PROCEEDINGS VOLUME II

11TH WORLD LAKES CONFERENCE NAIROBI, KENYA, 31 OCTOBER TO 4TH NOVEMBER 2005 PROCEEDINGS VOLUME II EDITED BY: Eric O. Odada, Daniel O. Olago, Washington Ochola, Micheni Ntiba, Shem Wandiga, Nathan Gichuki and Helida Oyieke

Feeding habits of introduced Largemouth Bass around Lake Biwa and in ponds without Mesopredators in Japan Masahiro Nakagawa Department of Fisheries, Faculty of Agriculture, Kinki University., Nakamachi 3327-204, Nara 631-8505, Japan Email : gf998002@yahoo.co.jp Abstract To understand the feeding habits of largemouth bass Micropterus salmoides in Japanese inland waters, the stomach contents of fish from three rivers, one satellite lake around Lake Biwa, and two closed water systems without mesopredators that differed in catchment landscape, were examined. Specimens from lotic areas were collected from Iso River in spring 2001; from Etsura River, spring 2002 and 2003; and from Kawamichi River, summer 2001. Specimens from lentic areas were collected from Katada-naiko Lake from October 2001 to May 2005; Pond A, summer 2001; and Pond B, summer 2002. To estimate the frequency of occurrence of prey, the stomach contents were categorized into fishes, shrimps, aquatic and terrestrial insects, daphnia and midges, and others. The main prey in Iso River was fishes, including Gymnogobius isaza, which is an endemic goby of Lake Biwa and is categorized as near threatened in the Japanese Red Data Book. The main prey in Etsura River remarkably changed from fishes (86.0%) in 2002 to shrimps (87.1%) in 2003. The main prey of fry in Kawamichi River was fishes and shrimps. The main prey in the lentic areas were fishes in Katada-naiko Lake, aquatic and terrestrial insects in Pond A, and daphnia and midges in Pond B. These findings indicate that depending on their established habitats, largemouth bass mainly prey on fishes and shrimps around Lake Biwa; they dramatically change their main feeding habits within a short time in the same habitat, and they normally prey on fishes and/or shrimps irrespective of the habitat area, i.e., rivers or lakes. These findings also demonstrated that the largemouth bass residing in closed water systems without mesopredators frequently prey on aquatic and terrestrial insects or daphnia and midges, and the top-down effects on account of direct predation will lead to confusion in the trophic cascade. Key words: endangered species, Gymnogobius isaza, invasive alien species Introduction Largemouth bass Micropterus salmoides is a typical invasive alien fish species that belongs to the family Centrarchidae of the order Perciformes. The largemouth bass, which is naturally distributed in the northern part of America, was primarily introduced into Lake Ashinoko in central Japan in 1925 and was then artificially dispersed in Japanese inland waters. The species is now widely distributed over 74% of the waters out of the examined 227 water bodies, including 94 dam lakes (Environmental Agency, 2004), because it possesses the ability to adapt and thereby establish itself in various environmental conditions due to its predominant feeding, growth, and reproductive habits (Yodo, 2002). 242 Largemouth bass has already established in Lake Biwa, which is the largest lake in Japan (surface area, 670.5 km 2 ; maximum depth, 104 m), a hotspot of biodiversity (more than 50 fish species) with some endemic species. The fish was first collected in 1974 from the northern part of the lake, and since the early 1980s, mass propagation was recorded throughout the coast of the lake (Maehata, 1993). It is common knowledge that the yields from fisheries specific for other fish species as well as the biodiversity in the lake has shown a decline with the increase in the population of largemouth bass in the lake. On one hand, the species must have occupied many ponds out of more than 200,000 ponds in Japan, although very little information on this subject is currently available in the published literature. Some ponds have recently been considered to be important because they have functioned as refuges for endangered fishes in terms of in situ conservation (e.g., Kawamura & Hosoya, 1997). In some ponds with limited capacity, mesopredators are often exhausted