Double-crested Cormorant Conflict Management And Research On Leech Lake 2005 Annual Report Prepared by Leech Lake Band of Ojibwe Division of Resources Management Minnesota Department of Natural Resources US Department of Agriculture Wildlife Services Photo by Steve Mortensen This report summarizes cormorant conflict management and research that occurred on Leech Lake during 2005. The overall goal of this project is to determine at what level cormorant numbers can be managed for on the lake without having a significant negative effect on gamefish and on other species of colonial waterbirds that nest alongside cormorants on Little Pelican Island. Work conducted this summer went much better than anticipated and we are well on the way towards resolving cormorant conflict issues on Leech Lake. Cormorant Management Environmental Assessment An environmental assessment (EA) was prepared that gathered and evaluated information on cormorants and their effects on fish and other resources. We compiled, considered, and incorporated input from all parties that have an interest or concern about the proposed controlling of cormorants on Leech Lake. This document could have been written specifically so the Leech Lake Band could address the Leech Lake cormorant colony, but it was decided to prepare it as a joint effort between LLBO Division of Resources Management (DRM) and USDA Wildlife Services (WS), MN Department of Natural Resources (MN DNR), and US Fish and Wildlife Service (FWS). WS served as lead agency in this effort. By preparing it in this manner, it could be utilized by other agencies elsewhere in Minnesota if resource damage was documented. The EA proposed to reduce the Leech Lake colony more than 10% so, under provisions of the Federal Depredation Order, the Leech Lake Band submitted a request to the US Fish and Wildlife Service to go to a higher population reduction limit (reduction to 500 nests). This was done with the understanding that a diet study will be completed that will help to determine what level cormorant numbers can be maintained without having significant negative effects on walleye populations. Decision notices were signed and published by the DRM, FWS and WS outlining the work to be done over the next few years at this colony. Cormorant Diet Study Grant Increasing numbers of cormorants on Leech Lake and concerns about predation on walleye and perch populations prompted LLBO to submit a grant proposal to the FWS Tribal Wildlife Grant Program. Partners and cooperators in this project include the MN DNR, WS, FWS, University of Minnesota (UM), and several local organizations and individuals. This grant has been combined with other funds and in-kind support to conduct a two-year study. Walleye Stocking The MN DNR stocked 7.56 million walleye fry into Leech Lake this past spring (2005). The fish were marked with oxytetracycline, a chemical that allows biologists to identify a fish as stocked. Stocking marked walleye fry provides an opportunity to evaluate the success of natural reproduction in Leech Lake by analyzing the ratio of marked (stocked) to unmarked (natural) fingerlings. This ratio can be tracked over time and may prove useful in the cormorant diet study. Catch rates for standard trawling efforts this year were nearly twice the long-term average and are indicative of a strong walleye year- class (Figure 1). Marked fish accounted for 39% of the fingerlings sampled this year. Oxytetracycline marked otolith (Photo from MNDNR) 2 600 500 400 300 Y 200 No. Walleye YOY sampled No. Walleye 100 0 1987 1988 1989 1990 1991 1992 1993 1994 1995 1996 1997 1998 1999 2000 2001 2002 2003 2004 2005 Year Y OY w alleye Long term average Figure 1. Long term walleye young-of-the year trend on Leech Lake . (Trawling data from MN DNR) Cormorant Culling Efforts to cull cormorants were initiated on May 4, 2005, at the nesting site on Little Pelican Island. LLBO contracted this work to WS, with the MN DNR providing over 50% of the funding. A total of 2993 birds were shot and removed from the lake for the culling and diet sampling projects. This reduced the number of cormorant nests to about 700, down from 2524 nests in 2004 (Figure 2). To reduce the disturbance to other colonial waterbirds on the island and to 500 cormorant nests that were to be conserved, culling was conducted early in the nesting season, 1-2 times per week. Carcasses were collected each day of culling and transported to shore where they were frozen. The frozen carcasses that were not retained for the diet study were transported to the Mille Lacs Wildlife Management Area for disposal by incineration. All culling efforts at the nest sites were conducted from elevated blinds using precharged, .22 caliber air rifles. Birds