Zooplankton Size Selection Relative to Gill Raker Spacing in Rainbow Trout

Transactions of the American Fisheries Society 134:1228 1235, 2005 Copyright by the American Fisheries Society 2005 DOI: 10.1577/T04-159.1 [Note] Zooplankton Size Selection Relative to Gill Raker Spacing in Rainbow Trout PHAEDRA BUDY* AND TYLER HADDIX 1 U.S. Geological Survey Utah Cooperative Fish and Wildlife Research Unit, Department of Aquatic, Watershed, and Earth Resources, 5210 Old Main Hill, Utah State University, Logan, Utah 84332-5210, USA ROGER SCHNEIDERVIN Utah Division of Wildlife Resources, Post Office Box 145, Dutch John, Utah 84023-0145, USA Abstract. Rainbow trout Oncorhynchus mykiss are one of the most widely stocked salmonids worldwide, often based on the assumption that they will effectively utilize abundant invertebrate food resources. We evaluated the potential for feeding morphology to affect prey selection by rainbow trout using a combination of laboratory feeding experiments and field observations in Flaming Gorge Reservoir, Utah Wyoming. For rainbow trout collected from the reservoir, inter gill raker spacing averaged 1.09 mm and there was low variation among fish overall (SD 0.28). Ninety-seven percent of all zooplankton observed in the diets of rainbow trout collected in the reservoir were larger than the interraker spacing, while only 29% of the zooplankton found in the environment were larger than the interraker spacing. Over the size range of rainbow trout evaluated here (200 475 mm), interraker spacing increased moderately with increasing fish length; however, the size of zooplankton found in the diet did not increase with increasing fish length. In laboratory experiments, rainbow trout consumed the largest zooplankton available; the mean size of zooplankton observed in the diets was significantly larger than the mean size of zooplankton available. Electivity indices for both laboratory and field observations indicated strong selection for larger-sized zooplankton. The size threshold at which electivity switched from selection against smaller-sized zooplankton to selection for larger-sized zooplankton closely corresponded to the mean interraker spacing for both groups ( 1 1.2 mm). The combination of results observed here indicates that rainbow trout morphology limits the retention of different-sized zooplankton prey and reinforces the importance of understanding how effectively rainbow trout can utilize the type and sizes of different prey available in a given system. These considerations may improve our ability to predict the potential for growth and survival of rainbow trout within and among different systems. * Corresponding author: phaedra.budy@usu.edu 1 Present address: GEI Consultants, Inc., 127 East Front Street, Suite 216, Missoula, Montana 59802, USA. Received September 13, 2004; accepted April 6, 2005 Published online August 10, 2005 Rainbow trout Oncorhynchus mykiss often demonstrate good growth and catchability (Varley and Regenthal 1971), making them the most extensively stocked species for sportfishing in the western United States and the most frequently stocked species in Utah (Hepworth et al. 1999). However, even lakes within a geographical area can vary tremendously in both physical characteristics and lake productivity, thus resulting in large differences in fish growth and production among systems (Hanson and Leggett 1982; Carpenter and Kitchell 1993). When forage fish are limited, larger trout may be forced to fulfill their dietary needs with a diet composed primarily of zooplankton, a potentially energetically inefficient food source for a large, particulate feeder (Larkin et al. 1957; Beauchamp 1990; Hubert et al. 1994). Further, the general morphology (e.g., large mouth, small eyes) and particularly the widely spaced gill rakers of rainbow trout likely result in more efficient feeding on large-bodied prey in the nearshore littoral zone of lakes and reservoirs (Scott and Crossman 1973), as compared with pelagic zooplankton (Hyatt 1980). Thus, despite the popularity of rainbow trout, their growth and survival rates are highly variable (Tabor et al. 1996; Hepworth et al. 1999). Many studies suggest that gill raker morphology sets limits on the ability of a fish to retain zooplankton of certain sizes (Wright et al. 1983; Link and Hoff 1998; Brunger Lipsey and Stockwell 2001; but see Langleland and Nost 1995). Large zooplankters (e.g., Daphnia spp.) often make up a substantial portion of rainbow trout diets (Schneidervin and Hubert 1987), zooplankton greater than 1.3 mm in total length composing the largest proportion of the diets of young rainbow trout (Gailbraith 1967; Taylor and Gerking 1980). Although it is firmly established that larger-sized zooplankton appear more frequently in the diet of planktivorous rainbow trout, it remains unclear whether 1228

