Great Bas;n Naturalist 56(2), e 1996, pp. 157-161 EFFECTS OF TURBIDITY ON FEEDING RATES OF LAHONTAN CUTTHROAT TROUT (ONCORHYNCHUS ClARKI HENSHAWI) AND LAHONTAN REDSIDE SHINER (RICHARDSONIUS EGREGIUS) Gary L. Vinyard l and Andy C. Yuanl ABSTRACT.-The spawning population of Lahontan cutthroat trout (Oncorhynchus clarki henshawi) in Summit Lake, Nevada, has reportedly declined since the early 19705, coincident with the appearance of Lahontan redside shiner (Richardsonius egregius) in the lake. We investigated the relative predatory abilities of these 2 fish species foraging on live Daphnia magna in turbidity conditions commonly observed in Summit Lake. Experiments were performed under controlled light and temperature conditions. In separate trials we fed trout and shiner 1 of3 size classes of D, magna (1.7 mm, 2.2 mm, and 3.0 mm) at 6 levels of turbidity ranging from 3.5 to 25 NTU. Feeding rates for both species varied inversely with turbi.dity for all prey sizes. Feeding rates ofshiner were greater than trout at all turbidity levels. In low turbidity (5 NTU), shiner consumed approximately 3% more prey during 2-b feeding trials. However, at higb turbidity levels, the diHerence in feeding rates between species was proportionally higher (l0%). At high turbidity levels ~ 20 NTU) trout predation rates were relatively insensitive to prey siZe. However, shiner continued to consume more, larger prey at the highest turbidity levels. These results indicate that Lahontan redside shiner may be superior to Lahonlan cutthroat trout as zooplankton predators at high turbidity levels, and may explain the recent success of shiner in Summit Lake. Key l.IXW'd.s: Dapbnia, Lahontan cutthroat trout, Oncorhynchus clarki henshawi, Lahontan redside shiner; Richardso­ NUS egregius, planktioory, predation, size selectivity, turbidity. The Lahontan cutthroat trout (Oncorhynchus (Cowan and Blake 1989, Valeska 1989). Other clm'!d henshawi) is an inland subspecies endemic lacustrine populations are either maintained to the physiographic Lahontan basin in northern by artificial stocking Or are subject to higher Nevada, eastern California, and southern Ore­ levels ofharvest and disturbance. Conservation gon. These trout were once widespread through­ of this population is compelling, and it has out the basins of Pleistocene Lake Lahontan been identified as important for recovery of (USFWS 1995). Currently, they occupy <1% the subspecies (USFWS 1995). of their former lacustrine range and 11% of Cutthroat trout spawning runs at Summit their former stream habitat within the native Lake have generally declined since the late range (USFWS 1995). Listed as endangered in 1970s (Cowan and Blake 1989). Collection of 1970, the fish was subsequently reclassified as roe during the 1960s and 1970s and excessive threatened in 1975. This facilitated manage­ loss of spawning habitat in Mahogany Creek ment and permitted regulated angling (USFWS from livestock overgrazing (Cowan and Blake 1995). 1989, Vinyard and Winzeler 1993) have beeo Summit Lake is located in the Summit Lake blamed. However, coinciding with the decline Paiute Indian Reservation in northwestern in trout, Lahontan redside shiner (Richara.o­ Humboldt County, Nevada (41°N latitude nius egregiu.s) also increased in abundance in 119°W longitude), at an elevation of 1828 m. the lake, suggesting a competition effect. Formed by a landslide about 20,000 years ago, Redside shiner are native to the Great Summit Lake is relatively shallow (maximum Basin, but they do not occur naturally in Sum­ depth 12 m) and has historically been subject mit Lake. Origins of the present shiner popu­ to high turbidity levels during summer months lation in the lake are unknown, but they have from suspended algae and silt (LaRivers 1962). been used frequently as live bait. Lahontan It contains the most secure remaining lacus­ redside shiner feed on drift in streams and are trine population of Lahontan cutthroat trout, zooplanktivorous in lakes (Vmyard and Winzeler and no other salmonids occur in the basin 1993). Laboratory observatious suggest they lDepmment ofBioJogy, UnNeoity ofNevada, Reno, Ne~ S9S57.oo15. 157 158 GREAT BASIN NATURALIST [Volume 56 may also prey on larval trout (Vinyard and by botb species. Lighting was provided by a Winzeler 1993). Analysis of stomach contents bank of three 56-watt fluorescent tubes con­ suggests that Lahontan cutthroat trout and trolled by an automatic timer (lOL14D). Light Lahontan redside shiner probably consume intensity at the water surface averaged 93 tiE similar foods both in Summit Lake and in m-2 S-l. An airstone in the center of each of Mahogany Creek, the primary spawning tribu­ four 38-L aqnaria provided aeration and kept tary for trout from Summit Lakc (Vinyard and turbidity in suspension. Turbidity (nephelo­ Winzeler 1993). Both species consume drift in metric turbidity units, NTU) was measured the stream, and mostly amphipods in Summit with an HF Instruments Model DRT 15 