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Diet of Adult Olympic Mudminnow, Novumbra hubbsiAuthor(s): Roger A. Tabor , Andi M. Kopit , Frithiof T. Waterstrat , Christina M. Meisterand Bradley E. ThompsonSource: Northwest Science, 88(1):33-43. 2014.Published By: Northwest Scientific AssociationDOI: http://dx.doi.org/10.3955/046.088.0106URL: http://www.bioone.org/doi/full/10.3955/046.088.0106

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33Northwest Science, Vol. 88, No. 1, 2014

Roger A. Tabor1, Andi M. Kopit2, Frithiof T. Waterstrat, Christina M. Meister3, and Bradley E. Thompson. U.S. Fish and Wildlife Service, Washington Fish and Wildlife Office, 510 Desmond Drive, Suite 102, Lacey, Washington 98503

Diet of Adult Olympic Mudminnow, Novumbra hubbsi

Abstract

Olympic mudminnow (Novumbra hubbsi) is a small species endemic to western Washington State that inhabits marshy, wetland-type areas with abundant aquatic macrophytes. Because they can be locally abundant and may be the only fish species present, they may have important effects on the aquatic community. However, little is known about their diet under natural conditions. To get a detailed account of their feeding ecology, Olympic mudminnow were sampled from six sites. Three sites were sampled monthly from February to August 2012 and the other three sites were sampled once in June 2012. Gastric lavage was used to collect stomach samples from fish ≥ 40 mm total length (TL). Stomach samples were collected from 477 fish (40-75 mm TL). Olympic mudminnow diet was comprised primarily of aquatic invertebrates. By weight, oligochaetes and chironomid larvae were the predominant prey types but by number, copepods were the predominant prey type. Of the three monthly sample sites, diet overlap between seasons was high at one site, low at another site, and variable at the third site. Diet overlap was generally low between sites. Olympic mudminnow 40-49 mm TL primarily displayed a generalist feeding strategy, whereas larger fish displayed more specialization. A consistent pattern of ontogenetic diet shifts was not apparent. Results of this study and an earlier study indicate Olympic mudminnow consume a wide variety of types and sizes of aquatic invertebrates and their diet can vary widely between sites and seasons.

Keywords: Olympic mudminnow, western Washington, diet overlap, feeding strategy

1 Author to whom correspondence should be addressed. E-mail: roger_tabor@fws.gov2Current address: Washington State Department of Agricul-ture, 1111 Washington Street SE, PO Box 42560, Olympia, Washington 985043Current address: U.S. Fish and Wildlife Service, 4401 N. Fairfax Drive, MS 330, Arlington, VA 22203

Introduction

Olympic mudminnow (Novumbra hubbsi) is a small fish that is endemic to western Washington State. Olympic mudminnow is generally found in marshy, wetland-type habitats with mud substrate bottom, low water velocities, and abundant aquatic vegetation (Meldrim 1968, Harris 1974, Mongillo and Hallock 1999). They are tolerant of a wide range of environmental conditions (Meldrim 1968). The low dissolved oxygen levels and high water temperatures tolerated by Olympic mudminnow can be detrimental to other native fishes of west-ern Washington. Olympic mudminnow is listed as “sensitive” by Washington State because of its restricted range and highly vulnerable habitat (Mongillo and Hallock 1999).

Where Olympic mudminnow occur, they are often locally abundant and can be the only fish

species present. Therefore, they may be an im-portant predator and may affect the community structure of wetlands and other aquatic ecosys-tems. However, limited information is available on their feeding ecology (Meldrim 1968) and little is known on how they may influence aquatic invertebrate populations and other aspects of the aquatic community. Preliminary work conducted by Meldrim (1968) found Olympic mudminnow consumed a variety of invertebrates; however, all sites and fish sizes were combined into one sample, expressed as percent by volume. More detailed baseline information on their diet is needed to better understand their functional role in wetlands and other aquatic habitats. To get a detailed account of Olympic mudminnow feed-ing ecology, we assessed their diet in relation to spatial, seasonal, and ontogenetic changes.