TECHNICAL NOTE 1229 their prey selection is determined primarily by (1) morphological limitations (e.g., gill raker spacing; this study), (2) optimal foraging behavior (e.g., predation risk or prey behavior; Werner and Hall 1974), (3) abiotic factors (e.g., light or turbidity; Vogel and Beauchamp 1999), or (4) some combination of these factors. Our understanding of the relationship between prey size and gill raker morphology may benefit from a combination of controlled laboratory experiments (where available prey composition and size can be effectively quantified and controlled) and field observations of actual prey selection (where other factors are also influential in determining prey choice). Our objectives were to (1) evaluate diet composition and prey selection in the field with regard to prey size and gill raker spacing; (2) determine experimentally if rainbow trout differentially select or retain large zooplankton, and, if so, whether this selectivity is based on a morphological limitation (e.g., gill raker structure); and (3) compare results from our controlled laboratory experiments with those from our natural field observations to obtain a more comprehensive understanding of the factors that affect prey choice of rainbow trout. Methods Study area. Flaming Gorge Reservoir is located in the northwest corner of Utah and southwest corner of Wyoming. When filled to capacity the reservoir is 145 km long, impounds 17,000 ha of water, has a maximum depth of 134 m, and a surface elevation of 1,841 m (Schneidervin and Hubert 1987). For management and research purposes, the reservoir has been divided into three areas with distinct differences in general productivity and morphology: the Inflow, Open Hills, and Canyon areas (Yule 1992; Haddix and Budy 2005). All sample collections described below occurred in each of these three study areas. Overall, the reservoir is considered mesotrophic; the most productive area is in the Inflow area and the least productive is at the southern end (Canyon), adjacent to the dam (Yule 1992). Haddix and Budy (2005) detail the differences in physical characteristics, productivity, and concordant effects on fish growth and survival across the three areas of the reservoir. Rainbow trout are thought to only rarely reproduce naturally; the Utah Division of Wildlife Resources (UDWR) and the Wyoming Game and Fish Department (WGFD) stock a combined, annual sum of 450,000 subadult and fingerling rainbow trout in Flaming Gorge Reservoir. Zooplankton collection and enumeration. For field observations of zooplankton abundance and size structure in Flaming Gorge Reservoir, zooplankton samples were collected in the three study areas in June, July, August, and November 2001 and in June, July, August, and October 2002. All zooplankton samples were collected with a 153- m mesh, 0.3-m-diameter zooplankton net; all zooplankton data were pooled for subsequent analyses and graphical description. In Flaming Gorge Reservoir, two zooplankton tows were taken on each date, in each of the three areas, with one epilimnetic tow (10 0 m) and one of the entire water column (bottom depths ranged from 20 to 100 m). Samples were collected in the daytime, usually in the afternoon (1200 1600 hours), and preserved in a 10% solution of formalin. All zooplankton samples were enumerated, identified to order for copepods and to genera for cladocerans; zooplankton dimensions (total length) were measured to the nearest 0.01 mm (from the top of the head to the base of the tail spine) using an ocular micrometer on the first 30 specimens of each taxonomic group per sample. The zooplankton community in Flaming Gorge Reservoir is dominated by large Daphnia pulex and Diaptomus, and lesser contributions of Ceriodaphnia and cyclopoid copepods (Teuscher and Luecke 1996; Budy et al. 2003). For laboratory feeding experiments, zooplankton obtained from Porcupine Reservoir were composed primarily of Daphnia pulex (there were only a few cyclopoid and calanoid copepod and Bosmina individuals). Diet evaluation. Rainbow trout were collected in Flaming Gorge Reservoir with variable-mesh (1.27 5.08-cm) horizontal, sinking gill nets set in the same three areas described above for zooplankton and during the same sample