SH 10.? :A7 R65 i9B9 This document is made available electronically by the Minnesota Legislative Reference Library as part of an ongoing digital archiving project. http://www.leg.state.mn.us/lrl/lrl.asp (Funding for document digitization was provided, in part, by a grant from the Minnesota Historical & Cultural Heritage Program.) THE ROLE OF LOW FLOW HABITAT AND INTERSPECIFIC COMPETITION 1 IN LIMITING ANADROMOUS PARR ABUNDANCE IN NORTH SHORE STREAMS by Tracy L. Close Fisheries Biologist and David A. Belford Fisheries Biologist and Steven E. Colvin Fisheries Biologist and Charles S. Anderson Coldwater Fisheries Research Supervisor Minnesota Department of Natural Resources Section of Fisheries 1 This project was funded in part by the Federal Aid in Sport Fish Restoration (Dingell~Johnson) Program. Completion report, Study 222, D-J Project F-26-R Minnesota. ABSTRACT We investigated the roles of summer low flow habitat and interspecies competition as limiting factors of chinook salmon (Oncorhynchus tshawytscha), Atlantic salmon (Salmo salar), and steelhead (£. mykiss, formerly Salmo gairdneri) parr in North Shore streams. Macro-and micro-habitat availability and use by allopatric and sympatric populations were analyzed by analysis of variance (ANOVA) and multiple regression. Sunnner densities of age 0 and age 1 or older (l+) age classes were highly variable and were independent of measured low flow habitat variables and the densities of potentially competing species. Steelhead and Atlantic salmon parr preferred fast-water habitats and avoided pools. Chinook salmon parr were found in deeper water than steelhead parr and Atlantic salmon parr, but snout velocities were similar. Overhead cover was a significant component of habitat use models for all ages of steelhead and Atlantic salmon, but overhead cover did not limit abundance. We concluded that low flow habitat is not a major limiting factor, and we found no evidence of interspecific competition in sympatry. The year-to-year and stream-to-stream variations in fish density suggest that other environmental factors normally limit salmonid abundance to less than the low flow carrying capacity. i INTRODUCTION Understanding the dynamics and behavior of steelhead Oncorhynchus mykiss (formerly~ gairdneri), Atlantic salmon ~· salar, and chinook salmon £· tshawvtscha parr is the key to intensively managing the Lake Superior anadromous fish stocks intensively. Steelhead residing 2-3 years in streams contribute 88% to the returning adult stocks (Hassinger et al. 1974). Hassinger (1981) and Close et al. (1984) suggest adult returns are primarily a function of the number and fitness of smolts. To increase the fitness and production of anadromous smolts, the factors affecting growth and survival of parr must be determined. Smolt production could be influenced by the amount of suitable summer rearing habitat. Binns and Eisermann (1979) found that trout abundance was limited primarily by the amount of summer habitat and flow stability, and they developed a model that precisely predicted fish density from habitat characteristics. Summer habitat in North Shore streams may have a similar impact on juvenile salmonid densities because there is little groundwater input for summer base flow. We investigated the relationship between low flow macrohabitat variables and juvenile salmonid densities to determine the limiting factors in North Shore streams. Interspecific competition for space can also influence species distribution and abundance (Fausch and White 1981; Whitworth and Strange 1983; Larson and Moore 1985). Competition for rearing space between chinook salmon, Atlantic salmon, and steelhead could limit parr abundances in North Shore streams, so the importance of competition for space must be determined to guide multispecies 1 management. We measured microhabitat availability and use by each species in allopatric and sympatric populations. An overlap in microhabitat use by allopatric species indicated there was a potential for competition while a change in microhabitat use by sympatric species was assumed to indicate the occurrence of competition (Fausch and White 1981). This study examined the role of summer habitat and interspecific competition as factors limiting abundance of anadromous salmonid parr. Our goal was to apply our findings by developing recommendations for stocking, for multi-species management, and for habitat improvement to maximize smelt output of North Shore streams. STUDY AREAS This study included 12 study sectors in five streams on the North Shore of Lake Superior (Table 1). North Shore streams are characterized by small watersheds and little groundwater input. Headwater reaches are inhabited by brook trout Salvelinus fontinalis, and natural reproduction of anadromous salmonids is limited by water ~ falls and cascades to 156 km in 58 streams (Hassinger et al. 1974). Stocking of salmonid fry above the barriers and barrier modification (step pockets) are