter (Etheostoma spp.) 5.6 0.3 Spottail shiner (Notropis hudsonius) 2.8 0.7 Fig. 3. Lengths of fish (FL, mm) captured in Kagawong Lake by: 1) cormorants (black Brown bullhead (Ameiurus nebulosus) 0.9 0.9 bars); 2) angler in the open water fishery (white bars), and 3) winter fishery (gray Sucker (Catostomus spp.) 0.9 2.9 bars). Fish captured by anglers were yellow perch and smallmouth bass in the open Unknown 23.2 17.7 water fishery and yellow perch in the winter fishery. Author's personal copy

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harvested 107.8% (90% CI, 61.4–154.2%) of potential large fish production. Excluding Lake Wolsey leads to a cormorant stress of 40.7% of medium fish production and an angling stress of 94.0% of large fish production. Employing the upper prediction limit on fish production reduces angler stress to a mean of 43.1% (90% CI, 24.5–61.7%; Table 5). Even with this optimistic estimate of total fish production, estimated angler harvest removes most of the large fish production from Lake Mindemoya and Pike Lake (Table 5). Comparing indices of angler stress to cormorant stress for the either the mean or upper 80% prediction of total fish production indi- cates that in 8 of 11 lakes angling stress exceeded cormorant stress. Lake Manitou showed similar levels of stress between cormorant consumption and angler harvest (Table 5). Angling stress in Oneida Lake was >100% of potential large fish production based on the mean prediction of total fish production and 58% under the upper Fig. 4. Lengths of fish (FL, mm) captured by anglers from the Manitoulin Island lake set. 80% prediction limit of total fish production (Table 5). Yellow perch (black bars), smallmouth bass (light gray bars), walleye (dark gray bars), The comparison of cormorant and angling stress on fish produc- and combined lake whitefish, northern pike and lake trout (white bars) are shown. tion in Table 5 assumes no linkage between medium and large fish production. However, medium fish production must occur before large fish production can occur for species growing through the two yr−1 (90% CI, 1.85–3.25 kg∙ha−1∙yr−1) and large fish production was size categories. Because of the effects of mortality on medium fish, 1.24 kg∙ha−1∙yr−1 (90% CI, 0.90–1.58 kg∙ha−1∙yr−1). For the upper not all residual medium fish production can be transferred to large 80% prediction limit, medium fish production was 6.38 kg∙ha−1∙yr−1 fish production. We estimated that 50% of residual medium fish pro- (90% CI, 4.65–8.11 kg∙ha−1∙yr−1), and mean large fish production duction was available for large fish production (see Methods). Angling was 3.11 kg∙ha−1∙yr−1 (90% CI, 2.27–3.95 kg∙ha−1∙yr−1). In compar- stress was then re-calculated as angling harvest relative to residual ison to Oneida Lake, total fish production as well as fish production large fish production. partitioned between medium and large fish was lower in Manitoulin When cormorant consumption of medium-size fish is accounted Is. lakes and similar to Wolsey Lake, an embayment of the North for by this approach, the potential production of large fish is reduced Channel (Table 4). and angling stress increased under both the mean model of Downing Stress indices for cormorants (consumption relative to medium et al. (1990) and its upper 80% prediction limit (Fig. 5). Sustaining fish production) and anglers (harvest relative to large fish produc- current harvest levels would result in angling stress far exceeding tion) indicated differences in levels of demand for fish production. 