Methods

Sample Sites and Collections

We sampled fish from different habitat types, seasons, and size classes. Olympic mudminnow were sampled from six sites representative of the different habitat types where they are found

34 Tabor et al.

(Figure 1). Three of the sites (Green Cove wetland, Hopkins Ditch, and Woodard Creek pond) were sampled monthly from February to August 2012. The other three sites (Adna wetland, Conner Creek, and Steamboat Creek bog; hereafter referred to as supplemental sites) were sampled once in late June 2012. Adna wetland and Hopkins Ditch are part of the Chehalis River basin, while the other sites are within small independent basins. Following the habitat classification system of Cowardin et al. (1979), Hopkins Ditch and Conner Creek were classified as riverine-lower perennial-aquatic beds; Adna wetland, Green Cove wetland, and Woodard Creek pond were classified as palustrine-aquatic beds; and Steamboat Creek bog was classified as a palustrine-moss/lichen wetland. No other fish species were present at the Green Cove wetland, Steamboat Creek bog, and Woodard Creek pond. Other fish species at the other sites included threespine stickleback (Gasterosteus aculeatus),

riffle sculpin (Cottus gulosus), cutthroat trout (Oncorhynchus clarkii), juvenile coho salmon (O. kisutch), or juvenile sunfish (Lepomis spp.).

Dip nets were used to collect 20-24 fish for each sampling event and we attempted to collect at least seven fish from each of three size classes: 40-49 mm, 50-59 mm, and ≥ 60 mm TL (total length). Size classes represented three convenient classes that encompassed the size range of adult Olympic mudminnow (40-75 mm TL, Hagen et al. 1972, Mongillo and Hallock 1999). Generally, the water depth where the fish were collected was 0.1 to 1.0 m. Fish were collected in the morning or early afternoon.

Fish were anesthetized, measured, and weighed to prepare for diet analyses. MS-222 (80 mg/l, Summerfelt and Smith 1990) was used to anesthe-tize the fish and total length (mm) and weight (0.1 g) were measured after the fish lost equilibrium. Stomach contents were removed using gastric

Figure 1. Map of western Washington displaying the six study sites. Sites in the circle were used as monthly sample sites, Febru-ary to August 2012. The other three sites were supplemental sites and were only sampled once in June 2012.

35Diet of Olympic Mudminnow

lavage as described in Foster (1977) and flushed into a 177-micron sieve. The nozzle inserted into the fishes’ stomach was a 1.5-mm-diameter copper tube. Stomach samples were frozen for later laboratory analysis. Preliminary testing on 10 fish (sacrificed and stomachs examined in the laboratory) indicated gastric lavage was effective in removing all the stomach contents. Initial testing also indicated the nozzle size worked effectively for fish ≥ 40 mm TL but may be harmful to smaller fish. Additionally, 10 fish (range 42-60 mm TL) that had their stomach contents removed by gastric lavage were placed in a net pen to evaluate any delayed mortality. All fish from this group and a control group (only anesthetized, length measured, and weighed) were actively swimming and did not show any obvious negative signs of gastric lavage after 24 hours. Therefore, we assumed the gastric lavage treatment did not result in mortality for at least 24 hours post handling.

In the laboratory, stomach contents were sepa-rated into major prey taxa. Aquatic insects and most crustaceans were identified to family while terrestrial insects and some crustaceans were identified to order and other invertebrate prey items were identified to the class level. Each prey group was enumerated and weighed (blotted for 10 s on a paper towel and weighed to the nearest 0.0001 g).

Additionally, 12 Olympic mudminnows were collected at the Woodard Creek pond site (April 3, 2013) to document the size range of consumed prey. Stomach contents were removed by gastric lavage and samples were immediately processed at the lab. All intact prey items were measured with an ocular micrometer to the nearest 0.02 mm. Copepods were measured from the anterior of the cephalothorax to the anterior of the uro-some. Ostracods were measured along the long axis of the external shell. Chironomids were measured from the anterior of the head capsule to the terminal prolegs.

Data Analyses

Diet data were pooled by size class and seasons (based on large changes in water temperature). Seasons were winter (February-March; 5-7 °C),

spring (April-June; 10-12 °C), and summer (July-August; 14-19 °C). To quantify diet composition, we calculated percent composition by weight (%Wi), percent frequency of occurrence (%Oi), and percent composition by number (%Ni) as follows:

%Wi =

%Oi =

%Ni =

where n is the total number of prey categories found in a given sample, and Wi, Oi, and Ni are the total wet weight, occurrence, or number of prey type i in a category (Liao et al. 2001).

