Culverts Often Present Problems for Fish
Nat Gillespie, fisheries scientist with Trout Unlimited, recently wrote an article on the threat culverts pose on survival of brook trout throughout their native range. Hundreds of thousands of culverts have the potential to block fish movement because they’re oftentimes poorly placed and alter the natural flow regime of a stream.
While suitable habitat may exist both above and below a culvert, and many fish populations thrive despite such a barrier, movement plays an important part in the long term persistence of healthy fish populations. Free movement along the length of a stream system allows fish to recolonize areas where populations may have been reduced or are threatened by human impacts or environmental factors. Fish that move upstream into headwaters to spawn, such as anadromous salmon, have their habitat significantly reduced when a new culvert presents an additional barrier.
Many biologists and members of the public are just beginning to realize the importance of fish passage through culverts, and this is shown in the push to install more fish-friendly culverts, or bridges as an alternative.
I find this funny…anybody who had ever ventured out into the woods of Maine. Will tell you that culverts and wild brookies go together like bread and butter. The deep pools usually found downstream from them hold more fish than any other hole on the brook. They often provide ideal holding water for trout all year long. The only thing better is a deep beaver pond. The culverts themselves prevent erosion and silting of these minor waters and in fact preserve the fishery. This article is way off in it’s basis that culverts harm fisheries. As long as man makes roads.. he had better protect the waters with culverts…or be prepared to kill another brook through silting.
September 25th, 2007 at 7:14 am
There is no question that culverts, especially hanging culverts, create pool habitat for fish on the downstream side of the culvert. Fishermen (including myself) often love culverts because they create a nice pool to fish right near the road that holds lots of fish. However, I would argue (and experts have agreed) that the one pool that’s gained as a result of culvert installation doesn’t outweigh the losses to the rest of the stream as a result of the culvert, especially if you think long term.
It’s a well documented fact that culverts can prevent fish passage. This is fine in some cases, but in others, fish need to move upstream to find suitable spawning habitat, and therefore the culvert has just divided a stream into two smaller segments, making it harder for fish populations to recolonize upstream in the case of a depressed population or catastrophic event.
Culverts do not prevent erosion. This argument has absolutely no scientific or logical basis. The only way culverts would prevent erosion would be in a case where no stream crossing structure existed previously and water was just running off without any drainage. Many would argue that culverts increase erosion, epsecially in relation to other stream crossing structures. Proper drainage ditches and sediment catch pools prevent erosion.
Culverts not only impede fish movement, they can also stop the movement of woody debris, which is incredibly important to a stream’s ecosystem because it provides direct staging habitat and cover for trout, and is an important food source for aquatic invertebrates, which usually represent about 90 percent of a trout’s diet during most of the year.
Many other stream functions require connectivity which can be disrupted by culverts. There are a number of great alternatives to culverts at stream crossings, including bridges and half-culverts, both of which allow the stream to function properly and with less disruption.
While fish like to hang out in these roadside culvert pools, they don’t need them, and experience has shown that undercut banks and large debris piles providing complex habitat and cover provide much more benefit to fish populations than a culvert pool.
Here are some guidelines for road passage developed by the Washington Department of Fish and Wildlife in 2003:
Guiding Principles for Water Crossings:
1. Culverts result in permanent, direct loss
of instream and riparian habitat.
2. Installation and maintenance of water crossings
that confine or constrict the channel or floodplain
will break ecological connectivity, alter channel
processes and change adjacent channel character
and shape by affecting the movement of debris,
sediment, channel migration, flood waters,
and aquatic and terrestrial organisms.
3. Water crossings may create an entry point
for road-runoff pollutants.
4. Fish passage can be hindered or blocked
at water crossings.
5. Water crossings increase the risk of damage to the
downstream habitat due to water crossing failure.
6. Cumulative impacts and risks of water crossings
can be avoided or minimized by consolidating
water crossings; employing full-span bridges,
by simulating a natural channel through culverts;
or removing water crossings. Access solutions
that do not require water crossings are preferred.
That being said, culvert size and the way culverts are constructed and installed can greatly affect their functionality and how they influence fish and stream habitat. I think that improvements in culvert placement and installation can be very effective. See the following link for the Idaho Dept. of Fish and Game policy on culvert installations:
https://research.idfg.idaho.gov/Fisheries%20Research%20Reports/Volume%20060_Article%2009.pdf
September 25th, 2007 at 10:14 am
I willl dispute the “erosion” issue by simply saying that nobody crosses a brook by “driving through it” up here as long as they have a culvert to pass over. The lack of the culvert causes the banks to get eaten away and the silting has killed many a trout. But hey…. in your view the pipe is to blame for the lesser of the evils..outweighing it’s true benefit to road and trail. What do you propose to use intead of the pipe…..or should we look into levitation.
September 25th, 2007 at 2:43 pm
I think a bridge or half-culvert would be much better in most cases. The stream bottom is less disrupted, less fill (which can erode away) is required, and you don’t have the problem with blocked fish passage from a hanging culvert.
Having a culvert is certainly better than nothing at all, but I believe there are better alternatives to just sticking a pipe in and filling around it.
September 25th, 2007 at 5:31 pm