Youichi Yasuda¹, Iwao Ohtsu¹, Tatsuo Hamano² and Yasuhiko Miya³

¹Dept. of Civ. Engrg., Coll. of Sci. and Tech., Nihon University

Kanda Surugadai 1-8, Chiyoda-ku, Tokyo, 101-8308, Japan

²Laboratory of Aquatic Biology, Dept. of Applied Aquabiology,

National Fisheries University

Nagata-honmachi, Shimonoseki, 759-6595, Japan

³Dept. of Environmental Conservation, Faculty of Environmental Science,

Nagasaki University

1-14 Bunkyo-machi, Nagasaki, 852-8521, Japan

Abstract: For fishes, freshwater shrimps, and crabs, weirs and drop-structures without fishways are obstacles to both upstream and downstream migrations. In particular, it is important for diadromous shrimps and crabs to migrate upstream over drop-structures from estuaries where their larvae develop and metamorphose to juveniles. A fishway for shrimps and crabs should be required in drop-structures in order to conserve the local populations of such aquatic animals. This paper proposes a type of fishway for shrimps and crabs that has a stepped channel with a trapezoidal cross-section. The effect of this fishway on the upstream migration of freshwater shrimp is discussed on the basis of experimental results concerning flow fields. Also, changes in the number of freshwater shrimps and crabs migrating upstream via the fishway during a 24-hour period are clarified.

 

Keywords: freshwater shrimps and crabs, fishway, migration, stepped channel, drop-structure

Many diadromous aquatic animals live in Japan. In particular, many diadromous shrimps, crabs, and fishes live in rivers. For such diadromous animals, it is necessary to migrate to an estuary during their lifetime, and a migrating course should be kept in the rivers for them, but dams, weirs, and sabo works without fishways are obstacles to their migration. Such drop-structures have caused the extinction of local populations in upstream habitats, and they affect the ecosystems of rivers [1]. Because, fishways have been constructed mainly for fishes that have prominent jumping and swimming abilities, most of fishways are not usable by freshwater shrimps and crabs because diadromous crustaceans migrate to their upstream habitats by walking [2]. The existence of freshwater shrimps and crabs is most significant for keeping a food web in a river, because juveniles of shrimps and crabs are food for other aquatic animals.

In designing drop structures, it is necessary to develop fishways in which diadromous animals can migrate to their upstream and downstream habitats. Although a study concerning fishways for fishes has been conducted, there is little information concerning fishways for freshwater shrimps and crabs. A method for improving the drop-structures should be required so as to enable shrimps and crabs to migrate to their upstream and downstream habitats.

In this paper, a fishway for freshwater shrimps and crabs is proposed. The fundamental structure of the fishway and a method for leading shrimps and crabs to the entrance of the fishway are presented, and the flow characteristics of the proposed fishway are shown. Also, changes of shrimps and crabs migrating upstream via the fishway during a 24-hour period are clarified.

The migration course of freshwater atyid and palaemonid shrimps and the Japanese mitten crab Eriocheir japonica has been observed around drop-structures with sloping channel chutes, and their upstream migration has been characterized as follows:

(1) Positive rheotaxis is the most important orientation mechanism for the upstream migration of shrimps and the crabs.

(2) The upstream migration of freshwater shrimp occurs mainly at night, whereas crabs migrate to upstream habitats not only at night but also in the daytime.

(3) In the case of upstream migration, freshwater shrimps walk along brink, the Japanese mitten crab walks along not only brinks but also along channel beds or out of the water.

(4) Immediately after their upstream migration, freshwater shrimps plunge into the water at a critical section where the flow is critical.

(5) In channels where slopes are small, shrimps and crabs can migrate easily. The upper limit of the slope of a channel for their migration is about 50 degrees.

(6) In order for freshwater shrimps and crabs to be able to migrate, a roughness wall that can be caught by the tips of their walking legs is required.