tur­ Lake (Cowan and Blakc 1989). In contrast, simi­ bidimeter. Six turbidity levels (3..5, 6, 10, 20, larly large Lahontan cutthroat trout in Pyramid 22, and 25 NTU) were produced using sus­ Lake are piscivorous (USFWS 1995). Because pensions of bentonite. Bentonite concentra­ most fish species depend on vision to locate tions (mg/L) were significantly correlated with prcy (Hobson 1979, Guthrie 1986), it is possi­ measured turbidity (NTU = 2.583 + 0.162 B, ble that high turbidity in Summit Lakc limits r2 = 0.99). This material is nontoxic and the visibility ofprey and impedes the ability of remains in suspension for long periods. trout to catch redside shiner and other large Feeding rates were determined for fish ex­ prey. posed to single-sized groups ofDaphnia rruzgna OUf experiments compared the relationships at each turbidity level. Laboratory-reared D. of feeding rate, turbidity, and prey size for magna were sorted into 3 size groups using a Lahontan cutthroat trout and Lahontan red­ dissecting microscope: 1.7 mm, 2.2 mm, and side shiner, with the primary focus being to 3.0 mm (top of bead to base oftail spine, + 0.3 examine the relative performance of both mm). Before each feeding trial, a single fish species under various turbidity levels. was placed into each experimental tank and allowed to acclimate for 24 h. A group of 200 METHODS AND MATERIALS Daphnia were introduced into the tank and the fish allowed to feed for 2 h. Fish were then Lahontan redside shiner were captured from removed and the water and remaining prey Mahogany Creek, Humboldt Connty, Nevada, siphoned through a 363-micron mcsh net. Prey and transported to the University of Nevada. retained on the net were counted to deter­ Lahontan cutthroat trout from the current mine consumption rates. This procedure was Pyramid Lake stock were acquired from the repeated for each of the 3 prey size classes Lahontan National Fish Hatchery, Gardner­ and 6 turbidity levels with 4 fish from each ville, Nevada. Although the historical origins species, yielding a total of 144 feeding trials. of the existing Pyramid Lake stock are mixed, Fish nsed in the feeding trials ranged from 70 Summit Lake fish were heavily planted into mm to 93 mm SL. Analysis of variance and lin­ Pyramid Lake for a number of years, and they ear regression were used to assess the eflects likely constitute the dominant component of of Hsh species, prey size, and turbidity level the population (USFWS 1995). Fish were OIl predation rates. housed in 19-L tanks and acclimated to local water conditions for at least 3 wk prior to RESULTS experiments. Experiments were conducted in a secluded An analysis ofoverall predation rates for both section of a greenhouse at the University of fish species consuming all prey sizes (Figs. la, Nevada. The experimental protocol was simi­ lb) indicates that feeding rates varied inversely lar to that employed by Vinyard and Winzeler with turbidity (multiple regression, F = 1894, (1993) and Li ct '11. (1985). Visnal isolation of P < 0.001) and between fish species (F = experimental tanks was ensured by opaque 28.4, P < 0.001), and that larger prey gener­ black polyethylene sheeting (10 mil, 2.5 m ally were consumed at greater rates (F = 38.3, high), which enclosed all sidcs of the experi­ P < 0.001). Significant results were observed mental area and controlled external light. for both the species*NTU and species'daph­ Temperatures ranged between 12°C and 17°C nia size interaction terms, indicating that the 2 during the experiments, and diel variation fish species differ in their responses to these 2 never exceeded 4 0 C, a range easily tolerated variables. Lahontan redsidc shiner consumed 1996J TURBIDITY EFFECTS ON FISH FEEDING 159 significantly more prey than Lahontan cut­ Our results demonstrate that turbidity throat trout. At the lowest turbidity level (3.5 reduces predation rates for all prey sizes for NTU), approximately 90% of all prey were both Lahontan redside shiner and Lahontan consumed by both fish species. However, even cutthroat trout. Larger prey were generally small increases in turbidity reduced predation consumed with greater frequency, although rates. This decrease in predation with turbid­ this frequency varies with turbidity and fish ity was strongly linear, and there was no indi­ species. The effect of prey size was most con­ cation ofa minimum value having been reached sistent for Lahontan redside shiner. These fish by 25 NTH At that turbidity level, predation consumed more large (3.0 mm) prey at all tur­ rates declined by approximately 80% for trout bidity levels than did Lahontan cutthroat trout (Fig. la) and by 60% to 80% for shiner (Fig. (Figs. la, Ib). In contrast, prey size had little Ib), depending on prey size. Predation rates effect on the relative numbers of prey of each for trout were significantly affected by prey size consumed by trout at turbidity levels of size and turbidity (multiple regression F = 20 NTU or above (Fig.