To compare the diet between sites, seasons, and size classes, we calculated diet-overlap index values using the equation of Horn (1966):

s s s

C = 2 ∑ Xi Yi / (∑ Xi2 + ∑ Yi

2)

i=1 i=1 i=1

where C is the index value, s is the number of food categories; Xi is the proportion of the total diet by site, season, or size class X contributed by food category i; and Yi is the proportion of the total diet by site, season, or size class Y contributed by food category i. Index values can range from 0 (no overlap) to 1 (complete overlap). An overlap index level of 0.6 or more is used to indicate a significant overlap in diet (Zaret and Rand 1971, Johnson 1981). To compare between monthly sites and supplemental sites, we pooled June and July data of the monthly sites because supplemental sites were only collected in late-June and monthly sites were collected in early-June and mid-July.

We used a graphical analysis tool (Amundsen et al. 1996, Garvey and Chipps 2012) to determine Olympic mudminnow feeding strategy and the inter- and intra-individual components of niche width. This tool plots frequency of occurrence with prey-specific abundance. Prey-specific abundance (%) was calculated as:

36 Tabor et al.

Pi = (∑Si/∑Sti) 100

where Pi is the prey-specific abundance of prey i, Si the stomach content (weight) comprised of prey i, and Sti the total stomach content in only those predators with prey i in their stomach (Amundsen et al. 1996). Plots were developed for each monthly sample site and size class. Data were pooled across months to display an overall feeding strategy.

Results

Diet Composition

Olympic mudminnow diet at all sample sites was comprised primarily of aquatic invertebrates (Figures 2 and 3). Terrestrial invertebrates were occasionally consumed but made up a minor portion of the diet. At most sites, larval aquatic dipterans were the most important prey category by weight. Of the aquatic dipterans, chironomid larvae (Chironomidae) was the most common prey type; they were present in 66% of all fish examined and made up 14% of the overall diet by weight for all size classes, sites, and seasons combined. Other aquatic dipterans consisted primarily of crane fly larvae (Tipulidae, %W = 4.9) and biting midge larvae and pupae (Cerato-pogonidae, %W = 3.0). Consumption of biting midges was observed at every site but was most apparent at the Steamboat Creek bog site where 247 biting midges were present from 18 stomach samples (%O = 88.9) and comprised 23% of the diet by weight. Consumption of mosquito larvae (Culicidae) was only detected at Woodard Creek pond in July and August. A total of nine larvae was observed from 42 stomach samples (%O = 14.3) during this time period. Mosquito larvae represented 1% of the diet by weight in July and 5% in August. Other aquatic insects (primarily Lepidoptera, Odonata, and Coleoptera) comprised only 3% of the prey items but made up 26.4% of the overall diet by weight.

For all sites combined, small crustaceans made up 68% of the prey items by number (copepods 50%, cladocerans 9%, and ostracods 9%) but only 5% of the diet by weight. Consumption of small crustaceans was substantially more pronounced at

the palustrine-aquatic bed sites (n = 3; mean %N = 68.9) than the riverine-aquatic bed sites (n = 2; mean %N = 19.7). Other crustaceans consisted primarily of amphipods, which made up 3.9% of the combined diet by weight and 8.5% of the diet at Hopkins Ditch site.

Diet-Overlap Indices

Most diet-overlap index values (C) between size classes and between seasons at the Green Cove wetland site were greater than 0.6 (Tables 1 and 2). In contrast, the diet at Hopkins Ditch varied between size classes and between seasons and only one diet-overlap index value was greater than 0.6. Hopkins Ditch diet-overlap index values of winter versus spring and winter versus summer were generally quite low (Table 1). This was be-cause Olympic mudminnow were only collected in the floodplain in the winter and their diet was comprised largely of oligochaetes. In subsequent sampling efforts, Olympic mudminnow were col-lected along the stream margin and oligochaetes made up a small percentage of the diet. Diet-overlap index values at Woodard Creek pond varied widely. For example, index values during the winter between size classes ranged from 0.03 to 0.07 but were 0.73 to 0.91 during the spring (Table 1). Overall, diet-overlap index values were generally not lower for the two most dissimilar size classes (40-49 versus ≥ 60 mm TL) than for more similar size class comparisons. Similarly, diet-overlap index values for winter versus sum-mer were usually not much lower than other comparisons, except for some Woodard Creek pond comparisons.

