Evaluation of Three Types of Artificial Habitats for Fishes in a Freshwater Pond in Maine, Usa

BULLETIN Or MARINE SCIENCE. 55(2-3): 1149-1159. 1994

EVALUATION OF THREE TYPES OF ARTIFICIAL HABITATS FOR FISHES IN A FRESHWATER POND IN MAINE, USA

John R. Moring and Peter H. Nicholson

ABSTRACT Three types of artificial structures (brush bundles, cinder blocks, tire bundles) were studied along transects in Lac D'or, a boreal, freshwater pond in central Maine. Observations were made of 1,397 adult and juvenile fishes by means of 18 dives during the day and 6 dives during the night in 1990. Cover attracted the five species of fishes in the pond (pumpkinseed, Lepomis gibbosus; chain pickerel, Esox niger; brown bullhead, Ameiurus nebu/osus; common shiner, Luxi/us comutus; and golden shiner, Notemigonus crysoleucas). Numbers of fishes were significantly higher in areas with artificial cover (70% of fishes, average counts pcr transcct), or in areas with natural weed beds (29%) than in areas without cover «1%). Pumpkinseeds were distributed about equally in areas with tire bundles (38%), cinder blocks (34%), and brush bundles (28%), whereas golden and common shiners were attracted primarily to brush bundles (62%). Numbers of fishes associated with artificial habitat werc significantly higher at night. Common and golden shiners occupied locations on the periphery of structures, whereas pumpkinseeds frequently inhabited recesses of cinder blocks and brush bundles. Associations with artificial habitat decreascd rapidly in Octobcr when water tcm- perature declined to below 12°C, By early November, as water temperatures decreased to 7°C, fishes moved to the bottom, away from cover, and markedly reduced their movements. Artificial habitats were re-examined in June 1991 following winter ice cover. All structures remained intact, except several branches from submerged brush bundles had been removed by animals, probably beaver Castor canadensis (Castoridae). Although artificial habitat may only serve to redistribute fishes in a lakc or pond, such structures arc rccommendcd as a long-term option for freshwaters where cover limits carrying capacity of fishes.

Artificial habitat has long been used by management biologists to improve local sportfisheries by providing cover and protection for fishes and attachment sites for macroinvertebrates and other prey (Wege and Anderson, 1979; Moring et aI., 1989), although the magnitude of the added food organisms is not always clear (Bohnsack et aI., 1991). Both predatory and prey fishes are attracted to artificial reefs (Wege and Anderson, 1979) and such concentrations of fishes, in turn, attract anglers (Polovina, 1991). Although most artificial reefs have been constructed in marine waters, freshwater reefs have been utilized since before the 1930s (Stone, 1985). Initially, these were "brush shelters." During the 1930s and 1940s, the State of Michigan placed piles of brush in numerous lakes (Hubbs and Eschmeyer, 1938; Rodeheffer, 1939, 1945). Other types of artificial material, such as submerged pulpwood and other woody debris (Moring et aI., 1989), plastic "trees," "lily pads," and tubes (1. Warnecke, Arizona Game and Fish Department, Phoenix, Arizona, unpubl. data) and rocks to increase spawning areas (Trendall, 1988) have also been introduced to freshwater environments. In recent years, logs (48%), brush (27%), and tires (21 %) have been the most common materials introduced to freshwater lakes (Sport Fishing Institute, 1984). Most reefs have been constructed in the South and Midwest as attractants for black basses and other centrarchids and for catfishes (Ictaluridae). But, evaluations of this management technique in freshwater ponds of boreal waters have been

1149 1150 BULLETIN OF MARINE SCIENCE, VOL. 55, NO. 2-3, 1994 limited (Haley et aI., 1987), The only study of artificial structures in Maine (with only limited analysis) occurred in the early 1970s, in Sand Pond, near Baldwin (DeRoche, 1973). Natural materials tended to attract largemouth bass (Mierop- terus salmoides), yellow perch (Perea fiaveseens), and pumpkinseed (Lepomis gibbosus), Waters of northern New England have long been managed primarily for salmonid fishes. Yet, the recent increase in popularity of fishing for black basses (Hartley and Moring, 1991), esocids (Herke, 1988), and other non-salmonid ga- mefishes in ponds and lakes of northern New England has led to the need for new management techniques to meet angler demands. The objectives of this study were to evaluate three types of artificial structures as attractors of freshwater fishes, and to evaluate seasonal changes in fish abundance.