by the indiscriminate introduction of fish. Therefore, the feeding habits of the largemouth bass might be unusual. In order to accumulate case studies and obtain new findings regarding the feeding habits of the largemouth bass and for the assessment of its negative effects on the ecosystem, this study of the stomach contents of the species focused on prey selectivity, attack on propagation of an endemic goby, and the top-down effect on low trophic levels in closed water systems. In this study, the author has reported several novel findings and suggestive data pertaining to the above-mentioned aspects. Material and methods Sampling sites This study was performed in Lake Biwa water system in Kinki region, Japan. The sampling sites around Lake Biwa in Shiga Prefecture were as follows: the Iso River (E136 15'50'', N35 18'15''; width of the river, 5 m) in Sakata county, the Etsura River (E136 12'55'', N35 15'15''; width, 20 m) in Hikone city, the Kawamichi River (E136 14'30'', N35 23'30''; width, 6 m) in Higashi-azai county, and a part of the Katada-naiko lake (E135 55'30'', N35 07'10''; surface area, 2340 m 2 ) in Otsu city. The population size of the largemouth bass in the Katada-naiko Lake had been constant at low levels

until 2003, but has increased recently (Nakagawa, unpublished data). The sampling sites of the ponds were as follows: Pond A (E135 44'45'', N34 46'45''; 150 m 2 ) in Tanabe county of Kyoto Prefecture and Pond B (E135 53'40'', N34 52'40''; 200 m 2 ) in Otsu city of Shiga Prefecture. In both the ponds, no fishes, except the largemouth bass and a few big carps (only Pond A), and no shrimps, except, crayfish were observed. Sampling methods All the specimens from the 3 rivers and the Katadanaiko Lake were collected using a casting net. Specimens from Pond A and Pond B were collected by fishing, except for 3 specimens that were colleted using a hand net and 7 specimens that were collected using a casting net, respectively. In order to prevent largemouth bass from swallowing fishing baits, which could result in mistaken assumption of the bait as part of the stomach contents, sausage and angleworm were used as baits. The specimens were collected during the daytime, with the exception of some specimens from the Etsura River in 2003 and the Katada-naiko Lake. Following their capture, the specimens were immediately fixed with 10% formalin solution to prevent expulsion and digestion of the stomach contents. Specimens In 2001, 166 fish specimens were obtained from the Iso River (Standard body length ± SD, 147.5 ± 22.6 mm; Min Max, 115 256 mm); 130 fish on April 14, 15 fish on May 13, and 21 fish on June 21. In 2002, 296 fish specimens from the Etsura River were caught (142.1 ± 32.3 mm, 68 256 mm; N = 260, except 36 specimens that were not measured); 27 fish on May 26, 67 fish on May 29, 63 fish on June 2, 19 fish on June 9, 7 fish on July 21, 27 fish on September 22, 64 fish on September 26, and 22 fish on September 29. In 2003, 105 fish specimens were obtained from the Etsura River (146.0 ± 36.2 mm, 95 298 mm); 16 fish, 61 fish, and 28 fish on June 15, June 19, and June 20, respectively. In 2001, 99 fish specimens were obtained from the Kawamichi River (86.6 ± 14.9 mm, 60 124 mm; N = 76, except 23 specimens that were not measured); 14 fish and 43 fish on August 22 and August 16, respectively, 29 fish on September 16, and 13 fish on October 13. From September 2001 to May 2005, 30 fish specimens were obtained from the Katada Lake (135.4 ± 69.9 mm, 59 360 mm). A total of 13 fish specimens were obtained from Pond A (116.6 ± 15.6 mm, 98 150mm); 10 fish and 3 fish on July 31 and August 3, 2002, respectively, and 26 fish specimens were obtained from Pond B (12.7 ± 4.1 mm, 72 254mm); 4, 2, 10, and 2 fish on 8, 10, 11, and 31 of August, respectively and 8 fish on September 4, 2003. Analysis of stomach contents To understand prey selectivity in largemouth bass, the stomach contents were examined using the specimens from the above-mentioned sampling sites. Frequencies of the occurrence of the prey were examined after measuring the standard body length. The preys in the stomach were categorized as follows: fishes, shrimps, insects (not including midges), daphnia and midges (DM), and/or others. Fishes were categorized into the following categories: osmerid genus, cyprinid genus, and gobiid genus. Shrimps were categorized into the following categories: river shrimp and/or freshwater prawn (river shrimps) and crayfish. Egg count in Biwa goby To clarify the negative effect of the feeding on Biwa goby (indigenous goby fish population of Lake Biwa), 7 specimens that preyed on Biwa goby without digestion from the Iso River were used in this study. Biwa goby was identified by inspecting the maxillary length, interorbital width, and caudal peduncle, and they were measured in terms of their body weight. Each ovary of the preyed goby was picked from the stomach of the largemouth bass and then counted the eggs in the ovary one by one. DM count In order to analyze the effect of the feeding on small organisms, the stomach contents of all the 24 specimens that preyed on the DM from Pond B were investigated in detail. The weight of the stomach contents was measured after removing the moisture by using an absorbent paper towel. In order to determine the correlation coefficient between the weight and the number of DM, the prey from one randomly selected specimen of the 24 specimens were counted one by one on graph paper. The numbers of DM in the remainder of the specimens were determined using the correlation coefficient. Daphnia and midges were equated in terms of weight due to the ease of calculations. In the case that the stomach contents included any insects, the weight measurements were repeated to ascertain the DM weight. Results Frequency of prey occurrence at each sampling site Figure 1 shows the frequencies of all specimens, except those with an empty stomach, at each sampling site: Iso River, Etsura River in 2002 and in 2003, Kawamichi River, Katada-naiko Lake, Pond A, and Pond B. The frequencies and prey categories differed among the sampling sites and sampling years. The diet composition also differed significantly. 243

Iso R. (2001) N = 124 Etusra R. (2002) N = 243 Etsura R. (2003) N = 101 Kawamichi R. (2001) N = 79 Katada L. (2001Ğ2005) N = 14 Pond A (2001) N = 9 Pond B (2002) N = 24 0% 20% 40% 60% 80% 100% F%; Frequency of prey occurrence Fishes Fishes and Shrimps Shrimps Fishes and Insects Shrimps and Insects Insects Daphnias and Midges Insects, Daphnias, and Midges Others Figure 1. Frequency of prey occurrence of largemouth bass Micropterus salmoides in Iso River, Etsura River, Kawamichi River, Katada-naiko lake, and two ponds without mesopredators. In this study, the main prey components of the largemouth bass were fishes and shrimps, except in the ponds. In the Iso River, stomach contents were obtained from 124 specimens (74.7%). Fishes, shrimps, and insects were obtained from 75 specimens (60.5%), 88 specimens (71.0%), and 7 specimens (5.6%), respectively. In the fishes category, the osmerid genus (Ayu Plecoglossus altivelis altivelis and Japanese smelt Hypomesus nipponensis), cyprinid genus (stone moroko Pseudorasbora parva), and gobiid genus (common freshwater goby Rhinogobius sp. and Biwa goby Gymnogobius isaza) were obtained in 3, 3, and 38 specimens, respectively. In the shrimp category, river shrimps and crayfish were obtained from 89 and 6 specimens, respectively. In the Etsura River, stomach contents were obtained from 243 specimens (82.1%) in 2002 and 101 specimens (96.2%) in 2003. The stomach contents showed a significant change between the two years. Fishes were obtained from 209 specimens (86.0%) in 2002 and from 28 specimens (27.7%) in 2003. Shrimps were obtained from 68 specimens (28.0%) in 2002 and from 88 specimens (87.1%) in 2003. Insects were obtained from 3 specimens (1.2%) only in 2002. In the fish category, in 2002, fish of the osmerid genus (Ayu) and gobiid genus (common freshwater goby) were obtained from 53 and 23 specimens, respectively. On the other hand, in 2003, fish of the osmerid genus (Ayu) and gobiid genus (common freshwater goby) were obtained from 6 244 and 4 specimens, respectively. In the shrimp category, in 2002, river shrimps were obtained from 52 specimens, whereas crayfish were obtained from only 1 specimen. On the other hand, in 2003, river shrimps were obtained from 87 specimens, whereas crayfish