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All Fish Are Not Created Equal

by Simon Blanford
Fly Fishing Science

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Fishermen spend a lot of time thinking about how to catch fish. It’s in the nature of our pursuit. If we were  golfers it would be about getting the ball in the  hole. But because we are fishermen it’s all about the catching.

We rarely voice this fact  quite so baldly but we do, nevertheless, spend time, particularly on the eve of a trip, considering the factors influencing the vulnerability of fish to our flies. For example, before a trip I will certainly be glued to forecasts from the National Weather Service and, so as not to put all my eggs in the meteorologist’s basket and because I’m originally from England, I’ll be keeping an eye on whether or not the cows are lying down. I’ll also track the height and likely color of the river and compare it to past water levels and how fish have behaved previously, imagining what they might do given X, given Y, given A or B or C. I will relate this information to the season, the type of hatches that I might expect to occur. I might even ponder the skill and disposition of other anglers on my chosen day on my chosen water. I will consider my flies.

All of this preparation is designed to help me land a few fish. The water I choose, the tactics use, perhaps even the species of fish I seek, will be influenced by this collation of information, my experience and, not to put too fine a point on it, a hunch or two. I consider the fish, if I really do at all, only as an amorphous mass whose tendency to take my imitations will vary along with the factors I’ve been obsessing about. If I am after discerning trout in my local river, I imagine them all as a collective “trout” who will, broadly speaking, move, eat and respond to flies in a similar manner. If I decide to pursue rotund carp in the nearest warm water creek I think of them only as a collective “carp” who, my research may tell me, will all be feeding on molting crayfish and might well take a carefully presented imitation.

So, whatever the species and wherever I end up fishing, this mental effort is directed not to a single fish, not at an individual, but at the collective body of fish that swim in the chosen water. All trout, all carp, smallmouth, bonefish are alike—each to their own kind—and will react alike to the prevailing conditions and my attempt to catch them. I don’t tend to shape my tactics to an individual fish unless I am pursuing a particular vendetta.

Mind you, pursuing a vendetta is an instructive thought. It is tacit acknowledgement that some fish behave differently to the general hoi polloi. Most often this occurs with a fish who has refused all the flies in your commodious box of tricks, when even the most hopeful “thing with feathers”  fails to elicit a response. These difficult fish are one of the delights of angling (though possibly more so when considered in retrospect, beer in hand) and a cornerstone of fly fishing conversation and debate. We love to chew over how the “wise” old trout can so cleverly discriminate between our flies and those that are real (not to discriminate against other species but “the educated trout” remains a leitmotif for this particular topic). Naturally, we recognize places where whole populations of fish are, by and large, harder to catch—Henry’s Fork trout, Florida Keys bonefish—but we also recognize single fish, particularly if we fish one body of water regularly, as fantastically difficult: Herbert beneath the bridge; Maude tucked up against the alder roots; that low-down-son-of-a-sucker under the willow fronds.

Now here’s the interesting part: while we conjecture and theorize about these post-graduates, we rarely consider whether individual fish are predisposed to be easy or hard to catch. That is, are some fish, from the egg, having never seen an artificial fly before – much less having been caught on one—harder to catch than others that have grown up under the same conditions?

Fisheries biologists have been interested in this question for many years and there are reports on the subject going back to the 1930s. Then, in the late 1970s a study was carried out on a lake in Illinois, a study originally designed to look at whether catch and release would lead to lower catch rates because of “learnt hook avoidance” by largemouth bass. What they showed was something slightly unexpected. Over the five year study period, during which bass were being caught regularly, there was large variation in the number of times individual fish were landed. One bass was caught twelve times in one year and another fifteen. At the other end of the scale there were fish that were never caught—not once during the whole study period. Given the level of angling, the accessibility of the lake, and the fact that these fish had no initial experience of being fished for prior to the experiment starting, this was surprising

The result got the biologists thinking. From a fisheries management point of view (and in the opinion of some anglers) the fact that there are fish that are difficult, or even impossible, to catch from the outset is not necessarily a good thing. It makes those fish inaccessible to anglers. The variation in catchability the bass showed could not be explained simply by prior experience or strain. The data actually suggested there might be some underlying trait, some more fundamental difference in make-up between these two kinds of fish. If so it meant that the trait could be selected for by careful segregation and breeding. In other words it might be possible to rear one line of fish that are highly vulnerable and a different line relatively immune to angling. The fishery management possibilities with lines of fish that are easy or difficult to catch are clear. So that is what they set out to do.