(7) The migration ability of the Japanese mitten crab is greater than that of freshwater shrimps.

A fishway for freshwater shrimps and crabs has been proposed on the basis of the characteristics of their migrations and the flow conditions of channels (Fig. 1 and Photo 1). The proposed fishways has the following features:

Fig.1  Proposed Fishway

(1) The fundamental structure of the fishway is that of a stepped channel with a trapezoidal section. The stepped channel with a 19-degree channel slope is effective in dissipating high-velocity flows [3],[4]. Because a stepped-channel flow becomes an aerated flow with a water-surface fluctuation [5],[6], the sidewall near the brink of the fishway is always wet. In order to enable freshwater shrimps and crabs to migrate safely via the fishway, a sidewall with ?= 45°is recommended, because that enables it difficult for predators, such as herons, to perch on the sidewall.

(2) The trapezoidal section can keep the low-velocity flow near the brink of the sidewall, which makes it easy for freshwater shrimps and crabs to walk along the brink. On the top of the fishway, the flow transits from a subcritical flow to a supercritical flow in the trapezoidal channel, which makes it easy for freshwater shrimps and crabs to plunge into the water.

(3) A roughness wall that can be caught by the tips of their walking legs should be utilized. Also, if the sidewall is made of concrete, the surface of the concrete should be rough. In this experiment, watertight sandpaper was used as the material of the sidewall.

The fishway shown in Fig. 1 [0.88 m height, 1.0 m wide (the stepped-channel portion is 0.6 m wide), 22 steps (step height S = 0.04 m)] was used. Stepped channels with ? = 19°and 45°were used. Experiments on upstream migration were conducted in June (water temperature was about 26－28℃). The fishway was settled in the pool (2.0 m wide 5.0 m long) as shown by Fig. 2, and the pool depth was adjusted to form a plunging flow immediately downstream of the fishway. The flow rate in the fishway was about 0.0066 m³/s. For each experiment of upstream migration, an atyid shrimp Caridina japonica, a palaemonid shrimp Macrobrachium nipponense, and the Japanese mitten crab Eriocheir japonica were used. The number of experiments conducted regarding the three animals was 600, 60, and 200, respectively. In order to investigate the flow characteristics immediately downstream of the fishway, the velocity and the air concentration were measured by using an electromagnetic velocity-meter (sampling time: 60 s, sampling frequency: 40 ms) and a void probe (sampling time: 60 s). Also, in order to observe the migration of freshwater shrimps and crabs via the fishway during a 24-hour period, an infrared video camera was utilized.

Fig. 3 shows an example of the velocity field at the downstream portion of the fishway. In the case of the proposed fishway, the velocity decays within a short distance, and the velocity near the sidewall is less than that at the center because, due to the trapezoidal section. Immediately after the plunging position (X≈20cm) the main flow separates from the sidewall and lifts to the water surface within a short distance.

Fig. 4 shows an example of air-entrainment distribution at the downstream portion of the fishway. The air-concentration ratio in Fig. 4 was defined as the volume of air/(volume of air + volume of water) 100 (%) for the space of 3.14 10^－2 cm³ (0.1 cm 0.1 cm 1.0 cm).

As shown by Fig. 4, the air-entrainment portion is distributed until 10 cm downstream of the first step (X = 0 cm) and there is little air entrainment near the bed, except at the toe of the plunging flow. The air entrainment near the sidewall is less than that at the center, because the main flow separates from the sidewall immediately after the plunge.

According to observations of the upstream migrations of atyid shrimps, palaemonid shrimps, and the Japanese mitten crab, all three animals could easily find the entrance of the fishway, because the aerated flow plunges into the pool and water-surface fluctuations occur immediately downstream region of the fishway. Further, the air entrainment near the sidewall is less than that at the center. For a fishway with a trapezoidal section, a low-velocity flow is formed near the sidewall, and the animals could easily walk along the brink (Photo 2). At the upper end of the fishway, they could plunge into the water near the critical flow (Photo 3). It is believed that by using their anntenal flagella of rostrum, they could judge whether or not they could plunge into the water.