STUDY SITE

Three types of artificial reef material were evaluated in Lac O'or, a freshwater pond that is part of Hirundo Wildlife Refuge, near Alton, Maine (Fig. I). Area and maximum depth of the pond are 1.4 ha and 3 m, respectively. Water temperatures during the growth season ranged from 7°C in November to 26°C in August. The non-natural impoundment has contained a fish community for almost 25 years and the fishes have been studied since 1972 (Moring, 1988). Five fish species are currently in the pond: pumpkinseed, chain pickerel (Esox niger), brown bullhead (Ameiurus Ilebulosus), common shiner (Luxilus cornutus), and golden shiner (Notemigonus crysoleucas). The pond is ideally suited for studies of artificial habitat because it is logistically convenient, closed to angling, and protecled from wind and other disturbances. The fish fauna is representative of many non-salmonid fish communities of northern New England.

METHODS AND MATERIALS

Prior to the introduction of artificial structures in Lac O'or, four transects were established in the deepest part of the pond on 8 June 1990 (Fig. 1). Each transect was 46 m (150 ft) long and marked by a rope, staked at each end. Each metal stake was attached to a clear plastic fishing bobber with monofilament line so that the position of each transect could be easily located. At the same time, the markers were inconspicuous and did not detract from the natural setting of the refuge. Pre-introduction surveys were conducted on 15 and 18 June 1990 by two divers who swam along the length of each transect and counted all fishes within sight during two passes, one near the bottom and one just below the surface. Artificial habitats were introduced on 20 and 21 June 1990 to three of the transects; the fourth transect served as a control. One unit of each of three types of artificial structure was placed at 15 m intervals along the length of each transect: bundled tires, brush bundles, and concrete blocks. Thus, there were three replicates of each treatment. The sequence of structures was different for each transect (Fig. 1). Tire bundles consisted of four tires placed end-to-end in a "+" pattern. Holes were drilled in the tires, and each tire bundle was secured with coated rope. No anchor was necessary bccause the protected conditions in the pond prevented displacement by winds or waves. Two 4-tire units were placed in an interlocking pattern on each transect. Concrete cinder blocks (each 20 cm X 41 em, with two open recesses), were stacked three layers high, but in an irregular pattern on each transect rope. Brush bundles consisted of tripod wooden frames, each 1.2 m X 1.2 m X 1.2 m of 5 em X 10 em wooden boards, to which bundles of white birch and white spruce limbs were tied. The tripod frames were then anchored to the bottom. On each sampling date, divers swam slowly along the bottom and recorded the species, number, and approximate size of all fishes associated with each type of structure or along the control transect. Because of the limited visibility, the divers then made a second pass near the surface to record fishes associated with the upper parts of the structures. Graham (\992) found that counts of fishes made by divers near freshwater artificial structures can be influenced by diver disturbance and diver adaptation to low light levels. Our structures were in comparatively shallow water and, because divers approached laterally, along a transect, the effect of disturbance should be equal for each structure, thus making diver counts appropriate as a technique. Data were recorded on underwater writing slates. In addition to fish counts, water temperature, weather conditions, time, underwater (horizontal) visibility, and Secchi disc measurements from the surface were also recorded. Following the two dives prior to installation of structures, on 15 and 18 MORING AND NICHOLSON: FRESHWATER ARTIFICIAL HABITATS 1151

j, ':, DAM ,~i, '/,".'

; •••• I r.',: .

: ... J' ." :1 .

. . ','. LAC O'OR

ALTON I MAl NE

SCALE; SOm

.. -. , .

Figure I. Lac D'or, Hirundo Wildlife Refuge, near Alton, Maine, and locations of three experimental transects (I, 2, 3) and a control transect (4) for studies in June-November 1990. C = cinder blocks; T = tires; and B = brush bundles,

June 1990, 16 dives were made during the day at about weekly intervals from 21 June (4 h after final introduction of habitat) to 7 November 1990. In addition, six dives were made at night between 3 August and 16 October to count fish, Only two transects could be surveyed on two of the night dives because underwater lights failed (transects I and 2 on 3 August and 20 September). Sampling was terminated after the 7 November dive because the water temperature declined to 7°C and diving conditions became more difficult. A final daytime dive was made on 19 June 1991. The purpose of this dive was to examine the 1]52 BULLETIN OF MARINE SCIENCE. VOL. 55. NO. 2-3, 1994

