AERIAL SURVEYS FOR BEAVER IN MACKENZIE DISTRICT NORTHWEST TERRITORIES"' CANADIAN ~iiLDUFE SERVICE, FORT SMI'IH, N. W. To The purpose of this paper is to describe the methods used in making extensive aerial surveys for beaver .in wilderness areas and to indicate briefly what ty:pe of results are obtained and hovr they may be used. The beaver, Castor canadensis, still occupies an important place in the economy of the aborigines (Indians and Eskimos} of the Mackenzie District, North;~est Territories, and accordinglY,, new or improved methods of beaver management are constantly being sought. Knowledge of the distribution and abundance of the beaver is considered to be of prime importance in establishing seasons and regulating harvests. The area of the Mackenzie District is 527,000 square miles~ of which about 300,000 square miles are south of timberline. Ground transportation in the area is slow, and the aeroplane was the only possible way in which adequate coverage could be obtainedo o:>i The first experiments with aerial surveys were carried out in Nc.o ~I 1947 and 1948. The object or these early surveys was merely to locate areas ~ in which the beaver population was depleted. In 1949, Kelsall (unpublished 0• l!) LO report in the files of the Canadian Wildlife Service} attempted to locate r-- M M ARLIS -' ALASKA RESOURCES LIBRARY & INFORMATION SERVICES 3150CSTREET, SUITE 100 ANCHORAGE, ALASKA 9950.3 - 2 - and ~let t~e active colonies in two areas in the southern part or the Mack~zie Jistrict. Kelsall's basic methods have been extended and refined during subsequent surveys in 1951 and 1952. The writer is indebted to D.R. ~look and E.H. McEwen of the Canadian Wildlife Service who have taken part in aerial surveys and contributed to the development of the· techniques employed. 03JECT!VES The two major objectives of the surveys were -- 1. TO examine all parts of the Mackenzie District and evaluate the ;uality of beaver habitat. 2. To dev:elop an index: of abundance which can be used either to compare the beaver populations of different areas, or to follow the trends in the beaver population of a given area over a period of time • .METHODS Before descrihing methods in detail, it is necessary to point out certain difficulties which were encountered. First~ because of the large­ ness o~ the area, the coverage had to be extensive rather than intensive. Second, as many of our flights exceeded the normal range of the light aircraft used, it was necessary to carry gasoline and oil in the cabin of the aero­ plane. Certain areas'- e.g. the north shore of Great Bear Lake, could not be reac~ed even then because gasoline caches are few and widely· scattered in that area. Third, the standard map sheet is on a scale of eight mil .. to one inch~ l~t~ough the general quality of these maps is good, it is impossible .•. .. .. 3 - to portray the smaller features accurately on this scale. Finally, there is a scarcity of physiographic data for this entire area. Few elevations and ~o contours are shown on the standard maps, and the personal knoNledge of the inY.estigators is therefore importanto The method of surveying consists in flying along a pre- arranged route ~~th an observer on each side of the aircraft recording in a notebook all evidence of the presence of beavero Parallel colwnns ure ruled on a page in tlle notebook and headed: Time, Feed Beds, Lodges, Dams, Runways, . Remarks. For each observation, the time is recorded to the nearest minute and then a stroke is entered in the appropriate column. Where two or more beaver "signs" are obviously associated - for example: a feed bed {1} in front of a lodge, the observations are entered in the same lineo However, if the "signs" are not related· - for example: a lodge in each of two adjacent lakes, each is entered on·a separate line in the notes. Whenever large, easily identified landmarks are passed, the time is recorded and the nature of the reference point is entered in the Remarks column. Brief note& on topography, vegetation, recent fires, or other observations of possible significance are also made from time to timeo Table 1, which contains part of a specimen page from the field notes, will help to clarify the methodo (1} This is the term applied almost universally by trappers in the North­ west Territories .to the mass of sticks ~vhich the beavers store up in advance for winter useo The term is used throughout this paper. - 4 T'o.Jle 1 - ~ract from field not·es for 19 September, 1952, to illustrate the method of recording observations. Time ~eed Lod.ges Dams Runways Remarks Beds 1438 Crossed Yuk:on-N.W.T. boundary 40 1 1 41 1 1 1 42 1 1 1 43 1 ) ) In same lake. 1 ) 1 In a small lake 1 " " " It· Picked up small creek leaving lake. Good habitat. Bordered 44 1 by vlillows therefore visibility - \ goodo Broad. valley. 