Fig. 5 shows a change of the numbers of the atyid shrimp Caridina japonica and the Japanese mitten crab migrating upstream via the fishway during a 24-hour period. In Fig. 5, the vertical axis shows the total number that migrated upstream in 1 hour. In the case of the atyid shrimp, upstream migration occurred in the time from sunset to sunrise, and the number of them migrating upstream was at its maximum immediately after sunset [Fig. 5 (a)]. In the case of crabs, as shown in Fig. 5 (b), upstream migration occurred both in the daytime and at night. Also, the number of them migrating upstream was at its maximum immediately before sunset. In addition, it has been confirmed experimentally that the effect of the channel slope on the upstream migration of the atyid shrimp C. japonica and the Japanese mitten crab is small, within the range of 19°≤?≤45°.

The proposed fishway for freshwater shrimps and crabs has been studied, and the results concerning it are summarized as follows:

(1) The fishway for freshwater shrimps and crabs has been proposed on the basis of the characteristics of their migrations and the flow conditions of channels. The fundamental structure of the proposed fishway is that of a stepped channel with a trapezoidal section. A roughness sidewall that can be caught by the tips of the animals' walking legs should be utilized.

(2) The flow at the immediately downstream portion of the fishway has been characterized by a velocity field and air-entrainment distribution. The stepped-channel flow of the proposed fishway is effective in leading freshwater shrimps and crabs to the entrance of the fishway. A low-velocity flow field, in which air entrainment is small, exists near the sidewall of the entrance, and the flow field enables freshwater shrimps and crabs to migrate along the brink of the fishway.

(3) The upstream migration of freshwater shrimps and crabs via the fishway has been observed during a 24-hour period, and changes in their numbers migrating upstream during that time period has been clarified. The upstream migration of freshwater shrimps occurs from sunset to sunrise, and the number of them migrating upstream is at its maximum immediately after sunset. The migration of the crabs occurs both in the daytime and at night, and the number migrating upstream is at its maximum immediately before sunset.

The authors should like to express their grateful thanks to the local public officials in Nagasaki prefectural office and Ohseto town office for their kind cooperation.

[1]  Miya,Y. and Hamano,T. (1988) “The influence of a dam having no fishway on the distribution of decapod crustaceans (in Yukinoura River, Nagasaki, Japan),” Nippon Suisan Gakkaishi (Bulletin of the Japanese Society of Scientific Fisheries), Vol. 54, No. 3, pp. 429-435 (in Japanese with English abstract).

[2]  Hamano,T. and Hayashi,K.-I. (1992) “Ecology of an atyid shrimp Caridina japonica migrating to upstream habitas in the Shiwagi Rivulet, Tokushima Prefecture,” Research on Crustacea, No. 21, pp. 1-13 (in Japanese with English abstract).

[3]  Yasuda,Y. and Ohtsu,I. (1999) “Flow resistance of skimming flows in stepped channels,” Proc. 28th IAHR Congress, Graz, Austria, Session B14 (CD-ROM).

[4]  Yasuda,Y. and Ohtsu,I. (1997) “Characteristics of plunging flows in stepped channel chutes,” International Workshop on Hydraulics of Stepped Spillways, Zurich, Session of Internal flow features, Balkema, Rotterdam, pp. 147-146.

[5]  Ohtsu,I. and Yasuda,Y. (1997) “Characteristics of flow conditions on stepped channels,”Proc. 27th IAHR Congress, San Francisco, USA, pp. 583-588.

[6]  Ohtsu,I., Yasuda,Y., and Takahashi,M. (2001) Discussion of “Onset of skimming flow on stepped spillways”, Journal of Hydraulic Engineering, ASCE (to be published).