60 • 150 0 0 • DAY 50 Z • o NIGHT 0 0 Z • DAY •... 120 0 0 o NIGHT u 0 :::> • ... + CONTROL .0 Q U ::;) 0 90 C '"•... '"•... 0 •• '"•... Z •.. • Z 30 ::) '" 0 :J Z 0 0 60 V • 0 0 U I •+ 20 •I • 0 + I • I 0 • 30 I 10 I 0• + I • I • 0 ++ • I JU JUL AUG 5EP OCT NOV JUN JUl AUG SEP I OCT NOV MONTH MONTH Figure 2. Numbers of pumpkinseed (Lepomis gihhosus) inhabiting artificial rccfs in Lac D'or, Hi- rundo Wildlife Refuge, 1990. Numbers are combined for the three transects. (Night divcs did not begin until August 3, 1990.)

Figure 3. Numbers of golden (Notemigonus crysoleucas) and common shiners (Luxilus comullls) inhabiting artificial reefs in Lac D'or, Hirundo Wildlife Refugc, ]990. Numbers arc combined for the three transects. No shiners were encountered in the control transect cxcept for 25 on thc night of September 12, all in a weedy area. (Night dives did not begin until August 3, 1990.)

physical condition of the three types of habitat almost one year ]atcr (364 days) and aftcr more than four months of ice cover on the pond. Because of the single control transect for comparison, most experimental analyses arc mensurative (Hurlbert, 1984) only comparing colonization of fishes with different introduced habitat structures. We have made statistical comparisons, where appropriate, with the Kruskal and Wallis Test for unmatchcd samples and Friedman's Test for matched samples (Kruskal and Wallis, 1952) at the 0.05 level of significance. Die] and size class comparisons were made with paired (-tests to examine differences in combined means. In such comparisons, the three transects were combined to produce average counts per 46 m to compare with counts along the single 46 m control transect.

RESULTS Overall Results.-Visibility for divers in Lac D'or was limited, ranging from 0,6 to 1.4 m, but adequate to observe species, numbers, and relative sizes of fishes. Generally, horizontal visibilities were highest in mid-summer. Secchi depths were relatively consistent throughout the study (1.2 to 1,8 m). Despite these visibilities, observations of fishes could be made with confidence. Water temperatures peaked in mid-August (26°C) and declined rapidly after mid-October to 7°C in November. Fish Abundance.-During the first pre-structure survey on 15 June only one chain pickerel (about 40 cm total length) was encountered along one of the four transects. In the second survey on 18 June, one chain pickerel (18 cm) was also observed. In contrast, after the artificial habitat was introduced, fishes rapidly colonized the structures and used the protective crevices and recesses of all three types of material (Figs. 2, 3; Table 1). All fish species in Lac D'or used the artificial structures. Because of fish size and poor visibility in the water, divers could tend to confuse common shiners with golden shiners, particularly juvenile sizes of these species. Thus, the two species were combined as "shiners." The three types of structures attracted mostly pumpkinseed and shiners, especially juvenile stages. Shiners used brush bundles more than the other types of structures (P < 0.05; Table 1). Because of low sample MORING AND NICHOLSON: FRESHWATER ARTIFICIAL HABITATS 1153

Table I. Average numbers of fish per three transects (fish per 46 m transect), and total numbers of lishes associated with three types of artificial habitat during 18 day and 6 night dives, Lac D'or, Hirundo Wildlife Refuge, June-November 1990 (there were no natural weed areas associated with artiticial habitat transects)

Tmnsects (day) Transects (night) N = 18 dales N = 6 dales

Ilahit

•••Avcmgc counts of fish per 46 III transect per date for comparison with 46-m long control transect. t Includes golden shiner. NOlemigonus cry.wleucas, and common shiner, Luxi/u.\' comutus. sizes, statistical comparisons could not be made of artificial habitat selection by chain pickerel or brown bullhead. More fishes were observed along transects with artificial structures (70% of fishes, using average counts per experimental transect compared to the control transect) than along the control (open) transect. (Friedman's Test was not significant if counts of fishes in natural weeds along transects are included: X2 = 3.87 for day, 3.87 for night; but significant if natural weed areas are excluded: X2 = 9.60 for day, 32.00 for night). Of the 121 fishes along the control transect, 98% (119) were observed within a few natural weed beds (Table 1). Only one fish (chain pickerel: brush bundle) was observed near artificial structures during the observation dive one and three days after the introduction of the structures. Eight days later, shiners were observed readily using brush bundles and tires, and these species continued to use the artificial reefs until 10 October, with peak abundance when water temperatures exceeded 21°C in early summer. 1154 BULLETIN OF MARINE SCIENCE, VOL. 55, NO. 2-3, 1994

COUNTsrrRANSECT/DIVE

1 2 3 4 5 6 7 8 9 10 11 12 13 14 TIRES ~------.