times (2001 2002; Haddix and Budy 2005); data were pooled across dates and sites for subsequent analyses and graphical description. Five gill nets were set in the evening and retrieved the following morning, each month, in each of the three areas of the reservoir. A minimal number ( 16% of the total collected fish) of rainbow trout were also collected from anglers at cleaning stations, also located at each of the three study areas. Angler-caught fish were collected from sunrise to 1000 hours, and most rainbow trout were caught along shorelines in the epilimnion; therefore, these fish mimic gill-net-sampled fish in terms of location and timing of collection. All collected fish were immediately placed on ice, and fish weights (g) and total lengths (TL, mm) were measured within approximately 4 h. Rainbow

1230 BUDY ET AL. trout stomachs (n 342) were removed and preserved in a 10% solution of formalin. Zooplankton found in the diets of rainbow trout were processed and measured as described above. Gill raker measurement. Gill raker measurements were made on a subset of collected fish (n 86 total), and samples were chosen to adequately span the size range of fish collected. The first gill arch of rainbow trout collected as described above was removed and preserved in a 10% solution of formalin. The right side of the first gill arch was removed and pinned on a flat surface in a position that mimics the natural orientation of attachment within the fish s buccal cavity. The space between each of five gill rakers was measured to the nearest 0.01 mm using an ocular micrometer; this included three inter gill raker spaces on the long side of the gill arch and two on the short side. Measurements were taken one-third up the gill raker from the base of the gill arch. The mean of the five individual measurements was calculated as an index of gill raker spacing for each individual fish. Based on size-at-age determinations, collected rainbow trout were composed of fish aged 1 5 years; detailed information regarding size and age for Flaming Gorge Reservoir rainbow trout are available in the study by Haddix and Budy (2005). Linear regression was used to evaluate relationships between inter gill raker spacing and fish length, and between the length of cladocerans in the diet of field fish and fish length. Laboratory experiments. Laboratory feeding trials were conducted between June and September 2002 in the laboratory at Utah State University (USU) for logistical reasons. Experimental fish were taken from the same hatchery group of rainbow trout stocked into the reservoir. Zooplankton for experiments were collected from a reservoir near USU (Porcupine Reservoir) on the same day that trials were run. Daytime, eplimnetic tows were repeated until sufficient zooplankton densities were collected; samples were returned to the laboratory immediately, and experiments were initiated that same day. Three to five rainbow trout were used for each of five trials (185-L glass aquaria each holding one rainbow trout per trial). Water temperature was recorded at the beginning and end of each trial to assure temperatures were similar among trials and throughout each individual trial. Fish were held in aquaria at 15 C, acclimated to a diet of zooplankton, and then starved for at least 48 h preceding experiments. A known volume of water containing zooplankton was added to each tank; starting zooplankton densities were kept high to allow selection ( 389 cladoceran zooplankters/ L on average as compared with highs of 10 cladoceran zooplankters/l in the reservoir). Prior to start, a subsample of known volume zooplankton was removed from each aquaria and preserved in 10% solution of formalin for later analysis according to identification and enumeration methods described above. Trials were run from 2 to 5 h, depending on when fish initiated feeding. Once rainbow trout were observed feeding on zooplankton, they were allowed to feed for approximately 2 h. At the end of the each trial, all fish were sacrificed and their stomachs were removed and preserved in a 10% solution of formalin. Stomachs were dissected, and all consumed zooplankton were identified, counted, and measured as described above. For each trial, mean cladoceran size in each rainbow trout stomach following a trial was compared with the mean size of cladocerans present in the aquaria at the beginning of each trial. Only cladocerans were used in analysis because they made up the majority ( 95%) of prey items found in the diets of rainbow trout used in laboratory