100% of large fish production in all but four lakes on Manitoulin Island In two lakes (Mindemoya and Silver), cormorant stress on fish pro- based on the mean model of Downing et al. (1990) (Fig. 5; circle duction exceeds the level observed for Oneida Lake (Table 5). Cormo- symbols above 1:1 line). Angling stress was reduced assuming that rants consumed on average 39.4% (90% CI, 24.1–54.7%) of potential fish production follows the upper 80% prediction limit for total fish medium fish production which approaches the level for Oneida Lake production, adjusted for cormorant consumption. But several lakes (62%).(Table 5). Based on the upper 80% prediction limit, cormorant remain (e.g., Mindemoya, Wolsey) where large fish production is stress is reduced to an average of 15.7% (90% CI, 9.7–21.7%). largely removed by anglers (Fig. 5; square symbols). Under both sce- Based on the mean prediction for total fish production, annual narios of total fish production (mean model and upper 80% prediction angler harvest exceeded potential large fish production (>100%, limit) Mindemoya, Wolsey, and Pike Lakes have high angling stress Table 5) for four lakes on Manitoulin Island (Kagawong, Mindemoya, (Table 5) and this stress increased when cormorant consumption of Tobacco, Pike) as well as Lake Wolsey. For Lake Wolsey, winter medium fish production was incorporated in angling stress (Fig. 5). harvest by anglers exceeded the 14 kg/ha estimate of production Kagawong and Tobacco lakes appear to have approximately 50% of provided in Leach et al. (1987) for Lake Huron. On average, anglers large fish production harvested by anglers under the upper prediction limit of fish production (Fig. 5). Most lakes have lower angling stress under the upper 80% limit scenario of fish production (Fig. 5; square symbols). Table 3 Annual lake-specific estimates of cormorant density (number∙km−2), cormorant consumption (kg·ha−1·yr−1), and angler effort (hr·ha−1·yr−1) and angler harvest (kg·ha−1·yr−1). Discussion Cormorant consumption is based on an active foraging season of April 20–Sept 30. Angler fi harvest incorporates open water and winter ice shery except for Wolsey Lake where When examined on a lake-by-lake basis, the approach adopted only winter yield is listed. See Methods for source information on Oneida Lake. in this study demonstrated that in several lakes (e.g., Manitou, Lake Cormorant Cormorant Angler effort Angler harvest Mindemoya, Kagawong, Silver, Pike, and Wolsey) total fish production −1 −1 −1 −1 density consumption (hr·ha ·yr ) (kg·ha ·yr ) of medium and large fish size segments was largely consumed by (birds·km−2) (kg·ha−1·yr−1) cormorants or harvested by anglers, respectively (Tables 4 and 5). Manitou 2.39 1.89 4.2 0.93 Anglers generally harvested a greater percentage of fish production Kagawong 0.65 0.51 7.3 1.11 in the large size segments they targeted than cormorants consumed Mindemoya 1.1 0.87 8.5 1.27 Silver 2.75 2.17 5.5 0.68 of the medium size segments. Variation in yield to anglers was greater Windfall 0.23 0.18 3.9 0.36 than for cormorants largely reflecting angler preference for lakes with Bass 1.67 1.32 7.9 0.86 sport fish production (e.g., Kagawong) compared to lakes with appar- Loon 0.89 0.70 0.2 0.02 ently little recreational fishing interest (e.g., Loon). Cormorants as Tobacco 0.42 0.33 5.6 0.66 fi Pike 0.49 0.39 19.0 4.27 generalist predators consumed more consistent levels of sh produc- Sucker 0.61 0.48 2.9 0.36 tion among lakes. Wolsey 1.65 1.30 6.3 5.87a Estimated harvest levels by anglers exceeded potential fish produc- Oneida 3.81 2.88 14.4 3.17 tion in five lakes and exceeded 50% of potential production in another a Winter yield only. two lakes using the mean prediction for fish production (Table 5). Author's personal copy