The level of diet overlap between all sample sites (n = 6; June-July comparisons) varied sub-stantially. The Adna wetland, Green Cove wetland, and Woodard Creek pond had some level of diet overlap with each other (Table 3). However, the other three sites had little overlap. At the Conner Creek site, most of the diet by weight was com-posed of aquatic lepidopteran larvae (Crambidae) (percent by weight= 82.6; percent occurrence = 40.9), which was not observed at any other site. Therefore, diet-overlap index values between this site and the other five sites were quite low (range, < 0.01-0.20).

37Diet of Olympic Mudminnow

Figure 2. Diet composition (percent by weight) of Olympic mudminnow at three sites in Thurston County, Washington, February to August 2012. Number of fish sampled is given above each bar. The “Other” category includes plant material, detritus, amphipods, water mites, and terrestrial invertebrates. ND = no data.

38 Tabor et al.

Figure 3. Diet composition (percent by weight) of Olympic mudminnow at three supplemental sites, June 2012. Number of fish sampled is given above each bar. The “Other” category includes plant material, detritus, amphipods, water mites, and terrestrial invertebrates.

TABLE 1. Diet-overlap index values (C) of Olympic mud-minnow between three size classes at three monthly sample sites for three seasons, February to August 2012. Bold values indicate comparisons that have a significant overlap (C > 0.6). Size classes: 1 = 40-49 mm TL; 2 = 50-59 mm TL; 3 = ≥ 60 mm TL.

Site ______Size comparison______ Season 1 vs. 2 1 vs. 3 2 vs. 3

Green Cove wetland Winter 0.69 ND ND Spring 0.72 0.51 0.68 Summer 0.71 0.67 0.90

Hopkins Ditch Winter 0.58 0.57 0.36 Spring 0.29 0.39 0.26 Summer 0.19 0.24 < 0.01

Woodard Creek pond Winter 0.13 0.07 0.03 Spring 0.87 0.73 0.91 Summer 0.96 0.58 0.29

TABLE 2. Diet-overlap index values (C) of Olympic mud-minnow between three seasons at three monthly sample sites for three size classes (mm TL), February to August 2012. Bold values indicate comparisons that have a significant overlap (C > 0.6). Seasons: Win = winter; Spr = spring; Sum = summer.

_______Season comparison_______Site Win vs. Win vs. Spr vs. Size class (mm) Spr Sum Sum

Green Cove wetland 40-49 0.68 0.65 0.75 50-59 0.70 0.61 0.80 ≥ 60 ND ND 0.82

Hopkins Ditch 40-49 0.10 0.11 0.49 50-59 0.72 0.04 0.03 ≥ 60 0.14 < 0.01 < 0.01

Woodard Creek pond 40-49 0.66 0.24 0.77 50-59 0.38 0.22 0.88 ≥ 60 0.06 0.08 0.31

39Diet of Olympic Mudminnow

Feeding Strategy

Feeding-strategy plots of the monthly sample sites indicated Olympic mudminnow 40-49 mm were primarily generalists (prey-specific abun-dance was < 50% for almost all prey items) (Figure 4). Olympic mudminnow 50-59 mm and ≥ 60 mm TL displayed more specialization as some prey types had a high between-phenotype component; specifically, different individuals specialized on different prey types (low frequency of occurrence and high prey-specific abundance). Similar to the small size class, there were few dominant prey types.

Prey Size

Length measurements of prey consumed by 12 Olympic mudminnow from Woodard Creek pond indicated they consumed a wide size-range (n = 161; median = 1.31 mm; range = 0.56-13.0 mm; Figure 5). Most prey were small copepods (range = 0.56-1.68 mm), but a wide size-range of chirono-mids (range = 2.24-13.0 mm) were also consumed. Prey size-range of small Olympic mudminnow (n = 3; TL = 41-45 mm; prey size-range = 0.56-11.5 mm) was similar to large fish (n = 9; TL = 53-71

mm; prey size-range = 0.56-13.0 mm); however, the ratio of copepods to chironomids was higher for the small Olympic mudminnow (ratio = 4.75) than for the large fish (ratio = 1.17).

