By definition, fish live in water and with few exceptions (e.g., lungfish, arapaima) can only extract oxygen from water. As such, it should be no surprise that air exposure is not a “good” thing for fish. Of course, a little air exposure is not going to kill a fish. But what defines “a little”? The answer varies based on many factors. For example, some fish species are simply more tolerant to air exposure than others. Species like common carp and bullhead catfish are able to live in water with very little oxygen and therefore tend to also be fairly tolerant of air exposure — as much as 10 minutes of air exposure or more. And others (such as bluefin tuna, bonefish, and salmonids) are extremely sensitive to low levels of oxygen whether they are exposure to air or exposed to waters with little oxygen.
Sensitivity to oxygen can also vary for a variety of reasons for a given species. For example, water temperature influences how much air exposure a fish can withstand. For any given species, fish tend to be able to handle longer periods of air exposure at cooler temperatures than at warmer temperatures. Take bluegill sunfish for example — for a given duration of air exposure, the extent of impact to the fish is always lower for the cooler temperatures than it is for warmer temperatures, and the extent of that difference increases with longer periods of air exposure. Water temperature is SO important for fish that it is referred to as the “master factor” (See Finsights 14), and it affects all biological processes and is also the reason why some fisheries close when water temperatures exceed a given threshold. The closer the fish are to the upper end of their thermal tolerance range, the more important it is to minimize stress from air exposure. Sensitivity to air exposure can also vary depending on life stage. For example, Pacific salmon are quite sensitive to air exposure during early phases of their upriver migration yet when the approach spawning grounds (literally about to spawn), they become quite resilient to air exposure.
The idea that the context matters makes it very difficult to identify a single duration of air exposure to guide anglers in how to handle fish. The default should be “as little as possible”. We are unaware of a biological explanation nor a single scientific study showing that air exposure is good for fish. So, what is a “little”? If we are looking for a single number to apply across the board, the 10 second limit proposed in a synthesis of available data remains the most useful value. The Keepemwet Fishing mantra is about minimizing air exposure and keeping fish wet. Even we scientists love to admire fish and capture the moment just like anyone else. In fact, that was the entire premise for the #keepemwet movement — how fish can be admired in a way that also ensures that they are released in a state where there are likely to survive and thrive.
A couple of recent studies have been critical of the existing literature that suggests air exposure is bad for fish. Indeed, some of the older work was done in the laboratory for experimental purposes and some of those studies use absurdly long air exposure durations. Nonetheless, the patterns that emerged from those studies stand — and the patterns are clear — more air exposure is worse than less air exposure. There are some studies that have failed to demonstrate a negative effect of air exposure. For example, a recent press release from a study by researchers at the University of Idaho used the headline “brief air exposure not a threat to fish survival”. The air exposure durations used in that study were 30 and 60 seconds and involved adult cutthroat trout as they approached spawning grounds. The researchers revealed that there was no difference in survival or reproductive success for control fish (no air exposure) and those exposed to air. However, this study, just as many that have come before it, has its own limitations (e.g. they held the fish in tanks before simulating angling, which we know to be stressful,) and is very context dependent (e.g. survival is a whole different ball game when there are predators around). This context was absent from the press release and we are concerned that anglers and the angling media are left with the impression that “fish are tougher than we give them credit for” when in reality the message is that the impacts of C&R are varied and depend highly on species, location, how a study is performed. “Spinning” such findings to get media headlines does nothing to help improve how fish are handled by anglers. In fact, it does the opposite — it creates confusion. We greatly encourage all anglers to be careful and not take press releases such as this at face value.
We await a study that provides evidence that air exposure is good for fish — that it benefits them in a biologically meaningful way. There are many ways in which anglers can interact with fish and capture the moment forever without extending air exposure beyond 10 seconds. Let’s #keepemwet.