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Table 4 Estimates of total fish production (kg∙ha−1∙yr−1) from Equation 4 in Downing et al. (1990) and for the upper 80% prediction limit. Total fish production was partitioned into medium and large categories of the fish community exploited by cormorants and anglers.

Lake Mean prediction Upper 80% prediction

Total fish production Medium fish production Large fish production Total fish production Medium fish production Large fish production (kg∙ha−1∙yr−1)a (kg∙ha−1∙yr−1) (kg∙ha−1∙yr−1) (kg∙ha−1∙yr−1)a (kg∙ha−1∙yr−1) (kg∙ha−1∙yr−1)

Manitou 9.18 3.04 1.48 22.80 7.56 3.68 Kagawong 5.44 1.80 0.88 13.53 4.48 2.18 Mindemoya 3.32 1.10 0.54 8.49 2.81 1.37 Silver 6.74 2.23 1.09 16.95 5.62 2.74 Windfall 4.92 1. 63 0.79 12.37 4.10 2.00 Bass 11.19 3.71 1.81 28.07 9.30 4.53 Loon 7.09 2.35 1.14 17.74 5.88 2.86 Tobacco 3.59 1.19 0.58 9.07 3.01 1.46 Pike 12.97 4.30 2.09 32.28 10.70 5.21 Sucker 5.44 1.80 0.88 13.53 4.48 2.18 Wolsey 14.8b 4.90 2.39 37.0 12.26 5.97 Oneida 14.1 4.67 2.28 33.77c 11.19 5.45

a From Equation 4 in Downing et al. (1990). b Based on estimate of Leach et al. (1987) for Lake Huron. c Based on multiplier of 2.5 (ratio of mean upper 80% prediction to mean prediction for Manitoulin Is. lakes).

Cormorant stress exceeded 75% of medium fish production in two lakes In a number of lakes, adjusting angling stress by incorporating (Mindemoya and Silver) and was comparable to angler stress in Lake cormorant consumption greatly exceeded total fish production for Manitou where both cormorant and angler demand exceeded 50% of large fish based on the mean prediction model for fish production. It fish production in their respective fish size segments. Measures of cor- is not realistic to assume anglers could harvest fish production at sus- morant and angler stress were reduced for each lake assuming the tainable levels exceeding what is available. We interpret the patterns upper 80% prediction limit defines fish production in this system of in Fig. 5 to indicate that fish production is most likely bounded by lakes (Table 5). However, in three lakes (Mindemoya, Pike and Wolsey) the mean and upper 80% prediction limit for fish production on angler demand for fish production nearly matched total fish production Manitoulin Is. Whatever scenario is most likely, cormorant consump- for the upper 80% prediction limit for the large fish size segment tion of fish production exacerbates angler stress on lakes. Assuming (Table 5). the upper 80% prediction limit of total fish production from Fig. 2 We inferred a transfer of 50% for medium fish production to large reduced this outcome but two lakes, Mindemoya and Wolsey, still fish production based on interpolating instantaneous mortality (Z) indicated that angling stress exceeded total large fish production from weight of fish (Randall and Minns, 2000). This is a reasonable (squares, Fig. 5). In Pike Lake, approximately 80% of large fish produc- assumption as production ought to compensate for mortality rate. tion was harvested by anglers under the more optimistic assumption Because cormorants have been present in this region of Lake Huron of fish production. In Kagawong and Tobacco lakes, approximately for three decades (Weseloh et al., 2002), it could be argued that the 50% of large fish production was harvested by anglers under the opti- current distribution of production among fish size segments exam- mistic assumption of the upper prediction limit. ined in this study already incorporates the 50% discount imposed by Foraging cormorants in this region of Lake Huron move among cormorants on larger fish production. Whether this discount exists Manitoulin Is. lakes, the North Channel and the South Shore of currently in the data or through our assumption of a discount, the Manitoulin Is. facing the main body of Lake Huron (Ridgway and conclusion drawn from this study is that anglers impose greater stress on their preferred sizes of fish (large fish) than cormorants do on their preferred sizes of fish (medium fish). This pattern was present for both the mean and upper 80% prediction limit for fish production.

Table 5 Estimates of cormorant and angling stress based on the consumption of medium ﬁsh production and harvest of large ﬁsh production, respectively. Cormorant consumption for Oneida Lake is the 1997 value in Table 3 of Rudstam et al. (2004).

