J. Great Lakes Res. 27(3):300–311 Internat. Assoc. Great Lakes Res., 2001

Diets and Diet Overlap of Nonindigenous Gobies and Small Benthic Native Fishes Co-inhabiting the St. Clair River, Michigan

John R. P. French III1 and David J. Jude2,* 1U.S. Geological Survey Biological Resources Division Great Lakes Science Center 1451 Green Road Ann Arbor, Michigan 48105 2Center for Great Lakes and Aquatic Sciences University of Michigan 501 East University Ave. Ann Arbor, Michigan 48109-1090

ABSTRACT. Round gobies (Neogobius melanostomus), after successfully reproducing in the early 1990s, decimated populations of mottled sculpins (Cottus bairdi) and possibly logperch (Percina caprodes) in the St. Clair River. Studies were conducted during 1994 to determine whether diets of round and tubenose (Proterorhinus marmoratus) gobies overlapped with those of native forage fishes. In the nearshore zone (depth ≤ 1 m), round and tubenose gobies, logperch, and rainbow darters (Etheostoma caeruleum) of similar sizes (total lengths < 75 mm) consumed mainly small-sized macroinvertebrates (dipterans, Caenis, and amphipods) during June 1994. Logperch and rainbow darters were present in the nearshore zone only during this month. At the crest of the channel slope (depth = 3 m), round gobies and northern madtoms (Noturus stigmosus) ate mostly ephemeropteran nymphs (Hexagenia and Baetisca), while predation on zebra mussels (Dreissena polymorpha) and other mollusks by round gobies was mini- mal. Northern madtoms did not feed on mollusks. Diet overlap between round gobies and native fishes was not observed at the channel slope (depth = 5 m and 7 m) due to heavy predation on mollusks by round gobies. Young-of-the-year (YOY) round gobies migrated to deeper water in autumn and became prey of mottled sculpins and northern madtoms. Eggs and YOY of mottled sculpins may have become vul- nerable to predation by both round gobies and native fishes in deeper water, since adult mottled sculpins were apparently confined to the channel with limited home range because aggressive round gobies occu- pied preferred shallow habitat, including spawning sites. INDEX WORDS: Round goby, tubenose goby, diet, diet overlap, predation, St. Clair River, northern madtom, mottled sculpin, Hexagenia, logperch.

INTRODUCTION eggs and larvae by rainbow smelt (Osmerus Populations of native forage fishes (TL < 30 cm) mordax) and yellow perch (Perca flavescens) larvae in the Great Lakes have declined during the 20th by alewives (Alosa pseudoharengus) has been doc- century after nonindigenous fishes of similar size umented in the Great Lakes (Crowder 1980, Brandt were accidentally introduced into their habitats et al. 1987); alewife predation on larval stages in (Crowder 1980, Mills et al. 1994, Bronte et al. winter may have decimated the population of deep- 1998). Predation on eggs and larvae and competi- water sculpins (Myoxocephalus thompsoni) in Lake tion for food by nonindigenous fishes are factors Michigan. Competition with alewives for zooplank- advanced to explain declines in some Great Lakes ton may have reduced recruitment of coregonines forage fish populations. Predation on coregonine (Wells and McLain 1973). Bloaters (Coregonus hoyi), however, can avoid this competition with *Corresponding author: E-mail: [email protected] alewives by feeding on large zooplankters or ben-

300 Diets of Gobies and Native Species in the St. Clair River 301

thic prey that are inaccessible to alewives (Crowder gobies ontogenetically shift their diet mainly to and Binkowski 1983). mollusks. Jude et al. (1995) observed that the im- Another recently introduced, nonindigenous fish portance of mollusks in diets of round goby col- species, the ruffe (Gymnocephalus cernuus), was lected from the St. Clair River in August 1993 first suspected to cause some declines in native for- increased directly with round goby length; dipter- age fishes in Duluth Harbor, Lake Superior through ans were common in small round gobies (TL = egg predation and competition for food (Ogle 1995, 47–59 mm). Data from this study will be useful to Selgeby 1998). Preference for soft-bodied benthos document potential impact on native species not was similar in both ruffe and yellow perch in exper- previously studied in this river. These findings then imental aquaria (Fullerton et al. 1998). However, can be used to make predictions for other ecosys- early available evidence suggests the ruffe so far tems across the Great Lakes. Therefore, the objec- has had little impact. Bronte et al. (1998) suggested tives of this research were to 1) document the diet that declines in some native forage fishes in west- of round and tubenose gobies and small, benthic, ern Lake Superior might be due to