Fisheries biologists report trout population declines while anglers tout great days on the water - How can both be true?

In Spring 2023, fisheries biologists and managers in Southwest Montana released alarming data for wild trout populations in Southwest Montana rivers, including the Jefferson Basin. FWP data revealed unprecedented, historically low population numbers and evidence that populations have been declining for several years. This alarming trend and historic population lows were reiterated in this year’s statewide fisheries management plan.

Nevertheless, as skilled anglers continue to catch plenty of fish, including big fish, a disconnect could be forming between fishery managers and anglers, hindering necessary management decisions needed to restore these world class cold-water fisheries.

Personal experience can offer a compelling but subjective perspective, and sometimes personal experience is contrary to empirical data examining a fishery at the population and watershed scale. As disconnects between fishery science and narratives of successful days on the river grow deeper, some within the fishing community bemoan management decisions that would restrict fishing opportunities, which in their eyes is overzealous, or even unsupported by data. While anglers do retain a wealth of practical knowledge earned by extensive amounts of time on the water, and fishery biologists may only spend a few days a year sampling a population, making management decisions for a fishery based on anecdotal catch rates - as opposed to scientific data and empirical population projections - is a recipe for disaster. 

So, how can trout fishing be good and fish continue to be caught in robust numbers while a population is simultaneously declining? 

This phenomenon is not uncommon, and ample scientific research performed under a variety of conditions with various species reveals that catch rates are often misleading. Research confirms that catch per unit effort (CPUE) can remain sufficiently high and stable long enough to forge a false narrative, until fish populations suddenly collapse. At times, CPUE can be incorrectly perceived as a proxy for overall population status, which causes angler optimism, and a stubborn bias and resistance to negative, yet scientifically-defensible, findings and management decisions. A more defensible approach is to deduce the health of populations by region and fishery by fishery using scientific assessments, which collate all sorts of data – from the results of ecological surveys, to the catch per fishing effort, and the age and size distributions of the fish caught. (In Montana, FWP does not use CPUE in assessing fishery conditions.)

In situations where CPUE remains consistently high during population instability and/or downturns, a condition known as stock hyperstability occurs. This phenomenon certainly could be the case on certain rivers or river stretches in Southwest Montana. Under stock hyperstability situations, catch rates remain sufficiently stable because fish such as trout prefer particular habitats, say in tailwaters below dams or in yet-to-be degraded areas, and replacement of individuals within those habitats continually occurs despite broadscale declines, until sudden upset in the system is detected. Hyperstability at prime fishing locations continues to attract anglers, but the continued fishing success and high catch rates ultimately lead to deceptive perceptions of population status by anglers. 

The danger of this dilemma lies in ignoring robust scientific study and experts, and utilizing CPUE as a proxy for population abundance, or population sustainability. Consequently, without a full understanding of the range conditions and broadscale status trends, as outlined in Montana FWP’s annual statewide fisheries management plan, angler perceptions can be understandably skewed into believing robust stock conditions exist. Another way to perceive the phenomenon is to consider an hourglass, where the thin waist of the hourglass represents the perspective of the anglers and the CPUE represents the grains of sand. The CPUE will remain constant as do the grains of sand passing through the bottleneck, until the sand is depleted and time has run out. No doubt that catch and release with safe handling practices can perpetuate the anecdotal CPUE phenomenon. 

While anecdotal fishing reports certainly have value for trout population managers, empirical stock assessment and ultimate fisheries management decisions must be based upon proven scientific methodology. Fishery managers use robust scientific monitoring methods that account for environmental variation and habitat, changes in fishing pressure, fluctuations in forage opportunities, recruitment dynamics, age and growth, and mortality rates that provide data capable of creating accurate estimates of stock structure, spawning stock biomass, and reproductive potential, all of which are involved in managing wild trout populations.

Scientific experts and FWP use probability-based sampling methods from which mathematical estimates of sampling bias, statistical error, sampling error, and confidence intervals can be determined. Surveys not utilizing probability-based statistical methods will fail to accurately capture the true stock structure, or health, of a fish population. The absolute worst outcome for Southwest Montana’s wild trout - and anglers alike - is for wild trout populations to collapse without a roadmap to recovery, which is precisely why trained scientific experts must communicate the extent of the problem and manage our fisheries using science, not anecdotal information.

This summer Save Wild Trout’s lead scientist, Dr. Kyle Flynn, in collaboration with fish health and mortality studies conducted by state fishery managers, will be performing basin-wide water quality monitoring to understand river health conditions. A working model of where baseline river health is poor, in conjunction with physical habitat assessment capable of identifying essential trout refugia, can assist population scale protection and restoration efforts. While various state and federal agencies have considered pieces of the puzzle, such a individual river stretches, over the past 40 years, neither resource agencies or the public have a holistic working model and road map for wild trout protection in the Jefferson subbasin, home to some of the Lower 48’s last, wild trout fisheries. The anticipated analysis will offer an invaluable baseline regarding the location and extent of refugia for salmonids during seasonal warm-weather conditions. This data will be shared with the Governor, FWP, DEQ, and other resource agencies to inform science-based management solutions and restoration efforts necessary to protect and restore wild trout populations in Southwest Montana.


Reprinted and paraphrased with permission from Riverkeeper Inc.