Lake Mean prediction Upper 80% prediction

Cormorant Angling Cormorant Angling stress (%) stress (%) stress (%) stress (%)

Manitou 62 63 25 25 Kagawong 28 >100 11 51 Mindemoya 79 »100 31 92 Silver 97 62 39 25 Windfall 11 46 4 18 Fig. 5. Angling stress adjusted for cormorant consumption plotted against angling Bass 36 48 14 19 stress in the absence of cormorants for lakes on Manitoulin Is. The diagonal line repre- Loon 30 2 12 1 sents the 1:1 relationship. Circles represent the stress index based on the predicted Tobacco 28 >100 11 45 mean fish production (Downing et al., 1990;solidlineinFig. 2). Squares represent the stress Pike 9 »100 4 82 index based on the 80% upper prediction limit of fish production (upper dotted line in Sucker 27 41 11 16 Fig. 2). Lakes are designated under both production scenarios by the first three letters of Wolsey 27 »100 11 99 the lake name. Lakes where stress exceeded 200% are designated as 200%. Oneida Lake is Oneida 62 >100 26 58 shown for comparative purposes. Author's personal copy

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Middel, 2011). Seasonal patterns of consumption differed among Lake study (Rudstam et al., 2004). Our results were consistent with these regions but aggregate prey demand over a season was similar other studies on cormorant prey size. Using Kagawong Lake as a between the North Channel and large lakes on Manitoulin Is case study, cormorants consumed yellow perch and other fish species (1.2 kg∙ha−1∙yr−1; Ridgway and Middel, 2011). Because within at sizes smaller than perch caught by anglers in summer and winter basin movements of cormorants are extensive in the Great Lakes (Fig. 3). Cormorant consumption could therefore remove perch (Guillaumet et al., 2011), foraging on inland lakes that are in close production prior to operation of the angling fishery. This may be a proximity to the Lake Huron coast is likely occurring in other areas common occurrence as the size of yellow perch consumed by cormo- (e.g., Dorr et al., 2010). Patterns detected in this study may occur in rants appears to be consistent across the Laurentian Great Lakes other locations given site differences in lake productivity and stress region and Kagawong Lake (Burnett et al., 2002; Johnson et al., imposed by cormorants and anglers. Reductions in cormorant abun- 2002), Les Cheneaux Is., Lake Huron (Diana et al., 2006), and Oneida dance in one coastal lake adjacent to Lake Huron resulted in increased Lake (Rudstam et al., 2004; VanDeValk et al., 2002). walleye year-classes (Dorr et al., 2010). In Dore Lake, Saskatchewan, sizes of yellow perch consumed by This study is based on two premises. First, the Downing et al. cormorants were smaller than those observed in the diets of cormo- (1990) total phosphorus model can be used to predict total fish pro- rants from the Great Lakes and Kagawong Lake (Fig. 4 in Barks duction and the size spectrum approach can partition this production et al., 2010). Yellow perch growth was limited in Dore Lake with into size segments on a relative basis. Second, cormorant demand for perch achieving final sizes smaller than those preferred by anglers fish production falls on different sizes of fish than that preferred by (Barks et al., 2010). Given this observation, food web limitations anglers and this demand may exacerbate angler demand for larger and/or risk sensitive foraging in relatively unproductive littoral fish production. habitat may have limited any possible gains through compensatory With respect to the first premise, the total phosphorus model has survival and expected higher growth for perch in Dore Lake (Barks performed better than other indices in predicting total fish produc- et al., 2010). The magnitude or even occurrence of compensatory tion (Downing et al., 1990; Hanson and Leggett, 1982; Ryder, 1965). growth for yellow perch in Manitoulin Island lakes is unknown. Providing lake-specific estimates of fish production without utilizing Sizes of yellow perch harvested by anglers on Manitoulin Island the total phosphorus model would require estimating age-specific (Fig. 4) exceed most perch sizes found in Dore Lake (Barks et al., growth, mortality and abundance of all fish species in each lake. 2010), so food web or habitat limitations on growth present in the This task would be challenging for each lake and one likely to work Saskatchewan lake are not apparent in Manitoulin Island lakes. His- against a multi-lake comparison of angler and cormorant demand torical data on perch growth in the Manitoulin Island lake set prior for fish production. Limitations of the total phosphorus model are to the presence of cormorants are lacking so the presence of compen- likely based on the subset of lakes used to help define the relationship satory growth in response to cormorant predation is