Experimental tests of intercohort competition for food and cover in the tidepool sculpin (Oligocottus maculosus Girard)

Abstract

In the field, the tidepool sculpin, Oligocottus maculosus, has been observed to display a vertical intertidal distribution characterized by increasing mean standard length with decreasing distance above mean tidal height. It was hypothesized that this distribution is related to intercohort competition for food and (or) cover and, as a result, competi tive exclusion of smaller fish by larger fish from the lower pools. To test this hypothesis, several competition experiments were conducted that involved pairing O. maculosus from three size classes in a laboratory setting and observing their behaviour when presented with food or cover. When both fish attempted to procure food, the success rate of the larger fish was significantly higher than that of the smaller fish (Wilcoxon's signed-ranks test, T = 0, N = 6, p < 0.025). Furthermore, as the size difference between the two fish increased, the smaller fish made significantly fewer simultaneous attempts to procure the food (hierarchical log-linear test, partial χ2[5] = 28.326, p < 0.001), and utilized the cover significantly less (ANOVA, F[5] = 3.387, p = 0.008). The results from these experiments indicated that larger fish have a competitive advantage over smaller fish which extends to the acquisition of both food and cover. This is presumed to be the result of both greater competitive ability due to increasing size and an increased need for smaller fish to avoid detrimental aggressive interactions. In view of evidence from other papers indicating that the lower pools are more desirable to O. maculosus, it is suggested that intercohort competition is in part responsible for limiting the smaller fish to the upper tide pools.