Physical tidepool characteristics affect age- and size-class distributions and site fidelity in tidepool sculpin (Oligocottus maculosus)

Rapidly changing environments pose unique challenges to the resident organisms. Tidepools in coastal environments vary in biophysical characteristics spatially and temporally, and how they vary determines their short- and long-term suitability as habitats and therefore influence on the distributions of tidepool organisms. Biophysical effects on distribution could differ between age classes, depending on their intrinsic ontogenetic requirements and dominance relations. In this study, we investigate the influence of physical pool characteristics on the site fidelity and population distribution of tidepool sculpin (Oligocottus maculosus Girard, 1856). We assessed short-term recapture of marked individuals and size-class distribution among four pool sets. The proportion of adults varied between pools primarily in association with water temperature and pool volume. Smaller adult and larger juvenile fish occupied warmer, small-volume pools, whereas larger adults occupied larger, cooler pools. Between 24% and 56% of marked fish were recaptured, with a higher probability of recapture in pools with “smooth” basins than in those with more rugose basins. Few fish moved among study pools, but the proportion of adults declined with repeated sampling, suggesting greater pool fidelity of juveniles. These results illustrate that intrinsic habitat features influence age- and size-class distributions in a resident tidepool sculpin species, with corresponding consequences for site fidelity.

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

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.