natural popula- co-occurring forage fishes collected along a tran- tion dynamics rather than interactions with ruffe. sect at several depths in the St. Clair River in 1994, Ruffe and Eurasian perch (Perca fluviatilis) co- and 2) determine whether diets of round and exist in some lakes in Europe (Winfield et al. tubenose gobies overlapped with those of selected 1998), and Adams and Maitland (1998) found no native fishes. significant diet overlap between ruffe and native fishes in Loch Lomond in Scotland. STUDY SITE AND METHODS The round (Neogobius melanostomus) and tubenose (Proterorhinus marmoratus) gobies are Round and tubenose gobies and native forage freshwater Eurasian gobiids that were first found in fishes were sampled along a transect perpendicular North America during 1990 in the St. Clair River, to the shoreline in the St. Clair River adjacent to the interconnecting channel between Lakes Huron Algonac State Park, Algonac, Michigan at depths of and St. Clair (Jude et al. 1992). These nonindige- 1 m, 3 m, 5 m, and 7 m during the day and night nous species are benthic with fused pelvic fins, from May to December 1994. This part of the 63- which can be used as a suctorial disk for anchoring km-long strait, which connects Lake Huron with in fast-flowing water. Populations of mottled Lake St. Clair, is near the outflow to Lake St. Clair, sculpin (Cottus bairdi), and possibly other small features considerable freighter traffic, a maximum benthic fishes such as logperch (Percina caprodes), depth in that area of 11 m, velocities up to 1.8 m/s may have decreased in the St. Clair River after (Derecki 1984), and a mean annual discharge rate round gobies successfully reproduced (Jude et al. of 5,182 m3/s (USACOE 1990). Fishes were sam- 1995). A similar mottled sculpin decline caused by pled from the riprapped shoreline by kicking the round gobies was also documented in southern substrate and collecting all disturbed fishes in a Lake Michigan by Janssen and Jude (2001). These small 4.7-m, specially designed seine (Jude and authors suggested that round gobies competed with DeBoe 1996). Here a larger seine (15.2 m × 1.2 m) mottled sculpin and other native species for food was used to catch fish in a shoal area (< 1 m deep) resources at small sizes, for space at intermediate 4 to 8 m from the shore (Jude et al. 1995). Both sizes, and for spawning habitat at large sizes. shoreline sites were directly adjacent to trawling Therefore, competition for food may be a mecha- sites. Tows with a bottom, semi-balloon otter trawl nism contributing to the decimation of mottled with a 4.9-m headrope and 5.8-m wide footrope sculpin and logperch in the St. Clair River (Jude et (Jude et al. 1995) were made at 3-, 5-, and 7-m al. 1995, Jude and DeBoe 1996). depths for approximately 10 min. The 3-m-deep site Round gobies eat a wide variety of benthic inver- was located near the crest of the shipping channel, tebrates and small fishes (Charlebois et al. 1997). and the deeper sites were on the sloping side (Hud- Molariform teeth enable the round goby to crush son et al. 1986). Fishes were removed from the shells, and adults feed primarily on bivalves catch and injected with and stored in ethanol for fu- (French 1993, Ghedotti et al. 1995, Ray and ture stomach content analyses. Gear bias is an obvi- Corkum 1997). Resource partitioning (Werner et al. ous concern, both in comparing seine with trawl 1977, Laughlin and Werner 1980, French 1988) catches and comparing day with night catches. may allow round gobies and some native forage However, specimens were used only for determina- fishes to co-exist in the St. Clair River after round tion of diets, so differential catch rates should not 302 French and Jude

be a problem. Mass movements of fishes from one large, with SD ranging from 25% to 46%. Diets of depth during the day to another at night would also small round gobies changed throughout the sam- bias diet comparisons. Undoubtedly, some migra- pling season, and were mainly composed of dipter- tions occurred, but these appeared to be minimal; ans and mollusks (36% and 32% of mean total fish such as round gobies at 1 m, 3 m, and 5 to 7 m stomach volume, respectively; Table 1). In late depth were eating substantially different food items June, dipterans consumed by small round gobies at each depth, which would not transpire if substan- were so numerous (40 ± 24 SD; range = 0 to 101) tial migrations were occurring. that they dominated diets. Consumption of mol- In the laboratory, fishes were identified to species, lusks by small round gobies peaked in September. measured to the nearest mm total length, and Zebra mussels and native mollusks (Amnicola, weighed to the nearest 0.1 g. Round gobies, northern Physa, and