were collected for the diet study off of the eastern end of Big Pelican Island using shotguns. Contrary to news reports, silencer-equipped firearms were not used. Legislation to permit their use was not passed in time and, as of the date of this report, we are still trying to determine whether the wording of the new law will permit use of silencer-equipped firearms on tribal lands or whether this legislation will need to be amended. Cormorant nest on Little Pelican Island (Photo from LL DRM) 3 6000 5000 4000 3000 2000 No. nests and fledglings and nests No. 1000 0 1992 1993 1994 1995 1996 1997 1998 1999 2000 2001 2002 2003 2004 2005 Year Es timated Number Of Nes ts W ithout Culling Nests Number Fledged Figure 2. Numbers of cormorant nests and fledglings on Leech Lake in recent years. (Data from LL DRM) Bird Counts We contracted with MN DNR pilots to conduct flights over Leech Lake in an effort to estimate cormorant numbers and to identify locations where they frequently forage. In addition, we contracted with Pro-West and Associates for high-resolution aerial photographs of Gull and Pelican Islands. These images were used to get accurate counts of birds and nests on the island. In general bird numbers were down this year from last year with many resident cormorants leaving the lake early this year. Example of high-resolution aerial photos that are used for bird counts (Pro-West and Associates) 4 Diet Study Sampling To determine what cormorants are consuming on Leech Lake, stomach contents were collected from foraging adults as well as juvenile birds. We collected adult birds by shooting them (with shotguns) on their return flights. This operation was conducted approximately weekly off tribal land on the eastern end of Big Pelican Island to minimize disturbance to the nesting colony and to bald eagles that nest on the island complex We sampled juvenile birds by approaching them on their nests and then collecting the material they regurgitate when disturbed. Juveniles were sampled about two weeks after hatching and then up until they began leaving their nests and moving about the colony. We preserved all samples with formalin and shipped them to University of Minnesota where a graduate student is examining the collections and quantifying the stomach contents. Diet samples collected from adults as well as from juvenile birds this summer have found that the vast majority of the fish they consume are small perch with the second most common species being shiners. As of the date of this report several hundred cormorants have been examined and only ten walleyes have been found. Diet studies on cormorants have consistently found that diets vary considerably from lake to lake and it appears that Leech Lake will be no exception. Combining this lake-to-lake variation with compensatory loses (the vast majority of small fish die or are eaten anyway) of young fish and you can see why it is so difficult to determine what is going on with a fish population in a lake. Peter Hundt collecting juvenile diet samples. (Photo from LL DRM) Fish Population Sampling The MN DNR collects a large amount of fish data annually from Leech Lake as part of their Large Lake sampling program. This includes shoreline seining, trawling, electrofishing and gill netting. These data document changes in fish populations over time. The MN DNR also collected diet data for yellow perch, walleye, and northern pike. We will use these data to assist computer modeling cormorant impacts on Leech Lake. Figures 3 and 4 show some of the fish population trends in Leech Lake. 5 18 16 14 12 10 8 6 4 Pounds Of Walleye Sampled Per Net Per Sampled Walleye Of Pounds 2 0 1983 1984 1985 1986 1987 1988 1989 1990 1991 1992 1993 1994 1995 1996 1997 1998 1999 2000 2001 2002 2003 2004 2005 Year Walleye Main Lake Walleye Deep Figure 3. Pounds of walleye sampled by gill net in the main and deep water bays of Leech Lake, 1983-2005 (Data from MN DNR, 2005) 45 40 35 30 25 20 Number per set 15 10 5 0 1987 1988 1989 1990 1991 1992 1993 1994 1995 1996 1997 1998 1999 2000 2001 2002 2003 2004 2005 Year Deep Water Main Lake Figure 4. Numbers of yellow perch sampled by gill net in the main and deep-water bays of Leech Lake, 1987-2005. (Data from MN DNR, 2005) 6 Walleye fingerlings sampled by trawling on Leech Lake in 2005 (Photo from MN DNR) Pat and Ozge shoreline seining for young-of-the year walleye (Photo from LL DRM) Fish Population Modeling When sufficient information has been collected from diet sampling, Large Lake sampling, and other sources, we will use a computer program to model the effects of cormorant predation on yellow perch and walleye. In addition, we will try to assess crayfish predation, cannibalism, and other fish mortality factors.