experiments, and they are the main ( 98%) zooplankton prey of rainbow trout in Flaming Gorge Reservoir (Haddix and Budy 2005). The mean size of cladocerans available in the aquaria was compared with the size of cladocerans in the diet based on analysis of variance (ANOVA) with trial (or date) as a factor. All statistical analyses were conducted with SAS software (SAS Institute 2000); data were tested for assumptions of normality before analysis, and we assessed statistical significance at 0.05. Electivity indices. Electivity indices were used to calculate and compare the degree of size-selective predation rainbow trout exhibited when feeding on cladocerans in feeding experiments and in field observations. Cladoceran zooplankton were measured and counted as described above and then categorized into 0.1-mm size bins based on their total length. Ivlev s index of electivity (Ivlev 1961) was computed using the size of cladocerans as a surrogate for prey type, that is, E (r p )/(r p ), i i i i i where r i is the proportion of a certain size cladocerans in the diet, and p i is the proportion of a certain size in the environment. Results Field Observations Inter gill raker spacing on Flaming Gorge Reservoir rainbow trout averaged 1.09 mm, and there

TECHNICAL NOTE 1231 FIGURE 1. Relationship between rainbow trout total length and inter gill raker spacing (y 0.002x 0.29; r 2 0.296; df 85; P 0.001). was low variation among fish overall (SD 0.28). Inter gill raker spacing generally followed the expected pattern of increase with increasing rainbow trout total length (Figure 1), but with a moderately shallow slope (y 0.002x 0.29; r 2 0.296; df 85; P 0.001). Although small overall, variation in mean inter gill raker spacing across similarsized individuals generally increased with fish length. The size of zooplankton retained did not increase as a function of increasing rainbow trout length (y 0.001x 1.26; r 2 0.06; df 39; P 0.12; Figure 2). Rainbow trout consistently consumed the largest zooplankton of those available in Flaming Gorge Reservoir (Figure 3), and the majority (97%) of zooplankton consumed by rainbow trout were larger than the inter gill raker spacing of 1.09 mm. These larger zooplankton composed a much smaller proportion of the total available zooplankton FIGURE 2. Mean (symbols) and range (vertical bars) of cladoceran zooplankton lengths from diets of rainbow trout of differing lengths collected in Flaming Gorge Reservoir (y 0.001x 1.26; r 2 0.06; df 39; P 0.12).

1232 BUDY ET AL. FIGURE 3. Lengths of cladoceran zooplankton found in the environment (left) and in rainbow trout diets (right) from Flaming Gorge Reservoir. Samples of zooplankton in the environment were pooled from all zooplankton tows taken during the study. Samples of zooplankton in rainbow trout diets were pooled from rainbow trout collected throughout the study. Vertical bars represent inter gill raker spacing for rainbow trout (mean 1.3 mm; SD 0.28; n 86). existing in the reservoir (29%). Of the diets collected from anglers and gillnets, 1% of anglercaught fish and 17% of gill-net fish were empty; more detailed information on other, nonzooplankton diet items and total diet composition are available in Haddix and Budy (2005). Laboratory Experiments When comparing the size distribution of zooplankton consumed with the total zooplankton available in the aquaria, rainbow trout consumed the largest zooplankton available. The mean size FIGURE 4. Box plot diagram of cladoceran lengths in aquaria versus the lengths of cladocerans consumed by rainbow trout in feeding experiments (all trials pooled). The upper and lower boundaries represent the quartiles, the horizontal bar represents the median, and the dots represent the 5th and 95th percentiles. of cladocerans consumed was significantly larger than the mean size of cladocerans in the aquaria (treatment effect: F 21.96; df 1, 17; P 0.05; Figure 4); neither trial nor the interaction between trial and treatment were significant (trial effect: F 4.35; interaction: F 4.24; df 1, 17; P 0.05 for both). The rainbow trout used in the feeding experiments averaged 225.0 mm TL (SD 22.81; n 20) and thus were smaller than the average rainbow trout sampled in the reservoir (mean 384.4 mm TL; SD 70.48 mm; n 86). Therefore, we calculated the inter gill raker spacing for rainbow trout used in feeding experiments based on the allometric relationship in Figure 1 (see above). When we compared the predicted inter gill raker spacing (0.94 mm) of experimental rainbow trout with the size distribution of zooplankton in the stomachs of rainbow trout, 64% of zooplankton consumed by rainbow trout were larger in length than the intergill raker spacing. The selection for larger zooplankton in both laboratory experiments and field observations was also confirmed with Ivlev s index of electivity, which compares size categories of zooplankton in the environment with those found in the stomachs. Electivity indices ranged from 1.0 at the smallest sizes of zooplankton prey (0.03 mm) to 1.0 at the largest sizes, and there was a distinct switch towards positive selection, retention, or both between 1.0 and 1.1 mm for laboratory fish and fish

