The unintentional but tragic loss of freshwater fish during water diversions - otherwise known as fish entrainment - has become a big problem in many parts of the world - particularly in the Western U.S.
The Trout Conservancy based in Missoula, Montana recently released an interesting and useful report titled Trout Entrainment in Montana: Status and Solutions. While the report specifically addresses the trout entrainment problems in the Big Sky state, the issues and recomendations discussed will likely apply to other fish in other geographic settings.
CM: Can you describe how fish entrainment specifically occurs?
JZ: Fish entrainment is a pretty simple process. In essence, whenever people divert stream, river, pond or lake water, the opportunity is created for fish to follow the flow of diverted water and become entrained. In Montana, what generally happens is that trout (and other fish) move into irrigation systems; when irrigation ceases in the fall, fish are stranded and die.
CM: How big of a problem is trout entrainment in Montana and what are the ecological consequences?
JZ: The entrainment problem in Montana is both substantial and complicated. Very few diversions have been studied to quantify seasonal entrainment problems, but, of those that have, analysis indicates several thousand trout (and tens of thousands of other fish species) per season are entrained by each diversion, and there are almost 180 thousand known diversions in Montana.
The math indicates that hundreds of thousands of trout (and much larger numbers of other fish species) are entrained each year and consequently lost to reproducing populations.
The complications arise from having big introduced populations of trout (mostly rainbows, browns and brook trout) in Montana watersheds for over a century. These non-natives now are self-reproducing (as opposed to being regularly stocked), and apparently suffer little harm at the population level from entrainment, despite the large annual losses, as is indicated by regular population surveys.
Many of the wild trout entrained annually in Montana are young fish, and normal mortality among this age group is so high as to often completely mask the effects of entrainment - at least among non-native populations.
Populations of native trout species (Yellowstone and westslope cutthroat trout, bull trout, and arctic grayling), on the other hand, are in decline, and these losses (especially for migratory populations) are often considered to be a big problem by fisheries managers. Entrainment in these situations is generally mitigated by approaches like fish screens.
CM: What is the goal of your report with respect to the trout entrainment issue in Montana and the larger issue of fish entrainment in general?
JZ: The goal of the report is to help prevent the needless loss of wild trout in Montana (regardless of whether they are native or introduced populations) to irrigation diversions.
Wild trout only exist in certain regions of the United States, and as such possess intrinsic value. Economically, wild trout are an important attraction in Montana, generating hundreds of millions of dollars in tourism and fishing activity by virtue of their presence.
From these perspectives, it is wasteful and foolish to allow large numbers of wild trout to be annually lost to entrainment in Montana, and, in the case of native species, wantonly destructive to the point of causing extinction.
CM: Can you outline the different sections of the report?
JZ: The first section describes the how's and why's of trout entrainment in Montana - including the influences of trout biology, water law, climate change and the design of diversions.
The second section describes (and illustrates) different types and designs of fish screens (the most common remedy for preventing entrainment).
The third section identifies some known entraining diversion sites in Montana.
The fourth and final section describes some funding sources available for fish screens and other remedies for those interested in preventing entrainment in Montana (funding assistance is often key, because fish screens generally cost $10,000 or more for every cubic foot per second of water diverted).
CM: preventing fish entrainment. How effective are screens in addressing the issue and what are some considerations that managers should keep in mind when using them?
JZ: Fish screens (structures which screen fish, but not water, from entering diverted flows) have been demonstrated to be highly effective in preventing entrainment for trout and salmon - provided that the correct design is used, and that regular maintenance is done to ensure proper screen function.
Maintenance is a key issue - unless a regular and routine long-term maintenance program is part of any installation, fish screens should not be used. In all cases, only an experienced professional should determine the type of fish screen, the installation and use, and the maintenance required.
Also, fish screens are generally the remedy of last resort for entrainment, even in states with well-funded screening programs (such as Idaho), owing to the problematic issues of fish screen operations and the lasting benefits of (and often far less complicated) approaches like retiring or reducing diversions.
CM: To what extent is the report applicable to entrainment issues with other types of fish in other geographic areas?
JZ: Entrainment affects all species of fish wherever surface flows are diverted for human use.
A prime example are delta smelt, which have declined precipitously in the Sacramento River Delta of California due to enormous intra-state diversions for agricultural, industrial and domestic human uses. Hundreds of millions of taxpayer dollars have been spent to mitigate entrainment impacts to delta smelt, with questionable success.
In other regions of the west coast of the United States, vast amounts of money have been spent to reduce entrainment impacts to anadromous salmon and steelhead stocks - again with questionable results.
Entrainment remedies like fish screens are not the sole answer to the survival problems faced by many fish species, and big entrainment programs in places like the Pacific region states are largely supported by federal subsidy (which is not necessarily a secure thing).
However, we still advocate fish screening as preferable to the unfettered diversion of surface waters.
--by Rob Goldstein