PhD candidate Didi van Trijp researches: When is fish fish?

Bird, butterfly, fish: when you look through a children’s book, you usually don’t think about the fact that humans divided these animals, depicted in bright colours, into categories. Yet, this division has been discussed for centuries. In her PhD dissertation, Didi van Trijp shows how natural scientists looked at definitions in the eighteenth century. When is a fish a fish? Promotion on 28 September.

‘In sixteenth-century Northern Europe, the word fish was used for everything that lived in water, so also for jellyfish, lobsters, otters and hippos,’ says Van Trijp. ‘This happened in popular speech, but also in more scientific texts, such as in the thick books in which natural scientists described the species that they encountered. At the end of the seventeenth century, a more restrictive definition of fish, animals with fins that live in water and never come out, appeared for the first time. As a result, several other animals were no longer fish.’

Complex object of study

This more narrow definition came about during a period in which nature was considered the ‘second book of God’. Natural scientists ‘read’ that book by very carefully studying the world around them. By categorising animal species based on their physical characteristics, they mapped every species in the Creation. This way, they wanted to understand how God intended the world. For natural scientists that studied fish, this wish added additional challenges. There are plenty of fish available in Northern Europe, but they were difficult to study.

‘During that time, it was near impossible to look at fish under water,’ Van Trijp explains. ‘But when you take fish out of the water and they die, their colours, and sometimes their shapes, change. In addition, fish spoils easily. This is what people continuously complain about in letters that I have found. A collector in Berlin, for example, had a missionary in India collect fish for him, but sometimes this went wrong. He would then receive a barrel full of spoiled fish.’

Who tells the true story?

Yet, a lot of researchers persisted. ‘Self-interest played a part in this. By portraying themselves as trustworthy researchers, they claimed their spot in the scientific field,’ argues the PhD candidate. ‘In every book about fish that I researched, authors strongly mention that they rely on their own observations, or on those of people that they trust. They also mention that they cannot simply rely on illustrations, because it is not always clear how trustworthy the choices of the illustrator were. Did they, for example, properly depict every detail?’

Van Trijp considers this dependence on others to be one of the surprises of her research: ‘On the cover of such books, only the name of the researcher is mentioned, but I found it to be striking how many people were involved in the creation of a scientific work about fish. Someone created the illustrations, someone took care of transport, and someone caught the fish.’ Researchers regularly got their information from that last group. ‘Because of their profession, fishermen could often tell whether a fish was rare, and whether a particular specimen was representative of its species.’

Here, the question of who exactly is trustworthy regularly comes up. Van Trijp: ‘Do the fishermen look at things the ‘right’ way? And is it better to ask an older, experienced fisherman for information instead of a younger one? We can see that researchers struggled with the question of how to create a trustworthy story when you have to rely on the observations of others. Very interesting, also because this continues to be an important theme in science today.’

But what exactly is a fish?

And the debate about what a fish is exactly? It continues to this day. Even though a toddler will be perfectly capable of distinguishing a fish from a bird or otter, the term ‘fish’ has no biological meaning anymore for current scientists; a salmon is genetically more related to a camel than to a lamprey. Van Trijp: ‘Scientist will probably always keep questioning categories.’

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