BRUSH BUNDLES ------

CINDER BLOCKS ------

CONTROL (Natural .------Weeds) DAY NIGHT ---- CONTROL • (Open)

Figure 4. Average counts of all fishes per transect for each dive date, compared to counts in the natural weed areas and the open areas of the control transect for each dive date.

After 10 October, water temperatures dropped below 12°C, and all fish species disappeared from the structures. Use of artificial habitats by fishes differed between day and night, with higher numbers at night (P < 0.05). Thirty-eight percent of the fishes were associated with cinder blocks in the day (the three transects combined), but only 24% were observed with cinder blocks at night (Fig. 4). Thirty-six percent of the fishes were associated with tires in daytime, and 21% with tires at night. In contrast, 27% of the fishes were in or near brush bundles in day and 56% at night. Five of the six observations of chain pickerel were during daylight hours (distributed among the three types of artificial habitat), but 91 % of the brown bullhead were observed at night, primarily with brush bundles; 59% of the shiners were encountered at night, but only 41% of the pumpkinseed (Table 1). Shiners only used brush bundles from late June to mid-September. Beginning in late July, pumpkinseeds were always encountered with artificial reefs and their numbers remained high until late September; numbers declined rapidly after that time. Day and night observations showed similar trends (Figs. 2, 3). Observations were too infrequent for conclusions about seasonal trends of habitat use by chain pickerel and brown bullhead. MORING AND NICHOLSON: FRESHWATER ARTIFICIAL HABITATS 1155

Table 2. Comparison of numbers of adult and juvenile pumpkinseed, Lepomis gihhosu.\·, associated with three types of artificial habitat during daytime and nighttime observations, Lac D'or, Hirundo Wildlife Refuge, June to November 1990

Arti ficial habitat

Silc/limc Blocks Brush Tires Total

Adults Day* 28 23 38 89 Night 37 II 16 64 Total 65 34 54 153 Juveniles Day 142 59 157 358 Night 74 105 88 267 Total 216 164 245 625

• Day divc~ N :., 16 with three transects combined; Night dives N = 4 with three transects combined and N = 2 with two trnnseCls (I Hnd 2) comhined (underwater lights failed on August 3 and September 20 dives).