were obtained from 3 specimens. In the Kawamichi River, stomach contents were obtained from 79 specimens (79.8%). Fishes and shrimps were obtained from 67 (84.8%) and 20 specimens (25.3%), respectively. In the fish category, fish of the osmerid genus (Ayu), cyprinid fish (field gudgeon Gnathopogon elongatus elongatus and dark chub Zacco temminckii), and gobiid genus (common freshwater goby) were obtained from 1, 2, and 26 specimens, respectively. In the shrimp category, river shrimps were obtained from 22 specimens. Although all the specimens were not adults, none of the specimens preyed on insects or DM. In the Katada-naiko Lake, stomach contents were obtained from 14 specimens (46.7%). Fishes and shrimps were obtained from 13 specimens (92.9%) and 1 specimen (7.1%), respectively. In the fish category, gobiid genus (common freshwater goby) was obtained from 2 specimens, whereas in the shrimp category, 3 crayfish were obtained from 1 specimen. None of the specimens preyed on insects or DM. In Pond A, stomach contents were obtained from 9 specimens (69.2%). Shrimps and insects were

obtained from 2 specimens (22.2%) and 9 specimens (100%), respectively. The shrimps were identified as crayfish. Furthermore, 1 frog was obtained from 1 specimen (11.1%). In Pond B, stomach contents were obtained from 24 specimens (92.3%). Crayfish, insects, and DM were obtained from 1 specimen (4.2%), 7 specimens (29.2%), and 22 specimens (91.7%), respectively. None of the specimens preyed on the fry of largemouth bass in both ponds. Number of eggs in the abdomen of goby from the stomach of largemouth bass Table 2 shows the number of eggs in the abdomen of Biwa goby from the stomach of specimens. The standard body length of 7 specimens that were used in the study was 192.7 ± 44.2 mm (min max: 145 253 mm). The weight of the stomach contents of the specimens was 5.9 ± 2.9 g (min max: 2.7 10.2 g). The stomach contents consisted of Biwa goby, Japanese smelt, stone moroko, an unidentified fish, and river shrimp (data not shown). All the specimens directly preyed on 1 3 mature females of Biwa goby and indirectly preyed on 377 4339 eggs of Biwa goby. Table 2. Stomach contents and the number of eggs of Biwa goby in the stomach of largemouth bass Micropterus salmoides in the Iso River No. Body length (mm) Weight of stomach contents (g) Category No. of Female/s No. of eggs of the goby 1 157 3.1 Fishes/Shrimps 1 1252 2 159 4.23 Fishes/Shrimps 2 1781 3 176 2.66 Fishes/Shrimps 1 377 4 145 4.66 Fishes 1 1662 5 244 7.92 Fishes/Shrimps 2 2406 6 253 8.42 Fishes/Shrimps 3 2168 7 215 10.24 Fishes/Shrimps 3 4339 Table 3. Stomach contents of largemouth bass Micropterus salmoides in Pond B. No Body length Weight of stomach %DM by weight to stomach. (mm) contents(g) Category (Numbers) contents 1 98 0.08 Insects (1)/ DM (240) 95.4 2 109 0.22 Insects (10)/DM (510) 79.0 3 100 0.17 Insects (4)/DM (440) 89.4 4 127 0.55 DM (1600) 100 5 88 0.35 DM (1000) 100 6 106 0.61 DM (1770) 100 7 143 0.41 Insects (1)/DM (780) 66.0 Shrimps (1)/Insects 8 254 4.60 (3) 0 9 146 0.11 DM (330) 100 10 113 0.60 DM (1750) 100 11 145 0.20 DM (570) 100 12 110 0.24 DM (700) 100 13 86 0.16 DM (470) 100 14 128 0.42 DM (1210) 100 15 92 0.19 DM (550) 100 16 135 0.64 UI (1)/DM (1110) 59.8 17 190 0.52 DM (1500) 100 18 139 0.35 Insect (1)/DM (620) 60.7 19 196 1.49 Others 0 20 95 0.07 DM (190) 100 21 72 0.04 DM (100) 100 22 100 0.13 DM (380) 100 23 112 0.07 DM (190) 100 24 135 0.14 DM (400) 100 245

UI: Unidentified. Other: rubber goods used in fishing (lure) Number of DM in the stomach of largemouth bass Table 3 shows the number and weight of DM from the stomachs of the specimens. DM was dominant in the diets of largemouth bass of Pond B in terms of quantity. The specimen that is selected as a monitor for determining the correlation coefficient between the weight and numbers of DM preyed on 137 individuals of DM per 0.047 g. In other words, the correlation coefficient is 29 individuals per 0.01 g. The standard body length of the 26 specimens used in the study was 126.9 ± 40.9 mm (min max: 72 254 mm). The weight of the stomach contents of the specimens was 0.5 ± 0.9 g (min max: 0.01 4.6 g). The stomach contents consisted of DM, strider, backswimmer, crayfish, and