¤ 2009 Zoological Society of Japan
ZOOLOGICAL SCIENCE 26: 778–782 (2009)
Behavioral Responses by Migratory Chum Salmon to Amino Acids in Natal Stream Water Yuzo Yamamoto and Hiroshi Ueda* Division of Biosphere Science, Graduate School of Environmental Science and Laboratory of Aquatic Bioresources and Ecosystems, Field Science Center for the Northern Biosphere, Hokkaido University, North 9, West 9, Kita-ku, Sapporo 060-0809, Japan
We propose that amino acids in natal stream water have important roles in Pacific salmon homing. This study hypothesized that amino acids found in natal stream water have a role in the ability of mature male chum salmon (Oncorhynchus keta) to home to the Osaru River (OR), Hokkaido, Japan. Behavioral experiments were conducted in a two-choice test tank using various combinations of control water (natural Toya Lake water; NLW and three artificial stream waters using amino acids: 1) artificial OR water (AOR); 2) AOR without L-glutamic acid, the major amino acid in OR water (AOR-E); and 3) artificial water matching another stream (ALS) that had much higher amino acid concentrations than OR. In behavioral tests, the fish did not select between AOR and AOR-E, but still chose AOR over NLW, AOR-E over NLW, and AOR over ALS. These results suggest that migratory male chum salmon respond to amino acid mixtures in their natal stream water and appear to be affected by multiple amino acids. Key words:
amino acids, homing migration, olfaction, selective behavior, chum salmon
INTRODUCTION After several years of oceanic feeding, mature salmon display an amazing ability to migrate thousands of kilometers to their natal stream for reproduction. It is hypothesized that adult salmon primarily use olfactory cues during this homing migration from the coastal sea to their natal stream. The olfactory function in salmon homing has been reviewed in many behavioral, electrophysiological, and neurobiological studies (Hasler and Sholz, 1983; Stabell, 1992; Nevitt and Dittman, 1998; Quinn, 2005; Ueda et al., 2007). Two different olfactory hypotheses have been proposed: the imprinting hypothesis, developed by Wisby and Hasler (1954) from studies on coho salmon (Oncorhynchus kisutch), and the pheromone hypothesis, developed by Nordeng (1971, 1977) primarily from studies on Arctic char (Salvelinus alpinus). Although the pheromone hypothesis assumes that streamdwelling juvenile salmon release population-specific odors that guide homing adults, there are no stream-dwelling juvenile chum salmon (O. keta) or pink salmon (O. gorbuscha) present when adults return. It is now widely accepted that juvenile Pacific salmon imprint on specific natal stream odorants during their downstream migration, and later use these odors to recognize the natal stream when migrating as adults. Using behavioral analyses, Idler et al. (1961) reported that natal river odors were volatile, heat-labile, neutral, and dialyzable, whereas other studies proposed that the non-volatile fraction was important (Fagerlund et al., 1963; Bodznick, * Corresponding author. Phone: +81-11-706-2598; Fax : +81-11-706-2598; E-mail :
[email protected] doi:10.2108/zsj.26.778
1978). Based on olfactory bulb electro-encephalogram (EEG) recordings, Cooper et al. (1974) reported that the stimulatory portion of natal stream water was non-volatile. Spectral analysis of EEG suggested that natal stream water odorants were absorbed on activated carbon and ion-exchange resins, insoluble in petroleum ether, dialyzable, non-volatile, and heat-stable (Ueda, 1985). Olfactory organs of fish respond only to a limited number of chemicals that dissolve in water, such as L-amino acids, bile acids, sex steroids, prostaglandins, and nucleotides (Hara, 1992). In general, amino acids are potent odorants for fish, and the salmon olfactory organ responds sensitively to various species of amino acids (Hara, 1982, 2005). Shoji et al. (2000) analyzed the compositions of amino acids, inorganic cations, and bile acids in waters from three streams flowing into Lake Toya, and prepared artificial waters containing equivalent amounts of these compounds; the olfactory response of masu salmon (O. masou) to these artificial stream waters closely resembled olfactory responses to the corresponding natural waters. We previously conducted behavioral experiments to determine whether artificial natal stream water matching the amino acid composition of natal streams had attractive effects on chum salmon upstream selective movement, and found that chum salmon selected the artificial natal stream water (Shoji et al., 2003). These results from previous electrophysiological and behavior experiments suggest the possibility that amino acids dissolved in the natal stream water have important roles in the homing migration of Pacific salmon. However, we did not test whether mature salmon can response to changes in the composition and concentration of amino acids in their natal stream water. The purpose of the current study was to investigate the role of amino acids in natal
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stream recognition by testing the responses of chum salmon to various artificial stream waters that varied in amino acid composition.
related substances were either gifts from Ajinomoto Co. or were purchased from Wako Pure Chemical Industries (Tokyo, Japan), Sigma Chemical Co. (St Louis, USA), and Nacalai Tesque (Kyoto, Japan).
MATERIALS AND METHODS
Behavioral experiments Behavioral experiments were carried out in October and November from 2002 to 2006 according to the methods of Shoji et al. (2003), using a two-choice test tank constructed at the Toya Lake Station, Hokkaido University. The experimental tank consisted of two upstream arms (12 m×1.8 m, 0.6 m water depth) in which water flowed at 50 liters/sec, and one pool (3 m×1.8 m, 0.6 m water depth) with one outlet at the end. Before each experiment, 2 to 4 chum salmon (mean=2.97±0.37) were acclimated for 3 hours in the pool, which was covered with boards and fitted with a gate to prevent the fish from entering either arm. After the acclimatization period, one of five combinations of test solutions was introduced into the left or right arm for a 9 hour period: (1) NLW in both arms, (2) AOR and NLW (data from Shoji et al. 2003), (3) AOR and ALS, (4) AOR and AOR-E, and (5) AOR-E and NLW. All test waters were 2,000 times the concentration of their natural counterparts when added at the upper end of each arm but became diluted to the same concentration as the respective natural water at the gate. The experiment was conducted between 1900 h and 0400 h to reduce stress from light (Banks, 1969). The experiment was started by opening the gate about 30 minutes before beginning to introduce the test and control waters, which were switched at each experiment. Fish were allowed to migrate into and out of either of the two arms. The number of entries of the test fish into each arm was counted for 9 hours after opening the gate. Preferences of the test fish for odors were compared by using chi-square tests, with the results for NLW in both arms used to generate expected values. The significance level was set at P