Adult and juvenile pumpkinseeds used all three types of artificial habitat, but were observed in significantly higher numbers with tires (43% of total fishes; P < 0.05), and higher numbers with cinder blocks (33%) than with brush bundles (24% of total) in daylight (Table 2). Of ]53 adults, 42% were observed with cinder blocks and 35% with tires, but only 22% with brush bundles. Of 625 juveniles, 35% were observed with cinder blocks and 39% with tires, but only 26% with brush bundles. The distribution of adult and juvenile pumpkinseed in daylight was similar: juveniles and adults, 40% and 31 %, respectively, with cinder blocks; 44% and 43% with tires; and 16% and 26% with brush bundles. Use of artificial habitats by juvenile pumpkinseed had a different pattern at night than in daytime: 39% of the juveniles at night were observed with brush bundles, whereas 33% were with tires; and 28% were with cinder blocks. Adults were observed primarily with cinder blocks (58%), and less at tires (25%) and brush (17%). Because age and size in shiners were sometimes difficult to judge at a distance underwater, juvenile-adult distinctions were not made for these two cyprinid species. All chain pickerel observed near artificial structures (all adults) were encountered in daylight hours: two (both about 10 cm in length) with tires, one (15 cm) with cinder blocks, and two (each 15 cm) not associated with any type of cover. The three chain pickerel along the control transect (sizes about 10, 13, and 23 cm) were in natura] weed beds. For brown bullheads, 9] % were observed at night; 56% of the fishes were adults and all of those were observed at night. Brown bullheads were encountered with all three types of artificial habitat, a]- though 68% of the adults and 75% of the juveniles that were encountered at night were within or near brush bundles. Fish Behavior.-Shiners were seldom observed within the confines of artificial habitat (recesses, or intrabrush areas) but, rather, they schooled in close proximity to cover, particularly in the upper portions of brush bundles. At night, juvenile shiners were found immediately at the tops of brush bundles. Pumpkinseeds exhibited quite different behavior. By August, individuals generally occupied the openings in cinder blocks, protective areas in tires, or spaces in brush bundles. By August, almost all cinder blocks contained one adult pumpkinseed or one or two juveniles. By September, each cavity was occupied by a single fish, and each tire well was occupied by one or two pumpkinseed. At night, individuals moved out of the protective recesses, but remained close to the upper 1156 BULLETIN OF MARINE SCIENCE, VOL. 55, NO. 2-3, 1994 surfaces of cinder blocks or tires, In October, when water temperatures declined to 12°C, this behavior changed. Fewer fishes were near artificial structures and more were located away from the introduced reefs, and resting on the bottom. Observations of chain pickerel and brown bullhead were too limited to make adequate conclusions as to behavior. Examination after One Year.-There was limited fish activity around artificial reefs when the structures were re-examined in June 1991, I year after the initial introduction. All fish were observed along the experimental transects rather than along the control transect. Two pumpkinseeds occupied an opening in a cinder block, and a shiner was observed with a brush bundle. All but one artificial structure remained intact and had some colonization by aquatic macroinvertebrates and anuran tadpole stages. One brush bundle was partly dismantled, probably by beavers (Castor canadensis: Castoridae), which are found in high numbers in a nearby stream and are visitors to the pond. The frames of the bundles remained functional, but several branches had been removed and some were scattered on the bottom. Potential Prey.-Although estimates of macroinvertebrate colonization densities were not made, gastropods and aquatic insects were first observed at brush bundles on 13 July. Densities of these macroinvertebrates remained high until early September. Notonectidae (water boatman) were extremely abundant at night on 8 August, but were uncommon at night on 6 September. Numbers of aquatic macroinvertebrates at all types of artificial habitat were always higher at night than during day time. Anuran tadpole stages attached to artificial reefs almost as soon as the material was introduced and remained in dense numbers throughout the summer, but disappeared or declined profoundly by early September.

DISCUSSION This study clearly demonstrates the ecological value of cover for freshwater fishes of boreal waters. No one type of artificial habitat was clearly selected by all fishes, although tires and cinder blocks were used more than brush bundles by pumpkinseed, and brush bundles and cinder blocks were clearly preferred by shiners. Part of the species preference may be related to cover associations influenced by low light intensity, as has been noted for bluegills, Lepomis macrochirus (Johnson, 1993). The three types of artificial reef materials examined in this study are not new as freshwater reefs. Carl Hubbs, for example, advocated the use of brush bundles in the 1930s (Stone, 1985), and these structures are now being used in many waters of the southwestern United States to attract various species of Centrarchi- dae (Mabbott, 1991; Johnson and Lynch, 1992; T. J. Hassler, Calif. Coop. Fish. Res. Unit, Humboldt St. Univ., Arcata, California, unpubl. data). Concrete blocks have been used in Lake Ontario (Gannon et aI., 1985), and polyethylene pipes, stakes, and tires have been used to attract various species of centrarchids (Prince et aI., 1975, 1985; Walters et aI., 1991; Johnson and Lynch, 1992). A survey in 1984 indicated that some type of artificial structure had been introduced to over 1,000 lakes in the United States (Sport Fishing Institute, 1984). Angler effort and harvest of warmwater fish species have increased dramatically in Maine in recent years, but the introduction of artificial reefs to waters of the state has been almost nonexistent. DeRoche (1973) examined tire bundles, concrete cinder blocks, chimney tiles, and tile pipes as artificial habitat for fishes in Sand Pond, Maine, from 1969 to MORING AND NICHOLSON: FRESHWATER ARTIFICIAL HABITATS ] 157