Gyraulus) composed 51% of large madtoms (Noturus stigmosus), logperch, and mot- round goby diets on average over the year. In late tled sculpins were divided into two size classes (35 June, the diet was primarily dipterans (Chironomi- to 74 mm and ≥ 75 mm). Tubenose gobies and rain- dae) and ephemeropterans (chiefly Hexagenia and bow darters (Etheostoma caeruleum) only occurred Caenis); mollusks (fingernail clams and Amnicola) in small size classes. Up to 20 fish per species, and zebra mussels composed 22% and 12% of the depth, date, and diel period were removed from col- diet, respectively. Round gobies from both size lections for diet analyses; only those with measur- groups ate two young-of-the-year (YOY) round go- able stomach contents were included in analyses. bies and one unidentified fish (unmeasurable due to Total volume of all prey items in each individual heavy digestion). In addition, one large (139 mm) fish was measured to the nearest 0.1 mL using a round goby ate a 41-mm trout-perch (Percopsis graduated burette to obtain a volumetric estimate omiscomaycus), the only evidence of predation on (Jude et al. 1995). Stomach content items were iden- an adult native fish. tified to genus when possible, and counted. For each Dipterans, small-bodied insects such as Caenis major group (ephemeropterans, crustaceans, etc.), and trichopterans (Cheumatopsyche), and crus- percent contribution by volume to the total mea- taceans (isopods and the amphipod Gammarus) sured volume was visually estimated. Percentages were common food of tubenose gobies (Table 1) for each group were then used to calculate a volume from May through late June and in December. Like estimate (in mL) for each major prey group. Diets small round gobies, tubenose gobies, rainbow were then summarized by averaging the volume darters, and small logperch consumed numerous consumed of each major prey group and reporting dipterans. Significant diet overlap was observed be- this number as the percent contribution to the total tween 1) tubenose gobies and rainbow darters stomach content volume for each date and depth. (Schoener index = 0.74) on 2 June, 2) small round Mean volume of total contents consumed was also gobies and rainbow darters (Schoener index = 0.83) reported. The Schoener (1970) method was used to on 27 June, and 3) small round gobies and small analyze diet overlaps between round and tubenose logperch (Schoener index = 0.83) on 27 June. Three gobies and native fishes. Some bias may be intro- of nine tubenose gobies consumed round goby eggs duced in the diet determinations from retention of during the day on 27 June. Diet overlap between organisms with hard structures such as zebra mus- large round gobies and logperch was moderate in sels (Dreissena polymorpha); however, Ghedotti et late June (Schoener index = 0.54) when mollusks al. (1995) indicated that round gobies sometimes in- (gastropods), ephemeropterans, and trichopterans gested zebra mussels, removed the flesh, then were common in both diets. Tubenose gobies and ejested the shells through their mouths. rainbow darters showed significant diet overlap for amphipods (Schoener index = 0.79) in December. RESULTS Diets in the Nearshore Zone Diets at the Crest of the Slope There were 304 round gobies analyzed from the All 90 northern madtoms, 53 of 57 logperch, and nearshore zone (≤ 1 m) throughout the collection 178 of 339 round gobies were captured from the season (Table 1); 17 had empty stomachs. Individ- crest of the channel slope (3 m) at night (Table 2). ual variations in the mean volume consumed of At this site, empty stomachs were found in 20 major prey items in diets of both small and large northern madtoms, 25 logperch, and 25 round gob- round gobies over time in the nearshore zone were ies captured at night and 21 round gobies and 1 log- Diets of Gobies and Native Species in the St. Clair River 303 Food item Mollusks Ephemeropterans Trichopterans Dipterans Crustaceans Fishes Miscellania Dreissena polymorpha 1 m) during 1994. No. Stom. (E) = total number of stomachs examined (number of stomachs examined that ≤ Lg. round goby19(1)Tubenose goby21(0) 0/19 1/20 0.37Lg. round goby24(0) 0.04Tubenose goby17(0) 33(37) 4/20Sm. logperch 5/12 —Lg. logperch 0.36 14(22)Rainbow darter 0.02 5(3) 24(1) 50(39) 7(1) 0/5 9(21) 9/15 — —Lg. round goby40(4) 0/7 12(25) 0.09 0.07Tubenose goby14(1) 20/20 0.12Sm. logperch 0.26 9/5 27(27)Lg. logperch 15(25) — 14(0) — — 9(20)Rainbow darter 0.02 12(23) — 14(0) 12/2 5(0) 9(15) 12/2 0.05 22(21) 6(13)Lg. round goby14(0) 0/5 13(21) — — — 2(7) 0.05Lg. logperch 11/3 8(17) — 0.13Rainbow darter 14(27) — 20(23) 0.79 1(0) 1(0) 63(40) — — 50(71) 8(18) — 5(20) — 2(8)Lg. round goby13(3) 1/0 49(42) 1/0 2(4) 18(28)Tubenose goby2(0) — 