rubber goods. Each specimen preyed on approximately 100 to 1770 individuals of DM. Based on the remarkably high value (59.8% 100%) of the frequencies of DM by weight; it is supposed that the specimens mainly preyed on DM. Discussion The relationship between predation by largemouth bass and disruption in the distribution of fish fauna that is simultaneous with the increase in largemouth bass population in Japanese inland waters has been receiving increasing attention since the species was introduced into the Japanese waters. In this study, the stomach contents of largemouth bass from 4 points around Lake Biwa and 2 ponds without mesopredators, were examined from 2001 to 2005 in order to understand the feeding habits, their effects on native ecosystem, and the top-down effects. The important findings from this study are as follows: (1) remarkable changes in stomach contents among sampling sites, (2) mass predation of endemic goby in the Iso River, and (3) direct impact on low trophic levels. These points have been discussed below, in the same order: 1. In this study, the largemouth bass mainly preyed on fish and/or shrimps around Lake Biwa. On the other hand, the largemouth bass mainly preyed on insects or DM in the two ponds. This result is consistent with other studies that suggest that largemouth bass that have been introduced are flexible in terms of feeding habits and can change the target of predation in each environment (Yodo & Kimura, 1998). However, the frequency of prey occurrences differed among the sampling sites of Lake Biwa and also differed between sampling years at the same sampling site. In Izu-numa of Miyagi Prefecture, the largemouth bass mainly prey on daphnia up to 20 mm body length (BL) and gradually prey on fishes during further growth (Takahashi, 2002). This report is consistent with the data in the Kawamichi River, whereas it is completely inconsistent with the data from the two ponds. 2. In the Iso River, Biwa goby Gymnogobius isaza, which is an endemic goby of Lake Biwa and is categorized as near threatened in the Japanese Red Data Book, was largely preyed on by largemouth bass. The mass predation of Biwa goby is the first record, although freshwater common goby was one of the main diets, as reported by the previous studies (Maehata et al., 1987; Yodo & Kimura, 1998). Biwa goby is an easily available vulnerable prey for the largemouth bass in the breeding season; this is similar to that observed in the case of other migratory gobiid fishes (Azuma & Motomura, 1998). Furthermore, the specimens preyed on the mature females of the Biwa goby and indirectly preyed on the eggs before they were laid. These results indicate that predation by largemouth bass virtually equals that of the predation of fertilized eggs by egg feeders such as introduced bluegill and leads a great loss in the propagation of Biwa goby. 3. Various field studies have reported the direct and indirect effects of bass occurrence in reducing the diversity of small-bodied fish species; thereby creating more homogenous fish communities and alternating planktonic and benthic communities (Jackson, 2002; Maezono & Miyashita, 2003). In this study, largemouth bass intensively preyed on DM irrespective of the body size in the closed water system without mesopredators. It must be emphasized that in this study, of the 26 collected specimens, 22 specimens (BL greater than 75 mm) preyed on DM, and the frequency of DM in the stomach contents was very high. This result indicates that the predation by largemouth bass primarily affects the low trophic levels and secondarily causes remarkable changes in the ecosystem and water quality. Further studies are necessary for obtaining a complete report on the topdown effect due to the predation of largemouth bass in the study areas. The above-mentioned results suggest that largemouth bass is a euryphagous species; therefore, extermination of this species from nonnative habitats is of utmost importance. Acknowledgements The author wishes to thank Dr. T. Suzuki of the Faculty of Agriculture, Kinki University, for providing valuable advice and assistance with regard to collection specimens. 246

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