current management practices to increase smolt output. METHODS Stocking and Fish Density Chinook salmon, Atlantic salmon, and steelhead swim-up fry were stocked in 1984-1986 in 300 m study sectors at a total density of 2 5 salmonid fry/m (wetted surface area) in allopatric and sympatric combinations (Table 1). The 300 m study sectors were subdivided into 2 Table 1. Stocking quotas (swim-up fry) for study sectors. Steelhead Atlantic Chinook a Stud~ stream Sector salmon salmon Lester River 1 5,000 5,000 5,000 Lester River 2 5,000 5,000 Lester River 3 5,500 5,500 Gooseberry River 1 5,000 5,000 5,000 Gooseberry River 2 8,500 8,500 Gooseberry River 3 13,500 Two Island River 1 3,500 3,500 Two Island River 2 7,500 Two Island River 3 9,000 w. Br. Split Rock 1 9,000 E. Br. Split Rock 1 9,000 a Sector number increased upstream. 25 m sections. Eggs were incubated at stream temperatures so that fry swim-up coincided with naturally hatched fish. Chinook salmon and Atlantic salmon were stocked in May and steelhead were stocked in June. Salmonid densities in each 25 m section were estimated once each year from June to October in 1984-1986 by electrofishing using the removal method of Carle and Strub (1978). Captured fish were given a caudal fin clip and released back to the area where they were captured. We alternated upper, lower, and both lobe fin clips to validate our assumption that movement of fish between sections was minimal. We assumed that migration into and out of the 300 m sectors was minimal during the sampling interval. Typically, two electrof ishing passes spaced 24 h apart were used to estimate the population size. Average densities (unweighted) for each 300 m sector are reported here, but the 25 rn section densities were used in 3 regression analyses. Electrofishing was performed using a Smith-Root Type 11 backpack unit. Macrohabitat Use Water velocity-depth categories and overhead cover types (Table 2) were diagrammatically mapped using the method of Oswood and Barber (1982). Oswood and Barber suggested two depth categories, shallow (<0.5 m) and deep (~0.5 m). North Shore streams have large areas of shallow water so we added a very shallow fast category that included depths <0.15 m. The percent area of each substrate type was estimated in each 25 m study section and the length of the thalweg was measured. The relationship of macrohabitat variables to juvenile densities was determined by step-wise multiple regression analysis (Draper and Smith 1981). For regression Model 1, dependent variables were section densities of similar aged salmonid cohorts and independent variables were percent of total section area of each macrohabitat category and thalweg length. Only significant variables (P <0.05) B were included in the final model. Transformations of the dependent and independent variables were investigated with the methods suggested by Weisberg (1985), but the models could not be improved significantly (P >0.05). We assessed multicolinearity in the independent variables by noting coefficient stability during stepwise regression and concluded multicolinearity was not excessive. Regression Model 2 included year and stream indicator variables (Neter et al. 1985) to determine the possible effects of unmeasured year-to-year and stream-to-stream variations. We compared the 2 coefficients of determination (R ) of the two models with the same 4 Table 2. Water velocity-depth, overhead cover, and substrate categories used in diagrammatic mapping. Category (code) Description Very shallow slow (VSS) Area of water <0.15 m deep and water velocity <0.3 m/s. Very shallow fast (VSF) Area of water <0.15 m deep and water velocity ~0.3 m/s. Shallow slow (SS) Area of water >0.15 m and <0.5 m deep and water velocity <0.3 m/s. Shallow fast (SF) Area of water >0.15 m and <0.5 m deep and water velocity >0.3 m/s. Deep slow (DS) Area of water >0.5 m deep and water velocity <0.3 m/s. Deep fast (DF) Area of water >0.5 m deep and water velocity ~0.3 m/s. Forest debris (FD) Area of fallen trees and branches in the sampling station (fish cover). Undercut banks (UB) Area of eroded stream banks which offer overhead cover for fish. Riparian vegetation (RV) Area of overhanging vegetation along stream banks. Boulder (1) Rock particles >250 mm diameter. Rubble (2) Rock particles .250 mm and >75 mm diameter. Gravel (3) Rock particles .75 mm and >3 mm diameter. Sand (4) Rock particles • 3 mm diameter. Silt (5) Fine material with little grittiness. Clay (6) Compact, sticky material. Muck (7) Decomposed organic matter, usually black. Detritus (8) Organic material composed of sticks, leaves, decaying plants, etc. 5 dependent variable to determine the relative importance of unmeasured variables. Microhabitat Use Microhabitat available to parr was determined in 1987 by making measurements at 1 m intervals along transects (perpendicular to the center line) spaced 25 m apart. Our goal was to sample 100 locations in each 300 m study sector.