From the study above the biologists took bass that had never been caught and those that had been caught four times or more, separated each group into their own ponds, allowed them to breed, let the offspring grow up for three or four years, moved them to new ponds, fished for them, recorded what was caught and how many times, drained the ponds, separated them by capture frequency again, and repeated the whole process another two times.

It took fourteen years to get the results for just three generations of bass, but the outcome was as clear as they could have wished. Over the selected generations the offspring of bass that had rarely been caught were even harder to catch than their parents. Conversely, the offspring of bass that had been caught a number of times were even easier to catch than their parents. This was a clear demonstration that not only is there initial variation in a fish’s response to angling but also that the variation can be selected to increase, or decrease.

What was driving this difference? Was it feeding behavior? Were the low-vulnerability group not stimulated to strike at prey in the way we associate with this species? Did they mooch along feeding on plainer food items, the highjinks of the bass bug just background noise? Did they perhaps become vegetarian?

It turns out that the two lines differed markedly in their “pace of life.” The fish that were hardest to catch, had lower metabolic rates compared to those that were easy to catch. The hard-to-catch bass also had higher food conversion efficiencies: more of what they ate was used for growth rather than burned away on fast living. As a result they grew more quickly, ironically making the ones that are harder to catch also bigger. Additionally, these bass, to go with their low metabolic rates, didn’t show the classic fast-startle response (the rapid swimming burst fish use to escape predators or attack prey) that we would expect to see from such a confirmed predator. They didn’t exactly amble but nor did they give it the big hurry-up either. This was in stark contrast to bass in the easy-to-catch group which shot-off in quick energetic bursts in response to every bit of persuasion.

Nevertheless, the easily catchable, live-fast-and-die-young bass (there was a suggestion that survival in vulnerable fish was lower) turned out to be better parents than the laid-back group. They fanned their nests (to provide oxygen and keep the eggs clear of algae, bacteria etc.) at a higher rate than the hard-to-catch bass, stayed at the nest for longer, were more vigilant and attacked and drove off intruders more aggressively.

Ultimately, which fish (and don’t forget it is a continuum not simply that there are two categories of fish, one “easy”, the other of ”difficult”) are ultimately more successful will depend on a myriad of factors specific to each body of water. And though these studies dealt with bass, the results are applicable to different species of fish, since the same variation in pace of life has been shown for a range of other animals. In the end, the fact that there clearly is a basis for differential susceptibility to angling in the absence of experience is rather neat. And it provides a possible explanation for our often intimated sense that there are “easy” and “difficult” fish in our target waters.

So recall my putative trip. It seems I am wrong to consider the fish as an amorphous mass hidden beneath the surface. Whether I look out over a lake, or river or even estuary, slough, beach or flat—whether I am fishing for trout, carp or bass or redfish, bonefish or permit—I have to allow that some of them will be easier to catch than others. There will be some who launch themselves at the fly I present and then there will be a bunch who simply don’t—those who don’t care a wit about the real jungle cock I laboriously and expensively added to my fly.

The kind of behavioral differences shown by the bass has begun to get a lot of attention in other systems in recent years. Previously variation between individuals in a population had been dismissed as noise—maladaptive behavior by a few extreme individuals. What was important, so it was thought, was the general pattern of behavior, the average for a particular group of fish. Nowadays there is much more interest in the range of behaviours individuals show. With this shift comes another way of describing the two kinds of bass. For example, one might be tempted to say that those bass that are difficult to catch show all the symptoms of being timid or shy, and the bass that are easier to catch appear much bolder, much more aggressive. But this kind of classification is more commonly used in psychology when talking about human personality and not when describing wild animals.

If I had made the suggestion ten to fifteen years ago that fish had different personalities I would have been told, in no uncertain terms, that wild animals do not have “personalities.” But time and research move on. If I voiced the same thought now I would find a wave of new research that would agree that some bass really are “shy” and some other very “bold”. Something to look at in the next article.

MidCurrent Fly Fishing
Simon Blanford has been a biologist for twenty years, fished for more than forty and has long been interested in how research on fish behavior helps us understand the fish we try to catch. He lives in central Pennsylvania where the trout, carp and bass routinely snub his carefully presented flies. He can be contacted at
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