8(14) 48(33) — 6/7 0.05 0.30 50(71) 22(23) t(t) 1(3) 20(45) 2/0 39(38) 16(31) — 0.11 26(36) 37(29)Lg. round goby5(0) 78(39) 0.07 — —Tubenose goby2(0) — 8(19) 7(27) 64(36) 12(27) — 26(34) t(1) 0/5 — 4(11) 1(2) 0/2 20(40) — — 14(23) 0.16 31(33)Lg. round goby3(0) 5(22) — — 0.01Tubenose goby18(0) — 4(13) 2(4) — 29(40) 14(23) — 0/3 —Sm. logperch 23(32) 15/3 t(1) 20(27) — — 0.30 89(29) 32(34) t(2) 0.14 1(0) 24(37) 26(43) — t(1) 7(10) — — 5 93(11) 27(40) 1/0 13(30) 40(55) t(1) 7(13) — 1(2) 4(11) — — — 0.01 — 2(5) — 3(6) — 3(12) — 9(11) 99 95 — — 15(21) — 1(5) 1(2) 60(28) — 5(12) — — t(1) — — — — 3(4) 10(10) — — 17(25) — 14(23) 1 33(46) — 25(7) t — 20(45) 50(71) — 60(14) — — — — — — 19(30) — — — — 76(33) — 70 — — 5(19) 30 — — Rainbow darter 24(0) 20/4 0.11 — t(2) 1(5) 7(14) 32(32) 60(32) — — Fish No. Mean TABLE TABLE 1.nearshore Mean zone (depth percent composition of food items by volume (SD) in stomachs of fishes collected from the St. Clair River in fish stomachs mL, t = trace, sm. small, lg. large. Mollusks include only native species. Date9 MaySm. round goby22(0) CategoryStom.(E) D/N 2/20 Volume 0.082 Jun 5(20) Sm. round goby40(5) 20/20 12(17) 0.07 10(26) 10(21)27 Jun Sm. round goby40(1) 25(38) 20/20 28(32) 0.09 11(26) 37(38) 9(14)15 Aug 11(26) round goby23(0) Sm. 8(22) 6(10) 17/6 27(38) — 0.05 11(15) 12(29)14 Sep 16(20) round goby20(0) Sm. — 20/0 — 20(15) 4(11) 0.0727 Oct 5(20) round goby20(1) Sm. 67(23) 22(25) 8(22) 0/20 0.065 Dec 38(25) 4(6) round goby21(2) Sm. 18(32) 9(12) 1/20 — 7(22) 19(23) 0.04 43(29) t(1) 15(29) 3(7) 6(11) 1(2) 7(15) — 16(15) 15(27) t(t) t(t) 3(12) 21(23) 10(23) 21(32) 4(14) 8(15) — 50(37) t(1) 17(33) 3(11) — were empty), D/N = number of fish captured during day/number of fish captured during night, = Mean mean Volume volume of food 304 French and Jude h Food item Mollusks Ephemeropterans Trichopterans Dipterans Crustaceans Fishes Miscellania Dreissena polymorpha Lg. round goby34(3)Sm. n. madtom 14/20 11(4)Lg. n. madtom 0.21Sm. logperch 20(10) 0/11 0/20 43(45) 0.02 8(2) 0.37Lg. round goby3(1) 0/8Lg. n. madtom 9(16) 3/0 — — 8(4) 0.03 0.10Lg. round goby10(3) 25(32) 0/8Lg. n. madtom — — 0/10Sm. logperch 0.15 — — 5(0) 0.39 10(19) 3(2) 0/5 —Lg. round goby39(10) 29(36) 21(39) — 10(11) 19/20 0/3 72(27) 11(20)Lg. n. madtom 0.40Lg. logperch 0.15 7(1) 0.03 — 48(55) 1(5) — — 17(37) 3(1) 27(27) 0/7 — —Lg. round goby40(2) —Sm. n. madtom 0/3 0.08 20/20 — 12(24) 10(11) 69(45)Lg. n. madtom 1(2) 59(48) 4(0) 0.13 79(39) — 0.10Sm. logperch 20(0) —Lg. logperch 32(33) 0/4 51(42) — — — 8(26) 0/20 3(3) — — 1(3) — 0.06 0.27 3(5) 5(1) 91(10) — 0/3Lg. round goby20(6) 14(23) 11(26) — 91(22)Lg. n. madtom 0/5 20/0 0.00 — — — —Sm. logperch — 15(1) 17(35) — — 0.16Lg. logperch 0.17 — 52(44) 6(9) 9(22) 5(12) 0/15 9(6) 21(43) 40(38) — — — 18(7) 0.25 — — — — — 0/9 — —Lg. round goby26(3) 5(19) 100 19(30) 2/16 — 3(4) — 0.01 19(40) 6/20 0.06 — t(1) — — 0.17 82(25) — — 8(18) 5(17) — 95(5) — 37(36) — — — — 100 — — 3(16) — 40(39) 15(24) — — 25(50) — 2(3) — — — t(t) 90(16) 5(16) — 2(2) 49(40) 4(10) — 67(47) 1(3) — 2(3) — — 15(30) 9(30) 1(3) 3(6) — — 8(15) — — t(1) — 47(41) — 76(41) — 2(3) — 33(58) 1(4) 8(17) 23(39) — 8(27) — — — — — — — 3(17) 6(21) — — 7(23) — — 67(58) — t(1) — — — — — Lg. logperch 8(4) 2/6 0.07 — — 9(17) 66(47) 25(50) — — — Fish No. Mean TABLE 2.TABLE Mean percent crest composition of food (depth items by volume = (SD) in stomachs 3 of fishes collected m) from the St. during Clair River channel 1994. No. Stom. = mean volume of food in fis empty), D/N = number of fish captured during day/number night, Mean Volume (E) = total number of stomachs examined (number of stomachs in mL, t = trace, sm. small, lg. large, n northern. Mollusks include only native species. stomachs examined that were Date9 MaySm. round goby38(1) CategoryStom.(E) 20/18 D/N Volume 0.10 3(12)2 Jun 10(18) Sm. round goby12(2) 12/0 23(33)27 Jun 0.14 Sm. round goby10(5) 0/10 22(38) — 0.0715 Aug round goby30(5) Sm. 39(38) 5(8) 4(6) 10/20 1(2) 0.10 11(17)14 Sep 64(48) 2(12) 3(16) round goby29(2) Sm. 57(46) 9/20 — 11(23) 32(39) 0.07 46(40) 8(9) 4(19) —27 Oct round goby20(1) Sm. 13(22) 19(20) 11(24) 20/0 — 0.07 56(41) 17(26) —5 Dec — 11(24) — round goby28(2) Sm. — 8(16) 1(5) 8/20 — 0.05 6(16) — 18(25) t(t) 17(31) — 1(2) — 44(45) — t(1) 29(40) 7(25) 4(11) — 50(43) — 42(42) 1(2) — — Diets of Gobies and Native Species in the St. Clair River 305