For all stomach samples examined, the largest prey observed was aquatic lepidopteran larvae (n = 17; range = 11-17 mm in length) from Conner Creek. Lepidopteran larvae were observed in fish as small as 45 mm TL and represented up to 9.6% of the body weight for an individual fish.

Discussion

Olympic mudminnow consumed a wide variety of prey types and sizes and their diet often varied between sites and from season to season, likely in response to differences in prey availability. For example, cladocerans were not consumed to any degree until the summer, which likely corresponds to the period of time when they are abundant (Whiteside et al. 1978). Our results are markedly different to those of Meldrim (1968) that found ostracods, isopods, mysids, and megalopterans made up over half of the diet by volume; whereas in our study, these prey items were either absent or made up a minor part of

TABLE 3. Diet-overlap index values (C) of Olympic mudminnow between six sample sites for three size classes, June-July 2012. Bold values indicate comparisons that have a significant overlap (C > 0.6).

__________Size class (mmTL)_________Site comparison 40-49 50-59 ≥ 60

Adna wetland vs. Conner Creek 0.01 0.01 0.03

Adna wetland vs. Green Cove wetland 0.54 0.72 0.84

Adna wetland vs. Hopkins Ditch 0.14 0.22 0.03

Adna wetland vs. Steamboat Creek bog 0.35 0.21 0.39

Adna wetland vs. Woodard Creek pond 0.55 0.75 0.78

Conner Creek vs. Green Cove wetland 0.01 0.05 0.07

Conner Creek vs. Hopkins Ditch 0.01 0.07 0.20

Conner Creek vs. Steamboat Creek bog <0.01 0.01 0.19

Conner Creek vs. Woodard Creek pond <0.01 0.02 0.13

Green Cove wetland vs. Hopkins Ditch 0.29 0.51 0.03

Green Cove wetland vs. Steamboat Creek bog 0.40 0.22 0.07

Green Cove wetland vs. Woodard Creek pond 0.90 0.81 0.84

Hopkins Ditch vs. Steamboat Creek bog 0.14 0.07 0.01

Hopkins Ditch vs. Woodard Creek pond 0.20 0.20 0.47

Steamboat Creek bog vs. Woodard Creek pond 0.44 0.15 0.05

40 Tabor et al.

Figure 4. Feeding-strategy plots (Amundsen et al. 1996, Garvey and Chipps 2012) for three classes of Olympic mudminnow at three sites (February to August 2012). An explanatory diagram for interpretation is shown at the top. Data points in the upper left and lower right quadrants indicate different feeding strategies and those in the lower left and upper right indicate prey dominance in the diet.

41Diet of Olympic Mudminnow

the diet. Prey availability in sites sampled by Meldrim (1968) was likely quite different than in our sample sites. Concurrent diet and prey availability sampling has not been done but we hypothesize that changes in their diet will reflect changes in prey availability.

The feeding strategy of Olympic mudmin-now appeared to vary between size classes and sites. Small Olympic mudminnow primarily had a generalist feeding strategy, whereas larger fish displayed more specialization. Also, the diet at Hopkins Ditch displayed more specialization than at the other two monthly sites. Increased specialization was usually due to the consumption of a few large prey items, such as oligochaetes or odonates, by a small portion of Olympic mudmin-nows. These large prey items may be relatively rare in the environment and the feeding strategy of Olympic mudminnow may be better described as opportunistic. For gape-limited predators, such as Olympic mudminnow, larger fish generally select larger prey (Zaret 1980). Therefore, some degree of specialization on large prey as fish size increases should be expected.

Overall, Olympic mudminnow did not show consistent ontogenetic diet shifts. For example, fish 40-49 mm TL often consumed relatively large prey. Also, small crustaceans (< 1 mm) were often present in the largest fish. At some sites, the importance of small crustaceans in the diet did appear to be reduced in large fish. Thus, mean prey size may increase with fish size. However, consistent ontogenetic diet shifts may be difficult to detect given the large variability between sites and among seasons. Measuring the size of all prey items from a large number of samples from each site may be necessary to accurately determine ontogenetic diet shifts. Additionally, we only sampled Olympic mudminnow ≥ 40 mm TL and major ontogenetic diet shifts may primarily occur between juvenile (< 40 mm TL) and adult fish (≥ 40 mm TL).

