Water temperature, vertical distribution, and risk of ectoparasitism in juvenile sticklebacks

We tested the hypothesis that the risk of juvenile sticklebacks (Gasterosteidae) being parasitized by the crustacean ectoparasite Argulus canadensis decreases with increasing water temperature. In the laboratory, juvenile sticklebacks and parasites were acclimated to three different temperatures, 18, 22, and 26 °C. The attack performances of single parasites on groups of 20 fish were measured at each temperature. We found no significant effect of temperature on the attack success (percentage of attacks successful) or the attack rate (number of attacks per minute) of parasites. In addition, we sampled sticklebacks in their natural habitat, salt marsh tide pools, three times per day, when water temperatures were lowest, intermediate, and highest. The relative abundance of parasites on fish (total number of A. canadensis divided by the total number of sticklebacks sampled) was twice as high during midafternoon, when temperature was highest, than in the early morning, when temperature was lowest. Sticklebacks usually swam close to the bottom of tide pools, where parasites were resting, in midafternoon, and at the water surface in the early morning. These changes in the vertical distribution of the fish may result from diel fluctuations in temperature or in other abiotic factors, and small differences in these factors between the bottom and the surface of the pools. Our results indicate that, while it had no direct effects on the attack success and attack rate of the parasites, water temperature, or other abiotic factors, may have indirectly affected the sticklebacks' chances of being parasitized by changing their vertical distribution, and thus their spatial overlap with parasites.