perch captured during the day. Ephemeropterans 12% and 13% of the diets, respectively, at these (Hexagenia and Baetisca) dominated diets of small depths. Predation on zebra mussels by round gobies round gobies from June to September; trichopterans at 7 m was more consistent than at all other depths, (Brachycentrus) were eaten in fall. An unidentified except for in December (range of SD = 5 to 37%). 23-mm-long fish larvae was found in the stomach Small round gobies ate a wide variety of prey at of a 74-mm round goby captured in May; two fish both depths. At 5 m, the diet on average was com- eggs were found in one small round goby in Au- posed of 20% zebra mussels, 13% native mollusks, gust. Predation on zebra mussels and native mol- 21% trichopterans (Brachycentrus and Cheuma- lusks by large round gobies decreased considerably topsyche), and 26% dipterans. At 7 m, zebra mus- throughout the sampling season at the crest. sels and native mollusks formed a little over half of Ephemeropteran nymphs (chiefly Hexagenia and the small round goby diet (30% and 23%, respec- Baetisca) were the most important food of round tively). Trichopterans (24%) and amphipods (11%) gobies in summer. In fall, round gobies shifted their were also important diet constituents for small diet to trichopteran larvae (Brachycentrus). In De- round gobies. Also found was a round goby that cember, three large round gobies ate YOY round was cut off behind the pelvic and anterior dorsal gobies. The greatest individual variations in diets of fins in the stomach of a 60-mm round goby; the round gobies occurred at the crest (range of SD = length of the victim was estimated to be 30 mm. 34 to 55% for mean volume of food items eaten). The diet of large northern madtoms on the slope Both sizes of round gobies showed significant (5 to 7 m) was composed of 48% Hexagenia and diet overlap with small northern madtoms for Hexa- Baetisca, 21% trichopterans (mostly Brachy- genia in September (Schoener’s Index = 0.67 for centrus), 14% crustaceans (amphipods), and 11% small gobies, 0.70 for large ones). Large northern fish on average (Tables 3, 4). Fishes (round gobies, madtoms fed mainly on Hexagenia and Baetisca northern madtoms, and unidentified) composed an throughout the collection season (Table 2). From average of 16% of the northern madtom diet at 5 m June to August, diet overlap of large madtoms with in May and August and 22% of the diet at 7 m in small round gobies was greater than diet overlap December. with large round gobies. Four YOY round gobies The logperch diet (both size groups) on average and one unidentified larval fish were found in stom- was composed of 47% trichopterans, 11% achs of four large northern madtoms collected in ephemeropterans, 6% dipterans, 30% crustaceans, August (n = 2) and September. and 5% fish eggs by volume (Tables 3, 4). Four log- Small logperch ate dipterans in May and tri- perch ate 69 fish eggs at 7 m in May. To determine chopterans in June and October. Large logperch fed their identity, 20 eggs were measured that had been mainly on trichopterans (> 67% of the diet) in Sep- removed from gravid females (three round gobies, tember and October. In October, diet overlaps be- three logperch, and one mottled sculpin) that were tween logperch and both size groups of round gobies captured on the same date. Eggs in the logperch were not significant (Schoener’s Index = 0.58, 0.59). stomachs (1.03 ± 0.13 mm SD) resembled mottled Logperch, however, ate only Cheumatopsyche, while sculpin eggs (1.14 ± 0.07 mm SD) more than log- Brachycentrus on average composed 93% of tri- perch (0.45 ± 0.07 mm SD) or round goby (0.76 ± chopterans in round goby diets. Schoener’s Indices 0.09 mm SD) eggs. were 0.21 (logperch vs. small round gobies) and 0.13 Mottled sculpins were collected only at 7 m. (logperch vs. large round gobies) in September and Diets of small mottled sculpins were dominated by October, respectively. caddisflies and crustaceans in June, September, and December, with fish (21%) being eaten in Decem- Diets at the Channel Slope ber. Large mottled sculpins ate mayflies (52%) and fish (45%) in May. During December, fish com- At 5 m and 7 m, 31 round gobies had empty posed 93% of the large mottled sculpin diet; 11 of stomachs during the sampling season, while 10 of 18 large mottled sculpins ate 11 YOY round gobies, 55 madtoms had empty stomachs, mostly in spring 1 mottled sculpin, and 3 unidentified fish. (Tables 3, 4) Predation on mollusks by round gob- ies increased with water depth. Zebra mussels on average composed 50% and 74% of the diet of large DISCUSSION round gobies at 5 m and 7 m, respectively, while In the nearshore zone, diet overlap occurred in native mollusks (fingernail clams, snails) composed June when an abundance of newly hatched dipter- 306 French and Jude h Food item Mollusks Ephemeropterans Trichopterans Dipterans Crustaceans Fishes Miscellania Dreissena polymorpha Lg. round goby5(0)Lg. n. madtom 0/5Sm. logperch 21(6) 0.32 7/14 3(0)Lg. round goby6(0) 0.48 82(21) 0/3Lg. n. madtom 2/4Sm. logperch 2(1) 0.01 — 8(18) 0.32 2(0) 0/2Lg. round goby7(1) — 77(39) 