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Authors requiring further information regarding Elsevier’s archiving and manuscript policies are encouraged to visit: http://www.elsevier.com/copyright Author's personal copy Journal of Great Lakes Research 38 (2012) 514–523 Contents lists available at SciVerse ScienceDirect Journal of Great Lakes Research journal homepage: www.elsevier.com/locate/jglr 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 M.S. Ridgway et al. / Journal of Great Lakes Research 38 (2012) 514–523 515 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 produc- tion (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.