1 . I 45 1 1 1 1 5 - Flight lines are plotted to sample the greatest amo~t and variety ot beaver habitat in the area to be covered. Where the beaver habitat consists mainly or streams, a number ot these are followed, and the course 1a planned in such a way that unproductive travel between watersheds is kept to a minimum. The pilot is instructed to tly along one side or the stream so that one observer devotes his tull attention to the stream while the other observer vi~vs some ot the tributaries, and usually, a number or small lakes which lie along the courseo Follovdng the main stream requires the intense concentration ot the observer which produces tatigueo When this occurs, the pilot shirts the aircratt to the other side or the s.tream which reverses the tasks ot the observerso A course such as this can be tollowed accurately and reflown in a similar manner in subsequent surveyso Vlhere the beaver habitat consists mainly ot lakes, the same method will otten succeed. Sometimes, however, the lakes are so numerous that they cannot all be portrayed on the map, and as individual lakes cannot be identified it is impossible to :f'ollovt a pre-arranged routa. To avoid this difficulty a technique is being developed whereby an attempt is made to fly along a straight line between recognizable points. At the same time the proportion ot lakes occupied by beaver is noted. The altitude at which the surveys are carried out varies with conditions. Stream courses are usually followed trom an elevation or 500 to 800 teet above the ground o When flying over lake country, however, it has -- heen found th::t beaver n:3ignrt is easily seen from an elevation of 2, 500 feet. DISCU S.SI m:; JF E'lliODS S·.qank and Glover (1948} and Crissey .(1949) apparently plotted their obse:'Vations directly on large scale maps of the areas in Vlest Virginia and Ne-.'; Yo::-k, respectively, which they surveyed. This_ was tried in the North­ west Terri :ori es and discarded in favour of the method described for two reasons. First, the standard maps show insufficient detail, and second, detailed map reading re:uires the observer to divide his attention betw·een the map and the ground and may result in beaver "sign" being overlooked. Map reading is, therefore 1 left to the pilot who follows the flight line marked on the map, infor:J.s tte observers of their position on request, and points out prominent landmarks as they are passedo An examination of Table 1 will show that the field notes may be readily cc~verted into the number of colonies seen. With few exceptions, each line in tte notes represents a separate colony. The decision whether two or more obse~vations represent one colony, or more than one, must be made at the time and ~~tered in the notes accordinglyo Plotting colonies directly on the map does ~ot relieve the investigator of the responsibility of making similar decisions, The colony is the unit of beaver management in the Northwest Territories. 3ach trapper is entitled to trap, each year, a number of beavers not to e:xcaed the number of beaver colonies in his area. For this reason all census fi,;~es are shown as the number of colonies seen, and no attempt. is made to estima:e t~a total number of beavers they represent. 7 One problem, whic4 is always present when taking a census of beavers, is how to allow for "bank beavers" that live in burrows in theatream bank and do not build lodges. This difficulty has been partly overcome by delaying all survey flights until after Septanber l5o By this date most of the beavers are at work accumulating their winter food supply. These feed beds, or storage piles, are easily seen from the air and are taken as certain evidence of the presence of a.beaver colony.Crissey {epo cito] utilized this fact in making a beaver census in New York. Swank and Glover {op. cit.) carried out their census in April when feed beds were used up or carried away by spring · freshetso They considered that late autumn, after the leaves had fallen, would be a better season for taking a census because of increased visibility, but they apparently overlooked the more important factor of the presence of winter foodo In certain cases, it is difficult to distinguish between an active and an inactive lodge when flying over themo If a feed bed is present, all doubt is removed.