Have you ever heard of a prawn conveyor belt? Read on to find out more!
Over the past few weeks, we have learned about what causes tides, what affects the strength of the tides and why the River Mersey’s tides are so unique. In this blog we’re going to take a look into how the thousands of animals that call the Mersey their home are able to live in such a dynamic and powerful environment.
During the early 1980’s the River Mersey was considered one of the most polluted rivers in the UK. Following extensive work by the Mersey Basin campaign, the river slowly started to recover and we now have over 60 species of fish living in the river as well as porpoises, dolphins, seals and octopus! But how do these animals deal with the intense flows and muddy environment that is so characteristic of the river?
Go with the flow
For animals living in estuaries things very rarely stay the same, rapid changes in salinity, flow speed, light levels, temperature and access to feeding areas can change literally minute by minute. This means that many of our estuarine animals need to move a lot throughout the day as the tides ebb and flow. It’s this flow that many aquatic animals use to find food and conserve energy.
Many species use a method called selective tidal stream transport. This is a process where if an individual wants to move quicky and a long way, they will move up in the water column to where the faster water is. If the animal wants to move less or stay in the same place they will drop to the river bed where the flow is much slower. The water speed is slower closer to the riverbed as the friction of the water passing over the riverbed helps to slow the current down. This means in some cases it’s possible to migrate upstream or downstream using very little energy by simply rising up or staying close to the riverbed. Many species have been recorded using this conveyor belt method (I prefer to see it as an escalator) including eels, flatfish (flounder), cod, crabs, shrimps and prawns (figure 1)!
For some of the species that we find in the upper estuary, the moon jellyfish (figure 2), the strength of the current is so strong that they just cannot swim again it, this means they are being constantly washed up and down stream throughout the day.
Figure 1: Estuarine prawn caught from the conveyor belt of a flooding tide under the jubilee bridge
Figure 2: Moon jellies use the tides to move up and down the river. They ae not strong ehough to swim against the currents so are constantly being washed up and down the estuary.
During a study in 2024, MSc student Sophie found that there are certain times during a tide where large numbers of invertebrates are present at a location, this is likey down to the shrimps using the tide to move upstream to access feeding opportunities. When sampling outside of these key windows there were very few animals caught in the plankton nets.
Fish
Some species of fish, like flounder and dab, are perfectly adapted to life in fast moving estuaries. Their flat bodies help them stick to the bottom and when the tide comes in they are often the first to re-enter the rockpools and channels as their flat bodies allow them to swim in very shallow water! Keep an eye out for flounder and dab around Spike and Wigg island as they hunt in the shallow pools!
Figure 3: Flounder are perfectly adapted to life in estuaries. We caught this one during some beam trawling surveys and was returned to the river after it had been measured.
Figure 4: Oystercatcher on the sandflats with a freshly caught lugworm
Birds
For many of the wading birds we find in the estuary, the ebbing tide is the bet time to see them. As the water recedes from the mud and sand, aquatic invertebrates like brown shrimp and mysis shrimp become trapped in the small creeks and pools. You can often see many birds squabbling around these pools as they try to catch their lunch! Burrowing worms such as lugworm and ragworm are also closer to the surface when the mud is still wet so you can see species like oystercatchers and curlew probing for them in the deep mud. As the tide comes in the birds are pushed off the exposed mud and sandflats and move inland to rest and wait for the tide to start dropping again. If you are very lucky you may even see an osprey catching mullet at low tide!
Mammals
The best time to see two of our most iconic species, the harbour porpoise and the grey seal, is the first couple of hours of a flooding or ebbing tide – this is when the prey items (flatfish and eels) first start to use the current to move up or down stream. The porpoises and seals follow the fish upstream as far as Howley weir! At low tide you can sometimes see the grey seals hauled out as they wait for the next tide to either bring food to them or allow them to use the flow to move around the river.
We are currently trying to monitor the Mersey marine mammal population to try and better understand how frequently these amazing animals use the river. Please submit the date, time, and location of any marine mammal sightings you have to us as MGET or to your local record centre.
Figure 5: Harbour porpoises use the strong flows to help them hunt. Porpoises are very strong swimmer and are able to chase their prey down in the fast moving currents
I hope the past few blogs about the Mersey tides have been informative, Hannah and I have really enjoyed writing them. For anyone wanting to see any of our amazing wildlife I would highly recommend visiting either Wigg Island or Spike Island at low water. You can find the tide times for both of these places using the Tide Times app or website.
Please continue to send your amazing photographs to us and keep an eye out on our social media pages for updates on our community science plankton survey days over the summer months! Any one of any age can come along to help and learn more about our amazing estuary animals!
Andrew Wolfenden, Biodiversity Manager