TECHNICAL NOTE 1233 collected in the reservoir. This switch from selection against smaller-sized zooplankton to selection for larger-sized zooplankton corresponds closely with the mean inter gill raker spacing for both groups of fish. Discussion While many studies make the assumption that gill raker spacing places a minimum threshold on the size of the prey that a predator can consume, most research has focused on obligate planktivores and not more generalist, particulate-feeding fish such as rainbow trout (MacNeill and Brandt 1990; Link and Hoff 1998; Brunger Lipsey and Stockwell 2001). Given the opportunistic feeding behavior of rainbow trout and the combination of suction and ram prey capture methods they employ (Rubenstein and Koehl 1977; Moyle and Cech 2004), gill raker spacing likely influences the minimum size of prey that can be efficiently retained. Results from this study indicate that rainbow trout are selecting for and retaining the largest zooplankton available in both Flaming Gorge Reservoir and in laboratory feeding experiments. Mean gill raker spacing is almost exactly equal to the minimum size of zooplankton consumed, which suggests a minimum morphological limitation on prey size. As may be expected based on allometry, we observed a positive relationship between adult ( 220-mm) rainbow trout total length and gill raker spacing (MacNeill and Brandt 1990; Etner and Skelton 2003). However, there was no relationship between rainbow trout length and the sizes of zooplankton they consumed. If gill raker spacing were the sole factor limiting the size of zooplankton consumed by rainbow trout, one would expect smaller fish (which would have smaller spacing between gill rakers) to have smaller zooplankton in their stomachs (MacNeill and Brandt 1990). Thus, these results suggest that the size of zooplankton consumed by rainbow trout is not determined solely by gill raker spacing. However, there were a few individual fish with more closely spaced intergill rakers, and we note that, in general, the size range of fish evaluated here is relatively narrow and does not include juveniles, which may show a different ontogenetic pattern (Mummert and Drenner 1986). In comparison with the field observations (3%), a larger proportion of the zooplankton consumed by rainbow trout in the aquaria experiments were smaller in size than the average rainbow trout gill raker spacing (38%). There are, however, several factors that may have influenced these laboratory results. Hunger level, or motivation to feed, can influence the behavior of food intake such that an animal may become more selective as it reaches satiation (e.g., Gill 2003). Our static analysis of gut contents does not allow us to address shifts in feeding behavior with increasing gut fullness; however, for adult ( 220-mm) rainbow trout feeding exclusively on plankton prey, and over a relatively short laboratory feeding period, these potential shifts are likely insignificant, if they occur. Nevertheless, laboratory fish were starved prior to the experiments and may have thus demonstrated less selection and therefore attempted to feed on smaller zooplankton more often as compared with fish in the field. Similarly, some animals have been shown to demonstrate a delay in prey size discrimination such that they must sample the prey available for some time period before they become size discriminate (Harper 1982; Croy and Hughes 1991). In our laboratory experiments, fish may not have had enough time to adequately sample the range of prey sizes and may have therefore been less size discriminate. Despite these relatively minor differences between field and laboratory results, both sets of observations suggest that prey size selection by rainbow trout is influenced by a combination of both behavioral selection for larger prey (e.g., Eggers 1977) and a greater probability of retention of larger prey, as determined