Whenever nations need to signal peace, they traditionally release doves into the air. When India and Pakistan need to do so, they release human beings. Human beings who they needn't have been in jail in the first place. These human beings are fishermen who have violated an invisible international boundary in the high seas. They violate it often, both Indian and Pakistani fishermen. They get arrested. When India and Pakistan signal peace – the resumption of dialogue or a high-level meeting or a new agreement – they are released like doves.

Almost nobody in either country cares about them. They live both literally and metaphorically on the margins of the nation. They are poor and low caste. They are blamed for violating maritime boundaries. They don't get the sort of sympathy and outrage that we reserve for detained spies or dead soldiers.

When Narendra Modi invited Pakistani Prime Minister Nawaz Sharif to witness his swearing-in, Sharif said he'd take his time to decide (meaning that he'd ask the Pakistani army for permission). But even before he knew he was going to India, he ordered the release of 151 Indian prisoners. Only one was a man who had accidentally strayed across to the Sindh border. The other 150 were all fishermen. Ninety-two were from Junagadh, 10 from Valsad, five from Jamnagar, one from Navsari and the rest from Diu and Daman, Maharashtra and some boat crew hailing from Uttar Pradesh and West Bengal.

Before they crossed the Wagah border on Monday, one of them, Nanji Soma, told Pakistani journalists that he would go back to fishing even if this means that he will be arrested again. What else could he do? He has to pay back his friends who made sure his ten-year-old son went to school even as his father was in a Pakistani jail. Given that his son is getting an education, Soma promised Pakistani journalists, he wouldn't become a fisherman.

These 151 Indians took a 24-hour bus journey to Lahore and crossed the Wagah border. By the time Nawaz Sharif shook hands with Narendra Modi, they would not even have had reached their homes in Gujarat, Modi's home state. Why couldn't they have been put in their boats in Karachi and sent back they way they had come? That's because Pakistan confiscates their boats, their source of livelihood. These mid-size fishing trawlers cost as much as Rs 50 lakh. There are around 850 Indian boats in the custody of Pakistan and 190 Pakistani boats in Indian custody.

When an Indian fisherman is in a Pakistani jail, the Gujarat government pays his family a princely compensation of Rs 150 a day. Despite such a low compensation the Gujarat government has paid out Rs 4.93 crores since 2004. A total of 4,682 Gujarati fishermen have been released till date, according to the Gujarat government.

There are many reasons the fishermen fail to observe the international boundary. They do have GPS devices, at least the ones one in the larger trawlers, but sometimes these fail, or there's bad weather. There's also the problem of Sir Creek, where India and Pakistan are yet to agree upon where exactly the maritime boundary lies.

What happens most often, though, is that the fishermen follow the fish, and take a risk with the maritime boundary. By doing so, they assert their own history over the political geography of two nation-states less than seven decades old. Indian and Pakistani fishermen both say they have no problem with colleagues from the other side coming in and fishing. It's the maritime security forces, valiantly saving one's own fish from the enemy.

More Indian fishermen get caught than Pakistani ones. That's because there is more fish on the Pakistani side, and it's of better quality too. That's because of the delta of the Indus river is on that side, but also because of industrial pollution on this side.

After fishermen are arrested, neither India nor Pakistan devotes much attention to their wellbeing, nor does anything to secure their release. They continue to languish in jail even after serving their sentences, sometimes for years after their terms have lapsed. Under an agreement to recognise prisoners as human beings, the governments of Pakistan and India exchange lists of prisoners only biannually, so their families don't receive official word about their arrests for months. As of January 1, 2014, India had 396 Pakistani prisoners, of whom 139 were fishermen. Pakistan had 281 Indian prisoners, of whom 232 were fishermen.

In Gujarat, there are 32 Pakistani fishermen who have completed their sentences but have not been released and repatriated as their nationality has not yet been verified. They will be lucky if their identification can take place while they are still breathing. If an imprisoned fisherman dies, the governments of these great countries show no urgency in getting their dead bodies home. Among the few people who write letters and petitions to merely bring home the bodies of dead fishermen is Mumbai-based Jatin Desai of the Pakistan-India People's Forum for Peace and Democracy.

Desai tells me of some recent cases of dead imprisoned fishermen. Bhikha Lakha Shival, 35, hailing from Junagarh district, died in Karachi on December 19, 2013. His body came to India on February 15, 2014, nearly two months later. Ramjibhai Vala died in Karachi in 2012 and his body was sent to India after 45 days. Nawaz Ali died in Ahmedabad in 2013; his body reached Pakistan after 25 days. These are just a few of many such names.

Still, it isn't clear why both India and Pakistan expend so much energy and spend so many resources on detaining fishermen. Once it has been ascertained they are not terrorists but merely fishermen who have strayed past an international boundary, why not just turn them back? Is there an answer to this other than fish nationalism? The answer is that India and Pakistan need symbols when they need to signal a thaw in relations. They catch fishermen only so that they are able to look magnanimous at the appropriate time.