sarvatrāsya yato mṛtyur
matsyānām udake yathā
The material body, whether in the higher or lower planetary system, is destined to die. In the lower planetary system or lower species of life one may die soon, and in the higher planets or higher species one may live for a long, long time, but death is inevitable. This fact should be understood. In the human form of life one should take the opportunity to put an end to birth, death, old age and disease by performing tapasya. This is the aim of human civilization: to stop the repetition of birth and death, which is called mṛtyu-saṁsāra-vartmani. This can be done only when one is Kṛṣṇa conscious, or has achieved the service of the lotus feet of the Lord. Otherwise one must rot in this material world and accept a material body subject to birth, death, old age and disease.
The example given here is that water is a very nice place for a fish, but the fish is never free from anxiety about death, since big fish are always eager to eat the small fish. Phalgūni tatra mahatām: all living entities are eaten by bigger living entities. This is the way of material nature.
phalgūni tatra mahatāṁ
jīvo jīvasya jīvanam
“Those who are devoid of hands are prey for those who have hands; those devoid of legs are prey for the four-legged. The weak are the subsistence of the strong, and the general rule holds that one living being is food for another.” (Bhāg. 1.13.47) The Supreme Personality of Godhead has created the material world in such a way that one living entity is food for another. Thus there is a struggle for existence, but although we speak of survival of the fittest, no one can escape death without becoming a devotee of the Lord. Hariṁ vinā naiva sṛtiṁ taranti: one cannot escape the cycle of birth and death without becoming a devotee. This is also confirmed in Bhagavad-gītā (9.3). Aprāpya māṁ nivartante mṛtyu-saṁsāra-vartmani. One who does not attain shelter at the lotus feet of Kṛṣṇa must certainly wander up and down within the cycle of birth and death.