Migrations have always been gruelling, from the Herculean flights of bar-headed geese to the epic odysseys of salmon returning upriver to their spawning grounds. However, when humans began altering river systems, taming them with dams, the channels of many migrating fish became utterly impassable. To mitigate the disruption, fish ladders – or fishways – are often installed to expedite the passage of migrating species. However, many of the current fishway designs are based on our understanding of a few poster-child species, such as the salmon, and few have considered the impact on other, less athletic species. ‘Sturgeons are one of the most threatened groups of fish in the world’, says Jason Thiem, from Carleton University, Canada, adding, ‘Little research has been conducted on fishway use by sturgeon, despite the recognition that barriers to migration represented a threat to the process’. With the future of these species in jeopardy, Thiem, Jeff Dawson and Steven Cooke decided to track the progress of lake sturgeon (Acipenser fulvescens) through a fishway to find out how much exertion the ungainly swimmers require to negotiate these obstacles.
After selecting the Vianney-Legendre Fishway on Quebec's Richelieu River for their study with the help of sturgeon and fishway experts Pierre Dumont and Daniel Hatin, Thiem and colleagues had only a brief 2-week period to capture sturgeon as they congregated below the fishway, ready to negotiate the obstacle in a bid to swim upstream to spawn. ‘It took 3 years to get the right field conditions for us to be able to undertake this field study’, says Thiem, who recalls battling adverse conditions. Having previously drained the 85 m long channel and fitted 15 antennae at intervals along its course to track the fish's progress, the team then fitted electronic tags and accelerometers to 44 sturgeon to monitor their movements, before releasing the animals and recording their progress as they ascended the fishway.
Retrieving the tags from the fish and releasing them at a spawning site, Thiem realised that although seven of the fish had successfully negotiated the 85 m channel – after as many as 16 attempts – 23 failed to reach the end and 14 never even attempted the climb. Calculating the fish's swimming speeds, the team discovered that most of the fish swam at relatively leisurely rates (≤1.25 m s−1); ‘We rarely observed high speed swimming and hyperactivity’, recalls Thiem. And when Thiem, Andy Danylchuk, Adrian Gleiss and Rory Wilson analysed the fish's energy consumption, there was little difference between the fish that succeeded in reaching the top and those that did not. However, the successful fish used more energy per meter (42.75 J kg–1 m–1) than the unsuccessful sturgeon (25.85 J kg–1 m–1). And when the team calculated the fishes’ energy consumption as they negotiated the two turns in the channel, they saw that the sturgeons’ cost of transport increased significantly. The team also compared the plucky sturgeons’ energy consumption as they tackled the ascent with the energy consumption of sturgeon swimming at similar speeds through a tranquil lake – their usual environment – and realised that as they repeatedly attempted to ascend the channel, the sturgeon consumed as much energy as sturgeon swimming 5.8–28.2 km in peaceful lakes.
‘Swimming uphill is energetically demanding’, says Thiem, adding, ‘We expected sturgeon to be poor swimmers… [however,] this is not the case and swimming performance is unlikely to be a limiting factor in fishway passage success’. Thiem also adds that sturgeon would benefit enormously if fishways were redesigned and the turn structures removed. ‘We identified that the absence of turning basins in the current fishway design would reduce passage time and correspondingly reduce energy expenditure’, he says.