FISHERY BULLETIN: VOL. 75, NO. 4 



materials permit a relatively uninhibited flow of 

 ground water that is the main source of water 

 for all ponds. Hoglot and Clubhouse springs are 

 on state-owned land and Maxwell Springs is 

 privately owned. The ponds are located in wooded 

 lowlands and all three drain into trout streams 

 that are part of the Wolf River drainage, a major 

 Lake Michigan watershed. 



The ponds are similar in size and have rel- 

 atively short exchange times due to large inflows 

 of ground water (Table 1). Because all ponds are 

 supplied by the same aquifer, concentrations of 

 common ions are similar. Bottom materials con- 

 sist mostly of marl and organic matter. About 

 10% of the shorelines in Maxwell and Hoglot 

 springs are composed of gravel with emerging 

 ground water and brook trout spawn in these 

 areas. Numbers of trout redds in Hoglot Springs 

 ranged from 85 to 105/ha of pond area, and in 

 Maxwell Springs redd densities ranged from 165 

 to 230/ha. Clubhouse Springs lacks gravel areas 

 with upwelling ground water and brook trout do 

 not spawn there. 



Continual inflow of ground water and rapid 

 exchange times tend to moderate pond tempera- 

 tures and maintain relatively high concentrations 

 of dissolved oxygen. Ground water temperatures 

 typically range from 6° to 7°C and concentrations 

 of dissolved oxygen, from 8 to 9 ppm. Pond tem- 

 peratures in summer at depths of 15 cm rarely 

 exceed 16°C. Concentrations of dissolved oxygen 

 rarely fall below 5 ppm at any depth throughout 

 the year and they usually exceed 7 ppm. Ponds 

 are ice-covered from early November to late 

 March. 



All ponds supported dense beds of aquatic vege- 

 tation. Chara vulgaris covered about 40% of the 

 bottom in Clubhouse Springs and 15% in Hoglot 

 Springs. Anacharis canadensis, the only common 



TABLE 1. — Some physicochemical features of study ponds in 

 north central Wisconsin. Chemical measurements were taken 

 in April 1970. 



Item 



Clubhouse 

 Springs 



Hoglot 

 Springs 



Maxwell 

 Springs 



Surface area (ha) 



Mean depth (m) 



Outlet discharge 1 (m 3 /s) 



Exchange time 2 (days) 



Specific conductance (umbolcm) 



Total alkalinity (mg/l as CaCOs) 



Calcium (mg/l) 



Nitrate (mg/l-N) 



Dissolved phosphorus (mg/l-P) 



'Summer base flow. 

 2 Pond volume/discharge. 



plant in Maxwell Springs, extended over 50% of 

 the bottom. 



Fish communities in the three ponds were sim- 

 ilar. Brook trout composed the major portion of 

 fish biomass. A small population of brown trout, 

 Salmo trutta, in Clubhouse Springs never ac- 

 counted for more than 10% of the total number 

 of trout. The white sucker, Catostomus commer- 

 soni; mottled sculpin, Cottus bairdi; Central 

 mudminnow, Umbra limi; and brook stickleback, 

 Culaea inconstans , were common in all ponds. The 

 brook stickleback was an important food source 

 for age 3 and older trout; however, benthic inver- 

 tebrates composed the major portion of the diet for 

 trout of all sizes. 



METHODS 



Trout populations were estimated in spring and 

 fall using Bailey's modification of the Petersen 

 mark and recapture method (Ricker 1975). Trout 

 were captured at night with electrofishing gear 

 and held overnight in screen cages. The following 

 day, fish were anesthetized, measured to the near- 

 est 2 mm (total length), weighed to the nearest 

 gram, given a temporary mark by clipping the 

 tip of the caudal fin, and released. A second 

 electrofishing run was made two or more days 

 later. Proportions of marked trout captured dur- 

 ing the second electrofishing sample were used 

 to calculate confidence limits for population esti- 

 mates (Adams 1951). 



Age structures of trout populations were deter- 

 mined from length distributions of known age fish 

 and scale analyses. Fall fingerlings and spring 

 yearlings, determined from length-frequency 

 distributions, were permanently marked by fin 

 removal. Estimated numbers of trout in each 

 25-mm length group were placed in appropriate 

 age-groups based on relative proportions of known 

 age fish. The electrofishing gear was size selective. 

 Efficiency was lowest for smallest fish and in- 

 creased until fish size reached about 12 cm. Sep- 

 arate estimates for 25-mm length intervals 

 avoided bias due to size selectivity of electro- 

 fishing gear. 



Maxwell outlet and Elton Creek, the stream 

 into which Clubhouse Springs flowed, were 

 sampled with electrofishing gear to obtain data 

 on growth rates of trout in outlet waters and on 

 movement of trout between ponds and adjoining 

 streams. A 1-km section of Elton Creek was 

 sampled five times from 1968 to 1971; Clubhouse 



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