How do we know salmon behavior?

�T．A microscale study: Behavior of individual salmon studied with data loggers

Yasuaki Takagi

Otsuchi Marine Research Center, Ocean Research Institute, The University of Tokyo

and

Hideji Tanaka，Yasuhiko Naito

National Institute of Polar Research

Chum salmon Oncorhynchus keta is one of the seven species of Pacific salmon. Like other salmonid species, chum salmon exhibit a typical anadromous migration: they are born in the river, migrate to a feeding area in the Bering Sea and the northern North Pacific Ocean and return to their natal river to spawn after four to six years. They are typical cold water fish; their upper lethal temperature does not extend beyond 24℃, and the critical temperature which defines the southern boundary of their feeding area is 10.2℃．Therefore, global warming due to the greenhouse effect of increasing CO2 level in the atmosphere is a potential threat to the salmon life. It is important to clarify the physiological and ecological impacts of high water temperature on the salmon.

Coastal area of Iwate Prefecture, which is located at the central part of Sanriku Coast in northern Japan, is close to the southernmost natural spawning area of Pacific salmon. Returning season of chum salmon in this area is generally September to December. The surface sea water temperature is usually more than 18℃ at the beginning of the season and decreases to about 12℃at the end, which appears to be unsuitably high for chum salmon. Moreover, in addition to the cold water mass or Oyashio, two other warm water masses of Tsugaru Warm Current and Kuroshio Current distribute off the Sanriku Coast complicatedly. How do chum salmon behave in such unsuitably warm water masses? Understanding of the homing behavior of chum salmon around the Sanriku Coast may provide an important insight to estimate the impacts of global warming on the salmon life.

To study salmon behavior in Sanriku Coast, we employed data logger tagging. The data logger tagging enables simultaneous recording of various data，such as depth, temperature, and swimming speed of free ranging individual salmon, over along time period in a broad area. Furthermore, the homing migration of salmon to their natal river promises high recovery of released loggers.

We found that the behavior of chum salmon drastically changed responding to the vertical thermal condition of seawater. In October, when the surface temperature was more than 18℃ and sea water was thermally stratified, they preferred to stay deep water, making long dives to the bottom, but showed frequent ascents to the surface. In December, when the Surface temperature was under 14℃ and the thermally mixed layer was developed, they basically stayed less than 50m deep，making restless descents and ascents. These data indicate that in thermally stratified water with high surface temperature, chum salmon seek the coolest thermal refuge as they can exploit by their vertical movement. Since salmon are poikilothermic, the coolest thermal refuge leads salmon to minimize the metabolic energy cost．Thus the salmon regulate their body temperature, and hence the metabolic energy consumption rate, through seeking the coolest thermal refuge.

Then, why do salmon prefer shallow water in December? Why do they come back to shallow water even the temperature is unsuitably ”hot” for them in October? It is generally accepted that salmon use olfactory cues to find their natal river. Since the river water is 1ighter than sea water, it distributes near the surface. Probably salmon are searching the odor of their natal river in the shallow water even if it is ”hot”. The minute to minute record by the logger allowed us to analyze the behavior of chum salmon in the surface water in detail, and we found the evidence which indicated that salmon were smelling the river water. In October, data loggers recorded the cool water masses that covered warmer sea surface in patches. These cool water masses are the water draining from the river system, since river water at this season is cooler than sea water. Salmon stopped their vertical movement for minutes in such cool water masses.

As described above, chum salmon reduce their metabolic energy consumption by making deep dives in thermally stratified water. We closely analyzed the diving behavior of salmon with newly developed loggers which can record swimming speed, depth, temperature, and acceleration in two directions, parallel and perpendicular to the body axis. Acceleration parallel to the body axis is a good indicator of an angle of fish body and the other is that of swimming activity. Our study revealed that chum salmon swam much more actively when they were ascending than descending, where as swimming speed was similar. The angle of fish body was more acute when they were descending. These data suggest that the density of the salmon is equivalent to that of sea water in the surface layer, but becomes larger in the deeper layer.

Application of data logger tagging to the salmon made us possible to know their minute to minute behavior. The detailed data on the response of homing chum salmon against the “hot” water masses are now accumulating. However, the study site is restricted to the coastal area and the study subject to homing adults at present. Further studies on the salmon at different life stages, at broader area, and in the different sea water condition are indispensable.