As regular readers will know, most fish can climb ladders (well, the ones that follow an upstream migratory path anyway). We've looked at fish ladders in the past, mostly because they're actually quite interesting and the mental image of a fish climbing a ladder is amusing! If you haven't seen it before I highly recommend our post in December last year where we gave some fun examples of the North American autumn Salmon migrations and their ladders. Fish ladders, for the uninitiated, have several types of construction, but the basic premise is a series of obstacles or pools which slow the water flow down and provide a bypass for man-made obstacles on the rivers such as dams.  The fish swim or jump from section to section in order to get upstream past the dam or other blockage.

How to Build Ladders for Fish

Salmon are one of the easiest fish to build ladders for. Many salmon fish ladders exist and it is commonly known what type of ladder they get on best with, as well as the optimum speed for outflow from the ladder to draw them in.  However, not all fish get on with a salmon ladder, because they are different in size, strength and swimming style. On riverswhere there is a lot of different species of fish, conservationists have to construct several ladders or use different types of ladder to serve the different species.  Building a fish ladder involves asking some of the same questions as you would when deciding what (human) ladder to buy: What is it for? How high does it need to reach? Who is going to be using it? Should I get the model with handrails? Where am I going to put it so it's not in the way, but will still get used? In Thompson Falls, Montana, there is a 48-section pool and weir type fish ladder that has been operational since 2010. They have no problem getting most of the species in the river up the ladder, but bull trout are posing a bit of a headache. The top chamber of the Thompson Falls ladder is monitored by scientists. They count the fish, note the species and sometimes attach radio transmitters to the fish.  This is so they can track the upstream journeys and also so they know what use the fish ladder is getting, and how much of an impact is has on the conservation and movements of certain species. Species such as pike and walleye are removed from the top chamber and returned to the river below, as their passage up the river is not necessary for them to spawn, and they can be predatory towards other fish. Last year they saw 2,660 fish come up the 913 foot long ladder and 1,700 the year before.  In all that time, only four of those fish have been bull trout (the rest being made up of suckers, northern pike minnows and a small number of westslope cutthroat trout, a species that conservationists in the area are working hard to protect).

So - Who Can't Climb The Ladder?

The bull trout is also a protected species and has been since 1998. Overfishing of the species by white settlers led to their decline, but they are a good species to have in a river: often their presence is indicative of a clean river system with good connectivity and a diverse biology. After the listing of bull trout as a threatened species, PPL Montana (the company that built and operates the present fish ladder) installed a metal fish ladder, designed to catch and hold the fish so they could be moved over the dam. Damage to the metal ladder and constant repairs meant that a more permanent solution was sought after a few years, leading to the construction of the ladder they have today. Although PPL Montana haven't quite cracked the secret of getting bull trout onto a fish ladder, they are still trying and will keep making improvements to the ladder in order to encourage the threatened species. Research has been carried out on this in the past and is still happening. Given that the Thompson Falls ladder has only been operational since 2010, it is encouraging that they have found any bull trout at all. Now all they need to do is wait for some more and study their habits in order to build the right ladder for them.

And If A Ladder Doesn't Work?

Hopefully, PPL Montana will make progress soon, otherwise they could find themselves in the same situation as people on the Saugatucket river, Rhode Island. Bill McWha, a truck driver, has always been fascinated by the concept of fish returning upstream to spawn and so co-ordinated a group of volunteers for the past three years in order to help river herring get upstream past two dams where they struggle. The volunteers use nets to catch and move fish and so far they have moved more than 50,000 fish already this year. That's much more than the Thompson Falls fish ladder gets through! The reason McWha and his volunteers have to do what they do is due to the inefficiency ofthe fish ladders that are supposed to provide a way around the dams on the Saugatucket. The lower dam has a very weak outflow which stops the fish from even finding the entrance to the dam (as they are drawn to strong currents), but the upper dam has too strong an outflow, meaning even though the fish can find it, they can't physically get into the ladder. The authorities are looking for solutions and new ladders to help overcome these problems, and are also looking at installing a ladder for eels. So we can see that while fish can climb ladders, they need the right one for the job, just like humans do.  If only there were a Midland Ladders for fish ladders, the bull trout and river herring that are struggling at the moment would be much better off!