How fishing changes catchability

Posted on March 27, 2017

The bolder members of a fish population – often defined as those most willing to continue to forage under the threat of predation – tend to be relatively exploratory, aggressive, territorial, and enjoy higher rates of food intake and specific growth than more timid individuals. However, their larger size makes bold fish more vulnerable to fishing, which is often strongly size-selective. This suggests that, by removing the boldest fish, angling may have an impact on size-linked features of the population and on future catch rates. Working in two streams in the Fuji River system in central Japan, one of which had a history of high angling pressure and the other a history of low angling pressure, researchers conducted angling experiments, using a stonefly larvae as a natural bait for amago salmon (Oncorhynchus masou ishikawae). The day after their angling trials they estimated salmon densities by using electrofishing to progressively remove fish. Although fish densities were similar in the two streams, catch rates were significantly lower in the stream under high fishing pressure. And in laboratory experiments with a simulated predator, salmon from the highly-fished stream were significantly more likely to hide under cover and grew more slowly than those from the lightly-fished stream. It seems that the selective removal of bold fish from the heavily-fished stream had indeed resulted in a measurable population bias towards increased shyness and lower vulnerability to angling. As well as having implications for angling success and stock assessment, such behaviour-mediated changes in population composition are also likely to have flow-on impacts on food webs and ecosystem structure.

Reference: Tsuboi, J. et al. 2016. Behaviour-mediated alteration of positively size-dependent vulnerability to angling in response to historical fishing pressure in a freshwater salmonid. Canadian Journal of Fisheries and Aquatic Sciences 73, 461–468.