2/0Lg. n. madtom 0.15 5(11) 0/7 —Lg. logperch 7(0) 0.05 0.29 — — 1(0) — 0/7Lg. round goby20(1) 4(5) 59(27) — 49(49)Lg. logperch 0/1 0.16 20/0 2(3) — 1.00 0.22 5(0) 4(9) — 33(21) 1(2) —Lg. round goby19(3) —Lg. logperch 27(44) 5/0 19/0 — 20(39) 32(41) 0.82 t(t) 0.10 — 65 5(0) 13(23) — 65(41) — 15(32) 5/0 1(1) — — 31(37) 1(2) 8(12) 35(41) — 0.10 38(33) 20(30) — 7(18) — 92(4) — — 1(2) 16(21) — — — 7(17) — 6(6) — 6(10) 15 8(4) — 2(8) 21(41) — 45(47) 2(3) — — 75 12(31) — — 20 59(35) 7(19) 13(12) — — 2(3) 26(34) — 15 — 5(13) 1(2) 15(16) — 27(42) — — — 19(22) 10 1(2) — — — — 71(41) — — — Lg. round goby3(1) 0/3 0.27 22(33) 48(38) 3(9) 8(18) t(1) 19(29) — — Fish No. Mean TABLE TABLE 3. Mean percent composition of food items by volume (SD) in stomachs of fishes slope collected from (depth the St. = Clair 5 River channel m) during 1994. No. = mean volume of food in fis Stom. empty), D/N = number of fish captured during day/number night, Mean Volume (E) = total number of stomachs examined (number of stomachs in mL, t = trace, sm. small, lg. large, n northern. Mollusks include only native species. stomachs examined that were Date9 MaySm. round goby 7(2) CategoryStom.(E) D/N 4/3 Volume 0.052 Jun Sm. round goby5(0) — 0/5 0.0515 Aug 1(2) round goby14(0) Sm. 0/14 3(5) 0.03 4(9)14 Sep 11(25) 40(34) round goby19(1) Sm. 19/0 17(35) 12(12) 4(7) 0.0627 Oct round goby14(0) Sm. 52(30) 53(44) 14/0 1(5) 53(34) 4(4) 0.115 Dec 9(25) round goby4(2) Sm. 26(23) 18(19) 9(21) — 2/2 25(42) 3(7) <0.01 22(32) 4(6) 3(5) — 9(27) — 24(28) 24(35) — — 45(64) 2(8) 32(29) t(1) 2(9) — 26(24) — 6(15) 5(7) — — 50(57) — — — — Diets of Gobies and Native Species in the St. Clair River 307 h Food item Mollusks Ephemeropterans Trichopterans Dipterans Crustaceans Fishes Miscellania Dreissena polymorpha Lg. round goby8(0)Sm. logperch 0/8Lg. logperchLg. mot. sculpin 2(0) 0.50 2(0) 2(0) 0/2 0/2 95(5)Lg. round goby11(0) 0/2 0.10Lg. n. madtom 0.15 0/11 0.20Sm. logperch 1(0) 1(4)Lg. logperch 0.26 —Sm. mot. sculpin 2(0) — 0/1 — 2(0) 83(31) 2(0) 0/2 0.30 0/2 —Lg. round goby36(1) 0/2 — 0.03 t(1)Sm. logperch — 20/16 0.01 — 0.10 —Lg. logperch 0.34 1(0) — — 1(0) 2(4) 71(35) — — 52(67) 1(5) 1/0Lg. round goby40(1) — —Lg. n. madtom 1/0 20/20 0.01 11(21)Lg. logperch — 5(1) 0.55 1(2) 0.05 —Sm. mot. sculpin — 3(4) 15(30) 4(0) 35(5) 7(0) 0/5 — 86(32) 1(4) 40 0/4 3(4) — 1(4) 55(35)Lg. round goby21(0) 0/7 0.13 15(21) t(1) 43(22)Lg. n. madtom 65(35) 20/1 0.01 — 4(13) 0.16Lg. logperch 15(1) — — — 0.28 5(10) 40(40) — — — 0/15 70(42) — — 10 1(0) 35(35) 2(12) — —Lg. round goby20(5) 87(29) 0.27 — Sm. n. madtom 22(18) 1/0 t(1) — — 0/20 — —Lg. n. madtom — — 1(1) 45(64) 12(30) — 0.20 — —Sm. logperch — — 0.08 — 5(15) — 3(0)Lg. logperch 12(20) 0/1 — 97(4) 5(5) 15(21)Sm. mot. sculpin — 22(33) 1(1) 19(0) 0/3 30(22) — — 0.00 9(1) — — 50 — 50 — — 0/1 0/19 12(32) 0.50 — 48(38) — — 90 0/9 0.13 0.00 — t(t) 2(3) — 5 0.10 — 3(16) — — 90(16) 77(38) — 1(4) 3(9) 5 — — 75(50) — — — — — — — — — 8(15) — 30 8(18) 2(5) — 45 — — 10 68(21) — 11(26) — 2(3) 42(49) 25(50) t(1) t(1) — — 1(3) — — 5 — — t(t) 19(29) 1(2) — 23(32) — — — — — 7(23) — — 13(23) — — 5 29(33) — — t(t) — — — 2(7) 55 22(37) 69(41) — — 68(33) 21(37) — — — — — — — — — — — Lg. mot. sculpin 6(0) 0/6 0.33 — — 1(2) 1(2) 5(8) — 93(10) — Fish No. Mean TABLE 4.TABLE Mean percent slope composition of food (depth items by volume = (SD) in stomachs 7 of fishes collected m) from the St. during Clair River channel 1994. No. = mean volume of food in fis Stom. empty), D/N = number of fish captured during day/number night, Mean Volume (E) = total number of stomachs examined (number of stomachs in mL, t = trace, sm. small, lg. large, n northern, mot. mottled. Mollusks include only native species. stomachs examined that were Date9 MaySm. round goby20(1) CategoryStom.(E) D/N 0/20 Volume 0.08 58(41)2 Jun Sm. round goby20(2) 1(2) 0/20 0.08 — 27(36)15 Aug round goby39(1) Sm. 20/19 1(3) 25(29) 0.0514 Sep 37(34) 6(11) round goby40(5) Sm. 7(12) 20/20 19(23) 11(31) 0.05 61(42) — 23(33)27 Oct — round goby26(2) Sm. 1(2) 49(41) — 20/6 0.03 4(12) 20(27) 4(14)5 Dec 19(35) round goby20(2) Sm. — 4(8) 0/20 46(44) 13(25) 0.01 t(t) 20(27) 2(7) 13(30) — t(1) 22(31) 9(21) t(1) 9(19) — 5(21) 17(36) — 15(27) 5(13) 6(15) 4(20) 39(38) — — t(1) 308 French and Jude