morphologically by gill raker spacing. Although it has been shown in this study (as well as in others) that rainbow trout (in some situations) can consume prey smaller than their gill raker spacing (Wankowski 1979; Dervo et al. 1991; Langleland and Nost 1995), gill raker morphology and other morphological features (large mouth, small eyes) may still have a biologically significant influence on how efficiently rainbow trout consume prey of different sizes (Rubenstein and Koehl 1977; Scott and Crossman 1973; Hyatt 1980; Wright et al. 1983). Electivity indices for both field observations and laboratory experiments indicated strong selection for larger-sized zooplankton. If rainbow trout were in an environment with a prey base composed solely of zooplankton of sizes less than 1 mm, they may be forced to expend additional energy capturing zooplankton due to low capture and retention rates (Wright et al. 1983). Studies designed to address how feeding efficiency changes with prey size and gill raker structure would contribute to our understanding of the role gill rakers play in a particulate feeding fish such as the rainbow trout. Despite the influence of other ecological factors

1234 BUDY ET AL. in prey selection, gill raker features have proven useful for delineating between species and inferring ecological function (Etner and Skelton 2003). In this study, rainbow trout (generalist particulate feeders) have much wider inter gill raker spacing (86% wider) than kokanee salmon O. nerka, an obligate planktivore. Kokanee salmon typically do not demonstrate plasticity in their feeding behavior, preying largely on freshwater zooplankton and yet still exhibiting high growth rates (Teuscher and Luecke 1996; Clarke et al. 2004). Because of their long, narrowly spaced gill rakers (Haddix 2004), kokanee salmon are more efficient than rainbow trout at retaining smaller-sized zooplankton that are often more abundant than larger forms of zooplankton (Brunger Lipsey and Stockwell 2001; Clarke and Bennett 2002). This is the case in Flaming Gorge Reservoir, where kokanee salmon demonstrate high growth rates, good condition, and large sizes at maturity, while rainbow trout grow rather slowly and often become stunted at maturity (Teuscher and Luecke 1996; Haddix and Budy 2005). In summary, for planktivorous fishes, fish growth, biomass, or both can often be predicted based on the abundance and composition of zooplankton (Budy et al. 1995; Teuscher and Luecke 1996; Johnston et al. 1999). Based on these types of relationships, rainbow trout are often stocked into freshwater lakes and reservoirs with the assumption that they will utilize abundant zooplankton resources (Hepworth et al. 1999; Haddix and Budy 2005). The results from this study indicate that when fish are expected to be largely zooplanktivorous, estimating the availability of zooplankton prey of size-classes that are efficiently retained and are preferred may provide a more reliable estimate of fish growth and production capacity as compared with total zooplankton abundance (Mills and Schiavone 1982). Our results also reinforce the importance of understanding how effectively rainbow trout can utilize the type and sizes of different prey available in a given system. These considerations may improve our ability to predict the potential for growth and survival of rainbow trout within and among different systems, recognizing that realized growth and survival rates are likely also influenced by a combination of other environmental factors. Acknowledgments This research was funded by the Utah Division of Wildlife Resources, Project XIV, Sport Fisheries Research, Grant Number F-47-R, Amendment 17, and by the U.S. Geological Survey (UCFWRU). Matt Helm and Mike Hadley provided field and laboratory assistance, and Nick Bouwes, Jr., EcoLogical Research, provided statistical advice and oversight. We thank Gary P. Thiede for logistical oversight of field and laboratory operations and for extensive help with manuscript development. Chris Luecke, Theodore Evans, John Janssen, and three anonymous referees reviewed previous drafts of this manuscript. Reference to trade names does not imply endorsement by the U.S. Government. References Beauchamp, D. A. 1990. Seasonal and diel food habits of rainbow trout stocked as juveniles in Lake Washington. Transactions of the American Fisheries Society 119:475 482. Brunger Lipsey, T. S., and J. D. Stockwell. 2001. Gill raker morphology for three age-classes of kokanee. 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