The diet of Olympic mudminnow included aquatic insects of two families (mosquitoes, Cu-licidae and biting midges, Ceratopogonidae) that contain many species that are human pests as adults. We did not identify these ingested insects to species and therefore, we are not certain that

Figure 5. Size frequency of prey consumed by Olympic mudminnow (n = 12; range, 41-71 mm TL) at Woodard Creek pond, April 3, 2013. All intact prey items were measured. The number of prey items measured is given in the legend.

42 Tabor et al.

any nuisance species were actually consumed. However, our results do provide a preliminary indication that Olympic mudminnow could have a beneficial role in wetlands and other aquatic habitats by consuming nuisance aquatic insects. Olympic mudminnow do have some charac-teristics (small fish, inhabit shallow waters, lo-cally abundant, associated with dense aquatic macrophytes, tolerant of a wide range of envi-ronmental conditions) of other fish that are used to control nuisance insects (Ahmed et al. 1988, Walton 2007, Pyke 2008). Olympic mudminnow may impact nuisance insect populations directly through predation or indirectly by affecting adult insect choice in oviposition sites (Petranka and Fakhoury 1991, Walton et al. 2009), especially in locations where they are the only fish species present. Alternatively, Olympic mudminnow may indirectly enhance nuisance insect populations by having some type of negative effect on predatory invertebrate populations (Batzer et al. 2000). Ad-ditional studies on Olympic mudminnow popula-tion sizes and consumption rates and their role in structuring aquatic communities are needed to better understand their potential as a biological control agent.

Observations of Olympic mudminnow in aquaria suggest they respond strongly to prey movements (Meldrim 1968, McPhail 1969, Baugh 1980). Typically, Olympic mudminnow remain motionless and then stalk and finally strike at passing prey (Meldrim 1968). By number the most common prey item consumed at our monthly sample sites were copepods, which are described as active, proficient swimmers (Williamson 1991). The swimming patterns of copepods may partly explain why large numbers are often consumed. It is difficult to characterize the movements of the other prey types but prey movements are likely an important part of Olympic mudminnow prey detection. In contrast, Olympic mudminnow are attracted to baited minnow traps and did consume

some sessile invertebrates such as fingernail clams (Sphaeriidae), suggesting they are able to locate prey by other mechanisms beside prey movement. However, the final strike at prey may be largely dependent on prey movement (Meldrim 1968, McPhail 1969).

Our results appear to be similar to information on the diets of other mudminnows including the eastern mudminnow (Umbra pygmaea) (Panek and Weis 2013); the central mudminnow (Um-bra limi) (Peckham and Dineen 1957, Keast 1978, Martin-Bergmann and Gee 1985); the Euro-pean mudminnow (Umbra krameri) (Wanzenbock 1995); and the Alaska blackfish (Dallia pectoralis)(Ostdiek and Nardone 1959, Gudkov 1998). All species appear to be carnivorous and are often described as generalist predators. They consume a wide variety of aquatic invertebrates but also occasionally terrestrial invertebrates and fish. Typi-cally, the diet is variable between sites, seasons, and fish sizes. The primary prey items commonly include chironomids and small crustaceans.

Acknowledgements

Funding for this study was provided by the U.S. Fish and Wildlife Service (USFWS), Washington Fish and Wildlife Office. We would like to thank Molly Hallock (Washington Department of Fish and Wildlife), Kira Mazzi, Dan Spencer, and Zachary Moore (USFWS) for their assistance with the field sampling. We would also like to thank John Trobaugh (Washington Department of Natural Resources) and Capital Land Trust for their assistance with access to some sample sites. Suggestions by Shannon Brewer (U.S. Geologi-cal Survey) and Denise Hawkins (USFWS) and three anonymous reviewers greatly improved earlier versions of the manuscript. The findings and conclusions in this report are those of the authors and do not necessarily represent the views of the USFWS.

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Received 07 June 2013

Accepted for publication 07 November 2013