The second image was drawn by a 9 year old boy to commemorate a specific event, a rite of passage; that of the catching, killing and eating a wild creature and the important lessons on conservation and respect for the natural world that that implies.
Different raison d'être as I said, but the two are similar in quite a crucial way. Both represent an important aspect of the drawn depiction of anything which is often forgotten; that of replacing the verbal aspect of description.
What follows is a piece written in the 1930's regarding the protocols of scientific illustration - a 'how to do it' publication, printed during a period when newspapers and magazines carried a lot of adverts for correspondence courses in drawing and illustration..."make money as an artist" :
"It would be impossible to present a clear understanding of the physical characteristics of a bird or other creature based alone on measurements, colour, and the usual diagnostic characters, or to describe adequately the form and details of many organisms, without illustrations. Both the writer and the reader would be burdened with undue descriptive matter. Illustrations, if properly prepared, relieve that condition...
Such illustrations make the text less open to dispute and thus serve the double purpose of depiction and corroboration".
The 18th Century trout obviously fulfils these criteria - the author does not have to burden the reader with longwinded descriptive passages, you can see what a trout looks like, allowing the author to concentrate on other aspects of a trouts habits. Now, book in hand, there is no doubt that a fish pulled from a river which resembles the engraved image is a trout; it cannot be anything but.
The childs' drawing fulfils quite a different purpose, one of commemoration, but also deals with replacement of verbal or written description, especially when the child is probably unable to verbalise all the emotions and physical sensations involved in catching and killing a living creature. But you can see the blood, not only in the gaping mouth, but also oozing from the gills of the trout, there's a sense of the rigor mortis which every fisherman would recognise in a recently killed fish and you can see the slightly bulging eye, again, common in a newly caught fish. Descriptive and accurate, yes, but also corroborative - we took photographs but this is the image which truly captures and corroborates the event.
And as with the engraving, if you chanced upon an unknown fish on the riverbank and it had red spots on it's flank and you could refer to the child's drawing, you would be in little doubt that the fish you had found was...a Brown Trout.