ans was available for small fishes (TL < 75 mm). at night after this trichopteran left cryptic daytime Rainbow darters and tubenose gobies were two hiding places. species that sought food and refuge in the more di- Jude et al. (1995) found that mottled sculpins in verse, rocky habitats near shore. Tubenose gobies the St. Clair River were apparently decimated by avoided predation by round gobies and other preda- round gobies in shallow water, while mottled tors by hiding in small crevices in rocky substrates sculpins apparently found a refugium in deeper wa- and macrophytes (Jude et al. 1995, Jude and DeBoe ters (7 m) that exposed them to stronger currents. 1996). Rainbow darters apparently preferred shal- As active nocturnal feeders, mottled sculpins feed low waters where populations of small-sized prey mainly on aquatic insects, crustaceans, annelids, were dense and they could avoid predation by large fishes, and fish eggs (Scott and Crossman 1973, fishes in deeper water (Fisher and Pearson 1987, Hoekstra and Janssen 1985). Harding et al. (1998) Greenberg 1991, Stauffer et al. 1996, Gray et al. and Welsh and Perry (1998) found that rainbow 1997). However, logperch tended to spend most of darters preferred to seek refugia in spaces between their time in deepwater habitats (Greenberg 1991), rocks with current velocities that were substantially where food items such as trichopterans were eaten. lower than adjacent velocities. This suggests that At the slope crest (3 m), the significant diet over- strong currents in the main channel of the St. Clair lap between northern madtoms, listed as an endan- River may confine rainbow darters to the nearshore gered species in Michigan (Evers 1994), and round area. Logperch were the only small native benthic gobies may not portend real competition for food fish that displayed movements between nearshore resources. As active nocturnal feeders, northern and offshore waters. madtoms successfully use their barbels to detect, There was evidence of predation on YOY fishes then capture Hexagenia and Baetisca nymphs bur- by both round gobies and native fishes in deeper rowing in soft sediments (Bardach et al. 1967, Scott waters. Round goby YOY migrated from nearshore and Crossman 1973, Edmunds et al. 1976); Hexa- to the slope in autumn and became easy prey of mottled sculpins and northern madtoms at the slope genia was more abundant at the crest (3 m) than on and bottom of the channel. Round gobies in their the slope (≥ 5 m) (Hudson et al. 1986). Northern native habitat fed on young fishes in fall (Miller madtom foraging behavior might cause nymphs to 1986) when they migrated to deep water (< 70 m) emerge from sediments and drift in currents. Wait- and in winter in the Sea of Azov (Skaskina and ing on the bottom, round gobies probably captured Kostyuchenko 1968). The poisonous, serrated drifting ephemeropterans or detected them in or spines in pectoral fins may protect northern mad- near their burrows in the dark (Jude et al. 1995; S. toms from predation by round gobies (Birkhead Carman, personal communication, Loyola Univer- 1967, Taylor 1969). Mottled sculpins might not sity, Chicago, IL). The low predation on mollusks move more than 15 m from their home sites (Brown by round gobies was attributed to their feeding on and Downhower 1982, Hill and Grossman 1987, the apparently more available ephemeropterans in Greenberg 1991). Mottled sculpin eggs, therefore, 3-m depths. may have been vulnerable to predation by highly Logperch were easily captured at the slope crest mobile logperch in spring (Smart and Gee 1979, with the trawl at night when they were inactive and Gray et al. 1997) because adult mottled sculpins hid on the bottom (Greenberg 1991). Resource par- were confined to the channel bottom with limited titioning between logperch and small round gobies movement from home sites. Mottled sculpins, like occurred in autumn when logperch skillfully round gobies, nest in shallow water with rocks and snatched Cheumatopsyche from top surfaces of slow currents (Greenberg 1991). Round gobies may rocks (Smart and Gee 1979), while round gobies have forced mottled sculpins deeper, exposing eggs were eating ephemeropterans, dipterans, and and YOY to predation by native fishes and round Brachycentrus that are generally associated with gobies, which may be an important factor in the de- water currents (Wiggins 1996). The interesting di- cline of mottled sculpin populations in the St. Clair chotomy among these three fish species, round gob- River (Baltz and Moyle 1993). No evidence was ies and northern madtoms eating Brachycentrus and found of substantial diet overlap between round go- logperch eating Cheumatophyche, might be ex- bies and native fishes (logperch, northern madtoms, plained by logperch being less benthic oriented and and mottled sculpins) at the slope due to heavy pre- able to feed off the rocks, while round gobies and dation on mollusks by round gobies. northern madtoms may have fed on Brachycentrus Round gobies have the potential to detrimentally Diets of Gobies and Native Species in the St. Clair River 309

