By Sean Mitchell
Freshwater fish are challenging to find, to count, and to sample. Whether in lakes or ponds, rivers or brooks, they are adept at avoiding detection and capture. Though this is true throughout the year, during the winter months — a time of cold water, long periods of darkness, usually ice cover — these challenges become particularly acute. It behooves us as investigators to understand fish in this season so we might better be able to sample them.
One of the key ecological discoveries in the last twenty years of research on rearing salmonids has been the 6°C cutoff — a distinct temperature boundary through which, as the fish passes, the behaviour changes. Above a mean daily temperature of 6°C, juvenile salmonids are active, or at least less cryptic. As the water cools in the autumn though, these fish seek substrate cover and spend the daytime hours concealed under boulders and cobble. While there is light in the water, illuminating them to warm-blooded mammalian and avian predators, they remain in their shelter. Once darkness descends, however, they emerge for activity through the night: feeding, holding position, interacting.
Historically, we thought that fish were physiologically limited in the cold water; that their energy-budgets would not allow them to be active as ambient temperatures dropped and approached the lower single digits. We assumed they hunkered down, entering a state of torpor, and remaining in place through this challenging period. This, we now know, is naïve. In a country such as Canada, where winter may extend for more than a third of the year, cold-water adapted fish have responded by limiting their energy-expenditure to only those necessary to maintain body condition, but they do remain active; muted activity to be sure compared with warmer temperatures, but active. And — most importantly for those of us trying to sample them — available.
This behaviour has an obvious influence on our ability to detect presence or estimate abundance during the cold seasons. If we electrofish or snorkel an area during the day, when visibility is best and we are in our working hours, the fish are simply not available to be found. They are there, only hiding deep into the substrate beyond our ken. At night however, when the darkness provides them a ubiquitous and effective form of cover from visual predators, they emerge for their limited daily activities. This is the time we should be looking for them. Obviously, night work in streams under cold conditions carries a new suite of challenges, from visibility to safety to simply access. But when done the evidence shows a frequent doubling, or more, of catch.
Appreciating our time-of-day bias is very important in our conclusions when doing winter work. Zero to low catches may not reflect actual absence. A wise sampling program would include some night work in order to measure or calibrate this bias. We need to be aware of what we are missing when we sample in addition to what we are catching.
1 Title shamelessly stolen from my PhD supervisor’s paper: Cunjak et al. 2011. Atlantic salmon (Salmo salar) in winter: ‘the season of parr discontent’? Canadian Journal of Fisheries and Aquatic Sciences. 55(S1): 161-180.