Science Friday: Are some fish caught more than others, and if so, why?
We’re deep in the heart of winter steelhead season now. But if you’re like us, you are probably suffering from high-wateritis right about now — a common affliction for winter steelheaders.
Living on the Olympic Peninsula, I have it bad. Non-stop rain for days. Rivers punched. Headaches due to lack of chrome-ium. Conditions better improve quickly…or I might not be able to focus enough to prepare another Science Friday post.
Fortunately I’m not that far gone yet. So here we go.
In recent posts we have focused on carrying capacity and the importance of this concept in wild steelhead management. Time to cast our lines into a topic that is more directly related to fishing.
Are some steelhead more prone to biting than others? Many anglers have wondered this over the years, and perhaps there is even anecdotal evidence that the answer is yes. If so, it might be possible that in our angling efforts we are selecting against those fish that are more likely to strike, and thus, be caught.
We are not aware of research on steelhead that has addressed this question, but the topic has been evaluated for largemouth bass. Bass aren’t steelhead,obviously, but the underlying mechanisms and behavioral factors are likely similar among different species of predatory fish.
Three studies, in particular, look at various aspects of this topic.
The first study, by David Phillip and several co-authors, conducted a long-term experiment to determine whether some fish were more prone to biting than others and then evaluated whether there was a heritable component to that tendency. From 1977-1980, anglers fished several ponds and the fish they caught were marked and released. After three years, in 1980, the ponds were drained and the fish captured.
The fish were delineated into two groups. Those that were never caught were identified and kept. These were described as LV, or low vulnerability to being captured by anglers. Fish that were caught four or more times were also identified and kept. These were described as HV, or high vulnerability to angling. A subset of those fish in each group were then bred and fish from the LV and HV lines were planted in the lakes and given unique fin clips so they could be identified by anglers and scientists. This process was continued for three generations of fish.
The results are interesting. First, the vulnerability response increased with each successive generation of breeding. In other words, HV fish became increasingly vulnerable to being caught.
Second, there was a heritable component to vulnerability, indicating there is an underlying genetic reason that partly explains why some fish were caught a lot and some fish were not caught at all.
The second study, on the same fish, by Tara Redpath and colleagues, evaluated whether vulnerability to angling was related to physiological traits that have a strong genetic basis, such as metabolism. They found that the HV line of fish had higher metabolic rates than the LV line. Further, the HV line required a longer period of time to recover from exhaustive exercise than the LV line.
What does this mean? Basically, it appears that individual fish with higher metabolic rates were more prone to being caught, and it took those fish longer to recover from angling. It is not clear why this happens, but the most logical explanation is that individuals with higher metabolic rates require more food to grow at the same rate as fish with lower metabolic rates (all else being equal). Basically, they need more food, which likely requires that those fish take more risks to acquire more food. As a result, they are more likely to be caught.
There are a number of implications, but most importantly, the studies suggest that some fish are more vulnerable to being caught. The more times a fish is caught, the greater the chance of mortality. Accordingly, it is possible that angling could select against individuals with faster metabolisms because they are more prone to being caught.
The last study, by Jan-Michael Hessenauer and others, looked at this very topic in largemouth bass. They collected juvenile bass from reservoirs that anglers had not fished for almost a century and two lakes that were regularly fish by anglers. Hessenauer et al compared metabolic rates among juveniles from lakes that had been fished versus lakes that had not.
What they found is that individuals from the unfished lakes had higher metabolic rates, on average, than those from lakes that had a long history of angling. This suggests that recreational fisheries can act as an evolutionary force that influences metabolic rates of fishes.
Largemouth bass are not steelhead, but the underlying premise could be applicable to O. mykiss. Research on steelhead indicates that metabolism plays an important role in life histories. Fish with faster metabolisms tend to take more risks to acquire more food and they also tend to take different life history paths to maturity compared to fish with slower metabolisms.
The upshot is that there appears to be a genetic explanation for why some fish are caught more than others. Of course, adult steelhead, once they enter freshwater, are not metabolizing food for survival — at least until after spawning. It would be interesting to see whether metabolic rate continues to influence the biting tendency of fish when they are in a life stage where they don’t need to feed to sustain themselves.
Still, we are not trying to draw inferences to steelhead in particular here. In this day and age, effects would likely be minimal since most wild steelhead fisheries are C&R and most fish survive C&R. Nonetheless, given that we are all anglers, we thought you might enjoy reading something that is actually focused on fishing.
Have a great weekend!