impact native fishes through competition for food prey with their neuromasts (Jude et al. 1995, resources and predation on young. Round gobies Janssen 1997). However, mottled sculpins may were found to be cannibalistic in this study, but have experienced higher mortality because of sub- they also ate a large adult trout-perch and other optimal spawning substrate and higher predation unidentified fishes. Some fish were also eaten by rates. Eggs, fish larvae, and adult fish were found in round gobies in the Detroit River (L. Corkum, per- the stomachs of round gobies, and cannibalism was sonal communication, University of Windsor, documented, suggesting that predation could have Windsor, Ontario, Canada), and Weimer and Sowin- an important impact when susceptible YOY or lar- ski (1999) found white perch (Morone americana) vae of native species co-occur with round gobies. larvae in stomachs of four round gobies trawled Round gobies could adversely affect lake trout re- from the bottom of dredged harbors (depths = 8 to cruitment (Chotkowski and Marsden 1999), and the 10 m) in Lake Erie during summer 1998. Round go- recent dispersal of round gobies to deep water > 30 bies may also feed on eggs of native fish species as m in Lakes Michigan and Huron could portend dire recorded here and may reduce lake trout survival by consequences for slimy and deepwater sculpins, feeding on eggs (Chotkowski and Marsden 1999) or where they might compete for similar food re- larvae on spawning reefs where these two species sources or disrupt spawning. co-occur. Round gobies have recently been found in deep water (> 30 m) in Lake Michigan (Jude, un- ACKNOWLEDGMENTS published data) and Lake Huron (G. Curtis, per- We thank Gary Crawford for field assistance and sonal communication, USGS, Great Lakes Science some preliminary diet data, and Kerstin Hanson and Center, Ann Arbor, MI), setting up the potential to Tomas Hook for laboratory assistance. Scott negatively impact populations of deepwater (see DeBoe, Donna Francis, Heang Tin, and Howard Vass et al. 1975), spoonhead (Cottus ricei), and McCormick helped with trawling and seining. The slimy (C. cognatus) sculpins in the oligotrophic specimens were collected with EPA grant No. upper Great Lakes. CR821052 to D. Jude and partial support was pro- vided by National Sea Grant. We gratefully ac- CONCLUSIONS knowledge the donation by Mercury Marine of two Previous studies have documented that mottled outboard engines, which were used to collect fish sculpin populations have declined precipitously during this study. Gerald Smith confirmed identifi- concurrent with the arrival and expansion of the cation of the northern madtoms. We thank Douglas nonindigenous round and tubenose gobies (Jude et Wilcox, Jerrine Nichols, Patrice Charlebois, and al. 1995, Janssen and Jude 2001). To determine anonymous reviewers for their critical comments on what mechanisms might be responsible, the stom- the manuscript. This is contribution no. 1137 of the achs of round and tubenose gobies and those of Great Lakes Science Center, USGS, Ann Arbor, MI common native species that co-inhabited several and 620 of the Center for Great Lakes and Aquatic areas were examined to determine temporal and Sciences, University of Michigan. spatial overlap in diets. Nearshore (≤ 1 m) in June, fish < 75 mm of both species of gobies, logperch, REFERENCES and rainbow darters consumed common resources, Adams, C.E., and Maitland, P.S. 1998. The ruffe popula- mainly macroinvertebrates. At 3 m, round gobies tion of Loch Lomond, Scotland: its introduction, pop- and northern madtom diets overlapped signifi- ulation expansion, and interaction with native species. cantly; mainly ephemeropteran nymphs were eaten. J. Great Lakes Res. 24:249–262. Few zebra mussels were consumed by round gobies Baltz, D.M., and Moyle, P.B. 1993. Invasion resistance at 3 m. However, at 5 to 7 m, round gobies ate large to introduced species by a native assemblage of Cali- quantities of zebra mussels, which may have fornia stream fishes. Ecol. Appl. 3:246–255. buffered their predation on other organisms, result- Bardach, J.E., Todd, J.H., and Crickmer, R. 1967. Orien- ing in no diet overlap at these depths with native tation by taste in fish of the genus Ictalurus. Science 155:1276–1278. species that co-occurred with them. Round gobies Birkhead, W.S. 1967. The comparative toxicity of stings may have forced some species (mottled sculpin) to of the ictalurid catfish genera Ictalurus and deeper water where they established a refugium in Schilbeodes. Comp. Biochem. Physiol. 22:101–111. the faster current of the St. Clair River because of Brandt, S.B., Mason, D.M., MacNeill, D.B., Coates, T., their superior ability over round gobies to detect and Gannon, J.E. 1987. Predation by alewives on lar- 310 French and Jude

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