Gender‐dependent effects of water quality and conspecific density on the feeding rate of fish – factors behind sexual growth dimorphism

Sexual growth dimorphism is common among animals, growth rate differing between the genders. Growth dimorphism is common also in fish, but the regulatory mechanisms remain unclear. Variations in feeding rate may lead to sexual growth dimorphism in fish, the growth rate of females decreasing steeper than the growth rate of males when feeding rate decreases. Because water quality strongly affects the prey detection by fish, variations in water quality could affect sexual growth dimorphism. Additionally, variations in fish density could affect dimorphism through food competition. We studied experimentally with perch Perca fluvialitis, whether the effects of decreasing water transparency and increasing fish density on the feeding rate of planktivorous fish are gender‐dependent. We expected that the feeding efficiency of females decrease steeper with increasing water colour and increasing fish density than the feeding rate of males. Additionally, we collected field data and studied the effects of water colour on the growth rate of male and female perch. The results showed that the effect of water colour on the feeding rate of perch was gender‐dependent, while perch density had no effect on the feeding rate difference between males and females. In highly humic water, the feeding rate of male and female perch did not differ, but in clear water females showed a significantly higher feeding rate than males. The results suggested that due to their high energy demand, female perch were feeding at high rate in both water colours, while the feeding rate of males in the clear water experiments was much lower than their possible maximum rate. This was probably due to the decreased feeding activity of males to reduce predation risk. The results were supported by field data, which revealed a significant effect of water colour on the gender growth difference in planktivorous 3‐year‐old perch. The results suggested that variations in water quality may be a factor behind the population‐dependency of dimorphism in fish.