Despite the apparently idyllic setting, when a fisherman settles down beside a peaceful lake he is embarking on a battle of wills, where the ultimate reward is to catch a bite. ‘I find the question of what makes a fish hit a lure to be really interesting’, says Michael Louison, from the Illinois Natural History Survey, USA. ‘I've done a lot of fishing and wondered why in some cases I can pitch a lure near a fish and get an immediate strike, while in other cases the fish ignores it or swims off’. Intrigued by the factors that might drive one fish to lunge while another hangs back, Louison says, ‘We know in all sorts of animals that physiology drives many decisions and we wanted to know if certain behavioural or physiological traits were key in leading a fish to be more likely to attack a lure’.
Fortunately, Louison and his colleagues, Jeffery Stein and Cory Suski, had access to two populations of largemouth bass – one descended from fish that readily snapped at bait and a second descended from a population that was more reluctant to bite – which had been nurtured by the Illinois Natural History Survey since the late 1970s. However, before Louison could settle down to his favourite pastime and ‘net’ some fish, he and Shivani Adhikari had to assess the animals’ boldness and responses to stress.
Isolating individuals from both populations in a tank divided into four zones, Louison and Adhikari monitored how eager each fish was to explore its surroundings. They then measured the amount of stress the animal experienced a day later, by holding the fish out of water for 3 min and then recording the levels of the stress hormone – cortisol – in the fish's blood. In addition, the duo tagged the fish so that they could identify each individual and measure its oxygen consumption while it was recovering from fast swimming to calculate its metabolic rate. Once Louison and Adhikari had completed the measurements, they released the animals into a pond that had been specially prepared for the study, and then Louison and Ryan Solomon collected their rods and went angling for a couple of hours a day for a week during the summer, logging each fish that they caught before returning them to the water.
However, after analysing the catch rates of the fish descended from the population that had been difficult to catch in the 1970s and those descended from the second population, which had been keen to bite and were highly vulnerable to capture, Louison was surprised that they were the same. ‘We had expected that our “high vulnerability line” would actually be more likely to be caught’, he says, although he suspects that they are still more catchable, but possibly at other times of year or when using different baits. The characteristic that seemed to be linked most strongly to the fish's ability to avoid capture was their response to stress. ‘Fish that showed a more pronounced rise in cortisol levels after an air exposure were less likely to be caught’, says Louison. He suspects that the fish that are more sensitive to stress may be shyer and less prepared to risk snapping at tasty morsels dangling before them, helping them to avoid capture.
But what could this mean for fish populations that are constantly under the fisherman's eye? ‘We can definitely expect to see selection favouring fish that are harder to catch’, says Louison, which could make it trickier for anglers to land fish in the future.