unknown. near the origin where most north temperate lakes reside in the Anglers and cormorants targeted fish size segments with different Downing et al. (1990) data set. Small oligotrophic lakes near functional roles in lake ecosystems. For cormorants, their prey consti- Manitoulin Island (Turkey and Wishart lakes, no nearshore develop- tuted species with small body size (e.g., sticklebacks; cyprinids) or ment, Table 2 in Downing et al., 1990) help define the relationship juvenile stages of species with larger body sizes (e.g., smallmouth closer to the origin of the total phosphorus: total fish production bass). Small species have high P/B ratios characteristic of shorter life regression. On Manitoulin Island, dolomitic limestone that defines spans, higher mortality, and more rapid turnover of biomass. Many base geology of the lake set combined with nearshore human devel- can be characterized as shoaling species (Suter, 1997). Anglers gen- opment provides for higher TP measures than on Canadian Shield erally target adult size segments of recreational fish where mortality landscapes (Appendix 1 in the Supplementary data). This is why we is lower, life span longer and biomass turnover is relatively slow included the upper 80% prediction limit to help scope total fish pro- (Randall and Minns, 2000). In the case of recreational fish, adult duction in this study of relative demand for fish production. Because size segments are potentially contributing to year-class production recreational fisheries have persisted to some degree on Manitoulin whereas juvenile stages are not. Cormorant consumption of juvenile Island for many years, even in the face of occupancy by double- recreational fish may exacerbate effects of angling on these target crested cormorants, the mean regression model of Downing et al. populations. (1990) may underestimate total fish production in this lake set. We Information from Oneida Lake on angling harvest and cormorant maintain that the lower 80% prediction limit for total fish production consumption were incorporated here for comparative purposes. Size was unreasonably low. On the other hand, assuming the upper differences among fish taken by cormorants and anglers have been 80% prediction limit of total fish production demonstrated that in examined for Oneida Lake (VanDeValk et al., 2002). Although cormo- some lakes, anglers and cormorants are still likely competing for rants removed smaller yellow perch compared to angler harvests, fish production. sizes were above the level where compensatory mechanisms could We used the relationship between production (P/B) and mass to operate (Rudstam et al., 2004). The level of cormorant consumption partition total fish production by body size into size segments of on young fish was sufficient to reduce cohorts and ultimately reduce fish analogous to a size spectra approach (Banse and Mosher, 1980; angling harvest of this species (Rudstam et al., 2004; VanDeValk et al., Dickie et al., 1987; Leach et al., 1987). The application of size spectra 2002). In this study, incorporating a similar direct effect of cormorant has been employed previously to assess size structure of fish commu- consumption on the yields of fish to anglers is therefore a reasonable nities subject to fishing relative to a non-fished state (Jennings and assumption. This was accomplished initially by recognizing the differ- Blanchard, 2004). Furthermore, size spectra reveal relative changes ence in general levels of fish production between medium and large in abundance of small, medium and large fish with slopes of the fish (Leach et al., 1987), and subsequently by reducing potential fish relationship between biomass and mass categories of individuals production to anglers based on cormorant consumption of medium becoming more negative with increasing numbers of small fish and/ size fish. or decreasing numbers of large fish (Bianchi et al., 2000). The effect The strength of the Oneida Lake study was the duration of fish of increasing negative slopes in size spectra with decreasing large population monitoring and process-based research on predator–prey fish abundance is analogous to the concept of population indicators dynamics of walleye and yellow perch (Rudstam et al., 2004). Con- of stress defined as the loss of large fish (Shuter, 1990). sumption of yellow perch and walleye by cormorants matched reduc- A second premise of this study was that cormorant demand for tions in yield to anglers in that large lake system. Similar long-term fish production of medium size fish could potentially exacerbate de- monitoring of the yellow perch fishery in the Les Cheneaux Islands mand for fish production by anglers. This was observed in the Oneida of Lake Huron revealed that mortality of yellow perch increased with Author's personal copy

