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Fisheries Behavioral Ecology Program - Newport Laboratory

Light, Temperature, and Food Control the Vertical Distribution of Juvenile Pacific Cod

Figure 8. experimental thermocline sea water tank
Figure 8.  An experimental thermocline sea water tank (2.2 m depth, 2.2 m wide, 1.1 m across) containing five juvenile Pacific cod. Note fish swimming downward at right side of the tank. (Arrows point to fish.) White feeding tube extends to bottom third of the tank.  Click photo to enlarge.


Understanding the factors that control vertical distribution of commercially important fishes is essential for effective fisheries management. Vertical distribution of juveniles impacts survey accuracy, avoidance of bycatch, evaluation of habitat and growth conditions for recruitment, and development of accurate models for juvenile distribution and growth. Pacific cod is a species of increasing importance in north Pacific fisheries.

Groups of five similarly sized Pacific cod juveniles that were 0+, 1+, or 2+ years old and between 7 and 28 cm in length were observed during lighted conditions in an experimental sea water tank. Thermoclines were constructed in the tank and light and food conditions controlled (Fig. 8). Fish responses to low and high light, warm and cold temperature, and food were observed in a series of choice tests. Fish were tested in isothermal conditions (9C) and in thermocline conditions (9C in top third, 9-3C in the middle third, and 3C in bottom third of tank). Food was introduced through a tube that extended into the tank.

Figure 9, click to enlarge
Figure 9.  Click image to enlarge.
  Figure 10 chart, see caption
Figure 10.  Bar graph showing juvenile Pacific cod feeding behavior in low light comparing isothermal and thermocline condidtions.  Points and bars indicate number of 0+, 1+, and 2+ year old fish feeding (+) 1 standard error.
  Figure 11, click to enlarge
Figure 11.  Click image to enlarge.

Pacific cod in low light avoided deeper, colder water (3C) in a thermocline (Fig. 9). When food was introduced into the bottom third of the tank, fish swam lower in warmer water (9C, isothermal), grouped more, and fed; while fish avoided deeper, colder water in a thermocline. Fish in low light thermocline conditions generally had a difficult time finding food and feeding (Fig. 10 above). Pacific cod in isothermal conditions (9C) and low light initially moved deeper in high light (on for 1, 30, and 60 min) and then returned to shallower depths after 30 and 60 minutes in high light (Fig. 11). When a thermocline was present, fish moved deeper in response to high light, but not into colder water. Fish grouped more in response to high light and then dispersed after 60 minutes.

The results of this study suggests several field-testable hypotheses: Juvenile Pacific cod remain in warmer water above thermoclines; they form groups in the presence of food or bright light stimuli; and they avoid higher light and colder temperatures, but can adapt to these conditions rapidly, suggesting that they could make excursions into areas where light and temperature are not optimal, but food is present or predators are absent.

By Michael Davis

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