1970. He found no increase in fish numbers along the control transect, but rapid increases in numbers of largemouth bass, yellow perch, brown bullhead, chain pickerel, and pumpkinseed around artificial reefs within the first year (some species were too numerous to count). Although, in the short-term, this may reflect a re-distribution or concentration of fishes, rather than a population increase, species used these reefs for feeding and protection, and there was evidence that largemouth bass and pumpkinseed had spawned in or around the structures. Fishes tended to prefer chimney tiles and tile pipes to other materials, notably tires. Our findings were similar, although DeRoche did not make precise counts of all fishes and did not separate observations by adult and juvenile sizes in his one experimental and one control transect. Pumpkinseeds have few fish predators in Lac D'or. Although chain pickerel prefer spiny-rayed fishes as prey, Herke (1988) found no pumpkinseed in the diet of chain pickerel in a Maine lake. But some mammals are known to feed on pumpkinseeds and several species of aquatic birds can be significant predators (Crivelli and Mestre, 1988). Therefore, cinder blocks and tires may provide protection from predators, especially for juvenile pumpkinseed, allowing the smaller fish to feed on a preferred food item, gastropods (Osenberg and Mittelbach, 1989), attached to those habitats. Except for the nocturnal feeding of a restricted size class of pumpkinseeds (100-140 mm TL: Keast and Welsh, 1968), most pumpkinseeds feed in daylight (Hanson and Qadri, 1984). Brush bundles are inexpensive to construct and provide a more natural type of habitat. Although these structures were completely submerged and were not vis- ible to the casual observer at Lac D'or, such habitat sometimes is placed in ponds with fluctuating water levels (T. J. Hassler, pers. comm.). Even if exposed, brush bundles appear quite natural. Brush bundles are not durable, however, and are known to decay over time (Bohnsack et aI., 1991). Our examination after I year indicated that brush bundles could be an intact, functional type of artificial habitat for at least a second open-water season in a pond environment. Tire bundles of the type used in our study and DeRoche's (1973) study are also valuable to fishes; pumpkinseeds seek the recesses of the tire wells for protection. In larger bodies of water, tires can be displaced by storms and become shore litter. This would not be the case for a small pond, as we studied, but could be a factor in management applications of this type of structure for a large lake. Cinder blocks were found to be valuable in DeRoche's study and in our study, particularly by pumpkinseed, but also by shiners. These structures can be consid- ered permanent and, after colonization by algae and sessile organisms, they appear natural. DeRoche (1973) found that all the artificial habitats that he examined were rapidly covered with algae and aquatic plant material. Our examination of the reefs after I year indicated that cinder blocks and tire structures remained intact, but some brush bundles were dismantled, probably by beavers. As durable habitat in boreal waters, we recommend cinder blocks and secured, anchored tire bundles. Fish species diversity is low in many bodies of water in northern New England (Kircheis and Stanley, 1981). Because of reduced habitat, the introduction of artificial habitats can potentially increase fish numbers for the long term, following successful reproductive cycles and subsequent use of new habitat, although studies of populations elsewhere have shown mixed results (Polovina, 1991). Whether an existing population is simply re-distributed or whether a population actually increases may be species- and site-specific. For example, Walters et al. (1991) did not conclude that artificial habitat will result in increased numbers of small centrarchids. However, the value of artificial reefs for concentrating prey and predator 1158 BULLETIN OF MARINE SCIENCE. VOL. 55. NO. 2-3. 1994

fish species has yet to be fully explored in terms of improving sport fisheries in this region. The disadvantage of freshwater artificial reefs is that the material can attract and aggregate fishes. This can improve catch rates, but could also cause over- harvest of an existing fish community because concentrated fish are more avail- able to anglers (Polovina, 1991). Our results suggest that these three types of artificial structures, particularly cinder blocks and brush bundles, may provide benefits for management of centrarchid fisheries in northern ponds and lakes. Longer-term studies will be required to document how much artificial reefs increase carrying capacity and population numbers of these species.

ACKNOWLEDGMENTS

These investigations were funded by the Hirundo Wildlife Trust. We thank O. Larouche, Hirundo Wildlife Refuge, for his many courtesies, and P. Bourque and J. Dennis McNeish for helpful discus- sions and background material. S. Szalkowski, M. Cartwright, O. van den Ende, M. Moring, and S. Moring participated in various aspects of the field work. We thank W. Krohn, National Biological Survey, for reviewing the manuscript.

LITERATURE CITED

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DATE ACCEPTED: July 29, 1993.

ADDRESS: National Biological Survey, Maine Cooperative Fish and Wildlife Research Unit, Depart- ment of Zoology, University of Maine, 575/ Murray Hall, Orono, Maine 04469-575/.