522 M.S. Ridgway et al. / Journal of Great Lakes Research 38 (2012) 514–523 increases in cormorant abundance, larger perch were reduced in Buckland, S.T., Anderson, D.R., Burnham, K.P., Laake, J.L., Borchers, D.L., Thomas, L., 2001. Introduction to Distance Sampling: Estimating Abundance of Biological number over time and a reduction in cormorant consumption led to Populations. Oxford University Press, Oxford, U.K. an increase in catches of age-2 fish (Fielder, 2008, 2010b; but see Burnett, J.A.D., Ringler, N.H., Lantry, B.F., Johnson, J.H., 2002. Double-crested cormorant Diana, 2010; Fielder, 2010a). In the St. Lawrence River, CPUE of yellow predation on yellow perch in the eastern basin of Lake Ontario. J. Great Lakes Res. fi 28, 202–211. perch was signi cantly negatively correlated with increases in abun- Chapman, L.J., Putnam, D.F., 1973. The Physiography of Southern Ontario, 2nd ed. University dance of nesting double-crested cormorants (Smith et al., 2007). of Toronto Press, Toronto. Our results indicate that fish consumption by cormorants on Christie, G.C., Regier, H.A., 1988. Measures of optimal thermal habitat and their rela- fi Manitoulin Is. lakes is low in absolute terms relative to Oneida Lake tionships to yields of four commercial sh species. Can. J. Fish. Aquat. Sci. 45, 301–314. −1 −1 (kg∙ha ∙yr , Table 3), where cormorant control has been imple- Cowx, I.G., 2003. Interactions between fisheries and fish-eating birds: optimizing the mented to protect the recreational fishery. However, when differ- use of shared resources. In: Cowx, I.G. (Ed.), Interactions Between Fish and Birds: – ences in lake productivity are accounted for, the cormorant stress Implications for Management. Blackwell Science Ltd., Oxford, pp. 361 372. Diana, J.S., 2010. Should cormorants be controlled to enhance yellow perch in the Les level on some Manitoulin lakes exceeds that of Oneida Lake and the Cheneaux Islands? A comment on Fielder (2008). J. Great Lakes Res. 36, 190–194. average level is close to that implied for Oneida Lake (Table 5). Diana, J.S., Maruca, S., Lowe, B., 2006. Do increasing cormorant populations threaten sport Long-term monitoring on Oneida Lake has demonstrated that cormo- fishes in the Great Lakes? A case study in Lake Huron. J. Great Lakes Res. 32, 306–320. Dickie, L.M., Kerr, S.R., Schwinhamer, P., 1987. An ecological approach to fisheries rants played a role in depleting yellow perch and walleye available to assessment. Can. J. Fish. Aquat. Sci. 44 (Suppl. 2), 68–74. anglers (Rudstam et al., 2004). Thus, we conclude that cormorant Dorr, B.S., Moerke, A., Bur, M., Bassett, C., Aderman, T., Traynor, D., Singleton, R.D., abundance on a sub-set of Manitoulin Island lakes plays a role in Butchko, P.H., Taylor, P.D., 2010. Evaluation of harassment of migrating double- fi fi crested cormorants to limit depredation on selected sport sheries in Michigan. reducing the abundance of sh species supporting the recreational J. Great Lakes Res. 36, 215–223. fishery. Given the observed levels of yield in the recreational fishery, Downing, J.A., Plante, C., LaLonde, S., 1990. Fish production correlated with primary anglers may be harvesting at or above sustainable levels on several productivity, not the morphoedaphic index. Can. J. Fish. Aquat. Sci. 47, 1929–1936. Fielder, D.G., 2008. Examination of factors contributing to the decline of the yellow lakes of Manitoulin Island. perch population and fishery in Les Cheneaux Islands, Lake Huron, with emphasis Cormorant consumption of fish production from size segments on the role of double-crested cormorants. J. Great Lakes Res. 34, 506–523. below that preferred by anglers is a general pattern (e.g., Johnson Fielder, D.G., 2010a. Response to Diana