When early Christians thought of fish, they remembered Jesus. In Greek, the word for fish is ἰχθύς. The Greek letters in that word are an acronym for “Ἰησοῦς Χριστὸς Θεοῦ Υἱὸς Σωτήρ” (Jesus Christ, Son of God, Savior).
During Jesus’s last supper with his students and friends, he used symbols of bread and wine to remind them of important lessons he had taught them before. Breaking bread and pouring wine foreshadowed the heavenly bread that would be broken and the innocent blood that would be poured out the next day.
If we want to have His spirit to be with us, we are to always remember him. There are many other things that can remind us of him. Indeed, if we have eyes to see and ears to hear, all things can remind us of him. Moses 6:63:
63 And behold, all things have their likeness, and all things are created and made to bear record of me, both things which are temporal, and things which are spiritual; things which are in the heavens above, and things which are on the earth, and things which are in the earth, and things which are under the earth, both above and beneath: all things bear record of me.