Updated: Jun 8
If you're interested in fishing, especially fly-fishing, you've likely heard about, felt influenced by, or even practiced catch and release. It has been said that catch and release fishing allows anglers to "best" nature (catch the fish) and be "healed" through nature (release the fish) [Arlinghaus et al. 2013]. Yet, by practicing catch and release (C&R) we actively alter our favorite C&R rivers and lakes by influencing the dynamics of the fishery. Several groups of scientists have studied the effects of C&R and I believe these findings are worth looking at for the inquisitive angler.
There is strong evidence that exploited fish (subject to C&R) adjust their behavior in response to human entanglement, inducing more timid behavior. This has been studied in many freshwater fish species, including bass, pike, and trout (Arlinghaus et al. 2016). Timidity leads fish to favor refuge habitat over navigating the open water column, making fish harder to find and stalk. Young and Hayes (2004) even went as far as to say that restricting access may be required to maintain naiivity in fish populations and maximize quality angling experiences (i.e., catching many fish instead of one or none on an outing). A bold claim, especially when considering that many of the famous western trout tailwaters are C&R rivers with public access. The overwriting message is that fish are forced to rapidly learn and tamper bold behavior in intense C&R environments. Another researcher, Askey, and his team of anglers... I mean scientists... found that sustained angling for 8 hours per day led to a drop in catch rates in as soon as 7-10 days (2006). When you think about high-pressured famous waters, some stretches surely experience this type of prolonged angling exposure, especially in the summer and fall months.
There is even some evidence that timidity is passed down through generations. If bold behavior in fish (like chasing lures and flies) leads to more encounters with anglers, then that fish might have expended too much energy eluding its captor that would otherwise be spent on foraging or survival. As a result, excessive fatigue could decrease a trout's fitness: their ability to successfully reproduce. [Side note: human-induced evolution is nothing new.] This is usually why you hear suggestions to leave spawning fish alone.
These bold fish are less likely to pass on their genes to the next generation, while the timid fish, who adapted to angling pressure by learning to avoid anglers, are more likely to pass on their timidity to the next generation (Alioravainen et al. 2019). But, bolder fish are what anglers want to catch. Bold behavior, without C&R pressure, promotes greater foraging, and thus larger size. These are the fish people want to catch!
So are all C&R fisheries going to become more and more difficult for anglers as time goes on? Yes, and no.
I'll start with the no. One overwriting factor that will keep fish biting is starvation. Fish can act as timid as they please, but eventually, they will need to forage to survive. A study was conducted examining timidity in two different groups of juvenile trout where one was starved longer than the other, and the results suggest that starvation likely increases catchability and decreases hook avoidance (Yoneyama 1996). For angler success with timid fish, it might boil down to timing, like fishing as run-off recedes when starvation is likely increased, or at night when angling pressure is negligible. Another supporting reason for why the fish will keep biting, may be found in streams that maintain robust populations. With more fish available to catch, the chances for hooking a single individual is reduced.
One final case for no: Innovation in fishing techniques and fly patterns can knuckle-ball trout into overwriting their precautions. Technology and human ingenuity are often used as bail-out solutions, but as long as there are picky fish, there will be an angler with a new approach to fool him.
Now for the yes... which really depends on angling pressure, and the sustained popularity of a C&R fishery. The more pressured a section of C&R water, the more encounters feeding fish have with anglers. And so it follows, with more encounters come more hook-ups and catches. With more hooked fish, more timid fish, and increased angling difficulty. So we may be faced with a positive feedback loop where C&R fishing creates fly-shy fish with highly timid and selective behavior, that could influence the evolutionary adaptations of future generations of the fishery. (This assumption only works if fish can naturally reproduce in the river system. Stocked fish are not going to be as timid).
Here's another trout-specific example: I'd argue the homogenization of river flows for human needs has increased timidity in trout. With regular controlled flows (think of your average tailwater), fish are not faced with unique, changing habitat as the water rises for runoff and falls in the autumn, but have the same habitat and surroundings much of the year. Therefore, they can get very receptive to changes in their static environment, i.e. knowing where threats are coming from, and where to hide. While in an unregulated river, runoff can completely change where fish rest and eat, and though trout are highly adaptive and capable of navigating these conditions, the threats (anglers' hooks) aren't repeatedly presented in the same ways, prolonging the naiivity of trout. Not to mention bug diversity is greater in unregulated rivers, compared to regulated rivers. In the graph below, note the predictably consistent flow after dam installation (red-dash) compared to the natural runoff spike in the unregulated pre-dam river (blue-solid).
This article in no way intends to suggest that C&R is a bad management technique. We simply wished to shed some light on how fish respond to pressure from angling. Humanity is in the age of the Anthropocene, where humans have greatly altered their environment, and this presents us (anglers) with new questions about ecological dynamics and human-nature interactions that we must consider if we are to continue fishing successfully.
Arlinghaus et al. 2013. Understanding the Complexity of Catch-and-Release in Recreational Fishing: An Integrative Synthesis of Global Knowledge from Historical, Ethical, Social, and Biological Perspectives. Reviews in Fisheries Science, 15(1-2), 75-167.
Arlinghaus et al. 2016. Passive gear-induced timidity in wild fish populations and its potential ecological and managerial implications. Fish and Fisheries, 18(2), 360-373.
Young & Hayes. 2004. Angling Pressure and Trout Catchability: Behavioral Observations of Brown Trout in Two New Zealand Backcountry Rivers. North American Journal of Fisheries Management, 24(4).
Askey et al. 2006. Linking Angling Catch Rates and Fish Learning under Catch-and-Release Regulations. North American Journal of Fisheries Management, 26(4).
Alioravainen et al. 2019. Behavioural effects in juvenile brown trout in response to parental angling selection. Canadian Journal of Fisheries and Aquatic Science, 77(2), 365-374.
Yoneyama et al. 1996. The Effect of Starvation on Individual Catchability and Hook-avoidance Learning of Rainbow Trout. Nippon Suisan Gakkaishi, 62(2), 224-236.