commentary. J. Great Lakes Res. 36, 195–198. et al., 2002, 2006, 2010; Lantry et al., 2002; VanDeValk et al., 2002; Fielder, D.G., 2010b. Response of yellow perch in Les Cheneaux Islands, Lake Huron to declining numbers of double-crested cormorants stemming from control activities. this study). For many fish species, cormorant consumption may fall J. Great Lakes Res. 36, 207–214. on fish size segments where compensatory processes are difficult Guillaumet, A., Dorr, B.S., Wang, G., Taylor, J.D., Chipman, R.B., Scherr, H., Bowman, J., Abraham, K.F., Doyle, T.J., Cranker, E., 2011. Determinants of local and migratory to detect or not operating (e.g., Lantry et al., 2002; Rudstam et al., – fi movements of Great Lakes double-crested cormorants. Behav. Ecol. 22, 1096 1103. 2004). A hypothesis for the effects of cormorant consumption of sh Haddon, M., 2001. Modelling and Quantitative Methods in Fisheries. Chapman and on fisheries can be based on this generality and the magnitude of Hall/CRC Press, Boca Raton, FL. fisheries exploitation detected in this study. Angler harvest near Hall, S.R., Rudstam, L.G., 1999. Habitat use and recruitment: a comparison of long-term recruitment patterns among fish species in a shallow eutrophic lake, Oneida Lake, (or above) maximum sustainable yield levels will be exacerbated NY, USA. Hydrobiologia 408/409, 101–113. and may appear as a fish collapse when cormorants consume fish Hanson, J.M., Leggett, W.C., 1982. Empirical prediction of fish biomass and yield. Can. J. production destined for size segments preferred by anglers. This was Fish. Aquat. Sci. 39, 257–263. fi Harris, C.M., Calladine, J.R., Wernham, C.V., Park, K.J., 2008. Impacts of piscivorous birds on essentially the process that reduced sh abundance in Oneida Lake salmonid populations and game fisheries in Scotland: a review. Wildl. Biol. 14, 395–411. (Rudstam et al., 2004), the Les Cheneaux Islands (Fielder, 2008, Harvey, H.H., 1978. Fish communities of the Manitoulin Island lakes. Verh. Internat. 2010b), and possibly Lake Ontario (Lantry et al., 2002). It also appears Verein. Limnol. 20, 2031–2038. Harvey, H.H., Coombs, J.F., 1971. Physical and chemical limnology of the lakes of to be the case for some lakes on Manitoulin Is. Manitoulin Island. J. Fish. Res. Board Can. 28, 1883–1897. Jackson, D.A., Harvey, H.H., 1989. Biogeographic associations in fish assemblages: local vs regional processes. Ecology 70, 1472–1484. Acknowledgments Jennings, S., Blanchard, J.L., 2004. Fish abundance with no fishing: predictions based on macroecological theory. J. Anim. Ecol. 73, 632–642. We thank the following people for their contributions: 1) Bud Johnson, J.H., Ross, R.M., McCullough, R.D., 2002. Little Galloo Island, Lake Ontario: a review of nine years of double-crested cormorant diet and fish consumption infor- Hebner, Wayne Selinger, and Holly Simpson for assistance with logistics mation. J. Great Lakes Res. 28, 182–192. and background information (OMNR Espanola Area office); 2) Nelson Johnson, J.H., Ross, R.M., McKenna, J.E., Lewis, G.E., 2006. Estimating size of fish con- Deschenes and Liane Hebner for angler interviews and biological sumed by double-crested cormorants: considerations for better understanding cormorant–fish interactions. J. Great Lakes Res. 32, 91–101. samples; 3) Coop. Freshwater Ecology Unit (Laurentian University), Johnson, J.H., Ross, R.M., McCullough, R.D., Mathers, A., 2010. Diet shift of double- for fish survey work and data summaries; 4) Marg Watson and staff crested cormorants in eastern Ontario associated with the expansion of the inva- of Sudbury Aviation for conducting aerial angler counts, and 5) Mike sive round goby. J. 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