#32: Empirical evidence of fish phenology shifts related to climate change
Dan Isaak
Are Fish Like Flowers?
Hi Everyone,
The answer to the question is probably not in most regards, but perhaps so with their response to climate change. Recall that last time we highlighted the Parmesan and Yohe review study describing the global rates at which species distributions have been shifting poleward and life history phenologies accelerating as global warming proceeds (graphic 1; blog #31). This time we’re examining the empirical evidence for the latter in fish populations, because unfortunately, none of the 1,700 species Parmesan and Yohe reviewed were freshwater fish.
To see biological associations with climate trends what’s usually needed are long-term monitoring records. Maintaining such efforts on shoestring budgets is always a challenge (because it just monitoring data, right?) but these data do exist and have a way of popping up once their importance becomes better appreciated. And as we’ve seen time and again, these “just monitoring” data often prove invaluable since there’s no way to ever go back in time and measure something once we realize later that it’s important. As a brief aside, graphic 2 shows the famous atmospheric carbon dioxide monitoring record initiated by Charles Keeling on Mauna Loa in the late 1950’s that’s been instrumental to understanding this whole global warming business. Without that record (& I’m sure at the time he heard plenty of, “you want to measure what? where? and for how long?!!!”) and global air temperature records over the last century, we’d have little insight regarding what’s presently afoot with the Earth’s climate system.
But back to biology—long-term records—phenologic shifts. The basic idea is that warmer temperatures accelerate metabolic processes, which in turn accelerate growth and development of individuals, so those individuals tend to do things earlier and earlier over time across generations. Intuitively this makes sense—we’re all accustomed to seeing flowers bloom at different times in the spring depending on whether it’s a warm or cool year; the grass needing to be mowed sooner or later depending on whether it’s been a late or early spring; the geese migrating south depending early or late depending on when winter came that year. With climate change, it’s the same thing; we’re just stretching the time horizon so that the climatic variation occurs as tiny increments each year rather than relatively large, inter-annual changes. And that’s why long-term monitoring efforts are needed, that inter-annual “noise” easily swamps the climate trend “signal” that may be there unless we’re there for a long time.
So one of the places we’ve tended to hang around fish for a long time consistently is when they’re on or near their spawning grounds and we’re either trying to eat them, count them, or capture them to steal their gametes. Kindof creepy if you think about it, but in some instances now we do have good datasets with which to assess long-term trends in fish showing up for their annual harassment. One study based on such a dataset is by Wedekind & Kung in which they describe 60 year trends in the timing of spawning for European grayling in a Swiss lake river outlet (graphic 3). Over that period, the dates at which the first-, median-, and last- reproductively mature females arrived at the spawning ground has steadily advanced at the rate of 6 days/decade so that now the fish are getting busy a full month earlier than they once did. During the 40 year period for which river temperature data were monitored, a clear warming trend was also apparent. In a twist to the normal situation, few nearby air temperature data were available to determine whether this warming trend was indicative of local climate trends, so the authors used the sugar content of grapes from a nearby vineyard as a surrogate for air temperatures. Apparently grapes are more sugary when it’s warmer so they track air temperatures. And those same air temperatures drive river temperatures, which drive grayling spawning, so it’s all one big ball of wax & maybe fish are like grapes too?
Another place we’ve long hung around fish are at dams and other impediments we or nature have constructed along the migratory routes some species take to their spawning grounds. Say what you will about dams in terms of their effects on fish populations, but they do sometimes make wonderful weirs & counting facilities, especially when equipped with fish ladders and windows where people sit and count fish all day for decades on end. Talk about a job for a fish voyeur, and talk about some good long-term data! So the second study today by Crozier and colleagues is based on one of these datasets, from Bonneville dam on the Columbia River in the Northwest U.S. Here, sockeye salmon have been counted passing the dam since 1938 and clear trends toward earlier migrations have emerged as temperatures increased during the last 70 years (graphic 4). The authors go beyond simply describing these trends, however, to also develop a technique for assessing the relative roles of phenotypic plasticity and genetic changes in causing the migration trend. They argue that a significant portion of the trend could be due to genetic changes forced by the selective gradient that temperature increases are causing. If that’s the case, it’s not just that fish are plastic and are shifting their timing, but they may also slowly be becoming different fish through evolutionary processes.
What a great example of how good monitoring data can feed directly into powerful science & facilitates the development and testing of theory. I wish we were running climate-aquatics experiments in different ways this century that didn’t risk so much biodiversity, but it is what it is. A non-stationary climate will provide many such natural experiments that allow us to learn a great deal about ecology and biophysical relationships if we’re watching the right things in the right places for a long time.
Until next time, best regards,
Dan