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  • 1. If the seawater density difference between depths of 5 m and 35 m is small (i.e., the inflow of river water into Mutsu Bay is small),

    2. then vertical mixing in winter is promoted,

    3. the number of food organisms increases,

    4. more food is eaten,

    5. and nutrients are accumulated between the periods of body lengths of 6 mm and 10 mm, and they become fat.

    These could be proved statistically.

    However, under low water temperatures:

    6. the digestion rate slows down,

    7. the food intake increases and the energy intake decreases,

    but the effect of (3) (absolute values of β = 0.75 and 0.66) is greater than the effect of (6) (absolute values of β = −0.45 and −0.56[A1] ),

    so it was statistically shown that the water temperature environment of Mutsu Bay was not as strong an effect for decreasing (5) body fat amount (can be calculated as an effect of 0.60 times (= 0.45 ÷ 0.75) to 0.85 times (= 0.56 ÷ 0.66).