In February 2018, we analysed the fish communities of the River Frome as part of a research collaboration with Natural England. A total of 15 samples were collected, made up of five samples from each of three locations on the river:
(1) at the mouth of the Frome, where it opens into Poole Harbour,
(2) in the saline transition zone where freshwater mixes with salt water
(3) in the freshwater part of the river, upstream of the tidal limit
For each sample, between 600 and 1250 ml of river water was filtered on site using our manual filtration kits. Preservative solution was added to the filter units and they were posted to the laboratory. We carried out our fish eDNA metabarcoding analysis to amplify and sequence the fish DNA that had been captured on the filters.
The figure below shows the relative sequence abundance of each species in each of the 15 samples (for instance it shows that 44% of sequences from River Frome Estuary 1 originated from bullhead):
In total, 25 freshwater and marine fish species were detected, including protected species like the European eel (Anguilla anguilla) and the invasive sunbleak (Leucaspis delineates).
There are several species that are present in virtually every sample across all three sampling points. These include European bullhead, minnow, stone loach, three-spine stickleback, European eel, brown trout, and Atlantic salmon.
Several marine species are detected only in the estuary samples. These include Atlantic herring, a bream species, golden mullet, goldsinny wrasse, and rock goby. Each of these species was represented by just a small proportion of sequences in a single sample, which suggests that they were only present at the sampling point in small numbers. These species are likely intolerant of low salinity environments.
Bass and flounder were consistently detected in both the estuary and the saline transition zone samples, but were absent from the samples collected above the tidal limit. This indicates that these species are able to inhabit brackish water but were not travelling further up the river into the purely freshwater zone.
Grey mullet were found in all three sampling locations, but they were found in progressively fewer sample replicates as you move upstream from the estuary, which suggests a corresponding decrease in their numbers. Although predominantly marine, grey mullet are known to travel quite long distances inland along rivers. http://britishseafishing.co.uk/mullet-species/
Rainbow trout showed the same pattern as the grey mullet, being most concentrated in the estuary sample. Rainbow trout in the UK are usually from farmed populations but it is interesting that it is showing a similar pattern to the marine species here. There is a subspecies of rainbow trout - the steelhead - that lives most of its life at sea but returns to rivers to spawn. This has been recorded in UK rivers, although very rarely.