Here is an example of taking a track from an historical paper and modelling it. This means that new information can be derived from old published data. So, we take a modern hydrodynamic model of the area where the tag was deployed, we set it up to replicate the tide on that day many years before, and attempt to replicate the track. When this is done we can decompose the track into two components – tide and directed swimming. Thus we can estimate not only the speed that the fish was swimming, but also the direction in which it was swimming. This is new information out of the old track. The fidelity of the water model and its dynamic nature allow us to reliably extract the new information from the old track. Real and interesting insights can be drawn from this: was the fish swimming in a pre-determined direction, or was the fish following some kind of cue, a scent trail for instance? Was the fish swimming at a constant speed, or did it accelerate or decelerate (perhaps related to water depth), and so forth?

Here is an example we used from a track recorded by Andy Moore in his 1997 technical report “The movements of Atlantic Salmon (Salmo salar L.) and Sea Trout (Salmo trutta L.) Smolts in the impounded estuary of the R. Tawe, South Wales.” Andy very kindly discussed this research at length and I have great admiration for the scientific quality of the work but also the sheer physical effort required to follow and track the fish over many hours. The fish is about 15 cm long and the tag had been inserted into its peritoneal cavity (the tag was about the size of half a baked bean). This work was done as part of the fisheries impact consultancy for the Swansea Tidal Lagoon project.

Here is the original figure showing the track in Moore 1997.

Then, after some analysis and so forth (do contact me if you’d like to know all the details – they were also included in the EIA for the Swansea Tidal Lagoon), anyhow, after analysis we can include this track in our model of smolt leaving the Tawe. In this video, the smolt (little black dots) all leave the Tawe around high tide and swim off out into the estuary. Those that left around the same time as the tagged fish are coloured red, and the tag track and tagged fish are shown in grey and green. The red crosses are the ‘waypoints’ on the scent trail that the model fish are following – these are derived from the same water model (run beforehand to model a passive olfactory substance emanating from the river). So clearly the combined model shows that the tagged fish track could have been produced by the model. Therefore, we hypothesise that the model can be used to make tracks in circumstances that do not exist now, but may exist in the future – i.e. a tidal lagoon! Brilliant.